Characterization of intestinal fungal community diversity in people living with HIV/AIDS (PLWHA)

The natural course of HIV-1 infection is mainly characterized by persistent virus replication, progressive decline in CD4⁺ T cell count, ultimately leading to severe immune deficiency in the human body, causing various opportunistic infections, tumors, and even death [1‐3]. The gastrointestinal T cells are the primary target of HIV [4]. HIV-related diarrhea is one of the most common complications in the middle and late stages of AIDS [5]. The main pathological site is the intestine, which is a digestive symptom caused by damage to the intestinal mucosa, impaired intestinal barrier function (increased intestinal permeability) and impaired immune function [6, 7]. The main clinical symptoms include diarrhea, malnutrition, and weight loss, which is one of the main causes of decreased quality of life and even death in patients [8].

The human intestinal tract harbors a large number of microorganisms, and the dynamic balance of these microorganisms is closely related to normal human life activities [9]. Dysbiosis of the gut microbiota can lead to the occurrence of various diseases and promote disease progression [10, 11]. After the HIV invades the human body, it targets CD4⁺ T cells and disrupts the immune system by replicating extensively and changing the permeability of cell membranes, thereby increasing the likelihood of various opportunistic infections [12]. The gastrointestinal tract is the primary site of residence for CD4⁺ T lymphocytes [13]. Their destruction leads to damage to the intestinal mucosal barrier, allowing the overgrowth and displacement of gut microbiota, which can trigger other inflammation-related complications and worsen infections, ultimately leading to death [14]. In addition, since the intestinal mucosal barrier is not repaired, bacterial translocation and infection rates do not decrease [15]. Therefore, current AIDS treatment may also be achieved through in-depth research on changes in the gut microbiota. Studies have found that probiotics can significantly improve gut microbiota dysbiosis in PLWHA, especially those not undergoing HAART treatment [16]. Some researchers have compared the gut microbial communities of children living with HIV/AIDS (CLWHA) and healthy children and found that HAART can help restore or approximate the abundance of gut microbiota in CLWHA to that of healthy individuals [17, 18].

Intestinal mucosal damage is the core of the pathogenesis of AIDS, and the ectopic gut microbiota and its metabolites are key drivers of intestinal mucosal damage [19, 20]. Current research on the gut microbiota of PLWHA mostly focuses on bacterial communities and their metabolites [21‐23]. Although the proportion of fungal communities in the human microbiome is less than 1%, fungal communities play a crucial role in maintaining microbiome structure balance, stable metabolic function, and host immune response [24]. When the human immune system is damaged or the normal microbial community is dysregulated, the antagonistic effect of intestinal bacteria against fungi will weaken, and the balance between microorganisms will be disrupted, exerting negative effects on the host. The most common Candida in the human intestine can transition from a non-pathogenic symbiotic state to a pathogenic state [25]. The detection rate or infection rate of Candida is high in the oral mucosa, urogenital tract, and other areas that are connected to the outside world in PLWHA. Research has shown that damage to the physical structure of the intestinal mucosa, imbalance of gut microbiota, and decreased host immune function can all promote fungal infection [26]. In addition, it has shown that damage to the physical structure of the intestinal mucosa, imbalance of gut microbiota and decrease host immune function can all promote fungal infection. This indicated that PLWHA are at risk of conditionally pathogenic fungal infections [26]. Therefore, studying fungal communities in the gut microbiota is important for understanding the pathogenesis of AIDS and delaying disease progression. As a consequence, this study characterized the differences in gut fungal communities, immune function, and inflammatory factors between PLWHA and healthy individuals as well as their correlations through high-throughput sequencing technology. For the subsequent development of precise interventions targeting intestinal fungal community disorders, including correcting intestinal fungal community disorders by introducing exogenous microorganisms, or inhibiting the damage of related fungal to the intestinal mucosal barrier, providing a basis for improving gastrointestinal symptoms in PLWHA.

The role of gut microbiota in the gut homeostasis of PLWHA is receiving increasing attention [27]. Previous studies have primarily focused on determining the bacterial diversity in the gut of PLWHA [28‐30]. Although fungal communities account for less than 1% of the human microbiota, they play a crucial role in maintaining the balance of the microbial community, stable metabolic functions, and host immune responses [31]. This study compared the differences in gut fungal communities and cytokines between healthy individuals and PLWHA.

