Cutting edge: T cell–dependent plasmablasts form in the absence of single differentiated CD4 ⁺ T cell subsets

The T follicular helper (Tfh) cell subset of CD4 ⁺ Th cells promotes affinity maturation by B cells in germinal centers. The contribution of other Th cell subsets to B cell responses has not been fully explored in vivo. We addressed this issue by analyzing the T cell–dependent B cell response to the protein Ag PE in mice lacking specific Th cell subsets. As expected, PE-specific germinal center B cell production required Tfh cells. However, Tfh, Th1, or Th17 cell–deficient mice produced as many PE-specific, isotype-switched plasmablasts as wild-type mice. This response depended on Th cell expression of CD154 and Ag presentation by B cells. These results indicate that many Th cell subsets can promote plasmablast formation by providing CD40 signals to naive B cells.