Response of C3H/HeJ and C3H/HeN mice and their peritoneal macrophages to the toxicity of Chlamydia psittaci elementary bodies

Host cell death during infection with Chlamydia: a double-edged sword

The phylum Chlamydiae constitutes a group of obligate intracellular bacteria that infect a remarkably diverse range of host species. Some representatives are significant pathogens of clinical or veterinary importance. For instance, Chlamydia trachomatis is the leading infectious cause of blindness and the most common bacterial agent of sexually transmitted diseases. Chlamydiae are exceptionally dependent on their eukaryotic host cells as a consequence of their developmental biology. At the same time, host cell death is an integral part of the chlamydial infection cycle. It is therefore not surprising that the bacteria have evolved exquisite and versatile strategies to modulate host cell survival and death programs to their advantage. The recent introduction of tools for genetic modification of Chlamydia spp., in combination with our increasing awareness of the complexity of regulated cell death in eukaryotic cells, and in particular of its connections to cell-intrinsic immunity, has revived the interest in this virulence trait. However, recent advances also challenged long-standing assumptions and highlighted major knowledge gaps. This review summarizes current knowledge in the field and discusses possible directions for future research, which could lead us to a deeper understanding of Chlamydia's virulence strategies and may even inspire novel therapeutic approaches.

Interaction of chlamydiae and host cells in vitro

The obligately intracellular bacteria of the genus Chlamydia, which is only remotely related to other eubacterial genera, cause many diseases of humans, nonhuman mammals, and birds. Interaction of chlamydiae with host cells in vitro has been studied as a model of infection in natural hosts and as an example of the adaptation of an organism to an unusual environment, the inside of another living cell. Among the novel adaptations made by chlamydiae have been the substitution of disulfide-bond-cross-linked polypeptides for peptidoglycans and the use of host-generated nucleotide triphosphates as sources of metabolic energy. The effect of contact between chlamydiae and host cells in culture varies from no effect at all to rapid destruction of either chlamydiae or host cells. When successful infection occurs, it is usually followed by production of large numbers of progeny and destruction of host cells. However, host cells containing chlamydiae sometimes continue to divide, with or without overt signs of infection, and chlamydiae may persist indefinitely in cell cultures. Some of the many factors that influence the outcome of chlamydia-host cell interaction are kind of chlamydiae, kind of host cells, mode of chlamydial entry, nutritional adequacy of the culture medium, presence of antimicrobial agents, and presence of immune cells and soluble immune factors. General characteristics of chlamydial multiplication in cells of their natural hosts are reproduced in established cell lines, but reproduction in vitro of the subtle differences in chlamydial behavior responsible for the individuality of the different chlamydial diseases will require better in vitro models.

Enhancement of natural resistance to influenza virus in lipopolysaccharide-responsive and -nonresponsive mice by Propionibacterium acnes

Lipopolysaccharide-responsive C3H/HeN mice were rendered resistant to a mouse-adapted strain of influenza (Aichi, H(3)N(2)) virus when Propionibacterium acnes was given either intranasally or intraperitoneally several days before virus infection. The time of P. acnes treatment was important since no protection was demonstrated when this agent was given either on the same day as or several days after virus challenge. In contrast, lipopolysaccharide-nonresponsive C3H/HeJ mice were not protected when P. acnes was administered intranasally at any time before infection; however, protection was demonstrated when P. acnes was given by the intraperitoneal route. Depending on the route of inoculation, P. acnes induced several distinctive immunological responses in the lungs of both C3H/HeN and C3H/HeJ mice. Intranasal inoculation was more effective in activating pulmonary macrophages in C3H/HeN than in C3H/HeJ mice. In contrast, intraperitoneal inoculation activated pulmonary natural killer cells in both mouse lines but did not activate pulmonary macrophages.

Modulation of the host immune response as a result of Chlamydia psittaci infection

After intraperitoneal injection of mice with infectious, inactivated, or envelope preparations of the elementary body of Chlamydia psittaci, lymphocyte transformation of spleen cells to the mitogens concanavalin A, phytohemagglutinin, and lipopolysaccharide was significantly reduced 1 and 2 weeks postinjection. Lymphocyte response returned to the control values by 4 weeks. Similarly, transformation of cells by chlamydial antigen was not detected until 4 weeks postinjection. Injection of the noninfectious intracellular reticulate body, in contrast, had little effect on transformation of cells to concanavalin A. When control spleen cells were incubated with infectious or inactivated elementary bodies in vitro, response to all three mitogens was also reduced. The sooner the organisms were added after the addition of mitogen, the greater the reduction in transformation. Incubation with elementary body envelopes and reticulate bodies had no effect on lymphocyte transformation of the spleen cells to concanavalin A. The relationship between the observed ability to reduce the response in the in vitro assay of lymphocyte transformation and the actual in vivo establishment of infection is discussed.