Cellular immunity to Legionella pneumophila in guinea pigs assessed by direct and indirect migration inhibition reactions in vitro

The guinea pig as a model of infectious diseases

The words 'guinea pig' are synonymous with scientific experimentation, but much less is known about this species than many other laboratory animals. This animal model has been used for approximately 200 y and was the first to be used in the study of infectious diseases such as tuberculosis and diphtheria. Today the guinea pig is used as a model for a number of infectious bacterial diseases, including pulmonary, sexually transmitted, ocular and aural, gastrointestinal, and other infections that threaten the lives of humans. Most studies on the immune response to these diseases, with potential therapies and vaccines, have been conducted in animal models (for example, mouse) that may have less similarity to humans because of the large number of immunologic reagents available for these other species. This review presents some of the diseases for which the guinea pig is regarded as the premier model to study infections because of its similarity to humans with regard to symptoms and immune response. Furthermore, for diseases in which guinea pigs share parallel pathogenesis of disease with humans, they are potentially the best animal model for designing treatments and vaccines. Future studies of immune regulation of these diseases, novel therapies, and preventative measures require the development of new immunologic reagents designed specifically for the guinea pig.

In vivo regulation of replicative Legionella pneumophila lung infection by endogenous interleukin-12

The in vivo role of endogenous interleukin 12 (IL-12) in modulating intrapulmonary growth of Legionella pneumophila was assessed by using a murine model of replicative L. pneumophila lung infection. Intratracheal inoculation of A/J mice with virulent bacteria (10(6) L. pneumophila cells per mouse) resulted in induction of IL-12, which preceded clearance of the bacteria from the lung. Inhibition of endogenous IL-12 activity, via administration of IL-12 neutralizing antiserum, resulted in enhanced intrapulmonary growth of the bacteria within 5 days postinfection (compared to untreated L. pneumophila-infected mice). Because IL-12 has previously been shown to modulate the expression of cytokines, including gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), and IL-10, which regulate L. pneumophila growth, immunomodulatory effects of endogenous IL-12 on intrapulmonary levels of these cytokines during replicative L. pneumophila lung infection were subsequently assessed. Results of these experiments demonstrated that TNF-alpha activity was significantly lower, while protein levels of IFN-gamma and IL-10 in the lung were similar, in L. pneumophila-infected mice administered IL-12 antiserum, compared to similarly infected untreated mice. Together, these results demonstrate that IL-12 is critical for resolution of replicative L. pneumophila lung infection and suggest that regulation of intrapulmonary growth of L. pneumophila by endogenous IL-12 is mediated, at least in part, by TNF-alpha.

Legionella pneumophila Infection in Intratracheally Inoculated T Cell-Depleted or -Nondepleted A/J Mice

The inflammatory response and influence of T cell depletion on the pathogenesis of an experimental Legionella infection were studied. A/J mice were infected with 106 CFU of Legionella pneumophila intratracheally. With this dose all infected animals survived the infection and bacteria were cleared from lung, spleen, liver, and kidney within 10 to 11 days, leaving no residual changes in the affected organs. Inflammatory cells were recruited into the lung on the second day of infection, reaching a maximum on the third day and filling out predominantly the interstitial areas. During the first 3 days after inoculation, mainly macrophages, B cells, NK cells, and large mononuclear cells of an unknown phenotype were attracted into the lung interstitium, whereas T lymphocytes infiltrated subsequently. During the early phase of infection, serum concentrations of IFN-γ, TNF-α, IL-1β, IL-4, and IL-6 but not IL-2 increased dramatically. The cytokine secretion decreased on the third day after infection although bacteria were still present in the lung or even disseminated in different organs. Successful clearance of bacteria from the lung was not observed before recruitment of T cells into the lung. In mice depleted of both CD4+ and CD8+ T cells, control of infection was impaired and lethality of infection increased. Depletion of either subset left residual antibacterial mechanisms, which, however, were not sufficient to clear the Legionella as rapidly as in undepleted mice.

Secondary immunity to Legionella pneumophila and Th1 activity are suppressed by delta-9-tetrahydrocannabinol injection

