Severe combined immunodeficiency (SCID) is 1 of the most common indications for pediatric hematopoietic cell transplantation (HCT) in patients with primary immunodeficiency. Historically, SCID was diagnosed in infants who presented with opportunistic infections within the first year of life. With newborn screening (NBS) for SCID in most of the United States, the majority of infants with SCID are now diagnosed and treated in the first 3.5 months of life; however, in the rest of the world, the lack of NBS means that most infants with SCID still present with infections. The average survival for SCID patients who have undergone transplantation currently is >70% at 3 years after transplantation, although this can vary significantly based on multiple factors, including age and infection status at the time of transplantation, type of donor source utilized, manipulation of graft before transplantation, graft-versus-host disease prophylaxis, type of conditioning (if any) utilized, and underlying genotype of SCID. In at least 1 study of SCID patients who received no conditioning, long-term survival was 77% at 8.7 years (range out to 26 years) after transplantation. Although a majority of patients with SCID will engraft T cells without any conditioning therapy, depending on genotype, donor source, HLA match, and presence of circulating maternal cells, a sizable percentage of these will fail to achieve full immune reconstitution. Without conditioning, T cell reconstitution typically occurs, although not always fully, whereas B cell engraftment does not, leaving some molecular types of SCID patients with intrinsically defective B cells, in most cases, dependent on regular infusions of immunoglobulin. Because of this, many centers have used conditioning with alkylating agents including busulfan or melphalan known to open marrow niches in attempts to achieve B cell reconstitution. Thus, it is imperative that we understand the potential late effects of these agents in this patient population. There are also nonimmunologic risks associated with HCT for SCID that appear to be dependent upon the genotype of the patient. In this report, we have evaluated the published data on late effects and attempted to summarize the known risks associated with conditioning and alternative donor sources. These data, while informative, are also a clear demonstration that there is still much to be learned from the SCID population in terms of their post-HCT outcomes. This paper will summarize current findings and recommend further research in areas considered high priority. Specific guidelines regarding a recommended approach to long-term follow-up, including laboratory and clinical monitoring, will be forthcoming in a subsequent paper.

An international consensus conference sponsored by the Pediatric Blood and Marrow Transplant consortium entitled \"Late Effects Screening and Recommendations Following Allogeneic Hematopoietic Cell Transplant for Immune Deficiency and Nonmalignant Hematologic Disease\" was held in Minneapolis, Minnesota on May 10, 2016 and May 11, 2016. The purpose of the conference was to address the unmet need for greater understanding of and the screening for long-term complications in the growing population of survivors of transplantation for nonmalignant disorders. The conference focused on transplantation for hemoglobinopathy, immune deficiency, and inherited bone marrow syndromes. A multidisciplinary group of experts in the disease areas and transplantation late effects presented the current state of understanding of how the underlying disease, pretransplantation therapies, and transplantation-related factors uniquely interact to influence the development of late toxicities. Recommendations were put forth by the group for the late effects screening of survivors of transplantation for these nonmalignant disorders. The findings and recommendations that came from this conference will be presented in a series of 6 additional manuscripts in the upcoming months. In this manuscript, we explore the need for screening practices specific to the survivors of transplantation for nonmalignant diseases and the methodologic challenges associated with the study of these patients.