C. albicans can exist harmlessly in symbiosis with the host or transition to an opportunistic pathogen. It naturally colonizes various parts of the human body, including the oral cavity, intestinal tract, vagina, and skin. The growth of C. albicans is typically controlled by the host immune system and regulatory mechanisms facilitated by normal microbiota (3). However, when this delicate balance is disrupted, C. albicans proliferates excessively and becomes more virulent. Factors influencing clinical manifestations include the specific site affected, route of infection, intrinsic characteristics of the pathogen, and underlying health conditions of the patients. As a result, the clinical presentations can vary widely, ranging from mild superficial infections to severe systemic illnesses. Therefore, accurate diagnosis and appropriate treatment are crucial for managing C. albicans infection. A 12-year retrospective analysis of autopsies conducted at the University of Kentucky Medical Center revealed that 54% of patients with positive C. albicans cultures had invasive candidiasis. Among these cases, the proportion of infected sites included the kidneys (80%), the brain (52%), and the heart (48%) (4). C. albicans infection of the central nervous system(CNS), previously thought to be rare, appears to be increasing in incidence; in most cases, the disease typically originates from fungal hematogenous dissemination, where the fungus spreads through the bloodstream from distant primary sites of infection (4) and occurs predominantly in individuals with certain types of immunodeficiencies, often leading to neurological sequelae or death if not diagnosed and treated in a timely manner (5). The main receptor families involved in C. albicans recognition are C-type lectin receptors (CLR), RIG I-like receptors (RLR), NOD-like receptors (NLR), and Toll-like receptors (TLR), which recognize different pathogen-associated molecular patterns (PAMPs) (6). The successful clearance of C. albicans from host tissues largely depends on the phagocytosis of this fungal pathogen by innate immune cells (i.e., macrophages, neutrophils, and dendritic cells). Exposure to β-glucan of C. albicans triggers proinflammatory innate immune responses among CLRs, particularly those dependent on DECTIN1/CLEC7A (C-type lectin domain family 7 member A) (7, 8). When this pathway is impaired, it reduces the body’s clearance rate of Candida albicans.

Non-tuberculous Mycobacteria (NTM), which are members of the Mycobacterium genus, excluding Mycobacterium tuberculosis and Mycobacterium leprae, are ubiquitous organisms commonly found in natural environments such as water and soil (9). Although NTM encompasses more than 190 species, only a small subset is associated with human diseases, with lung diseases being the most prevalent (10). CNS infections are relatively rare, and NTM is traditionally viewed as an opportunistic infection closely linked to immune impairment. Disruption of the IFN-γ/IL-12 signaling pathway has been identified as the primary cause of disseminated NTM infection (11). A study on the US national managed care claims database showed that the incidence rate of NTM disease in the population ranges from 3.1 to 4.7 per 100,000 person-years (12), with Mycobacterium avium complex (MAC: M. avium, Mycobacterium intracellulare and Mycobacterium chimaera) being the most common NTM species causing CNS diseases (13).

Caspase recruitment domain-containing protein 9 (CARD9) is a class of proteins containing the caspase recruitment domain (14), and the gene encoding CARD9 is located on chromosome 9 at position q34.3 with 13 exons. Human CARD9 transduces signals downstream of different CLRs (e.g., DECTIN1/CLEC7A, DECTIN2/CLEC6A, DECTIN3/CLEC4D, and MINCLE/CLEC4E) from fungal constituents through Syk activation of ITAM, resulting in activation of the NF-κB and MAPK pathways as well as proinflammatory cytokine production, thereby triggering a comprehensive antifungal immune response (15). CARD9 deficiency follows an autosomal recessive pattern and results in an inadequate immune response to fungi, leading to subsequent infection. However, the heterozygotes did not exhibit abnormal phenotypes. CARD9 deficiency has been reported worldwide, including but not limited to China (16), Iran (17), Canada (18), Algeria (19), Morocco (20), Tunisia (21), and the United States (22). The three regions with the most reports are China, Algeria, and Iran. Among them, Trichophyton and Candida are the two most prominent pathogenic fungi (23).