Resistance to infection with Legionella pneumophila is primarily dependent upon cell-mediated immunity rather than humoral immunity. Recent evidence suggests that activation of cell-mediated immunity depends on Th1 cells and activation of humoral immunity depends on Th2 cells. In this report, delta 9-tetrahydrocannabinol (THC), the major psychoactive cannabinoid of marijuana and an immunomodulator, suppressed development of secondary immunity to L. pneumophila, which correlated with a reduction in Th1 activity. BALB/c mice, infected with a primary sublethal dose of L. pneumophila, developed resistance to a larger challenge infection 3 to 4 weeks later. However, intravenous injection of THC (4 mg/kg of body weight) 1 day prior to primary infection resulted in increased mortality after the challenge infection. The level of anti-L. pneumophila antibodies in serum increased in both THC-treated and control mice; however, in the THC group IgG1 antibodies which are stimulated by Th2 cells were elevated while Th1-regulated, IgG2a antibodies were depressed. Furthermore, cultured splenocytes from THC-treated mice had less L. pneumophila-specific lymphoproliferation, indicating a deficiency in cell-mediated immunity. Normal mouse splenocytes treated in vitro with THC and pokeweed mitogen showed suppressed production of gamma interferon, a cytokine associated with Th1 cells, but increased production of interleukin 4, a cytokine produced by Th2 cells. Splenocytes from THC-treated mice, stimulated in vitro with either pokeweed mitogen or anti-CD3 antibodies, also produced less gamma interferon, indicating less Th1 activity in these mice. These results suggest that THC decreases the development of anti-L. pneumophila immunity by causing a change in the balance of Th1 and Th2 activities.

Macrophage-activating T-cell factor(s) produced in an early phase of Legionella pneumophila infection in guinea pigs

Protective immunity of guinea pigs against Legionella pneumophila was studied by infecting the animals with a sublethal dose (about 2 x 10(4) CFU) of the organism. The bacteria multiplied in the liver, spleen, and lungs up to day 4 after the intraperitoneal infection. The live bacteria in these organs decreased quickly thereafter and were eliminated by day 7. A delayed-type skin reaction and lymphoproliferation of spleen cells to Formalin-killed L. pneumophila were detected from days 5 and 6, respectively, after infection. Peritoneal macrophages obtained from guinea pigs infected 6 days previously inhibited the intracellular growth of L. pneumophila. Antigen-stimulated spleen cell factor prepared from infected guinea pigs inhibited the intracellular growth of the organism in macrophages obtained from uninfected animals. Antigen-stimulated spleen cell factor prepared from spleen cells treated with anti-guinea pig T-cell monoclonal antibody did not inhibit growth. The activity of antigen-stimulated spleen cell factor was labile to pH 2 treatment, and the factor could not be absorbed by L. pneumophila antigen, suggesting that it contains gamma interferon. Our data show that T-cell-mediated immunity begins to work from an early period of infection with L. pneumophila in guinea pigs.

Legionella and Legionnaires' disease: a review with emphasis on environmental studies and laboratory diagnosis

Legionella pneumophila and related species are important causes of epidemic bacterial pneumonia and nosocomial infection. This review will discuss this new family of bacteria and the diseases they produce. The classification, general microbiologic characteristics, and ecology of the bacteria will be reviewed and the epidemiology and clinical aspects of the infection will be discussed. More emphasis will be given to issues that are more directly related to laboratory workers and with which the author has had more direct experience: pathology, laboratory diagnosis of human infection, pathogenesis of the infection, and virulence mechanisms of the bacterium. Therapy and prevention of the infection will be discussed more briefly.

Relative potency of virulent versus avirulent Legionella pneumophila for induction of cell-mediated immunity

Guinea pigs were infected with either high-passage, low-virulence Legionella pneumophila or low-passage, high-virulence organisms. On an infectious dosage basis, the high-virulence organisms were much more effective at sensitizing animals for positive skin test responses and splenic lymphocyte proliferation responses to homologous antigens. These results suggest that exposure to low doses of virulent L. pneumophila can effectively prime animals for cell-mediated immune responses.

Immunostimulation by Legionella pneumophila antigen preparations in vivo and in vitro

Injection of Legionella pneumophila antigen, either killed vaccine or soluble sonicate thereof, resulted in an enhanced antibody response by mouse spleen cells to sheep erythrocytes as determined by the hemolytic plaque assay. Enhancement was dose dependent and reached a peak response at a concentration of 10(7) bacteria or 50 micrograms of sonicate per animal. Larger doses of antigen were less stimulatory or even depressed the antibody response. Similar enhancement of antibody formation by normal spleen cell cultures to sheep erythrocytes in vitro occurred in the presence of graded amounts of L. pneumophila vaccine or sonicate. In addition, the L. pneumophila antigen stimulated enhanced background antibody formation in vitro in the absence of sheep erythrocytes or specific antigen. It appeared likely that the immunoenhancing activity of the L. pneumophila extract may be unrelated to the presence of lipopolysaccharide since boiling the antigen preparation eliminated much of the antibody-enhancing properties of the extract. A large-molecular-weight surface component from L. pneumophila was also immunomodulatory in vitro. Immunostimulation appeared to be related to effects on macrophages since adherent spleen cell populations rich in macrophages, when derived from spleen cell suspensions incubated with L. pneumophila antigen in vitro, stimulated enhanced antibody formation by normal mouse spleen cells in coculture experiments. Further investigations concerning the mechanism of immunomodulation by L. pneumophila antigen in vivo and in vitro appear to be warranted.