What question did this study set out to answer?

This research investigates how bystander T cell activation by vaccination can enhance anti-tumor immunity in solid tumors.

April 5, 2026

Abstract 1350: Vaccine stimulation of bystander t cells in the lymph node unexpectedly promotes tumor-specific host immunity to regress solid tumors.

Key Points

This research investigates how bystander T cell activation by vaccination can enhance anti-tumor immunity in solid tumors.
Analyzed T cell responses in a melanoma patient receiving a COVID-19 vaccine.
Conducted experiments in mouse models using a lipid polymer vaccine to activate bystander CAR T cells.
Measured tumor volume changes and cytokine responses after vaccination.
Tumor volume reduced by over 70% within 72 hours post-vaccination in mouse models.
Activation of bystander CAR T cells prompted a significant immune response in the tumor microenvironment.
Co-treatment with anti-PD-L1 improved long-term tumor control and survival in pancreatic cancer models.

Abstract

Abstract Personalized neoantigen cancer vaccines have been at the forefront of therapeutic vaccination. However, although these neoantigen vaccines do induce neoantigen-specific T cell responses, the clinical effects are limited. Logistical challenges, such as the time required for developing personalized neoantigen vaccines, also limit their clinical utility. We were recently intrigued by a case at Penn where one of our patients had a 7.6 cm melanoma tumor and experienced a complete response after receiving the COVID-19 vaccine alone, consistent with a recent report that cancer patients who received the SARS-CoV-2 vaccination had improved survival. Further studies identified abundant SARS-CoV-specific T cells present in both the draining lymph node (LN) and in the necrotic tumor bed. At the same time, our preliminary experiment in mice showed a similar phenomenon that the activation of tumor-irrelevant CAR T cells by vaccination reinvigorated the host’s tumor-specific immunity and regressed established solid tumors. Specifically, when we used our recently developed lipid polymer vaccine (Ma et al, Science, 2019; Cell, 2023; Grzywa et al, Nat BME, 2025) carrying a chemical ligand Fluorescein (FITC) to specifically stimulate adoptively transferred bystander FITC-targeting chimeric antigen receptor (CAR) T cells in vivo in the lymph node as a negative control, yet unexpectedly we observed distal tumor regression to a level that’s comparable to vaccine stimulation of tumor-specific CAR T in syngeneic mouse models of pancreatic cancer and melanoma. In both settings, tumor volume was reduced by70% within the first 72 hours, and this effect persisted for up to 2 weeks (with some cures of the melanoma). This marked antigen-independent tumor control is completely lost in Rag1-knockout (no endogenous T cells) and BatF3-knockout (deficient in antigen cross-presentation) recipient mice. FITC vaccine boosting of bystander FITC-CAR T induced a rapid and transient cytokine and chemokine release that distally remodels the tumor microenvironment. Notably, IFNg blockade completely abolished this tumor control. In the pancreatic cancer model, IFNg significantly upregulated PD-L1 expression in cancer cells. As a result, co-administration of anti-PD-L1 markedly extended long-term tumor control and animal survival. Collectively, these unexpected findings in both the melanoma patient and syngeneic mouse models of solid tumors suggested a previously unappreciated anti-tumor bystander effect associated with vaccine-mediated T cell activation. Citation Format: Leyuan Ma, Tomasz Grzywa, Ryan Tannir, Gregory L. Beatty, Alexander Huang. Vaccine stimulation of bystander t cells in the lymph node unexpectedly promotes tumor-specific host immunity to regress solid tumors abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1350.

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