SUMMARY

 

1.  Severe Combined Immunodeficiency (SCID) is one of the most serious primary immunodeficiencies because it is often fatal in the first year of life without proper diagnosis and therapeutic intervention.  The overall incidence is 1 in 50,000 newborns. 

 

2.  Although there are many different causes of SCID (more than 30 have been described to date), the unifying feature shared by all types is a complete absence of T-cell development.  Although some mutations allow for B-cell development to occur, B-cell function is invariably impaired due to the absence of T-cell co-stimulatory signaling.  Thus, patients have complete absence of cell-mediated and antibody-mediated immunity.  

 

3.  Clinically patients present early in infancy (before 6 months of age) with chronic candidiasis, protracted diarrhea and FTT, Pneumocystis jiroveci pneumonia, and chronic respiratory virus infections.  The thymic shadow is often absent in infants with SCID.  

 

4.  Lymphopenia (95% have an ALC < 2800 cells/mm3) is a laboratory hallmark of SCID.  Lymphocyte subset analysis reveals low T-cell numbers.  B-cell and NK-cell numbers may or may not be decreased depending on the type of SCID.  T-cell proliferation to mitogens is markedly decreased (<10% of control).    

 

5.  Classification of the different types of SCID based on the pattern of lymphocyte subpopulation depression is a useful strategy for determining the genetic defect.  T-B+NK+ SCID, T-B+NK- SCID, T-B-NK+ SCID, and T-B-NK- SCID are the 4 major categories.  

 

6.  X-linked SCID is caused by mutations in the common gamma chain of the IL-2 receptor and accounts for 50% of all SCID cases.  This form of SCID results in a T-B+NK- phenotype because the impaired signaling  impairs T-cell and NK cell development, but it does not affect B-cell development.  JAK3 deficiency (10% of all cases) results in an identical phenotype because JAK3 is a signaling molecule immediately downstream from the common gamma chain.  

 

7.  Adenosine Deaminase (ADA) deficiency accounts for 20% of all SCID cases and results in a T-B-NK- SCID phenotype.  Deficiency of ADA results in the accumulation metabolites that are toxic to all types of lymphocytes.  This particular type of SCID can be treated with enzyme replacement therapy (PEG-ADA).

 

8.  T-cell numbers can occasionally be normal if maternal T-cell engraftment has occurred or if a hypomorphic (leaky) SCID mutation allows for development of residual autoreactive T-cells that undergo clonal expansion and invade tissues such as the skin or liver (Omenns syndrome).  In both cases, circulating T-cells have a predominantly memory phenotype (CD45RO) and proliferation in response to mitogens is very low.  

 

9.  Once a patient with suspected SCID is identified, several immediate management steps must be implemented:

 

- Avoid all live viral vaccines (rotavirus, varicella, MMR, BCG)
- Only irradiated, CMV negative blood products should be used (to prevent GVHD and infections)
- Pneumocystis jiroveci prophylaxis with trimethoprim-sulfamethoxazole 
- IVIG replacement therapy
- Start HLA-typing for the patient and any siblings for possible hematopoietic stem cell transplantation     
  (HSCT)

 

10.  Even with supportive therapies, patients with SCID will not survive without a HSCT.  Patients transplanted before 3 months of age have a greater than 80% chance of survival while patients who are transplanted later and have suffered end organ damage from infections have a much lower success rate.   The source of stem cells also affects the outcome of a transplant (HLA-identical sibling transplants are the most successful).

 

11.  A successful newborn screening program for SCID utilizing T-cell receptor excision circles (TRECs) has been implemented in some states.  More widespread use of this program in the future will lead to earlier diagnosis of patients.  

 

 

 

OVERVIEW

 

     Severe Combined Immune Deficiency (SCID) is a genetically heterogeneous disorder that results in profound defects in T-cell and B-cell mediated immunity.  It should be considered an immunologic emergency because untreated patients rarely survive beyond the first year of life.  The overall incidence of SCID is between 1 in 50,000 and 1 in 100,000 neworns.  Both X-linked and autosomal recessive forms of disease have been described.  

