Dr. AndrÃ© Veillette has had a longstanding interest in elucidating the molecular mechanisms by which immune cells are regulated. In the past, he discovered that two important receptors expressed on T cells, CD4 and CD8, are physically associated with an intracellular enzyme known as the â??protein tyrosine kinaseâ? Lck. He and his team uncovered that Lck is implicated in the initiation of the activation of T cells by antigens. A similar role was demonstrated for Fyn, another protein tyrosine kinase present in T cells. Dr. Veillette's laboratory also found that Csk, a third protein tyrosine kinase, suppresses the activation of T cells, as a result of its capacity to inactivate Lck and Fyn. This function requires the association of Csk with PTPN22/Lyp/PEP, an enzyme referred to as a â?protein tyrosine phosphataseâ?. Mutations of PTPN22/Lyp/PEP that interfere with the ability to associate with Csk were shown to exist in humans with various types of auto-immune diseases including diabetes, rheumatoid arthritis and lupus.