Serial T-cell Receptor (TCR) Repertoire Sequencing to Predict Outcomes in Gastrointestinal (GI) Malignancies

Abstract

Severe treatment-induced lymphopenia is associated with worse prognosis in multiple solid malignancies. Emerging data suggests that increased TCR repertoire diversity (RD) is associated with improved survival. We hypothesized that increased TCR RD following severe treatment-induced lymphopenia would predict for improved survival. Patients within a prospective cohort were retrospectively analyzed for severe (grade 4) lymphopenia during radiotherapy (RT) at a single institution between 2007 and 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 a commercially available assay. A Gini diversity index reflecting TCR RD 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. 109 patients were enrolled. 20 patients were evaluable with complete data. Median follow up was 39.9 months. Median age was 68. 55% were male. 80% had ECOG 0 and 20% had ECOG 1. 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 baseline white blood cell (WBC) was 7 k/ul. 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 RD as defined by the change in Gini diversity index while 16 (80%) had decreased TCR RD post-RT. There were no significant differences in baseline characteristics between the two cohorts. On univariable analysis of OS, increased TCR RD 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 RD post-RT had a 3-year OS of 75% vs 22%, respectively (log rank p = 0.026). Increased TCR RD predicted for better OS in our cohort of GI malignancies. Our findings provide biologic insight and refine the association between ALC and cancer outcomes. TCR RD warrants further study as a predictive and prognostic biomarker in GI cancers. Therapeutic approaches to limit severe lymphopenia are ongoing and should include TCR assessment.