Residency · Residency · Allergy Immunology

Combined Immunodeficiencies - SCID and Beyond

Severe Combined Immunodeficiency (SCID)

Overview

Severe combined immunodeficiency represents the most severe form of primary immunodeficiency, characterized by absent T cell immunity with variable effects on B cell and natural killer cell development. The incidence, as determined by US newborn screening data, is approximately 1 in 58,000 live births. Without definitive treatment through hematopoietic stem cell transplantation (HSCT), gene therapy, or enzyme replacement, SCID is universally fatal within the first 1 to 2 years of life. SCID constitutes a pediatric emergency in which early diagnosis, before the onset of infections, dramatically improves transplant outcomes, with survival exceeding 90 percent when HSCT is performed before 3.5 months of age.

SCID Classification by Lymphocyte Phenotype

T-B+NK- SCID

X-linked SCID, caused by mutations in IL2RG encoding the common gamma chain, is the most prevalent SCID subtype, accounting for 40 to 50 percent of cases. The common gamma chain is shared by the receptors for interleukins 2, 4, 7, 9, 15, and 21. Loss of IL-7 receptor signaling causes T cell developmental failure, while loss of IL-15 receptor signaling prevents NK cell development. B cells are present but non-functional due to the absence of T cell help and impaired IL-21 receptor signaling. JAK3 deficiency is an autosomal recessive condition that phenocopies X-linked SCID because JAK3 transduces the intracellular signals downstream of the common gamma chain. The phenotype is identical (T-B+NK-), but critically, JAK3 deficiency affects both males and females.

T-B-NK+ SCID

RAG1 and RAG2 deficiency results from absent V(D)J recombination, preventing assembly of both T cell and B cell antigen receptors. Complete RAG deficiency produces a T-B-NK+ SCID phenotype. Hypomorphic RAG mutations, which retain partial recombinase activity, cause Omenn syndrome, characterized by oligoclonal T cell expansion, generalized erythroderma, eosinophilia, markedly elevated IgE, and hepatosplenomegaly. Artemis (DCLRE1C) deficiency impairs DNA repair during V(D)J recombination and produces a radiosensitive form of SCID. DNA-PKcs deficiency and DNA ligase IV deficiency are additional causes of radiosensitive SCID, with the latter potentially associated with microcephaly.

T-B-NK- SCID

Adenosine deaminase (ADA) deficiency accounts for approximately 15 percent of SCID. The absence of ADA leads to accumulation of deoxyadenosine and deoxyadenosine triphosphate (dATP), which are toxic to lymphocytes across all lineages, producing a T-B-NK- phenotype. A characteristic radiographic finding is widening of the costochondral junctions on chest X-ray. ADA-SCID is unique among SCID subtypes in having enzyme replacement therapy available as a bridge to definitive treatment (PEG-ADA, marketed as Revcovi). ADA-SCID was also the first disease treated with gene therapy, with Strimvelis becoming the first approved gene therapy product for this indication in the European Union. Reticular dysgenesis, caused by deficiency of adenylate kinase 2 (AK2), produces the most severe form of SCID, combining T-B-NK- immunodeficiency with severe congenital neutropenia and sensorineural deafness.

T-B+NK+ SCID

IL-7 receptor alpha (IL7R) deficiency is an autosomal recessive condition in which impaired IL-7 signaling specifically affects T cell development while sparing B and NK cells. CD3 chain deficiencies (affecting CD3delta, CD3epsilon, or CD3zeta) impair TCR complex assembly. CD45 deficiency disrupts the phosphatase activity required for TCR signaling. Coronin-1A deficiency impairs thymic egress, allowing T cells to develop within the thymus but preventing their export to the periphery.

