Peptide-based vaccines hold great potential for immunotherapy by stimulating pathogen- or cancer-specific T cell responses. These vaccines rely on the endocytosis of antigenic peptides by dendritic cells (DCs), followed by intracellular proteolytic processing, epitope loading onto major histocompatibility complex (MHC) class I and II molecules, and subsequent presentation to antigen-specific T cells. Despite promising immunogenicity in preclinical models, peptide-based vaccines have largely failed to demonstrate efficacy in clinical trials. A key limitation is the insufficient presentation of antigenic epitopes on MHC molecules. Enhancing intracellular antigen processing could therefore improve vaccine efficacy. In this project, we evaluated the technical feasibility and translational potential of a novel strategy: co-administration of antigenic peptides with phosphatase inhibitors to enhance cross-presentation. Specifically, we assessed the impact of phsophatase inhibition on antigen cross-presentation by human monocyte-derived macrophages and DCs, using T cell reporter cell lines specific for the tumor-associated antigens NY-ESO-1 and gp100. Our findings indicate that phosphatase inhibition, particularly with the SHP-1/2 inhibitor NSC-87877 significantly reduces the cross-presentation efficiency of both antigens. Mechanistically, we demonstrate that SHP-1/2 inhibition disrupts endo/lysosomal acidification and impairs the activation of cathepsin proteases, which are essential for antigen processing. Consequently, this approach does not enhance cross-presentation and may even limit the effectiveness of combination therapies with immune checkpoint inhibitors. In fact, these results challenge current strategies that combine phosphatase inhibitors with checkpoint blockade, as they could have unintended detrimental effects on tumor immune responses. Given these findings, we explored alternative approaches with potential for patentability and commercialization, including co-encapsulation of kinase inhibitors.
