A longitudinal analysis of TCRβ repertoire diversity in immune tolerance of pediatric patients post-UCBT: T cell clonal deletion over clonal anergy

Abstract

Abstract The mechanisms of long term immune tolerance after successful hematopoietic stem cell transplantation are not fully elucidated. We set up serial experiments to test central and peripheral mechanisms sustaining tolerance after HLA-mismatched unrelated cord blood transplant (UCBT) in the GvH direction. TCRβ ImmunoSEQ® (Adaptive Biotechnology®) was applied for tracking and characterizing host-reactive T cell clones from graft or patient samples. Seven UCBT recipients with non-malignant diseases were studied. All patients enrolled on the same reduced intensity conditioning trial (NCT01852370). Patients achieved tolerance if discontinued immunosuppression, exhibited immunocompetence without GvHD or graft rejection. Purified T cells responses to host APCs were measured by mixed lymphocyte reaction, cytokine bioplex, and micro CTL assays. T cells clones expanding >0.1% frequency in response to host APC were tracked and characterized with TCRβ ImmunoSEQ®. ImmunoSEQ® revealed the disappearance of host-reactive clones that expanded from an aliquot of the infused cord blood graft by 7 days of stimulation with purified host APC. Most of these allo-reactive T cell clones were not identifiable even in the presence of IL2 supplementation to break anergy. Serial experiments post-UCBT revealed novel host-reactive clones that were not identifiable within the alloreactive pool of the graft pre-UCBT. In summary, the acquisition of immune tolerance in the GvH direction post-UCBT is characterized by hypo-reactivity in all assays employed towards host APC. The disappearances of most host-specific clones in the graft indicate that deletion is a major mechanism of long term tolerance with possibly a small degree of anergy in contribution.