 

     Although more than 30 types of SCID have been described, most cases present with a similar phenotype.  Common clinical features include chronic candidiasis (30%), protracted diarrhea and failure to thrive (60%), and Pneumocystis jiroveci pneumonia or chronic respiratory virus infections (60%).  Patients can also present with severe infections resulting from live viral vaccines (BCG, rotavirus).  The SCID phenotype is similar to what is seen in an HIV infected infant.  Unlike patients with antibody deficiencies (who may initially be asymptomatic due to protection from maternal IgG), SCID presents early in infancy before 6 months of age due to combined immune dysfunction.  

 

     A key laboratory feature is the presence of lymphopenia on a CBC with differential [95% of SCID infants have an absolute lymphocyte count < 2800 cells/mm3].  Unfortunately, this laboratory red flag is often overlooked, leading to life-threatening delays in diagnosis.  Further lymphocyte subset analysis by flow cytometry typically reveals very low or absent T-cells (<100 cells/µL).  B-cells and NK cells may or may not be present depending on the type of SCID. However, even if B-cells are present, they are unable  to produce functional antibodies without T-cell co-stimulation.  The pattern of lymphocyte subpopulation depression can be a useful classification system to determine the specific genetic defect.  Below are the 4 main subgroups of SCID along with key representative members for each group.

 

1. T-B+NK- SCID:

-Common Gamma Chain Deficiency (IL2RG)
-JAK3 Deficiency (JAK3)

 

2. T-B-NK+ SCID:

-RAG1 Deficiency (RAG1)
-RAG2 Deficiency (RAG1)
-Artemis Deficiency (DCLRE1C)

 

3. T-B-NK- SCID:

-Adenosine Deaminase Deficiency (ADA)
-Purine Nuceloside Phosphorylase Deficiency (PNP)
-Reticular Dysgenesis (AK2)

 

4. T-B+NK+ SCID:

IL7-R Deficiency (IL7-R)
CD45 Deficiency (CD45, LCA)
CD3  Deficiency (CD3G)
CD3  Deficiency (CD3D)
CD3  Deficiency (CD3E)
CD3  Deficiency (CD3Z, CD247)

 

     Three types of SCID account for approximately 80% of cases and these deserve special attention.    Mutations in the common gamma chain of the IL-2 receptor cause X-linked SCID (50% of cases) and results in a T-B+NK- phenotype.  B-cell numbers are preserved because the impaired cytokine signaling pathways do not interfere with B-cell development.  JAK3 deficiency is an autosomal recessive form of disease (10% of cases) that also causes a T-B+NK- phenotype.  JAK3 interacts intracellularly with the common gamma chain to transduce cytokine signals, thus a deficiency results in a phenotype identical to X-linked SCID.  Adenosine deaminase deficiency (ADA) deficiency is an autosomal recessive form of disease that causes 20% of cases.  ADA deficiency results in accumulation of toxic metabolites that interfere with development of all lymphocytes and results in a T-B-NK- phenotype.  

 

     While a genetic diagnosis for SCID is ideal, approximately 15% of cases do not have an identified genetic defect.  A specific diagnosis is not required to proceed forward with a life-saving stem cell transplantation procedure.  

 

 

         

PATHOGENESIS

 

     Details for the pathogenesis of each type of SCID will be available on the individual disease pages.  The causes of SCID are quite varied and range from defective cytokine signaling to impaired V(D)J recombination to accumulation of metabolites that are toxic to lymphocytes.  Regardless of the molecular cause, T-cell immunity is severely compromised and as a result, there is profound B-cell immunodeficiency as well.  

         

 

 

EVALUATION

 

 

Evaluation for SCID should be initiated for infants who have classic clinical features (FTT, diarrhea, thrush, PJP pneumonia, severe respiratory virus infections, infections from live viral vaccines) and low absolute lymphocyte counts.  SCID should be on the differential when evaluating infants for HIV infection given both conditions can present with a similar clinical phenotype. 

 

STEP 1:  Immune Evaluation 

 

-CBC with Differential
-Lymphocyte subset enumeration by flow cytometry (CD3, CD4, CD8, CD19, CD16/56)
-Naïve (CD45RA) and memory (CD45RO) T-cell enumeration by flow cytometry
-T-cell proliferation to Mitogens (PHA)
-IgG, IgA, IgM levels 
-Specific Antibody responses (if older than 6 months)
-Chest X-Ray

 

