Murine Leishmaniasis Research Articles

Interferon‐gamma and interleukin‐4 in human Leishmania donovani infections

Clinical and immunological similarities between Leishmania donovani infections in humans and L. major infections in mice suggest that some of the pathophysiological mechanisms are the same in the two conditions. Both infections can result either in a fatal systemic disease or in a self-limiting infection with few and mild symptoms. In the murine model the outcome of the infection is critically related to the cytokines produced by T lymphocytes activated by leishmanial antigens. Activation of the IFN-gamma producing Th1 subset of CD4 positive T cells results in cure and survival, whereas activation of the IL-4 secreting Th2 subset results in a progressive disease with fatal outcome. A similar Th1/Th2 dichotomy in the cytokine response to L. donovani may exist in humans, and may have influence on the outcome of infection. In murine leishmaniasis the levels of IL-4 and IFN-gamma at the time of infection can direct the T cell response into Th1 or Th2 type. If similar mechanisms operate in humans, the outcome of L. donovani infections may depend on the local cytokine environment in which early activation of Leishmania specific T cells takes place. Cytokines secreted by cross-reactive memory T cells, activated by antigens from the invading micro-organism, may contribute to determine this environment.

Resistance of mice to experimental leishmaniasis is associated with more rapid appearance of mature macrophages in vitro and in vivo.

Abstract Resistance to murine leishmaniasis has been related to the propagation of specific Th cell subsets (Th1 and Th2). This study shows that there are differences between resistant and susceptible mice in the initial myelomonocytic infiltrate, which precede the specific T cell response. After subcutaneous injection of 2 x 10(7) Leishmania major into footpads of resistant C57Bl/6 and susceptible BALB/c mice we performed immunohistochemical studies on the infiltrate. Two days after infection the percentage of more mature, F4/80-positive macrophages in the lesion increased faster in C57Bl/6 mice (63%) than in BALB/c mice (29%). The same strain-specific differences were observed after infection of corresponding strains of athymic mice (57.2% in C57Bl/6 nu/nu; 33.6% in BALB/c nu/nu), thus excluding a T cell-controlled phenomenon. After 1 wk the infiltrate in susceptible mice began to reveal significantly more cells containing MRP14, which is expressed by granulocytes and less mature monocytes but not by mature macrophages. No corresponding differences were found between athymic strains, suggesting that at this point organization of the infiltrate falls under control of protective T cells. In bone marrow cultures of BALB/c and C57Bl/6 mice, the percentage of F4/80-positive macrophages was also increasing faster in C57Bl/6 mice than in BALB/c mice. Increased expression of the F4/80 Ag was associated with higher leishmanicidal activity of C57Bl/6 macrophages. MRP14-positive bone marrow cells on the other hand were rarely infected by parasites. We suggest 1) that the earlier appearance of leishmanicidal macrophages in lesions of C57Bl/6 mice could influence propagation of either Th1 or Th2 cells by reduction of parasite load or by differential secretion of decisive cytokines and 2) that the diffuse accumulation of granulocytes and inflammatory monocytes in susceptible mice facilitates spread of disease.

Effect of in vivo inhibition of nitric oxide production in murine leishmaniasis.

