An H2-T MHC class Ib molecule presents Listeria monocytogenes-derived antigen to immune CD8⁺ cytotoxic T cells

Mouse spleen T cells can adoptively transfer immunity to Listeria monocytogenes; this activity was markedly enhanced by stimulation with Con A in vitro before transfer. The enhanced and prolonged protection against L. monocytogenes in vivo was correlated with enhanced lysis in vitro of target cells infected with strains of L. monocytogenes that produce listeriolysin O (LLO). One of the targets of such cytotoxic cells from BALB/c (H2(d)) mice was a peptide that corresponded to amino acids 91 to 99 (p91-99) of the LLO molecule, which satisfies the binding motif of H2-K(d). Listeria-immune CD3⁺ CD8⁺, but not CD3⁺ CD8^-, cells could also lyse H-2-incompatible, infected target cells. Immune cells from C57BL/6 (H2^b) mice lysed allogeneic H-2(d) target cells infected with L. monocytogenes or a Bacillus subtilis transformant that secretes LLO, but did not lyse targets pulsed with p91-99. This H2-unrestricted cytolysis was therefore directed at a fragment of the LLO molecule other than p91-99. Listeria-infected bone marrow macrophages from congenic and recombinant strains of mice were lysed only when they shared the H2-T region or were Qa1-compatible with the immune cytotoxic cells; sharing of the H2-D, Q, or M region was insufficient. Thus, the immune response to L. monocytogenes included cytolytic CD8⁺ cells that recognized endogenously processed Listeria-derived Ags in the context of the class Ia H2-K molecule, as well as a class Ib H2-T molecule.