CD200 and membrane protein interactions in the control of myeloid cells.

OX2 (now designated CD200) is a membrane protein expressed by a broad range of cell types. It is the ligand for a receptor restricted to myeloid cells, with the potential to deliver inhibitory signals. This is indicated by the CD200-deficient mouse model, in which myeloid cells are more activated when stimulated immunologically than cells from normal mice. The unusual tissue distribution of CD200 indicates where myeloid cells can be restrictively controlled through cell-cell contact. Recent data on CD200 will be reviewed in the context of other proteins that might have similar roles, in particular, the interaction between CD47 and SIRPalpha (CD172a).