THE PROTEIN-TYROSINE KINASE P56(LCK) REGULATES TCR EXPRESSION AND T-CELL SELECTION

The role of the protein tyrosine kinase (PTK), p56(lck), in T cell dev elopment was evaluated by mating TCR transgenic mice with transgenic m ice that expressed lckF505, a constitutively activated form of p56(lck ) which is under the control of the lck proximal promoter element. The TCR transgenic mice expressed either a receptor specific for the male antigen presented by D-b (H-Y TCR) or a receptor specific for pigeon cytochrome c peptide presented by I-E(k) class II MHC molecules (AND T CR). The lckF505 transgene caused lower TCR expression in immature CD4 (+)CD8(+) thymocytes from normal and TCR transgenic mice. Consistent w ith the conclusion that activated p56(lck) causes lower TCR expression , the PTK inhibitor, herbimycin A, was able to restore TCR expression to normal levels in CD4(+)CD8(+) thymocytes from TCR/lckF505 doubly tr ansgenic mice. However, despite lower TCR expression, calcium mobiliza tion was only moderately reduced in CD4(+)CD8(+) thymocytes from H-Y T CR/lckF505 doubly transgenic mice. Furthermore, negative selection of CD4(+)CD8(+) thymocytes expressing the H-Y TCR occurred efficiently in H-Y TCR/lckF505 doubly transgenic male mice despite lower TCR levels. By contrast, analysis of H-Y TCR/lckF505 and AND TCR/lckF505 doubly t ransgenic mice showed that positive selection in these mice was reduce d by 4- to 5-fold by thelckF505 transgene. The smaller proportion of c ells that were positively selected in doubly transgenic lckF505 mice e xpressed normal levels of TCR but higher levels of the appropriate CD4 or CD8 co-receptor molecule. These results indicate that the positive selection of thymocytes is regulated by the enzymatic activity of p56 lck.