Date Published: September 22, 2016
Publisher: Public Library of Science
Author(s): Abirami Kugadas, Stig Hill Christiansen, Saiprasad Sankaranarayanan, Neeraj K. Surana, Stefanie Gauguet, Ryan Kunz, Raina Fichorova, Thomas Vorup-Jensen, Mihaela Gadjeva, Linda D. Hazlett.
The existence of the ocular microbiota has been reported but functional analyses to evaluate its significance in regulating ocular immunity are currently lacking. We compared the relative contribution of eye and gut commensals in regulating the ocular susceptibility to Pseudomonas aeruginosa–induced keratitis. We find that in health, the presence of microbiota strengthened the ocular innate immune barrier by significantly increasing the concentrations of immune effectors in the tear film, including secretory IgA and complement proteins. Consistent with this view, Swiss Webster (SW) mice that are typically resistant to P. aeruginosa–induced keratitis become susceptible due to the lack of microbiota. This was exemplified by increased corneal bacterial burden and elevated pathology of the germ free (GF) mice when compared to the conventionally maintained SW mice. The protective immunity was found to be dependent on both eye and gut microbiota with the eye microbiota having a moderate, but significant impact on the resistance to infection. These events were IL-1ß–dependent as corneal IL-1ß levels were decreased in the infected GF and antibiotic-treated mice when compared to the SPF controls, and neutralization of IL-1ß increased the ocular bacterial burden in the SPF mice. Monocolonizing GF mice with Coagulase Negative Staphylococcus sp. isolated from the conjunctival swabs was sufficient to restore resistance to infection. Cumulatively, these data underline a previously unappreciated role for microbiota in regulating susceptibility to ocular keratitis. We predict that these results will have significant implications for contact lens wearers, where alterations in the ocular commensal communities may render the ocular surface vulnerable to infections.
The importance of microbiota in regulating lymphocytic development and inflammatory responses in the gut has been demonstrated in studies using germ-free (GF) or antibiotic-treated (ABX) mice [1–5]. Loss of intestinal microbiota diversity alters the host resistance to gut pathogens such as Salmonella typhimurium, Listeria monocytogenes, and Clostridium difficile to name a few [6–8]. Consistently, reconstitution of commensal bacterial communities facilitates the clearance of enteric opportunistic pathogens . This suggests that transferring defined commensal bacterial populations into the host to re-establish microbiota offers an antibiotic–independent approach to combat infections. These approaches may not be exclusive for intestinal pathogens. A recent study demonstrated that antibiotic-treated mice showed increased sensitivity to viral infections. Housed under conventional conditions influenza virus–infected mice displayed lower viral titers and virus–associated mortality when compared to antibiotic–treated mice . In lieu with these data, murine gut microbiota, particularly the Segmented Filamentous bacteria, promoted pulmonary type 17 immunity and resistance to S. aureus pneumonia . Despite the growing understanding of the impact of the host–microbe alliance on immunity in the gastrointestinal tract, the extent to which individual microenvironments such as that of the eye are controlled by resident or distant microbiota remains unclear.
Unlike any other body site, the ocular mucosal surfaces harbor very few cultivatable bacterial species [17,21]. A lower percentage of the conjunctival swabs give rise to cultivatable bacteria that are in stark contrast to the number of recovered bacteria from the skin or oral mucosa where 100% of the swabs result in microbial growth [28–30]. The cultivatable commensal species from the eye are limited in repertoire and include Coagulase Negative Staphylococcus sp., Propionibacterium sp., Corynebacterium sp., S. aureus, Streptococcus spp., Micrococcus sp., Bacillus sp., and Lactobacillus sp. [12,15–19]. These observations prompted the inquiry into whether small numbers of bacterial species have measurable and significant impact on ocular immunity . We examined this question by comparing the ocular immune responses to P. aeruginosa, a frequent opportunistic ocular pathogen, in GF mice, in mice treated with topically applied antibiotics to reduce the ocular microbiota, in mice treated with oral antibiotics to reduce gut microbiota, and in GF mice monocolonized with CNS sp.