Interleukin-12: A critical regulator of protective CD8+ T cell immunity during Toxoplasma gondii infection

Immunity to Toxoplasma gondii infection is principally mediated by the activation of CD8+ T cells producing the protective cytokine IFN-gamma. To characterize primary CTL activation, cell surface expression of the activation markers CD62L and KLRG1 as well as effector cytokine expression were used to identify anti-Toxoplasma effector CTLs. At the peak of CTL activation, four subpopulations were distinguished by the co-expression profile of CD62L and KLRG1, three of which contained varying levels of IFN-gamma+ and granzyme B+ effector cells. Differential expression of CD27 and IL-7R among the subsets suggested that the KLRG1+ subpopulations were highly activated effector cells whereas the KLRG1- subpopulations contained memory precursors. CD8+ T cell-intrinsic IL-12 signaling was found to play a pivotal role not only in the activation of IFN-gamma production, but also in the differentiation of KLRG1+ subpopulations. To accurately track the differentiation of bona fide memory cells, we first screened for putative Toxoplasma epitopes to identify Ag-specific CTLs. Having discovered a population of CTLs specific for the antigen tgd057 that was induced by infection of both virulent and avirulent parasites, memory CTL activation was analyzed during primary and secondary infection. A signature of the secondary response was the early activation of effector memory phenotype CTLs that was not observed in the primary response. The role of IL-12 in effector memory CTL differentiation was then revisited. Like polyclonal anti-Toxoplasma effectors, tgd057-specific CTLs required IL-12 for primary effector differentiation, including KLRG1+ subpopulation activation and IFN-gamma production. The effects of IL-12 signaling in tgd057-specific CTLs were profoundly imprinted on effector memory cells, as IL-12 was required during the initial priming to generate competent IFN-gamma-producing effector memory CTLs. However, central memory differentiation may benefit from the absence of IL-12, as central memory CTLs from IL-12p35-/- mice were superior to those from WT mice in generating secondary effector cells. In conclusion, IL-12 is the quintessential pro-inflammatory cytokine driving effector and effector-memory differentiation of Ag-specific CD8+ T cells during T. gondii infection. Our data indicate that transient depletion of IL-12 during vaccination and subsequent boosting in IL-12 replete conditions may enhance parasite immunity.