Intracellular rate-limiting steps in MHC class I antigen processing

Quantitative aspects of the endogenous pathway of Ag processing and presentation by MHC class I molecules to CB8(+) CTL were analyzed over a wide range of Ag expression in recombinant vaccinia virus-infected cells expressing beta-galactosidase as model Ag, Only the amount of starting Ag was varied, leaving other factors unaltered, Below a certain level of Ag synthesis, increasing protein amounts led to a sharp rise in recognition by CTL, Higher levels of Ag expression led to a saturation point, which intracellularly limited the number of naturally processed peptides bound to MHC and thereby also CTL recognition, The rate-limiting step was located at the binding of the antigenic peptide to MHC inside the vaccinia virus-infected cell or before this event.