Pivotal Roles of Tumor-Draining Lymph Nodes in the Abscopal Effect from Combined Immunotherapy and Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> Currently, due to synergy enhancement of anti-tumor effects and potent stimulation of abscopal effects, combination therapy of irradiation and PD-1/L1 immune checkpoint inhibition (iRT) has revolutionized the therapeutic guidelines. It's been demonstrated that tumor draining lymph nodes (TDLNs) are essential for effective antitumor immunity induced by radiotherapy, immunotherapy or iRT. Given that the function of TDLNs in iRT is still unclear, we investigated the role of TDLNs in iRT induced abscopal effects and the possible mechanisms. <h3>Materials/Methods</h3> The function of TDLNs was evaluated using unilateral or bilateral MC38 and B16F10 subcutaneous tumor models with or without indicated TDLNs. The flow cytometry, multiple immunofluorescence analysis and multiplex analysis were utilized to detect the composition and function of the immune cells in the primary and abscopal tumor microenvironments. And we tempted to interrogate the possible mechanisms via RNA-seq of tumor infiltrated lymphocytes and TDLNs. Finally, we assessed the prognosis of breast cancer patients subjected to various numbers of TDLNs removal. <h3>Results</h3> We found that monotherapy or iRT deficient of TDLNs restored the tumor progression to a mild degree, and bilateral TDLNs removal rather than unilateral TDLNs removal substantially curtailed iRT-stimulated anti-tumor effects and abscopal effects. Moreover, we showed that with the absence of TDLNs, the infiltration of CD45⁺ cells and CD8⁺ T cells was substantially reduced in both the primary and abscopal tumors, and the anti-tumor function of CD8⁺ T cells was attenuated as well. Additionally, the polarization of tumor-associated macrophages in primary and abscopal tumors relied on intact bilateral TDLNs. RNA-seq data indicated that impaired infiltration and anti-tumor effects of immune cells may partially attribute to the altered secretion of tumor microenvironments. <h3>Conclusion</h3> Our results highlight that TDLNs play a critical role in iRT via promoting the infiltration of CD8⁺ T-cells and maintaining the M1/M2 macrophage ratio. Currently, surgical removal or radiation ablation of TDLNs are standard treatments in various types of cancers and are regarded to avoid tumor metastasis and promote prognosis. Based on our findings, whether the regional TDLNs should be preserved in clinical practice should be deliberately considered.