Immune Reconstitution After Allogeneic Haematopoietic Cell Transplantation: From Observational Studies to Targeted Interventions

Immune reconstitution (IR) after allogeneic haematopoietic cell transplantation (HCT) represents a central determinant of the clinical post-transplant course, since the majority of transplant-related outcome parameters such as graft-vs.-host disease (GvHD), infectious complications, and relapse are related to the velocity, quantity and quality of immune cell recovery. Younger age at transplant has been identified as the most important positive prognostic factor for favourable IR post-transplant and, indeed, accelerated immune cell recovery in children is most likely the pivotal contributing factor to lower incidences of GvHD and infectious complications in paediatric allogeneic HCT. Although our knowledge about the mechanisms of IR has significantly increased over the recent years, strategies to influence IR are just evolving. In this review, we will discuss different patterns of IR during various time points post-transplant and their impact on outcome. Besides IR patterns and cellular phenotypes, recovery of antigen-specific immune cells, for example virus-specific T cells, has recently gained increasing interest, as certain threshold levels of antigen-specific T cells seem to confer protection against severe viral disease courses. In contrast, the association between IR and a possible graft-vs. leukaemia effect is less well-understood. Finally, we will present current concepts of how to improve IR and how this could change transplant procedures in the near future.

T-replete HLA-matched Grafts vs T-depleted HLA-mismatched Grafts in Inborn Errors of Immunity

Haematopoietic cell transplantation (HCT) has become standard of care for an increasing number of inborn errors of immunity (IEI). This is the first report to compare the transplant outcomes according to T-replete HLA-matched grafts using alemtuzumab (n=117) and T-depleted HLA-mismatched grafts using TCR αβ/CD19 depletion (n=47) in children with IEI who underwent first HCT between 2014 and 2019. All patients received treosulfan-based conditioning except patients with DNA repair disorders. For T-replete grafts, the stem cell source was marrow in 25 (21%) patients, PBSC in 85 (73%) and CB in 7 (6%). TCR αβ/CD19 depletion was performed on PBSC from 45 haploidentical parental donors and 2 mismatched unrelated donors. The 3-year OS and EFS for the entire cohort were 85% (77-90%) and 79% (69-86%) respectively. Analysis by age at transplant revealed a comparable 3-year OS between T-replete grafts (88%, 76-94%) and T-depleted grafts (87%, 64-96%) in younger patients (<5 years of age at HCT). For older patients more than 5 years of age, the OS was significantly lower in T-depleted grafts (55%, 23-78%), compared to T-replete grafts (87%, 68-95%) (p=0.03). Grade III-IV aGvHD was observed in 8% of T-replete marrow, 7% of T-replete PBSC, 14% of T-replete CB and 2% of T-depleted PBSC (p=0.73). Higher incidence of viraemia (p<0.001) and delayed CD3 reconstitution (p=0.003) were observed after T-depleted graft HCT. These data indicate that mismatched donor transplant after TCR αβ and CD19 depletion represents an excellent alternative for younger children with IEI in need of an allograft.