ID: 164: Role of cyclophilins B & C in the assembly of IL-12 and IL-23 in the endoplasmic reticulum
Heterodimer formation of IL-12 and IL-23 is known to take place in the ER. Peptidyl-prolyl cis–trans isomerases B and C (PPIB/C; cyclophilins B and C) locate to the endoplasmic reticulum (ER). PPIB interacts with multiple ER residents including calnexin, calreticulin, GRP94, BiP, Erp72. Cyclosporine A (CsA) induces secretion of PPIB/C thus strongly reducing the intracellular concentration of both cyclophilins. Intracellular removal of PPIB/C can also be attained by siRNA knockdown. We used both approaches to assess the role of PPIB/C on assembly and secretion of the heterodimeric cytokines IL-12 and IL-23 in a recombinant HEK293 cell line permanently expressing the p40 subunit, and transiently transfected with either the p19 or p35 subunits. Removal of PPIB/C reduced secretion of IL-12 and blocked that of IL-23. This did not coincide with altered levels of mRNA suggestive for a ER-centered effect. Pull down of the p40 subunit via a C-terminal his-tag revealed association with PPIB but not PPIC. These observations were validated in monocyte-derived dendritic cells stimulated with IFN-gamma and zymosan, inducers of both IL-12 and IL-23. Knock-down of PPIB/C is known to enhance the oxidation status of the ER. Our data indicates that PPIB is involved in assembly of IL-12 and IL-23 heterodimer assembly either via its peptidyl-prolyl cis–trans isomerase activity and/or via facilitating disulfide-bond formation. This data show also that the known suppressive effect of CsA on IL-12/IL-23 secretion form natural producer cells may be due to both transcriptional repression and interference with ER assembly.