B cells can prime naive CD4+ T cells in vivo in the absence of other professional antigen‐presenting cells in a CD154‐CD40‐dependent manner

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Abstract
The role of B cells as APC is well established. However, their ability to prime naive T cells in vivo has been difficult to examine because of the presence of dendritic cells. The current studies were undertaken to examine this issue in a model of adoptive transfer of antigen‐specific B cells and T cells into histoincompatible Rag2–/– mice. By means of this system, we were able to demonstrate that antigen‐specific B cells are competent APC for naive CD4+ T cells specific for the same antigen. In vivo antigen presentation resulted in expansion of both CD4+ T cells and B cells. The antigen‐presenting function of the transferred B cells was dependent on the CD154‐CD40 interaction, as transfer of CD154‐deficient antigen‐specific CD4+ T cells or CD40‐deficient B cells failed to induce T and B cell expansion in response to immunization. These results indicate that antigen‐specific B cells have the capacity to induce primary T cell responses in the absence of other competent APC.