Murine B cell response to TLR7 ligands depends on an IFN-beta feedback loop.

Type I IFNs play an important, yet poorly characterized, role in systemic lupus erythematosus. To better understand the interplay between type I IFNs and the activation of autoreactive B cells, we evaluated the effect of type I IFN receptor (IFNAR) deficiency in murine B cell responses to common TLR ligands. In comparison to wild-type B cells, TLR7-stimulated IFNAR(-/-) B cells proliferated significantly less well and did not up-regulate costimulatory molecules. By contrast, IFNAR1(-/-) B cells did not produce cytokines, but did proliferate and up-regulate activation markers in response to other TLR ligands. These defects were not due to a difference in the distribution of B cell populations or a failure to produce a soluble factor other than a type I IFN. Instead, the compromised response pattern reflected the disruption of an IFN-beta feedback loop and constitutively low expression of TLR7 in the IFNAR1(-/-) B cells. These results highlight subtle differences in the IFN dependence of TLR7 responses compared with other TLR-mediated B cell responses.

Functional outcome of B cell activation by chromatin immune complex engagement of the B cell receptor and TLR9.

We have previously shown that rheumatoid factors produced by Fas-deficient autoimmune-prone mice typically bind autologous IgG2a with remarkably low affinity. Nevertheless, B cells representative of this rheumatoid factor population proliferate vigorously in response to IgG2a/chromatin immune complexes through a mechanism dependent on the sequential engagement of the BCR and TLR9. To more precisely address the role of both receptors in this response, we analyzed the signaling pathways activated in AM14 B cells stimulated with these complexes. We found that the BCR not only serves to direct the chromatin complex to an internal compartment where it can engage TLR9 but also transmits a suboptimal signal that in combination with the signals emanating from TLR9 leads to NF-kappaB activation and proliferation. Importantly, engagement of both receptors leads to the up-regulation of a group of gene products, not induced by the BCR or TLR9 alone, that include IL-2. These data indicate that autoreactive B cells, stimulated by a combination of BCR and TLR9 ligands, acquire functional properties that may contribute to the activation of additional cells involved in the autoimmune disease process.

p53 regulates thymic Notch1 activation.

Notch is crucial for multiple stages of T cell development, including the CD4+CD8+ double positive (DP)/CD8+ single positive (SP) transition, but regulation of Notchactivation is not well understood. p53 regulates Presenilin1 (PS1) expression, and PS1 cleaves Notch, releasing its intracellular domain (NIC), leading to the expression of downstream targets, e.g. the HES1 gene. We hypothesize that p53 regulates Notch activity during T cell development. We found that Notch1 expression and activation were negatively regulated by p53in several thymoma lines. Additionally, NIC was elevated in Trp53(-/-) thymocytes as compared to Trp53(+/+) thymocytes. To determine if elevated Notch1 activation in Trp53(-/-) thymocytes had an effect on T cell development, CD4 and CD8 expression were analyzed. The CD4+ SP/CD8+ SP T cell ratio was decreased in Trp53(-/-) splenocytes and thymocytes. This alteration in T cell development correlated with the increased Notch1 activation observed in the absence of p53. These data indicate that p53 negatively regulates Notch1 activation during T cell development. Skewing of T cell development toward CD8+SP T cells in Trp53(-/-) mice is reminiscent of the phenotype of NIC-overexpressing mice. Thus, we suggest that p53 plays a role in T cell development, in part by regulating Notch1 activation.