HIV and Drug Use: A Tale of Synergy in Pulmonary Vascular Disease Development

Abstract

Over the past two decades, with the advent and adoption of highly active anti-retroviral therapy, HIV-1 infection, a once fatal and acute illness, has transformed into a chronic disease with people living with HIV (PWH) experiencing increased rates of cardio-pulmonary vascular diseases including life-threatening pulmonary hypertension. Moreover, the chronic consequences of tobacco, alcohol, and drug use are increasingly seen in older PWH. Drug use, specifically, can have pathologic effects on the cardiovascular health of these individuals. The “double hit” of drug use and HIV may increase the risk of HIV-associated pulmonary arterial hypertension (HIV-PAH) and potentiate right heart failure in this population. This review explores the epidemiology and pathophysiology of PAH associated with HIV and recreational drug use and describes the proposed mechanisms by which HIV and drug use, together, can cause pulmonary vascular remodeling and cardiopulmonary hemodynamic compromise. In addition to detailing the proposed cellular and signaling pathways involved in the development of PAH, this article proposes areas ripe for future research, including the influence of gut dysbiosis and cellular senescence on the pathobiology of HIV-PAH.

INTRODUCTION

Although the adoption of highly active antiretroviral therapy (HAART) has significantly altered the course of the HIV pandemic, HIV infection remains a major global public health concern. While the total number of AIDS-related deaths has dropped substantially (down 48% between 2005 and 2016), one million people died from an AIDS-related illness in 2016 alone, and a total of 1.8 million new cases of HIV were diagnosed that same year (1). Between 2001 and 2014, in the age of HAART, the proportion of PWH increased by more than 100% (2). Nearly 37 million people are living with the infection, 991,000 of whom live in the U.S. In low-income countries, HIV prevalence is much higher. The burden of disease is highest in eastern and southern Africa, where 19.4 million people are living with HIV (1). Interestingly, between 1990 and 2016, Eastern European and Central Asian countries collectively saw a 60% increase in the number of new infections (1). Of the 37 million PWH across the world, only 59% are currently on ART, and 81% of those individuals have achieved viral suppression (3).

HIV mainly infects CD4+ T cells, monocytes, and macrophages by binding its envelope glycoprotein 120 (gp120) to the CD4 receptor and co-receptors CCR5 and CXCR4 (4) (Figure 1). Following entry into the cell, the virus may remain latent within the host cell or, alternatively, viral genes may be transcribed, translated, and replicated with the help of important regulatory and accessory viral proteins, such as Tat (transcriptional trans-activator) and Nef (protein negative factor) (1, 5). Tat is a regulatory protein that recruits proteins to relieve repression of the viral long-terminal repeat (LTR) that prevents HIV transcription. Nef, another important accessory protein, can downregulate cell surface expression of CD4 and major histocompatibility complex (MHC) class I, enhance HIV-1virion infectivity, and impair Fas- and tumor necrosis factor (TNF) receptor-mediated apoptosis (6). Importantly, Nef can also enhance viral infection by promoting the secretion of Nef-containing extracellular vesicles, which will be discussed further below (7).

Figure 1: HIV Life Cycle.

HIV is capable of infecting CD4+ T cells, monocytes, and macrophages. The virus enters a host cell by fusion with a CD4 + CCR5/CXCR4 receptor complex through interactions with surface glycoprotein gp120. Then, the ssRNA genome is reverse transcribed inside the host cell’s cytoplasm by HIV reverse transcriptase. The viral cDNA next enters the nucleus of the host cell where it can be integrated into the host DNA genome by HIV integrase. Integration into the host genome enables HIV to establish active and chronic infection, and in consequence, HIV proteins are also chronically expressed in conjunction with host cell proteins.

While HAART has significantly advanced HIV treatment and increased the lifespan of PWH, chronic HIV infection has increased the burden of comorbid conditions associated with aging. In a cross-sectional analysis of adults receiving outpatient HIV care conducted by the Coordinamento Italiano per lo Studio di Allergia e Infezioni da HIV (CISAI), more than 50% of patients had more than one comorbidity, and 17% had three or more. Multi-morbidity was associated with older age, higher body mass index, history of smoking, HIV disease stage, and longer ART duration (8). HIV-associated neurocognitive disorder (HAND), cardiovascular disease and chronic lung complications are some of the common non-infectious complications observed in PWH. Most importantly, longer lifespans in HIV patients because of HAART have led to an increased incidence of pulmonary arterial hypertension (PAH) in PWH (9). HIV-PAH is highly devastating because of its rapid pathogenesis and mortality that is not shown to be attenuated by HAART and well-managed HIV (9, 10). Current therapies for HIV-PAH are not curative and, while they may improve symptoms for some patients, are not shown to improve mortality (11, 12).

Furthermore, illicit drug use is associated with an increased risk of HIV transmission, disease progression, and non-compliance with ART (13) and the chronic consequences of drug use are increasingly seen in older PWH. Drug use, specifically, can have pathologic effects on the cardiovascular health of PWH. The “double hit” of drug use and HIV increases the risk of HIV-associated pulmonary arterial hypertension (HIV-PAH) and potentiates right heart failure in this population. In this review, we will discuss in detail the epidemiology, clinical presentation and diagnosis, and pathophysiology of HIV-associated pulmonary hypertension, one of the most devastating non-infectious complications observed in PWH and explore the relationships between HIV and drug use as they relate to the development of HIV-PAH.

OVERVIEW OF PULMONARY HYPERTENSION

Pulmonary hypertension (PH) can be associated with a wide range of comorbid conditions, but, regardless of etiology, the disease generally progresses along a common pathway that involves narrowing of the pulmonary vascular lumen due to chronic vasoconstriction and structural remodeling of pulmonary vessels and subsequent increase in pulmonary vascular resistance (PVR) and mean pulmonary arterial pressure (mPAP), that leads to progressive Right Ventricular (RV) failure, and death. In 2018, the 6^th World Symposium on Pulmonary Hypertension (WSPH) Task Force updated the hemodynamic definitions of PH to include pre-capillary, isolated post-capillary, and combined pre- and post-capillary PH.

The WSPH Task Force updated the clinical classification of PH into five groups according to shared clinical, histologic, pathophysiologic, and therapeutic characteristics. Broadly speaking, Group 1 PH consists of all forms of pre-capillary PH or pulmonary arterial hypertension (PAH) including HIV associated PAH; Group 2 is PH secondary to left heart disease; Group 3 is PH due to lung diseases and/or hypoxia; Group 4 is PH related to pulmonary artery obstructions (i.e., chronic thromboembolic pulmonary hypertension (CTEPH)), and Group 5 is PH with an unclear cause or multifactorial mechanism (14). The specific pathophysiology of each clinical group is beyond the scope of this review (15). This review will mainly focus on pre-capillary PH or PAH, which is defined as a mPAP greater than 20 mmHg (down from 25 mmHg in prior definitions), pulmonary artery wedge pressure (PAWP) of ⩽15, and PVR ⩾3 Wood Units (14) when more common causes of pulmonary hypertension, such as left heart disease, chronic lung disease, or venous thromboembolism are not present.

Clinical Presentation & Diagnosis of PAH

Clinically, patients with PAH present with progressive dyspnea, lower extremity edema, dry cough, fatigue, syncopal symptoms, and chest pain. A major determinant of functional and clinical outcomes in PAH is RV dysfunction. Hemodynamic assessment of mPAP, right ventricle systolic pressure (RVSP), PAWP and cardiac output with right heart catheterization is the gold standard for the diagnosis of PAH. Early on, the RV’s adaptive response to elevated pulmonary artery pressures results in mild RV dilation but preserves stroke volume and maintains normal filling pressures. As the RV adjusts to increased pulmonary vascular afterload, it maintains efficient energy transfer through hypertrophy and increased contractility, which preserves ventriculo-arterial coupling. As the disease progresses, the RV response becomes maladapted with further RV dilatation, reduced stroke volume, and increased filling pressures, resulting in ventriculo-arterial uncoupling. As such, RV ejection fraction (RVEF) has been shown to be predictive of worse outcomes in patients with PAH, especially RVEF < 0.35 (16). Right ventricle function is the key dictator of clinical outcomes in PAH, and failure of the right ventricle is the most common cause of death in patients with PAH (12, 17).

Pathophysiology of Pulmonary Hypertension

Pulmonary hypertension pathophysiology is characterized by abnormal pulmonary vascular remodeling, inflammation and fibrosis in vascular walls (12, 18) (Figure 2). The mechanisms of vascular remodeling themselves are complex and involve frequent switching from apoptotic to anti-apoptotic and pro-proliferative phenotypes among pulmonary vascular cells (endothelial cells, smooth muscle cells, and fibroblasts). The dynamic, non-uniform vascular remodeling, particularly with medial smooth muscle hypertrophy, adventitial fibrosis and venous remodeling, play a significant role in PAH (19). A histologic hallmark specific to PAH is the plexiform lesion, which is traditionally described as a glomeruloid-like, occlusive lesion of pulmonary arteries and is a direct result of pulmonary vascular remodeling.

Figure 2: Pulmonary Remodeling and Plexiform Lesions.

Progression of PAH is characterized by increased dysfunction of pulmonary endothelial (ECs) and smooth muscle (SMCs) cells. Initial remodeling of pulmonary arteries begins with early EC injury coupled to thickening of the tunica media with proliferation of SMCs, leading to constriction of the lumen. Over time, the endothelium shifts to pro-proliferative phenotype, SMCs begin to infiltrate the interior of blood vessels, and fibrous tissue from the tunica adventitia infiltrates the arteries. This causes increased stiffness and fibrosis of pulmonary arteries along with reduced blood flow through advanced-stage plexiform lesions. Severe constriction and obliteration of the arteries strains cardiomyocytes of the right ventricle (RV), initially causing hypertrophy of the RV that progresses to RV decompensation and subsequent heart failure.

