Bare lymphocyte syndrome
Rare genetic immunodeficiency affecting MHC expression.
Bare lymphocyte syndrome (BLS) is a rare genetic condition resulting from mutations in genes of the major histocompatibility complex (MHC) or in genes involved in MHC molecule processing and presentation. It is classified as a form of severe combined immunodeficiency.
In type II BLS, a group of genes known as MHC class II are not expressed. This severely compromises the immune system, as antigen-presenting cells cannot activate CD4⁺ T cells—T-cell receptors cannot recognize MHC II/peptide complexes. Clinically, this resembles severe combined immunodeficiency, where lymphocyte precursor cells are malformed. Absolute T-cell counts are also reduced due to impaired development in the absence of MHC II.
Type I BLS is marked by a lack of MHC class I molecules. Symptoms may include recurrent bacterial respiratory infections and chronic skin lesions. Severe complications include bronchiectasis, respiratory failure, tissue erosion of the nose, and cerebral abscesses. The absence of MHC I on cell membranes leads to self-immunity in NK and γδ T lymphocytes, which are normally downregulated by MHC I presence. Diarrhea can be an associated condition.
Genetically, type II BLS is not caused by defects in the MHC II genes themselves, but by mutations in genes coding for transcription factors that regulate MHC II expression. The responsible genes were cloned by the laboratories of Bernard Mach in Switzerland and Jeremy Boss at Emory University in Atlanta, Georgia. Mutations in any of four genes can cause type II BLS: class II trans-activator (CIITA), regulatory factor X5 (RFX5), RFX-associated protein (RFXAP), and RFX ankyrin repeats (RFXANK, also known as RFXB).
The rarer type I BLS, also called HLA class I deficiency, is associated with deficiencies in TAP2, TAP1, or TAPBP. TAP proteins normally pump degraded cytosolic peptides across the endoplasmic reticulum membrane so they can bind to HLA class I. Once the peptide:HLA class I complex forms, it is transported to the cell membrane. A defect in TAP proteins prevents peptide pumping, so no peptide:HLA class I complexes form, and no HLA class I is expressed on the membrane.
Diagnosis classifies the condition as type 1 (MHC class I deficiency) or type 2 (MHC class II deficiency).
- Type
- Genetic disorder
- Subtypes
- Type I (MHC class I deficiency) and Type II (MHC class II deficiency)
- Inheritance
- Recessive
- Associated genes
- CIITA, RFX5, RFXAP, RFXANK (Type II); TAP2, TAP1, TAPBP (Type I)
- Clinical manifestations
- Recurrent infections, respiratory tract infections, chronic skin lesions, diarrhea, bronchiectasis, respiratory failure, tissue erosion of nose, cerebral abscess
Lore & Background
Bare lymphocyte syndrome type I (BLS I), also called HLA class I deficiency, is associated with deficiencies in TAP2, TAP1, or TAPBP. TAP proteins pump degraded cytosolic peptides into the endoplasmic reticulum for binding to HLA class I. A defect in TAP proteins prevents this pumping, so no peptide:HLA class I complexes form and no HLA class I is expressed on the cell membrane. Symptoms can include recurrent bacterial respiratory tract infections, chronic skin lesions, bronchiectasis, respiratory failure, tissue erosion of the nose, and cerebral abscess.
Reader's Guide
Bare lymphocyte syndrome is significant as a model of how defects in gene regulation, rather than the structural genes themselves, can cause severe immunodeficiency. In BLS II, the absence of MHC class II expression prevents antigen-presenting cells from activating CD4+ T cells, leading to a condition clinically similar to severe combined immunodeficiency. The identification of the transcription factor genes responsible—CIITA, RFX5, RFXAP, and RFXANK—by the laboratories of Bernard Mach and Jeremy Boss advanced understanding of gene regulation in the immune system. BLS I, though rarer, highlights the critical role of TAP proteins in antigen processing and MHC class I expression. The condition underscores the complexity of immune system development and the severe consequences of its disruption.
Did You Know?
- Bare lymphocyte syndrome type II is caused by mutations in transcription factor genes, not the MHC class II genes themselves.
- In BLS I, a defect in TAP proteins prevents pumping of peptides into the endoplasmic reticulum, so no HLA class I is expressed on the cell membrane.
- Diarrhea can be among the associated conditions of bare lymphocyte syndrome.
Frequently Asked Questions
What is Bare lymphocyte syndrome?
BLS is a rare inherited immunodeficiency in which the body fails to properly express or process MHC molecules on cell surfaces. It is classified within the broader family of severe combined immunodeficiency disorders.
What are the two subtypes of Bare lymphocyte syndrome?
Type I involves absent MHC class I expression, while Type II involves absent MHC class II expression. In Type II, antigen-presenting cells cannot properly stimulate CD4+ T cells because the T-cell receptors have no MHC II/peptide complex to recognize.
How is Bare lymphocyte syndrome inherited?
It follows a recessive inheritance pattern, so a child must receive two defective copies of the relevant gene. The specific genes differ by subtype: CIITA, RFX5, RFXAP, and RFXANK are linked to Type II, while TAP1, TAP2, and TAPBP are linked to Type I.
What clinical manifestations does Bare lymphocyte syndrome produce?
Affected individuals typically endure repeated severe infections, chronic diarrhea, persistent skin lesions, and progressive lung damage such as bronchiectasis. In advanced cases the picture can escalate to respiratory failure, nasal tissue erosion, and cerebral abscesses.
Why is Bare lymphocyte syndrome grouped with severe combined immunodeficiency?
Because the absence of functional MHC molecules cripples adaptive immune activation, leaving the patient with profoundly reduced T-cell responses and overall immune competence. The resulting clinical trajectory—persistent, life-threatening infections from early life—mirrors that of classic SCID.
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