Overview

Congenital lymphopenia is a rare but serious condition marked by abnormally low lymphocyte counts present at birth.¹ Since lymphocytes are crucial for protecting the body against infections, individuals with this disorder often present with severe, recurrent infections early in life. While environmental factors and infections can cause lymphopenia, congenital cases are often rooted in genetic mutations that disrupt the immune system's development. Understanding the genetic underpinnings of congenital lymphopenia is key to early diagnosis, effective treatment, and improved patient outcomes.

The immune system and lymphocyte development

The human immune system is composed of two main parts: the innate immunity and the adaptive immunity.² Innate immunity is known as the ‘first line of defence’ and offers immediate, nonspecific defence against invading pathogens.^2,3 Adaptive immunity, on the other hand, provides specific, long-lasting protection, ensuring that the immune response is faster the next time the same, or a similar, pathogen is encountered.^2,3,4 Specific cells known as the T cells, the B cells, and the natural killer (NK) cells are essential components of the adaptive immune response. Collectively, these immune cells are called lymphocytes.^3,4

Lymphocytes are formed from immature stem cells in the bone marrow. B cells mature in the bone marrow, while T cells travel to the thymus (a specialised immune organ located in the upper chest and between the lungs)  for development.⁴ Proper lymphocyte function depends on a precise sequence of genetic instructions that control cell survival, differentiation, and receptor formation. ⁴ When these instructions are disrupted, lymphocyte numbers may fail to reach normal levels, resulting in lymphopenia.

What is congenital lymphopenia?

When the lymphocyte count is significantly below the normal levels in a newborn or an infant, it results in a disorder known as congenital lymphopenia.¹ It can further be categorised into two groups:

• Isolated lymphopenia, where only lymphocytes are affected
• Syndromic lymphopenia, where other parts of the body, such as other organs, are affected due to a broader genetic syndrome involving other body systems

Common symptoms include persistent infections, failure to thrive, and inadequate vaccine responses. Due to newborns having immature immune systems, the disorder can go unrecognised until infancy and even later without proper screening.

Genetic causes of congenital lymphopenia

As mentioned above, lymphopenia can occur as a result of other genetic syndromes. Some of the most common genetic diseases that result in congenital lymphopenia are:

Severe combined immunodeficiency (SCID)

SCID is the most severe form of congenital lymphopenia and is usually referred to as the “bubble boy disease”.⁵ It is caused by mutations in genes that are crucial for lymphocyte development or function, leading to the formation of defective T-cells and dysfunctional B-cells and NK-cells. ⁵ In the most severe cases of SCID, B-cells and NK-cells are absent. SCID can occur due to various mutations, and there are many types. For example, X-linked SCID, ADA-SCID, and RAG1/RAG2 mutations (see below).

X-linked SCID

This mutation is linked to the X chromosome due to mutations in the IL2RG gene, which leads to a lack of T and NK cells.⁶ It is a recessive disease, meaning both parents have to have the faulty X-chromosome for the child to have SCID.⁶ As it is directly linked to a sex gene, the child is born with a deficiency in their lymphocyte count, leading to congenital lymphopenia.

ADA-SCID

This form of SCID occurs due to a deficiency in a specific enzyme known as adenosine deaminase (ADA).⁷ This causes the body to become unable to properly clear out toxins produced by the metabolism of lymphocytes, which results in the accumulation of harmful molecules in the body, causing damage to healthy lymphocytes, and even the destruction of lymphocytes.⁷ Ultimately, this causes the count of lymphocytes in the body to fall below the normal threshold, leading to lymphopenia. 

RAG1/RAG2 mutations

RAG1 and RAG2 are proteins that act together as an enzyme that is essential for the rearrangement of certain genes to enable the formation of receptors in the T and B lymphocytes.⁸ RAG1 is responsible for DNA-cleaving (cutting the DNA), while RAG2 regulates RAG1's aggregation by controlling when and where RAG1 becomes active, as well as preventing it from clumping together to ensure RAG1 works efficiently.⁸ Mutation in one of these two genes impacts gene rearrangement that is crucial for antigen receptor development, leading to dysfunctional receptors, which makes lymphocytes defective.⁸

In most cases, SCID is diagnosed very early on as newborns with the disorder develop life-threatening infections within the first few months of life.

