We have recently proposed a new model for the differentiation pathway of al
pha beta TCR thymocytes, with the CD4 and CD8 coreceptors undergoing an une
xpectedly complex series of expression changes. Taking into account this ne
w insight, we reinvestigated the timing of thymic negative selection, We fo
und that, although endogenous superantigen driven thymic negative selection
could occur at different steps during double-positive/single-positive cell
transition, this event was never observed among CD4(low)CD8(low) TCRint CD
69(+) thymocytes, i.e., within the first subset to be generated upon TCR-me
diated activation of immature double-positive cells. We confirm a role for
CD40/CD40L interaction, and the absence of involvement of CD28 costimulatio
n, in thymic deletion in vivo. Surprisingly, we found that thymic negative
selection was impaired in the absence of Fas, but not Fast, molecule expres
sion, Finally, we show involvement in opposing directions for p59(fyn) and
SHP-1 molecules in signaling for thymic negative selection.