Recent studies involving the genetics of idiopathic PAH (IPAH) have suggested PH results from a cancer-like pathologic pathway. This new concept is primarily based on the monoclonal nature of plexiform lesions, suggesting that certain cell types could obtain a “genetic or mutational advantage for growth” through enriched proliferation, decreased apoptosis, or other signaling pathways that favor survival, such as altered cellular metabolism, extracellular matrix remodeling, chronic inflammation, oxidative stress, chronic hypoxia, or cell-to-cell communication (19). Plexiform lesions may form during angio-proliferative remodeling when pulmonary arterial endothelial cells (PAECs) are dysfunctional and observed to transition from a normal apoptosis-sensitive phenotype to a less differentiated phenotype that is highly proliferative and apoptosis-resistant in a process known as endothelial-to-mesenchymal transitioning (20-22). Although this upregulation of pro-angiogenic factors would suggest increased angiogenic potential, angiogenesis in PH seems to be impaired by the progressive obliteration of pre-capillary arteries resulting in a pattern of pulmonary vascular rarefaction (“dead-tree” picture) (15).

Additionally, dysregulated proliferative pathways have been implicated in the survival of pulmonary smooth muscle cells (SMCs) and fibroblasts. Cellular crosstalk between ECs and SMCs also appears to play an integral role in remodeling and PAH progression (23). In addition, a dysregulated immune and inflammatory response sets the stage for pulmonary vascular remodeling in PAH, though the specific instigator(s) of this response remain unclear. These episodic mechanisms of vascular remodeling can be triggered by environmental and genetic interactions, such as chronic HIV infection, Schistosomiasis, autoimmune disorders, and genetic and epigenetic mechanisms, or by external agents such as prescription and recreational drugs (24).

Although current therapies for PAH patients do decrease hospitalizations and improve functional status; the 7-year survival rate from the time of diagnosis by right heart catheterization for PAH patients is just 49% (25-32). Current therapies do not actually target etiological mechanisms of PAH, but rather are designed for symptom relief and to reduce the burden of the disease. While this provides some benefit to patients in the short-term, it remains imperative to continue exploring the underlying mechanisms of PAH to develop new therapeutics for PAH management.

HIV AND PULMONARY HYPERTENSION

HIV infection is the leading infectious cause of PAH worldwide (33). HIV-PAH was first described in a case report published by Kim and Factor in 1987. The duo detailed the case of a 40-year-old homosexual man with hemophilia and AIDS who presented with dyspnea and renal failure and was found to have PH with plexiform lesions and pulmonary arteriopathy on autopsy. Notably, there was no evidence of direct vascular infection with HIV (34). Later in a systematic review and meta-analysis, Bigna et al. (33, 35) estimated the prevalence of HIV-associated PAH (HIV-PAH) among adults to be 0.5% based on right heart catheterization (RHC) and 8.3% based on transthoracic echocardiography (TTE). Because more than 70% of PWH live in sub-Saharan Africa, it is likely that more than 2 million people in the region are living with both HIV and PAH. In Ethiopia, for example, PH appears to be a common complication of HIV. In a study looking at PH in 315 adults living with HIV at a single specialized referral clinic in Addis Ababa, the prevalence of PH by echocardiographic measurements was 14.0% (44 patients), with one-fifth (20.5%) of these patients classified as having moderate-to-severe disease by echocardiogram data (36). While another recent echocardiographic findings from a different hospital in Addis Ababa reported PH in 3.6 % of PWH (37).

HIV-PAH has a poor prognosis compared to other PAH subgroups. Data from 2003 show a 3-year survival ranging from 28 to 84%, depending on New York Heart Association (NYHA) functional status (38). In a study on 254 PH patients from the Pan Africa Pulmonary Hypertension Cohort (PAPUCO), Thienemann et al. reported PAH in 36% of PWH with poorer survival compared to only 15% in HIV-negative individuals (39).The development of PAH in PWH is an independent predictor of poor survival, particularly in those with low cardiac index or low CD4 count (10). Higher tricuspid regurgitant velocity (TRV), a measure of poor right heart function on echocardiogram, correlates with higher HIV viral loads and more frequent CD4 counts <200 cells/mm³ (40). In addition, a detectable viral load is independently associated with HIV-PAH on multivariate analysis (OR 3.3; 95% CI, 1.04-10; p=0.04) (41). Clinically and histologically, features of HIV-PAH are indistinguishable from IPAH. RHC parameters are comparable in IPAH and HIV-PAH, and positive vasodilator testing is rarely seen in HIV-PAH. Additionally, biopsies from HIV-PAH patients show concentric intimal fibrosis, medial hypertrophy, and plexiform lesions, similar to those of patients with IPAH (42).

Double Hit of HIV and Drug Use in PAH

Recreational drug use is associated with an increased risk of HIV transmission, disease progression, and non-compliance with ART (13). The CDC reports that injection of recreation drugs via sharing used syringes has added considerable challenges to HIV prevention in the U.S. (43). Among 11,437 respondents in the CDC’s 5^th National HIV Behavioral Surveillance (NHBS) Report from 2018, approximately 1 in 3 injection drug users (IDU) report using shared needles when injecting drugs (43). Sixty two percent of HIV-positive IDU endorse daily heroin injection, 54% of IDU endorse injection of “speedball” (a combination of heroin and cocaine), and 35% of respondents endorse injection of methamphetamine (43). Importantly, injection of illicit drugs is associated with higher HIV prevalence due to high-risk sexual behaviors. In the CDC’s 5^th NHBS 2018 Report, 35% of HIV-positive males who inject drugs and 57% females who inject drugs reported condomless vaginal intercourse with a person of the opposite sex. Twenty percent of HIV-positive males who inject drugs reported condomless anal sex with other males whereas 6% of HIV-negative males were engaged in condomless anal sex (43). People who inject cocaine are 3.6 times more likely to acquire HIV than non-injecting users of cocaine (44). However, a cross-sectional study conducted in Cambodia in 2017 found that the risk of HIV infection was 10 times higher also in people who used non-injecting drugs, primarily methamphetamine (95.1%), compared to the general population after adjusting for covariates (45). In the ongoing MACS/WIHS Combined Cohort Study, which consists of two cohorts (the Multicenter AIDS Cohort Study (MACS Study) and the Women’s Interagency HIV Study (WIHS)) combined into one in 2019, more than half (55%) of study participants reported ever using crack cocaine, cocaine, or heroin, with 8% of participants reporting current use (22). The prevalence of crack/cocaine and methamphetamine use among PWH has been estimated to be as high as 35.9% and 14.4%, respectively (23), and both types of drugs have been linked to increased risk of cardiovascular toxicities.

Several oral, inhaled, and intravenous drugs of abuse have been linked to the development of PAH, including methamphetamines, amphetamine derivatives, cocaine, heroin, and morphine. Cocaine was noted to be an independent risk factor for PH with fivefold greater odds of echocardiographic PH compared to matched controls (46-48) after excluding confounders such as left ventricular diastolic dysfunction and other known causes of PH. Methamphetamine also is an independent risk factor for PAH development (49-52). Histopathology samples taken from the lungs of patients with methamphetamine-associated PAH show changes like those in IPAH: plexiform lesions and proliferative capillaries. They also, however, have scant scattered intravascular collections of microcrystalline cellulose (filler associated with injectable amphetamines) with thrombotic changes (49). Compared to patients with IPAH, individuals with methamphetamine-associated PAH have increased risk of heart failure, transplantation, and death (49). Potential causes for this discrepancy include delayed time to diagnosis, poor compliance, socio-economic status, comorbidities such as ischemic heart disease or left ventricular failure, and poor treatment candidacy.

One of the first cohort studies examining the risk and incidence of PAH in PWH was published in 1989 by Himelman et al (9). Of 1,200 PWH included in the study, 6 (0.5%) had HIV-PAH, and half of those patients were IV drug users. In another early case-control study by Opravil et al. that included 19 patients with HIV-PAH and matched HIV positive controls from the Swiss HIV Cohort Study, 16 patients from each group were injecting drug users. Median survival in the HIV-PAH group was 1.3 years compared to 2.6 years in the control group, as one might suspect (53).

In a later and much larger prospective, cross-sectional cohort study known as the HIV-HEART Study, Reinsch et al. (54) studied 802 PWH and identified 38 patients meeting their criteria for HIV-PAH based on echocardiographic data (sPAP >35 mmHg). Interestingly, of the 38 patients in the HIV-PAH group, 7.9% were IV drug users, compared to 11.2% of the 394 patients without PAH who had satisfactory echocardiogram measurements.

Unlike this study, Quezada et al. (41) published data supporting IV drug use as a risk factor for HIV-PAH, not just HIV infection. They studied the demographics, clinical data, and echocardiographic data of 392 PWH attending one HIV reference clinic in Spain in 2011. Approximately 10% of patients included in the study were diagnosed with HIV-PAH using echocardiographic data, and factors associated with development of PAH in a univariate logistic regression analysis were female sex, former IV drug use (OR 1.4), detectable HIV-RNA load, and comorbid chronic hepatitis C infection. Furthermore, in a study by Degano et al, looking at HIV-PAH in a retrospective cohort in France, the prevalence of HIV-PAH was 8.2%, with IVDU being the main risk factor for HIV infection in 36% of all HIV-PAH patients (10). Intriguingly, IVDU was the main risk factor in 57% of the HIV-PAH cases that were diagnosed with New York Heart Association (NYHA) stage IV PAH in this study.(10). Later Araujo et al group from Portugal. also reported IVDU as a significant risk factor of infection in 77.8% of HIV-PAH patients (55). According to the recent data from the Pulmonary Hypertension Association Registry, 1.43% of all participants had HIV-PAH, which was characterized by greater heart rates, higher right arterial pressures, and higher pulmonary resistance compared to patients with idiopathic PAH. Notably, 36% of registrants with HIV-PAH used methamphetamine compared to 6% of those with idiopathic PAH (56). Additionally, the Spanish PAH registry (REHAP) recently reported previous or present IDU (78%) as the most frequent mode of HIV transmission in HIV-PAH patients (57). Studies from the past few decades that attempted to better characterize HIV-PAH and/or drug use, are listed in Table 1.

