AbstractInfection of B6 mice with the intracellular pathogen Listeria monocytogenes (LM) results in the activation of CD8+ T cells that respond to Ag presented by both MHC class Ia and class Ib molecules. Enzyme-linked immunospot analysis reveals that these CTL populations expand and contract at different times following a primary sublethal LM infection. Between days 4 and 6 postinfection, class Ib-restricted CTL exhibit a rapid proliferative response that is primarily H2-M3 restricted. The peak response of class Ia-restricted CD8+ T cells occurs a few days later, after the majority of bacteria have been cleared. Although class Ia-restricted CTL exhibit a vigorous recall response to secondary LM infection, we observe limited expansion of class Ib-restricted memory CTL, even in MHC class Ia-deficient mice (B6.Kb−/−Db−/−). Despite this lack of enhanced expansion in vivo, class Ib-restricted memory CTL retain the ability to proliferate and expand when provided with Ag in vitro. Furthermore, we demonstrate that in vivo depletion of CD8+ T cells in LM-immune B6.Kb−/−Db−/− mice severely impairs memory protection. Together, these data demonstrate that class Ib-restricted CTL play an important role in clearing a primary LM infection and generate a memory population capable of providing significant protection against subsequent infection.