ABSTRACTSublethal infection of BALB/c mice with the intracellular bacterial pathogenListeria monocytogenesleads to the development of antilisterial immunity with concurrent stimulation of major histocompatibility complex (MHC) class Ia- and Ib-restricted CD8+effector T cells. SecondaryL. monocytogenesinfection is followed by an accelerated increase in the number ofListeria-specific CD8+cells and rapid clearance of the bacterium from the murine host. Recovery from secondary infection is associated with increased levels of effector cell function, as measured by gamma interferon secretion following coculture of immune cells withL. monocytogenesinfected APCs in vitro, as well as antilisterial cytotoxicity, as measured by effector cell recognition ofL. monocytogenes-infected target cells. We assessed the frequency ofL. monocytogenes-specific MHC class I-restricted cells following secondary infection by ELISPOT assays utilizing coculture of immune cells withL. monocytogenes-infected antigen-presenting cells that express MHC class Ia and/or Ib molecules. We found that the antilisterial Qa-1b(MHC class Ib)-restricted effector subset is not detected as an expanded population following secondary infection compared to the frequency of this effector population as measured following recovery from primary infection. This is in contrast to the frequency of antilisterial H2-Kd(MHC class Ia)-restricted effector cells, which following recovery from secondary infection are detected as an expanded population, and appears to undergo a substantial expansion event 3 to 4 days post-secondary infection. These results are consistent with the conclusion that althoughListeria-specific MHC class Ib-restricted effector cells are present following recovery from secondary infection, this subset does not appear to undergo the expansion phase that is detected for the MHC class Ia-restricted effector cell response.