Identification of an Expressed Truncated Form of CD200, CD200tr, which is a Physiologic Antagonist of CD200-Induced Suppression

Earlier studies have indicated that cell surface expressed CD200, or a soluble form of this molecule, can induce immunosuppression in a number of biological systems, and promote increased graft acceptance, after binding to receptors (CD200Rs). Many groups have reported that the NH2-terminal region of CD200 is crucial for interaction with CD200R. A truncated form of the full-length, immunosuppression-inducing molecule, CD200, representing translation of an mRNA splice variant, CD200tr, was expressed in the CHO cells and the transduced cells used to produce mAbs unique to CD200tr. These mAbs, and mAbs to full-length CD200, were used to document physiologic expression of full-length CD200 and CD200tr on lipopolysaccharide-stimulated dendritic cells (DCs). The ability of a soluble form of CD200 and CD200tr, each linked to a murine IgG2aFc, to suppress allogeneic immune responses in vitro (mixed leukocyte cultures) or in vivo (skin graft rejection) alone, or in combination, was studied. CHO cells expressing CD200tr inhibited the suppression of mixed leukocyte reactions seen after addition of soluble CD200 (CD200Fc) to culture. In addition, a soluble form of CD200tr, prepared by fusing the extracellular domain of the truncated CD200 to a murine IgG2a Fc region, could block in a competitive fashion the CD200Fc-mediated suppression of cytotoxic T lymphocyte induced in mixed leukocyte reactions, of tumor necrosis factor-alpha produced by lipopolysaccharide-stimulated splenic cells, and of allogeneic skin graft rejection in vivo. Taken together our data support the hypothesis that an expressed splice variant of CD200, CD200tr, is a physiologic antagonist of CD200.