Introduction to chediak-higashi syndrome

It is an autosomal recessive disorder, characterised by:

1. Albinism affects the colour of the eyes, the skin, and the hair, also known as oculocutaneous albinism
2. Easy bruising
3. Abnormal functions of the natural killer cells
4. Recurring pyogenic infections

The disease is caused by a mutation in the lysosomal trafficking regulator (LYST) gene. This leads to decreased phagocytosis and susceptibility to recurrent bacterial infection. It may also result in neurological symptoms such as ataxia and neuropathies. 

Initially, the condition may appear as a variety of albinism. However, diagnosis should be considered when recurrent infections occur.¹

The clinical features of CHS are:   

• Albinism of the OCA2 type, giving a lighter complexion than unaffected family members²
• Impaired vision, photophobia
• Silvery sheen to hair, which may be fair in colour
• Epilepsy
• Frequent infections (skin, mucous membranes, respiratory)
• Mental retardation
• Ataxia causes incoordination and a typical ataxic gait
• Tremor
• Enlarged liver and spleen, jaundice

Below 500 cases of CHS have been reported worldwide. Mildly affected individuals may be unreported because of phenotypic variability. All age groups can be affected, but the onset of the disease is after birth and before the age of five. 

The mutation of the LYST gene interrupts protein synthesis, which impairs the storage and secretion of lysosomal granules across multiple cell types. This includes lysosomes of leukocytes and fibroblasts, dense bodies of platelets, azurophilic granules of neutrophils, and melanosomes of melanocytes. These interruptions give rise to enlarged vesicles, which cause lymphocytes to function poorly in the antibody-dependent cell-mediated cytolysis. It also causes alterations in neutrophils, leading to neutropenia, impaired chemotaxis, and delayed phagolysosomal fusion, giving rise to disrupted bactericidal activity. In addition, the functions of natural killer cells are also impacted negatively.³

Investigation is initiated if an individual shows signs of immunodeficiency, pigment dilution of the skin, hair, or eyes; congenital or transient neutropenia; and signs of unexplained neurologic symptoms or neurodegeneration. A diagnosis can be made based on the presence of abnormally large granules in cells such as melanocytes, leukocytes, and their bone marrow precursors, fibroblasts, the central and peripheral nervous tissue, and hair.⁴ In the majority of cases, Chediak-Higashi Syndrome develops into an accelerated phase when white blood cells divide uncontrollably and invade many of the body's organs, causing the build-up of lymphocytes and macrophages of the liver, spleen, and lymph nodes, known as lymphohistiocytic infiltration. Hence, the accelerated phase is accompanied by fever, episodes of abnormal bleeding, overwhelming infections, and organ failure.  

Only a few people with Chediak-Higashi syndrome experience a milder form of the condition later in life, where the pigmentation and recurrent infection are less severe. However, they have a risk of progressive neurological problems such as tremors, difficulty with movement and balance (ataxia), reduced sensation and weakness in the arms and legs (peripheral neuropathy), and a decline in intellectual functioning.⁵

Treatment of Chediak-Higashi syndrome is initiated by chemoimmunotherapy followed by transition to continuation therapy for the accelerated phase. An allogenic stem cell transplant is done to cure haematological and immunological defects. L-DOPA is considered a medication for individuals with Parkinsonism. If ataxia and other neurological complications are present, home modifications and intensive rehabilitation are carried out. Corrective lenses can be used to improve visual acuity. Sunglasses help in protecting sensitive eyes from UV light. Sunscreen can be used to prevent sun damage and skin cancer. As CHS is a rare genetic disorder with a very high mortality rate, it is managed by a team of specialists. The caregiver should be educated about the prevention of complications, such as hand washing, remaining compliant with medications, avoiding NSAIDs, and undertaking all the recommended immunisations. CHS has no cure. More than 80% of patients are dead before the first decade of life, and survivors tend to have physiological deficits, leading to poor quality of life.³

Understanding the immune system and natural killer (NK) Cells

The innate immune system, including the skin and the mucous membranes that keep germs out,  is the body's first line of defence against all threats. It also includes immune cells such as Natural Killer (NK) cells. NK Cells belong to the same family as T and B cells. Therefore, they are classified as group I Innate Lymphocytes (ILCs), which respond to different pathological challenges⁶

NK cells are an important part of the immune system and contribute to protection from pathogens and cancer cells. Without prior exposure, they can destroy harmful foreign substances entering the body, unlike cytotoxic T-cells. They can also destroy cells infected with viruses and cells that have become malignant in the early stages.  They constantly patrol the body to scan and label cells as ‘healthy’ or ‘diseased’. This occurs with the help of multiple receptors present on the surface of an NK cell; these receptors work together to activate or inhibit an NK cell from killing a target.⁷

MHC-1 is the most common cell marker recognised by  NK cells as belonging or ‘self’. When the MHC-1 on the target cell attaches to an NK cell’s inhibitory receptor, the NK cell’s killing function switches off. If the target cells are not recognised as belonging, NK cells get activated by releasing chemicals into the target cells to kill them. These include cells that release activating signals, like cancer cells and infected cells, and cells with absent or downgraded MHC-1.  NK cells communicate by releasing proteins called cytokines, directing other immune cells to attack harmful cells and pathogens.