On December 14, 2021, upon arrival at our hospital, the patient underwent a physical examination that revealed clear consciousness, chronic facial features, absence of neck stiffness, Kernig’s sign, and Brudzinski’s sign. No palpable lymph node enlargement was observed, and cardiovascular, pulmonary, or abdominal examination revealed no obvious abnormalities. Further investigations included CD4/CD8 cells, neutrophils, HIV, TB-IGRA, TB-DNA, CMV-DNA, EBV-DNA, blood CrAg, tumor markers, PCT, and erythrocyte sedimentation rate, all of which showed no significant abnormalities. In the initial blood test, the only notable increase observed was in IgE level, which was 443 IU/mL. Enhanced MRI of the head revealed a bilateral frontal lobe and small ischemic lesions in the left central semiovale. The cerebrospinal fluid (CSF) analysis was negative for CrAg, ink staining, smears, and culture. However, protein and nucleated cell counts in the CSF were significantly elevated, whereas glucose levels were decreased ( Figure 2 ). Despite empirical antimicrobial treatment, the patient’s temperature remained elevated. On December 21, 2021, the CSF was rechecked, showing similar results, with increased protein and nucleated cells and decreased glucose levels ( Figure 2 ). Autoimmune encephalitis antibodies were negative, meanwhile also ruled out the possibility of connective tissue disease; however, Next-generation sequencing (NGS) of the CSF revealed 387 sequences of C. albicans. Considering the abnormal CSF test results and fever, C. albicans meningitis was diagnosed. Consequently, fluconazole (400mg qd) and flucytosine (1.5g qid) were administered on December 23, 2021. After approximately 10 days of treatment, the CSF did not show significant improvement ( Figure 2 ). Therefore, amphotericin B (40mg qd) was added to the treatment regimen. However, owing to acute renal function injury, amphotericin B was discontinued and replaced with voriconazole (200mg bid) and flucytosine (1.5g qid) before discharge on January 24, 2022.

After discharge, the patient adhered to antifungal therapy and the headache and fever resolved. However, he was readmitted on March 4, 2022, due to dizziness, visual rotation, and nausea. CSF re-examination showed decreased protein and nucleated cell counts and increased glucose levels compared to the previous admission. CSF culture from the previous admission suggested non-tuberculous mycobacteria (NTM); however, the identification and drug sensitivity results were inconclusive. The second NGS of CSF showed C. albicans (22 sequences), Aspergillus (6 sequences), and Torque teno virus (4 sequences). Magnetic resonance imaging (MRI) of the head revealed meningeal thickening and ventricular enlargement with hydrocephalus. As CSF results improved, anti-NTM treatment was not administered. Antifungal therapy was continued and the patient was discharged.

The patient was readmitted to the hospital on April 8, 2022, for lower limb weakness with unsteady walking. Enhanced MRI of the head showed enlarged ventricular hydrocephalus with interstitial edema, which was aggravated from the last follow-up. CSF was reviewed, the amount of protein and nucleated cells was significantly decreased, and glucose was increased ( Figure 2 ). The symptomatic treatment of hydrocephalus yielded limited efficacy. On April 11, with the assistance of neurosurgery, lumbar large-pool drainage was optimized, resulting in a daily drainage flow of approximately 120mL. Subsequently, on April 18, the patient developed high fever (39 °C) accompanied by loss of consciousness. Blood and CSF cultures revealed the presence of Klebsiella oxytoca, leading to the initiation of effective antibacterial therapy. Finally, NTM was identified as M. intracellulare ( Figure 3 ). In accordance with the drug sensitivity results, the patient was administered a regimen of moxifloxacin (400mg qd), ethambutol (750mg qd), and clarithromycin (500mg bid) for anti-NTM treatment.

Subsequent hospitalizations were aimed at assessing and monitoring the treatment efficacy. CSF analysis was conducted during the 4th hospitalization in July 2022, 5th hospitalization in September 2022, 6th hospitalization in February 2023, 7th hospitalization in June 2023, and 8th hospitalization in October 2023. The results showed normalization and stabilization ( Figure 2 ). Additionally, NGS of the CSF during the 5th hospitalization revealed no C. albicans sequences, prompting the discontinuation of antifungal medication after September 2022 (total duration>9 months). Both the patient and his son underwent exome sequencing, which indicated that the patient was homozygous for CARD9 deficiency, with a mutation site at c.175C>T (p. Arg59Trp). The gene mutation was predicted to be “probably damaging” with a score of 1.000 by PolyPhen-2 (http://genetics.bwh.harvard.edu/pph2/).

