The T-Cell Receptor Repertoire Predicts Interim-PET in Patients with DLBCL Treated with R-CHOP: An Observational Study from a Prospective Clinical Trial

T-cell infiltration of the tumor microenvironment (TME) in DLBCL is a key determinant of response to chemo-immunotherapy (Keane, Lancet Haem 2015). We have previously shown that greater diversity of the T-cell receptor (TCR) repertoire within the TME is correlated with improved survival following R-CHOP in DLBCL (Keane, CCR 2017). There are limited data on the impact of the intratumoral TCR repertoire on interim-PET (iPET), the relationship between intratumoral and circulating TCRs, and on dynamic changes of the TCR during therapy. In this study, we interrogated the TCR repertoire in a subset of DLBCL patients treated on the prospective Australasian Leukaemia Lymphoma Group NHL21 study (Hertzberg, Haematologica 2017), in which all patients had 4x RCHOP prior to iPET risk stratification.

The CDR3 region of TCRβ chain underwent high-throughput unbiased TCRβ sequencing (Adaptive Biotechnologies). Metrics included: productive templates (total functional T-cells), productive rearrangements (functional T-cells with distinct specificity), productive clonality (repertoire unevenness due to clonal expansions), and maximal frequency clones (% most dominant single clone). Matched intratumoral diagnostic samples, blood at pre-therapy and post-cycle 4 (at the time of iPET) were tested. 42 patients (enriched for iPET+ cases) had sufficient material for testing.

Median age was 55 (range 22-69) years and 72% were males. IPI was low/intermediate/high in 13/63/25% respectively. Cell of origin (COO) by Lymph 2CX method (nanoString) was ABC in 30%, and GCB in 44%. 40% were iPET+. In tissue, there was a median of 4652 productive templates, translating into 2998 productive rearrangements identified.

Notably, the clonal repertoire of intratumoral TCRs in iPET+ patients was larger than iPET-ve patients (productive clonality 8.1 vs 5.1 x10^-2, p=0.04), whereas the numbers of functional T-cells did not vary between groups. Comparing the tumor with the blood samples showed a high, but variable, degree of overlap between peripheral blood and the TME - TCR repertoire. Median number of top 100 tumor tissue clones shared in peripheral blood was 53.5 (range, 1-97) in pre-therapy and 39.5 (range, 0-93) in post-therapy blood, indicating that the both the circulation and the tumor likely contribute to immune-surveillance.

In pre-therapy blood, the median productive templates and productive rearrangements were 44,950 (range, 6,003-273,765) and 29,090 (range, 5,190-152,706), and the median clonality was 8.5 (1.46-45.3) x 10^-2. There were no differences between iPET+ and iPET-ve patients in these parameters. However, there was a marked change in T-cell composition between time points. Interestingly, in iPET-ve patients clonality measures were increased, with productive clonality 9.4 vs 14.4 x10^-2, p=0.03; and % maximum productive frequency 3.39 vs 5.89, p=0.04.

These findings demonstrate that the intratumoral TCR repertoire, and sequential blood sampling provide important information on outcome in DLBCL treated with RCHOP. A highly clonal T-cell repertoire in the TME was associated with iPET positivity after 4 cycles of R-CHOP. In line with findings in solid cancers treated with checkpoint blockade, development of clonal responses in peripheral blood was associated with iPET negativity. These findings indicate that clones expanded during therapy may be important in tumor clearance but that highly clonal T-cell responses in the tumor at diagnosis may hinder expansion of other T-cell responses to neoantigens. The circulating TCR composition is representative of the TME. These findings will assist the rationale design and therapeutic monitoring of novel immuno-therapeutic strategies.