CD8 requirements for negative selection events are directly related to the TCR-antigen interaction

Positive selection of class I-restricted T cells has been suggested to always require the surface expression of CD8 molecules on CD4+8+ thymocytes, whilst negative selection was found to be differentially dependent, relating to the antigen being engaged by the T cell receptor (TCR). We have studied the CD8-dependency of positive and negative selection using two TCR-transgenic (Tg) mice models, which both react against the same allo-antigen, H-2kb, back crossed with mice which are deficient for the expression of CD8. Whilst CD8 expression was always required for positive selection of cells expressing high levels of the Tg-TCR, events of negative selection were differentially dependent upon CD8, reflecting the CD8-dependency of the original CTL clones. For one TCR-Tg model (derived from a CD8-dependent CTL clone), deletion of CD4+8+ thymocytes was partially dependent upon the expression of CD8, though there was still selection against Tg-TCR positive CD4+ cells, which normally exit to the periphery expressing low levels of Tg-TCR. For the other model (derived from a CD8-independent CTL clone) negative selection was unaffected by the absence of CD8. For both models the CD4-8- Tg-TCR positive population was unaffected by the absence of CD8, including, for the CD8-independent model, reduced expression of the Tg-TCR/CD3 complex. These results suggest that CD8 expression may be a prerequisite for positive selection, whilst negative selection events can occur in the absence of CD8, depending directly upon the TCR-antigen interaction.