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.

Keratinocytes are involved in protecting the body from infections and environmental challenges, but also in inflammatory conditions like psoriasis. DNA has emerged as a potent stimulator of innate immune responses, but there is largely no information of how keratinocytes respond to cytosolic DNA. In this study, we report that human keratinocytes are tolerant to cytoplasmic DNA. However, if treated with inflammatory cytokines, keratinocytes gained the capacity to respond to DNA through a mechanism antagonized by the antimicrobial peptide LL37, proposed to be involved in activation and regulation of skin inflammation. The DNA sensor IFN-inducible protein 16 (IFI16) colocalized with DNA and the signaling molecule stimulator of IFN genes (STING) in the cytoplasm only in cytokine-stimulated cells, correlating with recruitment of the essential kinase TANK-binding kinase 1. Moreover, IFI16 was essential for DNA-driven innate immune responses in keratinocytes. Finally, IFI16 was upregulated in psoriasis skin lesions and localized to the cytoplasm in a subpopulation of cells. Collectively, this work suggests that inflammatory environments in the skin can lead to breakdown of tolerance for DNA in keratinocytes, which could contribute to the development of inflammatory diseases.

Classic drug development strategies have failed to meet the urgent clinical needs in treating infections with Gram-negative bacteria. Repurposing drugs can lead to timely availability of new antibiotics, accelerated by existing safety profiles. Glatiramer acetate (GA) is a widely used and safe formulation for treatment of multiple sclerosis. It contains a large diversity of essentially isomeric polypeptides with the cationic and amphiphilic character of many antimicrobial peptides (AMP). Here, we report that GA is antibacterial, targeting Gram-negative organisms with higher activity towards Pseudomonas aeruginosa than the naturally-occurring AMP LL-37 in human plasma. As judged from flow cytometric assays, bacterial killing by GA occurred within minutes. Laboratory strains of Escherichia coli and P. aeruginosa were killed by a process of condensing intracellular contents. Efficient killing by GA was also demonstrated in Acinetobacter baumannii clinical isolates and approximately 50% of clinical isolates of P. aeruginosa from chronic airway infection in CF patients. By contrast, the Gram-positive Staphylococcus aureus cells appeared to be protected from GA by an increased formation of nm-scale particulates. Our data identify GA as an attractive drug repurposing candidate to treat infections with Gram-negative bacteria.