Table 1:

Prevalence and demographics of HIV associated pulmonary hypertension

Author, Year	Study Population (PWH, n) unless otherwise noted	Prevalence of PAH/PH n (%)	Demographics and characteristics of HIV-PH patients
			Mean Age (Years) unless otherwise noted	Female n (%)	Male n (%)	CD4 Count (cellμL) mean +/− SD unless otherwise noted	Illicit Drugs n (%)	Diagnostic Criteria	Ref.
Americas
Himelman et al., 1989	1200	6 (0.5)					3 (50)	Doppler ECG, RHC, RVSP >35mmHg	(35)
Aguilar & Farber, 2000	HIV-PAH, n=6	6 (100.0)	39.3	4 (66.7)	2 (33.3)	240.5	6 (100.0)	RHC, mPAP >25 mmHg (>30 mmHg with exercise at RHC), PCWP <15 mmHg	(211)
Hsue et al., 2008	196	69 (35.2)	–	–	–	–	$ IVDU (current): 6.1%, IVDU (Ever): 32%, Stimulant use (Ever): 46%	Echocardiography, PASP >30 mmHg	(212)
Mondy et al., 2011	656	183 (27.9)	–	–	–	–	$Cocaine: n=80 (13), Marijuana: n=200 (31), Heroin/Methamphetamine: n=44 (7)	Echocardiography, RVSP >30 mmHg	(68)
Morris et al., 2012	116	18 (15.5)	50.8	7 (38.9)	11 (61.1)	644 (median)	0	Echocardiography, PASP >40 mmHg	(88)
Simon et al., 2014	104	16 (15.0)	–	–	–	–	–	Echocardiography, PASP >35 mmHg	(69)
									(102)
Brittain et al., 2018	2,831	782 (27.6)	57.9	15 (1.9)	767 (98.1)	24.5% with CD4 < 200	–	Echocardiography, PASP >40 mmHg	(213)
Rajaratnam et al., 2021	62	PH=32 (52.0) PAH=15/32(46.8%)	51.0	10 (31.3)	22 (68.8)	374 +/− 252	8 (25.0)	RHC, mPAP ≥ 25 mmHg, PCWP ≤15 mmHg	(84)
Kolaitis et al., 2022	1604	23 (1.43)	45	8	15 (65)	–	8 (36)	RHC, mPAP >25 mm Hg	(56)
Africa
Silwa et al.,2012	518	42 (8.1)	Men: 51.0 Women: 39.0	11 (26.1)	31 (73.8)	–	–	Echocardiography, heart failure secondary to right-sided pathology with increased jugular venous pressure and liver size, tricuspid regurgitation, and/or RVSP ≥ 35 mmHg	(214)
Chillo et al.,2012	102	13 (12.7)	42.0	5 (40.0)	8 (60.0)	242 +/− 208	–	Echocardiography, PASP >35 mmHg with or without dilated and/or hypertrophied right ventricle and in presence of dyspnea	(215)
Isiguzo et al., 2013	200	8 (4.0)	39.0	4 (50.0)	4 (50.0)	Unavailable	–	Echocardiography, PASP ≥25 mmHg	(216)
Menanga et al., 2015	44	5 (11.4)	–	–	–	≥200	–	Echocardiography, PASP ≥35 mmHg, No evidence of LHD	(217)
Asia
Rasoulinejad et al., 2014	170	5 (3.0)	–	–	–	–	–	Echocardiography, RVSP >35 mmHg	(218)
Singh et al., 2018	100	12 (12.0)	–	7 (58.3)	5 (41.7)	<350 (n=9) >350 (n=3)	–	Unavailable	(219)
Europe
Speich et al., 1991	74	6 (8.1)					5 (83.3)	Doppler ECG, RVSP> 30 mmHg	(220)
Petitpretz et al., 1994	HIV-PAH, n=20	20 (100.0)	32.0	11 (55.0)	9 (45.0)	<200 (n=12)	12 (60.0)	RHC, mPAP ≥ 25 mmHg, PCWP ≤15 mmHg	(221)
Opravil et al., 1997	HIV-PAH, n=19	19 (100.0)	30.6 (median)	12 (63.2)	7 (36.8)	83 (median)	16 (84.2)	Echocardiography, RVSP over RAP>30 mmHg	(53)
Nunes et al., 2003	HIV-PAH, n=82	82 (100.0)	34.0	37 (45.1)	45 (54.9)	269 +/− 219	48 (58.5)	RHC, mPAP ≥ 25 mmHg (mPAP ≥ 35 with exercise at RHC), PCWP ≤15 mmHg	(38)
Ghofrani et al., 2004	HIV-PAH, n= 8	8 (100.0)	38.3	5 (62.5)	3 (37.5)	473	5 (62.5)	RHC, mPAP ≥35 mm Hg (severe PAH)	(222)
Zuber et al., 2004	11,894	47 (0.4)	34.4	22 (46.8)	25 (53.2)	169 (median)	33 (70.2)	Echocardiography, RVSP over RAP >30 mmHg	(64)
Reinsch et al., 2008	802	38 (4.7)	48.3	4 (10.5)	34 (89.5)	431 +/− 270	3 (7.9)	Echocardiography, RVSP over RAP pressure >30 mmHg + symptoms of dyspnea	(54)
Sitbon et al., 2008	277	35 (12.64)	41.5	10 (28.6)	25 (71.4)	37% with CD4 count < 200	18 (51.4)	RHC, mPAP ≥ 25 mmHg (mPAP ≥ 35 with exercise at RHC), PCWP ≤15 mmHg	(62)
Degano et al., 2010	944	77 (8.2)	41	32 (42)	45 (58)	302 (median)	28 (36)	RHC, Resting mPAP>25 mmHg, PCWP<15 mmHg	(10)
Quezada et al., 2012	392	39 (9.9)	46.9 (median)	13 (33.3)	26 (66.6)	277 (median)	17 (58.6)	Echocardiography, peak systolic velocity of tricuspid regurgitation > 2.8 m/s	(41)
Isasti et al., 2013	194	5 (2.6)	56.3	0 (0.0)	5 (100.0)	363.3 +/− 126.7	0 (0)	Echocardiography, RVSP>40 mmHg	(223)
Araujo et al., 2014	HIV-PAH, n=18	18 (100)	40.2	7 (38.9)	11 (61.1)	554 +/− 267	14 (77.8)	RHC, Resting mPAP>25 mmHg, PCWP<15 mmHg	(55)
ten Freyhaus et al., 2014	220	1(0.45) *17 (7.7) HIV-PH	*47 (median)	–	–	*340 (median)	–	Echocardiography, PASP >35 mmHg	(83)
Schwarze-Zander et al., 2015	374	PH=23 (6.1) PAH=3/23 (13%)	51.0	8 (34.8)	15 (65.2)	428 +/− 278	5 (21.7)	Echocardiography, PASP >30 mmHg	(224)
Knudsen et al., 2021	900	44 (5.0)	40.0	9 (20.5)	35 (79.5)	27% with nadir CD4 count <200	3 (6.8)	CCT, ratio of diameter of main pulmonary artery (at the level of its bifurcation) to diameter of the ascending aorta (PA:A) >1	(225)
Salvador et al., 2022	Total PAH Patients, n= 2562	182 (7.1) *132 (72.53%) HIV-PAH only **57 (31.3%) HIV-PAH with other concomitant PAH etiologies	*43.4	*69 (52.3)	*63 (47.7)	–	*74(77.9)	Resting mPAP>25 mmHg, PCWP≤15 mmHg, PVR≥3 Wood units	(57)

^$Values are based on total HIV participants, RAP= Right arterial pressure, CCT=Chest computed tomography.

It remains unclear whether drug use increases mortality in people with HIV-PAH as studies to date have been small in size (primarily case series). In a comparative study by Nunes et al. (38) published in 2003, 82 patients with HIV-PAH were grouped according to NYHA functional class. Patients with NYHA functional class III-IV disease had significantly worse survival over the course of three years compared to those with functional class I-II disease. Of those with functional class III-IV disease, 32 patients (55%) had an IV drug use risk factor, compared to 16 patients (67%) in the functional class I-II group. IV drug use was not associated with mortality on univariate analysis. Similarly, Bruno et al. conducted a retrospective review of data from 77 consecutive patients with HIV-PAH treated at a PH center in France between 2000 and 2008. Among these patients, the most common risk factor for HIV was IV drug use (36% of patients), however, acquiring HIV through IV drug use was not associated with survival when compared with other forms of HIV transmission (10).

Importantly, 70% or more PWH who have a history of IV drug use also test positive for hepatitis C (HCV), and 20-30% of PWH in sub-Saharan Africa also test positive for Hepatitis B (58). Zola et al. performed a cross-sectional analysis of more than 6,000 participants in the Veterans Aging Cohort Study (16% of whom were living with chronic HIV and HCV) and found that HIV/HCV coinfection was associated with higher absolute pulmonary arterial systolic pressures (PASP) but not with PH prevalence.(59). Similarly, many PWH living with other co-infections, including tuberculosis, hepatitis B virus (HBV) and Schistosomiasis (60) (61) seem to contribute to the development of HIV-PAH. One may posit that co-infections in PWH and drug abuse could promote development of a severe form of PAH.