In vitro experiments suggest that cytokine induced nitric oxide (NO) synthesis from L-arginine is a major effector mechanism against prokaryotic and eukaryotic intracellular pathogens. N omega monomethyl L-arginine (MLA), an active site inhibitor of the cytokine induced NO synthase, inhibits cytokine induced resistance of mammalian cells to intracellular microbes in vitro. In our experiments, we show that Leishmania infection markedly increases NO synthesis in the genetically resistant C3H/HeN mouse strain. In addition, administration of 50 mM MLA in the drinking water inhibits endogenous NO synthesis as well as natural resistance to footpad Leishmania infections in both susceptible BALB/c and resistant C3H/HeN mice. Leishmania parasites continued to proliferate in MLA-treated C3H/HeN mice after footpad inoculation. Similarly C3H/HeN mice treated for 3 wk with MLA had an increased parasite load and sloughing of the footpad. Footpad size of C3H/HeN mice not treated with MLA returned to base-line diameter and the regional nodes contained few amastigotes. These in vivo effects were paralleled by endogenous NO synthesis (high when Leishmania was controlled and low when Leishmania was not controlled). In addition, inhibition of NO synthesis in Leishmania-infected mice leads to a cachectic state not caused by infection alone or inhibition of NO synthesis alone. The cachexia appeared to be due to decreased food intake that occurs when NO synthesis is inhibited in Leishmania-infected mice. Our results show that cytokine induced NO has a major effector role in resistance of murine hosts to Leishmania infection.

TH1 and TH2 T-cell subsets are differentially activated by macrophages and B cells in murine leishmaniasis.

The role of antigen-presenting cells in the differential expansion of TH1 and TH2 T cells in murine leishmaniasis was investigated. In general, macrophages preferentially induced gamma interferon and interleukin-2 secretion by syngeneic Leishmania-specific T cells, whereas B cells were more efficient in activating interleukin 4 production. B cells from susceptible BALB/c mice were better in inducing TH2 responses than B cells from resistant C57BL/6 mice, whereas macrophages from C57BL/6 mice were superior to BALB/c macrophages in inducing TH1 responses.

Recombinant interleukin 12 cures mice infected with Leishmania major.

Resistant C57BL/6 mice infected with Leishmania major are self-healing, whereas susceptible BALB/c mice fail to contain cutaneous infection and subsequently undergo fatal visceral dissemination. These disparate outcomes are mediated by dissimilar expansions of T helper type 1 (Th1) and Th2 CD4+ T lymphocyte subsets in vivo during cure and progression of disease. Because interleukin 12 (IL-12) has potent T cell growth and interferon gamma (IFN-gamma) stimulatory effects, we studied its effect on CD4+ T cell differentiation during murine leishmaniasis. Treatment with recombinant murine (rMu)IL-12 during the first week of infection cured 89% of normally susceptible BALB/c mice, as defined by decreased size of infected footpads and 1,000-10,000-fold reduced parasite burdens, and provided durable resistance against reinfection. Cure was associated with markedly depressed production of IL-4 by lymph node cells cultured with antigen or mitogen, but preserved or increased production of IFN-gamma relative to untreated mice. IL-4 and IFN-gamma mRNA associated with CD4+ T lymphocytes isolated from infected lymph nodes showed similar reciprocal changes in response to rMuIL-12 therapy. A single injection of anti-IFN-gamma monoclonal antibody abrogated the protective effect of rMuIL-12 therapy and restored Th2 cytokine responses. We conclude that rMuIL-12 prevents deleterious Th2 T cell responses and promotes curative Th1 responses in an IFN-gamma-dependent fashion during murine leishmaniasis. Since BALB/c leishmaniasis cannot be cured with rMuIFN-gamma alone, additional direct effects of IL-12 during T cell subset selection are suggested. Because rMuIL-12 is uniquely protective in this well-characterized model of chronic parasitism, differences in IL-12 production may underlie heterogenous host responses to L. major and other intracellular pathogens.

The Cellular Immune Response Against Yersinia enterocolitica in Different Inbred Strains of Mice: Evidence for an Important Role of T Lymphocytes

