Signal Transduction in Lymphocyte Homeostasis
The laboratory is focused on the elucidation of the temporal and spatial subcellular distribution and activation of second messenger systems coupling the T cell antigen receptor complex (TcR/CD3) to mechanisms underpinning T lymphocyte homeostasis. We have characterized the roles mediated by transmembrane accessory activation molecules (AAM) including CD4/8, CD45, CD25, CD28 and glycosylphosphatidylinositol anchored proteins (GPI-AP) can be extreme, resulting in perturbation of cell growth control, anergy, or death. Our objectives are to: characterize the ordered interaction of AAM with TcR/CD3; define the sequence and temporal engagement of the second messenger generating systems engaged; and to characterize the TcR/CD3/AAM induced spatial re-distribution of the second messenger generating systems that govern cellular homeostasis. Our approach utilizes primary T cells; T cell clonal variants; transgenesis: and shRNA mediated targeted gene knock down enabling a genetic analysis and facilitating the biochemical characterization underpinning the signaling phenotype.
We have characterized the unique yet interdependent temporal and spatial regulation of two src -family protein tyrosine kinases, Lck and Fyn during proximal TcR/CD3 signaling. Specifically, lipid rafts (LR) function to segregate Lck and Fyn. Upon antigen receptor engagement, Lck is activated within seconds outside LR, followed by its translocation into LR where is physically associates with co-localized Fyn and induces its activation. Genetic models and structure function analyses revealed a c-terminal sequence common to all members of the Lck subfamily of src family PTK that predicates their partitioning to LR. The role of Fyn in TcR/CD3 signalling has been enigmatic and largely ignored, as Fyn deficiency had not been associated with immune dysfunction.
Our more recent endeavours are focused on the role of GPI-AP in T cell homeostasis. Using both clonal and primary T cell models, we have revealed that GPI-AP deficiency results in hyper-responsive TcR/CD3 signalling and dysregulated growth. Specifically, TcR/CD3 induced IL-2 production is enhanced roughly 100-fold and prolonged by days/weeks in GPI-AP T cell variants. Characterizing the mechanisms underpinning this phenotype proffers new insights into the regulation of lymphocyte homeostasis and the cancer phenotype/genotype.