Severe Combined Immunodeficiency (SCID)

Severe Combined Immunodeficiency (SCID) represents the most severe form of primary immunodeficiency, characterised by profound defects in both cellular and humoral immunity. It is a paediatric emergency: without definitive immune reconstitution, affected infants typically succumb to overwhelming infection within the first two years of life.

SCID encompasses a heterogeneous group of monogenic disorders, all of which disrupt T-cell development and, variably, B-cell and NK-cell maturation. Over 20 causative genes have been identified, with X-linked SCID (IL2RG mutations) and adenosine deaminase deficiency (ADA mutations) together accounting for approximately 65% of cases worldwide.

Australian incidence: SCID affects approximately 1 in 58 000–75 000 live births, translating to an estimated 4–6 new cases per year nationally. Since the introduction of newborn screening across all Australian jurisdictions (completed by 2023), case ascertainment has improved, with presymptomatic diagnosis now the expected standard.

Key Australian referral centres include the Royal Children's Hospital Melbourne, the Children's Hospital at Westmead Sydney, Queensland Children's Hospital Brisbane, and Perth Children's Hospital. Telehealth multidisciplinary teams (MDTs) enable early specialist involvement for patients in regional and remote areas.

SCID is classified by the immunological phenotype (T, B, NK cell presence) and the underlying genetic defect. The two types discussed in detail below represent the most common subtypes encountered in Australian paediatric practice.

Other SCID Genotypes

Less common but clinically important forms include:

• JAK3 deficiency — autosomal recessive; T−B−NK− phenotype (mimics X-linked SCID but affects both sexes)
• RAG1/RAG2 deficiency — autosomal recessive; T−B−NK+; Omenn syndrome variant presents with erythroderma, eosinophilia, lymphadenopathy, hepatosplenomegaly
• Artemis deficiency (DCLRE1C) — radiosensitive SCID; important for pre-transplant conditioning (avoid irradiation-based regimens)
• IL7Rα deficiency — autosomal recessive; T−B+NK+ phenotype
• CD3δ, CD3ε, CD3ζ deficiencies — rare; T−B+NK+ phenotype
• PNP deficiency — purine nucleoside phosphorylase; neurological features prominent

Symptoms typically begin by 3–6 months of age, coinciding with waning maternal antibody. The clinical presentation is characterised by recurrent, severe, persistent, or opportunistic infections in a previously well-appearing infant.

Classic Presenting Features

• Recurrent sinopulmonary infections — > 2 episodes of pneumonia, otitis media, or sinusitis in 6 months
• Chronic diarrhoea — often persistent and watery; may be due to opportunistic organisms (Cryptosporidium, rotavirus, norovirus)
• Failure to thrive — poor weight gain, crossing centiles downward on WHO growth charts
• Oral/perianal thrush — persistent, extending beyond the oral mucosa, refractory to standard antifungal therapy
• Opportunistic infections — PJP pneumonia, disseminated BCG-osis (in vaccinated infants), invasive candidiasis, CMV viraemia
• Skin manifestations — generalised rash (GVHD from maternal engraftment or Omenn syndrome), eczematous dermatitis
• Hepatosplenomegaly — particularly in Omenn syndrome or ADA-SCID

ADA-SCID–Specific Features

In addition to infectious manifestations, ADA deficiency causes toxicity from accumulated deoxyadenosine and deoxyadenosine triphosphate (dATP) in non-immune tissues:

• Skeletal abnormalities — costochondral junction widening, cupped ribs, rachitic changes visible on chest X-ray
• Hepatic dysfunction — transaminitis, hepatomegaly
• Neurodevelopmental delay — sensorineural hearing loss, motor delay, cognitive impairment
• Auditory toxicity — high-frequency hearing loss; audiometry recommended at diagnosis

Omenn Syndrome

A phenotypic variant seen in hypomorphic RAG1/2, Artemis, or IL7Rα mutations. Features include generalised erythroderma, alopecia, lymphadenopathy, hepatosplenomegaly, eosinophilia, and elevated IgE. Maternal T-cell engraftment can produce a similar picture in classical SCID.

Newborn Screening (NBS)

All Australian states and territories now include the T-cell receptor excision circle (TREC) assay as part of the newborn bloodspot screening programme. TRECs are a biomarker of thymic output; low or absent TRECs indicate reduced naive T-cell production.

An abnormal TREC result triggers immediate recall for confirmatory testing. In addition to SCID, low TRECs may be detected in other conditions with T-cell lymphopenia (DiGeorge syndrome, prematurity, certain syndromic conditions), so a low TREC is not diagnostic of SCID alone.

Confirmatory Diagnostic Workup

Diagnostic Criteria

A diagnosis of SCID is established when all three of the following are met:

1. Absent or severely reduced T cells (CD3+ count < 300 cells/μL) with evidence of impaired T-cell function (PHA lymphocyte proliferation < 10% of control)
2. No evidence of secondary cause of immunodeficiency (HIV — always exclude with HIV PCR; maternal medications; protein-losing enteropathy)
3. Confirmed pathogenic mutation in a known SCID gene (or clinical phenotype consistent with SCID pending genetic confirmation)

SCID results from defects in genes critical for lymphocyte development, survival, or function. The common γ chain (γc, encoded by IL2RG) is shared by receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. IL-7 signalling is essential for thymic T-cell progenitor survival and expansion; IL-15 is essential for NK-cell development. Loss of γc therefore abolishes T and NK lymphopoiesis.

