Severe Combined Immunodeficiencies

Indications for Testing

Early onset viral, fungal, or P. jirovecii infections

Laboratory Testing

Initial testing
- CBC – presence of lymphopenia; absolute lymphocyte count often <500 cells/µL
  - Normal lymphocyte count does not rule out SCID
- HIV molecular PCR testing in infants ≤15 months – rule out HIV infection
- Cell-mediated immune function (results within 24 hours) – recommended to determine necessity of immediate prophylactic treatment for children
- T-cell and B-cell immunodeficiency profile testing
  - T-cell testing at minimum should include CD4, CD45RA, CD45RO, CD8, CD4:8 ratio, CD3, CD19, and NK cell
- Quantitative immunoglobulins – screen for immunodeficiencies
- Lymphocyte antigen and mitogen proliferation with cytokine response – will be abnormal
  - Test requires at least 7 days for results
Molecular diagnosis available for prenatal testing
Second-line testing
- Analyte/enzyme testing – adenosine deaminase, purine nucleoside phosphorylase
- Genetic testing – ADA, PNP, IL2RG

Severe combined immunodeficiencies (SCID) are genetic disorders characterized by blocking T-lymphocyte differentiation in function and often are associated with abnormal development of other lymphocyte lineages (B cells and NK cells).

Epidemiology

Incidence – 1/75,000-100,000
Age – median 4-7 months
Sex – M:F, equal, except for X-linked forms

Identified Forms of SCID

X-linked SCID – 50-60% of SCID
Adenosine deaminase (ADA) deficiency – 30% of SCID

T- and B-Cell Variants of SCID

T- and B-Cell Variants of SCID
Gene	T-cell	B-cell	NK-cell	Genetics*	Comment
Impaired cytokine-mediated signalling
IL2RG	–	+	–	XL
JAK3	–	+	–	AR
IL7RA	–	+	+	AR	T-cell growth IL7
Defects in V(D)J recombination
DCCRE1C (Artemis)	–	–	+	AR	Recombinant of T- and B-cell receptors
RAG1	–	–	+	AR	Recombinant of T- and B-cell receptors Associated with Omenn syndrome
RAG2	–	–	+	AR	Associated with Omenn syndrome
PRKDC (DNA-PKcs)	–	–	+	AR	Radiosensitivity
LIG4 (DNA ligase IV)	–	–	+	AR	Radiosensitivity, dysmorphic facies, microcephaly, psychomotor delay
NHEJ1 (Cernunnos)	–	–	+	AR	Radiosensitivity, dysmorphic facies, microcephaly, psychomotor delay
Impaired signalling through pre-T-cell receptors
PTPRC (CD3Z)	–	+	+/–	AR	T-cell receptors only
CD3D, CD3F, CD3G	–	+	+	AR	T-cell receptors only
CK	–	+	+	AR
ZAP70	CD4 (+) CD8 (–) Lymphocytes	+	+	AR
Increased lymphocyte apoptosis
ADA	–	–	–	AR	Skeletal alterations, hepatitis, neonatal sensorineural deafness
AK2	–	+	–	AR
NP (PNP)	–	+	–	AR	Neurological problems
Defects in thymus embryogenesis
Winged Helix Nude (WHN)	–	+	+	AR	Alopecia, embryonic neural tube defects
*AR = autosomal recessive; XL = X-linked

Inheritance

All but the X-linked forms are autosomal recessive

Pathophysiology

Block in T-lymphocyte differentiation or growth and variable abnormal development of other lymphocyte lineages

Clinical Presentation

Most newborns appear normal
Early onset of severe infections (earliest are Pneumocystis jirovecii viral, also fungal, followed in many cases at 4-6 months by bacterial infection)
Growth failure
Persistent diarrhea
Desquamative skin rash, elevated liver enzymes, and GI bleeding
Occurrence of graft-versus-host disease upon exposure to maternal lymphocytes, during delivery, or by nonirradiated blood transfusion
- Most prominent in skin and liver
- May be associated with autoimmune thrombocytopenia or pancytopenia
- Rejection of hematopoietic stem cell transplant from father or donors
Omenn syndrome – atypical SCID associated with null mutations

Treatment

Prophylaxis for P. jirovecii, fungal infections, and, in some cases, bacterial infections
Hematopoietic stem cell transplant
For ADA deficiency – exogenous enzyme replacement
Gene therapy for X-linked SCID and ADA deficiency

