The role of Fyn in regulating T cell responses
We have addressed the role of Fyn in T cells using two different transgenic mouse systems. Using the class I-restricted F5 TCR transgenic model, we found that Fyn CD8 cells showed normal triggering and entry into cell division over a range of peptide concentrations, but subsequently proliferated more than WT CD8 cells. Increased proliferation of F5 Fyn cells was driven by their ability to produce more IL-2, which occurred only after cross-linking TCR and CD8 molecules with antibodies or pMHC, not by triggering through the TCR alone. Additionally, F5 Fyn cells showed differences in the induction of specific AP I members that could influence IL-2 production. F5 Fyn7 cells required only 1 hr stimulation with Ag to commit to IL-2 production, whereas F5 WT cells needed -5 hrs, and yet IL-2 production by Fyn cells remained sensitive to the Src-inhibitor PP2, added after 3 hrs. These data suggest that loss of Fyn uncouples Lck activity from the need for sustained exposure to antigen and therefore, that Fyn acts as a negative regulator to limit IL-2 production in CD8 T cells. Not only was proliferation of F5 Fyn7 cells increased compared to WT controls, but so was the CTL response as judged by effector molecule expression and also specific killing activity. Like the increase in proliferation, the improved generation/function of Fyn7 CTL compared to WT was dependent on the levels of IL-2 produced by these cells. In our second transgenic model, Fyn7 mice expressing reduced amounts of Lck were generated. These animals develop a fatal lymphoproliferative disorder, characterized by gut pathology and weight loss, an expansion of effector-memory like CD4*CD44" cells, and the appearance of anti-nuclear antibodies (ANA). Disease was ameliorated by turning off Lck expression, suggesting that an appropriate balance of Lck and Fyn activity is required in vivo to achieve cell survival without stimulating autoimmunity. Our data provides functional in vivo and in vitro evidence for a regulatory role for Fyn in both CD4 and CD8 T cells.