Gastrointestinal Dysfunction and HIV Comorbidities

Abstract

Purpose of Review

Gut dysfunction and resulting chronic low-grade inflammation have been linked to metabolic and chronic diseases in the general population. In this review, we present recently published studies of HIV-associated gut dysfunction and comorbidities including obesity, diabetes, cardiovascular disease, liver disease, and neurocognitive disease.

Recent Findings

Biomarkers of microbial translocation, dysbiosis, or intestinal epithelial integrity have been used to investigate relationships between HIV-associated gut dysfunction and metabolic, cardiovascular, and neurologic complications. Many studies point to worsened comorbidities associated with gut dysfunction in people with HIV (PWH), but some studies show mixed results, and thus, the data are still inconclusive and limited to surrogate biomarkers rather than direct intestinal assessments.

Summary

Inflammation and immune activation stemming from changes in intestinal epithelial integrity and dysbiosis are present in PWH and relate to metabolic, cardiovascular, and neurologic complications of HIV. However, future investigations, especially future studies that directly assess intestinal pathology, are needed to investigate the direct contributory role of gastrointestinal dysfunction to comorbidities of HIV.

Introduction

People with HIV (PWH) are living longer than before in the era of combination antiretroviral therapy (cART). However, as recently highlighted by Dr. Anthony Fauci of the National Institute of Allergy and Infectious Diseases, PWH have a significant heightened risk of HIV-associated comorbidities that need to be addressed in order to improve the health and lives of PWH [1]. The significant burden of comorbidities such as obesity, diabetes mellitus, cardiovascular disease, liver disease, and neurocognitive disease remains a challenge and causes significant morbidity and mortality for PWH [1, 2]. A major contributor in the pathogenesis of these comorbid conditions is unresolved chronic immune activation, which has been hypothesized to be in part due to microbial translocation in the gut associated with HIV [3].

Changes in the intestinal mucosa and gut immune system in HIV disease have been well documented and include depletion of CD4+ T cells, enterocyte death, loss of tight junctions, decreased immunoglobulin A, and dysbiosis [4]. Highlighting the detrimental effects of intestinal barrier damage in PWH, soluble CD14 (sCD14), a marker of monocyte activation in response to endotoxin lipopolysaccharide (LPS), has been shown to be an independent predictor of all-cause mortality in HIV infection, in which the highest quartile of sCD14 levels had a 6-fold higher risk of death than did those in the lowest quartile [5]. Furthermore, plasma gut epithelial barrier integrity biomarkers, intestinal fatty acid binding protein (I-FABP) and zonulin, also predicted mortality in treated HIV infection [6]. However, exact mechanisms underlying these trends have not been fully elucidated.

Gut dysfunction has also been implicated in the pathogenesis of metabolic diseases even in the absence of HIV. Termed metabolic endotoxemia, the LPS-CD14 system has been shown to trigger weight gain and insulin resistance in mice [7]. In humans, circulating LPS and sCD14 levels were significantly higher in the BMI-, sex-, and age-matched T2DM group compared to controls [8, 9]. The prevailing hypothesis underlying these observations is that high-fat diet induces intestinal dysbiosis resulting in aberrant metabolites and disruption of the intestinal epithelial barrier integrity [10]. Due to the increased permeability, microbial endotoxins and other metabolites enter the circulation and contribute to chronic inflammation in the liver and the adipose tissue, which are associated with the development of metabolic syndrome and related conditions [10]. This link between gut dysfunction and metabolic complications is an area of special interest in the context of HIV disease as PWH have high rates of metabolic comorbidities and intestinal barrier damage, placing this population at even greater risk.

Here, we review what is known about gut dysfunction and HIV comorbidities including obesity, diabetes, liver disease, cardiovascular disease, and neurocognitive disease. We highlight recent findings; many of which, however, rely on markers of microbial translocation rather than direct inspection of intestinal disruption.

Role of Gut Dysfunction in Insulin Resistance and Adiposity in PWH

Consistent with studies in the general population, microbial translocation in HIV has been associated with insulin resistance. In an observational study in Denmark, Pedersen et al. showed that circulating LPS levels were higher in treated PWH compared to healthy controls, and higher LPS levels were associated with lower insulin sensitivity estimated using the Matsuda composite index obtained by using the oral glucose tolerance test in PWH [11]. Another study in the USA by Timmons et al. also showed a positive association between LPS and insulin resistance represented by homeostatic model assessment-insulin resistance (HOMA-IR), with the strongest relationship in individuals with more advanced HIV disease and those in the ART treatment–naïve group [12]. More recently, bacterial translocation as measured by circulating bacterial DNA was associated with long-term impaired glucose homeostasis determined by hemoglobin A1c change after median follow-up of 8.5 years in non-diabetic men with HIV in Spain [13]. Development of diabetes is associated with dysbiosis including decreased butyrate-producing bacteria in the general population [14, 15]. Although correlation with diabetes has yet to be shown in PWH, a lower relative abundance of butyrate-producing bacteria and its association with markers of microbial translocation and immune activation have been demonstrated in PWH [16•].

