Effect of neonatal thymectomy on dental caries in rats

Oral microbial ecology and the role of salivary immunoglobulin A

In the oral cavity, indigenous bacteria are often associated with two major oral diseases, caries and periodontal diseases. These diseases seem to appear following an imbalance in the oral resident microbiota, leading to the emergence of potentially pathogenic bacteria. To define the process involved in caries and periodontal diseases, it is necessary to understand the ecology of the oral cavity and to identify the factors responsible for the transition of the oral microbiota from a commensal to a pathogenic relationship with the host. The regulatory forces influencing the oral ecosystem can be divided into three major categories: host related, microbe related, and external factors. Among host factors, secretory immunoglobulin A (SIgA) constitutes the main specific immune defense mechanism in saliva and may play an important role in the homeostasis of the oral microbiota. Naturally occurring SIgA antibodies that are reactive against a variety of indigenous bacteria are detectable in saliva. These antibodies may control the oral microbiota by reducing the adherence of bacteria to the oral mucosa and teeth. It is thought that protection against bacterial etiologic agents of caries and periodontal diseases could be conferred by the induction of SIgA antibodies via the stimulation of the mucosal immune system. However, elucidation of the role of the SIgA immune system in controlling the oral indigenous microbiota is a prerequisite for the development of effective vaccines against these diseases. The role of SIgA antibodies in the acquisition and the regulation of the indigenous microbiota is still controversial. Our review discusses the importance of SIgA among the multiple factors that control the oral microbiota. It describes the oral ecosystems, the principal factors that may control the oral microbiota, a basic knowledge of the secretory immune system, the biological functions of SIgA, and, finally, experiments related to the role of SIgA in oral microbial ecology.

Comparison of the indigenous oral microbiota and immunoglobulin responses of athymic (nu/nu) and euthymic (nu/+) mice

The role of the immune system in the homeostasis of indigenous oral bacterial populations is poorly understood. In this study, we compared the evolution of the indigenous oral microbiota of specific pathogen-free athymic nude (nu/nu) BALB/c mice with that of their corresponding phenotypically normal (nu/-) littermates. We also evaluated corresponding salivary and serum antibody activities (IgA and IgG) against the predominant indigenous oral bacteria. The bacterial species recovered from the two mouse strains were Lactobacillus murinus, Enterococcus faecalis, Streptococcus oralis and Staphylococcus epidermidis. From 27 days of age, nu/+ and nu/nu mice had significantly different proportions of oral bacterial populations. When the microbiota stabilized (at 40 days of age), the total cultivable microbiota of nu/+ mice was dominated by L. murinus (65-85%), while that of nu/nu mice was dominated by E. faecalis (40-60%). The precise factors that alter the oral resident microbiota in nu/nu mice are unknown. We found that total salivary IgA levels were significantly lower in nu/nu mice, but no association were observed between the level of salivary IgA antibody against indigenous bacteria and the proportion of these indigenous bacteria in the oral microbiota. The change in the microbiota of nude mice may have been caused by other factors such as defects in other immune functions or cold stress.

Dental caries in congenitally athymic rats

The importance of the immune response in dental infection was evaluated in heterozygous (rnu/+) normal and homozygous (rnu/rnu) congenitally athymic "nude" Rowett rats. Animals of both types were infected, or immunized and infected, with mutans streptococci (Streptococcus sobrinus strain 6715). The mean numbers of S. sobrinus cells recovered from the nude rats were higher than those from comparable (immune/nonimmune) normal rats in 10 of 12 possible comparisons. Also, S. sobrinus constituted a greater percentage of the total streptococci in the nude rats compared with normal animals (6 of 6 possible comparisons). Antibody to S. sobrinus whole cells or to S. sobrinus glucosyltransferase from nude rats in serum or in saliva was significantly lower (or absent) than that of comparable normal rats. This was seen after infection, but was most pronounced after immunization (and infection). Dental caries was also significantly elevated in the congenitally athymic animals. Immunologic deficiency of congenitally athymic rats can lead to a greater infection level with mutans streptococci and increased dental caries.

Aging effects on secretory IgA immune responses

Studies of cellular and humoral components of the secretory immune system indicated dramatic decreases in the SIgA response to T-dependent antigens (i.e. DNP-BGG) in aged rats. Senescent rats (18-20 months old) showed comparable levels of SIgA anti-DNP antibody to adult animals after oral immunization with a T-independent antigen, DNP-FICOLL. The salivary SIgA responses to both antigens were substantially decreased in weanling rats (21-35 days) versus the adult rats. Significant anamnestic SIgA responses were shown after oral immunization with DNP-BGG in adult rats, but was not observed in the senescent and midlife (10-12 months) rats. In contrast, the DNP-FICOLL appeared incapable of eliciting a secondary SIgA response in any of the groups. Examination of immunoglobulin-containing cells (ICC) in secretory and lymphoid tissues indicated a generally decreased proportion of IgAICC and IgGICC in the secretory tissues from the senescent rats. IgAICC and IgGICC were within normal limits in the lymphoid tissues of the senescent rats. The results demonstrate a defect in the ability of aged rats to manifest an IgA response to T-dependent antigens administered at mucosal surfaces.

Genetic control of the immune responsiveness to Streptococcus mutans by the major histocompatibility complex of the rat (RT1)

The lymph node cells from 11 strains of rats, differing in the genotype of the major histocompatibility complex of the rat (RT1), were examined on the basis of their proliferative response to the cell wall antigen of Streptococcus mutans. The 11 rat strains fell into three groups: high, intermediate, and low responders. To demonstrate the influence of the major histocompatibility complex on immune responsiveness to S. mutans, further experiments were performed using the RT1-congenic rat strains WKAH.1L(LEW), WKAH. 1AV1(ACI), and WKAH.1J(LEJ), which differ only in the genotype of the RT1 region. Although the background genes of each strain were of WKAH origin, WKAH.1L(LEW) and WKAH.1AV1(ACI) rats showed a low response whereas WKAH.1J(LEJ) rats showed a moderate response to the S. mutans cell wall antigen. The results indicate that the immune response is controlled by the class II gene(s) in RT1. Furthermore, the RT1.D locus products were shown to play an important role in the restriction molecule, since a monoclonal antibody, HOK7, directed to the RT1.Dk locus products reduced the proliferative response of lymph node cells.

Induction and delivery of mucosal immune responses

The oral cavity is relatively deficient in lymphoid tissue capable of directing the inductive and effector events required for effective mucosal IgA responses to local antigen. Recent evidence regarding a common mucosal immune system has raised new possibilities in immunization of mucosal surfaces. This review describes the role of the intestine in contributing to responses at distant mucosal sites and in particular emphasizes the immune potential of the intestine in determining the success of immunization strategies to control dental caries.