Interferon-beta enhances monocyte and dendritic cell expression of B7-H1 (PD-L1), a strong inhibitor of autologous T-cell activation: relevance for the immune modulatory effect in multiple sclerosis.

Antigen-presenting cells (APC) are considered to play a critical role in promoting the (re)activation of potentially autoreactive T cells in multiple sclerosis (MS), an inflammatory demyelinating disorder of the central nervous system (CNS). B7-H1 (PD-L1) is a novel member of the B7 family proteins which exert costimulatory and immune regulatory functions. Here we characterize the expression and functional activity of B7-H1 expressed on monocytes and dendritic cells (DC) of healthy donors and MS patients. B7-H1 is constitutively expressed on monocytes and differentially matured DC, but not on B cells. IFN-beta, the principle immune modulatory agent used for the treatment of MS, strongly enhances B7-H1 expression on monocytes and semi-matured DC, but not B cells, in vitro. Importantly, B7-H1 expressed on APC strongly inhibits autologous CD4 T-cell activation. Neutralization of B7-H1 on monocytes or differentially matured monocyte-derived DC markedly increases the secretion of the pro-inflammatory cytokines, IFN-gamma and IL-2, T-cell proliferation, and the expression of T-cell activation markers. B7-H1 exhibits strong inhibitory effects when expressed on monocytes, immature or semi-mature DC, but less so when expressed on fully matured DC. B7-H1-dependent immune inhibition is in part mediated by CD4/CD25+ regulatory T cells. There is no difference in the baseline expression levels of monocytic B7-H1 between untreated MS patients and healthy donors. However, both groups show a significant concentration-dependent up-regulation of B7-H1 mRNA and protein in response to IFN-beta in vitro. Serial measurements of B7-H1 mRNA in MS patients before and 6 months after initiation of IFN-beta therapy corroborated the relevance of these results in vivo: Nine of nine patients showed a significant increase in B7-H1 mRNA levels after 6 months of IFN-beta therapy (median 1.04 vs. 8.78; p<0.05, two-sided t-test). Accordingly, protein expression of B7-H1 on monocytes was up-regulated after 24 h of IFN-beta application. In summary, B7-H1 expressed on APC acts as a strong inhibitor of autologous CD4 T-cell activation and may thus contribute to the maintenance of peripheral immune tolerance. IFN-beta up-regulates B7-H1 in vitro and in MS patients in vivo and might represent a novel mechanism how IFN-beta acts as a negative modulator on APC T-cell interactions in the periphery.