Oral administration of a bacterial immunomodulator enhances the immune response to cholera toxin.

Attempts to achieve IgA responses in the intestine by oral immunization with non replicating antigens have been characterized by ineffective responses of short duration unless long term dosages are administered. Cholera toxin (CT) is an exception in that it is able to produce a high secretory and systemic immune response. We study the effects of a bacterial immunomodulator [3 x 10(10) Propionibacterium granulosum ml-1 and lipopolysaccharide (LPS) 5 mg ml-1] on the immune response to CT orally administered to Wistar rats. The immunomodulator was orally administered as follows: in schedule 1 during 7 days prior to the first dose of CT; and in schedule 2, 2 days before, together, and 3 days after the first dose of CT. Schedules 1 and 2 were effective in increasing the specific IgA in the intestinal fluid and specific IgG in serum (P < 0.001) when compared to controls. Besides, schedule 2 was more effective than schedule 1 when the levels of specific IgG in serum or specific IgA in intestinal fluid was measured (P < 0.05). Total IgA in the intestinal fluid was increased in rats receiving the immunomodulator (P < 0.01). However, the ratio of specific IgA per total IgA was higher in rats receiving treatment 1 or 2 when compared to controls (P < 0.01). The number of antitoxin antibody producing cells was not increased in the Peyer patches, but a significant increase was observed in the mesenteric lymph nodes and spleen when compared to controls (P < 0.05). The administration of LPS alone produced an increase in the antitoxin immune response when compared to controls, but it was lower than those produced by the administration of the immunomodulator. These results indicate that this immunomodulator is an effective adjuvant of the mucosal and systemic immune response to CT. The mechanisms of action possibly involve nonespecific and specific modulations of the immune response.