Characterization of mouse CD229 (Ly9), a leukocyte cell surface molecule of the CD150 (SLAM) family

Mouse CD229 (Ly9) is a cell surface molecule of the CD150 (signaling lymphocyte activation molecule) family. This family consists of nine leukocyte receptors of the immunoglobulin superfamily that are involved in leukocyte activation. CD229 binds to SAP, a protein encoded by the gene for X‐linked lymphoproliferative disease. In this study, mouse CD229 expression was assessed with a new CD229‐specific monoclonal antibody (mAb) (Ly9.ab3), raised using CD229‐transfected cells. CD229 was expressed on Sca‐1+c‐kit+Lin− hematopoietic stem cells, and this expression increased during lymphocyte maturation. Virtually, all T and B cells expressed high levels of CD229. CD229 was absent on granulocytes, bone marrow‐derived dendritic cells, platelets, and red blood cells (RBCs). However, it was expressed at significant levels on monocytes, indicating that it is also expressed on mouse myeloid cells. We also show that natural killer cells, natural killer T cells, and B1 cells express very high levels of this molecule. In vitro functional experiments showed that ligation of CD229 inhibited the expression of the activation markers CD69 and CD25 on T lymphocytes in response to anti‐CD3 stimulation. Moreover, this reduced activation was concurrent with a reduction in cytokine production. Our results show that CD229 is a pan‐lymphocyte marker and indicate that mAbs against CD229 are able to down‐modulate T‐cell activation.