The roles of IL-12 and IL-23 in CD8+ T cell-mediated immunity against Listeria monocytogenes: Insights from a DC vaccination model.

Listeria monocytogenes infection induces a strong inflammatory response characterized by the production of IL-12 and IFN-gamma and protective immunity against this pathogen is dependent on CD8+ T cells (CTL). Recent studies have suggested that these inflammatory cytokines affect the rate of memory CD8+ T cell generation as well as the number of short-lived effector cells generated. The role of the closely related cytokine, IL-23, in this response has not been examined. We hypothesized that IL-12 and IL-23 produced by dendritic cells collectively enhance the generation and function of memory cells. To test this hypothesis, we employed a DC vaccination approach. Mice lacking IL-12 and IL-23 were vaccinated with wild-type (WT), IL-12(-/-), or IL-12/23(-/-) DC and protection to Lm was monitored. Mice vaccinated with WT and IL-12(-/-) DC were resistant to lethal challenge with Lm. Surprisingly, mice vaccinated with IL-12/23(-/-) DC exhibited significantly reduced protection when challenged. Protection correlated with the relative size of the memory pools generated. In summary, these data indicate that IL-23 can partially compensate for the lack of IL-12 in the generation protective immunity against Lm.

Vesicular stomatitis virus M protein mutant stimulates maturation of Toll-like receptor 7 (TLR7)-positive dendritic cells through TLR-dependent and -independent mechanisms.

Wild-type (wt) vesicular stomatitis virus (VSV) strains stimulate plasmacytoid dendritic cells (pDC) through Toll-like receptor 7 (TLR7) and its adaptor molecule, MyD88. Granulocyte-macrophage colony-stimulating factor-derived DC (G-DC), which do not express TLR7, are unresponsive to wt VSV due to inhibition of cellular gene expression by the matrix (M) protein. In contrast to its recombinant wt (rwt) counterpart, an M protein mutant of VSV, rM51R-M virus, stimulates maturation of G-DC independently of MyD88. These results suggest that, as in the case of G-DC, rM51R-M virus may stimulate pDC by mechanisms distinct from that by rwt virus. Studies presented here demonstrate that both rwt and rM51R-M viruses induced maturation of TLR7-positive DC derived by culture in the presence of Flt3L (F-DC), with the subsequent expression of type I interferon (IFN). F-DC are a mixture of myeloid (CD11b(+)) and plasmacytoid (B220(+)) DC, both of which respond to TLR7 ligands. Separated CD11b(+) and B220(+) F-DC responded to both rwt and rM51R-M viruses. Both viruses were also defective at inhibiting host gene expression in F-DC, including the expression of genes involved in the antiviral response. The data from F-DC generated from IFN receptor knockout mice demonstrated that the maturation of F-DC induced by rwt virus was dependent on the type I IFN response, while maturation induced by rM51R-M virus was partially dependent on this molecule. Therefore, activation of the type I IFN pathway appears to be important for not only inducing an antiviral response but also for stimulating maturation of F-DC upon virus infection. Importantly, F-DC from TLR7 and MyD88 knockout mice did not undergo maturation in response to rwt virus, while maturation induced by rM51R-M virus was largely independent of both molecules. These results indicate that although both viruses induce F-DC maturation, F-DC detect and respond to rM51R-M virus by means that are distinct from rwt virus. Specifically, this mutant virus appears capable of inducing DC maturation in a wide variety of DC subsets through TLR-dependent and independent mechanisms.

IL-12 produced by dendritic cells augments CD8+ T cell activation through the production of the chemokines CCL1 and CCL17.

IL-12 family members are an important link between innate and adaptive immunity. IL-12 drives Th1 responses by augmenting IFN-gamma production, which is key for clearance of intracellular pathogens. IL-23 promotes the development of IL-17-producing CD4(+) T cells that participate in the control of extracellular pathogens and the induction of autoimmunity. However, recent studies have shown that these cytokines can modulate lymphocyte migration and cellular interactions. Therefore, we sought to determine the individual roles of IL-12 and IL-23 in naive CD8(+) T cell activation by addressing their ability to influence IFN-gamma production and cellular interaction dynamics during priming by Listeria monocytogenes-infected dendritic cells (DC). We found that IL-12 was the major cytokine influencing the level of IFN-gamma production by CD8(+) T cells while IL-23 had little effect on this response. In addition, we observed that IL-12 promoted longer duration conjugation events between CD8(+) T cells and DC. This enhanced cognate interaction time correlated with increased production of the chemokines CCL1 and CCL17 by WT but not IL-12-deficient DC. Neutralization of both chemokines resulted in reduced interaction time and IFN-gamma production, demonstrating their importance in priming naive CD8(+) T cells. Our study demonstrates a novel mechanism through which IL-12 augments naive CD8(+) T cell activation by facilitating chemokine production, thus promoting more stable cognate interactions during priming.