Inborn errors of immunity (IEI) are a heterogeneous group of rare diseases, characterized by a broad spectrum of clinical features including infections, inflammation, auto-immune phenomena, lymphoproliferation, and malignant disorders (1).

Advances in molecular genetics and improved knowledge in cellular immunology determined, as a consequence, a striking increase in the number of known monogenic immune disorders in the past decade, reaching the number of 485 clinical phenotypes reported in the 2022 updated classification from the International Union of Immunological Societies (IUIS) Committee on Inborn Errors of Immunity (2, 3).

IEI were recently further divided into two main groups: primary immune deficiency disorders (PIDD) or primary immune regulatory disorders (PIRD). This latter group comprises about 130 genes, whose mutations are responsible for disruption in immune regulation resulting in predominant clinical phenotypes such as autoimmunity, autoinflammation/hyperinflammation, lymphoproliferation, malignancy, and severe atopy, rather than infections (4).

There is a wide spectrum of medical approaches in the treatment of IEI, ranging from conservative therapy alone based on antimicrobial prophylaxis, immunoglobulin replacement therapy, allogeneic hematopoietic stem cell transplantation (allo-HSCT), or gene therapy. Different elements (e.g., clinical presentation, infectious history, immunophenotype, genotype, autoimmune manifestations, organ damage history, family history, psychological and social factors) should be considered to define the best therapeutic strategy (5).

Treatment by allo-HSCT is increasingly successful (6). Improved survival could lead to a further extension of the indications for transplantation in patients affected by IEI.

We report our experience in patients with IEI who received allo-HSCT in our institute for more than 30 years, in order to analyze the impact of evolving clinical practice on allo-HSCT outcomes for these patients.

All children or adolescents with a clinical diagnosis of IEI, with or without a conclusive genetic diagnosis, who underwent allo-HSCT at the Hematopoietic Stem Cell Transplantation Unit of “Istituto Giannina Gaslini”, Genoa, Italy, between January 1990 and December 2021, were included in this retrospective, single-center study. Follow-up was closed at 15 June 2022. Patients who underwent allo-HSCT in the same years due to genetic alterations and diseases classified as bone marrow failure according to the 2022 Update of the IUIS Phenotypical Classification were excluded (2).

Writter informed consent to clinical data collection for scientific purpose was obtained from all patients' parents and/or legal guardians in accordance with institution's ethical standards. The study and all analyses conformed to the 1975 Declaration of Helsinki.

Pre-transplantation pseudo-anonymized data were collected regarding patients’ demographics (sex, age, diagnosis, and clinical condition). Clinical data were obtained from medical reports. Diagnosis was categorized as PIDD and PIRD according to Chan et al. and to clinical phenotypes (4).

Since patients affected by IEI can suffer from clinical manifestations related to defective or overactive immunity, the clinical phenotypes were summarized according to four phenotypic groups: (i) immunodeficiency (ID), in case of significant infectious events in the clinical history; (ii) autoimmunity (AI), in case of the occurrence of an immune-mediated cytopenia, encephalitis, colitis, skin manifestations, or any disease-specific autoimmune event; (iii) inflammation (INF), in case of the occurrence of one or more systemic symptoms, such as non-infectious fever, inflammatory skin rash, organ involvement, and increased inflammatory markers; and (iv) lymphoproliferation (LP), when clinical features such as lymphoadenopathy, hepato-splenomegaly, and macrophage activation were reported.

Transplants were classified according to donor choice as “MRD” (matched related donor) for related HLA genotypically or phenotypically identical donors, “Haplo” for haploidentical related donors, and “MUD” (matched unrelated donor) for unrelated donors or cord blood units.

The conditioning regimen (CR) was categorized as myeloablative (MAC), based on busulfan (Bus) or treosulfan (Treo) as alkylating agent, or reduced intensity (RIC), according to “Defining the intensity of CRs: working definitions” (7). Treo was introduced in clinical practice at our Institute in 2006. For each transplant, the following variables were collected: year of HSCT, which was further categorized as ≤2006 and >2006, age at HSCT, which was further categorized as <3 or ≥3 years, donor (MUD, MRD, and Haplo), source of stem cells (bone marrow, peripheral blood, and umbilical cord), type of CR (MAC and RIC), in vivo T depletion strategy, post-transplant immunosuppression, and serological status for cytomegalovirus (CMV) in donor and recipient.