In ADA deficiency, the enzyme adenosine deaminase catalyses the deamination of adenosine to inosine and deoxyadenosine to deoxyinosine in the purine salvage pathway. Loss of ADA activity leads to accumulation of deoxyadenosine and its phosphorylated metabolite dATP, which:

• Inhibits ribonucleotide reductase, blocking DNA synthesis and lymphocyte proliferation
• Promotes apoptosis of developing thymocytes and peripheral lymphocytes
• Causes direct toxicity to hepatocytes, osteocytes, and auditory neurons through mitochondrial dysfunction

The degree of enzyme residual activity correlates with disease severity — complete absence causes classical infantile SCID; partial activity may present later (> 12 months) as a "leaky" or delayed-onset phenotype.

Immunological Consequences

All forms of SCID share a common functional outcome: absent adaptive immunity manifesting as:

• No T-cell–mediated immunity → no delayed-type hypersensitivity, no graft rejection, no control of viral/fungal/opportunistic infections
• Absent or non-functional B cells → no specific antibody production; immunoglobulin levels fall progressively as maternal IgG wanes
• Absent NK cells (in some forms) → impaired innate antiviral and anti-tumour surveillance

A tiered investigation strategy enables rapid presumptive diagnosis while genetic confirmation is awaited.

Transplant outcomes are heavily influenced by the infant's clinical condition at the time of HSCT. The following risk categories guide urgency and inform donor selection:

Treatment of SCID has two phases: immediate stabilisation (bridging therapy) and definitive immune reconstitution (HSCT or gene therapy).

A. Bridging / Stabilisation Measures

Initiate immediately upon clinical suspicion — do not wait for genetic confirmation:

Supportive Measures

• Reverse isolation — single room, HEPA filtration, strict hand hygiene; minimise visitors; avoid soil/plant exposure
• Nutritional support — nasogastric or gastrostomy feeding if oral intake inadequate; dietitian involvement essential
• No live vaccines — BCG, rotavirus (RotaTeq/Rotarix), OPV, MMR, varicella, yellow fever, live attenuated influenza vaccine (LAIV) all contraindicated. Household contacts should not receive OPV or live attenuated vaccines that shed.
• Irradiated blood products — all transfusions must be irradiated and CMV-safe (leucodepleted) to prevent transfusion-associated GVHD and CMV transmission
• Genetic counselling — refer family to clinical genetics for carrier testing, prenatal diagnosis options, and reproductive planning

B. Definitive Treatment — Haematopoietic Stem Cell Transplantation (HSCT)

HSCT remains the gold standard and most widely available curative therapy for SCID in Australia.

Donor Priority

Conditioning Regimens

• T−B−NK− SCID (e.g. ADA-SCID, JAK3): Reduced-intensity conditioning (RIC) usually required — busulfan (targeted AUC-based dosing) + fludarabine ± serotherapy (anti-thymocyte globulin or alemtuzumab). Avoid irradiation in radiosensitive SCID (Artemis/DCLRE1C).
• T−B+ NK− SCID (e.g. X-linked SCID): Reduced-intensity or no conditioning; NK-cell alloreactivity may assist engraftment. Some centres use no conditioning for MSD transplants with excellent results.
• Artemis/DCLRE1C deficiency: Absolutely avoid total body irradiation and busulfan is used with caution — alkylator-free regimens with fludarabine + treosulfan preferred.

C. Definitive Treatment — Gene Therapy

Autologous gene therapy using lentiviral vector–mediated gene transfer has transformed the treatment landscape, particularly for ADA-SCID.

Gene Therapy vs HSCT — Key Considerations

• Advantages of gene therapy: No risk of GVHD (autologous cells); no need for immunosuppressive conditioning; avoids donor-related complications; available when no matched donor exists
• Limitations: Currently TGA/approved only for ADA-SCID; requires specialised manufacturing; insertional mutagenesis risk (very low with modern lentiviral vectors but long-term monitoring required); single-centre access in Australia
• Current status for X-linked SCID: Lentiviral gene therapy trials ongoing internationally (e.g. in the US via FDA IND); not yet standard of care in Australia. HSCT remains first-line for IL2RG-deficient SCID.

D. Enzyme Replacement Therapy (ERT) — PEG-ADA

Pre-Transplant Monitoring

• Weekly FBC, CRP, liver function tests
• CMV, adenovirus, EBV viral load (PCR) — weekly from diagnosis
• Respiratory virus PCR panel if symptomatic
• Growth monitoring — weight, length, head circumference fortnightly on WHO centile charts
• IgG trough levels before each IVIg infusion — target > 8 g/L

Post-Transplant Monitoring

• Chimerism studies (STR or donor-specific PCR) — day +30, +60, +100, +180, +365, then annually
• Lymphocyte subsets (CD3, CD4, CD8, CD19, CD16/56) — monthly for first 6 months, then 3-monthly
• IgG levels and immunoglobulin subclass — continue IVIg until B-cell engraftment and specific antibody responses confirmed
• Vaccine responses — check post-immunisation serology (tetanus, Hib, pneumococcal) at 12 and 24 months post-HSCT
• CMV, EBV, adenovirus monitoring — weekly during first 100 days, then as clinically indicated
• GVHD surveillance — skin, gut, liver assessment at each visit
• Neurodevelopmental assessment — particularly in ADA-SCID; early intervention referral if delay identified

Long-Term Follow-Up

• Annual immunology review — lifelong at a specialist centre
• Catch-up vaccination programme — live vaccines only after confirmed immune reconstitution (typically > 12–24 months post-HSCT, with specialist clearance)
• Fertility counselling — conditioning may affect fertility; refer to reproductive endocrinology
• Secondary malignancy surveillance — particularly with alkylating conditioning regimens

Any febrile episode in an infant with known or suspected SCID must be treated as a medical emergency. Initiate empirical broad-spectrum antimicrobials immediately after blood cultures and relevant specimens are obtained.

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