Hypogammaglobulinemia

Etiology

Hypogammaglobulinemia can be of primary or secondary origin. Primary immunodeficiencies result from genetic disorders and/or chromosomal anomalies during the development of the immune system. Secondary causes are usually induced by an external or acquired factor such as a corticosteroid or immunosuppressant drug, nutritional disorders, infections, chemotherapy, malignancy, nephrotic syndrome, other metabolic diseases, and hazardous environmental conditions.[5] It is important that physicians distinguish between primary and secondary causes of hypogammaglobulinemia to provide appropriate treatment.[6]

Categories of primary humoral Iimmunodeficiencies include X-linked Agammaglobulinemia, CVID, Hyper IgM syndrome, selective/Isolated Ig deficiency, and Transient hypogammaglobulinemia of Infancy.

X-linked Agammaglobulinemia (XLA) or Bruton Agammaglobulinemia

It is the first primary immune deficiency disease with a genetic cause identified. Mutation noted in the Bruton tyrosine kinase gene (Btk) is an important component for the maturation of pre-B cells. This leads to a lack of plasma cells with low levels of Ig and therefor no humoral response.[7][8] Inheritance of primary agammaglobulinemia is X-linked, with most mutations being familial with few autosomal recessive forms.[9]

Common Variable Immunodeficiency (CVID, Variable Pan-hypogammaglobulinemia)

It is defined by immune dysfunction of B-cells, T-cells, and dendritic cells due to a defect in the ability of B-cells to differentiate into plasma cells decreasing the secretion of immunoglobulins. Approximately 25 to 50% of CVID patients have a single gene defect which can lead to molecular deficiencies, epigenetic changes affecting gene expression, immune cell abnormalities such as increased B-cell apoptosis, and impaired antibody production. Genetic defects involve mutations in either the nucleus, cytoplasm or cell surface and can be autosomal dominant or recessive.[10][11] Intrinsic B-cell defect can play a role by involving mutations in CD 19 at 16p11.2 leading to CD 19 deficiency. Another gene identified is the transmembrane activator calcium-modulator and cyclophilin ligand interactor (TACI) which impacts B cell maturation. TACI deficient CVID involving the TACI gene at 17p11.2 is another well-known mutation. Tumor necrosis factor (TNF) receptor gene defects have been studied and also are involved.[12][13] Other T-cell defects with mutations in ICOS, 2q33 (ICOS-deficient CVID) SH2DIA (responsible for X-linked lymphoproliferative disease [XLP]), CD19, CD20, CD21, CD81, BAFF-R (B-cell–activating factor of the tumor necrosis factor family receptors) and 2 genes that encode DNA methyltransferase (DNMT3B and ZBTB24) have been reported. Patients with CVID and IgA deficiency have a common genetic basis. CVID is common in patients with 1st degree relative with IgA deficiency, and some patients with IgA deficiency may later become panhypogammaglobulinemic.[14]

Transient Hypogammaglobulinemia of Infancy (THI)

It is characterized by hypogammaglobulinemia with adequate antibody response. It is observed during the first three to six months of life secondary to prolongation of the physiologic nadir of immunoglobulin levels. Ig levels normalize by three years of age in most children (age range 2-6 years).[15] The cause is unknown but some theories have been proposed including failure of T-cells to stimulate the synthesis of B -cells and antibodies, IgG production suppression by maternal IgG, low levels of critical cytokines, and familial genetic variations.[16]

Selective IgA Deficiency

Decreased levels or complete absence of IgA in both serum and secretory forms (IgA <10mg/dl). B-cells are phenotypically normal. Selective IgA deficiency has autosomal dominant inheritance with variable expressivity. Chromosomal abnormalities and mutations in JAK3, TACI, RAG1, RAG2, STAT1 have been reported as a potential cause of selective IgA deficiency. Patients treated with drugs like phenytoin, gold, D-penicillamine who developed CVID, were found to have secondary IgA deficiency. It is also seen in conditions like ataxia-telangiectasia. Infections like CMV, EBV, Rubella, and Toxoplasmosis may also cause transient IgA deficiency.[17]