Resistance of mice against infection with Yersinia enterocolitica has been shown to be related neither to the Ity locus coding for resistance against infection with Salmonella typhimurium and other pathogens nor to the H-2 locus. From other mouse infection models, e.g., murine leishmaniasis, there is evidence that a different T cell-dependent regulation of the host immune response in various inbred strains of mice determines the susceptibility to the infectious agent. However, until recently, little was known about the cellular immune response against Y. enterocolitica. Thus, in a first approach we used the highly virulent Y. enterocolitica strain WA of serotype O:8 and different inbred strains of mice (C57 BL/6, Balb/c and athymic T cell-deficient C57 BL/6 nude mice) to investigate the cell-mediated immunity against parenteral infection. Comparison of the median lethal dose and of the net-bacterial growth in the spleens of infected mice indicated that Balb/c mice could be considered as Yersinia-susceptible whereas C57 BL/6 mice were relatively resistant. However, in contrast to normal C57 BL/6, athymic T cell-deficient C57 BL/6 nude mice have proved to be highly susceptible to Yersinia infection suggesting that T cells are required for the elimination of the pathogen. This conclusion was supported by histomorphological and immunohistological results indicating that T lymphocytes were present in Yersinia-induced tissue lesions. Moreover, the adoptive transfer of Yersinia-specific T cell lines and clones into naive animals mediated significant protection against the pathogen in both Yersinia-resistant C57 BL/6 and in Yersinia-susceptible Balb/c mice. These findings emphasize an important role of T lymphocytes in the host response against Y. enterocolitica infection.

Cytokine regulation of murine leishmaniasis: interleukin 4 is not sufficient to mediate progressive disease in resistant C57BL/6 mice

Neutralization of interleukin 4 (IL-4) at the time of infection with Leishmania major allowed susceptible BALB/c mice to heal. Recombinant IL-4, however, had little effect on the course of L. major infection in resistant C57BL/6 mice, nor did coinfection with Nippostrongylus brasiliensis, despite marked elevation of endogenous IL-4 levels.

Production of interferon gamma, interleukin 2, interleukin 4, and interleukin 10 by CD4+ lymphocytes in vivo during healing and progressive murine leishmaniasis.

The expression of interleukin (IL) 2, IL-4, IL-10, and interferon gamma (IFN-gamma) by lymphocyte subsets was examined during infection of resistant C57BL/6 and susceptible BALB/c mice with the protozoan parasite Leishmania major. CD4+ and CD8+ T lymphocytes and B lymphocytes were isolated from the lymph nodes draining infectious lesions, and their RNA was examined for lymphokine transcripts. Distinct patterns of CD4+ cell cytokine expression were apparent: C57BL/6 CD4+ cells contained IFN-gamma and IL-2 mRNA, whereas BALB/c CD4+ cells expressed IL-4 and IL-10 message. CD8+ cells contributed little lymphokine expression during disease, but B cells were a major source of IL-2 mRNA in both strains of mice. BALB/c mice made resistant by treatment with anti-CD4 antibody at the time of infection repopulated lymph nodes with CD4+ cells that expressed IL-2 and IFN-gamma. Protective treatment with anti-IL-4 antibody in vivo also resulted in the appearance of CD4+ cells with increased IFN-gamma and diminished IL-4 and IL-10 expression. These data establish CD4+ cells as the primary source of IFN-gamma in healing mice and of IL-4 and IL-10 during progressive infection and confirm that the spectral extremes of this disease are characterized by the presence of CD4+ cells expressing Th1 or Th2 phenotypes in vivo.

Granulocytes in the inflammatory process of BALB/c mice infected by Leishmania amazonensis. A quantitative approach

We used previously immunized (partially resistant) and naive (highly susceptible) BALB/c mice infected with Leishmania amazonensis for evaluating the role of granulocytes in the course of murine leishmaniasis. The animals were examined at different times post-infection and granulocytes counted in lesion tissues examined ultra-structurally. Polymorphonuclear granulocytes predominated during the early phase of infection and their number decreased with progression of infection; their number was similar in both groups during the early and intermediate phases of infection, though slightly higher in immunized animals during the late phase. Eosinophils represented approximately 10% of cells in the inflammatory infiltrate, being higher during the intermediate phase, and not differing between the groups. Another approach was the evaluation of granulocyte migration to the peritoneal cavity of susceptible BALB/c mice or resistant C57BL/6 mice under several stimuli. There was no statistically significant difference between resistant and susceptible animals in any of the treatments. Despite the influx of granulocytes to the lesion and its possible role in the initial destruction of injected Leishmania, this aspect does not seem to have an important effect on the outcome of the leishmanial infection.

