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-gamma 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-gamma-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.