Kozik et al. hypothesized that inefficient antigen transport from endosomes to the cytosol may restrict DC cross-presentation. Screening over 700 FDA-approved drugs, several were identified (including prazosin and tamoxifen) that increased antigen import. Mapping subcellular protein localization indicated movement of soluble lysosome-associated proteins to the cytosol. Prazosin accumulated within lysosomes along with antigen, increasing lysosome permeability and leakage of contents. Prazosin treatment increased DC cross-presentation to CD8+, but not CD4+, T cells without DC maturation in vitro, and with anti-PD-1 slowed B16-OVA growth in vivo.
Contributed by Alex Najibi
ABSTRACT: Cross-presentation of antigens by dendritic cells (DCs) is critical for initiation of anti-tumor immune responses. Yet, key steps involved in trafficking of antigens taken up by DCs remain incompletely understood. Here, we screen 700 US Food and Drug Administration (FDA)-approved drugs and identify 37 enhancers of antigen import from endolysosomes into the cytosol. To reveal their mechanism of action, we generate proteomic organellar maps of control and drug-treated DCs (focusing on two compounds, prazosin and tamoxifen). By combining organellar mapping, quantitative proteomics, and microscopy, we conclude that import enhancers undergo lysosomal trapping leading to membrane permeation and antigen release. Enhancing antigen import facilitates cross-presentation of soluble and cell-associated antigens. Systemic administration of prazosin leads to reduced growth of MC38 tumors and to a synergistic effect with checkpoint immunotherapy in a melanoma model. Thus, inefficient antigen import into the cytosol limits antigen cross-presentation, restraining the potency of anti-tumor immune responses and efficacy of checkpoint blockers.