<image>A comprehensive classification table of SCID subtypes organized by lymphocyte phenotype. Four columns for T-B+NK- (IL2RG, JAK3), T-B-NK+ (RAG1/2, Artemis, DNA-PKcs, Lig4), T-B-NK- (ADA, reticular dysgenesis), and T-B+NK+ (IL-7Ralpha, CD3 chains, CD45, Coronin-1A). Each column shows: gene defect, protein function, inheritance pattern (X-linked or AR), molecular pathway diagram (showing where the block occurs in lymphocyte development), unique clinical features, and treatment options (HSCT, gene therapy, enzyme replacement where applicable). Highlight the most common subtype (X-linked SCID, IL2RG) with a colored border. Include a pie chart showing relative frequency of SCID subtypes.</image>

Newborn Screening for SCID

Universal newborn screening for SCID is now performed in all 50 US states, having been fully implemented since 2018. The screening assay measures T cell receptor excision circles (TRECs), which are circular DNA byproducts of TCR gene rearrangement in the thymus. TRECs are quantified by quantitative PCR from dried blood spot samples obtained during the standard newborn heel prick. Low or absent TRECs indicate the absence or severe reduction of recent thymic emigrants. This assay detects all SCID subtypes, as all produce low T cell numbers, as well as many non-SCID causes of T cell lymphopenia. False positives occur with prematurity, DiGeorge syndrome, congenital heart disease, Down syndrome, and idiopathic T cell lymphopenia. The TREC assay does not detect pure B cell deficiencies (such as XLA or CVID), NK cell defects, or late-onset combined immunodeficiencies.

Clinical Presentation (Unscreened)

In unscreened infants, SCID typically presents within the first 3 to 6 months of life with failure to thrive, chronic diarrhea, and persistent oral thrush. Pneumonia caused by Pneumocystis jirovecii (the hallmark opportunistic infection), CMV, RSV, or parainfluenza is common. In countries with routine BCG vaccination, disseminated BCG infection may occur. Dermatitis that may resemble atopic dermatitis should raise consideration of Omenn syndrome or maternal T cell engraftment causing graft-versus-host disease. Absence of the thymic shadow on chest X-ray is suggestive but not specific. Infections with opportunistic organisms including Pneumocystis jirovecii, CMV, Candida, and Cryptosporidium are characteristic.

Treatment

HSCT is the treatment of choice for SCID. Best outcomes are achieved with an HLA-matched sibling donor, yielding greater than 95 percent survival. Matched unrelated donors and haploidentical donors with T cell depletion are also used. Transplantation before 3.5 months of age achieves greater than 90 percent survival regardless of donor type, underscoring the critical importance of early diagnosis through newborn screening. Conditioning regimen requirements vary by SCID subtype; some T-B+NK- SCID patients can engraft without conditioning because the T cell niche in the thymus is empty.

Gene therapy has made significant progress. For ADA-SCID, lentiviral vector-based approaches are in trials alongside the approved Strimvelis product. For X-linked SCID, initial retroviral gene therapy trials were complicated by insertional mutagenesis leading to leukemia, but newer lentiviral vectors have demonstrated improved safety profiles. Gene therapy trials for Artemis SCID are ongoing. Enzyme replacement with PEG-ADA (Revcovi) serves as bridge therapy for ADA-SCID but is not curative.

Supportive care includes PJP prophylaxis with trimethoprim-sulfamethoxazole, antifungal prophylaxis, immunoglobulin replacement, use of irradiated and CMV-negative blood products, and isolation precautions. All live vaccines must be avoided, and breast milk from CMV-positive mothers should be avoided (though this remains somewhat controversial).

Combined Immunodeficiency (CID) - Not SCID

Distinguishing CID from SCID

Combined immunodeficiencies that do not meet criteria for SCID are characterized by reduced but not absent T cell number and/or function, producing a less severe but still clinically significant immunodeficiency. These conditions typically present later than SCID with a broader infection spectrum. Recognition is important because many patients with CID benefit from HSCT before progressive disease causes irreversible complications.

