PDCD1 and keratitis: The relationship between HSV and the PD-1/PD-L1 pathway was first reported in the herpetic stromal keratitis mouse model in which ocular HSV-1 infection was shown to induce PD-1 and PD-L1 expression in T cells and macrophages, respectively, in the cornea and lymph nodes, and administration of an antagonistic antibody to PD-L1 aggravated keratitis [14].