Dual inhibition of ACE-2 and TMPRSS2: A promising strategy to mitigate SARS-CoV-2 severity in smokers

Main text

The COVID-19 pandemic spurred an urgent global effort to understand why some individuals suffer far more severe outcomes than others. While risk factors such as age and comorbidities were quickly identified, the role of cigarette smoking remained controversially debated, clouded by conflicting epidemiological data.^1,2 In this issue of Molecular Therapy Nucleic Acids, Hussain et al. provide crucial mechanistic clarity, moving beyond correlation to causation.³ Using a compelling combination of ferret and human airway epithelial cell models, they demonstrate that cigarette smoke exposure upregulates the two primary SARS-CoV-2 entry factors, ACE-2 and TMPRSS2, leading to increased viral load. Critically, they show that only the simultaneous blockade of both pathways using an ACE-2 targeting antisense oligonucleotide (ASO) and the TMPRSS2 inhibitor camostat mesylate could effectively mitigate this smoke-induced vulnerability. These findings not only illuminate a biological basis for smoking as a key risk factor but also provide a strong rationale for the limited success of clinical monotherapies and champion a new, personalized combination approach for treating high-risk populations.

The entry of SARS-CoV-2 into host cells is a well-orchestrated process initiated by the binding of its spike protein to the angiotensin-converting enzyme-2 (ACE-2) receptor. For the virus to complete its entry, the spike protein must be primed by a host protease, most notably the transmembrane protease serine 2 (TMPRSS2).⁴ The expression levels of these two proteins in the respiratory tract have therefore been considered a critical determinant of host susceptibility. Early in the pandemic, several investigations found that ACE-2 and TMPRSS2 are elevated in the airways of smokers and patients with chronic obstructive pulmonary disease (COPD), a condition predominantly caused by smoking.^5,6,7 This created a plausible hypothesis: smoking primes the airways for more efficient viral invasion, linking the habit to the severe disease observed in many hospitalized patients.⁸ However, inconsistent clinical data and the complexity of collecting accurate smoking histories left the hypothesis in need of direct, controlled experimental validation.

Hussain and colleagues tackled this challenge head-on with a methodologically robust, multi-model approach. They first demonstrated that both chronic in vivo cigarette smoke exposure in a ferret model and acute in vitro exposure of ferret tracheal epithelial cells (FTECs) to cigarette smoke extract (CSE) led to a significant upregulation of both ACE-2 and TMPRSS2 at both the mRNA and protein levels. This key observation was then replicated in human models, showing that CSE exposure increased ACE-2 and TMPRSS2 expression in Calu-3 lung cells and, more importantly, in primary human bronchial epithelial (HBE) cells. Strikingly, HBE cells derived from COPD donors, who have a history of smoking, exhibited persistently elevated levels of these viral entry factors even without acute CSE exposure, suggesting a lasting molecular signature of injury.

The functional consequence of this upregulation was unequivocal. In both ferret and human cell models, pre-exposure to CSE resulted in a significantly higher SARS-CoV-2 viral load and replication upon infection. The study’s most significant contribution, however, lies in its therapeutic investigation. The authors hypothesized that if smoking enhances both primary entry pathways, then blocking only one might be insufficient. Using FTECs, they found that an ASO targeting ACE-2 or camostat mesylate targeting TMPRSS2 could each partially reduce viral load in smoke-exposed cells. Yet, the most profound effect was achieved when the two treatments were combined; the dual blockade led to a dramatic and highly significant reduction in SARS-CoV-2 infection that was far superior to either monotherapy (Figure 1). The implications of this work are substantial. First, it provides a compelling mechanistic explanation for the failure of clinical trials that tested TMPRSS2 inhibitors such as camostat mesylate as a monotherapy. Hussain et al. propose that in high-risk individuals such as smokers, the virus can exploit the smoke-induced upregulation of ACE-2 as a compensatory entry mechanism when TMPRSS2 is blocked. This highlights a critical principle: effective therapeutic strategies must account for the host’s environmental exposures and resulting molecular landscape. Second, the study advocates for the development of combination therapies tailored to specific patient populations. For smokers or those with a history of significant smoke exposure, a dual-pronged attack targeting both ACE-2 and TMPRSS2 may be required for a meaningful clinical benefit. The successful use of a targeted ASO also reinforces the potential of nucleic acid-based therapies in the fight against respiratory viruses.

Figure 1.

Proposed mechanism for increased SARS-CoV-2 severity in smokers and rationale for combinational therapy

In a healthy non-smoker, the baseline expression of ACE-2 and TMPRSS2 on airway epithelial cell is relatively low, which limits the efficiency of SARS-CoV-2 entry. Cigarette smoke exposure significantly upregulates the expression of both receptor and the TMPRSS2 protease. This creates a highly permissive environment, leading to increased viral entry and replication, which can result in a higher viral load and potentially more severe disease. A combinational therapy that simultaneously blocks the ACE-2 and TMPRSS2 effectively shuts down the primary viral entry routes, significantly reducing viral load and mitigating the deleterious effects of smoke exposure.

While the study is strengthened by its use of multiple models, including primary human cells, the authors rightly acknowledge its limitations, such as the inherent differences between ferret and human physiology and its focus on conventional cigarette smoke. Future research should explore whether other inhalants of modern concern—such as e-cigarette vapor and cannabis smoke exert similar effects on these viral entry pathways. Furthermore, these findings pave the way for human clinical trials designed to test combination therapies in high-risk, smoke-exposed populations, potentially using the very agents investigated here.

In conclusion, Hussain et al. have provided a foundational, mechanistically grounded study that clarifies a contentious issue in COVID-19 research. They demonstrate not just that smoking increases SARS-CoV-2 severity, but how it does so at a cellular level, and what is required to counteract it. Their work shifts the therapeutic paradigm from a one-size-fits-all approach to one of precision, where understanding a patient’s exposure history is key to designing an effective antiviral strategy. This research is a critical step toward developing personalized, combination therapies that could improve outcomes for the millions of smokers who remain at high risk for severe respiratory viral infections.

Declaration of interests

The authors declare no competing interests.