-The absolute lymphocyte count (ALC) should be calculated from the CBC (WBC multiplied by the lymphocyte percentage).  SCID presents with an ALC less than 2800 cells/mm3 in 95% of cases. 
-Low T-cell numbers are seen in most cases are SCID (although maternal T-cell engraftment and Omenns syndrome are notable exceptions).  B-cell numbers and NK cell numbers are useful for categorizing SCID into subgroups (ex. T-B+NK- SCID). 
-Very low naïve (CD45RA) T-cell numbers can be a useful clue for lack of thymic output (or absence of thymic tissue in complete DiGeorge syndrome).  In cases of maternal T-cell engraftment and Omenn's syndrome, the circulating T-cells have a predominantly memory (CD45RO) phenotype and have poor proliferation in response to mitogens.   
-Extremely low T-cell proliferation to mitogens is seen in SCID (<10% of control).  The large blood volume required to perform mitogen proliferation is often an issue with small infants.  Performing the proliferation assay with one stimulus (PHA) is acceptable and requires less blood.  
-Immunoglobulin levels before 6 months of age may reflect transplacentally aquired maternal IgG).  However, immunoglobulin levels can be low prior to 6 months in SCID due to accelerated consumption from recurrent infections. 
-A chest X-ray may reveal absent thymic tissue.  

 

STEP 2: Additional Immune Evaluation

The following tests may provide additional support for a diagnosis of SCID.
  
-TREC Analysis
-TCR Gene Rearrangement PCR (TCR Spectratyping)
-Maternal Engraftment Study 

 

-TRECs (T-cell receptor excision circles) are loops of DNA excised during TCR rearrangement in the thymus.  Because TRECs are not replicated with cell division, they are gradually diluted as T cells become activated and expand.  Thus, naïve T-cells that are recent thymic emigrants have high TREC numbers.  SCID patients typically have very low TREC numbers.
-TCR gene rearrangement is useful for identifying oligoclonally expanded T-cells.  This can be seen in maternal engraftment as well as Omenns syndrome.  
-Maternal T cells can occasionally undergo clonal expansion in patients with SCID.  Maternal T cells typically are CD8+CD45RO+ and proliferate poorly to mitogen stimulation.  Assessing for the presence of maternal cells in circulation (maternal engraftment) is useful because it can affect the selection of a stem cell donor and it may necessitate immunosuppression prior to transplantation.  

 

STEP 3:  Gene Sequencing to identify specific mutations.  

This may take 4 -8 weeks to be completed.  Transplantation should not be delayed if the above screening tests strongly suggest a diagnosis of SCID. However, testing is useful for future genetic counseling.  The following tests are commercially available through Correlagen Diagnostics:

 

1. T-B+NK- SCID:
               -Common Gamma Chain Deficiency (IL2RG)
               -JAK3 Deficiency (JAK3)

 

2. T-B-NK+ SCID:
               -RAG1 Deficiency (RAG1)
               -RAG2 Deficiency (RAG1)
               -Artemis Deficiency (DCLRE1C)

 

3. T-B-NK- SCID:
               -Adenosine Deaminase Deficiency (ADA)

 

4. T-B+NK+ SCID:
               
-IL7-R Deficiency (IL7-R)
               -CD3  Deficiency (CD3D)
               -CD3  Deficiency (CD3E)
               -ZAP70 Deficiency (ZAP70)  A T+B+NK+ SCID with absent CD8 T-cells

 

 

MANAGEMENT

 

Pending the completion of an immunologic evaluation for suspected SCID, it is critical to initiate certain measures to prevent life-threatening complications for patients.  The following precautions should be implemented immediately: 

 

1.  Avoid all live viral vaccines (rotavirus, varicella, MMR, BCG)
      - Severe vaccine strain disease can occur if SCID patients receive these vaccines. 


2.  Only irradiated, CMV negative blood products should be used 
      - Leukocytes from non-irradiated blood can cause graft versus host disease and CMV can  
         cause severe infections.   


3.  Pneumocystis jiroveci prophylaxis with trimethoprim-sulfamethoxazole
     - 4-6mg/kg/day of Trimethoprim component divided twice daily 3 days per week 


4.  IVIG replacement therapy


5.  HLA-typing for the patient and any siblings 
     - For possible Hematopoietic Stem Cell Transplantation (HSCT)

 

STEM CELL TRANSPLANTATION

 

1. High resolution HLA typing of the patient and any siblings should be performed as soon as a diagnosis of SCID is made.

 

2.In one study, infants transplanted before 3.5 months of age had better long term survival rates (94% vs 70%) and also had improved T cell development.  Transplantation prior to development of end organ damage also improves survival. 

