T cell diversity as a measure of immune recovery following CBT

From the Cord Blood Group and colleagues, Clinical Research Division

Hematopoietic stem cell (HSC) transplantation is a life-saving procedure for patients with terminal cancers, but comes with its own dangers, including graft-versus-host disease (GVHD) and prolonged immune dysfunction leading to opportunistic infections. Risks of these potentially fatal complications can be mitigated, in part, by utilizing HSCs from donors that are matched to the patient in terms of their human leukocyte antigen (HLA) type. Unfortunately, well-matched donors are not always available, a situation particularly common for patients in ethnic and racial minority groups. Cord blood transplantation (CBT), in which umbilical cord blood is used as a source of HSCs, is associated with lower rates of GVHD and provides a solution for patients without conventional donors. However, CBT is associated with higher rates of fatal infections within the first year after transplantation, attributed to failure of the immune system to adequately recover. Quantitative measurements of immune cell counts are predictive of survival in the years following CBT, but assays for measuring immune recovery early on after transplantation are lacking. Dr. Filippo Milano, Fred Hutch Associate Professor and Director of the Cord Blood Transplantation Program, and colleagues investigated clonal T cell repertoire diversity as a measure of immune recovery for predicting patient outcomes. “In this work we report an innovative approach to investigate how immune recovery impacts outcomes after cord blood transplantation,” explained Dr. Milano. Their findings were recently published in Frontiers in Oncology.

T cells are important immune effectors for fighting infections. During their maturation, T cells undergo DNA rearrangements in the genes encoding the T cell receptor (TCR), such that each newly emergent T cell carries a unique TCR sequence. These unique TCRs endow each T cell with the capacity to recognize and respond to a specific molecular signature associated with foreign invaders. As a T cell divides, its progeny inherit its TCR sequence, forming a T cell “clone.” High-throughput sequencing technologies allow scientists to measure the clonal diversity of T cell pools, a useful measure for tracking T cell responses. Milano et al. hypothesized that TCR diversity could be used as a functional measure of immune recovery following CBT, since the diversity of the repertoire would be dependent on TCR rearrangements during reconstitution of the peripheral T cell pool.  

T cell diversity as a measure of immune recovery.
Figure provided by Dr. Milano.

The investigators collected blood from 34 cancer patients that received CBT, once prior to transplantation and at 5 subsequent time points over the course of the first year. Blood was also taken from a cohort of healthy adults as controls. High-throughput sequencing of TCR rearrangements was used to determine repertoire diversity in each sample. Unsurprisingly, clonal diversity plummeted immediately after CBT, which involves myeloablative conditioning regimens that eliminate the body’s immune cells. T cell diversity began to gradually rebound in the months following transplantation of cord blood HSCs. However, even one year after CBT, T cell diversity fell far short of that found in healthy adults. Moreover, unlike healthy controls, the T cell clones found at each post-CBT measurement were not stable over time.

To examine the predictive capacity of TCR diversity, the authors compared their measurements with patient outcomes over time. There were 15 deaths occurring in the first year following CBT. Interestingly, patients who survived past one year exhibited TCR diversity recovery beginning about one month after CBT, while those that died had repertoires that continued to shrink and were significantly smaller at 100 days. “We showed that the number of T-cell receptors post-transplant (expressing immunological diversity), is strongly associated with clinical outcomes,” said Dr. Milano. “Cord blood transplant recipients with ‘lower diversity’ are at higher risk of mortality, especially in the first period after transplant.” Surprisingly, overall numbers of T cells could not predict the level of diversity across patients and did not differ between survivors and non-survivors. Thus, a low level of TCR diversity several months after transplantation could be an important measure to help alert clinicians of failing immune recovery, and the need for additional treatment interventions, in CBT recipients.

These findings could have great implications for CBT patients. “New methods to directly measure immune recovery in post-hematopoietic cell transplant recipients, as proposed here, are vital in tailoring the medical management of individual patients. The clinical utility of such foreknowledge will rely on further study: namely, the creation of a clinically meaningful scheme for stratifying patients into risk groups and the development of effective alternative therapies for high-risk patients,” said Dr. Milano. However, there is still work yet to do. “A thorough study of whether TCR diversity is an independent predictive measure of patient outcomes and whether low TCR diversity is directly causal of inferior outcomes must await an analysis with a larger cohort of patients. Furthermore, this work has stimulated more research to look for other predictive tools to identify patients at high risk of death,” added Dr. Milano.

This work was supported by the Department of Health and Human Services, the National Institutes of Health, Medac GmbH, the Damon Runyon Cancer Research Foundation, and the George and Fay Young Foundation.

UW/Fred Hutch Cancer Consortium members Filippo Milano, Rachel Salit, Ann Dahlberg, and Katherine Guthrie contributed to this work.

Milano F, Emerson RO, Salit R, Guthrie KA, Thur LA, Dahlberg A, Robins HS, Delaney C. Impact of T Cell Repertoire Diversity on Mortality Following Cord Blood Transplantation. Front Oncol. 2020 Oct 9;10:583349. doi: 10.3389/fonc.2020.583349.