In the present study, we compared the ability of the soluble adjuvants
concanavalin A (ConA),
muramyl dipeptide (MDP), and
peptidoglycan (PG) to enhance immune responses to orally administered particulate
antigens of Streptococcus mutans 6715 in gnotobiotic rats. The isotype and levels of antibody in saliva and in serum from experimental rats were determined by an
enzyme-linked
immunosorbent assay using S. mutans whole cells (WC) as the coating
antigen. The specificities of salivary and serum
immunoglobulin A (
IgA)
antibodies to particulate S. mutans
antigens,
lipoteichoic acid, S. mutans serotype g
carbohydrate, and
dextran were also determined. When 50 micrograms of ConA was used as the oral adjuvant with S. mutans 6715 WC immunogen, a slight enhancement of immune responses was obtained. A higher dose of ConA suppressed humoral responses to the immunogen. Enhanced immune responses, especially of the
IgA isotype, in both serum and saliva were induced in gnotobiotic rats given MDP and either S. mutans 6715 WC or purified cell walls (CW) by gastric intubation. Elevated
IgA antibody levels to CW,
lipoteichoic acid, and
carbohydrate were observed in rats given S. mutans WC and MDP by gastric intubation, whereas oral immunization with S. mutans CW and MDP resulted in higher antibody levels to CW and
carbohydrate and lower levels to
lipoteichoic acid when compared with the antibody levels in rats given
antigen alone. Rats orally immunized with either S. mutans WC or CW and MDP and challenged with virulent S. mutans 6715 exhibited significantly (P less than or equal to 0.05) lower plaque scores, numbers of viable S. mutans in plaque, and caries scores than did rats immunized with
antigen alone or in infected-only controls. In another series of experiments, a PG fraction derived from S. mutans 6715 CW was assessed for adjuvant properties. The
oral administration of PG and either S. mutans WC or CW induced good salivary and serum
IgA antibody responses. The specificity of the
antibodies was similar to that obtained in rats given
antigen and MDP. Rats receiving either S. mutans WC or CW and PG and challenged with virulent S. mutans 6715 had lower plaque scores, fewer numbers of viable S. mutans in plaque, and lower caries activity than did infected rats receiving S. mutans WC or CW immunogen alone. These results provide evidence that soluble adjuvants derived from the gram-positive bacterial CW, e.g., MDP and PG, are effective oral adjuvants and augment
IgA immune responses to particulate S. mutans
antigens which are protective against the mucosally associated
disease, dental caries.