Mycophenolic acid-mediated suppression of human CD4+ T cells: more than mere guanine nucleotide deprivation.

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.