Differential susceptibility of mouse lymphocytes to an immunosuppressant from group A streptococci

Bacterial interference with immunospecific defences

There are many descriptions of bacterial agents that can suppress different phases of specific immunity. Bacterial agents are frequently employed as tools to modulate the immune system in experimental models or clinical therapy. Little attention has been given to the implications of such immunoregulating properties for infectious processes or for the natural role of the microbial flora in the normal regulation of the immune system. There are very few studies attempting to investigate directly the possible role of bacterial immunosuppressant factors in acute infection. Several examples of natural and experimental chronic infections have been described in which progressive uncontrolled infection is associated with a depressed cell mediated immunity, but a causal relation is unproven. This depression is usually not antigenically specific and there is some evidence that it may be initiated and maintained by persistent cell-wall components of the microorganisms. The bacteria implicated in suppression are all facultative or obligate intracellular parasites that can multiply and survive in the monocyte/macrophage. Bacterial immunosuppressants may have a crucial role in the pathogenesis of progressive chronic inflammation that occurs after infection.

Immunomodulatory effects of Bacteroides products on in vitro human lymphocyte functions

Bacteroides spp. have been implicated in the pathogenesis of several diseases, including periodontal diseases. In this study sonic extracts of 6 Bacteroides spp. were examined for their abilities to alter human lymphocyte function. We found that soluble extracts from Bacteroides intermedius, Bacteroides endodontalis, Bacteroides asaccharolyticus, Bacteroides melaninogenicus, and to a lesser degree Bacteroides loescheii, caused dose-dependent inhibition of human lymphocyte responsiveness to both mitogens and antigens. Suppression involved altered DNA, RNA and protein synthesis as well as immunoglobulin production. In contrast, Bacteroides gingivalis did not suppress these responses; instead, it stimulated lymphocyte proliferation and enhanced immunoglobulin production. It has been proposed that impaired host defense may play a pivotal role in the pathogenesis of many infections. The data presented in this paper suggest that microbial mediated immunosuppression may conceivably alter the nature and consequences of host-parasite interactions in periodontal disease.

Immunosuppressive properties of Actinobacillus actinomycetemcomitans leukotoxin

Actinobacillus actinomycetemcomitans produces a leukotoxin that kills human polymorphonuclear cells (PMNs) and monocytes but not lymphocytes. In this study, we examined A. actinomycetemcomitans leukotoxin for its ability to alter human peripheral blood lymphocyte (HPBL) responsiveness. After a 90-min exposure to the leukotoxin, all monocytes were killed and HPBL responsiveness to mitogens and antigens was significantly inhibited. The ability of the leukotoxin to inhibit HPBL responses was not surprising, since monocytes and macrophages are required for many lymphocyte functions. However, we were unable to totally restore HPBL responsiveness when adherent autologous monocytes were added back to cultures of leukotoxin-treated lymphocytes. These studies demonstrate that A. actinomycetemcomitans leukotoxin may also exert nonlethal effects directly on lymphocytes. Furthermore, impaired lymphocyte function did not appear to be the result of indirect effects of products released by dying monocytes. Although it is not clear how A. actinomycetemcomitans acts to cause disease, several investigators have proposed that impaired host defenses may play a pivotal role. Several studies have demonstrated defects in PMN, monocyte, and lymphocyte function in patients with periodontal disease. These findings, along with the data presented in this paper, support the hypothesis that patients who harbor A. actinomycetemcomitans could suffer from local or systemic immune suppression. The effects of this suppression may be to enhance the pathogenicity of A. actinomycetemcomitans itself or that of some other opportunistic organism.

Immunologic dysfunction in the pathogenesis of periodontal diseases

Despite significant progress in our understanding of the pathogenesis and etiology of periodontal diseases, the nature and contribution of the immune system to this disorder remains unclear. Several studies provide evidence for either a protective or destructive rôle. These conflicting findings are difficult to reconcile, since most interpretations tend to argue for a static contributory rôle (i.e., either protective or destructive) of the immune system. Current theories on the rôle of the immune response do not address these conflicting findings as well as the contradictory observation of a detectable immune response in the face of persistent infection in these patients. In this article, we present a model, based on available data, for the contribution of the immune system to the pathogenesis of periodontal disease. This model ascribes a dynamic rôle for the immune response. As documented in other infectious diseases, it is entirely possible, for example, that a state of immunologic dysfunction may occur in the earliest stages of periodontal disease progression; this may then be followed by a period of active immune reactivity (humoral and/or cellular) that would represent either a delayed or depressed response. This model is discussed in conjunction with recent findings that several suspected periodontal pathogens are capable of producing immunosuppressive agents. Many of the apparently contradictory clinical observations concerning the host immune response to oral pathogens and its correlation (or lack of) with both the progression and severity of periodontal disease may be accounted for in this model.

Indirect blastogenesis of peripheral blood leukocytes in experimental gingivitis

The blastogenic response of peripheral blood leukocytes to lipopolysaccharide (LPS) was followed over a short course of experimental gingivitis, developed in human volunteers who strictly avoided oral hygeine procedures for periods up to 9 days. Eleven young males initially received thorough dental prophylaxes and supervised oral hygeine until they acquired optimal gingival health. At this point, leukocytes (5 X 10(5)) incubated with 1.5 to 25 mug of LPS in serum-free media showed no response as measured by tritiated thymidine uptake. Coincubation of cells with LPS and phytohemagglutinin (PHA), however, caused synergistic enhancement of blastogenesis in every LPS-PHA dose combination tried. With progressive accumulation of dental plaque and the concomitant development of gingival inflammation, this synergistic response was lost and replaced, proportionately, by a direct response to LPS. The leukocyte response to PHA was marginally enhanced with gingivitis.