Human B Cell Lines and Primary B Cells Actively Secrete Granzyme B in Response to IL-2 Family Cytokines.

It is widely believed that the main function of B cells is antibody secretion, but not cellular cytotoxicity. Recently we found that human B cells activated with interleukin 21 (IL-21) and antibodies to the B cell receptor (BCR) or immunostimulatory oligonucleotides (CpG ODN) develop a phenotype similar to that of cytotoxic T lymphocytes. B cells treated in such a way start to secrete large amounts of granzyme B (GrB) instead of antibodies and, as in the case of B-chronic lymphocytic leukemia (B-CLL), acquire the capability to induce apoptosis in bystander B-CLL cells in a GrB-dependent manner. Using FACS and ELISpot analyses we could now demonstrate that GrB is actively secreted by B cells in a time-dependent manner and that IL-21 is not the only cytokine that induces GrB in B cells. Also cytokine combinations such as IL-10 and IL-4 as well as IL-10 and IFN-alpha induce GrB in normal B cells and various B cell lines including MEC-1 (CLL), ARH-77 (plasma cell leukemia) and Namalwa (Burkitts lymphoma). We conclude that IL-21 and further cytokines can induce B cells to produce functional granzyme B. Further studies are required to elucidate the interactions with B lymphocytes of cells producing these cytokines such as CD4+ T cells, regulatory T cells, NKT cells and plasmacytoid dendritic cells. Our unexpected findings could have significant implications on our understanding of the role of B cells in immune regulation and for a variety of immune phenomena including auto-, cancer and infectious immunity.