Detection of nonspecific resistance to Listeria monocytogenes in rabbits infected with Treponema pallidum

Listeria monocytogenes infection in pregnant macaques alters the maternal gut microbiome†

OBJECTIVES

The bacterium Listeria monocytogenes (Lm) is associated with adverse pregnancy outcomes. Infection occurs through consumption of contaminated food that is disseminated to the maternal-fetal interface. The influence on the gastrointestinal microbiome during Lm infection remains unexplored in pregnancy. The objective of this study was to determine the impact of listeriosis on the gut microbiota of pregnant macaques.

METHODS

A non-human primate model of listeriosis in pregnancy has been previously described. Both pregnant and non-pregnant cynomolgus macaques were inoculated with Lm and bacteremia and fecal shedding were monitored for 14 days. Non-pregnant animal tissues were collected at necropsy to determine bacterial burden, and fecal samples from both pregnant and non-pregnant animals were evaluated by 16S rRNA next-generation sequencing.

RESULTS

Unlike pregnant macaques, non-pregnant macaques did not exhibit bacteremia, fecal shedding, or tissue colonization by Lm. Dispersion of Lm during pregnancy was associated with a significant decrease in alpha diversity of the host gut microbiome, compared to non-pregnant counterparts. The combined effects of pregnancy and listeriosis were associated with a significant loss in microbial richness, although there were increases in some genera and decreases in others.

CONCLUSIONS

Although pregnancy alone is not associated with gut microbiome disruption, we observed dysbiosis with listeriosis during pregnancy. The macaque model may provide an understanding of the roles that pregnancy and the gut microbiota play in the ability of Lm to establish intestinal infection and disseminate throughout the host, thereby contributing to adverse pregnancy outcomes and risk to the developing fetus.

Experimental syphilis in guinea pig

The infrequent use of guinea pig in experimental syphilis, the not well genetically and immunologically characterized strains of animals originating from places with unspecified conditions of husbandry, and the various strains of Treponema pallidum used for infection provided inconsistent and discouraging results. For eight decades the rabbit has been the major animal model in studies of syphilis. However, the lack of readily available inbred strains of rabbits--necessary for adoptive transfer experiments--has been a stumbling block in revealing the mechanisms responsible for immunity, susceptibility, and resistance to T. pallidum infection. These difficulties have recently been overcome by demonstration of inbred strains susceptible to T. pallidum infection, paving the way to studies of adoptive immunity. The guinea pig may also be a better model than the rabbit for immunomanipulations (irradiation, injection with antibodies specific to various cell populations), allowing a closer insight into the immunopathologic mechanism operating during the course of syphilitic infection. The "rediscovery" of the guinea pig as a model for experimental syphilis and recent years of intensive studies justify a review summarizing older data and providing the most recent information. The authors, having first-hand experience with this model, will provide detailed information on (1) historical background; (2) course of infection with T. pallidum in inbred and outbred strains of guinea pigs; (3) the ID50 for various strains; (4) various routes of infection; (5) age and sex-dependent susceptibility to infection; (6) kinetic of the humoral response to specific and non-specific treponemal antigens; (7) appearance of autoantibodies and immune complexes; (8) cellular response, including lymphoproliferative response, macrophage inhibitory factor(s) production, chemotaxis and adoptive transfer of immunity by purified T cells; and (9) a complete list of references.

Infiltrate of syphilitic lesions before and after treatment

An immunohistological study of skin biopsy specimens from patients with early syphilis was undertaken before and after treatment (one day after intramuscular administration of 2.4 MIU benzathine penicillin and eight days later, after a total administration of 3.6 MIU. In chancres from seronegative patients treatment with 3.6 MIU usually resulted in fewer immunocompetent cells in the infiltrate. In lesions of secondary syphilis treatment with 2.4 MIU benzathine penicillin produced a significant decrease in immunocompetent cells. After treatment with 3.6 MIU there was no further decrease. It was worth noticing that even eight to nine days after the initial pretreatment biopsy, when 3.6 MIU had been administered, the overall lymphohistiocytic infiltrate was not substantially diminished. Significantly more suppressor (T8+) cells were found in lesions of primary syphilis than of secondary syphilis, and they showed remarkable exocytosis. Activated local T8+ cells may release immunosuppressive lymphokines.

