Antigen conformation determines processing requirements for T-cell activation.

Abstract

These studies determined the difference in requirements for processing and presentation to a single T-cell clone of 4 different forms of the same epitope of sperm whale myoglobin: the native protein, 2 conformationally altered forms of the potein, or a 22-residue antigenic peptide fragment. The [mouse] T-cell clone was I-Ed-restricted and specific for an epitope on the CNBr fragment 132-153 involving Lys-140. As inhibitors of macrophage processing of antigen, several agents that inhibit lysosomal function were used: the weak bases chloroquine and NH4Cl, the cationic ionophore monensin and the competitive protease inhibitor leupeptin. When these agents were used to inhibit processing of antigen by presenting cells and then washed out before T cells were added to culture, they inhibited the presentation of native antigen but not of fragment 132-153. The intact but denatured form, S-methylmyoglobin, behaved like the fragment, not like the native protein. Apomyoglobin was intermediate in susceptibility to inhibition. Thus, native myoglobin requires a processing step that appears to involve lysosmal proteolysis, which is not required by fragment 132-153 or the denatured unfolded forms. For an antigen the size of myoglobin (MW 17,800), it appears that unfolding of the native conformation, rather than further reduction in size, is the critical parameter determining the need for processing. Since a major difference between native myoglobin and the other forms is the greater accessibility in the latter of sites, such as hydrophobic residues, buried in the native protein, processing may be necessary to expose these sites, perhaps for interaction with the cell membrane or the Ia of the antigen-presenting cell.