A new study involving researchers from South Korea and Australia has validated that not only does SARS-CoV-2 exhibit ocular tropism and is able to get to the eyes of the host through various ways, but it also causes vision issues.
The study findings were published in the peer reviewed journal: Nature Communications.
A previous study has highlighted the neuro-tropism of SARS-CoV-2 and suggested the need to identify the route of viral invasion into the brain39. Olfactory nerves form bundles that provide an anatomical connection between the brain and nasal passage through the foramina in the cribriform plate and synapse on the glomeruli in the olfactory bulb40.
In addition, branches V1 (ophthalmic) and V2 (maxillary) of TN (Fig. 5c) innervate both the respiratory and olfactory regions of the nasal passage, connecting the brain 41,42. Although SARS-CoV-2 infection and transmission via the olfactory nerves have been demonstrated by several research groups26,27,43, those of TN remains to be elucidated.
Based on data obtained from a patient with COVID-19 who suffered from trigeminal neuralgia, neuronal invasion via TN has been suggested in humans 44. In this study, we demonstrated that TN can be infected by SARS-CoV-2 and used for transmitting the virus to the brain and eyes, along with the ON.
As the nasolacrimal duct provides an anatomical connection between the ocular surface and respiratory tract45, the spreading of the virus to the eyes could have occurred via the nasolacrimal duct, and not the neurons. Although we used IT infection to prevent early transmission to the eyes via the nasolacrimal duct (Fig. 4), post-inoculation progeny viruses may move toward the eyes via the nasolacrimal duct following viral replication in the lungs, which has not been investigated in this study.
Further studies on viral replication in the upper respiratory tract under conditions that stop spreading to the eyes via the nasolacrimal duct are warranted.
A recent clinical study revealed that patients with COVID-19 showed lesions at the ganglion cell layer and inner plexiform layer in both eyes46. In addition, another clinical study reported that the patients presented flame-shaped haemorrhages along the retinal vascular arcades and peripheral retinal haemorrhages 47.
The breakdown of the blood-retinal barrier can cause immune cells infiltration and abnormal fluid accumulation within the retina following an increase in retinal thickness 48. In this study, we have observed immune cells infiltration in the inner nuclear layer and ganglion cell layer, but there also was a marked increase in the outer-nuclear layer along with general retinal thickness increases in SARS-CoV-2 IN-infected mice (Fig. 2a, c and Supplementary Fig. 5).
The increases in the neighbouring retinal layers may be due to extracellular fluid accumulation resulting from the breakdown of the blood-retinal barrier in the deep retinal vascular plexus 49. We thus assume that both abnormal fluid accumulation within the retina and immune cells infiltration might be involved in the increase in retinal thickness following SARS-CoV-2 infection in this animal model.
Retinal inflammation, diagnosed based on retinitis, can occur following the direct or indirect invasion of pathogens such as cytomegalovirus50, chikungunya virus 51, and West Nile virus 52. When retinitis involves the fovea or optic disc, it can exacerbate to reduction or loss of vision 53.
In addition, blurred vision was one of the most common ocular symptoms in patients with COVID-192. Hence, we chose the visual cliff test, which is easy to perform in a BSL-3 facility, to assess the reduction or loss of vision of SARS-CoV-2-infected mice with ocular symptoms (Fig. 3b–d) as the functional consequences of retinal inflammation.
Although the number of mice with first foot on the cliff side did not change, the latency to dismount increased in the infected mice with ocular symptoms. We believe that this may be due to the blurring of vision because of ocular discharge. Despite the increased latency, the mice were still able to see the cliff and move toward the safe bench side.
Considering that viral infection, including that caused by SARS-CoV-2, can cause ocular manifestations in humans, several studies have investigated the spread of viruses to ocular tissues. Influenza viruses within the H7 subtype show ocular tropism and use the eyes as an entry route, which was confirmed by administering the virus onto the corneal surface in ferrets 54.
