Influenza antigen-sparing by immune stimulation with Gram-positive enhancer matrix (GEM) particles

Gram-positive enhancer matrix (GEM) particles, produced from non-genetically modified Lactococcus lactis bacteria have an inherent immunostimulatory activity. It was investigated whether co-administration of GEM particles can reduce the amount of influenza subunit vaccine (HA) necessary to protect mice from viral infection. Decreasing HA amounts of 5, 1, 0.2 and 0.04 μg admixed with GEM particles were tested in intramuscular immunizations. Combinations of GEM and seasonal HA (A/Wisconsin/67/2005 [H3N2]) induced significantly higher systemic and better Th1/Th2-type balanced immune responses than HA alone. Addition of GEM to 0.04 μg HA resulted in similar HI titers as 1-5 μg non-adjuvanted HA. To test the protective efficacy of the adjuvanted combination, mice were immunized with influenza subunit vaccine A/PR/8/34 (H1N1) and then challenged with live virus (A/PR/8/34). Mice immunized with 1 μg HA + GEM showed undetectable virus titers in the lungs 5 days after challenge, whereas mice immunized with 1 μg HA alone showed detectable levels of virus in the lungs. Interestingly, mice vaccinated with the 0.04 μg HA + GEM vaccine demonstrated reduced lung virus titers and a reduction in weight that was similar as that in mice vaccinated with 1 μg non-adjuvanted HA. These results indicate that the use of GEM as immunostimulant allows for a strong reduction in the antigen dose as compared to the benchmark vaccine by using GEM particles. Thus, addition of GEM can strongly potentiate immunogenicity of influenza subunit vaccine both quantitatively and qualitatively.