Differential contribution of Lck and Fyn protein tyrosine kinases to intraepithelial lymphocyte development

The developmental stages and the role of protein tyrosine kinases (PTK) in the maturation of CD3⁺CD8αα⁺ intraepithelial lymphocytes (IEL) have not been extensively characterized. However, comparisons of thymic and extrathymic T cell development indicate that these processes involve some distinct signaling and selection events. We used mice deficient in Lck, Fyn, or both Lck and Fyn to analyze the role that these src-family PTK play in IEL development. In contrast to thymocyte development, we found that all IEL subsets develop in mice deficient for either kinase alone. However, lck^-/- animals exhibited reduced numbers of TcRαβ⁺CD8αα⁺ IEL, indicating that Lck is important in the development of these cells. Mice which lack both Lck and Fyn fail to generate TcRαβ⁺ IEL, suggesting that signaling through the preTcR, mediated by Lck and, to a lesser extent Fyn, is required for maturation of all TcRαβ⁺ IEL lineages. Interestingly, a small population of TcRγδ⁺CD8αα⁺ cells are apparent in lck^-/-fyn^-/- animals, demonstrating that TcRαβ⁺CD8αα⁺ and TcRγδ⁺CD8αα⁺ IEL have distinct PTK requirements for their development or expansion. CD3⁻-CD8α⁻CD44⁺ and CD3⁻CD8αα⁺CD16/32⁺B220⁺ cells comprise the majority of IEL in both lck^-/-fyn^-/- and rag^-/- mice, while they are poorly represented in wild-type controls. Comparison of the cell surface phenotype of these putative precursor IEL in lck^-/-fyn^-/- and rag^-/- animals suggests that IEL maturation in these animals is arrested at an equivalent developmental stage. Overall, the data presented demonstrate that signals mediated by Lck or Fyn direct TcRαβ⁺CD8αα⁺ IEL maturation but are dispensable for the development of TcRγδ⁺CD8αα⁺ IEL.