Tests generally appear in the order most useful for common clinical situations

Test name: CBC with Platelet Count and Automated Differential

ARUP #: 0040003

Methodology: Automated Cell Count/Differential

Use:

Determine presence of lymphopenia

Test name: Human Immunodeficiency Virus Type 1 (HIV-1) Antibody with Reflex to Human Immunodeficiency Virus Type 1 (HIV-1) Antibody Confirmation by Western Blot

ARUP #: 2005375

Methodology: Qualitative Chemiluminescent Immunoassay/Qualitative Western Blot

Use:

Rule out HIV

Test name: Cell-Mediated Immune Function Screen

ARUP #: 0051273

Methodology: Cell Culture/Quantitative Chemiluminescent Immunoassay

Use:

May be useful as a screen for immunodeficiency and for determining the necessity of immediate prophylactic treatment in children

Limitations:

Time sensitive

Assay does not directly quantify level of immunosuppression

Test name: Lymphocyte Subset Panel 6 - Total Lymphocyte Enumeration with CD45RA and CD45RO

ARUP #: 0095862

Methodology: Quantitative Flow Cytometry

Use:

Preferred lymphocyte subset panel for the investigation of primary immunodeficiency disorders

Test enumerates the percent and absolute cell count of lymphocyte subsets in whole blood for CD4 (helper T cells), CD45RA (naive helper T cells), CD45RO (memory helper T cells), CD8 (suppressor T cells), CD4: CD8 ratio, CD3 (total T cells), CD19 (B cells), NK cells

Test name: Lymphocyte Subset Panel 7 - Congenital Immunodeficiencies

ARUP #: 0095899

Methodology: Quantitative Flow Cytometry

Use:

Acceptable lymphocyte subset panel for the investigation of primary immunodeficiency disorders

Test enumerates the percent and absolute cell count of lymphocyte subsets in whole blood for CD4 (helper T cells), CD45RA (naive helper T cells), CD45RO (memory helper T cells), CD8 (suppressor T cells), CD4: CD8 ratio, CD3 (total T cells), CD19 (B cells), NK cells

Test name: B-Cell Immunodeficiency Profile

ARUP #: 0095940

Methodology: Quantitative Flow Cytometry

Use:

Determine presence of B-cell deficiency

Measures circulating B cells (CD19), their surface immunoglobulins (total Ig, IgG, IgD, IgM, and IgA), and HLA-DR

Test name: Immunoglobulins (IgA, IgG, IgM), Quantitative

ARUP #: 0050630

Methodology: Quantitative Nephelometry

Use:

Initial test in the workup of immunoglobulin disorders

In adults and older children with suspected hypogammaglobulinemia, order in conjunction with serum protein electrophoresis and immunofixation

Quantitative nephelometry determines serum immunoglobulin concentrations of IgG, IgM, and IgA

Test name: Lymphocyte Antigen and Mitogen Proliferation Panel

ARUP #: 0096056

Methodology: Cell Culture

Use:

Screen for immunodeficiencies

Panel includes testing for phytohemagglutinin, concanavalin A, pokeweed mitogen, Candida antigen, and tetanus antigen

Limitations:

Time sensitive

Test name: Lymphocyte Antigen and Mitogen Proliferation Panel with Cytokine Response to Mitogens, 12 Cytokines

ARUP #: 0051584

Methodology: Cell Culture/Multiplex Bead Assay

Use:

Screen for inheritance of immunodeficiencies

Components include cytokine production by mononuclear cells in response to mitogen stimulation, which include: interleukin 2, interleukin 2 receptor, interleukin 12, interferon gamma, interleukin 4, interleukin 5, interleukin 10, interleukin 13, interleukin 1 beta, interleukin 6, interleukin 8, and tumor necrosis factor alpha

Limitations:

Time sensitive

Additional Tests Available

Test name: Adenosine Deaminase, RBC

ARUP #: 0083001

Methodology: Kinetic Spectrophotometry

Comments:

May be used as a marker of SCID; lack of ADA allows deoxyadenosine to accumulate and kill lymphocytes

Test name: Toll-Like Receptor Function Assay

ARUP #: 0051589

Methodology: Cell Culture/Quantitative Multiplex Bead Assay

Comments: