Increased diabetes development and decreased function of CD4+CD25+ Treg in the absence of a functional DAP12 adaptor protein.

Prior to the development of type 1 diabetes, T cells are primed in the pancreatic lymph nodes (PLN) where they interact with APC displaying beta cell-derived peptides. The details concerning the regulation of autoreactive T cell responses in the PLN are unclear. BDC2.5/B6g7 TCR transgenic mice represent a simplified model of type 1 diabetes, in which beta cell-specific CD4+ T cells expressing a diabetogenic transgenic TCR are first activated in the PLN and subsequently home to the pancreas where they mediate killing of beta cells. DNAX-activating protein of 12 kDa (DAP12) is an adaptor molecule carrying an ITAM motif. It associates with receptors on lymphoid and myeloid cells, including APC. We here show that introduction of a DAP12 null mutation in BDC2.5/B6g7 mice accelerated diabetes development and promoted an augmented activation state of PLN T cells expressing the transgenic TCR. Transferred BDC2.5 T cells proliferated more efficiently in the PLN of DAP12-deficient B6g7 recipients, which correlated with a decreased impact of co-transferred BDC2.5+CD4+CD25+ T cells. We propose that signaling through a DAP12-associated receptor on APC facilitates activation of Treg in the PLN and by this contributes to the maintenance of peripheral tolerance to beta cell-derived antigens.

RIP2 contributes to Nod signaling but is not essential for T cell proliferation, T helper differentiation or TLR responses.

Receptor-interacting protein 2 (RIP2), also known as CARDIAK and RICK, has been reported to play a role in both adaptive T cell responses and innate immunity as a mediator in TLR signaling and nucleotide-binding oligomerization domain (Nod) signaling. Because initial reports remain controversial, we have further examined both innate and adaptive immune responses in RIP2-deficient mice on the C57BL/6 background. Despite the up-regulation of RIP2 after T cell activation, we could not detect any defect in T cell proliferation or Th1/Th2 responses in RIP2-KO mice. Furthermore, we found that TLR responses in RIP2-deficient macrophages were normal. However, our analysis showed that Nod signaling was impaired in macrophages from RIP2-deficient mice. In conclusion, our data demonstrate a critical role for RIP2 in Nod signaling, while T cell proliferation, T helper differentiation and TLR responses were unaffected by the absence of RIP2.

Reduced antigen concentration and costimulatory blockade increase IFN-gamma secretion in naive CD8+ T cells.

CD8+ T cells are killer cells but also major producers of IFN-gamma. We have investigated the effects of peptide antigen titration and costimulatory blockade on IFN-gamma production and proliferation by naive CD8+ T cells. Mature dendritic cells (DC) pulsed with high amounts of agonist peptide triggered proliferation but little IFN-gamma secretion in individual T cells. In contrast, immature DC pulsed with similar amounts of peptide induced IFN-gamma secretion in a larger fraction of T cells but triggered less proliferation. Blocking B7.2 or lowering the amount of peptide on mature DC led to a response similar to that induced by immature DC, suggesting that differences in stimulatory strength were responsible for the different responses. Using splenic antigen-presenting cells (APC) we demonstrate that reducing the amount of peptide in combination with B7 blockage enhanced IFN-gamma secretion and decreased proliferation in naive CD8+ T cells in an additive way. Our data suggest that IFN-gamma secretion and proliferation are independently and inversely controlled by stimulatory strength in naive CD8+ T cells. This may enable CD8+ T cells to respond with IFN-gamma secretion to immature APC with few peptide ligands consistent with an early immunoregulatory role of CD8+ T cells.