Regulation of cytokine expression in human peripheral blood mononuclear cells by the periopathic bacteria A. actinomycetemcomitans and P. gingivalis.
Periodontal disease (also known as periodontitis), is a chronic inflammatory condition resulting from advance of the bacterial plaque normally present on the exposed surface of the tooth, below the gumline onto its root surfaces. Two species of gram negative bacteria P. gingivalis (Pg) and A. actinomycetemcomitans (Aa), identified as the major etiologic agents of periodontitis, have adapted to the microbicidal environment of inflamed periodontium by cytokine-dependent modulation of the host innate immune response. It is known that host macrophages respond to bacterial products by rapid upregulation of interleukin-12 (IL-12), which acts on T lymphocytes (and other leukocytes) to promote high levels of IFN-gamma expression. IFN-gamma is essential for microbicidal action of macrophages---which in turn establishes conditions for long-term immunity to a particular microorganism. We hypothesized that a major mechanism by which these bacteria are able to mediate this effect involves preferential repression of IFN-gamma production within the inflamed periodontium, while allowing expression of cytokines which maintain a chronic inflammatory state. For the empirical testing of our hypothesis, we used an in vitro model of bacteria-host interaction involving exposure of human peripheral blood mononuclear cells (PBMC) to soluble products of both periopathic species organisms (Aa and Pg). We evaluated expression of immunoparameters relating to critical events in the initial host response to microbial challenge. We measured expression responses of cytokines to bacterial products by enzyme-linked immunoassay (ELISA) analysis of cell culture supernatents; expression of cytokine mRNA by reverse transcriptase polymerase chain reaction (PCR) amplification; and measured expression of critical leukocyte surface antigens by flow cytometry. Our results demonstrated that both organisms induce interleukin-10 (IL -10) expression by PBMC sufficient to inhibit expression of IFN-gamma, while allowing production of 3 "pro- inflammatory" cytokines known to be the major agents of tissue damage: IL-1beta, IL-6 and TNF-alpha. The adaptive advantage for the bacteria, is that whereas inhibition of IFN-gamma would be expected to blunt the host innate antimicrobial response, expression of pro-inflammatory cytokines would create tissue damage allowing for expanded bacterial colonization. These findings provide insight into the fundamental mechanisms of chronic inflammation and suggest approaches for development of new therapies.