Major CIDs

Omenn Syndrome

Omenn syndrome results from hypomorphic RAG1 or RAG2 mutations (or, rarely, Artemis or IL-7Ralpha mutations) that retain partial recombinase activity. The limited V(D)J recombination produces oligoclonal, autoreactive T cells that expand dramatically while B cells remain absent. The clinical presentation is striking, with generalized erythroderma, desquamation, alopecia, hepatosplenomegaly, lymphadenopathy, markedly elevated IgE, and eosinophilia. The clinical appearance may closely resemble graft-versus-host disease or severe atopic dermatitis in infancy. Although T cells are present, they are oligoclonal and incapable of providing effective immune defense against infections. Treatment requires HSCT with a conditioning regimen to eliminate the autoreactive T cell clones.

MHC Class II Deficiency (Bare Lymphocyte Syndrome Type II)

MHC Class II deficiency results from defects in transcription factors that regulate MHC-II expression, including CIITA, RFXANK, RFX5, and RFXAP. In the absence of MHC-II expression on antigen-presenting cells, CD4+ T cells fail positive selection in the thymus, producing profound CD4 lymphopenia. CD8 T cells and B cells are present, but antibody responses are severely impaired due to the absence of T cell help. The condition follows autosomal recessive inheritance and is more common in North African and Mediterranean populations. Clinical features include severe infections beginning in infancy, with cryptosporidial cholangitis being a particularly characteristic complication. Treatment requires HSCT, and prognosis without transplant is poor.

Wiskott-Aldrich Syndrome (WAS)

Wiskott-Aldrich syndrome is an X-linked disorder caused by mutations in the WAS gene encoding the WASP protein, which regulates the cytoskeleton in hematopoietic cells. The classic triad consists of thrombocytopenia with characteristically small platelets (low mean platelet volume, which is a key diagnostic clue), eczema, and progressive immunodeficiency. The immunodeficiency initially manifests as impaired humoral responses to polysaccharide antigens, then progresses to include cellular immune dysfunction. Immunoglobulin patterns show elevated IgE, elevated IgA, and low IgM, and isohemagglutinin titers may be absent. Complications include autoimmunity in 30 to 40 percent of patients, lymphoma in 12 to 15 percent, and severe infections. HSCT is curative, and gene therapy trials have shown promising results. An attenuated form, X-linked thrombocytopenia (XLT), results from milder mutations in the same gene.

Ataxia-Telangiectasia (AT)

Ataxia-telangiectasia is an autosomal recessive condition caused by mutations in the ATM gene, which encodes a kinase essential for DNA damage repair. The clinical hallmarks include progressive cerebellar ataxia (onset at 1 to 2 years) and oculocutaneous telangiectasias (onset at 3 to 6 years). The immunodeficiency is variable, with IgA deficiency present in 50 to 80 percent, IgG subclass deficiency, and T cell dysfunction. Elevated alpha-fetoprotein (AFP) is the hallmark laboratory finding, present in more than 95 percent of patients. Cellular radiosensitivity is a critical feature, and unnecessary ionizing radiation (CT scans, radiation therapy) must be avoided. Malignancy risk is markedly increased (100-fold), primarily lymphoma and leukemia, and heterozygous female carriers have increased breast cancer risk. Immunoglobulin replacement is indicated for patients with recurrent infections and demonstrated antibody deficiency.

DiGeorge Syndrome (22q11.2 Deletion)

DiGeorge syndrome is the most common microdeletion syndrome, occurring in approximately 1 in 4,000 live births, and is caused by deletion at chromosome 22q11.2, detectable by FISH or chromosomal microarray. The T cell deficiency is variable, ranging from complete DiGeorge syndrome (representing fewer than 1 percent of cases, with an absent thymus producing a SCID-like phenotype) to partial DiGeorge syndrome (the majority of cases, with low T cells in infancy that improve with age). Complete DiGeorge syndrome uniquely requires thymic transplantation rather than HSCT, because without a thymic microenvironment, transplanted stem cells cannot undergo T cell development. Associated features include congenital heart disease (especially conotruncal defects such as tetralogy of Fallot, interrupted aortic arch, and truncus arteriosus), hypocalcemia from hypoparathyroidism, palatal abnormalities, and facial dysmorphism. DiGeorge syndrome may be detected on TREC-based newborn screening.

