The mycelium of the Trametes versicolor (Turkey tail) mushroom and its fermented substrate each show potent and complementary immune activating properties in vitro

Medicinal mushrooms describe a category of edible members of the kingdom Fungi, traditionally associated with health-supporting properties. They have been used for centuries to treat an array of ailments, particularly in traditional Asian medicine and Eastern European traditions. They are well regarded for supporting longevity, treating infectious disease and cancer, and promoting overall well-being [1, 2]. Contemporary research has mainly focused on the broad immune activity of mushrooms. Several preclinical findings suggest that mushrooms may specifically support NK cell upregulation, [2‐5] enhancement of T-cell and NK cell cytotoxicity [6], and the induction of immune-regulating cytokines such as TNF-α, IL-2, IFN-y, and IL-10 [7‐10]. As a category, medicinal mushrooms stimulate host defense and immunity due to the complex and varying polysaccharides; some well-studied examples include (1,3;1,6)-β-glucans, proteoglycans, heteroglucans, that comprise the chitin-based fungal cell wall [11‐13]. Mushrooms are also the source of other pharmacologically relevant compounds, such as proteins like Ling zhi-8 in Ganoderma lucidum [14] and lectins in several species [15], triterpenes [16, 17], phenols [18], and sterols [16]. While medicinal mushrooms generally confer broad immune activity, individual species often possess unique immunological properties. Trametes versicolor (Tv), commonly known as Turkey tail and previously named Coriolus versicolor, is known to enhance innate and adaptive immune responses [19, 20]. Recent clinical research involving consumption of Tv mycelium on rice substrate by Standish and colleagues [21] suggests NK cell induction in women with breast cancer. Other researchers cite antitumor effects [4, 22, 23], but this is generally considered to be a result of its underlying immunologic activity [24]. Tv contains precursors to the proteoglycans polysaccharide peptide (PSP) and polysaccharide-K (Krestin, PSK), the latter of which is frequently prescribed to gastric cancer patients in Japan [25]. The constituents responsible for these immunological effects are believed to be the polysaccharides. However, recent research suggests that the lipid fraction of PSK isolated from Tv is instrumental to its TLR-2 induction activity [26].

The natural ecological role of mushrooms is to assist the breakdown of dead plant matter, and therefore they engage in a highly dynamic interaction with the environment in which they grow. The fungal organism contains a vegetative state consisting of progressive extensions of tissue into a substrate, as well as a reproductive state for spore dispersal (Fig. 1). Mycelium is an aggregation of multinucleate hyphae that typically appear as strands or thin filaments. As a mycelium grows throughout its environment, it secretes an array of compounds into its substrate, altering the chemical nature of the substrate. This enzyme-rich exudate helps catalyze the breakdown of macromolecules for absorption—an example of which are the extracellular lignin-modifying enzymes laccase, lignin peroxidase, and manganese peroxidase [27]. Fungi that engage in this type of enzymatic lignin biodegradation and decompose wood are known as white-rot fungi, named for their white appearance. As a white rot fungus with notable laccase production, Tv has the capacity to enzymatically affect its environment, primarily by degradation of its substrate [28‐30]. This enzymatic activity is not limited to lignin-containing woody tissues; rice bran can also function as an efficient substrate for laccase production [31].

Of importance for the understanding of medicinal mushrooms for human consumption, fungal hyphae also secrete a wide variety of defense compounds to deter predators and pathogens [32]. Secreted defense compounds allow the fungi to maintain their territory and evade invasion by bacteria and molds. These compounds may be evolutionarily conserved and offer biological effects for other species such as humans. The medical significance of this is apparent in the famous example of the first antibiotic—penicillin, isolated from the Penicillium chrysogenum mold [33].

