Work

676 Total tumor mRNA-LPA vaccines and PD-1 blockade overcome myeloid resistance in metastatic osteosarcoma via CD4 + T cell activation

Year: 2026

Type: conference-abstract

Abstract: Background Osteosarcoma (OSA) is a prevalent cancer among pediatric and young adult that often metastasizes to the lungs. Canine OSA accurately recapitulates the human disease, serving as a reliable translational model. Although immune checkpoint inhibitors (ICI), such as anti-PD-1, have revolutionized oncology, they demonstrate limited efficacy in OSA. We previously developed a total tumor mRNA-lipid particle aggregate (tt-mRNA-LPA) vaccine that markedly extended survival in canines and murine models. Nonetheless, therapeutic resistance persists as a clinical challenge due to the highly immunosuppressive pulmonary microenvironment. Guided by observations in canine trials where sequential anti-PD-1 administration yielded salvage survival benefits, we utilized a preclinical mouse model to investigate the mechanistic synergy between tt-mRNA-LPA and PD-1 blockade.Methods A preclinical pulmonary metastasis model was established via intravenous injection of 1.25x10 6 K7M2 OSA cells in mice. Vaccines were generated using DOTAP liposomes complexed with amplified tumor-derived mRNA. Mice received either no treatment, tt-mRNA-LPA monotherapy, or tt-mRNA-LPA combined with anti-PD-1. To prime the immune landscape, the initial anti-PD-1 dose was administered two days prior to the first vaccine. Survival analytics, high-parameter flow cytometry, spatial immunophenotyping, and ex vivo co-culture assays were utilized to evaluate systemic and local tumor microenvironment (TME) responses.Results Combination therapy provided a significant survival benefit and reduced gross lung tumor nodule burden at day 70 compared to vaccine monotherapy ( p = 0.0165, n = 10). Early TME profiling revealed a marked depletion of immunosuppressive (CD206+ and Arg1+) tumor-associated macrophages (TAMs) in combination cohorts. Combination therapy mitigated T cell exhaustion, significantly reducing the frequency of terminally exhausted (PD-1+ TCF-1- TIM-3+) tumor-infiltrating lymphocytes (TILs) while preserving the stem-like memory progenitor pool. Functionally, combination therapy-derived CD4+ and CD8+ TILs demonstrated superior ex vivo tumoricidal capacity against K7M2 targets and enhanced CD4 activation profiles (CD40L+ CD25+) compared to the vaccine-alone group, which conversely exhibited higher regulatory T cell (FoxP3+ CD25+) infiltration. Spatial mapping of lung lesions from non-responding mice revealed an upregulation of the inhibitory macrophage receptor Vsig4, identifying it as a potential contributor to treatment resistance.Conclusions The combination of tt-mRNA-LPA and anti-PD-1 synergistically enhances survival in metastatic OSA by reprogramming the immunosuppressive myeloid compartment and rescuing T cell effector function. These preclinical findings validate our canine clinical observations, demonstrating that overcoming ICI resistance in solid tumors requires rationally timed, multi-modal approaches. Specifically, dual targeting of T cell exhaustion and Vsig4-mediated macrophage suppression represents a highly promising translatable strategy for treating metastatic osteosarcoma in both veterinary and human oncology.

Source: Journal for ImmunoTherapy of Cancer

DOI: 10.1136/jitc-2026-sitc2026.0676