X-Linked Agammaglobulinemia (XLA)

At a Glance

X-linked agammaglobulinemia (XLA) is a disease characterized by absence of B lymphoocytes and marked reduction of all classes of serum immunoglobulins.

Affected individuals are males with frequent infections of the respiratory and digestive tracts, which typically begin after the first 6 months of life, when the maternal IgG disappears from circulation. Typical causative agents are encapsulated bacteria and enteroviruses. There are no opportunistic infections, indicating presence of normal cellular immunity. However, severe meningoencephalitis can occur.

Most common presentations are otitis media and infections of the upper respiratory airways (sinusitis and pneumonia). Conjunctivitis, chronic recurrent diarrhea, and skin infections are also common.

These individuals have undeveloped secondary lymphoid organs, such as lymph nodes because of the absence of mature B-cells.

In some individuals, symptoms appear later in childhood or even in adulthood.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The work-up should start with a complete blood count (CBC) and differential. In XLA, the absolute number of neutophils is within normal limits, although severe neutropenia can be found in some patients if testing is performed during infection.

To detect defects in the B lymphocyte lineage, it is necessary to perform lymphocyte phenotyping of peripheral blood and bone marrow. In the blood, there is a complete absence (less than 1%) of circulating CD19+ CD20+ IgM+ kappa/lambda+ mature B cells (i.e., cells that carry CD19 and CD20 on their surface and express surface immunoglobulin M (IgM), and either kappa or lambda light immunoglobulin chain). Analysis of the bone marrow shows accumulation of immature B-cells without pre-B-cell receptors, confirming the blockage of B-cell differentiation from pro-B- to the pre-B-cell stage, and presence of less than 1% mature B-cells.

Evaluation of serum immunoglobulins IgG, IgM, IgA, and IgE usually show lack of all immunoglobulins. Quantitative Ig analysis shows IgG typically below 200 mg/dL and IgM and IgA levels below 20 mg/dL. However, it is possible that some residual IgM can be present.

There is a complete absence of antibodies to vaccinal and bacterial antigens.

What Lab Results Are Absolutely Confirmatory?

XLA is caused by mutations in the Bruton’s tyrosine kinase (BTK) gene, causing decreased levels of the BTK protein in the cells. It is possible to test for the presence of the BTK protein by flow cytometry or Western blot on circulating mononuclear cells. Molecular testing by sequencing of the BTK gene is also available.

Most BTK mutations result in deficient expression of BTK protein. The flow cytometry test can be a convenient screening tool for XLA. However, presence of the BTK protein does not rule out XLA, as some mutations of the BTK gene can result in aberrant protein function, despite normal protein expression.

A comprehensive assessment can be achieved by performing both protein and gene analysis and would enable appropriate genotype-phenotype correlations. Clinical phenotype can vary considerably, depending on the nature of the specific BTK mutation. Of the 600 detected mutations, some manifest early in life, some later in childhood or adulthood, and some cause no symptoms at all.

Presence of BTK mutation is confirmatory in male family members of affected individuals with known BTK mutations and is the preferred test for determination of carrier status of female relatives of male XLA patients with known BTK mutations, especially if the BTK protein flow test is normal. BTK gene testing is most effective if the familial BTK gene mutation is already known.

It is important to keep in mind that the mere presence of BTK gene mutations does not necessarily correlate with a diagnosis of XLA, unless the appropriate clinical and immunological features are present: early onset infections in a male child, hypogammaglobulinemia, and absent or low mature B-cells.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

A prenatal diagnosis is possible by evaluating presence of circulating mature B-cells in the fetal circulation. In XLA, there is a complete absence of mature B-cells (CD19+ Ig+ cells).

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?

Rare polymorphisms could potentially lead to false-negative or false-positive results. Also, some mutations are difficult to detect with current methodologies, which can also cause a false-negative result. If results obtained do not match clinical findings, additional testing should be considered. Any error in diagnosis or in pedigree provided to the laboratory could lead to an erroneous interpretation of results.

The decrease in numbers of peripheral B-cells is pathognomonic for XLA, although this finding can also be found in a small subset of patients with common variable immunodeficiency (CVID). Conversely, some BTK mutations can preserve small numbers of circulating B-cells.

A diagnosis of XLA should be suspected in males with early-onset bacterial infections, marked reduction in all classes of serum immunoglobulins, and absent B-cells (CD19+ Ig+ cells). Patients should be assessed for the presence of BTK protein by flow cytometry, although normal results by flow cytometry do not rule out the presence of a BTK mutation. Appropriate clinical history is required with or without abnormal BTK protein results by flow cytometry.