ICAM-1-targeted Imaging For Monitoring The Abscopal Effect Of Tumor Radiotherapy

Radiotherapy（RT）is one of essential parts of cancer therapeutics.More than half of patients with malignancies are estimated to receive RT at certain times after diagnosis,with indications spanning radical therapy to symptom alleviation.Although being generally considered as a local treatment strategy,compelling evidences indicate that RT has a systemic inhibitory effect on nonirradiated lesions（abscopal effect）in addition to eliminating irradiated tumors.Harnessing RT-induced abscopal effect could simultaneously eliminate metastatic lesions beyond the radiation field,however,this effect occurs only in few circumstances in the clinical practice.Most recently,the addition of combinational cancer immunotherapy has significantly increased the rates of RT-induced abscopal response.There is now a growing consensus that activation of the immune system is the key to inducing abscopal effect.However,the biological mechanisms underlying the abscopal effect are neither fully understood nor therapeutically utilized.This study aimed to explore a sensitive biomarker of RT-induced abscopal effect,facilitating the early monitoring of abscopal effect and improving its response rates.In this study,in order to mimic the patients with primary tumor and metastatic lesion,BALB/c mice were inoculated with a primary tumor（irradiated tumor）and an abscopal tumor（nonirradiated tumor）.After RT of the primary tumor,we discraminated the responders of abscopal effect through defined criteria.The results of quantitative proteomic analysis showed clear differences between the proteins of nonirradiated tumors from responder and non-responder groups.Notably,ICAM-1 was highly expressed in nonirradiated tumors those were responsive to RT.To investigate whether noninvasive imaging of ICAM-1 could predict an RT-induced abscopal effect,we synthesized the ICAM-1 targeted NIRF probe Dye-αICAM-1/Fab and PET imaging probe ⁶⁴Cu-NOTA-αICAM-1/Fab.Both the imaging of two probes could accurately monitor the ICAM-1 level at tumor sites.Importantly,it showed a strong negative linear correlation between the expression levels of ICAM-1 quantified by PET imaging and the growing tendency of nonirradiated tumors.To explore the mechanisms underlying the RT-induced abscopal response in tumors with high level of ICAM-1 expression,we firstly explored which cell subsets play a major role in linking ICAM-1 to the abscopal effect.Flow cytometry of nonirradiated tumor showed that the level of ICAM-1 expression on CD8⁺cells was significantly higher in tumor from responder.To further validate the role of ICAM-1 in the abscopal effect induced by RT,we blocked the function of ICAM-1 by using an anti-ICAM-1 antibody.The results showed that ICAM-1 blocking exhibited an almost identical effect on the tumor inhibition abrogation to that of CD8⁺T cells depletion.In vitro studies demonstrated that ICAM-1 blocking or down-regulation markedly inhibited the binding of dendritic cells and CD8⁺T cells,and also attenuated CD8⁺T cell-mediated cytotoxicity.To explore whether upregulating ICAM-1 could bolster the abscopal antitumor responses to RT,we utilized a variety of methods to intervene ICAM-1 expression in vivo.First,we generated ICAM-1 containing adenovirus vectors（ICAM-1 Ad）to achieve intratumoral transfection in nonirradiated tumors.The tumor monitoring curve results showed that combination therapy of ICAM-1 Ad and RT significantly improved the abscopal antitumor responses.Imiquimod,a TLR7 agonist and had been proved to up-regulate ICAM-1expression,could also synergize with RT to inhibit the growth of nonirradiated tumors.Together,in this study we identified that ICAM-1 was markedly up-regulated in nonirradiated tumors with an observed abscopal effect induced by RT only to the primary tumor.Noninvasive PET imaging of ICAM-1 expression exhibited a predictive value for the abscopal effect of RT at an early stage.Moreover,drugs upregulated ICAM-1 expression could amplify the abscopal antitumor responses of RT.Our study suggested that noninvasive PET imaging of ICAM-1 expression could be a powerful means to early predict the responses of metastatic tumors to local RT,which could facilitate the exploration of novel combination RT strategies to effectively control both primary and disseminated tumors.