HLA-DR-restricted presentation of purified myelin basic protein is independent of intracellular processing

Antigen presentation to CD4⁺ T cells involves intracellular antigen processing and loading of peptides onto newly synthesized major histocompatibility complex (MHC)-class II molecules. Some antigens, such as the lipid-bound, native form of myelin basic protein (LB-MBP) can also be presented by recycling of cell surface MHC class II molecules. The data reported here demonstrate that a preparation of highly purified, delipidated MBP (HP-MBP) follows yet another presentation pathway. Similar to LB-MBP, presentation of HP-MBP to HLA-DR1-restricted T cells was independent of HLA-DM, of newly synthesized proteins, and of invariant chain expression. However, in contrast to LB-MBP, presentation of HP-MBP was also independent of internalization of surface HLA-DR molecules. The different requirements for the presentation of the two molecular forms of MBP were further confirmed by use of the protease inhibitor E64: presentation of LB-MBP but not of HP-MBP was inhibited after treatment of target cells with E64. Furthermore, intact HP-MPB bound to isolated HLA-DR molecules in vitro with an association rate that was considerably faster than that of short peptides. These results show that presentation of HP-MBP is independent of intracellular processing and suggest that it may be presented to T cells by direct binding to surface HLA-DR molecules.