Role of antiretroviral drugs in HIV-PAH

The estimated prevalence of HIV-PAH has not changed after the introduction of HAART compared to pre-HAART times (62), though Opravil et al. did note a clear downward trend in incidence in the Swiss HIV Cohort Study as participant CD4 counts rose in the setting of HAART (63). Zuber et al. found that HAART decreases the progression of HIV-PAH compared to those taking NNRTIs alone or not receiving therapy at all, though this study was much smaller with only 47 patients enrolled in total (64). Some studies show long-term HAART may improve 6MWT distance but not hemodynamic measurements in patients with HIV-PAH (10, 65), thereby attenuating the severity of disease but not preventing the development of PAH (10, 53, 64). A descriptive study examining prevalence and risk factors of PH in 329 PWH receiving HAART found that use of nucleos(t)ide analogues tenofovir or abacivir, a protease inhibitor, or integrase inhibitors was not associated with PAH (41). Further, PWH taking non-nucleoside reverse transcriptase inhibitors (NNRTIs) are reported to have a lower incidence of HIV-PAH (41, 66). These results suggest that HAART may have a role in mitigating HIV-PAH pathophysiology; however, some studies have shown that HAART may also contribute to HIV-PAH pathophysiology.

In a study by Pugliese et al. (67), 2% of patients treated with HAART developed PAH compared to 0.7% of patients treated with nucleoside reverse transcriptase inhibitors (NRTIs) alone. The SUN study by Mondy et al. (68) demonstrated that two-thirds of patients treated with HAART develop cardiac dysfunction, though only ritonavir-boosted protease inhibitors were associated with PAH. Another echocardiography based study on PWH reported a strong association of estimated systolic PAP greater than 35 mm Hg with the current abacavir users (69). The effects of HIV infection on the cardiopulmonary health of children are largely unknown. Idris et al. (70) conducted a cross-sectional study of 102 PWH and 51 children living with HIV in Jakarta, Indonesia, to determine whether ART-naïve or ART-exposed HIV was associated with specific echocardiographic parameters. They found that ART-exposed HIV infection is associated with higher estimated PAP, and ART-naïve HIV-positive children have significantly reduced RV systolic function, which can be partly explained by lower respiratory tract infections in ART-naïve children (70) . Therefore, it may be possible for drugs that target HIV replication to also have unintended adverse effects on the pulmonary vasculature and contribute to the risk of HIV-PAH. Of note, most studies looking at the effects of HAART on the development of PAH in PWH have relied on TTE, not RHC, to define PAH. In addition, most data on HIV-PAH come from case reports and small cohort studies, and the impact of HAART on HIV-PAH remains controversial.

Ritonavir has been shown to inhibit bradykinin-dependent vaso-relaxation, contributing to endothelial dysfunction. Moreover, ritonavir, in addition to indinavir, lamivudine, abacavir, and zidovudine, decreases eNOS expression, increases Reactive Oxygen Species (ROS), and increases ERK1/2 activation in human pulmonary arterial endothelial cells (HPAECs) in vitro (71). A decrease in eNOS is associated with endothelial dysfunction and an insufficient NO-mediated vasodilatory response, which leads to increased vasoconstriction of blood vessels. Further, ROS and ERK1/2 activation are associated with the regulation of cell proliferation and promoting vascular remodeling (72, 73). Conversely, the protease inhibitors ritonavir, amprenavir, and nelfinavir have been shown to reduce PASMC proliferation in monocrotaline- and hypoxia-induced PH rats by inhibiting Akt phosphorylation (66).

Interestingly, in addition to the use of the above-mentioned illicit drug use, the recreational use of antiretroviral drugs has become a public health issue in developed and developing countries. The misuse of antiretroviral agents was first reported in the medical literature in 2007, though it gained media attention in 2010 when Larkan et al. (74) described the use of antiretroviral and recreational drug cocktails, or whoonga, in South Africa. There have been reports of individuals using efavirenz in this manner for its “intoxicating” effects and ritonavir to enhance the effects of recreational drugs like methamphetamine (75). Whoonga has been associated with the criminal diversion of important antiretroviral drugs, and its use by untreated PWH places them at risk of acquiring drug resistance. Finally, multiple HIV Cohort analyses report rates of polypharmacy (5 or more non-HIV drugs) in PWH ranging from 15 to 94% (76). Polypharmacy increases the risk of drug-drug interactions, and the resulting “pill burden” could influence ART compliance as well as development of comorbidities.

HIV & Other Forms of PH

HIV-PAH is a form of WHO Group 1 PH, though PWH may also develop WHO Groups 2, 3, 4, or 5 PH because of their comorbidities. In fact, in both PWH and non-HIV individuals, it can be difficult to delineate between the various forms of pulmonary hypertension and determine which WHO Group is contributing most to clinical presentation. Therefore, the prevalence of PAH in PWH may be under-reported or underdiagnosed. In the Veterans Aging Cohort Study, for example, veterans living with HIV had an increased risk for heart failure with reduced and preserved ejection fraction (77), as well as increased incidence of many cardiac and pulmonary conditions, including COPD and pulmonary fibrosis, diagnoses that can contribute to the development of WHO Groups 2 and 3 PH, respectively (78).

PWH experience higher rates of acute coronary syndrome and experience the effects of cardiovascular disease (CVD) at an earlier age (79). The risk of myocardial infarction correlates inversely with lower CD4 counts, and chronic elevated levels of inflammatory cytokines have been associated with CVD events (80) and all-cause mortality (81). Antiretroviral therapy may also contribute to the development of CVD by altering lipid metabolism and/or through drug-specific side effects, though this remains controversial (79). In PWH, WHO Group 2 PH has been associated with comorbid ischemic heart disease, diastolic dysfunction, hypertension, drug use-associated cardiomyopathy, and, less frequently, HIV-associated cardiomyopathy myocarditis (82). Because left heart dysfunction can impact pulmonary artery systolic pressure (PASP) measurements and is common in PWH, a diagnosis of HIV-PAH should not rely solely on transthoracic echocardiogram findings. In one study assessing the utility of screening PWH for PAH using echocardiogram detected HIV-PAH with a prevalence of 0.46%, whereas 7.7% of patients screened had moderate increases in PASP due to left ventricle diastolic dysfunction associated with arterial hypertension (83). In a recent study by Rajaratnam et al., 62 PWH and with a clinical likelihood for PH underwent RHC. Their hemodynamic data showed that 52% (n=32)) of the patients had PH and those with PH were more likely to have a detectable HIV viral load and lower CD4 count at the time of RHC (84). They further reported that n=15 of these PH patients met criteria for PAH and n=17 for pulmonary venous hypertension or group 2 PH (PCWP ⩾15 mmHg).

In addition to CVD, many PWH develop chronic lung diseases, such as COPD. Several studies have shown that HIV is an independent risk factor for the development of COPD after controlling for smoking status (85, 86). Moreover, airflow obstruction and impaired diffusion capacity of the lung for carbon monoxide (DLCO) are both associated with all-cause mortality in PWH (87). The largest study to examine COPD in PWH to date, the INSIGHT Strategic Timing of AntiRetroviral Treatment (START) trial, found that the overall prevalence of COPD in PWH with a CD4 count higher than 500 cells/mL is 6.8%, with a significantly higher prevalence (7.8 to 9.1%) in certain parts of the world, including Africa, Europe/Israel/Australia, and North America (86). The smoking-related chronic lung diseases complicated by hypoxia, particularly COPD is associated with group 3 PH. Elevated estimated PASP and total vascular resistance (TVR) on echocardiogram are associated with worse airflow obstruction and diffusing capacity. Morris et al (88) observed elevated tricuspid regurgitant jet velocity (TRV) and PASP measurements in a cohort of HIV-infected outpatients with impaired lung function. They reported that 15.5% study subjects had at least 40mmHg PASP and 7.8% had 3.0 m/sec TRV which was significantly associated with high viral load, reduced CD4 counts (<200 cell/ μl) and worse airflow obstruction.

A REVIEW OF PAH-ASSOCIATED SIGNALING PATHWAYS AND MOLECULAR MEDIATORS WITH AN EMPHASIS ON HIV-PAH AND THE SYNERGISTIC EFFECTS OF DRUG USE

The exact mechanism for the development of HIV-PAH is not known, though several inflammatory pathways have been implicated in response to HIV viral proteins (gp-120, Tat, and Nef), chronic inflammation due to HIV-associated immune dysregulation, and drug use (89). Thus far, there is no evidence to support direct HIV infection of the pulmonary endothelium, but HIV viral proteins such as gp120 or Tat do have the ability to activate endothelial cells by increasing expression of E-selectin (90), stimulating the release of pro-inflammatory cytokines (91) and vasoconstrictor endothelin-1 (ET-1) (92) and inducing apoptosis via the intrinsic pathway (93). HIV viral proteins also promote oxidative stress (94) by reducing the levels of anti-oxidant enzymes (95), which could contribute to pulmonary vascular remodeling and potentially PAH. Furthermore, Chelvanambi et al. (96) demonstrated that HIV-Nef protein persists in T cells, monocytes, and BAL EVs from the lungs of PWH even during treatment with ART and that Nef-containing extracellular vesicle (EVs) from BAL fluid have the ability to induce endothelial cell apoptosis. Nef can induce its own transfer into endothelial cells via nanotubes from Jurkat cells and THP1 monocytes (97). Once inside, Nef protein can induce ROS formation and apoptosis through NADPH oxidase activation and increase expression of monocyte chemoattractant protein-1 (MCP-1) through the NF-κB signaling pathway. Notably, Nef has been identified as a potential contributor to severe pulmonary vascular remodeling and development of plexiform lesions. In this study, Marecki et al. (98) demonstrated the presence of Nef in the endothelial cells of remodeled vessels from SIV-infected macaques as well as in lung tissues from HIV-PAH patients.

Endothelial dysfunction and compromise in endothelial barrier integrity in response to inflammation and HIV viral proteins may expose underlying vascular tissue such as SMCs within the tunica media (99) to the damaging effects of viral proteins and inflammatory mediators which in vivo would result in smooth muscle hyperplasia leading to increased pulmonary vascular resistance and subsequent RV dysfunction. At least one study suggests that HIV is capable of directly infecting SMCs via CD4 and chemokine co-receptor-mediated viral entry both in-vitro and in-vivo, further contributing to local inflammation within vascular tissue and dysfunction of SMCs (100).