Patterns of cytokine secretion in murine leishmaniasis: correlation with disease progression or resolution

Susceptibility or resistance to infection with Leishmania major correlates with the ability of mice to produce characteristic panels of lymphokines in response to the parasite. To investigate the role of antigen-presenting cells in this phenomenon, we developed a model system which used congenic (H-2d) susceptible and resistant mice. L. major-specific T cells were isolated from infected BALB/c and B10.D2 mice, and the cells were restimulated in vitro on syngenic or congenic antigen-presenting cells. BALB/c L. major-reactive T cells restimulated with either antigen-presenting cell produced high levels of interleukin-4 and low levels of gamma interferon. In contrast, T cells from B10.D2 mice produced gamma interferon. Radiation-induced chimeras reconstituted with BALB/c bone marrow also produced more interleukin-4 in response to L. major than did chimeras reconstituted with B10.D2 bone marrow. To test whether this pattern of cytokine secretion was unique to infection with L. major, we infected the mice with a second intracellular pathogen, Mycobacterium bovis BCG. Mycobacterium-specific T cells from both BALB/c and B10.D2 mice produced interleukin-2 and no interleukin-4. Finally, when BALB/c mice were vaccinated with avirulent L. major, the induced resistance correlated with reduced production of interleukin-4 but no increase in gamma interferon production. Instead, T cells from the vaccinated mice produced high levels of tumor necrosis factor. This suggests that tumor necrosis factor, in addition to gamma interferon, may be involved in resistance to L. major and that interleukin-4 may inhibit the leishmanicidal activity of tumor necrosis factor and/or gamma interferon.

Immunization of susceptible hosts with a soluble antigen fraction from Leishmania major leads to aggravation of murine leishmaniasis mediated by CD4+ T cells

This study was performed in order to define Leishmania major antigens that function as disease-modulating immunogens in susceptible BALB/c mice. A soluble leishmanial antigen preparation (S-SLA) derived from highly infective stationary-phase L. major parasites was fractionated by preparative gel electrophoresis. In vitro, the low molecular mass fraction (less than 31 kDa) of S-SLA fraction D (FR D) was found to be a potent stimulator of L. major-specific Th1 and Th2 helper cell clones. In vivo, immunization with FR D induced a Th2-biased immune response in BALB/c mice as determined by the numbers of splenic CD4+ cells secreting interleukin 4 and interferon-gamma according to limiting dilution analyses. In addition, FR D caused significant disease exacerbation in parasite-infected susceptible mice as assessed by the local lesion development and the numbers of parasites in lymph nodes and spleen. This effect was observed after local subcutaneous application of FR D as well as after systemic immunization (intrasplenic or intraperitoneal). Transfer experiments revealed, that the disease-aggravating effect of FR D was mediated by CD4+ T cells. From these results it is concluded that leishmanial protein preparations exist that not only fail to induce protective anitparasitic immunity, but can mediate disease exacerbation, independently of the primary application site of the immunogen. The existence of such structures may serve the parasite as a means to evade the host's immune attack and may also have implications for the development of vaccines.

Cure of murine leishmaniasis with anti-interleukin 4 monoclonal antibody. Evidence for a T cell-dependent, interferon gamma-independent mechanism.

