HER2 overexpression is a pivotal oncogenic alteration in NSCLC and is associated with poor prognosis. Existing HER2-targeted therapeutics, including antibody-drug conjugates, have limited anticancer efficacy and prominent systemic toxicity, whereas radiotherapy-mediated tumor immunosuppression further hinders therapeutic outcomes. This study aimed to construct a HER2-targeted mRNA vaccine delivered by lipid nanoparticles (LNPs), designated HER2-mRNA-LNP, and to explore its standalone or combined anti-tumor effects alone or in combination with radiotherapy (RT) in HER2-overexpressing NSCLC xenografts, as well as the underlying immune regulatory mechanisms. We first prepared the HER2-mRNA-LNP vaccine and constructed HER2-overexpressing Lewis lung carcinoma (LLC) cell line (HER2-LLC) to establish matching tumor-bearing xenograft mice. Tumor-bearing mice were divided into four groups: LNP control, HER2-mRNA-LNP, RT, and HER2-mRNA-LNP + RT (combination). Prophylactic and therapeutic effects were assessed by measuring tumor volume and weight, performing CCK-8 cytotoxicity assays, immunohistochemistry (HER2, Ki67), and flow cytometry for immune cell profiling. Our results demonstrated that HER2-mRNA-LNP significantly reduced tumor volume and weight compared with LNP controls and this suppression was further enhanced when combined with RT. Expression of HER2 and Ki67 decreased in both the HER2-mRNA-LNP and combination groups. HER2-mRNA-LNP increased the proportions of CD8+ T cells, NK cells, B cells, and M1 macrophages in tumors and spleens, elevated levels of IFN-γ, TNF-α, IL-4, and IL-2 in splenic lymphocytes, and reduced the proportion of regulatory T cells (Tregs) in tumor tissues. Combination with RT further augmented this immune remodeling. Collectively, prophylactic administration of HER2-mRNA-LNP provided safe and effective antigen-specific protection in mice and sensitized tumors to radiotherapy. Therapeutic administration of HER2-mRNA-LNP combined with RT exerted an enhanced antitumor effect on HER2-LLC xenografts. This effect may be mediated by the induction of a mixed Th1/Th2 immune response and by modulation of both the tumor immune microenvironment and systemic immunity.
