1141 - Conventional Fractionated Elective Nodal Irradiation Preserves Early Antitumor Immunity and Efficacy Compared to Hypofractionated Protocols

Purpose/Objective(s):

Recently, preclinical studies suggest that elective nodal irradiation (ENI) suppresses tumor immunity by reducing lymphocytes in tumor-draining lymph nodes (TDLNs) and tumor-infiltrating lymphocytes (TILs). Dose fractionation, a key radiotherapy parameter, differs between preclinical and clinical settings, with most preclinical studies using hypofractionation while clinical ENI employs conventional fractionation. We hypothesize that conventional fractionated ENI (Conv-ENI) would be less detrimental to immunity than hypofractionated ENI (Hypo-ENI) due to its lower initial radiation dose. This study compares their effects on antitumor immunity and efficacy in murine models.

Materials/Methods:

Splenocytes from C3H mice were irradiated (=20 Gy) to assess radiosensitivity. Proliferation capacity and allorecognition ability of irradiated lymphocytes were evaluated using T cell receptor stimulation and mixed lymphocyte reaction with B6C3F1 splenocytes. ENI was performed on MC38 and SCC7 tumor models, targeting inguinal lymph nodes with three schedules: no ENI (0 Gy), Conv-ENI (2 Gy × 8 fractions), and Hypo-ENI (9.8 Gy × 1 fraction). Tumors received 2 Gy × 12 fractions. CD8⁺ T cells depletion was achieved by intraperitoneal injection of anti-CD8 antibody. Lymphocyte egress was inhibited using FTY720. Antitumor effects, TILs, and TDLNs were analyzed. Flow cytometry was used for these experiments.

Results:

In vitro, irradiation (=3 Gy) reduced lymphocyte viability in a dose-dependent manner but did not impair proliferative capacity or allorecognition ability, whereas 9.8 Gy impaired both. In vivo, Hypo-ENI showed significantly inferior antitumor effects compared with Conv-ENI across tumor models (p <0.05, respectively), with the no-ENI group demonstrating the most effective antitumor control. When CD8⁺ T cells were depleted, the impaired antitumor efficacy in the Hypo-ENI group was no longer observed. In the MC38 model, CD8⁺ TILs were significantly reduced in the Hypo-ENI group on Day 1 (p <0.001) but not in the Conv-ENI group. CD8⁺ T cells in TDLNs showed no significant differences across groups on Day 1; however, following FTY720 administration, both Conv-ENI and Hypo-ENI groups exhibited a significant reduction in CD8⁺ T cells within TDLNs compared with the no-ENI groups (p <0.05, respectively), with the greatest reduction observed in the Hypo-ENI group. These findings suggest that the viability of lymphocytes egressing from irradiated TDLN was decreased in a dose-dependent manner.

Conclusion:

In the early phase of ENI, Conv-ENI maintains migrating viable and functional lymphocytes in TDLNs, preserving TILs and antitumor response. The negative impact of conventional fractionated ENI on antitumor immunity and efficacy may be less severe than previously assumed based on studies using hypofractionated protocols.