Actinobacillus actinomycetemcomitans mitogenicity for B cells can be attributed to lipopolysaccharide

The purpose of this investigation was to determine the component(s) of whole Actinobacillus actinomycetemcomitans bacteria responsible for B cell mitogenic activity. Congenitally athymic "nude" rats were used as a source of B cells devoid of T lymphocyte activity. Spleen cells were cultured with, or without, whole formalin-killed A. actinomycetemcomitans bacteria or with purified LPS from A. actinomycetemcomitans. Dose-response curves to A. actinomycetemcomitans cells or to A. actinomycetemcomitans-LPS showed that responses were dose dependent. If optimal quantities of both A. actinomycetemcomitans and A. actinomycetemcomitans-LPS were added in combination, the proliferative responses were the same as if either was added alone, i.e., the responses were not additive. Polymyxin B at 2 micrograms/well completely abrogated the proliferative response of athymic rat splenocytes to 10(7) A. actinomycetemcomitans cells or to 1.25 micrograms A. actinomycetemcomitans-LPS/well. Therefore, the in vitro early proliferative response of B cells to A. actinomycetemcomitans can be attributed to the presence of LPS on A. actinomycetemcomitans cells. A considerable portion of the in situ lymphocytic gingival response to A. actinomycetemcomitans infection seen in periodontal disease patients may be a B cell mitogenic response to the LPS of this bacterium.