MR1-tetramer+ MAIT cells accumulated in the lungs and were activated, with MAIT cell frequencies and CD25, CD69, and granzyme B upregulation peaking at day 5 post-infection, at least 48 h earlier than peak frequencies of conventional CD8+ T cells (defined as TCRβ+ CD45.2+ CD19− MR1-5-OP-RU tetramer− CD8+cells). The gene discussed is MR1; the disease is infection.