Differentiation And Activation Of Human Cd4 T Cells Is Associated With A Gradual Loss Of Myelin And Lymphocyte Protein

Upon generation of monoclonal antibodies to the T cell antigen receptor/CD3 (TCR/CD3) complex, we isolated mAb MT3, whose reactivity correlates inversely with the production of IFN-gamma by human peripheral blood T lymphocytes. Using eukaryotic expression cloning, we identified the MT3 antigen as myelin-and-lymphocyte (MAL) protein. Flow cytometry analysis demonstrates high surface expression of MAL on all naive CD4(+) T cells whereas MAL expression is diminished on central memory- and almost lost on effector memory T cells. MAL(-) T cells proliferate strongly in response to stimulation with CD3/CD28 antibodies, corroborating that MAL(+) T cells are naive and MAL(-) T cells memory subtypes. Further, resting MAL(-) T cells harbor a larger pool of Ser59- and Tyr394- double phosphorylated lymphocyte-specific kinase (Lck), which is rapidly increased upon in vitro restimulation. Previously, lack of MAL was reported to prevent transport of Lck, the key protein tyrosine kinase of TCR/CD3 signaling to the cell membrane, and to result in strongly impaired human T cell activation. Here, we show that knocking out MAL did not significantly affect Lck membrane localization and immune synapse recruitment, or transcriptional T cell activation. Collectively, our results indicate that loss of MAL is associated with activation-induced differentiation of human T cells but not with impaired membrane localization of Lck or TCR signaling capacity.