Hematopoietic cell transplantation for treatment of primary immune deficiencies

Hematopoietic cell transplantation (HCT) has the potential to cure primary immune deficiency syndromes (PIDS) that are a group of disorders primarily affecting a single lineage, e.g., lymphoid or myeloid lineage. Generally, implementation of various conditioning regimens depends the type of IDS. Some syndromes that cause profound immune deficiency may not require a conditioning regimen. There appears to be a barrier even in cases of severe combined immune deficiency (SCID), particularly in the situation of HLA mismatched or haploidentical grafts. For example, donor B cell chimerism is less likely in γ-chain deficiency (X-SCID), as host cells persistently occupy the B lymphocyte niche, than in syndromes without B cells such as adenosine deaminase (ADA) deficiency. The immune defect may be corrected by partial reconstitution of normal immune cells, in other words full donor chimerism of the affected cell subset may not be required. This concept may add further rationale to limiting the intensity of the conditioning regimen.SCID encompasses a broad range of inherited defects that individually cause a profound immune deficiency of both T and B cell function. The individual genetic defects give rise to various phenotypes, and, since the goal of HCT is to restore both T and B cell function, the SCID phenotype must be taken into consideration in addition to the degree of recipient-donor mismatch. Other biologic factors associated with the SCID phenotype may influence the barrier to engraftment, such as host NK cells, which may survive intensive conditioning regimens. One of the difficulties in analyzing outcome of HCT in SCID patients is the relative rarity of the condition, thus needing large multicentric studies. Recent studies show that the most important factor for improved survival after an HLA-identical sibling graft was younger age at time of HCT. Factors significantly associated with improved survival after haploidentical transplants were B+ SCID phenotype, protected environment, and lack of pulmonary infections before HCT. The advent of neonatal screening and in utero diagnosis has allowed early detection of SCID and therefore prompt intervention at an early age.Primary T cell immunodeficiency (PTCD) syndromes may be differentiated from SCID by virtue of reduced but not completely absent T cell function, or absent T cell function with the presence of B lymphocyte or NK cell function. Allogeneic marrow transplantation remains the only curative therapy available for these disorders. Worse outcomes were seen in patients with PTCD compared to other types of immune deficiencies, regardless of donor. Although life-threatening infections may be less common early in life, children with PTCD often develop organ damage from chronic infections, particularly lung disease, prior to HCT.In Wiskott-Aldrich syndrome, HCT offers significantly improved survival chances for patients. Achieving full donor chimerism was shown to be a favorable factor. In general, however, the studies suggest that low intensity regimens offer the potential for achieving donor cell engraftment with less morbidity than standard regimens, an important consideration for patients who currently may consider the risks of conventional transplants unacceptably high.