Severe Combined Immunodeficiency (SCID)

Pathogenesis and Immunologic Deficits

• Severe combined immunodeficiency (SCID) represents the most severe form of primary immune deficiency, caused by diverse pathogenic gene variants that lead to the profound absence of T-cell and B-cell function.
• The fundamental defect disrupts lymphoid cell development, resulting in very small thymuses that are devoid of thymocytes, lack corticomedullary distinction, and lack Hassall’s corpuscles.
• Peripheral lymphoid structures are also severely affected; the follicular and paracortical areas of the spleen are depleted of lymphocytes, and lymph nodes, tonsils, adenoids, and Peyer patches are either completely absent or extremely underdeveloped.
• Without immunologic reconstitution, SCID is considered a true pediatric immunologic emergency and is almost invariably fatal within the first 1 to 2 years of life.

Genetic Classification and Phenotypes

• SCID exhibits significant genetic heterogeneity and is primarily categorized based on the presence or absence of specific lymphocyte populations (T cells, B cells, and Natural Killer [NK] cells).
• T- B+ SCID (T-cell absent, B-cell present):
  • The most common form is X-linked SCID, caused by pathogenic variants in the IL2RG gene (CD132) encoding the common gamma chain ($\gamma c$) of cytokine receptors (IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21).
  • Patients with X-linked SCID typically present with absent T and NK cells but normal or elevated numbers of B cells (T- B+ NK-).
  • Pathogenic variants in JAK3, which signals downstream of the common gamma chain, cause an autosomal recessive form of SCID that presents with an identical lymphocyte phenotype (T- B+ NK-) affecting both males and females.
  • IL-7Rα deficiency, CD45 deficiency, and CD3 deficiencies (arrest of thymocyte differentiation) also cause T- B+ forms of SCID.
• T- B- SCID (Both T-cell and B-cell absent):
  • Autosomal recessive RAG1 and RAG2 deficiencies result in defective V(D)J recombination, leading to the absence of mature T and B cells.
  • Adenosine deaminase (ADA) deficiency causes an accumulation of toxic purine nucleosides leading to lymphocyte death, frequently accompanied by non-immunologic features such as pulmonary alveolar proteinosis, chondro-osseous dysplasia, and neurologic, hepatic, or renal abnormalities.
  • Defects in nonhomologous end joining (NHEJ) and DNA repair enzymes, such as Artemis deficiency, DNA-PK deficiency, and DNA ligase IV deficiency, cause T- B- SCID often associated with marked radiation sensitivity, microcephaly, and growth delay.
  • Reticular dysgenesis, caused by variants in adenylate kinase 2 (AK2), impairs mitochondrial energy metabolism and presents with severe combined immunodeficiency, profound neutropenia, and sensorineural deafness.

Clinical Manifestations

• If not detected by newborn screening, infants with SCID most often present with severe, persistent infections during early infancy.
• Common infectious presentations include chronic diarrhea, recurrent pneumonia, persistent otitis media, bacterial sepsis, and severe cutaneous infections.
• Patients demonstrate extreme susceptibility to opportunistic pathogens, classically presenting with severe oral thrush from Candida albicans, pneumonia from Pneumocystis jirovecii (PJP), and severe infections from parainfluenza 3 virus, adenovirus, respiratory syncytial virus (RSV), cytomegalovirus (CMV), and Epstein-Barr virus (EBV).
• Exposure to live-attenuated vaccines—such as rotavirus, measles-mumps-rubella-varicella (MMRV), oral poliovirus (OPV), or the bacille Calmette-Guérin (BCG) vaccine—frequently results in life-threatening, disseminated vaccine-strain infections.
• Maternal Engraftment: Infants with SCID cannot reject foreign tissues and are highly susceptible to severe graft-versus-host disease (GVHD) caused by the transplacental passage of maternal T cells during pregnancy.
• Maternal engraftment GVHD manifests with a generalized rash, hepatosplenomegaly, diarrhea, and expansion of allogeneic cells.
• Omenn Syndrome: Caused by hypomorphic pathogenic variants in SCID-associated genes, allowing the generation of a few oligoclonal T cells that expand in an unregulated manner, mimicking GVHD.
• Omenn syndrome is classically characterized by severe generalized erythroderma, desquamation, alopecia, lymphadenopathy, hepatosplenomegaly, striking eosinophilia, and elevated serum IgE levels.