BALB/c mice infected with Leishmania major develop fatal, progressive disease, despite an immune response characterized by expansion of CD4+ T cells in the draining lymph nodes. The immune response has been further characterized by a lack of IFN-gamma mRNA, but increased IL-4 mRNA in lymphoid tissues, and striking elevation of serum IgE. Treatment of infected BALB/c mice with rIFN-gamma at doses shown to be beneficial in other protozoan infections was insufficient to ameliorate L. major infection. In contrast, neutralization of IL-4 by six weekly injections of mAb 11B11 led to attenuation of disease in 100% of animals, and complete cure in 85%. Resolution of disease required the presence of T cells, and recovered mice remained resistant to reinfection at 12 wk. This immunity was adoptively transferable and was dependent on both CD4+ and CD8+ cells. Although administration of anti-IL-4 was associated with fourfold increase in IFN-gamma mRNA in lymph node cells draining the lesion, the coadministration of neutralizing R4 6A2 anti-IFN-gamma mAb had no effect on resistance to disease. This was in marked contrast to resolution of disease in both resistant C57BL/6- and GK1.5-pretreated BALB/c mice that was abrogated by in vivo treatment with anti-IFN-gamma. These data suggest a novel mechanism of cellular immunity established by interference with the development of Th2 cells during infection.

Vaccination trial against canine visceral leishmaniasis. Phocean Veterinary Study Group on Visceral Leishmaniasis.

In a double-blind study 393 seronegative dogs, residing in a holoendemic area for Leishmania donovani infantum infection, were randomly assigned to an immunization with a partly purified L.d. infantum-derived preparation, or received adjuvant only. During the first year of the study period the rate of infection was significantly higher in the vaccinated group than in the control one (P less than 0.05), but this difference disappeared during the second year (P = 0.44). Since a similar immunization protocol conferred resistance against experimental murine leishmaniasis, these results stress the differences that may exist between the natural hosts of Leishmania parasites and experimental animal substitutes.

Reciprocal expression of interferon gamma or interleukin 4 during the resolution or progression of murine leishmaniasis. Evidence for expansion of distinct helper T cell subsets.

We purified poly(A)+ mRNA from the spleen and lymph nodes at designated times after infection with Leishmania major in genetically susceptible BALB/c and resistant C57BL/6 mice. The steady-state levels of IL-2, IFN-gamma, IL-4, and IL-1 beta mRNA were determined using Northern hybridizations. IL-2 mRNA levels in the infected organs of BALB/c and C57BL/6 mice were comparable after infection, but IFN-gamma and IL-4 mRNA levels were reciprocally expressed. Levels of IFN-gamma mRNA in C57BL/6 draining nodes and spleen were significantly greater than in BALB/c mice except at 4 and 6 wk of infection, when splenic IFN-gamma mRNA levels were transiently comparable. In contrast, IL-4 mRNA was apparent only in BALB/c and not in C57BL/6 nodes and spleen. Tissue levels of IL-1 beta mRNA were 10-20-fold greater in BALB/c mice. BALB/c mice were pretreated with GK1.5 mAb, a manipulation that promotes healing of subsequent infection by transiently depleting L3T4+ cells. At 8 wk of infection, by which time lymphoid organs were repopulated with L3T4+ cells, GK1.5-pretreated BALB/c mice produced IFN-gamma, but not IL-4 message. Serum levels of IgE were markedly elevated in infected BALB/c, but not in infected C57BL/6 or GK1.5-pretreated BALB/c mice, consistent with in vivo biologic activity of IL-4 in nonhealing mice. Treatment of infected BALB/c mice with neutralizing anti-IL-4 antibody abolished the elevation of serum IgE and significantly attenuated the progression of disease as assessed by size and ulceration of the lesion, and by reduction in the number of tissue parasites. Both protective and deleterious responses to Leishmania infection have previously been shown to be L3T4+ cell dependent. Our findings are consistent with the differential expansion of protective, IFN-gamma-producing Th1 cells in healing mice, and the expansion of deleterious, IL-4-producing Th2 cells in nonhealing mice. The inverse relationship of IFN-gamma and IL-4 gene expression during leishmaniasis may underlie the divergence of cellular and humoral immunity that occurs during chronic infection with Leishmania and possibly other intracellular parasites.

