SELECTION OF DUAL V-ALPHA T-CELLS

Incomplete allelic exclusion of TCRa gene rearrangement permits the ge neration of dual V alpha T cells, though the issues of their frequency and whether both alpha beta pairs participate in thymic selection hav e not been resolved. Both questions have been investigated using lymph ocytes from mice hemizygous at the TCRa locus and consequently unable to express two rearranged TCRa genes, as background controls. The data presented show that both the frequency of dual V alpha T cells and th e relative expression levels of co-expressed V alpha chains are variab le and are determined by thymic selection. Possession of a V alpha cha in which is inefficiently positively selected appears to increase the likelihood that a second V alpha chain will be cc-expressed, whilst th e relative cell surface levels of a given pair of V alpha chains diffe r between CD4 and CD8 subsets. Further for some but not all V alpha pa irs, dual V alpha T cells appear to express elevated levels of surface TCR. Finally, contrary to previous claims, dual V alpha T cells do no t appear to be relatively frequent amongst immature thymocytes.