MED-NERD

Evaluation and diagnosis of Severe combined immunodeficiency disorder (SCID)

Immunological studies:

-Assessment of immunoglobulins: Quantitative Serum Immunoglobulins (IgG including IgG1, IgG2, IgG3, and IgG4, IgM, IgA, IgE)

-Assessment of antibody activity

-Assessment of post-immunization IgG antibodies (Diphtheria and Tetanus toxoid, Polio, and Pneumococcal polysaccharide)

-Assessment of post-exposure IgG antibodies ( Measles, Rubella, Varicella zoster)

-Assessment of isohemagglutinins (IgM): Anti-type A blood, Anti-type B blood

-Heterophile antibody

-Immunodiagnosis of infectious diseases (hepatitis B, hepatitis C, HIV, HTLV and dengue)

-Serum protein electrophoresis

-Anti-streptolysin O titer

-Total lymphocyte count

-B lymphocytes (CD19 and CD20)

and T-lymphocyte counts (CD3, CD4, and CD8)

-CD4/CD8 ratio

-NK cell count and function

-Lymphocyte stimulation assays (Phorbol ester and ionophore, Phytohemagglutinin, Antiserum to CD3)

-Assessment of Phagocytic function and activity "Nitroblue tetrazolium (NBT) test"

-Assessment of the complement system

-Immunoprecipitation tests, ELISA, or Western blotting to detect C3 and C4 serum levels, Factor B serum and C1 inhibitor serum levels

-Haemolytic assays (CH100, CH50, AH50)

Microbiological studies:

>Bacterial culture, PCR

>Cerebrospinal fluid culture and analysis

>Nasopharyngeal swab

>Sputum culture and PCR

>Urine testing for sepsis, proteinuria

>Stool culture

Autoimmune Studies:

-Anti-nuclear antibodies (ANA), anti-ds DNA, rheumatoid factor, anti-histones, anti-neutrophil, anti-RBC, antiplatelet

Coagulation tests:

-Prothrombin time, Thrombin time, Bleeding time

-Fibrinogen level

-Factor V assay

Other investigations:

-CT scan, Chest x-ray, Diagnostic ultrasound

-Bone marrow biopsy

-DNA testing

-Blood chemistry, Levels of cytokines, Complete blood cell count

-Histopathological studies

-Liver function test

-Tuberculin test

-Fluorescent in situ hybridization (FISH)

-Tumour markers

Genetic diagnosis:

Genetic testing of SCID is required for antenatal and genetic counselling. Some types of SCID especially radiosensitive types show higher toxicity to alkylator-based chemotherapeutic agents used in the treatment (pretransplant program) so genetic testing is necessary for those cases.

New-born screening:

1-Assessment of T cell receptor excision circles (TRECs):

The process of TRECs production performed briefly through the following steps:

-Each T cell has its own receptor (TCR) to recognize foreign antigens bound to its MHC molecule.

-In thymocytes, rearrangement of DNA gene and linear re-assembly to produce large number of unique TCRs with combination of TCR elements with each cell. Specific enzymes produce double strand breaks at specific sequences.

-Unique rearranged T cell receptors are produced after cutting and ligation of the DNA.

-Maturation of T cell precursors that express TCRs leading to the release of diverse repertoire of naïve T cells into peripheral blood.

-Some of the excised DNA fragments of the TCR are ligated at their ends to produce circular DNA by-products called T cell receptor excision circles (TRECs).

Using TRECs as a screening method:

An effective and immediate method used in the United States to detect SCID new-borns is through assessment of T cell receptor excision circles (TRECs) in the residual dried blood spots (DBS). Amplification of DNA isolated from the dried blood spots is performed through PCR, and then used to detect TRECs that act as a marker for naïve T cells. During T cell receptor recombination process, TRECs are the by-products from the process. To provide efficient interventions for infants with T cell insufficiency, determination of appropriate cut-off levels for TRECs is required. If the screening results are positive, laboratory investigations are required to determine the type of SCID. New-born screening (NBS) improved the prognosis of SCID over the last 10 years. Screening allows early detection and avoiding various complications as well as early therapy through HSCT or gene therapy. It also provides information about the incidence and the spectrum of the disorder and reflects certain aspects of the population. However, some types of SCID such as ZAP70 defect, MHC class II deficiency, and Late-onset ADA deficiency have been reported to have normal TREC levels.

2-Assessment of kappa-receptor excision circles (KREC):

Detection of delayed-onset ADA SCID and B cell lymphopenia can be performed through assessment of kappa-receptor excision circles (KREC).

Other methods for screening that were suggested before:

3-A simple complete blood count (CBC):

Previously, it was suggested that a simple CBC and differential can be used to detect lymphopenia and many cases of SCID. However, leaky SCID cases with oligo clonal T cell expansion (Omenn syndrome) and SCID cases with maternal T cell engraftment can not be detected using this simple method.

4-Flow cytometry with liquid blood samples:

Another method for screening of SCID that was suggested if flow cytometry with liquid blood samples but it is expensive to obtain liquid blood sample for every infant instead of using the dried samples (DBS).

5-Protein assays for T cell specific markers:

It was suggested that protein assay for specific T cell markers such as CD3 chains can be used as screening method for SCID. Low amount of protein that can be obtained from DBS samples limited the efficiency of this method.

6-Detection of DNA sequence:

New mutations in SCID frequently occur, therefore gene sequencing is not suitable as a primary screening test for SCID.

Differential diagnosis of SCID:

-Other forms of combined immunodeficiency including agenesis of the thymus or T cell deficiency (e.g., DiGeorge syndrome or CHARGE syndrome)

-Wiskott-Aldrich syndrome

-Malabsorption syndromes

-HIV/AIDS

-Zeta-chain-associated protein 70 deficiency

-Calcium channel deficiencies

-NF-kappa-B essential modifier (NEMO) deficiency

Prognosis of SCID:

The outcomes of SCID are extremely poor. Performing a successful transplantation or gene therapy may improve the outcomes. Long-term antimicrobial agents are required for the management which is mainly personalized management based on genetic studies.

References:

(1)Justiz Vaillant AA, Mohseni M. Severe Combined Immunodeficiency. 2023 Jan 1. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan–.
https://www.ncbi.nlm.nih.gov/books/NBK539762/
(2)Kumrah R, Vignesh P, Patra P, Singh A, Anjani G, Saini P, Sharma M, Kaur A, Rawat A. Genetics of severe combined immunodeficiency. Genes Dis. 2019 Jul 24;7(1):52-61.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7063414/
(3)Fischer A, Hacein-Bey-Abina S. Gene therapy for severe combined immunodeficiencies and beyond. J Exp Med. 2020 Jan 6;217(2):e20190607.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7041706/
(4)Puck JM. Newborn screening for severe combined immunodeficiency and T-cell
lymphopenia. Immunol Rev. 2019 Jan;287(1):241-252.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6324582/
(5)Haddad E, Hoenig M. Hematopoietic Stem Cell Transplantation for Severe Combined Immunodeficiency (SCID). Front Pediatr. 2019 Nov 19;7:481.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6877719/