Evidence for depletion of Ia+ macrophages and associated immunosuppression in African trypanosomiasis

Regulation of innate and acquired immunity in African trypanosomiasis

African trypanosomes are well known for their ability to avoid immune elimination by switching the immunodominant variant surface glycoprotein (VSG) coat during infection. However, antigenic variation is only one of several means by which trypanosomes manipulate the immune system of their hosts. In this article, the role of parasite factors such as GPI anchor residues of the shed VSG molecule and the release of CpG DNA, in addition to host factors such as IFN-gamma, in regulating key aspects of innate and acquired immunity during infection is examined. The biological relevance of these immunoregulatory events is discussed in the context of host and parasite survival.

Trypanosoma brucei brucei infection impairs MHC class II antigen presentation capacity of macrophages

During African trypanosomiasis, macrophages play a central role in T cell hyporesponsiveness to parasite-related and unrelated antigens. In this study, the ability of macrophages from Trypanosoma b. brucei-infected mice to present exogenous antigens to a major histocompatibility complex (MHC) class II-restricted CD4+ T cell hybridoma was analysed. We demonstrate that the antigen presentation capacity of macrophages from infected mice is markedly reduced as a result of a lower expression of [MHC class II-peptide] complexes on their plasma membrane. This defect did not result from a decreased antigen uptake/catabolism, a reduced MHC class II and intercellular adhesion molecule 1 expression on the surface of macrophages, a decreased affinity of MHC class II molecules for antigenic peptides, a competition between exogenous and parasite antigens, or the generation of inhibitory peptides. Our data indicate that the step resulting in coexpression of processed antigens and MHC class II molecules is affected in T. b. brucei-infected mice. Additionally, macrophages from infected mice secreted IL-10 that in turn contributes to the impairment of T cell activation.

Immune response of cattle infected with African trypanosomes

Trypanosomosis is the most economically important disease constraint to livestock productivity in sub-Saharan Africa and has significant negative impact in other parts of the world. Livestock are an integral component of farming systems and thus contribute significantly to food and economic security in developing countries. Current methods of control for trypanosomosis are inadequate to prevent the enormous socioeconomic losses resulting from this disease. A vaccine has been viewed as the most desirable control option. However, the complexity of the parasite's antigenic repertoire made development of a vaccine based on the variable surface glycoprotein coat unlikely. As a result, research is now focused on identifying invariant trypanosome components as potential targets for interrupting infection or infection-mediated disease. Immunosuppression appears to be a nearly universal feature of infection with African trypanosomes and thus may represent an essential element of the host-parasite relationship, possibly by reducing the host's ability to mount a protective immune response. Antibody, T cell and macrophage/monocyte responses of infected cattle are depressed in both trypanosusceptible and trypanotolerant breeds of cattle. This review describes the specific T cell and monocyte/macrophage functions that are altered in trypanosome-infected cattle and compares these disorders with those that have been described in the murine model of trypanosomosis. The identification of parasite factors that induce immunosuppression and the mechanisms that mediate depressed immune responses might suggest novel disease intervention strategies.

Suppressive macrophages occurring in murine Trypanosoma brucei infection inhibit T-cell responses in vivo and in vitro

Intraperitoneal injection of Trypanosoma brucei AnTat 1.1 into mice of the C3H.He, BALB/c or C57BL/6 strains resulted in impaired immune responses from day 3 onwards, as measured by the reduction in DNA synthesis in spleen cell populations stimulated with concanavalin A (Con-A) in vitro. Adherent cells from the peritoneum (PC) or from the spleen of infected mice, consisting predominantly of macrophages, caused a 60-80% reduction of the Con-A response in spleen cells from syngeneic recipients 3-4 days after transfer in vivo. Adherent PC from irradiated or athymic mice were equally suppressive. Spleen cells from infected mice reduced the proliferative response of spleen cells from uninfected mice upon co-cultivation in vitro. This dominant suppressive effect was abolished after the selective removal of macrophages from the spleen cell population by treatment with L-leucine methylester. Moreover, the macrophage-depleted spleen cells from infected mice responded normally to Con-A provided they were supplemented with splenic adherent cells from naive mice as a source of accessory cells. Both the cell transfer and co-cultivation experiments suggest that infection with African trypanosomes changes the properties of macrophages to a state which allows them actively to suppress immune responses.

