The phase III trial KEYNOTE-689 established perioperative PD-1 blockade as a standard in resectable head and neck cancer, whereas concurrent PD-1 with definitive chemoradiotherapy has failed to improve outcomes. These findings suggest that therapies directed at tumor-draining lymphatics may impair critical mechanisms of immune priming when delivered concurrently.

We previously demonstrated that surgical ablation or broad-field radiation of tumor-draining lymph nodes eliminates response to immunotherapy, supporting a model in which effective checkpoint blockade depends on intact lymphatic–immune architecture. However, the dynamics of immune cell migration from tumor to sentinel lymph node (SLN) during response remain undefined. We hypothesized that rational sequencing of tumor-directed, lymphatic-sparing radiotherapy with PD-1 blockade reprograms migratory dendritic cells to enhance SLN priming and drive durable antitumor immunity. To explore this, we developed an inducible reporter model enabling spatiotemporal labeling of immune effectors. Coupled with SLN mapping, this system tracks migratory immune populations. Using CITE-sequencing, we defined the “cancer immune migratome,” a repertoire trafficking from tumors to SLNs during active response. We then developed a tumor-directed, lymphatic-sparing radiotherapy model. Sequential radiotherapy followed by PD-1 blockade achieves complete and durable responses. Analysis of the migratome revealed a crucial role for CCR7⁺ dendritic cells. These cells are reprogrammed by treatment sequencing from immune tolerance to active antitumor function, enhancing T cell priming and clonal expansion. Disruption of sentinel lymphatic channels or blockade of CCR7-dependent trafficking to the SLN abrogates response.

This work represents the first characterization of the migrating immune repertoire from tumor to SLN during an active antitumor response, revealing a unique immunologic niche defined by distinct phenotypic and transcriptional profiles. Overall, this work supports rationally sequencing immune-sensitizing, lymphatic-preserving, tumor-directed radiotherapy followed by immune checkpoint inhibition to optimize tumor response to immunoradiotherapy by driving activated dendritic cells to draining sentinel lymph nodes.