Hyper-IgM Syndrome

It is a rare primary immunodeficiency disorder characterized by normal to elevated IgM levels with decreased levels of IgG, IgA, and IgE immunoglobulins caused by mutations in genes on the X-chromosome and autosomal chromosomes which are responsible for the B-cell class switch from IgM to other classes of antibodies.[18][19] On the X chromosome, the CD40 ligand (hyper-IgM syndrome type 1 [HIGM1]) and NEMO (nuclear factor κB essential modulator, XHM-ED) genes play an active role. Autosomal chromosomal involvement is seen with 3 genes including the activation-induced cytidine deaminase (AID) gene (hyper-IgM type 2 [HIGM2]) on chromosome 12, the uracil DNA glycosylase gene (UNG, hyper-IgM type 5 [HIGM5]) on chromosome 12, and the CD40 gene (hyper-IgM type 3 [HIGM3]) on chromosome 20.[14]

Epidemiology

For X-Linked Agammaglobulinemia, prevalence is approximately 1 in 379,000 live births (1 in 190,000 male births) reported from a United States registry.[8]

Of the primary immunodeficiencies, Common Variable Immunodeficiency is the most common after selective IgA deficiency and is noted to have a high prevalence of 1 in every 10000 to 50000 live births. CVID affects 1 in 25.000 individuals with some evidence of higher prevalence in those from northern Europe.[20][21][22] CVID in pediatric age groups presents mostly after puberty, but in all patients with CVID, about 25% present in childhood or adolescence with a peak age of diagnosis at 8 years of age.[23][24] The male to female ratio is approximately 5 to 3.[5]

THI is usually diagnosed retrospectively, predominant in males with a male to female ratio of 2 to 1. The cause is unknown.[15]

Pathophysiology

The immune system has the potential to produce several species of antibodies to remove a variety of different antigens. Immunoglobulins are glycoproteins produced by plasma cells known as antibodies that are present in extracellular fluid and serum. Different classes exist for Ig such as IgG with subtypes 1-4, IgA with subtypes 1-2, IgM, IgD, and IgE. Function and longevity of the subclasses differ with some living for weeks and others for hours. Anti-infectious immunity is primarily regulated by IgG, IgA, and IgM.[25] 

Ig heavy chain class switching takes place rapidly after the activation of mature naïve B cells. This results in a switch from expression of IgM and IgD to IgG, IgA, or IgE. This isotype switching of the antibody then enhances the immune reaction by adequate removal of the pathogen that initially induced the humoral response.[26] Different mechanisms exist for anti-infectious immunity such as antigen neutralization, activation of the classical complement pathway with bactericidal effect, Ab-dependent cytotoxicity, and opsonization with phagocytosis. On serum protein electrophoresis, Ig travels primarily in the region of gamma globulins and normal values are determined based on the age of the individual. Different categories and their pathophysiology are described below.

X-linked Agammaglobulinemia (XLA) or Bruton Agammaglobulinemia

Bruton tyrosine kinase gene (Btk) is essential for the development of mature B-cells. Mutations in this gene will cause low levels of plasma cells and immunoglobulins resulting in little to no humoral response.[26][8] This may cause immunoglobulin deficiency (hypogammaglobulinemia) or a complete absence of immunoglobulins (agammaglobulinemia).[18]

Common Variable Immunodeficiency (CVID)

Defected B-cells lead to low levels of plasma cells and therefore low immunoglobulins. Defects in T cells and dendritic cells are present causing both humoral and cell-mediated immune response disturbance. CVID is defined based on age-specific levels of serum Ig, response to immunization (poor vs absent), absence of profound T-cell immunodeficiency, and any other immunodeficiency states.[27][28][29]

Transient Hypogammaglobulinemia of Infancy (THI)

Seen in 3-6 months of life with low levels of immunoglobulins due to the presence of maternal antibodies crossing the placenta. This indirectly causes immunosuppression for the infant. In addition to IgG, IgM and IgA can also be suppressed during this stage. Ig levels normalize by 3 years of age in most children.[15] Patients with THI may present with recurrent infections or be asymptomatic. The diagnosis can be incidental when evaluated for other reasons.

Selective IgA Deficiency

The main defect consists of immunoglobulin class switching. IgA bearing B lymphocyte cannot convert into an IgA secreting plasma cell.[17] Deficient T helper cells, impaired B-cell signaling, and cytokine disturbance can play a role in defective antibody production. Infectious etiologies and drug-induced deficiency are some secondary causes.