Although Imai et al. introduced a combination of the IN- and ocular routes for SARS-CoV-2 inoculation, they detected an infectious virus in the eyelid tissues of Syrian hamsters36. In this study, we examined the dissemination of SARS-CoV-2 to ocular tissues following IN infection and assessed whether the infection route can be reversed by administering the virus to the eyes.
Our data suggest unidirectionality of the infectious route, from the lungs to the eyes, in these animal models where ACE2 was expressed in the corneas of the eyes (Supplementary Figs. 9, 10). In particular, the viral burdens of the eye globes were comparatively low and disappeared with time, suggesting the absence of viral proliferation in the eyes (Fig. 4c). Infectious viral particles were not detected in the eyes of Syrian hamsters following ED infection at 6 dpi (Fig. 6d).
The prolonged analysis of viral RNA levels following ED infection in the lungs, brain, eye globes, TN, and ON of K18-hACE2 mice and Syrian hamsters at 3, 6, 9 and 12 dpi revealed that the low-level infection of TN and ON at 3 and 6 dpi did not lead to brain infection with time. There was no loss of weight of ED-infected mice until 18 dpi and of ED-infected hamsters until 12 dpi (Supplementary Fig. 11).
This unidirectionality of the infectious route may be due to the formation of the tear film and blinking. The tear film plays an important role in the innate immune system of the eyes for protection against potential pathogens, including lysozyme, lactoferrin, secretory immunoglobulin A, and complement produced in the lacrimal gland 55.
Lysozyme is known to confer anti-HIV activity 56. Lactoferrin can act as a cationic detergent and disrupt the cell membrane of some bacteria, fungi, and viruses 57. Secretory immunoglobulin A possibly prevents pathogen adhesion to host cells, blocking further viral infection at mucosal surfaces 58, and is chemotactic for phagocytic neutrophils 59.
Functionally active complement factors in tears are involved in acute inflammatory responses, contributing to innate defence against pathogens 60. Thus, anti-microbial components in the tear film might provide an immunological and protective environment against viral infections. In addition, the act of blinking provides not only physical protection from outside contaminants, but also helps in the drainage of the tear film from the tear punctum 61.
Clinical studies have reported that the time required for ocular manifestation varied from 15 days to two months after the infection or symptom onset 8,62. Moreover, Colavita et al. detected viral RNA in the ocular swabs with lower Ct values than those in the nasal swabs of a patient with COVID-19 who suffered from ocular manifestations between 21 and 27 days from the onset of symptoms 63.
Interestingly, they also isolated live replication-competent viruses directly from the ocular fluid collected from the patient. Consistent with the results of these clinical studies, IN- and IT-administered viral copies were detected in the eye globes, which increased in a time-dependent manner (Fig. 4c).
In summary, ocular manifestation and retinal inflammation were promoted by SARS-CoV-2 infection in the mouse model, which increased cytokine production. The virus spreads from the lungs to the brain and eyes through a network consisting of TN and ON. This ocular tropism was also observed in wild-type Syrian hamsters. However, the elicitation of ocular inflammation by a viral infection of the eyes and its clinical relevance remains unknown and warrants further investigation.
Along with the respiratory system, eyes and TN should be considered SARS-CoV-2-susceptible organ systems. Our data increases awareness regarding ocular and neuronal infection-mediated disorders beyond respiratory diseases, which will assist in designing treatment strategies for patients with COVID-19.
Respiratory viral infections represent the most common cause of acute illness and physician visits in the United States, with disease ranging from mild influenza-like symptoms to life-threatening pneumonia (1). Shared features between the principal viruses associated with human respiratory disease include high transmissibility, global distribution, mucosal sites of infection, and several overlapping symptoms.
While human infection with respiratory viruses generally causes an acute but transient and resolving upper respiratory tract illness, progression to lower respiratory disease is possible, especially among individuals with compromised immune systems or other comorbidities. Respiratory viruses are typically spread by inhalation of virus-containing aerosols expelled by infected individuals or by direct or indirect contact with virus-contaminated fomites on environmental surfaces (1, 2).
