MHC class II presentation of endogenous tumor antigen by cellular vaccinesdepends on the endocytic pathway but not H2-M

We have developed cell-based cancer vaccines that activate anti-tumor immun ity by directly presenting endogenously synthesized tumor antigens to CD4() T helper lymphocytes via MHC class II molecules. The vaccines are non-con ventional antigen-presenting cells because they express MHC class II, do no t express invariant chain or H-2M, and preferentially present endogenous an tigen. To further improve therapeutic efficacy we have studied the intracel lular trafficking pathway of MHC class II molecules in the vaccines using e ndoplasmic reticulum-localized lysozyme as a model antigen. Experiments usi ng endocytic and cytosolic pathway inhibitors (chloroquine. primaquine, and brefeldin A) and protease inhibitors (lactacystin, LLnL. E64. and leupepti n) indicate antigen presentation depends on the endocytic pathway. although antigen degradation is not mediated by endosomal or proteasomal proteases. Because H2-M facilitates presentation of exogenous antigen via the endocyt ic pathway, we investigated whether transfection of vaccine cells with H-2M could potentiate endogenous antigen presentation. In contrast to its role in conventional antigen presentation. H-2M had no effect on endogenous anti gen presentation by vaccine cells or on vaccine efficacy. These results sug gest that antigen/MHC class II complexes in the vaccines may follow a novel route for processing and presentation and may produce a repertoire of clas s Ii-restricted peptides different from those presented by professional APC . The therapeutic efficacy of the vaccines, therefore, may reside in their ability to present novel tumor peptides. consequently activating tumor-spec ific CD4(+) T cells that would not otherwise be activated.