T Regulatory Cell Receptor Repertoire Focusing and Clonal Expansion Indicates Control of Acute GvHD after Donor Lymphocyte Infusion

Background: T effector cells (Teff) within the stem cell graft in allogeneic hematopoietic stem cell transplantation (HSCT) can elicit disabling acute graft-versus-host disease (aGvHD) contributing to transplant-related mortality. Teff as donor lymphocyte infusion (DLI) are a therapeutic option to re-induce complete remission (CR) after leukemia relapse. Usually DLIs are given in dose escalating regimens until CR is achieved or first signs of aGvHD develop. To monitor the DLI induced allo-immune response and the efficacy of aGvHD treatment is a clinical challenge, since no established biomarkers are available.

T regulatory cells (Tregs) are thought to play an important role in balancing immune responses and studies have shown effects of adoptive Treg transfer as a therapeutic option in GvHD. T cell receptor (TCR) sequencing of T cell subsets, such as Tregs, after DLI might allow the identification of clones inducing control of GvHD. Here we report data of 29 DLI patients with a median follow-up of >1 year allowing us to thoroughly analyze differences of the TCR repertoire in patients with or without aGvHD.

Aims: We aimed to analyze Treg TCR diversity and clonality changes over time as a potential biomarker for development and treatment response of aGvHD following DLI.

Patients and Methods: The study cohort consisted of 29 leukemia patients after HSCT who received DLI treatment for recurrence of disease, molecular relapse or high risk phenotype. Blood samples were taken before and after DLI. A median of 4 (range 2-6) blood samples per patient were available for TCR-sequencing. The last sample was taken at a median of 123 (27-530) days post DLI. After generation of PBMCs CD4+CD127-CD25+ Tregs were FACS-sorted for cDNA-based CDR3-region amplification of the TCR-β chain. CDR3 amplicons were sequenced on the Illumina MiSeq platform and annotated using the IMGT.org database. Further bioinformatic analyses were based on VDJtools and the tcR R-package.

Results: In 18/29 patients we observed clinical symptoms of aGvHD, with blood samples available of the acute onset in 12/18 cases. Treg frequencies and absolute numbers did not differ between aGvHD and noGvHD samples. Treg TCR diversity, assessed via inverse Simpson's diversity index (1/D) increased on average by 322% at the first occurrence of aGvHD compared to the previous sample (Figure 1A, B). Stratifying for aGvHD severity (total grade 1-2 vs. 3-4) did not reveal any group differences. However, stratifying by organ involvement (skin vs. GI/liver) showed a more pronounced increase of 1/D in patients with aGvHD of the GI tract and/or the liver. Moreover, in 11 subjects blood samples were available a median of 7 (3-14) days prior to aGvHD diagnosis. Already at this preclinical time point we detected an increased 1/D of +361% (Figure 1A, B). Again, aGvHD organ involvement significantly affected the result, with GI/liver involvement mainly driving this effect (+567% vs. +1% 1/D). In contrast, patients who did not develop aGvHD at any time after DLI showed on average a slight decrease of -8% 1/D compared to the previous time point. Next, we analyzed all aGvHD patients with at least one sample available after initiation of treatment (local and/or systemic steroids). Control or remission of aGvHD symptoms was accompanied by decreased 1/D of on average -58% compared to the previous samples (Figure 2). In 9/11 patients we saw a focusing of the Treg TCR repertoire; in the other two (Figure 2, red lines) no or only partial clinical response to systemic steroids was reported.

Conclusion: Our data describe detailed changes in the Treg compartment on the TCR level following development and treatment of aGvHD. Currently, aGvHD diagnosis following DLI treatment relies solely on clinical symptoms, deciding whether further dose increments of DLI can be administered. As our data suggest, Treg TCR sequencing may support transplant specialists with (1) detection of patients at risk for aGvHD, even prior to clinical symptoms, (2) identification of patients eligible for further dose increments (compare Figure 1B), and (3) assessment of aGvHD treatment with guidance whether higher dosage of steroids and/or alternative immunosuppressive treatment might be required (compare Figure 2, red lines). Future prospective studies are needed to replicate these findings in a large cohort, potentially enabling identification of specific Treg TCR clones controlling the allo-immune response in aGvHD.

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