The role of granulocyte-macrophage colony-stimulating factor in the pathogenesis of experimental murine leishmaniasis.

The role of granulocyte-macrophage colony-stimulating factor in the pathogenesis of experimental murine leishmaniasis.

Application of recombinant granulocyte-macrophage colony-stimulating factor has a detrimental effect in experimental murine leishmaniasis.

The purpose of this study was to evaluate the effect of recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) on BALB/c mice infected s.c. with the intracellular pathogen Leishmania major. Daily i.p. application of 1 microgram rGM-CSF for 21 days following the infection led to an aggravated course of the disease in most animals. In no case was a therapeutic effect observed. In vitro analysis revealed that the parasite burden was approx. 2- to 7-fold higher in the infected lesions, in the lymph nodes draining the infection and in the spleens of rGM-CSF-treated animals than in tissues from nontreated mice. L. major-infected macrophages obtained from chronically infected mice proliferated in the presence of rGM-CSF in vitro without gaining antiparasitic effector function. However, antiparasitic effector function increased and macrophage growth was inhibited in the presence of recombinant interferon-gamma (IFN-gamma). These data indicate that rGM-CSF-induced macrophage proliferation alone is not sufficient to overcome infections with intracellular pathogens like L. major, since simultaneous activation of macrophages by IFN-gamma is required.

Vaccination and treatment trials against murine leishmaniasis with semi-purified Leishmania antigens

Antigens with molecular weight ranges of 94-67 kDa (LiF2), 30-20 kDa (LiF5), or below 20 kDa (LiF6), isolated from lysates of Leishmania infantum promastigotes by electroelution from polyacrylamide gels were injected into mice which were genetically either partially resistant (C57BL/6) or susceptible (BALB/c) to Leishmania infection. One month after the completion of the intravenous (C57BL/6) or subcutaneous (BALB/c) schedules, the mice were challenged with 1 × 10 3 L. major promastigotes. All mice immunized with LiF2, LiF5 and LiF6 were completely resistant. Furthermore, the C57BL/6 mice immunized with LiF2 resisted a second challenge with 1 × 10 4 L. major amastigotes. 5 months later, LiF2 antigen was used for immunotherapy of L. major leishmaniasis; parasites disappeared from the treated skin lesions, although ensuing systemic infection could not be averted.

Leishmania major: Analysis of lymphocyte and macrophage cellular phenotypes during infection of susceptible and resistant mice

Genetically susceptible BALB/c and resistant C57 BL 6 mice were infected with Leishmania major and the phenotypes of the responding cells in the draining lymph nodes and cutaneous lesions were analyzed. As early as 1 week, significantly increased numbers of L3T4 + cells as compared to Lyt-2 + cells were present in BALB/c mice lymph nodes ( P < 0.005). Increases in L3T4 + and Lyt-2 + cells were comparable in C57 BL 6 mice, resulting in threefold lower L3 T4 Lyt-2 ratio than in BALB/c mice. T cell subsets were activated in both strains to express interleukin-2 receptor (IL2R) above resting values, although greater numbers of activated L3T4 + cells were present in the draining lymph nodes from BALB/c at 1 and 3 weeks of infection than in C57 BL 6 ( P = 0.02). Despite the presence of activated L3T4 + cells in both strains, macrophages differed in the expression of immunologically important surface molecules during infection. Tissue macrophages from BALB/c mice were IgG 1 G 2 b Fc receptor (FcR) + and Ia − late in disease, whereas macrophages in C57 BL 6 became FcR − and Ia + during healing. BALB/c mice, treated with monoclonal antibody GK1.5 to transiently deplete L3T4 + cells, became resistant to subsequent infection and developed a macrophage phenotype that was FcR − and Ia +. These differences in macrophage phenotype were closely linked to susceptibility during infection with L. major and may play a role in the pathophysiology of murine leishmaniasis.

Immunomodulation of murine leishmaniasis with cyclosporin A.

