SARS-CoV-2 Entry: The Potential EV Hitchhiker

You can find the original article here, and the Ya-Wen Chen Lab (Icahn Medical School at Mount Sinai) here.

Introduction

The COVID-19 pandemic, caused by SARS-CoV-2, is an event whose modern-day impact precludes introduction. In December 2019, an outbreak of SARS-CoV-2 emerged from a cluster of patients in Wuhan, situated in China’s Hubei province (1). By November 2025, upwards of 7.1 million deaths worldwide had been attributed to the pandemic, especially among adults above the age of 65 (2). SARS-CoV-2 infection is usually characterized, within the first 0-7 days post-infection, by its constitutional symptoms (the generic, body-wide symptoms associated with “normal” disease progression). In the vast majority of infections, this is followed by a gradual return to health following bodily elimination of the virus. For certain groups of individuals (especially older adults, immunocompromised individuals, and those with underlying pulmonary or respiratory illness), however, internal dysregulation or severe COVID-19 disease may manifest instead. This is usually due to the emergence of a “cytokine storm” (essentially an overproduction of molecules that cause inflammation), and SARS-CoV-2 infection of cell types outside of its normal hosts. There is also “Long COVID”, a poorly understood condition wherein patients experience internal dysregulation months to years after virus elimination (2). Overall, SARS-CoV-2 creates a very diverse set of illnesses and conditions, and the reasons for this remain poorly understood. Given these discrepancies, there has been a major focus in the scientific enterprise over the last several years to fully understand and characterize SARS-CoV-2 infection mechanisms, with the hope of producing more effective and specific therapies for affected individuals, especially given the large number of coronaviruses circulating in mammal populations. This paper, published from the Ya-Wen Chen Lab at the Icahn Medical School at Mount Sinai, explores a poorly understood but intriguing aspect of SARS-CoV-2 infection: how it can infect cells that do not possess its attachment receptors (molecules on cells that the virus uses to gain entry).

An Upheaval in SARS-CoV-2 Entry

Viruses build their outer proteins into super-specific shapes, and in the world of proteins, shape determines what you can do and what you can interact with. By doing so, viruses, under evolutionary pressure, design their outer proteins to specifically and strongly interact with host cells' outer proteins. This allows them to gain entry to these cells and determines which cells they can interact with. In the case of SARS-CoV-2, established models affirmed that the virus interacts with Angiotensin Converting Enzyme 2, or ACE2, on pulmonary epithelial cells (cells that comprise the surfaces of much of the lungs. This places the virus in an endosome (an internal cellular compartment) that then showers the virus with acids that allow it to release its genetic material. An alternate entry mechanism, however, involves TMPRSS2, a protein that changes SARS-CoV-2’s outer protein shapes, which in turn allows the virus’s outer membrane to fuse with the membrane of the host cell. 

The latter explanation generally assumes that these proteins are on the surface of cells, since the virus needs to access them first in order to gain entry. However, imaging of the cells reveals that TMPRSS2 is almost exclusively found on the inside of cells, near but not breaching the surface:

Fig. 1A, C: These results show that TMPRSS2 exists inside the cells. To the left, Figure 1A shows images of cells with dyes that stain TMPRSS2 and EPCAM, a surface protein. As can be seen, TMPRSS2 exists in an inner band within the EPCAM signals, indicating that it exists inside the cells, rather than on the surface. On the right, we can see the results of flow cytometry, where the further along an axis a signal is, the greater the amount of protein is in that sample. In the surface samples, we can see high levels of EPCAM-APC but low levels of TMPRSS2. In the inner samples, we observe high levels of both signals, indicating that TMPRSS2 is exclusively intracellular.

Fig. 2A and Fig. S5A: These results show that TMPRSS2 exists in extracellular vesicles or EVs (which are small compartments that carry stuff out of cells). To the left, there are more images showing TMPRSS2 in green and common “markers” of EVs in red. As can be seen, there is significant overlap between these signals. To the right, we can see that a great proportion of these EV markers are in the same places as TMPRSS2 (and vice versa), indicating that TMPRSS2 may exist inside of EVs.

