Major histocompatibility complex (MHC) class II tumor vaccines present endogenous antigens through a novel pathway

As novel tumor immunotherapies are being developed, much emphasis is being placed on the activation of tumor-specific T cells for treating malignancy. Tumor vaccines have been designed in this laboratory using autologous tumor cells expressing syngeneic MHC class II molecules. The vaccines are effective in eliciting antitumor immunity because they are antigen presenting cells to CD4+ T cells for endogenously encoded tumor antigens. Unlike the well-characterized MHC class II presentation pathway for exogenous antigens, the pathway for presentation of MHC class II-restricted endogenous antigens are poorly understood. Since elucidation of the endogenous antigen presentation pathway in the vaccines may reveal strategies for optimizing vaccine efficacy, we have addressed three questions that may clarify the underlying mechanisms. (1) From which subcellular compartments are potential MHC class II-restricted endogenous antigens derived? (2) What is the intracellular trafficking pathway for presentation of MHC class II-restricted endogenous antigens? (3) How is presentation of endogenous antigens regulated by MHC class II-associated accessory molecules (invariant chain, H2-M and H2-O)?; To address these questions, vaccine cells were transfected with a model antigen targeted to different organelles, including the nuclei, mitochondria, endoplasmic reticulum, and cytosol. Presentation of the model antigen in the vaccines in the presence or absence of the accessory molecules was measured by antigen-specific T cells. The intracellular trafficking pathway for antigens from the endoplasmic reticulum was determined using a variety of inhibitors. Our data demonstrated that the vaccine cells present antigens derived from diverse intracellular compartments. Furthermore, the presentation pathway for endogenous antigens by MHC class II molecules is different from the classical pathway for exogenous antigens. Finally, MHC class II-restricted endogenous antigen presentation is not affected by H2-M nor H2-M plus H2-O, but is inhibited by co-expression of invariant chain. Serendipitously, we discovered that H2-O reduces bacterial superantigen binding to MHC class II molecules and subsequent activation of T cells. Our data indicate that MHC class II molecules present endogenous antigens through a novel pathway in tumor vaccines and that H2-O may regulate bacterial superantigen-mediated immune responses.