Macrophage procoagulant activity in experimental African trypanosomiasis

Peritoneal macrophages from mice infected with Trypanosoma brucei brucei expressed a greatly elevated procoagulant activity (PCA) which could be reversed to normal levels after trypanocidal therapy. Comparison with infection caused by the non-pathogenic T. musculi suggested that the level of PCA related to parasite pathogenicity. Unstimulated macrophages, which generate only slight PCA upon stimulation with zymosan, become hyperresponsive to this stimulus during the course of infection. Hyperresponsiveness is not a generalized feature of these cells during infection, as they become progressively hyporesponsive to the same stimulus in terms of lysosomal enzyme secretion. We were unable to demonstrate a direct role of the trypanosomes in macrophage activation; however, artificial removal of the glycoprotein coat rendered the parasites highly stimulatory for macrophages even in the absence of opsonins. These results suggest that the parasites may activate macrophages indirectly, and that the resulting elevated PCA may play a role in the abnormal blood coagulation known to occur in this disease.

Suppression of immune response to Listeria monocytogenes: mechanism(s) of immune complex suppression

We have investigated possible mechanisms underlying immune complex suppression of resistance to Listeria monocytogenes. Inhibition of resistance was found when immune complexes were formed in vivo in immune mice or in nonimmune mice adoptively transferred with specific antibody. Suppression was also found when nonimmune mice were injected with immune complexes preformed in vitro. We investigated the role of complement by decomplementing mice with cobra venom factor purified by high-pressure liquid chromatography. Complete depletion of serum C3 did not eliminate immune complex suppression of resistance to L. monocytogenes, suggesting that complement activation is not required for immune complex suppression. Infection-induced changes in the surface phenotype and functional properties of macrophages from normal and immune complex-suppressed mice were also investigated. Macrophage expression of both H-2K and Ia molecules increased during the response of normal mice to L. monocytogenes. However, these changes were not found in immune complex-suppressed mice. In contrast, membrane interleukin 1 expression was increased in macrophages from suppressed mice compared with macrophages from normal mice. Macrophages from L. monocytogenes-infected normal and immune complex-suppressed mice expressed cytotoxicity against tumor cells in vitro. We conclude that immune complexes do not inhibit resistance to L. monocytogenes by activation of complement or decreasing macrophage cytotoxic activity. Rather, defects in Ia expression by macrophages from suppressed mice might be one component responsible for immune complex suppression of resistance to L. monocytogenes.

Suppressor factor of T-cell activation and decreased interleukin 2 activity in experimental African trypanosomiasis

Spleen cells from Trypanosoma brucei-infected BALB/c mice were unable to respond to a T-cell mitogen, concanavalin A. Moreover, they were unable to produce detectable amounts of the growth factor required for T cell proliferation, interleukin 2. In addition, supernatants from 24-h in vitro cultures of these cells produced a slight but detectable suppressive activity of the interleukin 2-dependent proliferation of a T-cell line. Infected spleen cells also suppressed the response of T. brucei-immunized spleen cells as well as normal spleen cells to concanavalin A. However, a major difference was shown in the mechanism of the suppression in both systems. Suppression of normal spleen cells required cell-to-cell contact. In contrast, suppression of 30-day T. brucei-immune cells could be mediated by a soluble suppressor factor released by in vitro culture of infected spleen cells. This molecule had an apparent molecular weight of 18,000. Finally, similar suppression could be generated in 30-day T. brucei-immune spleen cells but not in normal cells, with living cells but not with extracts of T. brucei.

In vivo analysis of impaired macrophage bactericidal capacity during experimental African trypanosomiasis

Since innate resistance of mice to Salmonella typhimurium depends on an intact macrophage system, we have used this bacterium to investigate the effect of Trypanosoma brucei subsp. rhodesiense infection on macrophage phagocytic and cytolytic function. CBA/CaJ mice infected with T. brucei subsp. rhodesiense have decreased resistance to S. typhimurium, since doubly infected mice rapidly succumb to sublethal doses of S. typhimurium. Although trypanosomiasis is known to suppress antibody formation, such a suppression of antibody does not seem to play a role in trypanosome-induced sensitivity to S. typhimurium. A trypanosome-induced blockade of the reticuloendothelial system also does not occur, since parasitized and control mice clear S. typhimurium from the blood equally well. Early killing (0 to 48 h) of S. typhimurium in the liver and spleen is mainly macrophage mediated, and mice infected with trypanosomes kill S. typhimurium in the liver and spleen very poorly. Apparently trypanosomiasis inhibits macrophage bactericidal activity, but has no effect on phagocytosis.

