CD84 is up-regulated on a major population of human memory B cells and recruits the SH2 domain containing proteins SAP and EAT-2

CD84 is a member of the CD2 subset of the immunoglobulin superfamily of cell surface receptors. Several members of this family are involved in the activation of T cells and NK cells. Although CD84 was originally cloned from a human B cell line cDNA library, very little is known regarding its biology on primary human leukocytes. We investigated the expression and biochemistry of CD84 on human B cells. CD84 was expressed on B cells in peripheral blood, spleen and cord blood. Two populations of splenic B cells could be resolved, CD84(lo) and CD84(hi). CD84(hi) B cells represented a subset of memory B cells as demonstrated by increased cell size, co-expression of the memory B cell-specific marker CD27, somatically mutated Ig variable region genes, and increased proliferation compared to CD84(lo) B cells. CD84 became rapidly phosphorylated on tyrosine residues following ligation with a specific monoclonal antibody and recruited the cytoplasmic adaptor proteins SAP and EAT-2. The ability of CD84 to undergo tyrosine phosphorylation and to recruit these SH2 domain-containing proteins suggests it may function in the activation of B cells, particularly memory cells, and its signal transduction pathway may utilize SAP and/or EAT-2. Thus, investigation of expression and function of CD84 and CD27 is likely to contribute to a greater understanding of the development and biology of memory B cells in normal and immunocompromised hosts.