Activation of macrophages by products of lymphocytes from normal and syphilitic rabbits

The production of soluble macrophage-activating factors by lymphocytes from syphilitic and normal rabbits was examined. Culture supernatants of splenic lymphocytes cultured with Treponema pallidum antigens or concanavalin A were incubated with rabbit peritoneal macrophages in vitro. The macrophage monolayers were then washed and infected with log-phase Listeria monocytogenes. Activation of the macrophages by lymphocyte products was measured by the ability of the macrophages to resist intracellular multiplication of Listeria and thus survive infection. Macrophages incubated with supernatants of unstimulated lymphocytes or T. pallidum-stimulated lymphocytes from normal rabbits were unable to resist intracellular multiplication of Listeria. Specifically stimulated lymphocytes from syphilitic rabbits and mitogen-stimulated lymphocytes from both normal and syphilitic rabbits demonstrated a clear ability to produce soluble factors which conferred upon macrophages the ability to limit the intracellular growth of the bacteria. Antigen or mitogen alone was unable to activate the macrophages; the presence of lymphocyte products was required.

Macrophages in immunity to syphilis: suppressive effect of concurrent infection with Mycobacterium bovis BCG on the development of syphilitic lesions and growth of Treponema pallidum in tuberculin-positive rabbits

Paired groups of male rabbits were challenged with Treponema pallidum and Mycobacterium bovis BCG. One group had been sensitized to BCG by inoculation 3 weeks before challenge. All animals were challenged intradermally at multiple sites with T. pallidum alone, BCG alone, and both organisms into the same sites. The resulting lesions were followed clinically and histologically. BCG lesions enlarged more rapidly in sensitized rabbits, but they were otherwise no different from those in the controls. T. pallidum lesions enlarged and regressed simultaneously in both groups, but in the BCG-sensitized animals they became twice as large as those in the unsensitized rabbits. Mixed BCG-T. pallidum lesions showed the greatest differences in the two groups of animals. Like the pure BCG lesions, they enlarged more rapidly in the sensitized rabbits but began to recede after 1 week. The corresponding lesions in the controls enlarged more slowly and reached their maximum size after 3 weeks when the receding lesions in the sensitized animals were much smaller. The most marked histological-histochemical difference between the two groups of animals was in the number and activation of macrophages. These cells were more numerous in the mixed lesions of BCG-sensitized animals than in similar lesions of the controls and more activated as determined by beta-galactosidase staining. Although sparsely distributed, activated macrophages were more numerous in the pur T. pallidum lesions of sensitized animals than in those of control animals. Silver-stained sections revealed fewer treponemes in mixed lesions of sensitized animals than in the mixed lesions of control animals. Quantitation of treponemes in pure T. pallidum versus mixed lesions was determined in two groups of rabbits challenged intratesticularly. The total number of treponemes per testis in the mixed lesions of BCG-sensitized rabbits was significantly less than the number in the mixed lesions of control animals, and also less than the number in pure T. pallidum lesions of both groups of animals.

Cell-mediated immunity during syphilis

Evidence is presented which reinforces the complexity of the host-parasite interaction during the course of syphilis. Infection with Treponema pallidum evokes a complicated antibody response and an assortment of cell-mediated immune reactions in the host. It appears that humoral immunity plays a minor role towards the complete elimination of syphilitic infection while the cellular limb of the immune response may be an important host defence mechanism. Information now available indicates that a state of anergy, or immunosuppression, exists in the early stages of human and experimental rabbit syphilis based upon negative skin reactions to T. pallidum antigen(s), the abnormal histological appearance of lymphoid organs, and impaired in vitro lymphocyte reactivity. It is also evident that in the later stages of the disease cellular immunity becomes activated as delayed type skin reactions can normally be elicited in tertiary syphilitics and lymphocyte behaviour in cell culture appears normal. Several mechanisms have been invoked to explain the delay in an effective immune response against syphilitic infection and the duration of the disease: (1) a capsule-like substance on the outer surface of virulant T. pallidum may act as a barrier against treponemicidal antibody; (2) this material and other biological properties of virulent treponemes could enable spirochaetes to escape being engulfed by macrophages and other phagocytic cells; (3) antigenic competition among different treponemal antigens causing partial tolerance; (4) T. pallidum infection may bring about the elaboration of immunosuppressive substances of host or treponemal origin which inhibit the proper function of lymphocytes, macrophages, and other cell types.