Hyper-IgM Syndrome

This is a rare primary immunodeficiency disorder with normal to elevated IgM levels and decreased levels of IgG, IgA, and IgE immunoglobulins.[30] 

History and Physical

Physical examination may be normal or patients may exhibit signs and symptoms of chronic illnesses.[23]

Some common clinical manifestations are listed below:

1. Infections

• Recurrent infections of the sino-pulmonary tract with encapsulated pyogenic bacteria (Streptococcus pneumoniae, Hemophilus influenzae type B, Streptococcus pyogenes, Pseudomonas species, and some atypical species).[14] Bacterial infections may be present both before with recurrent otitis media being the most common and after the diagnosis with pneumonia, acute or chronic sinusitis.[7][8][31][32] Patients requiring several rounds of antibiotics to eradicate infections is a common finding.
• Tympanic membrane scarring from recurrent otitis can be noted on the physical exam.
• In XLA by 2 years of age, more than 50% of patients have had serious infections with encapsulated bacteria, and virals including coxsackievirus, enterovirus, and echovirus infections are also common.[33][34]
• In CVID, sinopulmonary infections and complications like bronchiectasis and bronchial wall thickening being common in all age groups. History of at least one episode of bacterial pneumonia is present in almost 75% of the patients prior to diagnosis.[22]
• Urogenital infections by mycoplasma, gastrointestinal infections by Giardia lamblia (common in IgA deficiency), and Campylobacter jejuni have also been reported in CVID and XLA patients. CNS involvement with intracranial granulomas manifesting as seizures, ataxia, weakness, headaches, vision loss, and coma has been seen in children with CVID.[35]
• Opportunistic infections are common in Hyper-IgM syndrome. Congenital rubella syndrome may also cause elevated IgM levels.[36]

2. Autoimmune Manifestation

•  Cytopenias, polyarthritis, and other immune dysfunctions. Autoimmune cytopenias are more common in children with CVID compared to adults. 
•  Vitiligo, Psoriasis, ITP, Hyper/Hypothyroidism, SLE, JIA, IBD can be the presenting disorder in patients with CVID.[37][38][39] Hematologic disorders are common manifestations of autoimmunity in CVID.[40]
•  Antibodies to IgA are reported in almost 44% of patients with selective IgA deficiency with an increased incidence of autoimmune diseases and malignancies.

3. Hepatomegaly, Splenomegaly, and Peripheral Lymphadenopathy

• Physical exam findings in children and adults seen in hypogammaglobulinemia.[22]

4. Allergic Symptoms

• Allergic diseases such as asthma, eczema, urticaria, and allergic rhinitis have been reported in children with CVID.
• Children with IgA deficiency have higher rates of atopy and food and drug allergies.[41]

5. Growth Retardation or Developmental Abnormalities

• Failure to thrive, poor oral intake, GI symptoms like diarrhea, and abdominal pain seen as common findings.
• With XLA, the absence of tonsils and adenoids can be present in adulthood from prophylaxis treatment with antimicrobials in childhood.
• In CVID,  GI involvement presenting with diarrhea, weight loss and malabsorption in children and adults are common.
• In IgA deficiency, children and adults may present with the celiac-like syndrome, recurrent diarrhea with blunting of villi on jejunal biopsy leading to growth retardation in children.[25]

6. ENT Abnormalities and Cardiovascular Abnormalities such as Digeorge or CHARGE Syndromes in Pediatric Patients

• Tympanic membrane perforation, scarring, and possible hearing loss found on examination.
• Cobblestoning pattern of mucosa in the pharynx with the absence or underdevelopment of tonsillar tissue, nasal exudates.[42]

7. Malignancies in the Older Population

• Approximately 25% of patients diagnosed with chronic lymphocytic leukemia (CLL) developed hypogammaglobulinemia long term, this was demonstrated in a study conducted at the Mayo Clinic in the 2000s. IgG levels were subsequently lower during the course of the disease when compared to initial levels during diagnosis.[25][5][43] 
• Patients with CVID and XLA may also present with colorectal cancer, lymphoreticular malignancies, and non-Hodgkins lymphoma.[44][45]
• Adults with IgA deficiency may develop complications like adenocarcinoma of stomach and lymphoma.[41]

Evaluation

Diagnostic criteria for hypogammaglobulinemia by the European Society of immunodeficiency (ESID) require a substantial decrease in IgG concentration defined by at least two standard deviations below the average for the age group. The range for healthy adults is from 8 to 12 g/L. In this group, hypogammaglobulinemia is diagnosed by 2 standard deviations below the average with levels less than 5g/L.[25] 

Reduced IgA and IgM isotypes are also commonly seen with low IgG levels. IgM is the main immunoglobulin which is abundant during the primary immune response but is contained in the intravascular compartment. IgG is the most numerous in both vascular and extravascular compartments and interacts with different Fc fragments on immune cells to activate the complement pathway. Measurement of IgG is key in diagnosing Hypogammaglobulinemia. 

