Beta-glucan (β-glucan) Enhances the Memory Immune Response of Healthy Dogs

Background

Beta-glucans (β-glucans) are a type of polysaccharide derived from sources such as yeasts, fungi, mushrooms, algae, bacteria, barley, and oats. β-glucans are investigated for use in animals to manage disorders associated with immune-mediated and inflammatory mechanisms, including the chronic diseases Atopic Dermatitis (AD), Osteoarthritis (OA), and Inflammatory Bowel Disease (IBD), in dogs and cats. In aquaculture, they are also used to reduce high, varying, and unpredictable mortality and morbidity in stressed marine life, such as shrimp larvae and trout. β-glucans are valuable as supplements because they are recognized as biological response modifiers possessing immunomodulatory and anti-inflammatory effects. Studies indicate that undenatured, unaltered biological structure, beta-glucans (UDBG) positively influence gene transcription related to overall health, modifying inflammatory responses, protecting from cellular stress, and enhancing immune function without causing immune system over-activity. Additionally, β-glucans function as prebiotics, being selectively fermented by gut bacteria to produce short-chain fatty acids (SCFAs), molceules produced by gut bacteria, that help regulate inflammation, repair mucosal damage, and modulate the intestinal microbiota. Due to its immune-modulating and positive  gut heath properties, β-glucan is becoming a supplement of interest to treat a variety of ailments in animals.

Study objective

Investigate trained immunity in dogs by creating an in vitro model of canine macrophages to see if  β-glucan stimulation could induce an increased production of pro-inflammatory and anti-microbial compounds in response to bacterial stimuli. Researchers also screened different pathogen-associated molecular patterns (PAMPs) to identify the most effective training compounds. 

Methods

Study Design: Canine peripheral blood mononuclear cells (PBMCs), type of immune cells isolated from a dog's blood that includes lymphocytes and monocytes, crucial for the immune system's response to disease, were isolated from Beagle puppies and then treated with various  β-glucan supplements for 24 hours.

β-glucans in the study:

1. Euglena gracilis (BG-Eg): Algae derived β-glucan.
2. Whole Glucan Particle (WGP): Purified from yeast cells.
3. Laminarin: Brown algae derived β-glucan.

Researchers evaluated the following:

Results

No adverse reactions are observed in PBMCs

• No adverse reactions were assessed or reported.

The β-glucan supplement enhances the immune response 

• The β-glucan from E. gracilis (BG-Eg) was identified as the best training inducer, eliciting the highest production of TNF-α upon re-stimulation, significantly superior to all other compounds tested.
• BG-Eg training resulted in a drastic and significant increase in TNF-α secretion upon immune challenge compared to control cells. IL-6, IFN-γ, and IL-12p40 secretion were also significantly enhanced. 

The β-glucan supplement increased anti-microbial immune response

• BG-Eg training significantly increased intracellular ROS production (1.46-fold compared to control cells). Phagocytic activity against E. coli infection was also significantly enhanced in BG-Eg treated macrophages,

B-glucan supplementation alters metabolic profile of PBMCS

Trained macrophages showed hallmarks of the Warburg effect: lower extracellular glucose concentrations and higher lactate production compared to control cells. The metabolite fumarate, a regulator of immune response, was also increased in trained macrophages.

Takeaways

The study demonstrated that β-glucan successfully induces trained immunity in canine macrophages. This phenomenon relies on two intertwined mechanisms and pathways: epigenetic reprogramming, which alters gene expression without changing the underlying DNA sequence (specifically histone methylation) and associated metabolic changes (the Warburg effect).

1. Epigenetics: Histone methyltransferases, requiring cofactors like S-Adenosyl methionine (SAM), are essential for establishing the trained phenotype, allowing for sustained gene expression changes leading to faster and higher inflammatory responses.
2. Metabolic Shift (Warburg Effect): Trained cells increase glycolytic flux and lactate production even in the presence of oxygen. This metabolic change is necessary to maintain the trained phenotype. The enhanced glycolytic flux leads to the accumulation of metabolites, such as fumarate (a TCA cycle substrate), which can act as cofactors for chromatin-modifying enzymes, reinforcing the training. Fumarate also stabilizes HIF-1α, which regulates glycolytic genes and IL-1β production.
3. Receptor Recognition: β-glucans interact with the Pattern Recognition Receptor Dectin-1. Structural modeling revealed differences in canine Dectin-1 compared to human and mouse, notably an increased steric hindrance in the binding groove. This species-specific conformation may explain why linear β-glucans (BG-Eg, Curdlan) were significantly more effective in dogs than branched polymers.
4.  Cytokine Role: High IL-1β secretion observed early upon BG-Eg stimulation suggests a potential specific role for this cytokine as a trained immunity inducer, acting in an autocrine/paracrine manner to reinforce the phenotype.

Limitations

Limited clinical Interpretation

• This study relied entirely on an in vitro model using isolated monocytes/macrophages, meaning the full complexity of in vivo interactions, systemic effects, and long-term outcomes were not assessed. 

Metabolic and epigenetic mechanisms are not explored

Although mechanisms were investigated using inhibitors, the causality and precise timing of the metabolic and epigenetic changes require further documentation. Furthermore, the general lack of standardization regarding the nomenclature and characteristics (branching, molecular weight) of β-glucans complicates the interpretation of their differential biological activity across species.