Interaction of African trypanosomes with the immune system

African trypanosomes cause disease in man and domestic animals. The parasites have the ability to escape immune control by two means: by antigenic variation of the surface glycoprotein coat so that waves of variant parasites arise and by inducing a general immunosuppression affecting immune responses to the parasite as well as to parasite-unrelated antigens. The cellular basis of the immune dysfunction will be discussed in relation to a mouse model system - it is the result of proliferative stimuli to T- or B-cells which then become refractory to selection by antigen and normal control signals. Recent experiments have focused on macrophages as important direct target cells for parasite action. We have obtained no evidence for a parasite derived mitogen acting directly on B- or T-cells. In vitro cell proliferation is associated with accessory cells and relates only to T-cells. During infection, macrophages become activated with changes in receptor expression and mediator release, so that there is, for example, spontaneous IL-1 release (with a role in T- and possibly in B-cell proliferation) and several-fold increases in PGE2 secretion, with its immunosuppressive activities. We also find parasitaemia-associated release of alpha-beta and gamma interferon by various cells which in turn influences immune function. The active parasite component is associated with parasite membranes, but its nature has not been further defined. We proposed that the macrophage changes provide a general pathway causing immune dysfunction associated with many infections, be they parasitic or caused by other invading organisms.

T cell-mediated immunity in murine malaria. II. Induction of protective immunity to P. chabaudi by antigen-fed macrophages and antigen-educated lymphocytes

We previously described assay systems for generating antigen specific proliferating T cells to P. chabaudi antigens. In the present study we examine whether the various sensitization approaches confer immunity against a cloned virulent strain IP-PCI of P. chabaudi. We present data indicating that effective specific protective immunity can be induced through P. chabaudi antigen fed macrophages and antigen educated spleen cells (initiator lymphocytes). The expression of this protective immunity is proposed to depend on (a) antigen presentation and/or accessory function of macrophages and (b) the subsequent activation of T cell functions related to protection. Indeed analysis of different macrophage populations revealed a correlation between the expression of Ia molecules and IL-1 secretion with their capacity to induce antigen specific T cells in vivo and subsequent protective immune mechanisms. Thus these results emphasize the critical functions of accessory cells in determining the outcome of malaria infections.

Interference with anti-trypanosome immune responses in rabbits infected with cyclically-transmitted Trypanosoma congolense

Rabbits were infected with two clones of antigenically distinct stocks of Trypanosoma congolense transmitted through Glossina morsitans. Local skin reaction development and the appearance of neutralizing antibodies were followed in animals infected with one or other of the trypanosome stocks, with both stocks simultaneously or with both stocks consecutively. There was little difference in local skin reaction development on rabbits infected with a single stock or with both stocks simultaneously but, in rabbits exposed to a heterologous stock 14 or 21 days after a primary infection reactions were reduced in size or completely absent. Neutralizing antibodies against metacyclic-derived trypanosomes were detected 21 days after infection in animals infected with a single trypanosome stock and, in rabbits infected with both stocks simultaneously, antibodies against each stock were also detected 21 to 28 days after infection. In rabbits challenged 14 or 21 days after primary infection the appearance of trypanocidal antibodies against the stock used for challenge was delayed from 28 to 49 days.

Association of an inflammatory I region-associated antigen-positive macrophage influx and genetic resistance of inbred mice to Rickettsia tsutsugamushi

Strains of C3H mice differing in susceptibility to intraperitoneal infection with the Gilliam strain of Rickettsia tsutsugamushi were used to investigate the role of the I region-associated (Ia) antigen-bearing macrophage in the genetic resistance of mice to this organism. Resistant mice (C3H/RV) were found to produce a quantitatively greater Ia antigen-positive macrophage response after infection compared to mice (C3H/HeDub) which underwent a lethal infection. The macrophage influx produced in response to infection of the C3H/HeDub mice was deficient in Ia antigen-bearing cells, as evaluated by antigen presentation function and by the use of macrophages as stimulator cells in a mixed lymphocyte response. The resistance to infection, as well as the Ia-positive macrophage response in C3H/RV mice, was sensitive to 450 to 600 rads of irradiation. C3H/HeDub mice produced exudates rich in Ia-positive macrophages if stimulated with concanavalin A or after challenge with R. tsutsugamushi (if previously immunized), ruling out an innate inability of this strain of mice to produce Ia-positive macrophages exudates. Challenge of either strain of mice immunized by a prior subcutaneous infection resulted in a rapid (3 to 5 days) peak of Ia-positive macrophages responding to the peritoneal cavity. It also was noted that subcutaneous infection alone resulted in an increase in the proportion and number of "resident" macrophages which were Ia positive. These data suggest that the macrophage influx in terms of Ia-bearing cells is at least associated with the genetic resistance of C3H/RV mice to infection with this rickettsiae and may play a role in resistance. Furthermore, it would appear that the Ia-positive macrophage is a factor in acquired immunological resistance to reinfection.