<image>A clinical features diagram of major combined immunodeficiencies beyond SCID. Four panels arranged as quadrants: (1) Wiskott-Aldrich syndrome: clinical image showing eczematous rash, peripheral blood smear with small platelets, immunoglobulin pattern (high IgE, high IgA, low IgM), X-linked inheritance pattern. (2) Ataxia-telangiectasia: clinical image showing ocular telangiectasias and cerebellar ataxia (wide-based gait), elevated AFP level bar, radiosensitivity warning symbol, and chromosome breakage study illustration. (3) DiGeorge syndrome (22q11.2 deletion): cardiac defects (tetralogy of Fallot diagram), absent thymic shadow on chest X-ray, calcium level (low), facial features (micrognathia, low-set ears), FISH showing deletion. (4) Omenn syndrome: erythrodermic infant, desquamating skin, lymphocyte panel showing elevated but oligoclonal T cells with absent B cells, high IgE and eosinophilia. Each panel lists the gene/defect, inheritance, key labs, and treatment.</image>

Approach to Suspected CID

Laboratory Evaluation

The laboratory evaluation of suspected combined immunodeficiency follows a systematic approach. Initial studies include a complete blood count with differential, with particular attention to the absolute lymphocyte count (values below 2,500 in infants or below 1,500 in children are concerning). Lymphocyte subset quantification by flow cytometry measures CD3 (total T cells), CD4, CD8, CD19/CD20 (B cells), and CD16/CD56 (NK cells). T cell functional assessment includes mitogen proliferation assays (using PHA, ConA, or anti-CD3) and antigen-specific proliferation (using Candida or tetanus). Quantitative immunoglobulins (IgG, IgA, IgM, IgE) and specific antibody responses to pre- and post-vaccination titers are measured. TREC quantification is valuable when SCID is suspected early in life. Advanced studies include TCR spectratyping or V-beta repertoire analysis to assess for oligoclonality, chromosomal breakage studies for ataxia-telangiectasia and Nijmegen breakage syndrome, and telomere length measurement. Genetic testing through next-generation sequencing panels or whole-exome/genome sequencing provides definitive diagnosis.

Key Clinical Pearls

• X-linked SCID (IL2RG/common gamma chain) is the most common SCID subtype; phenotype is T-B+NK-
• All SCID subtypes are detected by TREC-based newborn screening (all have low/absent T cells)
• Transplant before 3.5 months of age yields >90% survival in SCID regardless of donor type; early diagnosis is critical
• Omenn syndrome (hypomorphic RAG) presents with erythroderma, elevated IgE, eosinophilia, and oligoclonal T cells; resembles GVHD
• ADA-SCID (T-B-NK-) is the only SCID with enzyme replacement therapy as a bridge (PEG-ADA) and has the most mature gene therapy data
• Complete DiGeorge syndrome requires thymic transplantation, NOT HSCT (no thymic microenvironment for T cell development from transplanted stem cells)
• WAS classic triad: thrombocytopenia with SMALL platelets (low MPV is the clue), eczema, and immunodeficiency; X-linked
• Elevated AFP is the hallmark lab finding in ataxia-telangiectasia; avoid unnecessary ionizing radiation

References

1. Puck JM. Newborn screening for severe combined immunodeficiency and T-cell lymphopenia. Immunol Rev. 2019;287(1):241-252.
2. Pai SY, et al. Transplantation outcomes for severe combined immunodeficiency, 2000-2009. N Engl J Med. 2014;371(5):434-446.
3. Notarangelo LD, et al. Primary immunodeficiency diseases: an update. J Allergy Clin Immunol. 2009;124(6):1161-1178.
4. Kwan A, et al. Newborn screening for severe combined immunodeficiency in 11 screening programs in the United States. JAMA. 2014;312(7):729-738.
5. Tangye SG, et al. Human inborn errors of immunity: 2022 update on the classification from the IUIS Expert Committee. J Clin Immunol. 2022;42(7):1473-1507.

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