The study found significant changes in gut fungal communities in PLWHA, with a significant increase in the relative abundance of Candida, Bjerkandera, and Xeromyces, while the relative abundance of Mycospaerella, Xeroxysium, Penicillium, and Glomerella significantly decreased. It is suggested that Candida, Bjerkandera, and Xeromyces may be key factors in further disrupting the microbial ecology. Candida is an opportunistic pathogen in humans, and its pathogenicity is associated with compromised host immune systems [32]. The formation of hyphae, phenotypic switching, and secretion of proteases contribute to its virulence, with 10 SAP genes encoding secretory aspartic proteases being key to its pathogenicity [33‐35]. After Candida infection, it is attacked by the host’s innate immune system, which degrades host complement components by secreting proteins such as Sap1, Sap2, and Sap3 to evade the host’s immune defenses [36]. Sap1 also plays an important role in Candida albicans mucosal colonization, increasing adhesion intracellularly and causing tissue infiltration extracellularly [33]. In addition, Candida utilizes the intestinal immune damage caused by HIV and the dysbiosis of bacterial flora in the intestinal environment to promote its own growth and high expression of toxic protein SAP1 [37]. The abundance of Candida in the intestines of PLWHA significantly increases, and the structure of gut fungal community changes [38]. Overexpression of the toxic gene SAP1 related to Candida albicans may enhance its ability to adsorb intestinal mucosa and colonize in vivo [39]. Meanwhile, SAP1 can further exacerbate mucosal barrier damage in PLWHA by degrading host complement factors and disrupting intestinal immunity [35, 40]. Developing precise interventions targeting the microbiota, correcting fungal dysbiosis in the gut by introducing exogenous microorganisms, and/or inhibiting SAP1 activity, may reduce damage to the intestinal mucosal barrier and delay disease progression [41]. In summary, Sap1 mediates multiple processes in Candida albicans, including adhesion to host mucosal epithelial cells, lysis of host cells, and immune evasion [42].

Debaryomyces may be a potential probiotic. The research results of Guo et al. showed that Debaryomyces has a good effect on antibiotic-related diarrhea. It can promote the growth of intestinal Bifidobacteria and Lactobacilli in mice and regulate the balance of intestinal microecology [43]. Another potential medical application of Debaryomyces involves their ability to express antipathogenic proteins known as killer toxins or mycocins of pathogenic yeasts (e.g. Candida), Banjara et al’s studies in vitro experimental have shown that Debaryomyces can produce killing toxins that inhibit the growth of Candida albicans and Candida tropicalis [41]. In this study, the abundance of Debaryomyces in PLWHA group was significantly reduced. This may be due to the drastic changes in the intestinal immune environment caused by HIV infection, leading to a significant reduction in the colonization of Debaryomyces. On the one hand, it may lead to its weakened control of Candida. On the other hand, these beneficial fungi may also be affected in synthetic metabolism of short-chain fatty acids and vitamins. Thereby affecting the intestinal health of the host.

The immune dysregulation resulting in microbial imbalance depends on the early study of imbalance [44]. This study found a correlation between the immune levels and gut fungal communities in PLWHA, with Candida exhibiting a positive correlation with inflammatory factors and a negative correlation with anti-inflammatory factors and CD4⁺/CD8⁺ T cells. Previous studies have found that HIV infection significantly damages intestinal immune cells, and as CD4⁺ T lymphocyte counts decrease, immune function is significantly impaired, leading to microbial dysbiosis, overgrowth of pathogenic bacteria in the intestines, immune escape, and accelerated intestinal smooth muscle contractions, leading to the occurrence of symptoms such as diarrhea [45]. HIV-associated diarrhea includes infectious diarrhea (protozoa, fungi, viruses, bacteria, etc.) and non-infectious diarrhea (intestinal diseases, fat absorption disorders, antiviral drugs, tumors), with gastrointestinal immune dysfunction and intestinal dysbiosis considered the two main causes of HIV-related diarrhea, with intestinal microbial infections in PLWHA occurring 20–100 times more frequently than in the general population [46, 47].

This study found that PLWHA have altered gut fungal profiles and immune functions. The increased relative abundance of Candida may be one of the potential factors triggering microbial dysbiosis and intestinal mucosal damage in PLWHA, and a risk factor for substantial intestinal mucosal damage leading to the onset of AIDS. The results of this study provide additional information on the structural characteristics of fungal communities in the gut microbiota of PLWHA and their correlation with gut immune function. In the future clinical treatment of AIDS-related digestive tract diseases, in addition to paying attention to bacterial communities, we should also pay attention to the impact of fungal communities, especially Candida, on the host intestinal immune microenvironment. For the subsequent development of precise interventions targeting intestinal fungal community disorders, including correcting intestinal fungal community disorders by introducing exogenous microorganisms, or inhibiting the damage of related fungal to the intestinal mucosal barrier, providing a basis for improving gastrointestinal symptoms in PLWHA. In addition, it should be noted that the small sample size and insufficient research depth are the limitations of this study, and it is necessary to expand the sample size and attempt to correct the disordered fungal community in future studies.

The fungal diversity in the gut of PLWHA undergoes significant changes, with a significant increase in the relative abundance of Candida, Bjerkandera, and Xeromyces and a significant decrease in the relative abundance of Mycospaerella, Xeroxysium, Penicillium and Glomerella being the main characteristics. These changes are accompanied by a significant decrease in the CD4⁺/CD8⁺ T cell ratio and the anti-inflammatory factor IL-4, as well as a significant increase in pro-inflammatory factors (IL-1β, IL-17, IL-23 and TNF-α).