Immune depression in bovine trypanosomiasis: effects of acute and chronic Trypanosoma congolense and chronic Trypanosoma vivax infections on antibody response to Brucella abortus vaccine

Cattle were vaccinated with Brucella abortus (S19) vaccine during acute (25 days) and chronic (25 weeks) Trypanosoma congolense and chronic Trypanosoma vivax (25 weeks) infections in order to determine the effect of such infections on the antibody response to the vaccine. It was found that the specific antibody responses of IgG1 and IgG2 sub-classes were profoundly depressed (80%) in both the acute and chronic infections with T. congolense. Whereas IgM antibody response was also profoundly depressed (90%) in cattle with the acute infection, it was only 50% depressed in those with chronic infection. There was no depression of IgG1, IgG2, or IgM in cattle infected with T. vivax. These animals, however, had no detectable parasitaemia at the time of vaccination and thereafter. These results suggest that during acute infection with T. congolense depressive mechanisms could be acting on the afferent arm of the immune response, namely, antigen recognition and/or processing.

Macrophage activation in murine African trypanosomiasis

African trypanosomiasis is accompanied by a profound general immunosuppression in which both suppressive T cells and macrophages (M phi) have been implicated. The present studies define changes in the M phi surface, endocytic and secretory properties, during the infection of mice by Trypanosoma brucei. Peritoneal M phi obtained after the control of the first wave of parasitemia displayed characteristics similar to those activated by intracellular pathogens, such as Mycobacterium bovis bacillus Calmette-Guérin, e.g., the enhanced expression of Ia antigen, decreased M phi-specific antigens, receptors mediating the pinocytosis of mannose-terminal glycoproteins, and an increased ability to secrete plasminogen activator, superoxide anion, and H2O2. Some markers of macrophage activation persisted during the subpatent period before the recurrence of parasitemia, whereas others reverted to normal. Mature T cell function appears not to be essential for M phi activation by T. brucei since the infection of athymic nude mice also induced Ia antigens and plasminogen activator. These studies show that M phi activated by different pathways express common features which may contribute to immune dysfunction observed in trypanosomiasis, as well as in other infections.

Ability of macrophages to process and present Treponema pallidum Bosnia A strain antigens in experimental syphilis of syrian hamsters

The ability of macrophages to process and present treponemal antigens to T-lymphocytes was studied in early stages of experimental syphilis produced by Treponema pallidum Bosnia A strain (the causative agent of endemic syphilis) infection of inbred Syrian hamsters (LSH/Ss Lak strain). A difference was noticed in the response of macrophages obtained from the peritoneal cavity, lymph nodes, and spleens of the infected animals. In all of these locations, a general increase in the population of Ia(k)-positive macrophage was seen during the entire period of infection, i.e., 3 to 18 weeks after inoculation. Peritoneal cavity-derived macrophages showed no difference in antigen presentation to sensitized and nonsensitized T-lymphocytes for the first 7 weeks of infection. However, at 18 weeks after infection, peritoneal macrophages lost their ability to process treponema antigens. Spleen- and lymph node-derived macrophages did not exhibit a parallel loss in their ability to process treponema antigens. A fluctuation without a consistent pattern was noticed in the antigen processing and presentation by macrophages from the spleen and lymph nodes. In general, the sensitized T-lymphocytes responded to treponema antigen presented by macrophages more vigorously than the nonsensitized T-lymphocytes. An increased ability of spleen-derived macrophages to process and present antigens was noticed throughout the entire period of infection. The macrophages from the lymph nodes showed such an increase only temporarily at 3 weeks after infection. These data suggest that the processing and presentation of treponema antigens by macrophages in acute syphilitic infection fluctuates considerably and depends on the source of macrophages and the duration of the infection. The differences in the response of peritoneal cavity-, spleen-, and lymph node-derived macrophages probably reflect the complex interactions between the macrophage and other cells involved in the immune response to treponema infection.