Expression of the negative co-stimulatory ligand sCD152 in the yeast, Pichia pastoris, and its regulation of antigen specific immune responses.

CD152, a ligand expressed on the surface of the activated T cells that inhibits co-stimulatory signals, is associated with negative regulation of T-cell activation in the antigen-presenting process. In order to interfere with immune signal transmission, obtain functional proteins with specific immunosuppressive effects, and regulate the immune response, we cloned the full-length extracellular domain of CD152 into the expression vector pPICTLA and transformed Pichia pastoris GS115 cells by electroporation. Yeast colonies expressing and secreting large quantities of the extracellular soluble fragment of CD152 (sCD152) were isolated, and the protein was purified and used in assays designed to investigate the ability of sCD152 to regulate the immune system. In a series of antigen specific immune response interference tests in mice, human allogeneic mixed lymphocyte reaction and antigen specific lymphocyte transformation tests, we found that sCD152 had a marked immunosuppressive effect. This protein reduced antibody titer in mice immunized with BSA, significantly inhibited the killing of antibody-dependent cytotoxicity and CTL-mediated donor target cells, and reduced the expression of IL2 in mice. In the absence of exposure of sCD152, long-term stimulation (>5 days) by the same antigen in vivo partially restored the immune response-correlated stimulation index (SI), showing the characteristics of a primary immune response. Following the deletion of sCD152, however, there was no obvious inhibitory effect on the primary immune response triggered by the nonrelevant antigen OVA, indicating that sCD152 had an antigen-specific immunosuppressive effect and that the inhibitory effect of sCD152 required the co-administration of antigen. These findings suggest that sCD152 may have clinical potential as an immunosuppressive agent.