**3. Potential systemic invasiveness of SARS-CoV-2 following ocular transmission**

The three human coronaviruses, SARS-CoV, MERS-CoV, and SARS-CoV-2 are highly infectious compared to HCoV-229-E, HCoV-NL63, HCoV-OV43, and HCoV-HKU1, which infect upper respiratory tract with mild symptoms. On the other hand, SARS-CoV, MERS-CoV, and SARS-CoV-2 cause severe lower respiratory tract infection, which then leads to pneumonia [49]. The transmission mechanism of SARS-CoV and SARS-CoV-2 are similar in many aspects. These viruses could be transmitted with direct contact, droplet, or aerosolized particle contact with the eye surface, nose, and mouth [9]. SARS-CoV and SARS-CoV-2 are genetically similar as well. However, the number of patients infected with SARS-CoV-2 is hundreds of times higher, indicating a significantly higher transmission rate compared to SARS-CoV and MERS-CoV [50]. It's also recently shown that the rate of SARS-CoV-2 replication in conjunctiva is higher than SARS-CoV and MERS-CoV [51].

Potency of viral infections are mainly affected by the virus invasiveness, receptor repertoire of the host cell membrane, and the immune system response. The first step for the viral invasion is the binding of the virus to the host cell by its receptors [52]. Glycoproteins and spike proteins are well-known proteins for all coronaviruses that bind to the receptor of the host cell and trigger the viral entry. The spike proteins are encoded in β-coronaviruses and today, it has been known that SARS-CoV-2 spike protein has the receptor-binding domain, mediating the interaction with the host cell membrane receptor, ACE2 [53].

The eye is an organ representing a large surface area and could be easily exposed to external pathogenic factors. The large surface area of the eye is a potential landing zone for viral particles [54]. Importantly, the expression of TMPRSS2, CD147, ACE2, and CTSL proteins in ocular tissues indicate their potential as SARS-CoV-2 entry route [55–57]. Confirmed expression of ACE2 and TMPRSS2 in conjunctival and corneal tissues [46] suggest conjunctiva and cornea as ocular regions for SARS-CoV-2 entry [8, 49].

Ocular exposure may lead to systemic transmission of the SARS-CoV-2 virus via two pathways. In first pathway, cornea, conjunctiva, lacrimal gland, meibomian glands could be directly exposed to the infection. Particularly, the conjunctival tissue could be easily infected via droplets or a close contact with infected individuals and contaminated hands. Due to its potency as an entry site of viruses, conjunctiva is accepted as an important pathway for infection of the respiratory viruses [52]. In second pathway, virus in tear can migrate through the nasolacrimal duct and infect the nasal or gastrointestinal epithelium [9].

SARS-CoV-2 may indirectly enhance the possibility of ocular complications as well. For instance, the cytokine storm, vascular endothelial dysfunction, and hypercoagulability may lead to not only retinal microangiopathic changes but also congestion of the central retinal vessels, or micro-vascularization of the optic nerve head [14, 58, 59]. It has been also reported that SARS-CoV-2 led to paracentral

#### *Potency of SARS-CoV-2 on Ocular Tissues DOI: http://dx.doi.org/10.5772/intechopen.97055*

acute middle maculopathy and acute macular neuro-retinopathy [60, 61]. In May 2020, retinal changes in 12 adult COVID-19 patients were analyzed by using optical coherence tomography (OCT). Hyper-reflective lesions were observed at the ganglion cell level and interestingly, inner plexiform layers were found more prominently at the papillomacular bundle in both eyes [14]. A 40-year-old man diagnosed with SARS-CoV-2 infection reported that he had right calf pain and blurred vision in both eyes. His ophthalmic exam revealed retinal vein occlusion (RVO) on both eyes, indicating COVID-19 as a potential cause for RVO [12].

On the other hand, the viral infection can occur at the upper respiratory tract and viruses can migrate to the nasolacrimal duct and to the conjunctiva, resulting in viral conjunctivitis [62]. Furthermore, SARS-CoV-2 infection via the conjunctival tissues may also occur in non-human primates that the SARS-CoV-2 inoculation has been shown to cause mild COVID-19 in rhesus macaques [63].

