L19-IL2 reverts radiation-induced lymphopenia in a mouse model of lung cancer

Over half of radiotherapy-treated cancer patients develop radiation-induced lymphopenia (RIL). Severe RIL has been associated with worse prognosis and survival, and recent studies suggested that RIL also affects immunotherapy efficacy. We aimed to develop murine grade 2 (≥20 % decrease in absolute lymphocyte counts (ALC)) RIL models and to examine the effects of RIL on progression-free survival upon radiotherapy-immunotherapy treatment. C57BL6/J mice received heart, large blood vessels (LBV) or thoracic vertebrae irradiation (10 Gy) and ALC were monitored weekly. In tumour-bearing animals, Lewis Lung Carcinoma cells were injected subcutaneously one day prior to RIL induction. When tumours reached 212 ± 45 mm3, tumours were locally irradiated (10 Gy), and animals were injected with L19-IL2 (1 mg/kg, 3 times QOD) or vehicle intravenously. Tumour growth was monitored until reaching > 4 times treatment starting volume. Flow cytometry-based immune cell profiling was done on blood collected 2 weeks post-tumour cell injection. Radiation treatment plans targeting lymphocyte-rich organs were optimized to achieve maximal target coverage while minimal dose to normal tissues. In naïve animals, LBV and vertebrae irradiation led to grade 2 RIL, however heart irradiation induced only grade 1 RIL. In tumour-bearing animals, RIL induction was confirmed by a 16 % and 20 % drop in ALC upon LBV and vertebrae irradiation, respectively. Grade 2 RIL did not negatively influence progression-free survival upon radiotherapy. Radiation combined with L19-IL2 induced a tumour growth delay compared to radiotherapy only (p < 0.0005). LBV or vertebrae irradiation did not affect radiotherapy-immunotherapy outcome, explained by the restored and increased lymphocyte and eosinophil counts upon L19-IL2 administration (p < 0.05). L19-IL2 increased inducible regulatory and CD8+ T cells, especially in vertebrae (p < 0.01) and LBV (p = 0.07) irradiated animals, respectively. Collectively, utilizing the developed murine RIL models, we observed that RIL did not negatively affect radiotherapy treatment outcome. L19-IL2 can be a promising strategy to restore lymphocyte counts and revert RIL.