However, the epithelia of the human eye represent an additional mucosal surface which is similarly exposed to infectious aerosols and contaminated fomites (3, 4). Viruses which are generally considered respiratory pathogens are nonetheless capable of causing ocular complications in infected individuals and establishing a respiratory infection following ocular exposure (Table 1). It is important to keep in mind that our use of “respiratory viruses” in this review encompasses a diverse range of pathogens, of which ocular disease is but one of many potential complications.
Principal respiratory viruses known to cause ocular disease in humans
|Virus||Subtype(s)a||Tropism in humans||Ocular disease in humans||Reference(s)|
|Adenovirus||Species D||Ocular||Frequently associated with epidemic keratoconjunctivitis||5–7|
|Species B, C, E||Respiratory||Occasional simple acute follicular conjunctivitis or pharyngeal conjunctival fever||1, 8|
|Influenza virus||H7||Ocular||Conjunctivitis||9, 10|
|H1, H3, H5||Respiratory||Rare but documented ocular complications||1, 11, 12|
|Respiratory syncytial virus||NA||Respiratory||Occasional reports of conjunctivitis concurrent with respiratory illness||13–16|
|Coronavirus||NL63||Respiratory||Rare reports of conjunctivitis||17, 18|
|Rhinovirus||NA||Respiratory||Rare but documented ocular complications||20–22|
|Human metapneumovirus||NA||Respiratory||Rare but documented ocular complications||23, 24|
aNA, not applicable (indicates that there is no association with any given subtype/serotype with ocular complications in humans).
Despite the anatomical proximity between ocular and respiratory tissues and documented reports of ocular disease following infection with most known respiratory viruses in humans, studies of respiratory pathogens and their role in ocular disease have been underrepresented in the literature.
Our knowledge is incomplete regarding the properties which confer an ocular tropism to particular respiratory viruses or virus subsets and the mechanisms which allow ocular exposure to viral pathogens to cause a respiratory infection. To appropriately control and treat disease presenting with ocular complications, a more rigorous understanding of the relationship between the development of ocular symptoms and respiratory disease is critical. In the sections below, we present a summation of ocular findings following respiratory virus infection in humans and the current innovations in laboratory modeling which will allow for a greater analysis of the properties which govern virus tropism.
ANATOMICAL AND HOST RECEPTOR LINKS BETWEEN OCULAR AND RESPIRATORY SYSTEMS
There are several properties which permit the eye to serve as both a potential site of virus replication as well as a gateway for transfer of virus to extraocular sites to establish a respiratory infection. This is achieved primarily by the nasolacrimal system, which provides an anatomical bridge between ocular and respiratory tissues (Fig. 1) (4, 25).
The lacrimal duct collects tear fluid from the ocular surface and transports it to the inferior meatus of the nose, facilitating the drainage of virus from ocular to respiratory tract tissues in a replication-independent manner, thus serving as a conduit for virus-containing fluid exchange between these sites (3, 26–28).
When placed on the eye, fluid can be taken up by the conjunctiva, sclera, or cornea, but the majority of liquid is drained into the nasopharyngeal space or swallowed; absorption of tear fluid through the epithelial lining of the lacrimal duct is also possible (29). This allows drainage of immunizing agents to nasal tissue following topical ocular administration as well as the spread of intranasally administered solutions to the conjunctival mucosal surface (28, 30).
The lining of nasolacrimal duct epithelial cells with microvilli additionally permits both secretion and reabsorption of tear fluid components (31). Linkage of the ocular mucosal immune system (composed of the conjunctiva, cornea, lacrimal glands, and lacrimal drainage system) with nasal cavity-associated lymphoid tissue in the nasolacrimal ducts further supports the immunological interdependence between ocular and respiratory tract tissues (28).
Despite the presence of antimicrobial peptides present in tear film, numerous viral agents have been detected in the tear fluid of symptomatic, chronic, and asymptomatic individuals (4, 32–36), underscoring the potential for ocular involvement following respiratory virus infection.