Further, in addition to the direct effect of viral proteins on vascular cells, chronic HIV infection contributes to a chronic inflammatory state that can alter endothelial function. It is evident that the abnormal expression and secretion of specific cytokines and chemokines influences outcomes in PAH and, specifically, in patients with HIV-associated PAH (15, 101). Elevated pulmonary artery pressures have been linked to higher levels of nitric oxide synthase inhibitors and elevated IL-6 levels in PWH (102). In addition, a study from the P. Hsue group reported independent association of elevated levels of asymmetric dimethyl arginine (ADMA) in HIV-infected individuals with the presence of PAH (102). Increased levels of ADMA, an endogenous inhibitor of eNOS and marker of chronic inflammation, reflect nitric oxide-mediated endothelial dysfunction and are a predictor of disease-related mortality in PAH.

In terms of pathophysiologic synergy, HIV and drugs act on many of the same pathways to exert their pathologic effects on the pulmonary vasculature. Consequently, it’s possible that the “double hit” of drug use in PWH could increase the risk of HIV-PAH and/or promote a more severe disease phenotype. Integrated HIV proviruses remain silent in latently infected cells, such as T cells, monocytes, and macrophages, until a stimulation factor is introduced (93). Previous literature provides evidence for the hypothesis that drug abuse in PWH stimulates expression of HIV proviruses and viral proteins, thereby contributing to disease progression (Figure 3). In addition, PWH who use drugs, such as opioids, cocaine, and/or methamphetamine, recreationally, risk exacerbating their existing chronic inflammatory state due to HIV.

Figure 3: Effects of recreational drugs on the HIV Life Cycle.

Recreational drug use is shown to increase viral load in PWH by several mechanisms. A) Cocaine induces HIV replication by activation of RSK1 and MSK1 of the ERK pathway which both induce activation of the p65 subunit of NF-κB by phosphorylation. The p65 subunit translocates to the nucleus and induces chromatin remodeling, increasing replication of integrated HIV cDNA at the HIV-LTR. Cocaine also activates AP-1 expression which activates NF-κB. B) Methamphetamine increases IL-1β and decreases Type I IFNs to enhance HIV replication and expression of viral proteins. AP-1 is known to increase in the presence of methamphetamine that may increase the NF-κB dependent expression of HIV proteins. C) Opioids enhance HIV replication by increasing the concentration of circulating LPS, subsequently activating an immune response that induces expression of NF-κB. Opioids also increase CREB activity and its interaction with CREB-binding protein (CBP) at the HIV LTR leading to HIV transcription. Morphine is also shown to increase the expression of CCR5 and CXCR4 to promote viral entry, replication, and expression of viral proteins by infected host cells.

Inflammation from drug use as well from continual reactivation of HIV proviruses augments injury to pulmonary vascular tissue, setting the stage for PAH to develop with severe phenotypes. Furthermore, drug abuse in PWH has a more potent effect on pulmonary micro vascular endothelial cell (PMECs) dysfunction and pulmonary arterial smooth muscle cell (PASMC) proliferation in the presence of HIV proteins (103, 104) (Figure 4 and 5). Potential mechanisms for how drug abuse and HIV may act synergistically in PAH pathophysiology are expanded upon below.

Figure 4: Dual hit of recreational drugs and HIV-infection augments pulmonary endothelial dysfunction.

HIV and recreational drugs act additively or synergistically to accelerate the progression of HIV-PAH. This is known as “The Two-Hit Model of HIV-PAH.” Recreational drug use can induce HIV replication and enhance expression of HIV proteins by activating host immune cells. Increased levels of HIV-Tat, Nef, and gp120 in the presence of illicit drugs augment the induction of inflammation and pulmonary vessel remodeling. Endothelial apoptosis and proliferation both contribute to pulmonary vascular remodeling associated with PH. Caspase 3, an effector caspase in apoptotic pathways, is increased in response to HIV proteins and illicit drugs which is followed by increased autophagy leading to shifting of endothelial cells to the proliferative phenotype. Proliferative factors, such as VEGFR2, are initially downregulated as cells undergo apoptosis but are subsequently activated, causing proliferation of endothelial cells. Furthermore, EVs that carry TGF-β, HIV-Nef, and miRNA can also contribute to endothelial dysfunction and HIV-PAH pathogenesis.

Figure 5: Dual hit of recreational drugs and HIV-infection augments pulmonary vascular smooth muscle dysfunction.

Use of illicit drugs potentiate the effect of HIV proteins on the activation of pro-proliferative TGF-β1/2 receptor and the downregulation of anti-proliferative BMPR signaling, causing hyperproliferation of pulmonary arterial smooth muscle cells (PASMCs). In addition, PDGF-βR expression on PASMCs is enhanced in both PDGF-BB-dependent and -independent manners. The circulating or macrophage derived EVs containing TGF-β, or miRNAs also contribute to activation of pro-survival signaling or silencing of anti-proliferative signaling in PASMCs. Together, in conjunction with endothelial dysfunction, these mechanisms of uncontrolled smooth muscle hyperplasia result in pulmonary vascular remodeling, pulmonary vasoconstriction, and formation of plexiform lesions in PAH.

Cocaine and HIV in PAH pathophysiology

Cocaine is known to activate NF-κβ, resulting in enhanced HIV replication and increased viral load in PWH. Specifically, cocaine activates ribosomal S6 kinase-1 (RSK1) via the extracellular signal-regulated kinase (ERK) pathway and activates mitogen- and stress-activated kinase 1 (MSK1). MSK1 induces NF-κβ nuclear translocation and chromatin remodeling to activate HIV replication at HIV-LTR (105-107). Activator protein-1(AP-1) also increases in the presence of cocaine (108) (109) and interacts with NF-κβ to increase the expression of HIV proteins, such as HIV-Tat (Figure 3). In PWH, HIV-Nef induces the secretion of macrophage inflammatory proteins (MIP)-1α and activation of NF-κB, like cocaine (110).

Our group demonstrated robust perivascular inflammation in lungs of Sprague-Dawley HIV-transgenic (Tg)rats (expressing 7 of 9 HIV proteins) treated with or without cocaine with inflammatory cells staining positive for the presence of Nef and Tat proteins (111). This perivascular inflammation in lungs was associated with increased pulmonary vascular remodeling and, elevated mean pulmonary arterial pressures and RVSP in HIV-Tg rats treated with cocaine. We have shown that HIV-Tat can increase the production of ROS, which can also be aberrantly produced following activation of inflammatory pathways, resulting in endothelial activation (e.g., pro-thrombotic phenotypes leads to clotting), dysfunction (e.g., exhaustion of NO-mediated vasodilation mechanisms leads to vasoconstriction), and cell death (94). Additionally, inflammatory cytokines expressed following the activation of NF-κβ leads to disruption of endothelial cell junctions and increased vascular permeability, which may create an easier route for HIV-infected monocytes and/or CD4+ T cells to migrate into vascular tissue and contribute to local inflammation and vascular remodeling (112). Not only do these pathways impact endothelial function, but they also may pave the way for dysregulated SMC growth.

Oxidative stress and chronic hypoxia are known causes of the pulmonary vascular remodeling that is characteristic of PH. Hypoxia-inducible factor-1α (HIF-1α) and HIF-2α regulate the pulmonary vasculature’s response to hypoxia (113, 114). Porter et al. (115) noted vascular remodeling, elevated RVSP, and increased HIF-1α expression in lungs from HIV- Tg rats exposed to chronic hypoxia. Furthermore, our group demonstrated that oxidative stress-induced upregulation of HIF-1α and PDGF-BB expression directly correlates to increased RV/LV septum ratio in HIV Tg rats (116), supporting the involvement of these pathways in the development of HIV-PAH as well. PDGF is a known mediator of vascular remodeling in PAH (117, 118) and HIV infection has been shown to induce activation of PDGF/PDGF-Receptor axis (103, 104). Interestingly, exposure of HIV-infected macrophages and Tat-treated endothelial cells to cocaine expressed more PDGF-BB than those treated with either Tat or cocaine alone (119) (Figure 4). Histological findings of lung sections from IVDUs living with HIV showing early pulmonary arteriopathy with increased expression of PDGF-BB and severe down-regulation of tight junction proteins (TJPs) further strengthened the additive effect of HIV-infection and illicit drug use (119). Additionally, generation of ROS and exacerbated disruption of TJPs and permeability of human pulmonary artery endothelial cells via activation of Ras/Raf/ERK1/2 pathway on simultaneous exposure to HIV-Tat and cocaine has also been reported (72). Comparing mono-treatments of cocaine or HIV-Tat to exposure to cocaine in the presence of HIV-Tat showed a synergistic increase in apoptosis of human pulmonary microvascular endothelial cells (HPMECs) after 48-hour exposure. Interestingly, after 6 days of chronic treatment, surviving endothelial cells showed increased proliferation with the combined treatment of cocaine and HIV-Tat having the strongest response (120). These studies clearly demonstrate how the use of cocaine in PWH could contribute to worsening of pulmonary arteriopathy associated with PAH phenotype.

