Priming Dogs’ Immune Systems with β-Glucan May Improve Rabies Vaccine Protection

Objective 

To assess the potential of β-glucans to induce innate immune system training in dogs and evaluate their capacity to enhance the adaptive immune response elicited by an inactivated rabies vaccine (uses a "killed" version of the germ that causes a disease to trigger an immune response without causing the illness itself) labeled as Rabisin®.  

Background

A vaccine adjuvant is a substance that helps intensify weak antigen-specific cellular or humoral immune responses elicited by a vaccine, or provides additional stimulatory effects on the innate immune system that the vaccine alone may not trigger, while lacking any specific antigenic effect itself. β-glucans are attractive candidates for new vaccine adjuvants due to their properties as Biological Response Modifiers (BRMs) and their capability to stimulate various aspects of the immune system. They function as pathogen-associated molecular patterns (PAMPs) that bind to receptors such as Dectin-1 and Complement Receptor 3 (CR3) on immune cells (monocytes, macrophages, dendritic cells, and NK cells), thereby activating both the innate and adaptive immune responses. Crucially, certain β-glucans, like those derived from Aureobasidium pullulans AFO-202, are reported to induce Trained Innate Immunity (TRIM) through epigenetic modifications at the central level in the bone marrow, leading to a long-lasting and wide-spectrum immune effector response. Their generally recognized safety profile and proven track record as immune enhancers further support their potential clinical use as vaccine adjuvants.

Methods

Study Design: 24 Beagles (aged 4.5 to 5 months) were randomly divided into four groups (n=6 per group).  The reference vaccine, Rabisin®, is an inactivated rabies vaccine adjuvanted with aluminum hydroxide. The β-(1-3)-glucan extracted from Euglena gracilis was administered subcutaneously.

Researchers evaluated the following:

Results

β-glucans are well tolerated in animals

• Dogs treated with  β-glucans showed no local reactions or swelling at the site of injection and displayed very good tolerance.

β-glucans induce trained immunity following rabies vaccination

• Macrophages isolated from β-glucan-treated dogs (Groups B/D) showed an increase in markers associated with trained immunity such as significant downregulation of CD14 expression (cell receptor that plays a role in activating immune response), upregulation of MHC-II expression( a group of proteins on the surface of antigen-presenting cells (APCs) that presents extracellular antigens to CD4 helper T cells to orchestrate an immune response), and Significant downregulation of intracellular arginase. Additionally, there was an increase in expression of the β-glucan receptor Dectin-1. A trend towards increased secretion of pro-inflammatory cytokines (TNF-α, IL-6, IL-1β) was also observed in Groups B&C, although this did not reach statistical significance due to intergroup variability. 

β-glucan increase IgG antibody titers and neutralizing antibodies

• All dogs in the study achieved seroconversion above the protection threshold.
• Group B (β-glucan D-28) showed substantially higher levels of neutralizing antibodies at D28 compared to Group A, the control.
• Group B demonstrated a significant increase in total IgG concentrations and IgG1 isotype concentrations compared to Group A.
• Group C (concomitant injection) showed a similar trend of increase in total IgG but did not pass the statistical threshold of significance.

Takeaway

The subcutaneous injection of β-glucan in dogs in conjunction with Rabies vaccination successfully induced changes in innate immune cell phenotypes (macrophages) consistent with mechanisms of trained immunity, including epigenetic reprogramming and metabolic changes (e.g., arginase downregulation/pro-inflammatory polarization).

1. The key finding regarding adjuvantation is that administering β-glucan 1 month prior to vaccination resulted in a consistent and significant enhancement of humoral immune responses (Total IgG and IgG1 concentrations), which is the main correlate of protection against rabies, and might be the optimal treatment plan.

Limitations

Rabies Vaccination induces high baseline metrics that may obscure β-glucan’s effectiveness

• The high efficacy of the reference vaccine (Rabisin®) challenged the ability to demonstrate statistically significant improvements in every parameter. 

The study size is small

• The substantial inter-individual variability in Groups B and C limited the statistical power of the comparisons. More studies with larger group sizes need to be conducted to confirm findings.

The optimal dose of β-glucan and duration of immune response were not evaluated

• The study suggests that the double injection protocol (Group D) may have had a negative immunomodulatory impact, possibly due to repeated injections or the presence of anti-β-glucan antibodies, which were not measured.