As structurally different as the mycelium is from the fruitbody, so too are their biological functions. Whereas the mycelium is the major biomass of a fungus and serves to gather nutrients and interact with the substrate during decomposition, the fruitbodies (the most commonly known form of edible mushrooms) are the instruments of spore dispersal in higher fungi (Basidiomycota). They commonly appear as a cap on top of a stem or stalk, with either gills or pore structures underneath the cap. Mycelium and fruiting bodies share similar cell wall structures and contain the polysaccharide complexes that enhance the innate and adaptive immune response [12, 34, 35]. However, concentrations vary, and β-glucans are considered to be present in higher concentrations in the fruiting body compared to the mycelium [36], whereas the mycelial tissue may contain a broader profile of bioactive compounds. Other metabolic variances may exist: recent proteomic research suggests that 40% more protein-coding genes in G. lucidum are expressed in the mycelial state, compared to the fruiting body [37].

The production of medicinal mushroom products utilizes a wide spectrum of substrates, including sawdust to mimic the natural habitat, as well as various grains. It is well known that the biological properties of raw grain are altered by fungal fermentation, likely due to the secreted enzymes. A simple fungal organism, namely yeast, grown on red rice, is considered a dietary supplement, and documented to reduce LDL in preclinical and clinical settings [38], properties not associated with consumption of plain unfermented rice. Another example is the Saccharomyces yeast-based fermentate EpiCor®, which is composed of the fungal cell walls as well as secreted metabolites produced during the fermentation. The aqueous extract of the dried fermentate has well-documented immune activating, anti-inflammatory, and antioxidant properties [39‐41]. Consuming the whole dried fermentate is associated with clinical benefits including improved mucosal immune health [42], and reduced incident and duration of colds [43] and allergies [44]. Research using the SHIME model for digestive health has shown beneficial effects on the gut microbiome [45].

Many medicinal mushroom products are sold as a crude powder consisting of mycelium and its fermented substrate. While pre-clinical and clinical studies have been performed on these products [19, 21], the immunological contributions of the fermented substrate have not been examined. The purpose of this study was to characterize the immunological activity of each of the components, namely the mycelia and the fermented substrate, using the initial substrate as a control. Tv was selected as a model organism for this effort because the physical structure of the mycelium is well defined and allows for harvest of both the mycelium and the fermented substrate (including secreted fungal metabolites) when cultivated in specially designed solid substrate fermentation (SSF) systems. The test model involved evaluation of the early activation marker CD69 on different subsets of immune cells and the induction of production of cytokines and growth factors. The choice of CD69 is based on its role in natural killer (NK) cell function, where CD69 is rapidly induced in NK cells shortly after activation [46] and has a direct role in NK cytotoxicity (killing of target cells) [47].

TvM triggered robust increases in the CD69 activation marker on lymphocytes and monocytes, unlike FM which did not induce CD69 on immune cells. The cell surface marker CD69 is rapidly upregulated on many immune cell types after activation, and correlations have been made between Natural Killer (NK) cell CD69 expression and NK cell-mediated tumor-killing activity in the classical target cell-based assay, by a number of research teams over the past 25 years. We have found the induction of the CD69 activation marker a helpful tool for natural products research, both in vitro [48‐54] and in clinical studies [55, 56]. When human NK cells are co-cultured with K562 target cells, CD69 expression is upregulated, and the increase significantly correlated with NK cell activity, as measured by today’s gold-standard CD107 mobilization assay [57]. CD69 has the capacity to activate the NK cytolytic machinery in the absence of other NK–target cell adhesion molecule interactions [58]. A direct and highly significant correlation between CD69 levels and NK cell activity was demonstrated by Clausen et al 2003 [59], in a study involving 14 breast cancer patients tested repeatedly during chemotherapy.

