The progesterone-induced blocking factor modulates the balance of PKC and intracellular Ca++

Problem Progesterone-induced blocking factor (PIBF) induces Th2 biased cytokine production; therefore, this study investigates the effects of PIBF on the protein kinase C (PKC)/Ca++ system - which plays a key role in Th1/Th2 differentiation. Method of study Proteins from PIBF-treated cells were reacted on Western blots with phospho-specific antibodies recognizing different PKC izoforms. Intracellular free calcium was measured by flow cytometry. Results Both interleukin (IL)-4 and PIBF induced PKC phosphorylation, which was abrogated by anti-IL-4R alpha or anti-PIBF immunoglobulin G pre-treatment. PIBF treatment did not alter intracellular Ca++ levels. Inhibition of PKC zeta or PKC theta phosphorylation, but not that of PKC alpha/beta resulted in the loss of STAT6 and Jak1 phosphorylation by PIBF. Conclusions Our data show that PIBF phosphorylates PKC via binding to the IL-4R, without affecting intracellular Ca++. Phosphorylation of PKC zeta and PKC theta is required for Jak1 and STAT6 activation, whereas PKC alpha/beta is not involved. These findings explain the mechanism by which PIBF supports a Th2 dominant cytokine pattern.