Lateral dynamics of major histocompatibility complex class II molecules bound with agonist peptide or altered peptide ligands.

We examined the lateral diffusion of I-Ad on A20 cells following the binding of ovalbumin-derived peptides. The peptides were OVA323-339 and OVA325-335 and a related peptide OVA325-335s substituted H331Q. Only OVA323-339 and OVA325-335 were effectively presented by A20 cells to DO-11.10/S4.4 T cells as assessed by IL-2 production. Fluorescence photobleaching recovery (FPR) measurements showed anti-I-Ad to have a lateral diffusion coefficient on untreated A20 cells of 1.8 +/- 1.0 x 10(-10) cm2 s-1 at 25 degrees C with fluorescence recovery after photobleaching greater than 50%. After 24 h incubation of A20 cells with OVA323-339 or OVA325-335, a subpopulation of A20 cells appeared that were approximately half the size of untreated A20 cells. Culture of A20 with OVA325-355s did not stimulate DO-11.10 cells or induce a size change in A20 cells. Class II molecules were laterally immobile on these small cells with fluorescence recoveries after photobleaching of less than 20%. The relative number of small cells in the A20 cell population was correlated with the immunogenicity of the peptides. These results suggest that immobilization of surface I-Ad may be an important event in antigen presentation.