Presentation of Listeria monocytogenes to CD8⁺ T cells requires secretion of hemolysin and intracellular bacterial growth

Cell-mediated immunity to Listeria monocytogenes (LM) involves both CD4⁺ and CD8⁺ T cell responses. An important virulence factor in the pathogenesis of infection and development of protective immunity to LM is secretion of the sulfhydryl-activated hemolysin (Hly), listeriolysin. Listeria secretion of Hly allows LM to escape the endosomal compartment and enter the cytosol of the cell where intracellular growth can occur. We developed a system using bone marrow macrophages cultured in CSF-1 or IFN-γ for comparing the response of CD4⁺ or CD8⁺ T cells to heat-killed, live Hly^-, and live Hly⁺ LM. Macrophages grown in CSF were permissive for intracellular growth of Hly⁺ but not Hly^- LM. CD8⁺ T cells recognized Hly⁺ LM but not HK or Hly^- LM pulsed macrophages. However, CD4⁺ T cells recognized all three Listeria preparations fed to IFN-γ-treated macrophages. These results suggest that both heat-killed LM and live LM efficiently enter the exogenous pathway for class II Ag processing and presentation. In contrast, only Hly⁺ LM activates the class I pathway, probably as a result of Hly⁺ bacterial replication within the cytosolic compartment.