## **3.1 Natural ocular defense mechanisms**

The eye has natural anatomical and physiological protection mechanisms that prevent the entry of large amounts of virus-loaded particles to the ocular surface [64]. The eye has three defense mechanisms against different types of microorganisms and toxic substances. These are; mechanical, immunological, and anatomical defense mechanisms which are critical to recognize and eliminate the pathogens from the ocular surface for eye protection [65].

**Mechanical defense** system is composed of eyelids, eyelashes, corneal epithelium containing tight intercellular junctions, and conjunctival mucosa. Corneal epithelial cells also protects the ocular surface by secreting cytokines and causing immune defense activation against the viral invasion [65]. Eyelid protects the eye surface against any mechanical injury. When the eyelids and lashes are closed, the eye is also protected from any exposure of pathogens and other foreign molecules such as dust, dirt, and any other debris [66].

**Anatomical defense system** is based on the barriers of anterior and posterior segments of the eye. The drugs administered to the eye is extensively drained by the precorneal barriers present in the anterior segment (around 90%) and tears migrate through the nasolacrimal duct [67]. Aqueous humor is secreted by the ciliary body and the flow direction of the aqueous humor is towards the cornea, which is an opposite direction of topically administrated drug. The aqueous humor can be a limiting factor for the drugs to show therapeutic effects. Sclera presents at the posterior segment of the eye and protects the eye from the exogenous substances. Surface charge, physicochemical properties, and molecular radius are the parameters affecting the drug permeability across sclera. The drug with greater molecular radius and lipophilicity can lead to inhibition of permeation across sclera [67]. On the other hand, the pathogens are also cleaned from the ocular surface with the lacrimal drainage system. However, this physical self-cleaning system may cause SARS-CoV-2 infection via the migration of infected tears throughout the nasolacrimal drainage system and this passage can function as an alternative entry route of the virus from ocular surface to the respiratory tract [68].

**Immune defense** at the ocular surface is important for preservation of the eye. Particularly, cornea have a variety of defense mechanisms; classified as native, nonspecific, and acquired immunological defenses.

Innate immunity is the first line of defense mechanism in corneal infection; presents at birth and provides a nonspecific defense system [65]. This system can function in case of viral load and pathogenesis. Innate immune response is given at first encounter with the pathogen and can vary between different pathogens. For instance, among SARS-CoV viruses, the replication of the SARS-CoV-2 has been

more extensive in the bronchus than SARS-CoV and the higher plasma concentrations of proinflammatory cytokines have been observed in the SARS-CoV-2 infected patients [9].

Tears, corneal nerves, epithelium, keratocytes, polymorphonuclear cells and some cytokines are other cellular and molecular elements for protection of cornea against microorganisms. The first function of tears is to keep the cornea not to be dried. Tears clean the foreign particles from the ocular surface and transports antimicrobial proteins lactoferrin, lysozyme, lipocalin, and beta-lysine to prevent the infections. In addition to these proteins, immunoglobulins protect the cornea from infections [65]. Lactoferrin is able to inhibit the binding of SARS-CoV-2 to ACE2, and IgA shows an effective immune response against different types of microorganisms [65, 69]. For instance, secreted IgA protects the corneal epithelium by binding to bacteria and prevents it from attaching to epithelium. Besides, IgG has the ability to bind bacteria and neutralize some viruses.

Corneal epithelial cells activate immune response by secreting cytokines to preserve the eye against microbial invasion. They store IL-1α to release it passively, when the trauma or any foreign agent stimulates the membrane. Keratocytes synthesize IL-6 and defensins as a defense mechanism. IL-6 and IL-1 show a synergetic effect against microbial activity. Defesin has antimicrobial activity in ocular infections and induce epithelial healing. It is also found in neutrophils located in conjunctiva. Corneal nerves send sensory information and therefore control the reflexive movements for protection of the eye. Furthermore, several other eye complements, composed of a variety of effectors and regulatory proteins activating each other to produce biologically active molecules, such as opsonins, enzymes and chemotaxins [65].

There are early and late defense stages of relevant innate and acquired immune responses. The immediate immune response takes minutes to several hours against microbial infection. When the innate immunity is unable to fight against the microorganisms or their antigens, acquired immunity can control microbial replication. Langerhans cells, antigen-presenting cells of the cornea, recognize the foreign antigen and can respond within 24–48 hours [65]. They recognize, process and present the antigens with MHC class II molecules, which are present on their surface. When they recognize an antigenic foreign molecule, they process the antigen and transport it to the surface by MHC molecules both class I and class II. The presentation of peptides by MHC molecules activates T cells and T-cell receptors, which then lead to the binding of MHC molecules to each other. If the MHC II molecule presents the antigen, then CD4 helper T cells kill the pathogen by secretion of cytokines that activates the other effector cells such as macrophages [65].