Genomic DNA from the proband, father, two siblings, and son was amplified by polymerase chain reaction (PCR) using the following primers for Sanger sequencing of the identified CARD9 missense mutation (p. Arg59Trp): forward, 5’-F: GCCCTCAGCTCCTCTGCCCATTCCA-3’; reverse, 5’-GGACCCAACACCACTGCCCGCTCC-3’, with an annealing temperature of 60 °C. All gene mutations are shown in Figure 4① . His father and son are heterozygotes; he and his brother are homozygotes; and his sister is wild type.

To date, the patient has discontinued anti-NTM medications for nearly a year (discontinued in August 2024, duration 2 years) and antifungal drugs (discontinued in September 2022, duration over 9 months) for more than two years, with no fever, headache, dizziness, or physical impairment. Head MRI examinations conducted during this period indicated gradual disappearance of the hydrocephalus ( Figure 4② ).

Fungal infections are prevalent in individuals with immune dysfunction. However, in clinical practice, greater emphasis is typically placed on fungal infections in HIV/AIDS patients, potentially overshadowing the significance of immune dysfunction-related fungal infections in non-HIV/AIDS patients. In addition to CARD9, this review discusses several other genetic factors associated with the susceptibility to fungal infections, including high IgE syndrome (HIES), Autoimmune Polyendocrinopathy–Candidiasis–Ectodermal Dystrophy (APECED) syndrome, Dendritic cell-associated C-type lectin-1 (Dectin-1) deficiency, STAT1 mutations, and IL-17 mutations. These factors collectively constitute the primary immunodeficiency disorders (PIDD).

The clinical features of HIES include markedly elevated serum IgE levels, eczema, and recurrent skin and lung infections. HIES presents two distinct genetic patterns, autosomal dominant inheritance (AD) and autosomal recessive inheritance (AR). Among these, AD-HIES is the most prevalent genetic form. It exhibits unique clinical manifestations, including distinctive facial features, persistent deciduous teeth, and skeletal/connective tissue abnormalities, which are infrequently observed in AR-HIES patients (24). Although the precise immunodeficiency mechanisms and underlying causes of HIES remain unclear, it is evident that immune system dysregulation contributes significantly to the pathogenesis of the disease. This dysregulation primarily stems from the Th1/Th2 imbalance and aberrant production of cytokines and chemokines (25).

Autoimmune Polyendocrinopathy–Candidiasis–Ectodermal Dystrophy (APECED) Syndrome, also referred to as Autoimmune Polyendocrinopathy Syndrome I (APS I), is a rare autosomal recessive autoimmune disorder arising from a single gene mutation. It is distinguished by autoimmune polyendocrinopathy, candidal infection susceptibility, and ectodermal dystrophy (26).Caused by loss-of-function variants of the autoimmune regulatory gene AIRE, occurring in either single or double allele forms, which compromises the thymus’s ability to perform negative selection against autoreactive T cells (27)and disrupts peripheral B cell tolerance (28). Research has demonstrated that JAK inhibitors are effective in managing these condition (29).

Dendritic cell-associated C-type lectin-1 (Dectin-1), also recognized as the β-glucan receptor, represents an emerging PRR within CLR family. It identifies ligands independently of calcium ions (Ca²⁺), and primarily facilitates the connection between innate and adaptive immunity. Dectin-1 is predominantly expressed in dendritic cells, macrophages, and neutrophils and recognizes ligands from the PAMP family, such as β-1,3-glucans. Human Dectin-1 deficiency is associated with vaginal candida infections (30). Additionally, Dectin-1-deficient mice are susceptible to Candida infections (31).

