REQUIREMENT FOR P56(LCK) TYROSINE KINASE ACTIVATION IN T-CELL RECEPTOR-MEDIATED THYMIC SELECTION

The nonreceptor protein tyrosine kinase p56(lck) (Lck) serves as a fun damental regulator of thymocyte development by delivering signals from the pre-T cell receptor (pre-TCR) that permit subsequent maturation. However, considerable evidence supports the view that Lck also partici pates in signal transduction from the mature TCR. We have tested this conjecture by expressing a dominant-negative form of Lck under the con trol of a promoter element (the distal kk promoter) that directs high expression in CD4(+)CD8(+) thymocytes, mature thymocytes, and peripher al T cells, thereby avoiding complications that result from the well-d ocumented ability of dominant-negative Lck to block very early events in thymocyte maturation. Here we report that expression of the catalyt ically inactive Lck protein at twice normal concentrations inhibits th ymocyte positive selection by as much as 80%, while leaving other aspe cts of T cell maturation intact. This effect was studied in more detai l in mice simultaneously bearing the male-specific H-Y alpha/beta TCR transgene and ovalbumin-specific DO10 alpha/beta TCR transgene, where even equimolar expression of the dominant-negative Lck protein substan tially vitiated the positive selection process. Although deletion of H -Y alpha/beta thymocytes proceeded normally in male mice despite the p resence of catalytically inactive Lck, modest inhibition of superantig en-mediated deletion was in some cases observed. These data further im plicate Lck in the propagation of all TCR-derived signals, and indicat e that even very modest deficiencies in the representation of function al Lck molecules could, in humans, profoundly alter the character of t he peripheral TCR repertoire.