Newborn Screening

• Newborn screening allows for the early detection and treatment of SCID prior to the onset of severe infections, which has dramatically improved survival rates.
• The standard screening assay utilizes a quantitative polymerase chain reaction (PCR) to measure T-cell receptor excision circles (TRECs) from dried blood spots.
• TRECs are episomal DNA byproducts formed during the V(D)J rearrangement of T-cell receptor genes; they do not replicate during cell division and serve as a highly accurate biomarker for enumerating recent thymic emigrants.
• A low or absent TREC count raises high suspicion for SCID and mandates immediate confirmatory immunologic evaluation.
• Some newborn screening programs additionally assay kappa excision circles (KRECs) to simultaneously identify severe B-cell defects like agammaglobulinemia.

Laboratory Diagnosis

• A complete blood count (CBC) typically reveals persistent and profound lymphopenia, defined as an absolute lymphocyte count (ALC) of less than 3,000 cells/µL in infants younger than 12 months.
• A normal ALC does not reliably rule out SCID, as the uncontrolled proliferation of maternal T cells, autologous B cells, or NK cells can mask an underlying T-cell lymphopenia.
• Flow cytometry is mandatory to precisely quantitate lymphocyte subsets, including T cells (CD3, CD4, CD8), B cells (CD19, CD20), and NK cells (CD16, CD56).
• Flow cytometric analysis of CD45 isoforms helps differentiate naive T cells (CD45RA) from memory T cells (CD45RO); a predominance of memory T cells strongly suggests maternal engraftment or Omenn syndrome.
• Functional T-cell evaluation is conducted through lymphocyte proliferation assays, where a proliferative response to the mitogen phytohemagglutinin (PHA) of less than 10% of a normal control confirms severe combined immunodeficiency.
• Fluorescence in situ hybridization (FISH) for X and Y chromosomes can be utilized in male infants to confirm the presence of engrafted maternal (XX) T cells.
• Definitive diagnosis requires targeted gene sequencing using a SCID or primary immunodeficiency gene panel to identify the specific pathogenic variant, which is critical for guiding conditioning regimens and evaluating gene therapy options.

Management and Treatment

• SCID necessitates immediate intervention, placing the infant in strict isolation to limit exposure to infectious agents.
• Supportive therapy must be initiated promptly, including regular immunoglobulin replacement (IVIG or SCIG) and initiation of prophylactic antimicrobials targeting PJP, viral, and fungal pathogens.
• Administration of any live-attenuated viral or bacterial vaccines (such as BCG, OPV, MMR, or varicella) is strictly contraindicated due to the high risk of fatal disseminated disease.
• To prevent fatal transfusion-associated GVHD from viable donor lymphocytes, all administered blood products must be exclusively irradiated or frozen.
• Breastfeeding should be withheld until the CMV and EBV statuses of both the mother and the infant are established, to prevent transmission of these viruses through breast milk.
• Hematopoietic Stem Cell Transplantation (HSCT): Allogeneic HSCT remains the most effective, definitive, and curative treatment for SCID.
• Survival rates approach 95% when HSCT is performed optimally within the first 100 days of life, prior to the onset of systemic infections.
• Transplantation utilizing an HLA-identical sibling donor is preferred; however, haploidentical parental grafts utilizing rigorous T-cell depletion techniques are frequently and successfully employed without the need for pre-transplant myeloablative chemotherapy in some subsets.
• Gene Therapy: Ex vivo gene transfer using lentiviral vectors has demonstrated significant success for patients with X-linked SCID and ADA-deficient SCID.
• The transition from retroviral to lentiviral vectors has successfully minimized the risk of insertional mutagenesis and subsequent leukemic-like clonal T-cell proliferation.
• Enzyme Replacement Therapy: For patients with ADA-SCID, regular intramuscular injections of polyethylene glycol-modified adenosine deaminase (PEG-ADA) provide a temporary bridge to definitive treatment, although it does not yield immune reconstitution equivalent to HSCT or gene therapy.