Fig 3A-B: These results show that TMPRSS2 is expressed on the surface of pulmonary epithelial cells in diseased lungs. In A, we see that healthy lung cells have low levels of TMPRSS2 compared to EPCAM, indicating little to no TMPRSS2 is on their surfaces. In B, we see that amongst patients with idiopathic pulmonary fibrosis (IPF), fibrosis, or interstitial lung disease (ILD), there are small but nonzero levels of TMPRSS2 on cell surfaces

From the above results, the authors begin to challenge the latter SARS-CoV-2 entry mechanism. Only in diseased lungs is TMPRSS2 on the surface, since in healthy lung cells, TMPRSS2 remains in extracellular vesicles. EVs are notable in that they tend to be absorbed by other cells, especially phagocytic cells (cells that engulf and consume given materials). Based on these findings, then, the authors began to look for cells that contain TMPRSS2 but aren’t capable of producing the molecule, as they may be absorbing it through these EVs.

A Striking Discovery: SARS-CoV-2 can enter Cell Types that do not Produce TMPRSS2

In their search, the authors find that alveolar macrophages, or AMs, and endothelial cells (ECs) expressed TMPRSS2 on their surfaces despite not creating the genetic material needed to produce TMPRSS2. Alveolar macrophages are phagocytic cells that patrol the alveoli for incoming disease threats. Endothelial cells are cells that comprise the lining of blood vessels, and do not normally express TMPRSS2 at all. However, the authors find that some of these cells nevertheless have TMPRSS2 on their surfaces:

Fig 5A-B: These results show that, in some diseased lungs, endothelial cells and alveolar macrophages possess TMPRSS2 on their surfaces, despite these cell types being incapable of doing so. We can see in A and in B that TMPRSS2 levels are high in both cell populations. 

Fig 6C, E: These results show that, within purified alveolar macrophages and endothelial cells, adding the aforementioned EVs caused the sudden appearance of TMPRSS2 on their surfaces. In C and E, we can see that adding EVs caused a massive increase in the TMPRSS2 signal. This perhaps explains why, in certain cases, cells have TMPRSS2 on their surfaces even if they are incapable of producing it, and a potential way in which they then acquire TMPRSS2. 

Fig 7B-C: These results show that adding EVs to stem cells increases their susceptibility to SARS-CoV-2 infection. We can see in both panels that viral titers (concentrations) are massively increased when EVs are added. Stem cells are cells that haven’t developed into any given cell type, so they shouldn’t normally produce TMPRSS2 since they haven’t developed in pulmonary epithelial cells. However, by adding EVs, SARS-CoV-2 was able to infect them nonetheless, suggesting that the EVs were installing TMPRSS2 onto their surfaces, and thus allowing the virus to gain entry.

The aggregate of these experiments reveals a striking new mechanism through which SARS-CoV-2 gains access to cells. Extracellular vesicles can carry TMPRSS2 and install it onto the surface of phagocytic cells, expanding the range of cells SARS-CoV-2 can infect. This newfound host diversity may begin to explain some of the greater discrepancies regarding COVID-19 disease, especially within severe COVID-19. As aforementioned, this installation of TMPRSS2 onto cell surfaces occurs more frequently in diseased lungs, providing potential directions regarding COVID-19’s abnormal pathologies (the manifestation of disease) in such cases. One major question that remains is whether this release of EVs is a deliberate mechanism of SARS-CoV-2, meant to prolong or intensify infection, or if this release is a hijacked or deliberate mechanism of the body. The authors posit that these EVs may serve as “decoys”, meant to absorb viruses so that they cannot enter actual cells. It’s also possible, as the authors also consider, that SARS-CoV-2 deliberately “hitchhikes” within the EVs and waits for them to be absorbed by cells, which would allow the viruses to enter undetected. More experiments in these regards will be needed to meaningfully explore these avenues, but this paper nonetheless explores a fascinating aspect of SARS-CoV-2 infection. In exploring such questions, it may be possible to create better treatments against coronaviruses, especially for vulnerable individuals, and to better understand SARS-CoV-2 infection. Coronaviruses are diverse and circulate in animal populations, so there is an omnipresent threat that a new virus may emerge in humans, as has occurred a few times this century (such as SARS-CoV in 2006, MERS-CoV in 2011, and of course, SARS-CoV-2 in 2019). By studying SARS-CoV-2 in its entirety, we can better safeguard ourselves against such new diseases, improving outcomes for untold numbers of people as we continue in our ever-advancing day.

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coronavirus disease (COVID-19): a mini-review.Frontiers in immunology, 14, https://doi.org/10.3389/fimmu.2023.1116131. 

Rea-Moreno, M., Tian, L., Tavakol, T.N. et al. (2026). Unveiling alternate pathways for 

SARS-CoV-2 infection via extracellular vesicle-mediated transfer of ACE2 and TMPRSS2. Nature Communications, https://doi.org/10.1038/s41467-026-71680-w.