Background: Radiotherapy (RT) has been shown to stimulate an antitumor immune response in irradiated tumors as well as unirradiated distant sites (abscopal effect). Previous studies have demonstrated a role for the tumor-draining lymph node (LN) in mediating an anti-programmed death-1 (PD-1)/programmed death ligand-1 (PD-L1) stimulated antitumor immune response. Here, we investigated whether the LN is also important in mediating a RT alone stimulated abscopal response. Methods: We used a subcutaneous modified B16F10 flank tumor model injected bilaterally. Our B16F10 cell line has an inserted viral glycoprotein which facilitated identification of tumor-specific T-cells. RT was directed at one flank tumor alone or one flank tumor and the tumor-draining LN. We evaluated response by tumor growth measurements and flow cytometry of both tumor-infiltrating and LN T-cells. Results: We show that local tumor irradiation improves distant tumor control (abscopal effect). Depletion of CD8⁺ T-cells significantly reduced this abscopal response. We have previously shown, in a chronic lymphocytic choriomeningitis virus (LCMV) infection, that the T-cell proliferative burst following blockade of PD-1/L1 is provided by a ‘stem-like’ CD8⁺ T-cell subset which then differentiate into terminally differentiated effectors. These terminally differentiated effectors have the potential to kill virally infected or tumor cells following PD-1/L1 blockade. In the chronic LCMV infection, stem-like CD8⁺ T-cells were found exclusively in secondary lymphoid organs. Similarly, here we found these cells at high frequencies in the tumor-draining LN, but at low frequencies within the tumor. The effect of RT on this T-cell subset in unknown. Interestingly, tumor irradiation stimulated total CD8⁺ and stem-like CD8⁺ T-cell proliferation in the LN. When the LN and the tumor were then targeted with RT, the abscopal effect was reduced, and we found a concomitant reduction in the number of total tumor-specific CD8⁺ T-cells and stem-like CD8⁺ T-cells in both the irradiated and unirradiated tumor. Conclusions: These correlative results suggest the tumor-draining LN may be an important mediator of the abscopal effect by serving as a stem-like CD8⁺ T-cell reservoir, a site for stem-like T-cell expansion, and a site from which they can populate the tumor.

Radiotherapy has been used for more than a hundred years to cure or locally control tumors. Regression of tumors outside of the irradiated field was occasionally observed and is known as the abscopal effect. However, the occurrence of systemic anti-tumor effects was deemed too rare and unpredictable to be a therapeutic goal. Recent studies suggest that immunotherapy and radiation in combination may enhance the abscopal response. Increasing numbers of cases are being reported since the routine implementation of immune checkpoint inhibitors, showing that combined radiotherapy with immunotherapy has a synergistic effect on both local and distant (i.e., unirradiated) tumors. In this review, we summarize pre-clinical and clinical reports, with a specific focus on the mechanisms behind the immunostimulatory effects of radiation and how this is enhanced by immunotherapy.

Background: Recent studies have demonstrated that earlier time-of-day infusion of immune checkpoint inhibitors (ICIs) is associated with longer progression-free survival (PFS) and overall survival (OS) among patients with metastatic melanoma and non-small cell lung cancer. These data are in line with growing preclinical evidence that the adaptive immune response may be more effectively stimulated earlier in the day. We sought to determine the impact of time-of-day ICI infusions on outcomes among patients with metastatic renal cell carcinoma (mRCC). Methods: The treatment records of all patients with stage IV RCC who began ICI therapy within a multicenter academic hospital system between 2015 and 2020 were reviewed. The associations between the proportion of ICI infusions administered prior to noon (denoting morning infusions) and PFS and OS were evaluated using univariate and multivariable Cox proportional hazards regression. Results: In this study, 201 patients with mRCC (28% women) received ICIs and were followed over a median of 18 months (IQR 5–30). The median age at the time of ICI initiation was 63 years (IQR 56–70). 101 patients (50%) received ≥20% of their ICI infusions prior to noon (Group A) and 100 patients (50%) received <20% of infusions prior to noon (Group B). Across the two comparison groups, initial ICI agents consisted of nivolumab (58%), nivolumab plus ipilimumab (34%), and pembrolizumab (8%). On univariate analysis, patients in Group A had longer PFS and OS compared with those in Group B (PFS HR 0.67, 95% CI 0.48 to 0.94, P_univar=0.020; OS HR 0.57, 95% CI 0.34 to 0.95, P_univar=0.033). These significant findings persisted following multivariable adjustment for age, sex, performance status, International Metastatic RCC Database Consortium risk score, pretreatment lactate dehydrogenase, histology, and presence of bone, brain, and liver metastases (PFS HR 0.70, 95% CI 0.50 to 0.98, P_multivar=0.040; OS HR 0.57, 95% CI 0.33 to 0.98, P_multivar=0.043). Conclusions: Patients with mRCC may benefit from earlier time-of-day receipt of ICIs. Our findings are consistent with established mechanisms of chrono-immunology, as well as with preceding analogous studies in melanoma and lung cancer. Additional prospective randomized trials are warranted.

