Characterization of a Novel Receptor CS1 in Human Lymphocytes; Studies in Natural Killer Cells and B-Lymphocytes

The purpose of this study was to investigate the roles of CS1 on human lymphocytes. The molecular and functional characterization of CS1 receptor in the natural killer (NK) cells and B lymphocytes was investigated. CS1 (CRACC, novel Ly9) is a novel member of the CD2 family receptor expressed on natural killer (NK), T cells, activated Bcells and dendritic cells. To examine the existence of isoform of CS1, library from NK cells was screened based on wild type of CS1 (CS1-L). A splice variant form of CS1 (CS1-S), which lacks immunoreceptor tyrosine-based switch motifs (ITSMs) in cytoplasmic domain, was identified. To demonstrate the function of CS1 on human NK cells, transfectants that stably express each isoform were generated. CS1-L was able to mediate redirect cytotoxicity of P815 target cells as well as intracellular calcium influx in the presence of monoclonal antibody against CS1 suggesting that CS1-L is an activating receptor. CS1-S showed no effect on the cytolytic function and calcium influx suggesting that CS1-L and CS1-S may differentially regulate human NK cell functions. Although CS1 was also cloned from cDNA library of human B-lymphocytes as well as of NK cells, very little is known regarding its biology on human B-lymphocytes. Here I investigated the expressions and functions of CS1 in human B cells. Human B cells expresses only CS1-L isoform and the levels of CS1 expression are upregulated after activation in vitro. Importantly, monoclonal antibody of CS1 (1G10 mAb) strongly enhances proliferation of both freshly isolated and activated B cells. The enhanced proliferation effects of CS1 were most prominent on B cells activated by anti-CD40 mAbs and/or IL-4. Human cytokine microarray results indicated CS1 enhanced mRNA transcripts of fms-line tyrosine kinase 3 ligand, lymphotoxin A, tumor necrosis factor, and IL-14 which are related with mostly growth promoting activity. These results suggest that autorine cytokines might be the mediators for the function of CS1 on B cell in which it can induce proliferation of activated B cells. This study suggests that CS1 plays important role in human NK cells and B-lymphocytes.