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The CD8beta ectodomain contributes to the augmented coreceptor function of CD8alphabeta heterodimers relative to CD8alphaalpha homodimers.

Within the lymphoid compartment, CD8 is expressed either as an alphaalpha homodimer or as an alphabeta heterodimer. Prior functional characterization of CD8alpha transfectants has demonstrated that CD8alphaalpha homodimers can reconstitute T cell responses in the absence of the CD8beta subunit. In order to now examine the role of CD8beta in TCR recognition, the CD8alpha cDNA alone or in combination with CD8beta cDNA was transfected into the mouse T cell hybridoma, N15wt, specific for VSV8/Kb. Comparison of antigen-induced IL-2 production reveals that CD8alphabeta+ transfectants are 100-fold more sensitive in molar terms of peptide than CD8alphaalpha+ transfectants. This enhancement of IL-2 production is independent of CD8alpha or CD8beta cytoplasmic tails as demonstrated by analysis of cytoplasmic deletion mutants CD8alpha'beta, CD8alphabeta', and CD8alpha'beta'. These results indicate that the ectodomain of the CD8beta chain greatly enhances the coreceptor function of the CD8alphabeta molecule, at least for certain class I MHC restricted alphabeta TCRs.