Increased T-cell receptor repertoire diversity to predict better overall survival in gastrointestinal malignancies.
Authors: Kim Daniel W , Heather James , Lee Grace , Van Seventer Emily E. , Fish Madeleine G , Corcoran Ryan Bruce , Hacohen Nir , Klempner Samuel J , Cobbold Mark , Hong Theodore S. 
Published: January 2021

474 Background: Severe treatment-induced lymphopenia is a poor prognostic factor for multiple solid malignancies. Emerging data suggests that increased TCR repertoire is associated with improved survival. We hypothesized that increased TCR repertoire following severe treatment-induced lymphopenia would be associated with better survival after accounting for known prognostic factors. Methods: Patients within a prospective cohort were retrospectively analyzed for severe (grade 4) lymphopenia during radiotherapy (RT) at a single institution between 2007-2016. Peripheral blood samples were collected weekly. TCR repertoire at 3 time points were evaluated: pre-RT, during or immediately post-RT at absolute lymphocyte count (ALC) nadir, and ~2 months post-RT. Peripheral blood was analyzed via multiparametric flow cytometry. TCR beta sequencing was performed on immunoSEQ platform from Adaptive Biotechnologies. A Gini diversity index reflecting TCR repertoire was calculated as well as percentage change between ALC nadir and ALC 2 months post-RT. Primary endpoint was overall survival (OS). Cox modeling and Kaplan Meier estimates were used for survival analyses. Results: 109 patients were enrolled. 20 patients were evaluable with complete data. Median follow up was 40 months. Primary malignancy was pancreatic adenocarcinoma in 40%, gastric adenocarcinoma in 30%, and cholangiocarcinoma in 30%. Clinical stage was I-II in 29%, III in 50%, and IV in 21%. 80% underwent chemotherapy (CTX) pre-RT and none received CTX post-RT. 80% had concurrent CTX during RT. Median RT dose was 56 Gy. Among those with pathology available (n = 12), 17% had a pathologic complete response (pCR) and 83% had R0 resection. Median ALC at each of the three study timepoints was 1.26 k/ul, 0.32 k/ul, and 1.02 k/ul, respectively. 4 (20%) patients had increased TCR repertoire as defined by the change in Gini diversity index while 16 (80%) had decreased TCR repertoire post-RT. There were no significant differences in baseline characteristics between the two cohorts. On univariable analysis, increased TCR repertoire from ALC nadir to 2 months post-RT significantly correlated with better OS (HR = 0.20 [95% CI 0.04-0.93]; p = 0.041). Other variables including ECOG, stage, RT dose, CTX use, pCR, R0 resection, baseline WBC, and ALC at three study time points did not correlate with OS. Patients with increased vs decreased TCR repertoire post-RT had a 3-year OS of 75% vs 22%, respectively (log rank p = 0.026). Conclusions: Increased TCR repertoire post-RT was an independent prognostic factor for better OS in our cohort of GI malignancies. Our findings provide biologic insight and refine the association between ALC and cancer outcomes. TCR repertoire warrants further study as a predictive and prognostic biomarker in GI malignancies. Therapeutic approaches to limit severe lymphopenia are ongoing and should include TCR assessment.