In addition to disturbances in insulin sensitivity, several markers of microbial translocation have been associated with adiposity. Soluble CD14 levels rose with BMI gain in over-weight or obese participants after ART initiation in a multinational cohort of PWH [17]. Baseline I-FABP, a marker of enterocyte damage, predicted increase in adiposity over 96 weeks following ART initiation in treatment-naïve participants [18•]. Beta D-glucan (BDG), a marker of fungal translocation, increased at 96 weeks after initiation of ART in AIDS Clinical Trials Group study A5260s, and higher BDG was associated with larger trunk and total fat gains [19•]. In highlighting the detrimental effects of dysbiosis in PWH, bacterial diversity measured by fecal 16S rDNA sequencing was reduced in visceral and general obesity compared with lean participants and was negatively correlated with sCD14 [20•].

However, studies looking at the association between sCD14 and obesity have not been consistent. Among PWH, similar levels of sCD14 were seen among both obese and non-obese individuals [21]. In a larger observational study, median sCD14 levels were unexpectedly found to be significantly lower in obese PWH population compared to lean participants without HIV [22]. The authors posit that hepatic steatosis in obesity may influence sCD14 levels due to decrease in hepatocyte-derived sCD14 when HIV and obesity co-occur, which also points to an important limitation utilizing sCD14 as a biomarker for microbial translocation and intestinal integrity.

Role of Gut Dysfunction in Nonalcoholic Fatty Liver Disease in PWH

Via the portal vein, the liver receives the majority of its blood supply from the intestines, and thus, the liver is exposed to gut-derived microbes and microbial products. The disruption of intestinal barrier function causes bacterial translocation of LPS across the intestinal endothelial barrier. Translocated LPS then binds to TRL4 on Kupffer cells, which in turn activates NF-κB via MyD88, inducing production of cytokines such as IL-1β and TNF-α in the liver. Thus, intestinal dysbiosis can result in an increase in intestinal permeability and microbial translocation that increase transcriptional activation of proinflammatory genes and cytokines in the liver, contributing to nonalcoholic steatohepatitis (NASH) [23].

Although the role of gut–liver axis has been elucidated in the pathogenesis of nonalcoholic fatty liver disease (NAFLD), there is little data on intestinal permeability and dysbiosis in development of NAFLD in PWH. However, due to increased intestinal mucosal damage, heightened immune activation, and higher prevalence of NAFLD in the HIV population, PWH are thought to be at a particularly high risk for NASH. Under this model, HIV-related changes in the intestinal microbiota and the mucosal immunity lead to increased intestinal permeability with translocation of microbial products into the portal system [24]. The end result is hepatic inflammation and fibrosis leading to the progression of NAFLD.

Similar patterns have been observed in other chronic liver diseases such as hepatitis C in PWH in which circulating levels of endotoxins and sCD14 have been shown to be associated with disease severity [25]. However, current published studies have not yet shown clear association between markers of microbial translocation and NAFLD in PWH. Although sCD14 level was elevated in a group of PWH with biopsy-proven NAFLD compared to control PWH, there was no difference in other bacterial translocation markers, LPS, LPS-binding protein (LBP), and bacterial DNA. Furthermore, sequencing of stool samples showed no distinct microbial profile in the NAFLD group compared to controls in PWH [26]. Future larger studies are needed to clarify the association between gut dysfunction and metabolic liver disease.

Role of Gut Dysfunction in Cardiovascular Disease and Dyslipidemia in PWH

In the general population, circulating LPS levels can predict adverse cardiovascular events in patients post-myocardial infarction [27]. Kallio and colleagues have found that high serum LPS activity is associated with cardiovascular disease and cardiometabolic disorders, independent of traditional cardiovascular risk factors like diet, obesity, and diabetes [28]. Gut dysbiosis has been found to be present in patients with atherosclerotic coronary artery disease in multiple case-controlled studies utilizing fecal samples from patients [29]. Sandek and colleagues have found that individuals with chronic heart failure have a 35% increase of small intestinal permeability by lactulose-mannitol test and a 210% increase in large intestinal permeability by lactulose test, which may contribute to chronic inflammation seen in patients with chronic heart failure [30].

With regard to the relationship between gut dysbiosis and cardiovascular disease in PWH, serum trimethylamine (TMA), a microbiota-derived metabolite of phosphatidylcholine, is associated with the presence of coronary plaque and calcified plaque in PWH [31]. In addition, Haissman and colleagues have found that elevated trimethylamine-N-oxide (TMAO), a proatherogenic substance formed in the liver from TMA, was associated with silent ischemia in PWH [32].

Recent studies have also investigated the relationship between intestinal barrier function and cardiovascular disease in PWH. These studies show that microbial translocation across the gut endothelial barrier may be associated with increased cardiovascular risk in PWH. For example, Timmons and colleagues have shown that LPS and sCD14 may contribute to elevated triglyceride levels and increased insulin resistance in PWH. Additionally, they found a negative correlation between sCD14 and high-density lipoprotein (HDL) cholesterol in PWH [12]. Elevated triglyceride levels, increased insulin resistance, and decreased HDL levels may contribute to the increased risk of cardiovascular disease in PWH on chronic treatment.

