IL-1 receptor (IL-1R)/Toll-like receptor (TLR) family and TNF receptor (TNFR) superfamily members are critical for regulating multiple aspects of dendritic cell (DC) biology. Several signaling pathways associated with each family utilize the adapter molecule, TRAF6, but its role in DCs is unclear. By examining TRAF6-deficient mice and bone marrow (BM) chimeras reconstituted with TRAF6-deficient fetal liver cells, we show that proper DC maturation requires TRAF6. In response to either microbial components or CD40L, TRAF6-deficient DCs fail to upregulate surface expression of MHCII and B7.2, or produce inflammatory cytokines. Moreover, LPS-treated TRAF6-deficient DCs do not exhibit an enhanced capacity to stimulate naive T cells. Interestingly, a major population of splenic DCs, the CD4(+)CD8alpha(-) subset, is nearly absent in both TRAF6-deficient mice and BM chimeras. Together these results indicate that TRAF6 regulates the critical processes required for maturation, activation, and development of DCs, the primary cellular bridge between innate and adaptive immunity.

The interaction of host cells with microbial products or their invasion by pathogens frequently results in activation of the NF-kappaB family of transcription factors. The studies presented here reveal that in vivo, infection with Toxoplasma gondii results in the activation of NF-kappaB. To determine whether host cells could activate NF-kappaB in response to invasion by T. gondii, Western blots, immunofluorescence, and electrophoretic mobility shift assays were used to assess the response of host cells to infection. In these studies, infection of macrophages or fibroblasts with T. gondii did not result in the activation of NF-kappaB. In addition, the ability of lipopolysaccharide to activate NF-kappaB was impaired in cultures of macrophages infected with T. gondii. Together, these data demonstrate that invasion of cells by T. gondii does not lead to the activation of NF-kappaB and suggest that the parasite may actively interfere with the pathways that lead to NF-kappaB activation.

The NF-κB family of transcription factors is critical in controlling the expression of a wide range of immune response genes. However, whether individual family members perform specific roles in regulating immunity and inflammation remains unclear. Here we investigated the requirement for NF-κB1, NF-κB2, and c-Rel in the expression of Th2 cytokine responses, development of host protective immunity, and regulation of intestinal inflammation following infection with the gut-dwelling helminth parasite Trichuris muris. While mice deficient in c-Rel mounted sufficient Th2 responses to expel infection, NF-κB1 knockout (KO) and NF-κB2 KO mice developed chronic infections associated with elevated production of Ag-specific IFN-γ. However, only infected NF-κB1 KO mice exhibited polarized IFN-γ responses associated with the loss of intestinal goblet cells and the development of destructive colitis-like pathology. Furthermore, blockade of IL-12 (previously shown to confer resistance in susceptible strains) recovered Ag-specific IL-13 responses and resistance to infection in NF-κB2 KO, but not NF-κB1 KO mice. Therefore, unique infection, immunological, and pathological outcomes were observed in different NF-κB KO strains. Taken together, these results provide direct evidence of nonoverlapping functions for NF-κB family members in the development of Th2 cytokine-mediated resistance to T. muris and the control of infection-induced intestinal inflammation.

The NF-κB family of transcription factors are involved in the regulation of innate and adaptive immune functions associated with resistance to infection. To assess the role of NF-κB2 in the regulation of cell-mediated immunity, mice deficient in the NF-κB2 gene (NF-κB2−/−) were challenged with the intracellular parasite Toxoplasma gondii. Resistance to this opportunistic pathogen is dependent on the production of IL-12, which is required for the development of innate NK cell and adaptive T cell responses dominated by the production of IFN-γ necessary to control replication of this parasite. Although wild-type controls were resistant to T. gondii, NF-κB2−/− mice developed severe toxoplasmic encephalitis and succumbed to disease between 3 and 10 wk following infection. However, NF-κB2 was not required for the ability of macrophages to produce IL-12 or to inhibit parasite replication and during the acute stage of infection, NF-κB2−/− mice had no defect in their ability to produce IL-12 or IFN-γ and infection-induced NK cell responses appeared normal. In contrast, during the chronic phase of the infection, susceptibility of NF-κB2−/− mice to toxoplasmic encephalitis was associated with a reduced capacity of their splenocytes to produce IFN-γ associated with a loss of CD4+ and CD8+ T cells. This loss of T cells correlated with increased levels of apoptosis and with elevated expression of the pro-apoptotic molecule Fas by T cells from infected NF-κB2−/− mice. Together, these results suggest a role for NF-κB2 in the regulation of lymphocyte apoptosis and a unique role for this transcription factor in maintenance of T cell responses required for long-term resistance to T. gondii.

The NF-kappaB family of transcription factors regulates the expression of a wide range of immune response genes involved in immunity to pathogens. However, the need for individual family members in regulating innate and adaptive immune responses in vivo has yet to be clearly defined. We investigated the role of NF-kappaB1 in the induction of protective IL-12-dependent Th1 cell responses following infection with the intracellular protozoan parasite Leishmania major. Whereas wild-type C57BL/6 mice controlled parasite replication, NF-kappaB1 knockout (KO) mice were susceptible to infection, developing chronic unresolving lesions associated with persistent parasites. There was a profound defect in Ag-specific CD4(+) T cell proliferation and IFN-gamma production in infected KO mice, although innate responses-including IL-12 production and control of intracellular parasite replication by macrophages-were intact. In vitro polyclonal stimulation of purified naive KO T cells revealed an intrinsic defect in CD4(+) T cell proliferation associated with reduced IL-2 receptor expression, but operating independently of APC function and IL-2 production. Critically, the frequency of proliferating KO CD4(+) T cells secreting IFN-gamma matched that of wild-type cells, suggesting that NF-kappaB1 was not required for efficient transcription of the IFN-gamma gene. Taken together, these results identify a novel role for NF-kappaB1 in CD4(+) T cell proliferation and the development of Th1 cell responses required for protective immunity against intracellular pathogens.