Angiotensin converting enzyme 2 (ACE2), the human receptor for severe acute respiratory
syndrome coronavirus (SARS-CoV-2), is expressed in healthy human conjunctiva (Grajewski
et al., 2020). Entry of SARS-CoV-2 into host-tissue occurs after attachment of the
viral hemagglutinin protein to ACE2 that is followed by cleavage of the hemagglutinin
by its coreceptor transmembrane serine protease, subtype 2 (TMPRSS2) to activate viral
entry (Stopsack et al., 2020). Here, we examine the expression of both ACE2 and TMPRSS2
in conjunctival neoplastic lesions. This is an important issue as cancer tissue has
been shown to have an impact on expression levels of these entry molecules for SARS-CoV-2
(Kong et al., 2020), while conjunctivitis has been reported as an ocular manifestation
of coronavirus disease 2019 (COVID-19)(Xia et al., 2020). Furthermore, ophthalmologists
and eye-care personnel have been described at risk for transmission of COVID-19 due
to close contact to patients during examination (Rokohl et al., 2020).
After obtaining written informed consent, ACE2 and TMPRSS2 were, for the first time
to our knowledge, detected in formalin fixed paraffin-embedded sections of healthy
and neoplastic conjunctival tissue using mouse IgG2a anti-human ACE2 (R&D Systems,
catalog # MAB933, clone #171606; dilution 1:100) at 5 μg/mL for 60 min at room temperature
(RT) and rabbit IgG anti-human TMPRSS2 monoclonal antibody (Abcam, catalog# ab92323,
clone EPR3861; dilution 1:000) at 0.477 μg/mL for 30 min at RT. Before incubation
with the primary antibody, tissue was subjected to heat-induced epitope retrieval
using Target Retrieval Solution (pH 9 for ACE2, pH 6.1 for TMPRSS2 Dako, catalog #
S2367, S1699). Tissue was stained using DCS DetectionLine, Polylink and Peroxidase
Label HRP (DCS, catalog # PD000RP) and as the substrate chromogen AEC+ High Sensitivity
(DakoCytomation, catalog # K3461). The counterstain was hematoxylin (blue).
Immunohistochemistry demonstrated ACE2 expression in healthy conjunctiva as well as
in conjunctival nevus and melanoma but not in conjunctival carcinoma (Fig. 1
A, D, H, and K). In contrast, we detected TMPRSS2 in all these conjunctival entities,
respectively (Fig. 1 B, E, I, and L).
Fig. 1
Angiotensin converting enzyme 2 (ACE2) is expressed in healthy human conjunctiva (A)
and in conjunctiva affected by nevus (D) and malignant melanoma (H) but not carcinoma
(K), whereas transmembrane protease, serin subtype 2 (TMPRSS2) is expressed in all
these tissues, respectively (B, E, I, and L). Scale bar, 100 μm.
Fig. 1
Immunopositivity for ACE2 and TMPRSS2 was specifically confined to the conjunctival
epithelium in healthy conjunctiva (Fig. 1A and B) but extended to neoplastic tissue
in nevus and melanoma (ACE2 and TMPRSS2, Fig. 1 D, E, H, and I) as well as carcinoma
(TMPRSS2, Fig. 1L). Similar results were obtained in all three sections that were
performed on the tissue samples of two patients per group.
Staining without the primary antibody (Fig. 1 C, F, J and M) demonstrated no staining.
Human kidney tissue served as a positive control and showed a specific staining of
epithelial cells in convoluted tubules, whereas human skin served as a negative control
(data not shown).
Our results clearly demonstrate expression of ACE2 and TMPRSS2 in both healthy human
conjunctiva as well as conjunctiva affected by the presence of a nevus and conjunctival
melanoma. Furthermore, it appears that conjunctival carcinoma might downregulate conjunctival
ACE2 expression, although this requires confirmation in a larger study for definitive
conclusions. Patients with different types of cancer were shown to have a higher incidence
of adverse events and this could also be related to differences in ACE2 and TMPRSS2
expression compared to patients without cancer (Kong et al., 2020). Ocular expression
of ACE2 and TMPRSS2 has been demonstrated in human primary conjunctival and pterygium
cell lines (Ma et al., 2020). Interestingly, this study showed a decrease of TMPRSS2
expression in association with pterygium, emphasizing the potential of conjunctival
pathologies to modify expression of entry factors for SARS-CoV-2.
In summary, our results demonstrate a clear and specific ACE2 and TMPRSS2 expression
in healthy and neoplastic conjunctival cells, providing the receptors for entry of
SARS-CoV-2. Together, these findings emphasize the urgent need for further research
regarding the eye as a possible alternative route for transmission of SARS-CoV-2 and
the potential interaction of viral entry and replication with neoplastic and other
conjunctival pathologies.
Financial support
German Research Foundation(FOR 2242 “(Lymph) Angiogenesis and Cellular Immunity in
Inflammatory Diseases of The Eye” to LMH (HE 6743/3-2 and 5-1 to LMH), German Research
Foundation to FP (PA 738/15-1).
Ethical statement
All authors agree upon standards of expected ethical behavior for all parties involved
in the act of publishing
Conflict of interest
No author has any financial/conflicting/proprietary interests to disclose.