Early neutrophil infiltration promotes TRIMELVax-induced antitumor immunity by linking local inflammation to tumor control.

Enhancing innate-adaptive immune crosstalk is key for improving cancer vaccine efficacy. TRIMELVax is a heat shock-conditioned whole-tumor-cell vaccine combining xenogeneic melanoma cell lysate, syngeneic B16F10 melanoma cell lysate, and Concholepas concholepas hemocyanin. Although TRIMELVax elicits robust antitumor responses in preclinical models, the mechanisms underlying its efficacy remain poorly defined. We characterized the early immune events triggered by TRIMELVax in mice using RT-qPCR, high-dimensional flow cytometry, immunohistochemistry, CFSE-based dendritic cell (DC) migration assays, and therapeutic melanoma models with transient neutrophil depletion. TRIMELVax elicited a rapid inflammatory response at the vaccination site, characterized by local upregulation of CXCL3, CXCL5, CXCL9, CCL3, CCL4, CCL12, IL-1β, IL-6/OSM, IL-12a, and G-CSF. This response drove an early influx of neutrophils and monocytes, followed by increased accumulation of cDC1, cDC2, and monocyte-derived DCs. Notably, we identified a transient population of neutrophils expressing markers associated with antigen-presenting cells (CD45⁺, CD11b⁺, Ly6G⁺, CD11c⁺, MHC-II⁺) that emerged within 12-24 hours postvaccination. These APC-like neutrophils colocalized with cDC1 at the injection site and subsequently migrated to the popliteal draining lymph nodes (pLN). Neutrophil depletion impaired cDC1 migration, reduced APC accumulation in pLN, and abolished the therapeutic efficacy of TRIMELVax. Together, these findings identify neutrophils as key early regulators of the innate inflammatory environment induced by TRIMELVax and suggest that neutrophils with APC-like features may impact DC trafficking and downstream antitumor immunity. Neutrophils, particularly those with APC-like phenotypes, emerge as promising cellular adjuvant targets for enhancing cancer vaccination strategies, offering a new avenue for rational vaccine design and combination with checkpoint blockade therapies.