IFN-γ Treatment Protocol for MHC-I/PD-L1 Pancreatic Tumor Cells Selectively Restores Their TAP-mediated Presentation Competence and CD8 T-cell Priming Potential

Background: Many cancer cells express a major histocompatibility complex class I low/ programmed cell death 1 ligand 1 positive (MHC-I/PD-L1) cell surface profile. For immunotherapy, there is, thus, an urgent need to restore presentation competence of cancer cells with defects in MHC-I processing/presentation combined with immune interventions that tackle the tumor-initiated PD-L1/PD-1 signaling axis. Using pancreatic ductal adenocarcinoma cells (PDACCs) as a model, we here explored if (and how) expression/processing of tumor antigens via transporters associated with antigen processing (TAP) affects priming of CD8 T cells in PD-1/PD-L1-competent/-deficient mice.

Methods: We generated tumor antigen-expressing vectors, immunized TAP-competent/-deficient mice and determined de novo primed CD8 T-cell frequencies by flow cytometry. Similarly, we explored the antigenicity and PD-L1/PD-1 sensitivity of PDACCs versus interferon-γ (IFN-γ)-treated PDACCs in PD-1/PD-L1-competent/deficient mice. The IFN-γ-induced effects on gene and cell surface expression profiles were determined by microarrays and flow cytometry.

Results: We identified two antigens (cripto-1 and an endogenous leukemia virus-derived gp70) that were expressed in the (ER) of PDACCs and induced CD8 T-cell responses either independent (Cripto-1:K/Cr) or dependent (gp70:K/p15E) on TAP by DNA immunization. IFN-γ-treatment of PDACCs in vitro upregulated MHC-I- and TAP- but also PD-L1-expression. Mechanistically, PD-L1/PD-1 signaling was superior to the reconstitution of MHC-I presentation competence, as subcutaneously transplanted IFN-γ-treated PDACCs developed tumors in C57BL/6J and PD-L1 but not in PD-1 mice. Using PDACCs, irradiated at day 3 post-IFN-γ-treatment or PD-L1 knockout PDACCs as vaccines, we could selectively bypass upregulation of PD-L1, preferentially induce TAP-dependent gp70:K/p15E-specific CD8 T cells associated with a weakened PD-1 exhaustion phenotype and reject consecutively injected tumor transplants in C57BL/6J mice.

Conclusions: The IFN-γ-treatment protocol is attractive for cell-based immunotherapies, because it restores TAP-dependent antigen processing in cancer cells, facilitates priming of TAP-dependent effector CD8 T-cell responses without additional check point inhibitors and could be combined with genetic vaccines that complement priming of TAP-independent CD8 T cells.