Session Item

Tumor draining lymph nodes (TDLNs) are a common site of early metastatic spread and therefore often irradiated as part of curative treatment. However, TDLNs are the primary site of anti-tumor immune response generation. Therefore, therapeutic sterilization of the TDLNs at the time of RT might abrogate the immunostimulatory effects of RT, especially in combination with immunotherapy. Here we investigate whether temporal distancing between RT of the tumor and RT of the TDLN maximizes the positive effects of RT on the anti-tumor immune response, while preserving the beneficial effect of metastatic tumor cell killing by delayed RT of the TDLN.

We use mice bearing subcutaneous tumors (B16F10-Luc melanoma cell line) with an early disease spread into the TDLNs. Using a small animal image-guided RT treatment platform, we precisely irradiate the axillary and inguinal TDLNs either concomitantly (CON) or at variable times after primary tumor irradiation (adjuvant treatment, ADJ) and investigate the effect of delayed TDLNs irradiation either with RT alone or in combination with immunotherapy.

Irradiation in absence of immune checkpoint inhibition (ICI) did not reveal any differential treatment response in between concomitant and adjuvant TDLN irradiation.  On combining irradiation with anti-CTLA-4 inhibition the concomitant TDLN irradiation regimen did not enhance the primary tumor response, indicating the relevance of an intact TDLN to support CTLA-4-oriented ICI. In contrast and surprisingly, a strongly improved primary tumor response was observed upon adjuvant TDLN irradiation, performed as early as 48 hours and up to 7 days after primary tumor irradiation when combining RT with anti-CTLA-4 inhibition (50% versus 0% complete responses in “ADJ” versus “CON” treatment groups). Upon tumor rechallenge in the mice with complete responses, all mice rejected the tumor (versus a 100% take rate in the control group), thereby demonstrating formation of immunological memory. Mechanistic-oriented immunophenotyping with the aim of identifying optimal timing and the immune cells that play key roles in the observed effect of delayed TDLN irradiation suggest the induction of an immunosuppressive environment on concomitant irradiation of the primary tumor and TDLN.

The benefit of TDLNs irradiation is being widely revisited, especially in the setting of radioimmunotherapy, with TDLNs as the primary site of anti-tumor immune response generation upon RT. However, with an extensive metastatic spread, it is unlikely that complete avoidance of TDLNs irradiation can be achieved without a negative effect on the survival. Delayed irradiation of the TDLNs will preserve the beneficial function of the TDLNs at the time of the primary tumor irradiation, while still eradicating the metastatic tumor cells. Here we demonstrate a strong, positive effect of a delayed TDLNs irradiation on the treatment response to RT combined with immunotherapy, but not to RT alone.