IL-27p28 specifically regulates MHC II expression in macrophages through CIITA

Antigen-presenting cells (APCs) constantly express major histocompatibility complex II (MHC II), including macrophages and dendritic cells (DCs) which deliver antigens to CD4⁺ T cells and play an important role in adaptive immunity. The expression of MHC II is controlled by the transcriptional coactivator CIITA. Interleukin-27 (IL-27), a newly discovered IL-12 family cytokine, is composed of p28 and EBI3 subunits. In this study, we used IL-27p28 conditional knock-out mice to investigate the regulatory effects of IL-27p28 on macrophage polarization and the expression of MHC II in macrophages. We found that MHC II expression was upregulated in the bone marrow-derived and peritoneal exudate macrophages (BMDMs; PEMs) from IL-27p28-deficient mice, with their inflammation regulating function unaffected. We also demonstrated that in the APCs, IL-27p28 selectively regulated MHC II expression in macrophages but not in dendritic cells. During Pseudomonas aeruginosa (P. aeruginosa) reinfection, higher survival rate, bacterial clearance, and ratio of CD4⁺/CD8⁺ T cells in the spleen during the specific immune phase were observed in IL-27p28 defect mice, as well as an increased MHC II expression in alveolar macrophages (AMs). But these did not occur in the first infection. For the first time we discovered that IL-27p28 specifically regulates the expression of MHC II in macrophages by regulating CIITA, while its absence enhances antigen presentation and adaptive immunity against P. aeruginosa.