Reactions in the periodontium to continuous antigenic stimulation in sensitized gnotobiotic rats

The purpose of our experiment was to evaluate the destructive potential of a strain of Actinomyces viscosus on the periodontium of sensitized rodents, describing the induced lesions on the basis of quantitative cytology. The experimental design comprised in principle the following procedures. Young germfree rats were immunized either by intravenous or intradermal injections with heat-killed cells of A. viscosus Ny 1 or sham immunized intradermally with physiological saline. After a period suitable for activation of the humoral and cell-mediated immune systems, Ny 1 was monoassociated in all animals by oral implantation. During the ensuing 42 days the animals were allowed to react against the continuous peripheral oral antigen challenge. Since A. viscosus is known as a heavy dental plaque-forming organism, the animals could be expected to develop local immunopathological lesions in the periodontal tissues. These lesions were then studied by quantitative cytology after sampling procedures allowing optimal tissue preservation. The degree of bone loss observed in all treatments was unrelated to the destructive capacity of the infiltrates, suggesting the presence of distinct mechanisms responsible for the activation of osteoclasts and factors interfering with fibroblast activity. Although the cellular composition of the infiltrated tissues was analyzed at the end of the experiment only, distinct stages of the lesions in the different treatments allowed reconstruction of the sequence of events. After an acute inflammatory phase, a classic delayed hypersensitivity reaction developed which was transformed after some time by a large superimposed plasma cell accumulation. The particular but undefined immune status was the significant factor determining the final type of peripheral infiltrative reaction.