Specific immunity induction at the mucosal level by viable Lactobacillus casei: A perspective for oral vaccine development

In previous papers, we demonstrated that viable Lactoacillus casei ssp. casei administered orally had a good adjuvant activity at the mucosal level and protected against Salmonella typhimurium infection. We also demonstrated that the mixture of L. casei plus LPS enhanced its protective capacity without side-effects. In this paper we study the frequency and duration of boosters with L. casei necessary to maintain a good mucosa immune response that protects against infection, without alteration in the immune cells associated with the lamina propia of small intestine. After priming with L. casei, different boosting protocols with single or double doses oft. casei were made. We determined on the lamina propia of the small intestine, IgA and IgM producing cells, CD4(+) and CD8(+) T-cells and the number of macrophages and polymorphonuclear (PMN) cells. Protective assays against S. typhimurium infection and assays of specific anti-Salmonella IgA present in the intestinal fluid were also performed We demonstrate that boosters with only one dose on the 15th or 30th day post-priming were more effective in protecting against S. typhimurium infection than a booster with two doses oft. casei, which was not effective. The secretory-IgA levels, IgA-secreting cells, macrophages, CD4(+), CD8(+) and T-cells were increased with the effective boosters. We also demonstrate that the ratio of CD4(+)/CD8(+) T-cell populations was not altered with the different boosting protocols. The conditions for the use of L. casei ssp. casei CRL 431 as an oral adjuvant or as a vaccine vector were determined.