BOTH HIGH AND LOW AVIDITY ANTIBODIES TO THE T-CELL RECEPTOR CAN HAVE AGONIST OR ANTAGONIST ACTIVITY

Anti-TCR antibodies can activate or block the activation of T cells. I n the present experiments, we have shown that different monoclonal ant ibodies to the same TCR can have either agonist or antagonist activity , and we have examined the relationship between these functional effec ts and the avidity of the antibody for the TCR. We show here that it i s not the avidity of an anti-TCR antibody that determines whether it a cts as an agonist or an antagonist. Moreover, we show that monovalent Fab fragments of agonist antibodies produce detectable changes in T ce ll behavior. These data suggest that T cell activation may involve not just aggregation of the TCR but also some induced change in individua l ligated receptors, and that agonists may produce this change while a ntagonists do not. We argue that similar effects may apply to peptide- MHC ligands as well.