Immunoreceptor tyrosine-based activation motif is required to signal pathways of receptor-mediated growth arrest and apoptosis in murine B lymphoma cells

The B cell Ag receptor is a multimeric protein complex consisting of the ligand binding mIg and the Igα/Igβ heterodimer. The cytoplasmic tails of Igα and Igβ both contain a consensus sequence termed the immunoreceptor tyrosine-based activation motif (ITAM). This motif is believed to play a critical role in the receptor-mediated signal transduction. To explore the role of ITAM in signaling for B cell death (apoptosis), we transfected CH31 cells, an immature B lymphoma cell line, with expression vectors encoding for the CD8 extracellular/transmembrane domains and the cytoplasmic signal- transducing domain (ITAM) of Igα or Igβ, respectively. Here, we demonstrate that cross-linking of CD8:Igα or CD8:Igβ with anti-CD8 mAb effectively induced cell growth arrest and apoptosis characterized by [³H]thymidine release and DNA fragmentation; in contrast, CD8:γ2a or truncated CD8:Igα lacking the ITAM could not do so. Moreover, selective point mutation of either of the two conserved tyrosine residues within the ITAM, but not the nonconserved tyrosine, completely abrogated the ability of this motif to mediate cell death signals. These findings clearly indicate that ITAM is a critical component required for transmitting growth arrest and apoptotic signals, and that these functions of ITAM are positively regulated by tyrosine phosphorylation.