Class II molecules are believed to influence immune responses by selectively binding antigen-derived peptides for recognition by T cells. In Goodpasture's (antiglomerular basement membrane) disease, autoimmunity to the NC1 domain of the ␣3-chain of type IV collagen (␣3(IV)NC1) is strongly associated with HLA-DR15. We have examined the influence of the peptide binding preferences of DR15 molecules on the selection of ␣3(IV)NC1-derived peptides displayed bound to DR15 molecules on the surface of ␣3(IV)NC1-pulsed DR15-homozygous Epstein-Barr virus-transformed human B cells. The preferences of DR15 molecules were investigated using a panel of 24 overlapping peptides spanning the sequence of ␣3(IV)NC1. The ␣3(IV)NC1-derived peptides selected for display to T cells were determined by biochemical analysis as reported previously (Phelps, R. G., Turner, A. N., and Rees, A. J. (1996) J. Biol. Chem.
271, 18549 -18553).Three nested sets of naturally presented ␣3(IV)NC1 peptides were detectable bound to DR15 molecules. Peptides representative of each nested set bound to DR15 molecules, but almost two-thirds of the ␣3(IV)NC1 peptides studied had as good or better DR15 affinity than those identified as naturally processed. Thus ␣3(IV)NC1 presentation to T cells is determined more by "processing factors" than by the preferences of relatively indiscriminate DR15 molecules. The results have important implications for the use of class II peptide binding data to aid identification of potential T cell epitopes, especially for antigens which, like ␣3(IV)NC1, contain many sequences able to bind class II molecules.
Antigen presenting cells (APC)1 potentially exert a profound influence on immune responses, including those to self antigens, because they regulate the way T cells recognize antigens. CD4 T cells recognize antigens in the form of processed peptides presented bound to MHC class II molecules on the surface of class II positive APC types (1, 2). These include cells important in the initiation of immune responses, such as dendritic cells, B cells, and macrophages, and cells important in regulating the T cell repertoire and self-tolerance, such as thymic epithelia. How APC select antigen-derived peptides for display to T cells therefore not only constrains the peptide specificity of responding T cells, but also influences the repertoire of T cells available to mount immune responses. Furthermore, the way APC present antigens may determine the immunodominant T cell response, which at least for some exogenous antigens is directed at the antigen-derived peptide displayed at the highest level (3). There is therefore great interest in understanding how APC generate antigen-derived peptides and make a selection for display to T cells, both as an approach to identifying T cell epitopes for specific immune modulation and toward understanding the basic biology of immune responses.APC, like many cell types, internalize extracellular proteins into their endosomal/lysosomal pathway where denaturing (low pH and reducing) conditions promote th...