Furthermore, the treatment of human PASMCs with both cocaine and Tat synergistically augments the smooth muscle hyperplasia when compared with only cocaine or Tat mono-treatment (119, 121). Blocking of PDGF receptor signaling using antagonist or small interfering RNA inhibits Tat and cocaine-mediated smooth muscle hyperplasia. Although this combined treatment does not result in an additive or a synergistic secretion of PDGF-BB by smooth muscle cells like in endothelial cells or macrophages as mentioned above (119), our group reported both ligand dependent and ligand independent activation of PDGF receptor β mediated downstream signaling in human PASMCs in response to cocaine and Tat exposure (121). Not only the dual treatment of Tat and cocaine increases expression of PDGF receptor β, it also enhances the Src/ROS-mediated ligand-independent phosphorylation of PDGFRβ at Y934 that ultimately leads to SMC hyperplasia (121) (Figure 5). Exposure to HIV-Tat and cocaine in cell culture and in-vivo has been reported to alter the TGF-β superfamily signaling. The TGF-β superfamily consists of both TGF-β- and bone morphogenic protein (BMP)-mediated signaling, which activate Smad signaling pathways and promote their translocation to the nucleus where they induce the expression of genes that regulate cell survival and proliferation (122). For more in-depth explanations of the interactions between TGF-β and BMP signaling pathways in PAH, we refer readers to the cited reference (122). Loss of or decrease in BMPR2 expression is one of the major contributors to PAH pathophysiology that results in endothelial apoptosis and PASMC proliferation (123). Cocaine augments the HIV-Tat, Nef and gp120 mediated decrease in the expression of BMP receptors (BMPRs) in PASMCs as demonstrated in both human cells and in HIV-Tg rats (111) (124). Additionally, loss of BMPR1A, −1B, and −2 expression corresponds with abnormal hyperactive, proliferative TGF-β signaling in PASMCs, promoting hyper-proliferation of these cells (125, 126). TGF-β is a fibrogenic cytokine that induces PASMC proliferation and increases angiogenesis and fibroblast activity resulting in reduced elasticity of pulmonary arteries and increased RV afterload (122, 127). In PAECs with mutations in BMPR2, such as in familial PAH, TGF-β production is increased, which signals PASMCs to proliferate and surrounding PAECs to undergo apoptosis and/or endothelial-to-mesenchymal (EndoMT) transition (122).

Treatment of PASMCs with both HIV-Tat and cocaine leads to increased expression of TGF-β receptors and TGF-β-1 ligand (128). Both SMAD- and non-SMAD-dependent TGFβ signaling cascades were found to contribute to hyper-proliferation of PASMCs in the presence of HIV-Tat and cocaine. Levels of phosphorylated SMAD2/3 and TAK1-mediated SMAD independent downstream signaling molecules, like p-MKK4 and p-JNK, were found to be significantly increased in these SMCs. Further, an increase in phosphorylated TGFβR1 and TGFβR2 were observed in pulmonary smooth muscle cells from cocaine injected HIV-Tg rats as well as in total lung extracts from HIV infected cocaine and/or opioid users and confirms the synergistic role of HIV Tat and cocaine on TGF-β superfamily dysfunction (128). Recently, our group (129) also reported higher levels of TGF-β1-loaded extracellular vesicles (EVs) (cell-derived nanoparticles) in plasma from PWH cocaine users compared to PWH non-users. The highest levels were observed in HIV-PAH patients, correlating with an increase in proliferation of both human PMECs and PASMCs. TGF-β1 levels were also higher in EVs isolated from HIV-infected monocyte-derived macrophages (MDMs) treated with cocaine, and injection of these EVs in rats resulted in pulmonary arteriopathy, increased RVSP, and myocardial injury (129). There is growing evidence that EVs mediate communication between SMCs and ECs and promote pulmonary vascular remodeling. TGF-β-mediated survival and increased proliferation of PASMCs causes smooth muscle hypertrophy within vessels, which results in increased mean pulmonary arterial pressure, right ventricular systolic pressure, and right ventricular fibrosis, all contributing to RV dysfunction (111, 130, 131).

Cocaine and HIV proteins also induce the expression of messenger RNAs and non-coding RNAs that contribute to the PMECs and PASMC dysfunction seen in PAH (132). Non-coding RNAs include micro RNAs (miRNAs), which contain 21-23 nucleotides, and long non-coding RNAs (lncRNAs), which are more than 200 base pairs in length. The investigation of dysregulated mRNAs and lncRNAs identified several lncRNA-mRNA relationships that were involved in controlling the hyperplasia of PASMCs in response to cocaine and Tat protein exposure (132). The bio-informatics, fold change, and conservation analysis of microarray data along with cell-culture findings identified ENST-536 as one of the top up-regulated lncRNA candidates, implicated in vascular diseases. The knockdown of ENST-536 resulted in the up-regulation of tumor-suppressive HOXB13 mRNA expression and decrease in the proliferation of PASMCs treated with both HIV-Tat and cocaine. Further the combined analysis of dysregulated miRNAs, mRNAs, and lncRNAs in these cells identified many lncRNAs seemed to have the capacity to behave as competing endogenous RNAs or sponges for many anti-proliferative miRNAs, up-regulating pro-proliferative mRNAs(132). Our group (124) also identified miRNAs capable of binding BMPR2 mRNA and studied the effects of these miRNAs on HIV and cocaine and mediated smooth muscle proliferation. Inhibition of BMPR2-targeting miRNAs prevented PASMC proliferation, while overexpression of these miRNA increased proliferation. Direct binding of miR-216a and miR-301a to 3’UTR of BMPR-2, specifically, resulted in translational repression of BMPR2 without degradation of its mRNA. This miRNA-mediated translational repression brought about a paradoxical increase in BMPR1A, BMPR1B, and BMPR2 mRNA expression and decrease in BMPR protein expression in HIV-infected rats treated with cocaine. EVs carrying coding and non-coding RNA have been shown to play a role in a number of disease processes, including cardiovascular disease and various malignancies (133, 134). Zeng et al. (135) described the transfer of miRNA from apoptosis-resistant endothelial cells seen in the plexiform lesions of PAH to vascular smooth muscle cells. Deng et al. (136) successfully reported EV-mediated transfer of miR-143 from PASMCs to HPAECs leading to HPAEC migration and angiogenesis. One of our previous studies also highlights the communication of EVs derived from HIV-1 infected inflammatory cells to un-infectable pulmonary vascular cells for the development of PAH. The EVs from HIV-infected and cocaine-treated macrophages contain higher levels of miR-130a compared to EVs from non-infected macrophages. When transferred to PASMCs, the EV-linked miR-130a down-regulates PTEN expression and activates P13K/AKT signaling, thereby promoting cell survival and proliferation. This again suggests a synergistic effect between HIV and drug use in the development of pulmonary vascular remodeling (137). The miR-130/301 family is known to promote multiple proliferative and vasoconstrictive pathways, including activation of PPARγ-ApoE-LRP8 axis to promote PASMC and PMEC proliferation, extracellular matrix remodeling, and progressive PH (138, 139).

Methamphetamines and HIV in PAH pathophysiology

PWH and who use methamphetamine have higher HIV viral loads regardless of HAART adherence (140, 141). Marcondes et al. (142) used an animal model to show that methamphetamine-treated SIV-infected rhesus macaques have significantly higher CNS viral loads and reduced numbers of CD4+ T cells than control animals, but no effects on peripheral viral loads. On the contrary, a separate animal study using JR-CSF/hu-CycT1 HIV Tg mice found that methamphetamine does increase plasma HIV-1 RNA loads (143). The exact mechanisms by which methamphetamine promotes HIV replication are not fully understood, but it may be dependent upon methamphetamine concentration. There is recent evidence that methamphetamine exposure activates an IL-1β feedback loop that, in concert with downregulation of type I interferons and associated genes, enhances HIV-1 replication and suppresses innate immune pathways in a dose-dependent fashion (144). Furthermore, AP-1 also increases in the presence of methamphetamine and interacts with NF-κβ to increase the expression of HIV proteins, such as HIV-Tat (109) (145) (Figure 3).

Similar to cocaine, acute exposure of HPMECs to methamphetamine is also reported to synergistically increase cell death in the presence of Tat (120). Our unpublished findings suggest a decrease in BMPR2 expression on simultaneous treatment of these cells with both HIV-Tat and methamphetamine. Further, chronic exposure of endothelial cells to both methamphetamine and HIV-Tat led to a considerable increase in cell proliferation comparable to that seen following combined exposure to cocaine and HIV-Tat as mentioned above (120). Therefore, similar mechanisms of pulmonary vascular remodeling to that of cocaine and HIV-Tat combined exposure may exist for methamphetamine use and HIV infection as well. Ramirez et al. report that ROS levels are increased with methamphetamine use and propose that higher ROS levels cause damage to PAECs and promote pulmonary vascular remodeling (146). Since methamphetamine is a potent inducer of ROS, this would increase expression of HIF-1α and lead to more severe PAH phenotypes in PWH who use methamphetamine. Moreover, HIV proteins Tat and gp120 increase the expression of HIF-1α in PAECs which further suggests a double hit mechanism of HIV and methamphetamine in the lungs and significance in PAH pathophysiology (116). Meth increases endothelial permeability by downregulating, fragmenting, and redistributing tight junction proteins like occludin, claudin-5, and ZO-1 (168, 169) by activating NADPH oxidase-dependent oxidative stress (170, 171) and therefore can potentiate the damaging effects of HIV-proteins on the vascular endothelium (Figure 4). Further, experimental models show maladaptive responses to chronic amphetamine use, resulting in mitochondrial oxidative stress, DNA damage, and vascular injury (147-149). Decreased superoxide dismutase expression in the lungs of methamphetamine users living with HIV results in prolonged oxidative stress, which can contribute to HIV-PAH progression via regulation of apoptosis, cell-survival, and proliferation of endothelial and smooth muscle cells (148, 150).

Concurrently, high doses of methamphetamine show a preferential interaction with serotonin transporters (146, 151), which are increased in the lungs and pulmonary arteries of rats exposed to methamphetamine, similar to hypoxia-induced changes in PASMCs (150). Serotonin has known vasoconstrictive and oxidative stress-dependent growth modulating effects on smooth muscle cells (152) and has been reported to induce ROS-dependent development of experimental PAH (153).

Opioids and HIV in PAH pathophysiology

There is strong evidence that the use of opioids among PWH promotes HIV-1 replication and expression of HIV proteins which leads to increased inflammation, effectively worsening a person’s chronic HIV infection. Disruption in intestinal integrity plays a key role in the immune activation seen in PWH and the gut translocation of lipopolysaccharide (LPS) into circulation is enhanced in opioid users, leading to systemic inflammation (154) via NF-κβ activation and cytokine production. NF-κβ is a transcription factor that, upon entering the nucleus of HIV-infected cells, can bind the HIV LTR enhancer region, activating viral transcription (105) (Figure 3). Similarly, opioids and psychostimulants increase cAMP response element binding protein (CREB) activity and its interaction with CREB-binding protein (CBP), a histone acetyl transferase, at the HIV LTR, supporting HIV transcription (106).