Purpose: Combination of chemotherapy with programmed cell death 1 (PD-1) blockade is a front-line treatment for lung cancer. However, it remains unknown whether and how chemotherapy affects the response of exhausted CD8 T cells to PD-1 blockade. Experimental Design: We used the well-established mouse model of T-cell exhaustion with chronic lymphocytic choriomeningitis virus (LCMV) infection to assess the effect of chemotherapy (cisplatin+pemetrexed) on T-cell response to PD-1 blockade, in the absence of the impact of chemotherapy on antigen release and presentation observed in tumor models. Results: When concomitantly administered with PD-1 blockade, chemotherapy affected the differentiation path of LCMV-specific CD8 T cells from stem-like to transitory effector cells, thereby reducing their expansion and production of IFNγ. After combination treatment, these restrained effector responses resulted in impaired viral control, compared with PD-1 blockade alone. The sequential combination strategy, where PD-1 blockade followed chemotherapy, proved to be superior to the concomitant combination, preserving the proliferative response of exhausted CD8 T cells to PD-1 blockade. Our findings suggest that the stem-like CD8 T cells themselves are relatively unaffected by chemotherapy partly because they are quiescent and maintained by slow self-renewal at the steady state. However, upon the proliferative burst mediated by PD-1 blockade, the accelerated differentiation and self-renewal of stem-like cells may be curbed by concomitant chemotherapy, ultimately resulting in impaired overall CD8 T-cell effector functions. Conclusions: In a translational context, we provide a proof-of-concept to consider optimizing the timing of chemo-immunotherapy strategies for improved CD8 T-cell functions. See related commentary by Vignali and Luke, p. 1705

There are a number of dynamic regulatory loops that maintain homeostasis of the immune and skeletal systems. In this review, we highlight a number of these regulatory interactions that contribute to maintaining homeostasis. In addition, we review data on a negative regulatory feedback loop between osteoclasts and CD8 T cells that contributes to homeostasis of both the skeletal and immune systems.

Combination radiotherapy (RT) and αPD-L1 therapy has potential to enhance local and distant (abscopal) tumor control, however, clinical results in humans have been variable. Using murine melanoma models, we found RT + αPD-L1 increases intra-tumor progenitor CD8+ PD-1+ TCF-1+ T cells. This increase depends on trafficking of the PD-1+ TCF-1+ cells from the tumor-draining lymph node (TdLN) to the tumor. RT alone promotes the expansion and differentiation of the TdLN derived PD-1+ TCF-1+ cells into TIM-3+ GZMB+ TCF-1- effector-like cells in the tumor with further enhancement after the addition of αPD-L1. In the TdLN, combination therapy enriches for a novel PD-1+ TCF-1+ TOX- LY6A+ subset with expression of a type I interferon and migratory signature. This subset is able to traffic to the tumor and differentiate into TIM-3+ TCF-1- cells. Finally, we found that ablation of the PD-1+ TCF-1+ T cell population attenuates the enhanced tumor control observed with combination RT + αPD-L1. These results suggest that abscopal response failures may be secondary to impaired stimulation of TdLN CD8+ PD-1 + TCF-1+ T cells or an inability of PD-1+ TCF-1+ cells in the TdLN to traffic to the tumor.