CD4(+) T cells from MHC II-dependent thymocyte–thymocyte interaction provide efficient help for B cells

Recently, a novel CD4(+) T-cell developmental pathway was reported that generates thymocyte–thymocyte (T–T) CD4(+) T cells. We established a mouse system (CIITA(tg)CIITApIV(−/−)) where thymic positive selection occurred only by major histocompatibility complex (MHC) class II(+) thymocytes. T–T CD4(+) T cells selected via MHC class II-dependent T–T interaction are comprised of PLZF-negative and innate PLZF-positive populations. Until recently, the functional role of the PLZF-negative population was unclear. In this study, we demonstrate that naïve T–T CD4(+) T cells provide B-cell help to a level comparable with that of naïve conventional CD4(+) T cells. Considering the absence of PLZF expression in naïve T–T CD4(+) T cells, these results suggest that PLZF-negative naïve T–T CD4(+) T cells are functionally equivalent to conventional naïve CD4(+) T cells in terms of B-cell help.