ACTIVATION OF EXTRACELLULAR SIGNAL-REGULATED PROTEIN-KINASE (ERK MAP KINASE) FOLLOWING CD28 CROSS-LINKING - ACTIVATION IN CELLS LACKING P56(LCK)/

T lymphocytes require two signals for activation. Recognition of antig en/MHC complexes by the T cell receptor delivers the first signal, whi le a second signal, delivered by the cell surface receptors CD80 and/o r CD86 binding to the T cell surface molecule CD28, has been shown to be effective for the initiation of effective T cell responses. While s ome of the cytoplasmic effector molecules involved in T cell receptor signaling is known, little is known regarding those involved in the co -stimulation of T cells by CD28. Using the T cell leukemic cell line J urkat as a model for T cell activation, we demonstrate that cross-link ing CD28 using monoclonal antibodies causes tyrosine phosphorylation a nd activation of MAP kinase/ERK. This activation was rapid, peaking at approximately 5 minutes post CD28 cross-linking, and transient. Activ ation of MAP kinase/ERK occurred 3 fold less efficiently in a Jurkat l ine lacking functional p56(Ick) (JCAM.1), and was almost undetectable in a line lacking CD45 (J45.01). These results suggest that CD28 cross -linking can activate intracellular signaling pathways via several dif ferent tyrosine kinases. Thus CD28 signaling can activate src family k inases Ick and fyn, as well as the Tec family kinase emt/itk. Activati on of any one or a combination of these tyrosine kinases may be suffic ient for the activation of MAPK following CD28 cross-linking. Activati on of MAPK has been shown to cause activation of AP-1 and other transc ription factors via serine and/or threonine phosphorylation. This obse rvation thus connects signaling via CD28 to activation of nuclear tran scription factors required for the transcription of T cell genes inclu ding lymphokines such as IL-2 and GM-CSF