2293 - Should Tumor-Draining Lymph Nodes be Spared from Irradiation during Immunotherapy? Insights from T Cell Analysis in Patients with Locally Advanced Rectal Cancer

Purpose/Objective(s):

Growing evidence demonstrates that tumor-specific memory T cells (Ttsm) and precursor exhausted T cells (Tpex) within tumor-draining lymph nodes (TdLNs) are primary responders to immune checkpoint blockade (ICB). The conventional practice of regional nodal irradiation, which diminishes these T cells and ICB efficacy in preclinical models, has therefore been challenged. This study compares Ttsm and Tpex in TdLNs from patients with locally advanced rectal cancer (LARC) following different neoadjuvant treatments, aiming to provide clinical evidence on whether to spare TdLNs irradiation when combined with immunotherapy.

Materials/Methods:

65 LARC patients receiving different treatments were retrospectively enrolled into four groups: upfront surgery (n=17), neoadjuvant 5Gy×5Fx radiotherapy (nRT, n=19), neoadjuvant chemoradiotherapy (nCRT, n=15), and immunotherapy-based total neoadjuvant therapy (iTNT, n=14). Propensity score matching (PSM) based on baseline characteristics was used to minimize confounding factors among groups. 83 non-metastatic (also negative at baseline) and 15 paired metastatic TdLNs were obtained from surgical specimens. H&E and multiplex immunofluorescence were performed on TdLNs sections to quantify Ttsm (CD8+ TCF-1+ TOX-) and Tpex (CD8+ TCF-1+ TOX+) subsets. Cell counting was performed across the entire TdLNs area using the HALO platform.

Results:

For non-metastatic TdLNs, both nRT and nCRT significantly reduced densities of total CD8+T cells, Ttsm, and Tpex to a similar extent compared to upfront surgery. However, no differences in these densities were found between the iTNT and upfront surgery groups. Only nCRT significantly decreased the proportions of Ttsm and Tpex in CD8+ T cells compared to upfront surgery, whereas nRT and iTNT had no effects on these ratios. Notably, the iTNT group exhibited significantly higher Tpex proportions than the nRT group. In metastatic TdLNs, both densities and proportions of total CD8+T cells, Ttsm, and Tpex were substantially decreased across all four groups (all p<0.01), compared to their non-metastatic counterparts.

Conclusion:

Our findings showed that the combination of ICB could reverse the detrimental effect of neoadjuvant chemoradiotherapy on Ttsm and Tpex in non-metastatic TdLNs and that these responsive T cells were severely compromised in metastatic TdLNs, regardless of neoadjuvant treatments. Therefore, in the iTNT setting for LARC, standard regional nodal irradiation to both non-metastatic and metastatic TdLNs should not be omitted to reduce the risk of metastasis.