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Rituximab is a chimeric anti‐CD20 monoclonal antibody (mAb) used in B‐cell malignancies, various autoimmune disorders and organ transplantation. Although administration of a single dose of rituximab results in full B‐cell depletion in peripheral blood, there remains a residual B‐cell population in secondary lymphoid organs. These nondepleted B cells might be altered by exposure to rituximab with subsequent immunomodulatory effects. Therefore, we analyzed in vitrothe effects of rituximab on proliferation, activation and differentiation of CD19+B cells by means of carboxyfluorescein succinimidyl ester (CFSE)‐based multiparameter flow cytometry. Rituximab inhibited the proliferation of CD27−naïve, but not of CD27+memory B cells. Interestingly, upon stimulation with anti‐CD40 mAb and interleukin‐21 in the presence of rituximab there was an enrichment of B cells that underwent only one or two cell divisions and displayed an activated naïve phenotype (CD27−IgD+CD38−/+). The potency of prestimulated B cells to induce T‐cell proliferation was increased by exposure of the B cells to rituximab. Of note, after stimulation with rituximab‐treated B cells, proliferated T cells displayed a more Th2‐like phenotype. Overall, these results demonstrate that rituximab can affect human B‐cell phenotype and function, resulting in an altered outcome of B–T cell interaction.

Mycophenolic acid is the active ingredient of the immunosuppressant mycophenolate mofetil that is widely used in transplantation medicine and autoimmunity. Mycophenolic acid inhibits inosine monophosphate dehydrogenase, an enzyme involved in biosynthesis of guanine nucleotides required for lymphocyte clonal expansion. Here, we present novel insights into the mechanisms underlying mycophenolic acid‐mediated suppression of human CD4+ T cells. Upon CD3/CD28 stimulation, mycophenolic acid inhibited T cell IL‐17, IFN‐γ and TNF‐α production but not IL‐2 production. Phenotypic analysis showed that drug treatment enhanced the expression of negative co‐stimulators PD‐1, CTLA‐4 and the transcription factor FoxP3 and decreased the expression of positive co‐stimulators CD27 and CD28, whereas CD25 was unaffected. Mycophenolic acid‐treated cells were anergic, but not suppressive, and at the same time proved hyperblastoid with high metabolic activity. Moreover, a reduced Akt/mTOR and STAT5 signaling was observed. Interestingly, the co‐stimulatory molecule CD70 was uniquely and dose‐dependently upregulated on mycophenolic acid‐treated T cells and found to be directly linked to target enzyme inhibition. CD70 on mycophenolic acid‐treated cells proved functional: an anti‐CD70 agonist was found to restore both STAT5 and Akt/mTOR signaling and may thereby prevent apoptosis and promote survival. These novel insights may contribute to optimization of protocols for MPA‐based immunosuppressive regimens.