CD200 Is a Ligand for All Members of the CD200R Family of Immunoregulatory Molecules

Discrete monoclonal antibodies define functionally important epitopes in the CD200 molecule responsible for immunosuppression function

BACKGROUND

Both murine and human CD200 fusion proteins (CD200Fc) act as immunosuppressants after engagement of cell-bound receptors (CD200R). Anti-CD200 monoclonal antibodies (mAbs) augment activity in mixed leukocyte cultures (MLCs) (increased cytotoxic T lymphocyte/cytokine production) after neutralization of endogenous CD200 activity. Previous studies documented critical regions in the N-terminal domains of both CD200 and CD200R1 for ligand:receptor binding and defined a number of synthetic CD200 and CD200R peptides that antagonize that interaction.

METHODS

We used a panel of mAbs to mouse and human CD200Fc to compare the rank activities of antibodies for binding (flow cytometric analysis [FACS] or enzyme-linked immunoadsorbent assay [ELISA]) to CD200 with their abilities to augment immune reactivity in MLCs.

RESULTS

Only mAbs defining epitopes in the N-terminal domain could augment MLC reactivity (or block immunosuppression by soluble CD200Fc), whereas mAbs targeting C-domain epitopes, although reactive in ELISA or FACS (targeting cell surface CD200), were inactive in MLCs.

CONCLUSION

In addition to defining the importance of N-terminal epitopes for CD200 function, rank comparison of mAbs for FACS staining of CD200 expressed on various cell types indicates heterogeneity in expressed CD200.

[Recognition of virus infected cells by NK cells]

NK cells show cytotoxicity against virus-infected cells and tumor cells and play an important role in host defense. Although mecheanism of target cell recognition by NK cells have been unclear for a long time, it has recently been elucidated that certain NK cell receptors specifically recognize virus products. Furthermore, expression pattern of NK cell receptors, which consist of activating and inhibitory receptors, determines susceptibility to virus-infection. Here, we review recent progress of mechanism of recognition of virus-infected by NK cells.

Structural and functional heterogeneity in the CD200R family of immunoregulatory molecules and their expression at the feto-maternal interface

PROBLEM

We have shown that CD200Fc, a chimeric molecule including the extracellular domain of CD200 and a murine immunoglobulin (Ig)G2a Fc region, regulates immune responses and prevents T helper (Th)1 cytokine-triggered spontaneous abortions in mice. CD200 is expressed on a subpopulation of uterine decidua cells and on trophoblast, both in the mouse and human. The receptor(s) for CD200, CD200R(s), was not previously well-characterized.

METHODS

5'-rapid amplification of cDNA ends (RACE), cDNA and genomic DNA clone analysis were used to identify a family of CD200Rs on mouse chromosome 16, juxtaposed to the CD200 gene, named CD200R1, R2, R3, and R4. Northern blot and reverse transcriptase polymerase chain reaction (RT-PCR) analysis was used to detect expression of different CD200R subtypes in different organs. Rabbit polyclonal and rat monoclonal antibodies (mAbs) to CD200R isoforms was used for fluorescence-activated cell sorter (FACS) analysis, to test for immunomodulatory effects on allogeneic mixed-lymphocyte responses in vitro, and for immunohistochemistry.

RESULTS

The CD200Fc was able to interact physically with each of the CD200Rs expressed on the cell surface. Northern blot and RT-PCR analyses indicated distinct patterns of CD200R isoform mRNA expression in different tissues and FACS analyses confirmed unique cell- and tissue-specific expression of the different CD200Rs. mAbs directed against the different isoforms modified the development of in vitro alloimmune responses. The addition of anti-CD200R1/R4 elicited immunomodulatory responses in vitro comparable to findings with CD200Fc, but different from the effects of anti-CD200R2-3.

CONCLUSIONS

These data provide evidence for a family of CD200R molecules in the mouse genome and defines the existence of previously unrecognized diversity in the CD200/CD200R immunomodulatory gene member family. Although this gene member family is clustered in the genome, the different CD200Rs and CD200 exhibit distinct expression patterns and functional properties. Restricted CD200R isoform expression at the feto-maternal interface suggests CD200:CD200R interactions may serve important function(s) determining the successful outcome of pregnancy.

CD200 receptor family members represent novel DAP12-associated activating receptors on basophils and mast cells

Modulation by balancing activating and inhibitory receptors constitutes an important mechanism for regulating lymphocyte and myeloid cell effector responses. Using a microarray screen during parasitic helminth infection, we identified CD200 receptor-like 3 as a transcript highly expressed in basophils. Novel splice variants were present that generated proteins that differed in surface expression. The second immunoglobulin-like domain, encoded by exon 4, was required for cell surface expression and recruitment of DAP12 to the cell surface. Splice variants also generated unique cytoplasmic domains, which contributed to efficient pairing with DAP12. Despite expression on basophils and mast cells, which are integral components of allergic immunity, the absence of DAP12 did not alter effector cell recruitment or the host response elicited by helminth infection with Nippostrongylus brasiliensis.