Post-transplantation data included graft failure (GF), further classified as primary or secondary, or engraftment of neutrophils (absolute neutrophil count >500/mmc for at least three consecutive days) and platelets (>50,000/mmc without any platelet transfusion in the previous 5 days), graft versus host disease (GvHD), grading of acute GvHD (aGvHD) according to Glucksberg criteria (8), chronic GvHD (cGvHD), grading, organ involvement, therapy related to aGvHD and cGvHD, post-transplant bloodstream infections or invasive fungal disease, CMV reactivations, other viral infections, donor chimerism, autoimmune complications occurrence, and survival. Hematopoietic chimerism was evaluated by multiplex PCR analyses of short-tandem repeats (STR) at engraftment, +6 months, +12 months, and at last follow-up and reported according to Clarck (9).

HSCT represents an effective strategy in the treatment of IEI and a heterogeneous group of rare diseases, characterized by a broad spectrum of clinical features.

It is well known that patients affected by IEI can display several different clinical manifestations. Most frequently, immunodeficiency features indicated that HSCT should be performed (about 80% in whole cohort), particularly in the PIDD group compared to the PIRD group (96.7% vs. 64.9%, p = 0.001). Signs and symptoms of inflammation and lymphoproliferation were detected more frequently in the PIRD group. Autoimmune features were described in less than 20% of patients but, unexpectedly, more frequently in the PIDD group compared to the PIRD group (p = 0.199). This finding can be related to 16 diagnoses of primary HLH reported in our cohort, representing almost half the PIRDs described: primary HLH is not typically characterized by autoimmune features.

An early clinical suspicion is required to guarantee an early admission to a tertiary care center, and in our cohort, an increased awareness of IEI led to an increased number of HSCTs performed in the last decades (53, >2006 vs. 20, ≤2006). Together with clinical diagnosis, an earlier genetic diagnosis is nowadays available due to improved molecular genetics. In our cohort, most patients underwent HSCT with a previous genetic diagnosis, and improved genetic tools led to a molecular diagnosis also for three patients who had previously received only a clinical indication for HSCT. The frequency of patients who did not receive a genetic diagnosis in their clinical history appears to be reduced in the last years (3/9, >2006 vs. 6/9, ≤2006).

Diagnosis of PIRD and related indication to perform HSCT increased in the last years compared to PIDD (p = 0.065). This result was associated with allo-HSCTs more recently performed at older age (p = 0.061). In fact, in our cohort, median age at HSCT was 2.5 years, but a trend for a higher age at HSCT (median: 2.7, >2006 vs. 1.9, ≤2006) was found in recent years. PIRDs are mainly associated with different and sometimes more subtle clinical pictures compared to severe infections occurring in the first days/months of life as in some PIDDs, and timing for diagnosis and treatment can therefore be delayed.

The principle of transplant in IEI may appear simple, as recipient hematopoiesis is replaced by donor hematopoiesis, and the genetic defects of the former, resulting in defects in either the number or function of mature immune cell progeny of the recipient HSCs, are corrected. However, despite the advances in HSCT technique and the significant improvement in patient’s survival, this treatment still carries a relatively significant rate of complications and side effects (e.g., GvHD, infections, GF, and long-term organ dysfunction, including gonadal failure and infertility). Actually, in most IEIs and in particular in PIRDs, the indication for HSCT can be influenced by many factors, such as the specific genotype–phenotype and comorbidities (e.g., previous infections, burden organ damage, donor availability, or alternative immunomodulation strategies). Advances in HSCT outcomes thanks to new transplant settings and improved supportive treatment could lead to further extension of the indication to perform HSCT in patients affected by IEI.

A satisfactory OS of transplanted patients was demonstrated in our cohort, resulting in 89.2% at 1 year and 83.4% at 3 years, stable over time, with significant improvement after 2006; this can be related to the improved clinical and genetic knowledge, and supportive care.

Overall, the most important factor for improved survival rate in HSCTs performed more recently is the introduction of the haploidentical TCRαβ/CD19-depleted platform.

HSCT from a Haplo donor is nowadays widely used for adult and pediatric recipients lacking either a related or unrelated HLA-matched donor, although the significant HLA donor–recipient disparities carry a high risk of GF and GvHD. The introduction of the PT-Cy platform represented a significant milestone (10–12), as it demonstrated an association with low rates of GvHD with no requirement of graft manipulations. Alternatively, new graft manipulation techniques, such as the ex vivo TCRαβ depletion, have been developed, offering the opportunity to selectively remove cells responsible to GvHD development while preserving those relevant to ensure engraftment, immune reconstitution, and graft versus tumor effect (13–15). This latter approach has recently been applied for both malignant and non-malignant disorders (16–22).

