Differing processing requirements of four recombinant antigens containing a single defined T-cell epitope for presentation by major histocompatibility complex class II

A set of predictive rules governing the likelihood of generating a particular peptide-major histocompatibility complex (MHC) class II complex from an intact antigen has not been fully elucidated. We investigated the influence of positional and structural constraints in the region of the epitope by designing a set of recombinant antigens that each contained the well-characterized T-cell epitope moth cytochrome c (MCC) (88-103), which is specifically recognized by the monoclonal antibody (mAb) D4 when complexed with H-2Ek. Our model antigens contained MCC(88-103) either peripherally, at or towards the C-terminus, or internally. Their abilities to bind directly to soluble H-2Ek, and the extent of D4 epitope formation from them by antigen processing-competent and -incompetent cell lines, were determined. Here we report that three of these four antigens yielded MCC(88-103)/H-2Ek complexes independently of the conventional MHC class II antigen-processing and presentation pathway, and in each case the epitope was carried peripherally; two bound directly as intact proteins, probably as a result of spatial separation of the epitope from the major globular domain, and one was processed to peptide by a cell-surface protease. One protein, which carried the epitope inserted into an internal loop, acted as a conventional processing-dependent MCC(88-103) delivery vehicle. Thus, this epitope has different presentation requirements depending on its context. These antigens constitute a panel whose framework could be modified to further define predictive rules for antigen processing for presentation through the different MHC class II complex-generating pathways.