 

3.Following transplantation, hematopoietic stem cells are educated in the patients thymus.  Thymic T cell development can be monitored using TRECs.  

 

4.HLA-matched sibling donors are the preferred choice because stem cells can be infused without any processing to remove mature T-cells.  The great advantage of an HLA-identical donor is the ability to transplant mature functional T cells along with the graft that expand and provide immediate immune reconstitution before de novo generation of T-cells from the thymus occurs.  The risk of developing GVHD is also very low.      

 

5.A matched unrelated donor or haploidentical (50% matched parent) donor is sometimes used when an HLA-identical donor is not readily available.  Unlike fully matched transplants, the stem cells have to be T cell depleted in order to prevent graft versus host disease.  It takes approximately 100 days for de novo T cells from the transplanted marrow to develop in the thymus.  The overall likelihood of engraftment and survival for haploidentical transplants is lower than HLA-identical transplants.  

 

6.SCID patients often require no conditioning prior to stem cell transplantation.  

 

7.Prior myeloablative conditioning does not appear to affect the rate of T cell engraftment in patients with T-B+ SCID.  However, myeloablation does appear to improve T cell engraftment and long-term survival in  T-B- SCID patients.       

 

8.Restoration of B cell function is more difficult and takes longer than restoration of T cell function following transplantation.  HLA identical sibling transplants are more likely to yield the development of normal B cell function than haploidentical transplants (90% vs 70% for T-B+ SCID).  Patients with T-B+ SCID have a higher rate of B cell reconstitution than patients with T-B- SCID.  Livelong IVIG therapy may be required if B cell reconstitution does not occur.   

 

                                                                       

RESOURCES

 

Diagnostic Resources       (LAB ORDER FORMS)

 

1.  Lymphocyte Subsets by Flow Cytometry for T-cell (CD3, CD4, CD8), B-cell (CD19), and NK cell (CD16/56).   

This test is commonly available at many academic centers as well as commercial laboratories including those listed below:

-Childrens Hospital of Philadelphia  Basic Lymphocyte Panel 
-Cincinnati Childrens Diagnostic Immunology Laboratory

-ARUP Laboratories (Lymphocyte Subset Panel 5)
          www.aruplab.com/guides/ug/tests/0095892.jsp
-Quest Diagnostics (Lymphocyte Subset Panel 1)
          www.questdiagnostics.com

 

2.  Naïve (CD45 RA) and Memory (CD45 RO) T cells by Flow Cytometry 

-Childrens Hospital of Philadelphia - DiGeorge Panel 
          The DiGeorge Panel is an advanced lymphocyte flow cytometry panel which includes CD45RA and CD45RO (this test is NOT a FISH for 22q11.2 deletion 
          or a SNP array)        
-Cincinnati Childrens Diagnostic Immunology Laboratory
-ARUP Laboratories (Lymphocyte Subset Panel 7)
          http://www.aruplab.com/guides/ug/tests/0095899.jsp

 

3.  T-cell proliferation to Mitogens

This test is available at many academic centers and commercial laboratories.

-Childrens Hospital of Philadelphia      
-Cincinnati Childrens Hospital
-ARUP laboratories

 

4.  TREC (T-cell receptor excision circle) Analysis

-Mayo Clinic TRECs:  
          Submit with sample and patient information sheet
          10ml lavender (EDTA) tube
          www.mayomedicallaboratories.com/test-catalog/Specimen/87959

 

5.  T-cell Recepror Gene Rearrangement (TCR Spectratyping)

-Mayo Clinic Laboratories 
-UCLA Diagnostic Molecular Pathology Laboratory

 

 

Literature Resources


1.  Kalman 2004
     SCID Review

 

2.  Baker 2009
     Newborn screening for SCID
    

3.  Antoine 2003
     HSCT for primary immune deficiency.  

     European experience 1968-1999
     

4.  Buckley 2010 
     HSCT for SCID
     

5.  Myers 2002
     HSCT in the neonatal period leads to superior thymic output and survival
     

6.  Bertrand 1999
     T-cell depleted non-identical HSCT for 214 SCID patients
   

7.  Buckley 2010
     B-cell function in SCID after HSCT
     

8.  Mueller 2001
     Maternal engraftment in SCID - study of 121 patients
 

9.  Veys 2010 
     Reduced Intensity Conditioning for PID HSCT