The human CD8β isoforms differentially affect signal transduction pathways and monoubiquitination of the M4 isoform regulates signaling through the mTORC1/2/pathway (IRM4P.497)

Abstract

Abstract The CD8αβ coreceptor contributes to T cell signaling by bringing the tyrosine kinase p56lck associated with CD8α to theTCR-CD3 complex. Four isoforms (M1 to M4) of the CD8β chain exist in the great apes and humans that differ in the amino acid sequence of their cytoplasmic tails and their expression pattern. We previously found that the M1 isoform, equivalent to murine CD8β, is the predominant isoform in naïve CD8+ T cells, while the M4 isoform is predominant in effector memory T cells. We expressed each isoform in either T cell tumor lines or peripheral blood T cells, stimulated the cells by crosslinking with anti-CD8 or anti-CD3 antibodies and determined effects on signal transduction pathways using phosphoflow. Differences were observed with anti-CD8 crosslinking. For instance, the M1 isoform showed greater amounts of phospho-Erk (2-3 fold) and the M4 isoform showed greater amounts of phospho-Akt (2-3 fold). In addition, because the M4 isoform uniquely binds the adaptor protein AP-2 and is mono-ubiquitinated on lysines, we analyzed mutant forms of the M4 isoform in signal transduction as well. Mono-ubiquitination appears to be a regulator of the mTORC1/2/pathway important in metabolism as the lysine mutant demonstrated a large increase in phosphorylation of the ribosomal protein S6, in the mTORC1 pathway. This supports the hypothesis that the different CD8β isoforms evolved to make unique contributions to CD8 coreceptor function.