Nitroblue tetrazolium test in experimental syphilis

The reduction of nitroblue tetrazolium (NBT) by phagocytic leucocytes in peripheral blood has been examined in rabbits injected intratesticularly with Treponema pallidum, T. pallidum-free supernatant of orchitic testes, or normal rabbit testes supernatant. A significant (P less than 0.01) increase in NBT reduction was observed only in the infected animals.

In vitro cell response of Treponema pallidum-infected rabbits. III. Impairment in production of lymphocyte mitogenic factor

Production of mitogenic factor was examined in rabbits infected intratesticularly with T. pallidum and in control animals injected with saline or saline extract of normal rabbits' testes. Lymph nodes and spleen from animals killed 2, 6 and 12 weeks after injection were used as the source of lymphocytes, cultured in serum-free medium in the presence of Reiter antigen. The active supernatants of lymph node cells (LNAS) and spleen cells (SPAS) were examined for the presence of mitogenic factor using normal rabbit peripheral lymphocytes. The LNAS of control animals showed a mitogenic index (MI) between 4 and 6 and the infected animals less than 2. The SPAS of infected and control rabbits showed an MI of less than 2. The lower mitogenicity in LNAS of infected and that of SPAS of infected and control animals seems to be due to the presence of inhibitors of DNA synthesis.

Development of of macrophage migration inhibition in rabbits infected with virulent Treponema pallidum

Peritoneal exudate cells from rabbits infected with Treponema pallidum Nichols were used as indicators of macrophage migration inhibitory factor activity. Between 3 and 15 weeks after infection, the migration of peritoneal exudate cells was inhibited in the presence of 3 to 25 microgram of T. phagedenis biovar Reiter protein per ml. Before this period, the migration patterns of peritoneal exudate cells from infected animals were uninhibited and similar to those from noninfected control rabbits. These observations were correlated with the development of active cell-mediated immunity during experimental T. pallidum infection.

Inability of spleen cells from chancre-immune rabbits to confer immunity to challenge with Treponema pallidum

Although several lines of evidence suggest that cellular immune mechanisms play a role in controlling infection due to Treponema pallidum, recent studies have shown that induction of acquired cellular resistance by antigenically unrelated organisms fails to protect rabbits against syphilitic infection, thereby casting doubt on this hypothesis. In the present paper we describe attempts to transfer immunity to syphilis by using spleen cells from chancre-immune rabbits. Intravenous infusion of 2 X 10(8) spleen lymphocytes was capable of transferring acquired cellular resistance to Listeria and delayed hypersensitivity to tuberculin. However, in eight separate experiments using outbred or inbred rabbits, 2 X 10(8) spleen cells from syphilis-immune animals failed to confer resistance to T. pallidum whether by intravenous or intradermal challenge. Mixing immune lymphocytes with treponemes immediately before intradermal inoculation also failed to confer resistance. Despite the fact that syphilitic infection stimulates cellular immune mechanisms and induces acquired cellular resistance to antigenically unrelated organisms, cellular immunity may not play an important role in immunity to syphilis.

Selective response of lymphocytes from Treponema pallidum-infected rabbits to mitogens and Treponema reiteri

The in vitro response of peripheral blood lymphocytes from rabbit infected with Treponema pallidum was examined using various mitogens and avirulent Treponema reiteri. For the first 4 weeks after treponemal infection, the response of lymphocytes from syphilitic rabbits to phytohemagglutinin and pokeweed mitogen was markedly reduced in comparison to uninfected controls. Lymphocytes from both groups of rabbits responded normally to class-specific immunoglobulin anti-sera (anti-immunoglobulin M and anti-immunoglobulin G) and T. reiteri.