The fungal metabolite, cyclosporin A (CsA), is an undecapeptide with novel immunosuppressive effects as well as anti-parasitic and antiinflammatory properties (1,2). CsA is particularly effective in preventing activation of T helper and T cytotoxic cells at the induction phase of T cell responses both in vitro and in vivo (3). Although the mode of action of CsA is not yet entirely clear, it has been shown to inhibit the production of IL-2 and may have effects on other lymphoid communication signals and responsiveness (2–4). Cyclosporin A has been shown to have unexpected beneficial effects on certain parasitic infections in animal models of human disease. In murine malaria, cyclosporin A exhibited potent activity against P. yoelii and P. berghei when administered to mice either prophylactically or therapeutically (5,6). Furthermore, CsA also markedly inhibited the growth of Plasmodium falciparum in cultured human erythrocytes (6). In this work, both the therapeutic and prophylactic effects of the drug were thought to be due to the direct effect of the CsA on the Plasmodium parasites. Surprisingly, CsA also had protective effects in mice infected with the helminth Schistosoma mansoni. Animals were protected both from primary infections and secondary challenges (7,8). In these studies, however, no direct effect of the drug on the parasite was observed, and it was concluded that the drug evoked a stimulation of host mechanisms directed against the parasite (8). In contrast to these studies, CsA treatment of Trypanosoma cruzi infected mice was found to exacerbate the disease (9).KeywordsCutaneous LeishmaniasisTrypanosoma CruziInfected MacrophageMicrobicidal ActivityLeishmania PromastigotesThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Parasite accessory cell interactions in murine leishmaniasis. I. Evasion and stimulus-dependent suppression of the macrophage interleukin 1 response by Leishmania donovani.

Interleukin 1 (IL 1) is a principal mediator of the host immune response to microbial challenge. Accessory cells of the monocyte-macrophage series are a major source of this cytokine and are also chronically parasitized by protozoa of the genus Leishmania. This suggests that characterization of the macrophage IL 1 response to Leishmania would increase our understanding of the regulation of host immunity to these organisms. In the present study, the macrophage IL 1 response to Leishmania donovani was examined because infections with this organism have findings consistent with parasite-specific T cell unresponsiveness. Cytokine activity was measured either by direct stimulation or by co-stimulation of thymocytes. Conditioned media from BALB/c resident peritoneal macrophages infected with amastigotes of L. donovani contained no more IL 1 than did supernatant fluids of control cells. In contrast, supernatants from cells stimulated with lipopolysaccharide or heat-killed Listeria monocytogenes had significantly increased cytokine content. Resident cells infected with L. donovani for 4 hr before being stimulated with Listeria demonstrated a suppressed IL 1 response (approximately 40% of Listeria alone) to this secondary particulate stimulus. In contrast, the secondary response of leishmania-preinfected cells to lipopolysaccharide was not affected. To examine whether accessory cell nonresponsiveness to L. donovani (with respect to IL 1) was related to the state of macrophage activation, elicited peritoneal macrophages obtained by injection of proteose peptone were also studied. These cells responded to stimulation with lipopolysaccharide and fixed Staphylococcus aureus with increases in intracellular, membrane, and secreted cytokine activities. In contrast, L. donovani failed to induce any of these activities. This was found to be the case irrespective of whether amastigotes were alive or killed or opsonized with specific antibodies. Elicited cells preinfected with Leishmania responded normally to secondary stimulation with lipopolysaccharide, but not S. aureus (64% of Staphylococcus alone). In addition, attachment and penetration of L. donovani promastigotes and their subsequent conversion to amastigotes within macrophages failed to induce IL 1 synthesis. The findings of this study indicate that L. donovani has the ability to both evade and suppress the macrophage IL 1 response. Because this monokine provides an obligatory signal during macrophage-dependent T cell activation, evasion of signal transduction for IL 1 synthesis may be related to defects in cell-mediated immunity which occur during infections with this organism.