Molecular mechanisms in the regulation of B7-1 and B7-2 co-stimulatory molecules in human monocytic cells

Abstract

The engagement of CD28 receptor with the B7-1 or B7-2 ligands on antigen presenting cells, provide the critical second co-stimulatory signal for the immune activation of T lymphocytes. The molecular mechanisms by' which B7-1 or B7-2 expressions are regulated are not well understood. I investigated the role of mitogen-activated protein (MAPKs) in the regulation of lipopolysaccharide (LPS)-induced B7-1 expression in human monocytes and the promonocytic THP-1 cells. My results show that LPS-induced B7-1 expression in monocytic cells did not involve the activation of either p38 or ERK kinases. By employing JNK inhibitors SP600125, dexamethasone and curcumin that inhibit JNK activation, LPS-induced B7-1 expression was determined to be regulated by JNK MAPKs in both monocytes and THP-1 cells. Also, I identified a distinct B7-1 regulatory element corresponding to the interferon regulatory factor-7 (IRF-7) binding site on the B7-1 promoter, responsible for the regulation of LPS-induced B7-1 transcription. Furthermore, SP600125 and dexamethasone inhibited LPS-induced IRF-7 activity as determined by the luciferase reporter and gel shift assays. Taken together, the results suggest that LPS-induced B7-1 transcription in human monocytic cells may be regulated by IRF-7 transcription factor through JNK MAPKs activation. I also studied the role of MAP kinases in the regulation of B7-2 expression in LPS stimulated human monocytic cells. LPS stimulation of human monocytes resulted in the down-regulation of B7-2 expression that could be abrogated by anti-IL-10 antibodies. My results reveal the distinct involvement of p38 in IL-10 dependent, and JNK in 1L-10independent regulation of B7-2 expression in LPS-stimulated monocytic cells. In addition, by deletion and mutant B7-2 promoter/luciferase gene analysis I identified 3 positive regulatory elements on the B7-2 promoter, NF-kappaB, IRF-2, and an unidentified binding site region that operate cooperatively to control B7-2 expression. Elucidating the signal pathway and molecular mechanisms in the regulation of B7-1 and B7-2 in antigen presenting cells is important as it could provide us with a better understanding of the molecular basis for CD28/B7 co-stimulation. This in turn, may lead to the development of strategies in treating autoimmune diseases, organ transplantation rejections, graft-versus-host disease, infections, and cancer.