Calcineurin inhibitors (CNI), used frequently in solid organ transplant patients, are known to inhibit T cell proliferation, but their effect on humoral immunity is far less studied. Total and naive B cells from healthy adult donors were cultured in immunoglobulin (Ig)A- or IgG/IgE-promoting conditions with increasing doses of cyclosporin, tacrolimus, rapamycin or methylprednisolone. The effect on cell number, cell division, plasmablast differentiation and class-switching was tested. To examine the effect on T follicular helper (Tfh) cell differentiation, naive CD4<sup>+</sup> T cells were cultured with interleukin (IL)-12 and titrated immunosuppressive drug (IS) concentrations. Total B cell function was not affected by CNI. However, naive B cell proliferation was inhibited by cyclosporin and both CNI decreased plasmablast differentiation. Both CNI suppressed IgA, whereas only cyclosporin inhibited IgE class-switching. Rapamycin had a strong inhibitory effect on B cell function. Strikingly, methylprednisolone, increased plasmablast differentiation and IgE class-switching from naive B cells. Differentiation of Tfh cells decreased with increasing IS doses. CNI affected humoral immunity directly by suppressing naive B cells. CNI, as well as rapamycin and methylprednisolone, inhibited the in-vitro differentiation of Tfh from naive CD4<sup>+</sup> T cells. In view of its potent suppressive effect on B cell function and Tfh cell differentiation, rapamycin might be an interesting candidate in the management of B cell mediated complications post solid organ transplantation.

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Clinical and Experimental Immunology
Department of Pulmonology

De Bruyne, R., Bogaert, D., De Ruyck, N., Lambrecht, B., Van Winckel, M., Gevaert, P., & Dullaers, M. (2015). Calcineurin inhibitors dampen humoral immunity by acting directly on naive B cells. Clinical and Experimental Immunology, 180(3), 542–550. doi:10.1111/cei.12604