An immunological role for the CD8 beta-chain

Mature T cells can be functionally divided into two categories distinguished by surface expression of either CD4 or CD8, which in turn corresponds to restriction by and binding to class II or class I major histocompatibility complex proteins, respectively. CD8 can be expressed as a homodimer of the alpha-chain, or as a heterodimer of alpha- and beta-chains on human and mouse T cells, although most peripheral T cells seem to express CD8 alpha beta heterodimers exclusively (reviewed in ref. 9). Functional characterization of CD8 has focused primarily on the effect of the alpha-chain, which enhances or reconstitutes T-cell responses in homodimeric form and may play a specific role in thymic selection. In contrast, no role has been ascribed to CD8 beta or alpha beta heterodimers specifically. Here we show that CD8 alpha beta transfectants produce more interleukin-2 than CD8 alpha transfectants in response to specific stimuli. Increased interleukin-2 is also observed in cells expressing hybrid CD8 beta-alpha molecules (extracellular CD8 beta plus CD8 alpha transmembrane and cytoplasmic regions) on their surface. These results indicate that external portions of CD8 beta could be critical and that they may act independently of CD8 alpha in mediating their augmentation effect.