| Thymically derived T cells bearing alphabeta TCRs found in the vasculature, lymph nodes and spleen are usually characterized as either CD8+CD4 - or CD8-CD4+ single positive (SP) T cells, commonly referred to as cytolytic T cells and helper T cells, respectively. These T cell populations, despite their exclusive expression of one co-receptor, develop from a common pool of thymocyte progenitors that are double positive (DP) for both co-receptors. This pool provides a diverse repertoire of TCR clones, of which only thymocytes bearing useful TCRs are positively selected to further mature into SP thymocytes. CD8 or CD4 SP lineage commitment requires the respective co-receptor for efficient selection and maturation. However, the co-receptor signals that guide differential commitment and development are not well understood. It is generally thought that Lck is involved, as it has been shown to bind the cytoplasmic tails of both CD8 and CD4. But studies involving Lck deficient co-receptors have questioned the need for this association. Other studies, however, suggest that it is important. Further complexity arises from the recent observation that LAT also binds both co-receptors. We have attempted to directly address the role of CD8 associated Lck in CD8 SP T cell development by generating a chimeric protein where the CD8alpha cytoplasmic tail, known to bind Lck or LAT, is replaced by Lck. The chimeric protein, denoted CD8alpha-Lck, possesses kinase activity and is competent in pairing with CD8beta. The CD8alpha-Lck protein was primarily analyzed in CD8alpha deficient mice harboring the MHC class I restricted TCR, 2C. Robust reconstitution of CD8 SP thymocytes was observed despite a CD8alpha-Lck expression level 2 logs less than CD8 wild type. The reconstituted CD8 SP thymocytes were phenotypically equivalent to their 2C wild type counterparts as assessed by CD2, CD3epsilon, CD5, CD24 and CD69 expression. Further, functional assessment of reconstituted CD8 SP T cells was comparable to their 2C wild type counter parts. The results suggest that CD8's role in mediating efficient CD8 SP thymocyte development is sensitive to associated Lck and that this association may be sufficient in driving the development of bona fide CD8 SP T cells. |
