Indeed, as shown in Fig. 6B similar Foxp3 staining could be observed

in the lamina propria of TNBS-treated mice that received PI when compared to TNBS-treated mice that received saline suggesting that Foxp3+ T cells are normally present in the lamina propria of PI-treated mice. Our data demonstrate that the phospholipid PI inhibits T-cell proliferation and Proteases inhibitor differentiation but does not affect Treg differentiation. In vitro experiments established that PI strongly inhibited PMA-stimulated T-cell activation. Moreover, T cells stimulated with plate-bound anti-CD3 and soluble anti-CD28 antibodies were also effectively inhibited by PI. The latter experiments excluded that PI acted through interference with processes such as transmembrane diffusion of PMA. Inhibition was dose dependent and did not involve anergy, apoptosis or deletion as reflected by the fact that suppression learn more was reversible when PI was removed (Supporting Information Fig. 3). Moreover, suppression was not restricted to a specific subset of T cells as both CD4+ and CD8+ T-cell activation was inhibited. In contrast, PI did not affect T-cell proliferation indirectly by inhibiting antigen-presenting DCs, as loading of DCs in the presence of PI before co-incubation did not

inhibit T-cell activation. These data strongly implicate an effect of PI on the intracellular pathways that are associated with T-cell activation. By determining phosphorylation activity of various intracellular checkpoints of T-cell activation we established that PI exerted its effects on the PKC–MAPK pathway. As we found decreased ERK1/2, P38 and JNK phosphorylation upon PI treatment, we hypothesize that PI targets ADP ribosylation factor one of the common inositol lipid pathways that precedes these downstream routes. Recently, it was established that inositol lipid signaling is regulated through cellular expression of a class of PI-transfer proteins PI-TPα and PI-TPβ 11. Modulation of this signaling has been associated with changes in cellular proliferation. In particular, overexpression of PI-TPα in fibroblasts induces an increased growth rate, whereas the growth rate of PI-TPβ overexpressing cells

is decreased 12, 13. Future studies into the specific regulation of expression of such transfer proteins may lead to the development of novel PI-based immunosuppressants. The inhibition of signal transduction in T cells cultured with PI led to reduced IL-2 mRNA expression and low levels of IL-2 protein release, which abrogated T-cell proliferation. In consequence, PI inhibited inflammatory CD4+ Th cell proliferation and cytokine release. Crucially, Foxp3+ Treg differentiation was not aborted by addition of PI, which is a pivotal characteristic for a potential immunosuppressant. As such, the immunosuppressive effects of PI share more similarity with the inhibitor rapamycin, which preserves Foxp3+ T cells while inhibiting inflammatory responses than with cyclosporine, which suppresses both inflammatory and Tregs 14, 15.