FAILURE TO DEMONSTRATE LONG-LIVED MHC SATURATION BOTH IN-VITRO AND IN-VIVO - IMPLICATIONS FOR THERAPEUTIC POTENTIAL OF MHC-BLOCKING PEPTIDES

Peptides that bind with high affinity to class II MHC molecules can in hibit T cell activation both in vitro and in vivo. Thus, they have bee n suggested as potential therapeutic agents for MHC-associated autoimm une diseases. We have constructed nonnatural peptides with high affini ty for certain disease-associated MHC alleles. More specifically, a pa rticular peptide, designated as CY-760.50, was found to have a high bi nding affinity for DR1, slow dissociation kinetics after binding to MH C, and prolonged stability in human serum. However, when the ability o f this peptide to block peptide presentation to an influenza hemagglut inin 307-319 peptide-specific, DR1-restricted T cell clone was examine d, it was found that MHC blockade could only be achieved when high con centrations of peptide were present along with Ag in the fluid phase. Thus, pretreatment of APC with MHC class II blocker, followed by remov al of unbound blocker, did not result in saturation of MHC molecules, because practically immediate reacquisition of Ag-presenting capacity was observed after removal of fluid phase blocker. The pharmacokinetic behavior and the duration of blocking activity of CY-760.50 were also examined in vivo, taking advantage of the fact that the mouse MHC cla ss II molecule I-A(b) also bound CY-760.50 with high affinity. CY-760. 50 administered i.v. to C57BL/6 mice was rapidly cleared from the circ ulation and virtually undetectable in the serum 10 min after injection . This fast clearance rate was paralleled by a similarly short duratio n of the MHC blockade effect. These in vivo results have implications concerning the biology of peptide-MHC interactions, and suggest that M HC blockade may not be feasible as a therapeutic approach unless effec tive concentrations of inhibitor can be maintained over extended perio ds of time in the extracellular fluids.