It has been reported that germ-free mice and antibiotic-treated mice show enhanced corneal barrier disruption, reduced goblet cell density, increased infiltration of lymphocytic cells (particularly CD4+ IFN-γ+ T cells) in the lacrimal gland, and higher levels of pro-inflammatory cytokines (i.e., IL-12 and IFN-γ), thus resembling the dry eye phenotype observed in Sjögren’s syndrome patients [144,250,251]. The gene discussed is CD4; the disease is Keratoconjunctivitis sicca.