IMAGING ANTIGEN RECOGNITION BY NAIVE CD4(-CELLS - COMPULSORY CYTOSKELETAL ALTERATIONS FOR THE TRIGGERING OF AN INTRACELLULAR CALCIUM RESPONSE() T)

Antigen recognition was analyzed at the single-cell level by using for the first time T cells which were not altered by in vitro selection, transfection or immortalization. The first consequence of antigen reco gnition by ex vivo naive CD4(+) T cells from T cell receptor (TCR)tran sgenic mice is the formation of a ''contact zone'' with the B cell pre senting the antigen. The T cell intracellular calcium (Ca2+) response begins after a delay of 30 s on average, following the formation of th e contact zone. The T cell response is entirely inhibited by either pr otein tyrosine kinase or actin polymerization inhibitors but, surprisi ngly, it is insensitive to inhibitors of phosphoinositide 3-kinase. Mo reover, inhibition of microtubule polymerization and use of Ca2+-free medium do not prevent the beginning of the T cell response, but do red uce the stability of the contact zone and/or the amplitude of the Ca2 plateau. The critical involvement of the cytoskeleton in antigen reco gnition on B cells introduces a checkpoint in T cell activation: the i nitial TCR engagement triggers a Ca2+ response only after an amplifica tion step corresponding to a cytoskeleton-controlled increase in the n umber of engaged TCR.