TNFα Inhibits the Expression of Germinal Center Phenotype on T-bet+ CD11c+ IgM Memory B Cells without Altering Clonal Diversity

Abstract

Abstract Our studies have identified a population of T-bet+ CD11c+ IgM memory B cells that arise within 30 days following infection with the intracellular bacterium, E. muris. The memory cells require for their generation T cell help and IL-21; however, they are generated in a GC-independent fashion, because GCs are suppressed during infection. Previous studies have shown that TNFα is necessary for proper GC formation. Given that we detected an increase in serum TNFα following infection, we addressed a role for this cytokine in IgM memory cell development. Relative to wild-type, we observed a substantial increase in the frequency of developing IgM memory. These B cells were positive for GL7+ CD38lo CD95+ on day 16 post-infection. Imaging studies corroborated these findings by demonstrating the appearance of GC-like structures in infected TNFα-deficient, but not in wild-type, mice. Increased GL7 expression on IgM memory cells in absence of TNFα was not correlated with changes in clonal diversity on day 30 post-infection. This suggests that TNFα does not affect B cell selection. To address mechanisms whereby the absence of TNFα may facilitate formation of pseudo-GC structures, we quantitated CXCL13, a chemokine known for its role as a chemoattractant during GC morphogenesis. CXCL13 expression was 10-fold lower in TNFα-deficient mice, indicating its requirement, in part, for splenic disorganization in wild-type mice during infection. Our findings suggest that although GCs are suppressed during ehrlichial and other bacterial infections, GC structures can, when present, magnify IgM memory cell generation. Ongoing studies will address whether changes in lymphoid architecture impact other properties and functions of T-bet+ CD11c+ IgM memory cells.