Depression of lymphocyte response to concanavalin A in rabbits infected with Treponema pallidum (Nichols strain)

Peripheral blood lymphocytes isolated from rabbits in the early stages of Treponema pallidum infection responded poorly when exposed to concanavalin A in vitro. Maximal depression of blastogenesis occurred when lymphocytes were cultured in the presence of autologous serum in comparison with fetal calf or normal homologous rabbit serum.

Effect of pretreatment with Mycobacterium bovis (strain BCG) and immune syphilitic serum on rabbit resistance to Treponema pallidum

Stimulation of the rabbit reticuloendothelial system with viable Mycobacterium bovis (strain BCG), and other agents, had no effect on the development of syphilitic lesions after intradermal or intravenous inoculation with graded doses of Treponema pallidum (virulent Nichol's strain; mean infective doses less than 10). The simultaneous administration of immune syphilitic rabbit serum retarded the development of lesions, but this appeared to be due solely to the immune serum, suggesting no synergism between the activated reticuloendothelial system and the anti-T. pallidum antibodies. The administration of two doses of BCG enhanced syphilitic lesion development in the rabbit.

Nonspecific resistance to Listeria monocytogenes in mice infected and elicited with Staphylococcus aureus

Normal mice and mice displaying delayed hypersensitivity to Staphylococcus aureus were challenged with a lethal dose of Listeria monocytogenes. Nonspecific antimicrobial activity was assessed by inhibition of bacterial growth in the spleen and by increased survival rates. Administration of specific staphylococcal antigen prior to challenge was a prerequisite for the induction of nonspecific resistance. Both the time of administering eliciting antigen and the route were important consideration for evoking the response. Similar responses were seen in 3 strains of inbred mice following immunization with both encapsulated and nonencapsulated staphylococci. Although enhanced resistance as measured by viable counts in the spleens was achieved after 2 injections, protection as measured by survival required 4 injections of S. aureus followed by elicitation with staphylococcal antigens. Nonspecific resistance could be detected as late as 9 weeks after the disappearance of delayed hypersensitivity.

Lymphocyte transformation in syphilis: an in vitro correlate of immune suppression in vivo?

Suppression of cellular immunity during primary and secondary infection may explain, in part, the unusual clinical evolution of syphilis. We have previously shown that lymphocytes from normal subjects undergo blastic transformation when exposed in vitro to Treponema refringens. This response was suppressed in patients with syphilis. the suppression being unrelated to serum factors. In the present paper we studied lymphocyte response in vitro to T. refringens, T. reiter, and T. pallidum as well as to monilia and trychophytins. The response to these antigens was suppressed in patients with syphilis although the response to phytohemagglutinin. pokeweed mitogen, and streptolysin was normal. These data support the hypothesis that human infection with T. pallidum is followed by a complex interaction between cellular and humoral immunity, the former being suppressed in primary and secondary stages.

New evidence for the non-infectivity of Treponema pallidum for mice

We have recently shown that syphilitic rabbits are resistant to challenge with Listeria monocytogenes. This resistance was thought to reflect stimulation of cell-mediated immunity by active infection with Treponema pallidum. We now report data which show that the growth of Listeria was not suppressed in mice inoculated with T. pallidum. Re-inoculation with T, pallidum or with a large dose of an avirulent treponeme also failed to suppress the growth of Listeria. These results contrast with those obtained in rabbits and provide additional evidence that T. pallidum is not infective for the mouse.

In vitro lymphocyte response to Treponema refringens im human syphilis

The response of lymphocytes from patients with syphilis and normal subjects was studied in vitro by using phytohemagglutinin (PHA), pokeweed mitogen (PWM), streptolysin O (SLO), and a preparation of Treponema refringens. Normal lymphocytes exhibited a dose-response curve to treponemes. Although lymphocytes from patients with primary and secondary syphilis responded normally to PHA and PWM, their response to SLO was suppressed and they failed to show significant stimulation by treponemes. Serum from syphilitic patients did not affect normal lymphocytes, and culturing lymphocytes from patients with syphilis in normal serum did not restore their responsiveness. Six to 10 weeks after syphilitic patients had been treated, the degree of stimulation by treponemes was the same as for normal subjects. These data give indirect support to the hypothesis that immunological suppression occurs during active infection with T. pallidum.