Morphine and other drugs that act on mu opioid receptors (MORs) induce upregulation of HIV-1 co-receptors CXCR4 and CCR5 facilitating viral cell entry and, ultimately, the progression of HIV-1 infection (155). HIV-1 Tg rats are more prone to opioid addiction than control rats and demonstrate increased expression of MORs (153-155). Moreover, expression of HIV-1 proteins gp120 and Tat is also increased in HIV-1Tg rats in the presence of morphine, suggesting that there is a bi-directional relationship between HIV-1 viral proteins and MOR function that promotes progression of HIV-1 viral infection in opioid users (156) (Figure 3).

Schweizer et al. (156) studied lung macrophage populations in the non-human primate model of HIV-associated PAH and found that PAH animals had increased frequency of pro-inflammatory, non-classical monocytes in peripheral blood and broncho-alveolar lavage fluid. In addition, these SIV-infected macaques with PAH had fewer anti-inflammatory macrophages and IL-10+ cells compared to SIV-infected animals without PAH. HIV is known to dysregulate chemokine and cytokine cascades by skewing the Th1-Th2 balance in favor of a Th2 immune response to evade immune detection, a process that also favors the development of PAH (157-160). Our group (120) reported presence of angio-proliferative advanced stage pulmonary vascular lesions in the lungs from SIV-infected macaques, an established animal model for studying HIV/AIDS, exposed to chronic morphine. We noted increased perivascular inflammation, including an influx of macrophages and elevated plasma levels of monocyte chemotactic protein-1(MCP-1) and interleukin-8, induced by activation of NF-κB, in SIV-infected macaques treated with morphine compared to infected animals with no drug exposure or uninfected animals with only morphine exposure. In addition, acute exposure of HPMECs to HIV-Tat and morphine downregulated the expression of vascular endothelial growth factor receptor-2 (VEGFR2) and decreased VEGFR2 phosphorylation (Figure 4). However, six continuous days of exposure to both Tat plus morphine caused significant increase in total and phosphorylated VEGFR2. This increase in VEGFR2 expression was associated with a switch in endothelial cells from early apoptotic stage to pro-proliferative stage. Vascular endothelial growth factors and their receptors have been implicated in PAH pathophysiology. VEGFR2 and 3 promote angiogenesis and the angio-obliterative lesions seen in PAH (161).

Simultaneous exposure of morphine and HIV proteins has also been shown to induce oxidative stress in the pulmonary microvascular endothelial cells, promoting a shift in the balance between cell death and cell survival that favors proliferation as seen in PAH (94, 120, 130). The initial combined treatment of HPMECs with HIV-Tat and morphine results in an increase in autophagy and apoptosis of HPMECs compared to exposure to either treatment alone due to increased production of superoxide (O₂⁻) and hydrogen peroxide (H₂O₂) radicals (130). However, continuous induction of autophagy by chronic exposure to morphine and Tat actually decreases oxidative stress, thereby attenuating apoptosis and promoting endothelial proliferation (130). Further studies found that increased expression of NADPH oxidase (NOX) is responsible for the initial robust generation of ROS in response to Tat and morphine exposure. Over time and with continued daily exposure to Tat and morphine, there is a trend toward decreased endoplasmic reticulum specific NOX2 and increased NOX4 expression. Knock-out of either NOX 2 or 4 prevented early apoptosis and later-stage HPMEC proliferation (94). Uncontrolled proliferation of cells within pulmonary vascular tissue leads to the formation of plexiform lesions and occlusion of pulmonary arteries, therefore worsening PAH phenotypes. In addition to opioids potentiating the HIV-proteins mediated endothelial dysfunction, opioids also act synergistically with HIV-Tat to cause smooth muscle hyperplasia which leads to pathophysiologic pulmonary vascular remodeling and the development and/or progression of HIV-PAH phenotypes (120).

In conclusion, PAH in recreational drug users living with HIV most likely results from repeated insults to the pulmonary vasculature. Morphine, cocaine, and methamphetamine all activate NF-κB and the expression of pro-inflammatory cytokines. The resultant inflammation and presence of any of these drugs in the bloodstream also activates the expression of HIV and HIV proteins, such as HIV-Nef, Tat, and gp120. Together, sustained use of drugs and HIV proteins causes repeated insults to the endothelium over time, which can lead to uncontrolled expression of angiogenic and fibrotic growth factors in an attempt to rescue damaged endothelial tissue.

ROLE OF SEX IN HIV /DRUG USE -PAH

While most cardiovascular diseases occur more commonly in men, the incidence of PAH is increased in women. This observable “sex bias” in PAH is especially notable in heritable, idiopathic, congenital and connective tissue disease-associated PAH (162, 163). The REVEAL registry from the late 2000s exhibited a female-to-male ratio of 4:1 compared to 3:1 in more recent Chinese registries (164). Although females are more likely to develop PAH than males, males tend to have poorer hemodynamics, functional parameters, and survival (165).

Unlike these subgroups of PAH, HIV-PAH is one group in which males are more affected than females (56, 57). Prevalence data from USA, France, and Spain show male-to-female ratios as high as 7.7:1 (163), which is attributed to the fact that most HIV patients in developed countries are men. Approximately 81% of new HIV cases in the U.S. in 2018 were among men (166), and 69% of those new HIV diagnoses were among men who have sex with men (167). In addition, adult men are two to three times more likely than women to develop a drug use or dependence disorder, although rates are similar among male and female adolescent drug users ages 12-17 (Substance Abuse and Mental Health Services Administration, 2014). This could also be one of the reasons why HIV-PH is more prevalent in males. Male gender was suggested as a predictor of PAH development in two studies evaluating characteristics of methamphetamine-associated PAH (49, 52) but two other studies showed female predominance (50, 51). A male predominance was also observed in cocaine-associated PAH cohort, though it was a small study (46). Overall, there appears to be a consensus of male predominance in drug-associated PAH given the male predominance in drug use.

However, this male predominance among PWH is not the case globally. In 2019, 48% of new HIV infections worldwide were diagnosed in women and girls (168). More than 50% of PWH in low- and middle-income countries are female, and, in eastern and southern Africa, young women are twice as likely to acquire HIV-1 than young men (169). These differences have been attributed to both socioeconomic and biologic factors. First, women exhibit an increased type I interferon (IFNα) response to HIV infection, indicating that women experience more robust chronic innate immune activation than men, which may promote HIV pathogenesis (170, 171). Further, progesterone increases the expression of HIV-1 receptors CD4, CCR5, and CXCR4 on human cervical CD4+ T cells, while estrogen has been shown to be protective against sexually transmitted viral infections like HIV (172, 173). Hormonal contraception may play a role in HIV acquisition; however, this remains controversial(174).

Several studies have explored the role of sex hormone signaling in PAH in an attempt to explain the difference in PAH susceptibility and survival between men and women. Interestingly, it appears that sex hormones also may influence the synergistic effects of HIV and drug use in the development of PAH. Male HIV- Tg rats treated with morphine and HIV-Tat developed pulmonary vascular remodeling as evidenced by increased RVSP and RV hypertrophy (Fulton’s index). These changes were absent in female HIV- Tg rats (94). Estrogen has previously been reported to attenuate NADPH oxidase (NOX) 1, 2, and 4 expressions, thereby reducing oxidative stress and ROS production (175, 176). Further, estrogen has an established vasodilatory effect due to upregulation of nitric oxide production in human endothelial cells (177). Classic rat models with monocrotaline-induced PH have established estrogen’s protective role. A study by Philip and colleagues shows ovariectomized rats receiving exogenous 17β-estradiol (E2) are protected from Sugen-hypoxia-induced PH, resulting in preserved transpulmonary gradient and distal PA distensibility with reduced PA wall remodeling compared to controls (178).

However, estrogen and its metabolites also play a pathologic role in humans and shown to increase vascular remodeling in experimental models (179, 180). It has also been suggested that while estrogen’s effects in the pulmonary vasculature may be pathologic, it may be protective in the RV under exposure to increased afterload (181, 182). The contradictory data in animal models and humans has implicated estrogen in both protective and pathologic roles in PAH, which is referred to as the “estrogen paradox” (183). Notably, pro-inflammatory cytokines are known to strongly induce aromatase and peripheral estradiol synthesis, which augments the pro-inflammatory Th2 and innate immune response in lung endothelium and, subsequently, promotes PAH progression. Tofovic and colleagues suggest a three-tier concept to explain the contradictory effects of estradiol in PAH: estradiol as initially protective in a healthy endothelium environment; estradiol stimulating endothelial proliferation and angiogenesis in response to injured endothelium; and, finally, estradiol protecting against maladaptive RV remodeling in progressive disease (183).

ROLE OF AGEING IN HIV& DRUG USE-PAH

Older age has recently been identified as an independent risk factor for mortality in PAH. A Swedish study evaluating risk assessment of PAH patients, including elderly patients, found that younger patients had better functional parameters, cardiac indices, diffusing capacity of the lungs for carbon monoxide (DLCO), and exercise capacity, and they were able to better tolerate treatment with combination PAH-targeted therapy. Patients over the age of 65 exhibited decreased DLCO, increased comorbidities, and worse five-year survival. While the number of associated comorbidities does not seem to affect survival, the presence of ischemic heart disease and kidney dysfunction portend poor prognosis (184). One study observed that mean pulmonary artery pressures (mPAP) at diagnosis declined with age, suggesting that the elderly are less able to adapt to increased afterload and may develop RV failure at a lower threshold (185).

