MR1 and viral infectious disease: Mucosal-associated invariant T cells are activated either by recognizing riboflavin metabolites produced by a wide range of microbes (including bacteria and fungi) presented on MHC-Ib-related protein (MR1) (40, 41), or independently of the TCR by stimulation by pro-inflammatory cytokine such as IL-12 and IL-18 (42, 43), which could occur during viral infections (23, 27).