Introduction

Fungal infections are a growing problem worldwide, with over a billion people suffering from various fungal diseases. These infections can either be mild and affect the skin, nails, and hair, or, in severe cases, they can enter the bloodstream and spread to different organs in the body. In healthy individuals, most fungi are reasonably harmless, and the infections can be easily treated with antifungal creams or tablets. However, in individuals with weakened immune systems, symptoms can be severe and may lead to hospitalisation or even death.¹ 

Persistent fungal infections are also an issue, especially with the limited antifungal treatments currently available on the market.² This can be more pronounced in patients diagnosed with CARD9 deficiency with impaired immune systems, making them more susceptible to fungal infections.³ This report looks into the disease and its impacts on the immune system in causing these infections. 

What is CARD9?

Caspase-recruitment domain 9 (CARD9) is a protein found in various tissues throughout the body, including the spleen, heart, brain, lungs, bone marrow, and blood. The protein is expressed in immune cells, such as macrophages and dendritic cells, which play a crucial role in the body’s immune system to trigger the inflammatory response and fight infections. 

CARD9 is involved in both innate and adaptive immune responses in targeting bacterial, fungal, and viral infections. An innate immune response is the body’s initial reaction to a pathogen (an organism that causes disease). This immune response works quickly to fight off non-specific pathogens and is classed as the body’s first line of defence. Whereas, in adaptive immune responses, the response is slower. The body can recognise and memorise the specific pathogen through previous exposure or vaccination to fight off the particular infection. 

During an infection, CARD9 regulates the production of innate and adaptive inflammatory markers called chemokines and cytokines to elicit an inflammatory response.^3,4

CARD9 in innate and adaptive immunity

During innate immunity, the C-type lectin receptor Dectin-1 signals CARD9 to produce various cytokines to cause inflammation. The receptor, located on the surface of macrophages, dendritic cells, and neutrophils, recognises β-glucan carbohydrates found in fungal cell walls. This activates CARD9 and the signalling cascade, triggering the production of pro-inflammatory cytokines and chemokines during a fungal infection.

In adaptive immunity, CARD9 plays a major role in the signalling cascade. When CARD9 signals to dendritic cells, this releases cytokines IL-1β, IL-6, and IL-23, which differentiate and activate naïve T cells (white blood cells that have not been activated yet) into T helper 17 cells (Th17). Th17 cells then produce IL-17, which activates neutrophils. Activated dendritic cells also release IL-18 and IL-12, capable of differentiating naïve T cells into Th1 cells. Th1 cells then produce the cytokines, Interferon-gamma (IFN-γ) and Tumour Necrosis Factor-alpha (TNF-α) to activate macrophages. The activated neutrophils and macrophages further promote Th1 and Th17 differentiation to trigger a strong immune response and fight the active infection. However, as CARD9 is greatly involved in both the innate and adaptive immune responses, problems can arise when patients are deficient in this protein.³

What is CARD9 deficiency?

Inheritance

CARD9 deficiency is a congenital, autosomal recessive condition.⁵ It is classed as autosomal recessive, because to inherit the disease, a child needs to have two copies of the abnormal gene, one from each parent. Therefore, it is rarer compared to autosomal dominant conditions, where a child only needs to carry one abnormal gene from either parent to have the disease.⁶ 

Pathophysiology of CARD9 deficiency

CARD9 deficiency is caused by mutations in the CARD9 gene on chromosome 9. Individuals with CARD9 deficiency have been found to possess various gene mutations on the CARD9 gene: missense (a nucleotide is substituted and replaces an amino acid, altering the protein’s function), nonsense (a nucleotide substitution which produces a stop codon and either inactivates the protein or causes it to lose its function), synonymous mutations (also referred as silent mutations, where a nucleotide is substituted but the same amino acid is produced and does not change the protein’s function), small deletions (one or more nucleotides are removed from the DNA sequence), small insertions (one or more nucleotides are added into the DNA sequence) and substitutions (can be either missense, nonsense or silent mutations). In patients diagnosed with CARD9 deficiency, the homozygous nonsense mutations Q289* and Q295* are the most common. Patients with Q289* are shown to be from North Africa, and those with Q295* are reportedly from the Middle East.^5,7

