Regulatory T-cells(Treg) are crucial for immune homeostasis and prevention of immune pathology. Yet, Treg may lose Foxp3 and start secreting IL-17, dependent on environmental cues. Our previous data revealed that Treg from severe psoriasis patients are particularly prone to such conversion. The question of how to maintain Treg stability in the context of inflammation awaits immediate resolution. The pan-PKC inhibitor sotrastaurin has shown efficacy in clinical trials of psoriasis. Here, we show that sotrastaurin inhibited effector T cell responses, whereas the regulatory response was enhanced. Sotrastaurin prevented TCR/CD28-induced T-cell activation and pro-inflammatory cytokine production, but preserved a stable Treg phenotype as evidenced by maintenance of suppressive capacity, high Foxp3 and CD25 expression, and lack of IL-17A and IFNγ production. Moreover, in both circulating and dermal psoriatic Treg, prone to rapid induction of IL-17, sotrastaurin enhanced Foxp3 expression and prevented IL-17A and IFNγ production even when stimulated in the presence of the Th17-enhancing cytokines IL-1β or IL-23. Thus, pharmacological inhibition of PKC may serve as a powerful tool to concurrently inhibit effector T cells and to facilitate Treg, thereby showing therapeutic potential for the treatment of psoriasis.Journal of Investigative Dermatology accepted article preview online, 5 November 2013; doi:10.1038/jid.2013.459.