Immunosuppressive effect of systemic administration of Lactobacillus rhamnosus KL37C-derived exopolysaccharide on the OVA-specific humoral response

Lactobacilli, including Lactobacillus rhamnosus, commonly used probiotic bacteria, show the ability to produce biofilm. Whole lactic acid bacteria, as well as their cell-wall components show strain specific immunomodulatory properties. Exopolysaccharides (EPSs) are major components of extracellular polymeric substances and the lactobacilli biofilm matrix. However, in contrast to peptidoglycans or teichoic acids, the role of EPS in the modulation of the immune system by lactobacilli is still obscure. Previously, we have shown that crude EPS (cEPS) isolated from L. rhamnosus KL37C can effectively stimulate production of inflammatory mediators by macrophages in vitro. Moreover, EPS inhibits the production of anti-collagen specific immunoglobulin G (IgG) in mice. Interestingly, the reduction of anti-collagen antibodies correlated with the amelioration of collagen-induced arthritis. However, the mechanism of EPS immunosuppressive activity remains unknown. In this paper we examine the ability of EPS to inhibit humoral response to ovalbumin (OVA). The experiments were performed in CBA mice, by immunizing animals with OVA in the presence of EPS or lipopolysaccharide (LPS). The results showed that EPS inhibited humoral response to OVA. Exopolysaccharide injected simultaneously with the antigen decreased the production of anti-OVA IgG, IgG1 and IgG2a antibodies. Interestingly, EPS given together with LPS diminished its adjuvant properties by inhibiting OVA-specific IgG production. Therefore, our data indicate that EPS may exert opposing effects on antibody production in vivo to the adjuvant-like effect of whole lactic bacteria strains, as reported by others. Moreover, this study expands our understanding of a role of EPS in the cross-talk between biofilm forming bacteria and the immune system.