NK cells do not function in a vacuum; they are regulatory cells engaged in crosstalk with other cell types [60]. Therefore, the work reported here focused not only on NK cells, but also on NKT cells, T cells, and monocytes. The TvM-mediated induction of CD69 expression on lymphocytes and monocytes was triggered by both aqueous and solid fractions, however, the CD69 expression on lymphocytes was more robust when cells were treated with the aqueous fraction than the solid fraction, with the aqueous fraction comparable to the induction caused by LPS. Given that the aqueous fraction contained almost 10 times less material than the solid fraction, this further demonstrates the potency of the aqueous compounds in the mycelium. For monocytes, this was reversed, where the solid fraction triggered a stronger CD69 expression than the aqueous fraction of the TvM. This is expected, since monocytes are known to be robustly activated by insoluble fungal beta-glucans through Toll-Like receptors (TLR), specifically TLR-2 and TLR-4 [61].

The cytokine induction by aqueous and insoluble compounds in the fermented substrate was seen in the absence of CD69 up-regulation, suggesting activation via alternate pathways, such as has been demonstrated for NK cell activation, which may involve CD69 expression or as an alternative mode of activation involve upregulation of the Interleukin-2 receptor CD25. This was demonstrated by Clausen’s team to be associated with distinct functional differences, where CD69 expression is associated with cytotoxicity as described above, whereas the CD25 expression is associated with increased cell proliferation [59].

Interestingly, the initial un-fermented substrate was not very bioactive, devoid of aqueous bioactive compounds, and only minor effects on cytokine induction by the solid fraction. This further helps demonstrate the uniqueness of the fermented substrate, in terms of fermentation of the rice along with fungal exudates. This is important for the many consumable products that are produced from mycelia along with their fermented substrates.

The aqueous extracts were produced by cold-water extraction, and not using heat or pressure. This is in contrast to other types of fungal extracts and teas, where heat and sometimes pressure is applied to produce the extract. Examples include the use of batch reactors and subcritical water extraction, with heat up to 300 °C, to produce extracts from golden oyster mushrooms (Pleurotus citrinopileatus) [62] and Chaga mushrooms (Inonotus obliquus) [63].

The results clearly demonstrate that most of the bioactivity for cytokine induction lies in the fermented substrate. Based on the known enzyme secretions by mycelium during active growth, the fermented substrate likely represents a broad array of fungal products, in conjunction with breakdown products from the substrate.

Tv has proved to be an effective model for demonstrating the bioactivities of mycelium versus its fermented substrate. This model will be useful for further evaluation of Tv and other medicinal mushrooms, and this model can help mycology research in the isolation and identification of bioactive compounds. There may be potentially pharmaceutical uses of isolated novel compounds form both mycelium and fermentate, however, one of the more unique and interesting challenges will be to understand their synergistic relationships in traditional medicinal use of the whole crude fermentate. Further work from our team is ongoing and includes this evaluation of synergy, by pretreating isolated NK cells and monocytes with TvM, FM, and a blend thereof, followed by the classical NK activity assay in co-culture with tumor cells, and by co-cultures of Tv-primed monocytes with lymphocytes to evaluate the role of NK cells and monocytes in the overall immunological cross-talk between cell types. In addition, a clinical trial will compare the TvM, FM, and the blend thereof. This will help further document the importance of the multi-faceted and complex actions of the natural mixture of TvM and its fermented substrate, traditionally used for immune support in the integrative medicine setting.

The work reported here has helped demonstrate that the mycelial fermentation of its substrate dramatically alters the biological effects of the fermented substrate. Furthermore, the mushroom mycelium has distinctly different biological and immune-modulating properties than its fermented substrate. The mycelium was very potent in terms of triggering immune cell activation, whereas the fermented substrate was very active in terms of cytokine induction. Complex immune-activating bioactivity of mycelial-based medicinal mushrooms go beyond effects of insoluble beta-glucans, as potent effects were also seen in the aqueous fraction. The results suggest that overall medicinal effects are associated both with the mycelium itself (including insoluble beta-glucans, but also water-soluble components), and the highly bioactive fermented substrate. Novel applications for animal and human immune health may be identified in the future for components isolated from fermented substrates, independent of mushroom mycelium.