CARD9 deficiency and its effect on the immune system

Those affected with CARD9 deficiency exhibit reduced production of inflammatory markers, especially the cytokines IL-1β, IL-6, and TNF-α, and chemokines CXCL1, CXCL2, and CXCL8. In addition, cytokine release is impaired in monocyte-derived macrophages, monocyte-derived dendritic cells, and peripheral blood mononuclear cells. Th1 and Th17 responses are also diminished. Some patients with CARD9 deficiency also have reduced Th17 cells circulating in the blood. This increases an individual’s susceptibility to fungal infections.^3,5

Moreover, CARD9 also reportedly provides immunity against bacterial and viral infections in mouse models. CARD9 provides innate and adaptive immunity and releases inflammatory cytokines against specific bacterial strains, such as gram-positive bacteria (bacteria with a thick peptidoglycan cell wall) like Listeria monocytogenes and Mycobacterium tuberculosis. In CARD9 deficiency, immunoglobulin G (IgG) release during a bacterial infection is greatly reduced. However, patients with CARD9 deficiency do not appear to be vulnerable to bacterial infections. In viral infections, CARD9 plays a role in activating and releasing IL-1β. However, in mouse models lacking CARD9, IL-1β levels are reduced, causing a weakened antiviral response.⁸

CARD9 deficiency and its effect on the body

CARD9 deficiency can contribute to many fungal infections that affect various parts of the body, including the central nervous system, liver, kidneys, lungs, skin, fatty tissues, mouth, eyes, and digestive tract.

Patients may develop brain infections from fungal strains such as Candida albicans, Aspergillus fumigatus, dark-pigmented fungi groups, and dermatophytes (fungi group that causes hair, skin, and nail infections). In CARD9 deficiency, there is a lack of chemoattractant (a substance that attracts white blood cells to a specific location in the body) produced by the glial cells in the brain, which causes fewer neutrophils to be recruited to the site of infection.

In the lungs, Dectin-1 and Dectin-2 signalling, along with CARD9, have been shown to provide Th17 immunity. CARD9 deficiency reduces the number of naïve T cells being activated to Th17 cells during lung infections caused by Cryptococcus neoformans, Histoplasma capsulatum, and Blastomyces dermatitidis.

In the skin and fatty tissues, CARD9 deficiency increases the possibility of contracting phaeohyphomycosis (a fungal infection that involves dark-pigmented fungi) and deep dermatophytosis (a severe fungal infection that can be invasive and penetrate the middle layer of the skin and the deepest fatty tissue layer). This could be due to reduced Th17 responses because of a lack of Th17 cells circulating in the blood. CARD9 also appears to be involved in sterile skin inflammation, such as. In atopic dermatitis, the signalling pathways produce IFN-γ and IL-17.

In the digestive tract, CARD9 deficiency has been linked to inflammatory bowel disease. CARD9 has been shown to promote the production of the cytokine IL-22 to provide anti-inflammatory effects, such as reducing colon (large intestine) inflammation and maintaining gut barrier health. Patients with CARD9 deficiency have an increased likelihood of developing fungal infections in the colon due to Candida colonisation.⁹

Summary

Fungal infections are an ongoing global issue, especially with limited antifungal treatments. CARD9 is a protein that plays an important role in fighting these infections by activating both innate and adaptive immune systems. CARD9 achieves this by recruiting cytokines and chemokines and activating neutrophils and macrophages to target fungal infections. However, in patients with CARD9 deficiency, the immune system is weakened and there is a marked reduction in innate and adaptive immune responses, causing the body to be more susceptible to fungal infections. This can cause persistent fungal infections that can be difficult to treat, such as phaeohyphomycosis, dermatophytosis, and candidiasis.