Suppression of antigen-specific CD4+ T cell activation by SRA/CD204 through reducing the immunostimulatory capability of antigen-presenting cell.

Pattern recognition scavenger receptor SRA/CD204, primarily expressed on specialized antigen-presenting cells (APCs), including dendritic cells (DCs) and macrophages, has been implicated in multiple physiological and pathological processes, including atherosclerosis, Alzheimer's disease, endotoxic shock, host defense, and cancer development. SRA/CD204 was also recently shown to function as an attenuator of vaccine response and antitumor immunity. Here, we, for the first time, report that SRA/CD204 knockout (SRA(-/-)) mice developed a more robust CD4(+) T cell response than wild-type mice after ovalbumin immunization. Splenic DCs from the immunized SRA(-/-) mice were much more efficient than those from WT mice in stimulating naïve OT-II cells, indicating that the suppressive activity of SRA/CD204 is mediated by DCs. Strikingly, antigen-exposed SRA(-/-) DCs with or without lipopolysaccharide treatment exhibited increased T-cell-stimulating activity in vitro, which was independent of the classical endocytic property of the SRA/CD204. Additionally, absence of SRA/CD204 resulted in significantly elevated IL12p35 expression in DCs upon CD40 ligation plus interferon gamma (IFN-γ) stimulation. Molecular studies reveal that SRA/CD204 inhibited the activation of STAT1, mitogen activated protein kinase p38, and nuclear factor-kappa B signaling activation in DCs treated with anti-CD40 antibodies and IFN-γ. Furthermore, splenocytes from the generated SRA(-/-) OT-II mice showed heightened proliferation upon stimulation with OVA protein or MHC-II-restricted OVA(323-339) peptide compared with cells from the SRA(+/+) OT-II mice. These results not only establish a new role of SRA/CD204 in limiting the intrinsic immunogenicity of APCs and CD4(+) T cell activation but also provide additional insights into the molecular mechanisms involved in the immune suppression by this molecule.