Comparative effect of the adjuvant capacity of<i>Lactobacillus casei</i>and lipopolysaccharide on the intestinal secretory antibody response and resistance to<i>Salmonella</i>infection in mice

Abstract

A comparative study was carried out to determine the effect of Lactobacillus casei lipopolysaccharide (LPS) and a mixture of L. casei plus LPS used as oral adjuvants on the increase of mucosal immunity. The ability of these substances to protect against enteric infection was also studied. It was demonstrated that treatment with viable L. casei mixed with LPS and administered for 2 days was more effective than L. casei or LPS alone in increasing secretory IgA (S‐IgA) levels, the number of IgA‐producing cells and the total number of T‐lymphocytes. These effects were not observed when non‐viable L. casei were used with LPS. The study examined the duration of stimulation with L. casei, LPS or a mixture. S‐IgA levels were measured and the effect of boosting (carried out at 15 and 30 days post‐priming with a single dose) against infection with Salmonella typhimurium was determined. L. casei or LPS alone were demonstrated to maintain the IgA levels determined in intestinal fluid for 3 days. Such levels increased with boosting, along with the number of IgA‐producing cells, thus favouring protection against S. typhimurium infection. When L. casei plus LPS were used, the IgA levels were maintained for 7 days. When boosting was effected with the same mixture, results similar to those obtained with L. casei were observed. No side‐effects (hepato and/or splenomegaly) were observed in any case, although the mixture of L. casei plus LPS was shown to induce an increase in polymor‐phonuclear cells in histological studies. This failed to occur when L. casei or LPS were administered separately. However, the mixture was an effective immunopotentiator in the prevention of Salmonella infection due to the association of the two adjuvant substances. The authors believe that viable L. casei, by either itself or associated with LPS (the most important structure of the Gram‐negative enteropathogen cell wall), administered at adequate doses, might act as a useful adjuvant in the mucosa by protecting the host from infections.