Beyond the anatomical linkage of ocular and respiratory tract tissues, the structure and distribution of cellular receptors in these systems likely contribute to the tissue tropism of respiratory viruses. Host epithelial cell glycoproteins bearing terminal sialic acids (SA) are distributed throughout the human respiratory tract and ocular tissue (Fig. 1) (reviewed in reference 37) and serve as the cellular receptor for several respiratory viruses.
In humans, α2-6-linked SA dominate in the nasal mucosa and trachea, whereas α2-3-linked SA are found at greater abundance in lower respiratory tract tissues and ocular tissue (37–39). Interestingly, the epithelium of the human lacrimal sac and nasolacrimal duct, which bridge ocular and respiratory tissues, express a diverse variety of lectin-binding sites, including both types of SA; α2-6-linked SA have been detected on both epithelial and goblet cells of the lacrimal duct, and α2-3-linked SA are restricted to epithelial cells (31). While the terminal SA linkage represents a critical determinant of tissue tropism and host range for influenza viruses, the composition of internal sugars of the glycan receptor can also influence receptor specificity (40, 41).
The pattern of cellular receptor distribution in ocular and respiratory tract tissues generally agrees with the tropism of numerous respiratory viruses. Human influenza viruses prefer α2-6-linked SA, and as such, their replication is typically restricted to the upper respiratory tract, whereas avian influenza viruses preferentially bind α2-3-linked SA and are capable of efficient replication in lower respiratory tract tissue, where these receptors are most prevalent.
The abundance of α2-3-linked SA on the corneal and conjunctival epithelium may partially govern the tropism observed with select influenza virus subtypes, although it has been shown that both human and avian influenza viruses can bind to human ocular tissue, demonstrating that receptor binding preference is not the sole determinant of this property (3, 42).
Similarly, the tissue distribution of cellular receptors may partially govern the tropism of adenoviruses (Ad). Generally, adenoviruses which exhibit a respiratory tropism use CD46, desmoglein-2 (DSG-2), or the coxsackievirus and adenovirus receptor (CAR) as a cellular receptor (43–45), while adenovirus serotypes which exhibit an ocular tropism use α2-3-linked SA and GD1a glycans present on the human ocular surface as host cellular receptors (46, 47), although the bread tissue distribution of these receptors indicates a role for additional tropism determinants.
Furthermore, the location of angiotensin-converting enzyme 2 (ACE2), the cellular receptor for severe acute respiratory syndrome (SARS) coronavirus, on cardiac and pulmonary tissue likely contributes to the severe respiratory disease associated with this virus (48–50). The continued identification of the cellular receptors utilized by respiratory viruses will allow for a greater understanding of the permissiveness of ocular tissue to infection with these agents (51). Collectively, it appears that the presence of permissive receptors can contribute to the tropism of a virus to ocular tissue but does not restrict respiratory viruses from using the eye as a portal of entry to gain passage to extraocular tissues to establish a productive infection.
Despite the body of work that has been focused on an understanding of the distribution of viral receptors present on the ocular epithelium, further research is needed to better understand the contribution of sialylated ocular mucins to host defense of these pathogens (3).
While there is great heterogeneity of mucins and secretory peptides synthesized and secreted by discrete regions and cell types within the human ocular surface and nasolacrimal ducts (29), the potential contribution of this localized distribution to viral infection is not well understood. The identification of differences in the spatial arrangement of α2-3- and α2-6-linked sialic acids on purified human ocular mucins highlights the potential importance of receptor distribution in mucosal antimicrobial defense in this tissue (52).
Examination of the structural topology and length of surface glycans present on human ocular tissue may also shed light on fine receptor differences present on this surface which influence the tropism of select virus subtypes, as was previously shown for respiratory tissues (40, 53). Due to the potential of zoonotic spread of select respiratory viruses to humans, it is prudent to extend the study of these properties in relevant nonhuman species (54).
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3591987/