Spontaneous Germinal Center Formation Research Articles

IL-21R is required for the systemic accumulation of activated B and T lymphocytes in MRLlpr mice (117.9)

Abstract IL-21 is a pleiotropic cytokine that can influence the activation, differentiation and expansion of B and T cells. Lupus prone MRL/MpJ-Faslpr/lpr/J (MRLlpr) mice have been shown to develop renal pathology and autoantibodies in an IL-21 dependent manner. We generated MRLlpr mice deficient in IL-21 receptor (MRLlpr.IL-21R-/-) to assess the impact of loss of IL-21R signaling on the systemic B and T cell lymphoaccumulation and activation that occurs in MRLlpr mice. Consistent with therapeutic blockade of IL-21, MRLlpr.IL21R-/- mice developed reduced proteinuria, renal pathology and serum IgG autoantibody levels. Lymphoaccumulation was significantly reduced in MRLlpr.IL-21R-/- mice, as evidenced by decreased lymphadenopathy and splenomegaly. Importantly, spontaneous germinal center formation and plasma cell accumulation were absent in IL-21R deficient MRLlpr mice. In addition, activated CD4+ CD44+ CD62Llo T cells failed to accumulate. We also demonstrate that T extrafollicular helper cells, which are a subset of activated CD4+ T cells that are reported to be the primary inducers of antibody production in MRLlpr mice, require IL-21R for their generation. T helper cell differentiation was also found to be impacted by loss of IL-21R, as IFN-γ producing CD4+ T cells were significantly reduced in MRLlpr.IL21R-/- mice. Together, our data highlight that IL-21 promotes multiple pathogenic B and T cell effector responses in MRLlpr mice.

Differential requirement of MALT1 for BAFF-induced outcomes in B cell subsets

B cell activation factor of the TNF family (BAFF) activates noncanonical nuclear factor κB (NF-κB) heterodimers that promote B cell survival. We show that although MALT1 is largely dispensable for canonical NF-κB signaling downstream of the B cell receptor, the absence of MALT1 results in impaired BAFF-induced phosphorylation of NF-κB2 (p100), p100 degradation, and RelB nuclear translocation in B220+ B cells. This corresponds with impaired survival of MALT1−/− marginal zone (MZ) but not follicular B cells in response to BAFF stimulation in vitro. MALT1−/− MZ B cells also express higher amounts of TRAF3, a known negative regulator of BAFF receptor–mediated signaling, and TRAF3 was found to interact with MALT1. Furthermore, phenotypes associated with overexpression of BAFF, including increased MZ B cell numbers, elevated serum immunoglobulin titers, and spontaneous germinal center formation, were found to be dependent on B cell–intrinsic MALT1 expression. Our results demonstrate a novel role for MALT1 in biological outcomes induced by BAFF-mediated signal transduction.

CD40L is crucial for the development of autoimmunity induced by TLR7 overexpression

Antibodies against nuclear antigens secreted by autoreactive B cells are a hallmark of systemic autoimmunity. Failure to eliminate autoreactive B cells in the germinal center reaction has been reported in lupus disease. We have shown that control of TLR7 expression is essential to restrict B cell autoimmunity and DC expansion. However, the underlying mechanisms of TLR7‐induced autoimmunity still need to be scrutinized. We found increased spontaneous germinal center formation in Tg.TLR7 mice. B cells from these mice showed enhanced H2‐Ab1, stat1, T‐bet, AID expression and increased IgG1 and IgG2b immunoglobulin transcript. To test whether these B cell phenotypes required T cell help, we bred Tg.TLR7 with CD40L deficient mice. In the absence of CD40L, Tg.TLR7 mice did not develop spontaneous germinal center or any autoimmune phenotypes. Reconstitution experiments with an equal mix of Tg.TLR7.CD40L−/− and WT bone marrow showed that CD40L provided in trans can induced the disease. All of these results suggest that autoreactive B cell and autoimmune phenotype of Tg.TLR7 mice is CD40L‐dependent. Future experiments will determine whether T cells are needed to provide help through CD40L in this system.

Phenotypes of TLR7-overexpressing B cells suggest a TLR7-mediated increase in antigen presentation (129.11)

Abstract Autoantibodies against nuclear antigens secreted by autoreactive B cells are a hallmark of systemic autoimmunity. Failure to eliminate autoreactive B cells in the germinal center reaction has been reported in lupus disease. We have recently shown that TLR7 duplication in Yaa mice accelerates the onset of lupus disease. To better understand TLR7 roles in the pathogenesis of lupus disease, we created transgenic TLR7 (B6.Tg. TLR7) mice. We found an increase of spontaneous germinal center formation in both B6.Tg. TLR7 mice and B6. Yaa which is TLR7 dependent. We have carried out microarray analysis of gene expression profiles of follicular B cells (CD9-CD43-) from B6.Tg. TLR7 compared to B6. The upregulated genes in B6.Tg. TLR7 consist of interferon inducible genes, cell cycle regulators, MHC, and immunoglobulin genes. We have confirmed increased protein expression of phosphorylated stat1 (Ser727) by western blot and of H2-Ab1 by flow cytometry in B6.Tg. TLR7. These results suggest the possibility of increased antigen presentation by TLR7-overexpressing B cells in the high interferon environment.

Negative Regulation of Lymphocyte Activation by the Adaptor Protein LAX

The membrane-associated adaptor protein LAX is a linker for activation of T cells (LAT)-like molecule that is expressed in lymphoid tissues. Upon stimulation of T or B cells, it is phosphorylated and interacts with Grb2 and the p85 subunit of PI3K. LAX, however, is not capable of replacing LAT in the TCR signaling pathway. In this study we report that upon T or B cell activation, the LAX protein was up-regulated dramatically. Although disruption of the LAX gene by homologous recombination had no major impact on lymphocyte development, it caused a significant reduction in CD23 expression on mature B cells. Interestingly, naive LAX(-/-) mice had spontaneous germinal center formation. Compared with normal T and B cells, LAX(-/-) T and B cells were hyperresponsive and had enhanced calcium flux, protein tyrosine phosphorylation, MAPK and Akt activation, and cell survival upon engagement of the T or B AgRs. Our data demonstrate that LAX functions as a negative regulator in lymphocyte signaling.