SARS-CoV-2 infection in conjunctival tissue.

In a recent study published in The Lancet Respiratory Medicine in May, 2020, Kenrie Hui and colleagues described the tropism, replication competence, and innate immune responses of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the human respiratory tract and conjunctiva. The authors concluded that SARS-CoV-2 replication in the conjunctiva was greater than SARS-CoV replication, and that the conjunctival epithelium was a potential portal of infection.

However, this finding could be misleading. The main issue with the ex-vivo experimental model used in this study is that it does not consider the fact that the ocular surface is covered by a tear film. The tear film forms a dynamic innate protection for the ocular surface that is regularly turned over at a rate of 5·4–20·6% of its volume per minute. Tears are secreted at a rate of 2 μL/min, which is even higher during reflex tearing in response to inflammation, such as in viral conjunctivitis. Hence, in a clinical setting, viral pathogens reaching the ocular surface are unlikely to inoculate the conjunctiva at a fixed viral load for 1 h, as in the described experimental model, since the viral titre will be diluted by tear fluid. The authors should test the effect of different viral concentrations on infectivity to mimic the dilution effect of the tear film. Constant tear production will also wash the viral load away from the ocular surface and into the nasolacrimal duct. Most published clinical studies2, 3 support this notion and have found that the proportion of PCR-positive patients with COVID-19 who test positive for SARS-CoV-2 RNA in conjunctival swabs or tears is very low (0–5·2%). In addition, the tear film contains several antimicrobial properties. Tear lactoferrin, mucins, lysozyme, lipocalin, IgA, and complement work synergistically to facilitate the clearance of viral pathogens. Lactoferrin, in particular, has been shown to have a protective function by inhibiting the early cellular attachment phase of SARS-CoV. All of these factors will substantially affect viral kinetics on the conjunctiva, but these factors were not taken into account in the experimental model or discussed in the study by Hui and colleagues.

Conjunctivitis has been reported as an ocular manifestation of COVID-19, with a reported prevalence ranging from 0·9% to 31·6%.2, 5 However, the number of conjunctival samples positive for SARS-CoV-2 RNA has been low. SARS-CoV-2 gains cellular entry through the angiotensin-converting enzyme 2 (ACE2) receptor after cellular protease priming by transmembrane protease serine 2 (TMPRSS2). By use of single-cell sequencing and high-throughput RNA sequencing, two independent studies6, 7 have shown that the expression of ACE2 and TMPRSS2 on the ocular surface is significantly lower than that in the respiratory tract. Immunohistochemical staining studies have shown no ACE2 expression in conjunctival tissue using two different monoclonal antibodies. However, the nasal cavity, which is anatomically contiguous with the conjunctival mucosa, has one of the highest levels of both ACE2 and TMPRSS2 expression in the human body. Comparing entry of SARS-CoV-2 in the conjunctiva with the nasal mucosa would have been key to understanding the potential portal of entry of this virus in the study by Hui and colleagues.

Finally, the appearance of the immunostaining results for SARS-CoV-2 in the conjunctiva was unusual in this study, as positive nucleoprotein staining was found in the conjunctival stroma, but not in the epithelium. One would have expected SARS-CoV-2 staining on the conjunctival epithelial surface to be consistent with the authors’ theory. Performing the analysis at an earlier timepoint (eg, at 24 h) could help to explain this finding. Another possible explanation could be that the mechanism of SARS-CoV-2 entry into the conjunctiva is different from other types of SARS-CoV. All of these issues should have been addressed, and the need for further studies should have been suggested, before concluding that the conjunctival epithelium was a potential portal of infection for SARS-CoV-2.