ROLE OF THIOLS IN THE PRESENTATION OF A SNAKE TOXIN TO MURINE T-CELLS

We isolated and characterized two T hybridomas specific for a highly s table snake toxic protein. One hybridoma, called T1C9, is I-E(d)-restr icted and stimulated by both the native and reduced and carboxymethyla ted (RCM) toxins and by synthetic fragments containing the region 24-3 6. The other hybridoma, called T1B2, is I-A(d)-restricted and stimulat ed by the native toxin, only. Neither the RCM toxin nor any of the ini tial synthetic peptides used in our study could stimulate it. We show that this lack of effect is associated with the presence, in the epito pe-containing fragment, of irreversible blocking groups on cysteine re sidues. Indeed, when the fragment 32-49 has its cysteines involved in either intra-(32-49SS) or mixed disulfides, a stimulation of T1B2 was observed. Fixed APC do not present native toxin to either hybridomas b ut present RCM toxin to T1C9. Strikingly, fixed APC present the peptid e 32-49SS to T1B2; however, we show that this is possible only because the peptide disulfide is reduced. The thiol dependence of this epitop e suggests that the native toxin can stimulate T1B2 only after disulfi de reduction. This reaction may constitute a major step during the pro cessing of the toxin and more generally of any disulfide-containing Ag .