Identification of a macrophage antigen-processing event required for I-region-restricted antigen presentation to T lymphocytes.

The mechanism of macrophage-antigen handling was studied using a system that involves the quantitation of the antigen-specific binding of Listeria monocytogenes-immune T cells to macrophages. Specific T cells did not bind to native antigen. Because the specific binding of T cells to macrophages could be measured during a short (5- to 15-min) interaction, it was possible to follow the temporal development of a T cell-binding substrate with increasing time of antigen-macrophage interaction. In contrast to the rapid (5-min) uptake of Listeria by macrophages, the development of T cell-binding ability required a 30- and 60-min period of antigen-macrophage interaction. During this processing period, Listeria organisms bound to the macrophage surface were ingested and partially catabolized. Unlike antigen uptake, antigen processing was a temperature-dependent and energy-requiring event. Although macrophages treated with paraformaldehyde before antigen processing did not develop T cell-binding activity, macrophages treated with paraformaldehyde after a 60-min antigen-processing period retained T cell-binding ability. The kinetics of antigen catabolism correlated with antigen processing, and inhibition of antigen catabolism was associated with a corresponding inhibition of antigen processing for T cell binding. Anti-Ia antibodies had no effect on Listeria uptake of catabolism. These results supply direct evidence for a macrophage-antigen processing event relevant to T cell recognition of antigen.