Some studies have shown that sCD14 is correlated with the progression of atherosclerosis in PWH. Kelesidis and colleagues have found that serum biomarkers of LPS and sCD14 predict subclinical atherosclerosis progression in PWH [33]. Longenecker and colleagues have found that sCD14 is correlated with coronary artery calcification in PWH on ART and can be used to predict the extent of subclinical disease in other vascular beds among those with detectable calcium [34]. Further research is needed to fully understand the relationship between sCD14 and the development of atherosclerosis in PWH.

Role of Gut Dysfunction in HIV-Associated Neurocognitive Disorder (HAND)

The majority of studies investigating gut dysfunction and neurological outcomes use surrogate markers for microbial translocation. The most studied surrogate markers for microbial translocation in PWH and neurological outcomes are sCD14 and LPS, both of which have been tested in multiple cohorts among PWH with mixed results. Early work in PWH and CD4+ T-cell count < 300 cells/μl suggested that both sCD14 and LPS were closely associated with HIV-associated dementia (HAD), compared to participants without neurocognitive impairment, and did not see observed associations among less-severe forms of HAND [35]. These data were replicated in cohorts among participants with incomplete viral suppression [36, 37]. In more recent studies, sex differences in the association between sCD14 levels and cognitive function have been reported with sCD14 levels elevated in women, but not men, who have cognitive impairment and after 48 weeks after ART initiation [38]. Higher sCD14 levels were associated with worse cognitive performance on tests of verbal learning, verbal memory, executive function, and psychomotor speed among participants in the Womens Interagency HIV Study (WIHS) [39] and associated with more severe global impairment in a cohort of ART-naïve women with HIV in Nigeria [40]. The underlying mechanism for this association remains unknown, but in a small pilot, higher sCD14 correlated with lower cortical volumes in frontal and temporal lobes in women with HIV [41].

The relationship between sCD14 and cognitive outcomes in suppressed PWH cohort is mixed. While a sub-analysis of women with viral suppression show sCD14 levels correlated with reduced executive function performance, this was not observed for other domains [39]. Other similar studies in virally suppressed cohorts or sub-analyses of participants with viral suppression do not demonstrate significant correlations with sCD14 or LPS and global cognitive function or HAND [37, 42, 43]. This suggests that sCD14 may have a limited association with global cognitive function in virally suppressed adults, but could be related to performance on cognitive subdomains, or that sex modifies the relationship between sCD14 and cognition. Interestingly, sex differences in sCD14 have been demonstrated in recent data from people without HIV infection enrolled in Framingham Heart Study (mean age 69) or Cardiovascular Health Study (CHS; mean age 72). In this study, sCD14 levels positively correlate with incident dementia, and greater progression of brain atrophy and cognitive decline in executive functioning; the effect of sCD14 and incident all-cause dementia was stronger in women (HR 1.53; 95% CI 1.23–1.89) than men (HR 1.03; 95% CI 0.71–1.48) in CHS [44].

Other tested gut-associated biomarkers in virally suppressed PWH include intestinal I-FABP [39] and (1-3)-β-D-glucan (BDG) [42]. Serum I-FABP is higher in people with chronic HIV infection on ART [45, 46] and elite controllers [46], and serum levels positively correlate with markers of immune activation (e.g., monocyte chemoattractant protein-1, sCD163; [46]). To date, few studies have investigated I-FABP in PWH for neurological outcomes, and the majority do not suggest significant associations with cognitive impairment [39, 42]; others show weak associations [37]. Microbial translocation has focused on bacterial flora in HIV and its associations with comorbidities, while the impact of fungal translocation is not well studied. BDG is a polysaccharide cell wall component of many fungal species and a potent stimulator of macrophages and T cells. Plasma, not CSF, BDG levels correlate with global cognitive deficits in virally suppressed PWH [42, 43], and in a small pilot study, the combined biomarker panel that included sCD14, IL-8, and BDG demonstrated a more robust correlation with the global cognitive deficit than BDG or sCD14 alone [43].

Conclusions

Inflammation and immune activation stemming from changes in the intestinal mucosal barrier and dysbiosis have been well documented in PWH and contribute to comorbidities associated with HIV. However, the exact degree to which gut dysfunction contributes to HIV-associated comorbidities has not yet been clearly elucidated partly due to the fact that the majority of studies investigating gut dysfunction and metabolic outcomes or other comorbidities use surrogate markers for microbial translocation and many do not involve direct inspection of the intestinal mucosal barrier or investigation of microbial populations. Another limitation is that the two most widely used biomarkers of microbial translocation LPS and sCD14 have been limited by technical difficulties with assays and non-specific response to stimuli other than endotoxins, respectively [47]. Therefore, future investigations of HIV-associated intestinal barrier dysfunction and comorbidities are needed in PWH in order to help identify therapeutic strategies to target the pathogenesis of HIV-associated comorbidities.