Abstract 1662: Immunogenomic mechanisms of response and resistance to combined radiation and immunotherapy in lung cancer
Authors: Belcaid Zineb , Theelen Willemijn S. , White James R. , Sivakumar Ashok , Balan Archana , Niknafs Noushin , Ferreira Leonardo , Adleff Vilmos , Velculescu Victor E. , Baas Paul , Karchin Rachel , Anagnostou Valsamo 
Published: July 2021

Abstract
Introduction: Radiation therapy (RT) has the potential to elicit a systemic immune response beyond the field of radiation (abscopal effect), which can be enhanced by immune checkpoint blockade (ICB) leading to improved anti-tumor immune responses. Immune cell repertoire dynamics during combined immunotherapy and radiation are not well understood.
Methods: We performed whole exome sequencing (WES) and T cell receptor (TCR) sequencing utilizing serial tumor biopsies and peripheral blood samples from 72 patients with advanced non-small cell lung cancer (NSCLC) treated on a phase 2 clinical trial of stereotactic body RT (SBRT) and pembrolizumab (NCT02492568). Best overall response was evaluated by RECIST criteria at 12 weeks on-therapy. Serial biopsies from a non-irradiated tumor site and peripheral blood samples were obtained prior to therapy and after SBRT and/or two cycles of pembrolizumab. 58 matched tumor/normal DNA pairs were analyzed by WES and 120 baseline/on-therapy tumor and peripheral blood mononuclear cell (PBMC) samples were analyzed by TCR-β CDR3 next generation sequencing. We performed differential intratumoral TCR clonotypic abundance analyses and examined the representation and dynamics of these clones in peripheral blood. The most expanded and regressed TCR clonotypes, corresponding to fold changes in productive frequency of TCR clones with an FDR&lt;0.05 were selected to determine intratumoral and peripheral T cell repertoire reshaping during therapy.
Results: Tumor mutation burden was not significantly correlated with response to combined immuno-radiation therapy (P = 0.16). Paired TCR-β CDR3 sequencing of baseline and on-treatment tumors revealed a significantly higher intratumoral TCR density in the on-treatment tumors after combined SBRT and pembrolizumab compared to pembrolizumab alone (P= 0.013). TCR clonotypic differential abundance analysis revealed a significantly higher fraction of expanding and regressing clones that were shared between the tumor and peripheral blood compartments after combination therapy (P= 0.045). Combination therapy induced reshaping of the peripheral TCR repertoire seen in significant TCR clonotypic expansions compared to pembrolizumab alone (P=0.0078). In all 3 patients with a complete response after combination therapy, a distinct pattern of TCR repertoire reshaping in the TIL and peripheral blood compartments was observed despite absent or &lt;50% tumor PD-L1 expression. While intratumoral TCR dynamic changes were appreciated in patients achieving a clinical response independent of therapy, reshaping of the peripheral TCR repertoire was more prominent in responders in the radiation therapy group.
Conclusion: Combined radiation with ICB induces unique dynamic changes in the peripheral and intratumoral T cell repertoire that may be reflective of an abscopal effect and linked with clinical outcomes.
Citation Format: Zineb Belcaid, Willemijn S. Theelen, James R. White, Ashok Sivakumar, Archana Balan, Noushin Niknafs, Leonardo Ferreira, Vilmos Adleff, Victor E. Velculescu, Paul Baas, Rachel Karchin, Valsamo Anagnostou. Immunogenomic mechanisms of response and resistance to combined radiation and immunotherapy in lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1662.