p56lck Signals for Regulating Thymocyte Development Can Be Distinguished by Their Dependency on Rho Function

The tyrosine kinase p56lck regulates the differentiation and proliferative expansion of pre-T cells. However, nothing is known about other signaling molecules that operate with p56lck to mediate the pleiotropic changes that occur at this stage of thymocyte development. We used a genetic strategy to examine the requirement for the GTPase Rho in p56lck-mediated signals in the thymus. By generating mice double transgenic for a constitutively activated form of p56lck (p56lck^F505) and the Rho inhibitor C3 transferase we were able to compare thymocyte development in mice expressing active p56lck on a wild-type or Rho⁻ background. Thymocytes expressing active p56lck show enhanced proliferation of pre-T cells resulting in increased numbers of late pre-T cells, however, this dramatic effect on pre-T cell proliferation is lost when the p56lck transgene is expressed in thymocytes lacking endogenous Rho GTPase function. Expression of active p56lck also generates double positive (DP) thymocytes with low levels of CD2 antigen expression. Again, p56lck cannot prevent expression of CD2 when expressed on a Rho⁻ background. CD4⁺CD8⁺ DP cells expressing active p56lck have been shown to lack functional α/β–T cell receptor (TCR) complexes due to p56lck-mediated inhibition of TCR gene Vβ-Dβ rearrangement. This inhibition of TCR expression by active p56lck is unimpaired in the absence of Rho function. The signaling pathways that are mediated by p56lck and control thymocyte proliferation, α/β-TCR and CD2 antigen expression can thus be distinguished by their dependency on Rho function.