DiGeorge syndrome

DiGeorge syndrome is caused by the deletion of certain genes on chromosome 22. This deletion affects the development of the thymus.⁹ The thymus is where T-lymphocytes migrate to and mature, and so, any developmental abnormalities within the organ also affect T-cell maturation.⁹ Other than T-cell lymphopenia, this syndrome is also characterised by congenital heart defects, facial abnormalities, and the level of calcium in the blood being too low (hypocalcemia).

DiGeorge syndrome can occur at varying levels, and as such, some patients can have partial function of their thymus, whereas others may need a thymus transplantation and targeted-cell therapy.

Wiskott-aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a disorder that is linked to the X chromosome.¹⁰ It is a recessive disorder that results from mutations in the WAS gene, which affects the structural support in immune cells, causing them to be faulty and dysfunctional. ¹⁰ In individuals with this syndrome, T and B cells are present but function poorly, leading to severe infections and increased autoimmunity risk.

Ataxia-telangiectasia (A-T)

A-T is caused by mutations in a gene called the ATM gene, which affects DNA repair and results in neurological symptoms and immunodeficiency.¹¹ In particular, it causes progressive lymphopenia as the immune cells can’t properly repair themselves after getting damaged, leading to a decrease in the number of effective lymphocytes in the body.

Clinical signs of congenital lymphopenia

The symptoms of congenital lymphopenia can vary depending on severity and the underlying genetic disorder.¹ However, they share some common features:

• Recurrent, unusual, and long-lasting infections (respiratory, gastrointestinal, fungal, or viral)
• Poor response to vaccines
• Skin rashes or eczema
• Autoimmune complications
• In syndromic form, where it occurs due to other genetic disorders, it can cause heart defects, neurological issues, or skeletal abnormalities

Diagnosis

It is crucial to identify congenital lymphopenia as early as possible to minimise danger and manage symptoms to enable the child to have a good quality of life. Many diagnostic methods have been implemented to diagnose signs of the disease, such as: 

• Newborn screening
• Genetic testing
• Immunological lab tests to count lymphocytes
• Lab tests to screen for the presence of immunoglobulins, which are highly specialised proteins produced by the immune system¹

Treatment and management

The primary goal of any treatment for congenital lymphopenia is to restore immune function and to prevent infections. Some treatments deployed to manage the disorder are:

Hematopoietic stem cell transplantation (HSCT)

This treatment requires the transplantation of stem cells that will specifically become blood cells once differentiated. Since lymphocytes are white blood cells, HSCT enables the body to recover its reserve of lymphocytes. ¹² It is the most effective treatment for severe forms of lymphopenia, such as SCID, and gives the best outcomes when performed as early in life as possible, such as in infants.¹²

Enzyme replacement therapy

This treatment is specific for lymphopenia that is directly linked to the malfunction of proteins and enzymes that are crucial for the formation of receptors in lymphocytes, such as ADA-SCID. ¹³ Although it can’t cure the disease permanently, it can temporarily reduce symptoms.

Gene therapy

This treatment involves directly working on the defective gene or chromosome.¹⁴ The treatment is still in its infancy, but it is growing as a promising curative option, especially with recent success in using viral vectors to deliver corrected IL2RG or ADA genes to the site of mutation and carrying out a replacement.¹⁴

Summary

Congenital lymphopenia can lead to severe immune impairment from birth. As such, genetic disorders leading to congenital lymphopenia need urgent medical intervention.  Early treatment may allow many children to live healthy lives. With advances in molecular diagnostics, early diagnosis and curative treatments like HSCT or gene therapy, early detection has become increasingly feasible. As genetic and immunological research progresses, there is hope for safer, more accessible cures for all forms of congenital lymphopenia.