#### **3.2 Nasolacrimal duct can play a role in SARS-CoV-2 systemic transmission**

The human tear ducts consist of the upper and lower lacrimal canaliculus, lacrimal sac, and nasolacrimal duct (**Figure 2**) [70]. The nasolacrimal system functions as a bridge between the ocular surface mucosa and upper respiratory tract for migration of the viruses with the help of tears to the inferior meatus of the nose. It allows the virus to move from the ocular surface to the respiratory tract throughout the nasolacrimal duct [52, 71]. The fluid may be taken up by the conjunctiva, sclera, or cornea but the highest percent of the liquid is drained into the nasopharyngeal space. Additionally, the epithelial lining of the lacrimal duct can absorb the tear fluid, which allows immunizing agents to be drained to nasal tissue [72].

In addition to the above-mentioned functions of nasolacrimal duct, it has a role in nonspecific immune defense. Nasolacrimal duct protects against dacryocystitis; thus, the epithelial cells produce a variety of antimicrobial substances, such as

*Potency of SARS-CoV-2 on Ocular Tissues DOI: http://dx.doi.org/10.5772/intechopen.97055*

#### **Figure 2.**

*Representative schematic of ocular surface and tear ducts (This schematic was created using Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License; https://smart. servier.com).*

lysozyme and lactoferrin. In case of an infection or an inflammatory dacryocystitis, antimicrobial peptides human inducible beta defensins 2 and 3 are produced. Moreover, the secretory products of the mucus component mucins (MUC1, MUC2, MUC4, MUC5AC, MUC5B, and MUC7) are produced by goblet and epithelial cells. Mucins preserve mucosal surfaces against pathogenic substances. It has been observed that MUC5B protects the patient against the SARS-CoV-2. There is a lower allelic frequency of the MUC5B genetic variant in the COVID-19 patient's body compared the healthy people, which is related to a higher level of MUC5B expression [73]. On the other hand, the expression of MUC1 and the soluble mucin MUC5AC were observed in cells that also express ACE2, indicating that the mucins may function in entry and transmission of the SARS-CoV-2 [46, 74]. However, it was revealed that the increased levels of secreted MUC1 and MUC5AC in the sputum cleaned from the trachea of COVID-19 patients [75]. The epithelium present in the nasolacrimal duct produces TFF peptides TFF1 and TFF3. The efferent tear ducts also contain lymphocytes and other defense cells that function in adaptive immune mechanisms [70].

Nasolacrimal duct has common entry receptors for some respiratory viruses. For instance, the glycoproteins of host epithelial cell, carrying terminal sialic acid (SA), are distributed through the ocular tissue and the respiratory tract through the lacrimal passage. Thus, the patient becomes infected with pneumonia [71]. α2–6-linked SA is significantly abundant in trachea and nasal mucosa, while α2–3- linked SA are more prominent in ocular tissues and the lower respiratory tract tissues [72]. There are several reports hypothesized that the exposure of the ocular surface to SARS-CoV-2 may lead to infection, because of the drainage of the virus particles via the nasolacrimal duct [76]. Siedlecki *et al.* has shown that SARS-CoV-2 can infect the ocular surface by migrating into the respiratory tract with the help of tears through the nasolacrimal duct [77]. Supporting to this, the highest expression level of SARS-CoV-2 entry factors was shown in nasal epithelial cells (clusters of goblet cells and ciliated cells), among all cells present in the respiratory tree [46]. Unlike all these infection routes, SARS-CoV-2 infection may also possible with the hematogenous spread from the lacrimal gland [8].

Consequently, the human eye has three roles in coronavirus infection. Firstly, it is one of the target organs for coronavirus infection. Secondly, the conjunctiva can function as a transporter for human coronavirus to enter the respiratory tract. SARS-CoV-2 can reach to nasal mucosa with nasolacrimal epithelium, gastrointestinal tract, and systemic circulation by leaving the conjunctivitis [19]. Thirdly, conjunctival secretions and tears can function to spread human coronavirus [52].
