H2-M3–Restricted T Cells in Bacterial Infection

Major histocompatibility complex (MHC) class Ib molecules have been implicated in CD8+ T cell–mediated defenses against intracellular bacterial infection, but the relative importance of MHC class Ib–restricted T cells in antimicrobial immunity is unknown. In this report, we use MHC tetramers to characterize T cell responses restricted by H2-M3, an MHC class Ib molecule that selectively presents N-formyl peptides. We find that sizeable H2-M3–restricted T cell responses, occurring earlier than MHC class Ia–restricted T cell responses, are mounted after primary infection with the intracellular bacterium Listeria monocytogenes. These H2-M3–restricted T cells are cytolytic and produce interferon γ. However, after a second L. monocytogenes infection, H2-M3–restricted memory T cell responses are minor in comparison to the much larger MHC class Ia–restricted responses. This first direct characterization of an MHC class Ib–restricted T cell response indicates that CD8+ T cells responding to L. monocytogenes infection can be divided into two groups: H2-M3–restricted responses, which provide rapid and quantitatively substantial effector function during primary infections but contribute relati