Approximately 50% of PWH in the U.S. are over the age of 50, and 17% of new HIV cases in 2018 were among people in this age group (186). The number of PWH aged 50 or older is expected to rise in the coming years, largely thanks to advancements in therapies. Unfortunately, PWH experience premature aging and early development of comorbidities. Contemporary studies report the average age of those living with HIV-PAH to be 38-41whereas the average age of those living with IPAH is 50, suggesting that HIV infection accelerates the onset of PAH in younger population (187, 188). As described above, several comorbidities are associated with increased mortality in PWH. Markers of various disease states also appear to progress or worsen earlier in PWH than their age-matched HIV negative counterparts. For example, the diffusion capacity of the lung naturally declines with age (189). A recent study examining lung function in middle-aged men with and without HIV found that men with HIV had worse DL_CO (adjusted difference 2.6% of predicted; 95% confidence interval: 4.7 to 0.6%) and higher risk of impaired diffusion (odds ratio for DL_CO < 60% of predicted 2.97; 95% confidence interval: 1.36–6.47) than men without HIV (190). An Italian case-control study found that PWH accumulate age-associated comorbidities like hypertension and cardiovascular disease about one decade earlier than the general population. The prevalence of non-infectious comorbidities in this study’s HIV population was associated with lower nadir CD4 count and longer exposure to ART (191). Using data from the Veterans Aging Cohort Study, which included 6,351 HIV-positive and negative adults, Green et al. assessed drug use among aging adults with and without HIV (192). They found that current and past multidrug use was more prevalent among PWH and AIDS-associated illness and multimorbidity was highest among multidrug users living with HIV. Furthermore, HIV-PAH patients in Spanish PAH registry (REHAP) & Pulmonary Hypertension Association Registry (US) have been identified to be significantly younger compared to other cohorts of PAH, such as IPAH/FPAH (57) (56).

Despite the benefits of HAART, chronic inflammation experienced by PWH can persist while on therapy, a phenomenon known as “inflammaging.” Zhao et al. (193) found that latently infected CD4+ T cells and lymphocytes in PWH on ART expressed more programmed death-1 (PD-1), a marker of cell exhaustion, than age-matched controls. Telomere length was also significantly shorter in latently infected CD4+ cells. These cells were noted to suppress DNA damage checkpoint kinase ataxia-telangiectasia mutated (ATM) and its downstream checkpoint kinase 2 (CHK2), resulting in DNA damage and cell death.

Cellular senescence is the transition of a cell from a proliferative, regenerative phase to a phase of growth arrest in response to DNA damage (194). Van der Feen et al. proposed a conceptual model linking PAH with a pro-inflammatory senescence-associated secretory phenotype (SASP) through common triggers and shared signaling pathways (195). They suggest that the cycle of ROS production, DNA damage, and loss of BMPR2 expression could lead to cellular senescence (195). Prolonged exposure to TNF-α, an inflammatory mediator known to promote pulmonary vascular remodeling by downregulating BMPR2, can induce senescence and an SASP in fibroblasts and PAECs (196, 197). Key HIV factors, including HIV-Tat and Nef, can modulate the DNA damage response (198). Tat interacts with the histone acetyl transferase Tip60 (Tat interactive protein 60kDa), inhibiting its activity and facilitating proteasomal degradation. This ultimately results in pro-survival phenotype and fosters long-term HIV infection. Moreover, because Tip60 is responsible for the acetylation and activation of the kinase ATM, a master regulator of the DNA damage response, Tat’s inhibition of Tip60 activity likely also suppresses ATM axis-mediated DNA repair (198). Likewise, HIV-Nef promotes viral persistence by inducing an anti-apoptotic state. Extracellular Nef is taken up by T cells, where it activates AKT serine/threonine kinase (AKT) and pro-survival pathway. As part of the DNA damage response, AKT interacts with ATM-mediated factors to facilitate DNA repair (198).

PWH also experience enhanced telomere shortening due to some antiretroviral therapies, specifically nucleoside analogue reverse transcriptase inhibitors (NRTIs) (199). NRTIs inhibit mitochondrial DNA polymerase gamma, which leads to mitochondrial DNA depletion, mitochondrial dysfunction, and oxidative stress (200). Protease inhibitors also can cause accumulation of prelamin A, which is known to promote premature senescence (199). Cellular senescence and related signaling cascades should be explored as potential “serotherapeutic” targets in HIV-PAH.

NEXT STEPS: INVESTIGATIVE CHALLENGES & AREAS OF FUTURE RESEARCH

Although PAH is a major global health concern associated with significant morbidity and mortality, researchers often find it difficult to conduct large-scale studies in the field. This is largely due to the limitations of studying a chronic, progressive disease in short-lived cell and animal models and due to the diversity of the PAH patient population. These patients have varied clinical presentations, disease severity, rate of progression, and therapeutic responses, posing a significant challenge to researchers (17). The study of HIV-PAH, specifically, has been further challenged by limited participation of PWH in randomized controlled trials. PWH are often excluded from PAH studies due to patient comorbidities and concern regarding potential drug interactions. Janda et al. (201) performed a systemic review of the HIV-PAH literature and, aside from case reports, included only 13 cohort studies, one case series, and two case-control studies in their analysis. None of these were interventional studies and, therefore, did not control for confounding variables and were subject to bias. Moreover, some HIV-PAH studies rely on PAH diagnosis by TTE rather than RHC. As previously discussed, measurements by TTE can be unreliable (202). Because the majority of PWH live in developing countries where testing and reporting capabilities are limited, it is difficult to determine the prevalence of HIV-PAH with accuracy and nearly impossible to conduct large studies.

Despite these challenges, investigators are forging ahead, working to grow our knowledge of the pathobiology of the disease. Several potassium (K+) channels help maintain vascular tone, and mutations in the genes that encode these channels can contribute to the development of PH (15, 203). However, only one study by Mondejar-Parreno et al. (204) reported reduced K+ currents and more depolarized PASMCs in HIV Tg (Tg26) mice compared with uninfected controls. While the Tg26 group did exhibit some pulmonary vascular remodeling and endothelial dysfunction, it was not enough to cause significant changes in hemodynamic parameters. Furthermore, Tat can modulate oxidative phosphorylation, ATP production, mitochondrial calcium uptake, and oxygen consumption resulting in an increase in oxidative stress in cardiomyocytes (205, 206) and neurons (207). HIV-gp120 also can cause oxidative stress through lipid peroxidation and hydroxynonenal ester production (208). These pathways may also be involved in pulmonary smooth muscle and endothelial dysfunction and can be examined in relation to HIV and recreational drug-mediated worsening PAH phenotypes. Furthermore, chronic hypoxia, mechanical-stress and inflammation-induced endothelial to mesenchymal transition (EndoMT) has been extensively reported in PH (209), but the direct role of EndoMT in HIV-PH development is yet to be explored. The cell growth and proliferation that occurs in PH is also associated with high metabolic demands, but very few studies have examined metabolic changes in the context of HIV-PH. ECM stiffening activates glycolysis and glutaminolysis, and, in the SIV-infected primate model, collagen deposition in vessels correlated with increased glutaminolysis and aspartate production (210). Additional research is needed to better characterize these pathways as well as to identify novel targets in HIV-PH and explore their therapeutic potential.

CONCLUSION

As the population of PWH continues to grow, we can expect the prevalence of HIV-PAH to rise as well, especially among illicit drug users. Not only are people with HIV living longer with comorbidities like pulmonary hypertension, but more people now meet PH diagnostic criteria as defined by the WSPH Task Force in 2018. It is noteworthy that not all recreational drug users with HIV will incur PAH, proposing that there are likely other genetic and environmental factors that contribute to the development of PAH in these individuals. Nonetheless, there is strong evidence to support the “double hit” model of drug use and HIV contributing to PAH pathophysiology. In terms of pathophysiologic synergy, HIV and drugs act on a number of the same or different interconnected pathways to exert their pathologic effects on the pulmonary vasculature. Consequently, it’s possible that the “double hit” of drug use in PWH could increase the risk of HIV-PAH and/or promote a more severe disease phenotype.

Unfortunately, HIV-PAH is underdiagnosed in developing countries where echocardiography and invasive heart catheterizations are often inaccessible. In addition, HIV and pulmonary hypertension are complex diseases independently; together and in the presence of drug use, their diagnosis and treatment can become increasingly challenging. Rarely is the etiology of PH clear-cut. More often, multiple risk factors for PH are present, and coinfections and comorbidities confound the diagnosis of HIV-PAH. It also remains unclear whether HIV therapies can cause or contribute to vascular remodeling in HIV-PAH. Future studies should investigate the effects of the various HIV drug classes on the heart and pulmonary vasculature.

Decades of work by dedicated researchers have enriched our understanding of the pathophysiology of both HIV and PH and the overlap of the two diseases. However, we need larger-scale studies to explore the synergistic effects of HIV infection and drug use on the development of PH. While current PAH therapies target the well-described nitric oxide, endothelin, and prostacyclin pathways, additional research efforts should focus on mapping the cellular and signaling pathways involved in the pathophysiology of HIV-PAH. A better understanding of parallel, overlapping PH and specific novel pathways is warranted to offer insight into the additive effects of HIV and drug use and reveal potential novel therapeutic targets.

DIDACTIC SYNOPSIS.

Major teaching points:

• Highly active antiretroviral therapy has decreased AIDS-related deaths and increased the prevalence of chronic illnesses among people living with HIV.

• Recreational drug use is associated with an increased risk of HIV transmission, disease progression, and non-compliance with ART.

• PAH is the consequence of an exaggerated immune response and dysregulated vascular remodeling. Together, drug use and HIV enhance these vascular changes.

• Exposure to illicit drugs, such as methamphetamine, cocaine and opioids, increases HIV protein-mediated pulmonary microvascular endothelial dysfunction, contributing to pulmonary arteriopathy.

• Drugs of abuse also act synergistically with HIV proteins to induce pulmonary arterial smooth muscle cell hyperplasia.

• Higher levels of TGF-β1 have been detected in extracellular vesicles from the plasma of cocaine users living with HIV or HIV-PH compared to non-users living with HIV. The circulating extracellular vesicles from HIV-PH patients have the ability to augment pulmonary vascular endothelial and smooth muscle dysfunction.