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-4Ralpha or anti-PIBF immunoglobulin G pre-treatment. PIBF treatment did not alter intracellular Ca(++) levels. Inhibition of PKCzeta or PKCtheta phosphorylation, but not that of PKCalpha/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 PKCzeta and PKCtheta is required for Jak1 and STAT6 activation, whereas PKCalpha/beta is not involved. These findings explain the mechanism by which PIBF supports a Th2 dominant cytokine pattern.