Evaluation should consist of:

1. Measuring the serum Immunoglobulin levels (IgM, IgG, IgA):

• Laboratory tests: CBC with differential, Quantitative serum immunoglobulin levels, Antibody titers in response to immunization (tetanus, diphtheria vaccines), or infections. If the results of the tests are suggestive of XLA, then do flow cytometry to determine the lymphocyte subsets to rule out combined immunodeficiencies.
• In adults and children with CVID, IgE and allergen-specific IgE may be undetectable or absent. For diagnosing CVID, decreased IgG in association with low levels of IgM and/or IgA is needed using age-specific values for pediatric patients.
• The level of IgG is at least two standard deviations below the expected controls in patients with THI.[46] Infants with THI make normal antibodies in response to diphtheria, tetanus, hepatitis A & B vaccines, MMR, and conjugated H.influenza. If there is a lack of response, then a complete immunological evaluation is warranted.[47][48][49][50]
• Hypogammaglobulinemia is the primary lab finding of CVID. It is important to exclude secondary causes and physiologic causes, before considering primary immunodeficiencies. Selective IgA deficiency has normal levels of other Immunoglobulins, with usually elevated IgM levels.[14] Hyper-IgM presents with normal to elevated levels of serum IgM and reduced levels of IgG, IgA, and IgE

2. Functional Antibody production (Isohemagglutinins, IgG antibodies level, post-immunization/post-exposure):

• Testing for B-cell function can be determined by the presence and titer of isohemagglutinins, and natural antibodies to type A and B red blood cell polysaccharide antigens. It measures IgM antibodies predominantly. The absence of Isohemagglutinins is normal in the 1st two years of life, and are always absent in patients with blood group AB.[14] In addition to the isohemagglutinins, the antibodies produced in response to the antigens during the routine vaccination are abnormally low in XLA but are normal in THI.
• Adequate antibody response in young children demonstrated by vaccination history, antibody titers to Diphtheria, tetanus, H.influenza excludes the diagnosis of CVID.

3. Flow cytometry for Immunophenotyping of B- cells and T-cell/B-cell evaluation:

• Conditions of B-cell deficiency mimic each other, hence to differentiate XLA from CVID, hyper–IgM syndrome, and THI flow cytometry is a critical test to demonstrate the absence of circulating B cells which is characteristic of XLA, and presence of B-cells in CVID, IgA deficiency, and hyper-IgM syndromes.[51][52]

4. Genetic testing:

• A genetic test to identify the mutation in the BTK gene once the decreased Ig and B-cells have been found is needed for confirmation. In the absence of family history, molecular testing is indicated for other B-cell defects and XLA to help with genetic counseling.[14] Genetic testing is usually not required for the diagnosis of CVID but is helpful in understanding the inheritance pattern.
• Combined immunodeficiencies involving cellular and innate immunity must be excluded in addition to secondary causes of humoral immunodeficiency.

5. Miscellaneous:

• Additional blood tests (CRP, ESR) and antinuclear antibody testing may be obtained in patients with active infections, and symptoms suggestive of autoimmune conditions respectively. Lymph node biopsy and bone marrow biopsy can be considered if there is a suspicion of lymphoma or myelodysplasia.[53]

Treatment / Management

1. Avoiding infections by practicing hand hygiene, drinking treated water, and having adequate respiratory protection.

2. IV Immunoglobulin replacement therapy has limitations as it only replaces IgG but not IgM or IgA. Immunoglobulin levels need to monitor every 6 months, and the dose should be adjusted based on the patient’s IgG production and weight. Immunoglobulin therapy is not indicated in THI or conditions with normal B cells but specific antibody impairment. Patients with high IgG and mild impairment in response to vaccination can be followed closely. Side effects with IVIG administration may range from inflammatory reactions to rarely anaphylaxis. Headache, AKI, hemolytic anemia, and neutropenia can also be seen. Patients receiving IVIG should be screened for anti-IgA antibodies to prevent anaphylactic reactions caused by a patient’s IgE antibodies to the IgA in the IVIG preparation in patients with CVID or IgA deficiency. Patients with IgA deficiency make IgG antibodies normally and therefore do not need IVIG with >99% IgG. Administration of IVIG with IgA deficient patients may cause anaphylaxis.[54](A1)

