Credit: NPG

Immune tolerance can be restored with autologous haematopoietic stem cell transplantation (HSCT), but how this is achieved remains unclear. “Autologous HSCT is a last-resort treatment for patients with autoimmune diseases refractory to available therapies, and the only approach to date that can induce long-term, drug-free remission in these patients,” states Femke van Wijk, the corresponding author of a new paper published in Blood. In this study, an increase in diversity of the T-cell receptor (TCR) repertoire of T regulatory (TREG) cells is shown to occur after HSCT—possibly a crucial mechanism for the re-establishment of immune tolerance.

“Although the underlying mechanisms [of successful HSCT therapies] are incompletely understood,” explains van Wijk, “we have previously shown that transplantation restores immune tolerance by renewal and modulation of the CD4+ T effector cell compartment in a mouse model, and that the presence of TREG cells after transplantation is crucial to prevent disease relapse.” With these previous findings in mind, van Wijk and colleagues analysed the TREG cell compartment in the proteoglycan-induced arthritis (PGIA) mouse model (in which arthritic disease is induced with two intraperitoneal injections of proteoglycan plus adjuvant) and in patients with juvenile idiopathic arthritis (JIA) or juvenile dermatomyositis (JDM) before and after HSCT.

The use of congenic markers enables the distinction between host and donor-derived TREG cells differentiated after lethal irradiation and bone marrow transplantation in mice. Using this approach, the researchers confirmed that the donor-derived thymic output of TREG cells after bone marrow transplantation was progressively replaced with those originating from the graft; by 7 weeks post-transfer, this reconstitution was virtually complete. Analysis of a TREG cell-specific demethylated region in the Foxp3 gene showed that donor-derived TREG cells were more stable (higher demethylation) than host TREG cells. Furthermore, donor TREG cells had greater capacity to inhibit the proliferation of effector T cells than host TREG cells as assessed in in vitro suppression assays.

These findings prove that bone marrow transplantation in mice successfully replaced the host TREG cell population with TREG cells with increased stability and function. To extend this analysis to human patients, van Wijk and colleagues analysed peripheral blood TREG cells from two patients with JIA and two with JDM (before and after HSCT) and from four healthy controls. The sorted cells were then analysed by next-generation sequencing of the complementarity determining region 3 (CDR3) of TCRBV to assess TCR diversity. Before HSCT, the number of unique TREG cell TCR sequences was lower in patients than in controls (4–19 versus 225–320), as was the estimate of TCR diversity (Simpson's index of diversity 0.114–0.687 versus 0.978–0.988). Notably, the number of unique TCRs and their diversity in TREG cells improved after HSCT, suggesting a successful renewal of the TREG cell compartment that correlated with the data from PGIA mice. These results could provide an explanation as to why most patients experience remission after HSCT.

Given the importance of TREG cells for the recovery of immune tolerance, the researchers then asked whether adding purified TREG cells to the stem cell inoculum could improve the ameliorating effect of HSCT further. To answer this question, Foxp3-expressing TREG cells (purified from mice in which Foxp3-expressing cells also express GFP) were added to the graft before transplantation into PGIA mice. Whereas these additional TREG cells could be detected in the host after transfer and inhibited proinflammatory cytokine production, their inclusion in the graft did not lead to additional improvement in clinical outcome, as disease scores were equally decreased in both groups of transplant-treated PGIA mice in comparison with control PGIA mice (which did not receive a bone marrow transplant). Importantly, repopulation with donor bone-marrow-derived TREG cells was slower in PGIA mice that received bone marrow plus TREG cells compared with mice that received only bone marrow.

The renewal of the TREG cell pool seems paramount to the establishment of productive immune tolerance observed in patients treated with HSCT. According to van Wijk, work is now underway to determine the TREG cell TCR repertoire prior to and at different stages after transplantation in other autoimmune diseases. “We will not only look at circulating TREG cells but also at those derived from sites of inflammation such as the gut and the joint,” adds van Wijk.