At our center, TCRαβ depletion of the graft has been considered as the favorite strategy in non-malignant disorders, according to the impressive results that have been published in the last few years (19, 23, 24), preferred over PT-Cy, in order to maximize the reduction in GvHD incidence in disease which does not require an anti-tumor action of the graft. In our cohort, 20/21 transplants from Haplo donors were performed based on the TCRαβ/CD19-depleted platform. A significant increase in the selection of a Haplo donor in most recent years was also found in our cohort (p = 0.005), and accordingly, PBSCs were increasingly used as SC source, as they are collected and ex vivo depleted before graft infusion. Meanwhile, also a shorter time is required to plan and perform HSCT thanks to the selection of Haplo donors. In fact, in our cohort after 2006, time from diagnosis to HSCT was reduced compared to previous years (median, 166 days vs. 280 days, p = 0.057). As far as a parent of the patient is the most frequently selected Haplo donor, time to complete the donor selection is shortened. This result can lead to important advantages to the patient, reducing the risk of severe pre-transplant infections or other complications. A very satisfactory 90% cumulative survival rate in this specific setting has been found, principally thanks to the absence of severe GvHD and despite a higher incidence of GF after haploidentical TCRαβ/CD19-depleted HSCTs (4/20) compared to HLA-matched HSCTs (4/49). However, all but two patients were successfully retransplanted in both cohorts. In our patients, no additional risk factors of proven relevance for GF were present, since none presented a significant level of anti-HLA antibodies against donor HLA before transplant; all received graft with an adequate cell dose and a Treo-based CR according to current recommendations (23). It is known that the risk of GF is higher after T-cell-depleted haploidentical transplant and, in our cohort, it was in line to that reported in previous studies using the same T-cell depletion approach (24).

Recently, data about some IEI (25–29) demonstrated an increased success for HSCTs performed earlier, leading to the introduction of neonatal screening programs in order to refer affected children to a transplant center as soon as possible; the first report found an improved outcome for HSCTs performed in children who received an early diagnosis through a positive newborn screening (30).

In our cohort, however, we could not find a significantly improved survival in children transplanted before 3 years of age. In fact, we found that most of our deceased children were transplanted when younger. However, this finding is severely influenced by the fact the most of our deceased patients were transplanted before 2006, when a lower number of HSCT was reported and OS was less satisfactory. The analysis of HSCT outcomes in our cohort in different historical periods showed that, compared to previous years, more HSCTs were performed after 2006, OS in recent years was more similar in the two age groups, and OS was >90% in children who received HSCT at age <3 years ( Figure 3B ).

The main limitations of the analysis are related to its retrospective nature and the wide period of time considered. In different historical periods, patients and clinical or HSCT features can display significant differences. For example, only a smaller number of the whole HSCT cohort was performed before 2006, based mainly on Bu-based CR and only by selecting HLA-matched donors, but follow-up is more significant. Follow-up for more recent HSCTs is shorter, but a higher number of procedures could have been analyzed. Additionally, more recently HSCTs such as haploidentical TCRαβ/CD19-depleted HSCTs were based on homogeneous approaches, leading to relevant data.

In recent years, only a few specific IEI have been studied in order to develop new gene therapy (GT) approaches (31). GT, compared to allo-HSCT, is not burdened by GvHD. However, clinical trials are ongoing to improve knowledge about the curative potential of a mixed chimeric state in non-SCID IEI and the possible risk for insertional mutagenesis. Data about long-term safety and efficacy are limited.

For this reason, HSCT remains the best therapeutic approach for IEI, and improved outcomes related to the development of new strategies could lead to the extension of the indications for transplantation in patients affected by IEI. The development of a safe and low-risk allo-HSCT approach such as the TCRαβ/CD19-depleted platform and the improved knowledge chemoterapic agents such as Treo can guarantee an effective therapeutic approach in IEI with reduced burden of toxicity and GvHD.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

Written informed consent was obtained from all patient parents and/or legal guardians in accordance with our institution’s ethical standards and with the Declaration of Helsinki Principles.

GD collected the clinical data and drafted the first version of the manuscript. GD and MF conceived the presented idea. FB performed the statistical analysis. SG, FP, GF, DD, FS, CM, DG, and SV reviewed the clinical and technical information reported. DD performed STR assays. FS was directly involved in stem cell manipulation. MG, MM, CD, and MF provided critical review of the manuscript. All authors contributed to the article and approved the submitted version.