Signal Transducer and Activator of Transcription (STAT) proteins serve as crucial components of both innate and adaptive immune responses against pathogenic microorganisms. Upon cytokine binding to hematopoietic receptors, signaling is initiated through Janus kinases (JAK1, JAK2, JAK3, and Tyk2) and subsequently via STAT proteins (STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B, and STAT6) (32). The molecular pathway mediated by JAK-STAT1 has garnered significant attention in recent years because of its involvement in the IFN-γ-mediated host defense against intracellular pathogens. A mutation in the coiled-coil domain (CC-D) of STAT1 was identified in an index patient from Ukraine, and has been observed in numerous patients from different familial backgrounds worldwide (33). Individuals with functional acquired mutations affecting STAT1 CC-D are predisposed to develop mucosal, skin, and nail candidiasis (34).

IL-17 is a proinflammatory cytokine that plays a pivotal role in the activation of various proinflammatory cytokines through signaling pathways. These include antimicrobial peptides that possess antifungal activity and neutrophil chemotactic factors (35). Th17 cells are a significant source of IL-17 during Candida infections. Genetic defects affecting the IL-17 pathway may impair the immune response against Candida, potentially leading to the progression of Chronic Mucocutaneous Candidiasis (CMC) disease (36).

The deficiency of CARD9 renders individuals vulnerable to numerous fungal infections, including Cryptococcus neoformans, Cryptococcus verrucosus, Aspergillus, Pneumocystis, Mucor irregularis, Rhizopus arrhizus, among others (37–41). Susceptibility to fungal infections in patients lacking CARD9 is primarily attributed to impaired production of pro-inflammatory cytokines and chemokines, compromised neutrophil recruitment, and dysregulated activation of pathways such as NF-κB and MAPK. Additionally, deficiencies in Th-related responses, particularly in Th17 and Th22 cells, contribute to susceptibility. Furthermore, emerging evidence suggests that CARD9 plays regulatory roles in B cell-mediated humoral immunity, further emphasizing its significance in antifungal defense mechanisms (42).

The antimicrobial signaling cascade mediated by CARD9 is initiated by TLRs as part of the PRRs. Among TLR family members, TLR2 and TLR4 play pivotal roles in recognizing the components of gram-positive and gram-negative bacteria, respectively. A study indicates (42) that CARD9-mediated signaling is crucial for anti-pneumococcal immunity by regulating neutrophil function and cytokine production. Both neutrophil phagocytosis and accumulation depend on CARD9. Although CARD9 typically serves as a key downstream connector molecule for PRR-triggered signaling, it is important to note that PRRs can also initiate CARD9-independent antimicrobial signaling pathways. These pathways often intersect with the CARD9-dependent mechanisms to collectively exert bactericidal effects (43).

The Dectin-1-Syk-CARD9 signaling pathway plays an important role in tuberculosis immunity (44, 45). Clecsf8 (MCL) is a FcRγ-coupled receptor (46). Cellular responses mediated by Clecsf8 depend on the Syk/CARD9 complex and encompass a spectrum of functions including phagocytosis, proinflammatory cytokine production, dendritic cell maturation, and T-cell initiation and bursts (47). Studies have demonstrated that Clecsf8-deficient mice are more susceptible to Mycobacterium tuberculosis infections. This susceptibility is characterized by increased load, hyperenhanced neutrophil infiltration, enhanced pathological injury, and early death. These findings underscore the critical role of the Clecsf8-Syk-CARD9 pathway in tuberculosis immunity and related immune responses (48). Similar Syk pathways are also present in Mycobacterium bovis, which promote the initiation of immune responses (49). Although M. intracellulare likely exhibits similarities, targeted research on its specific immune-mediated processes is lacking.

As a pivotal downstream molecule in the signal transduction of PRRs, CARD9 is increasingly being recognized for its significant role in viral infections. CARD9 integrates with the DNA sensor Rad50 and double-stranded DNA (dsDNA), leading to assembly of the dsDNA-Rad50-CARD9 complex. This complex ultimately triggers the activation of NF-κB and generation of a pro-IL-1β response to viral infections (50). Similar mechanisms have been observed for recognition of RNA viruses via the RIG-I pathway. RIG-I, along with the mitochondrial antiviral signaling proteins CARD9 and Bcl-10, activates NF-κB in response to RNA virus detection. These intricate signaling pathways highlight the multifaceted role of CARD9 in coordinating immune responses against both DNA and RNA viruses (51).