3. Antibiotics for active infections or prophylaxis and monitoring for chronic lung problems due to recurrent pneumonia.[55](B3)

4. Systemic glucocorticoids can be used for cytopenias, high dose IVIG, and rituximab as a steroid-sparing adjunctive agent have also shown benefits.[56](B3)

5. Hematopoietic stem cell transplant can also be done when weighing for cost and IVIG availability. In cases with no response to the above treatment, a splenectomy can be considered.[57](B2)

6. Vaccination recommendations: seasonal influenza vaccine for high-risk patients. The recommendations vary according to the severity of antibody deficiency. In severe deficiency, live attenuated vaccines are not recommended but routine vaccination with inactivated vaccines like Influenza, HPV, Anthrax, and Rabies can be given. The vaccination schedule for inactivated or subunit vaccines is the same as the general population in mild antibody deficiency but its important to weigh the benefit and possible harm in administering live-attenuated vaccines to this population.[58]

7. Allergy testing/ PFT  in children with continued sinopulmonary symptoms. They may need allergy testing to rule out asthma or allergic triggers contributing to these symptoms.

8. Repeated ear infections may lead to sensorineural hearing loss and need audio examinations for concerns of impaired hearing or poor school performance.[59]

Differential Diagnosis

Differential diagnosis includes:

• Drug-induced secondary causes such as antimalarial agents, Phenytoin, Penicillamine, Glucocorticoids. Stopping offending drug and rechecking immunoglobulins is warranted if patients are demonstrating an immunocompromised state.[17]
• Genetic disorders as mentioned above including but not limited to Hyper IGM immunodeficiency, X-linked agammaglobulinemia, X-linked SCID, Trisomy 18, and 21, ataxia-telangiectasia can be mistaken for hypogammaglobulinemia. A genetic evaluation would be the next step to distinguish any chromosomal involvement or defect.[14]
• Infectious etiologies such as CMV, EBV, HIV, Rubella, and congenital Toxoplasma gondii. Rule out by PCR is indicated if suspicion is high. Certain viruses and bacterial infections can cause low serum immunoglobulins and must be ruled out. Repeat Serum immunoglobulins once the infection is treated.[17]
• Malignancies like Chronic lymphocytic leukemia, Non-Hodgkins lymphoma, B cell malignancy can present similar to hypogammaglobulinemia. Oncological evaluation with proper tissue biopsies will be beneficial in certain cases if malignancy is high on the differential. 
• Systemic disorders such as severe burns, nephrotic syndromes, severe diarrhea, and malnutrition can also be causes of low immunoglobulins. Proper history and examination are essential for adequate evaluation and assessment.[40]

Pertinent Studies and Ongoing Trials

Role of B-cells in COVID-19

In a study involving seven patients with COVID-19 infection and underlying primary immunodeficiencies, it was found that patients with agammaglobulinemia had mild symptoms of COVID-19 as compared to patients with CVID who presented with severe symptoms needing antiretroviral drugs, IL-6 blocking drug therapy, and mechanical ventilation. There were significant changes in high-resolution chest CT noted in response to treatment in these two conditions.This was attributed to a possible role of B-cells in the inflammation caused by SARS-CoV-2. Also, the reason why children contain the SARS-CoV-2 better in the early infection phase because of the ability of the B-lymphocytes to produce natural antibodies in a timely manner in children when compared to the adults.[60][61][62]

Prognosis

Pulmonary complications and malignancy mainly determine the prognosis. Antibiotics usage and IVIG therapy have decreased the mortality from bacterial infections.[38] Timely intervention can delay or prevent these complications but the range of IgG replacement to prevent the complications is not clear.[63]

Patients with THI with low IgM and IgA levels were found to have a slow recovery, whereas those who breastfed for longer duration had an early recovery. In patients with Hyper-IgM syndrome, mortality is secondary to opportunistic infections, malignancies, and liver/biliary tract disease.[64]

Complications

Pulmonary complications can lead to shorter life expectancy even in early detection of hypogammaglobulinemia. The main complication involved is the development of bronchiectasis which is seen in approximately 20% of patients with a history of recurrent infections. This may cause worsening respiratory symptoms with increased bronchospasms presenting like an obstructive and/or restrictive pulmonary disease.

A high-resolution CT scan is recommended periodically to monitor pulmonary status. Another long term complication is malignancy that is usually seen in the fourth to fifth decades of life, for which close follow up is advised. High dose IVIG started early in the disease course can promote a healthier outcome with decreased pulmonary complications.[65]