NK cells start developing in bone marrow before either staying in there or moving to other tissues and organs in the lymphatic system, such as lymph nodes, spleen, tonsils, and thymus. After maturation, the body releases NK cells into the bloodstream. Mature NK cells are also present in lymph tissue and associated organs (liver and lungs). Adults have more than 2 billion NK cells, constituting about 5% to 10% of the lymphocytes circulating in the blood, with a life span of around two weeks. Lower levels of NK cells in the body can cause infections and cancer. Moreover, NK cells play a prominent role in conditions like autoimmune diseases, inflammatory bowel disease (IBD), and asthma.⁷

NK cells in chediak-higashi syndrome (CHS)

Due to the LYST gene mutation, the natural killer (NK) cell function is impaired in Chediak-Higashi Syndrome (CHS). The mutation causes abnormal lysosome-related organelle formation, predominantly lytic granules. These granules are essential for NK cell cytotoxicity, but they become enlarged and struggle to fuse with the cell membrane to release their contents in CHS. This leads to reduced NK cell killing ability.^6,7 

What is flow cytometry?

Flow cytometry is a lab technique for analysing single cells in solution. It is a powerful tool frequently applied in immunology, cancer biology, molecular biology, bacteriology, virology, and infectious disease monitoring. The procedure involves suspending a sample of cells or particles in fluid. The cells are treated with fluorescent antibody dyes to label specific protein cell surfaces. This method is known as fluorescence-activated cell sorting (FACS). Then, the cells are injected into a flow cytometer, which uses lasers to produce scattered and fluorescent light signals. The light signals are read by detectors, converted into electronic signals, and analysed by a computer. Hence, cell populations are analysed based on their fluorescent or light scattering characteristics. Different fluorescent reagents utilised in flow cytometry include antibodies, fluorescently conjugated DNA-binding dyes, ion indicator dyes, viability dyes, and fluorescent expression proteins. Flow cytometry helps in the characterisation of mixed populations of cells from blood. Also, bone marrow and solid tissues can be dissociated into single cells. Another application of flow cytometry is sorting cells for further analysis.⁸

Clinically, flow cytometry is used for cell counting, cell sorting, determining cell characteristics and function. It is also used for the diagnosis and treatment of blood and bone marrow cancers. A healthy cell will have an antigen pattern matching the type and maturity of the cell. On the other hand, an abnormal cell will show different patterns indicating the presence of leukaemia, lymphoma or other diseases. The flow cytometry results are interpreted in the context of the symptoms and health history of an individual.⁹

What are functional assays?

Functional assays are laboratory tests for assessing the activity or function of a molecule or a cell. These assays are essential in diagnosing diseases and understanding the interaction of biological components.¹¹  It enables direct observation of cellular defects associated with the disease, such as enlarged lysosomes and impaired immune cell function. These assays also characterise the severity of disease, identify potential therapeutic targets, and monitor treatment responses.¹¹  

How do these tests help in CHS

Flow cytometry is a valuable tool in diagnosing Chediak-Higashi Syndrome (CHS). It can identify abnormalities in lymphocyte subsets. Flow cytometry can assess the size and granularity of lysosomes and evaluate the expression of proteins involved related to lysosomal function and immune cell activation. In addition, flow cytometry plays a vital role in the evaluation of natural killer (NK) cell activity. It allows analysis of multiple parameters—NK cell phenotype, cytotoxicity, and cytokine production—on a single-cell level and thereby provides a comprehensive understanding of their function.^9,10 

Cell-based assays help in analysing CHS in the following ways:^3,11,12

• Diagnosis and Severity Assessment:

Functional cell-based assays utilising blood smears and bone marrow smears can identify the pathognomonic giant granules in leukocytes and their precursor cells, which is a hallmark of CHS. In addition, disease severity can be correlated with the extent of granule enlargement in different cell types. This provides insights into the clinical phenotype.  Cell-based assays can also assess impaired function of natural killer (NK) cells and cytotoxic T lymphocytes (CTLs) in CHS patients, a dysfunction associated with increased susceptibility to infections and malignancies. 

• Understanding Cellular Mechanisms:

CHS is characterised by defects in the transport and fusion of lysosomes and related organelles. These defects are visualised with the help of cell-based assays. The impact on cellular processes, such as melanosome transport in melanocytes, platelet function, and immune cell cytotoxicity, is also illustrated.

One of the key features of CHS is impaired lysosomal exocytosis, especially in response to wounding. Cell-based assays assist in demonstrating this defect along with its consequences. Cell-based assays can be utilised to screen potential therapeutic compounds for correcting the cellular defects in CHS. The effectiveness of treatments like hematopoietic stem cell transplantation or other immunomodulatory therapies can also be measured. This can be done by assessing the restoration of normal cellular function. In a broader context, understanding cellular phenotype related to specific mutations in CHS can promote personalised treatment strategies. 

Summary

Chediak-Higashi Syndrome (CHS) is a rare genetic disorder that impairs the immune system, making it harder for the body to fight infections. One of its key features is defective natural killer (NK) cell function. NK cells are special immune cells that normally destroy infected or cancerous cells. To assess how well NK cells are working, two main laboratory tools are used: flow cytometry and functional assays. Flow cytometry uses lasers and special dyes to identify NK cells in a blood sample and check if they are activated. Functional assays go a step further by testing whether NK cells can actually kill target cells or send immune signals. Together, these tests help confirm a CHS diagnosis, monitor disease progression, and guide treatment decisions such as stem cell transplantation. Advances in these techniques are improving early detection and offering new hope for better outcomes in people living with CHS.

Frequently asked questions (FAQs)

How can chediak-higashi syndrome affect the body in the long term?

Chediak-Higashi Syndrome can cause recurrent infections, bleeding problems, and neurological issues in the long term because of a damaged immune system.

Can flow cytometry detect NK cells?

Flow cytometry is one of the best methods to identify NK cells and analyse them for defects.

What treatment options are available for chediak-higashi syndrome?

It is treated by managing infections with immunotherapy, antibiotics or bone marrow transplant.

How can I prevent the recurrence of chediak-higashi syndrome?

Chediak-Higashi Syndrome is a genetic disorder. It does not have any known cure. Hence, disease management focuses on treating infections and giving supportive care.