Cellular immunity of mice to Leishmania donovani in vitro: lymphokine-mediated killing of intracellular parasites in macrophages

The ultrastructure of the parasitophorous vacuole formed by Leishmania major

Protozoan parasites of Leishmania spp. invade macrophages as promastigotes and differentiate into replicative amastigotes within parasitophorous vacuoles. Infection of inbred strains of mice with Leishmania major is a well-studied model of the mammalian immune response to Leishmania species, but the ultrastructure and biochemical properties of the parasitophorous vacuole occupied by this parasite have been best characterized for other species of Leishmania. We examined the parasitophorous vacuole occupied by L. major in lymph nodes of infected mice and in bone marrow-derived macrophages infected in vitro. At all time points after infection, single L. major amastigotes were wrapped tightly by host membrane, suggesting that amastigotes segregate into separate vacuoles during replication. This small, individual vacuole contrasts sharply with the large, communal vacuoles occupied by Leishmania amazonensis. An extensive survey of the literature revealed that the single vacuoles occupied by L. major are characteristic of those formed by Old World species of Leishmania, while New World species of Leishmania form large vacuoles occupied by many amastigotes.

Immunotherapy of murine visceral leishmaniasis with murine recombinant interferon-gamma and MTP-PE encapsulated in liposomes

The efficiency of immunotherapy with murine recombinant interferon-gamma (rIFN-gamma) in mouse visceral leishmaniasis caused by Leishmania donovani was examined. To avoid the side effects encountered after the in vivo administration of high dosages of free IFN-gamma, this lymphokine and muramyltripeptide (MTP-PE) were encapsulated into multilamellar liposomes. We established that a combination of 5 X 10(3) U of IFN-gamma and 6 micrograms of MTP-PE, encapsulated in liposomes and given i.v. in C56BL/6 and BALB/c mice activates macrophages from spleen and liver in vivo to kill L. donovani in vitro. Neither empty liposomes nor the same concentration of free IFN-gamma and/or MTP-PE injected i.v. resulted in a leishmanicidal activity of these macrophage populations. For verification of these results in an in vivo infection model, susceptible mice were infected with L. donovani and were treated with IFN-gamma and MTP-PE encapsulated in multilamellar vesicles. Treatment consisted of multiple i.v. injections beginning 4 and 2 days before infection (prophylactic), either simultaneously with the infection or at various times of the exacerbation and remission phases of visceral leishmaniasis. These mouse strains treated with IFN-gamma and MTP-PE in liposomes had significantly fewer splenic parasites than untreated mice or control animals treated with free drugs or empty liposomes. The targetting of multilamellar vesicles to liver and spleen make them particularly suited for the delivery of macrophage-activating substances used for treatment of visceral L. donovani infection.

Immune reactivity to fractionated Leishmania aethiopica antigens during active human infection

Fractionated antigen preparations of Leishmania aethiopica parasites were used to stimulate the peripheral blood lymphocytes of patients with active cutaneous leishmaniasis. In assays measuring lymphocyte proliferation, 9 of 10 patients with similar clinical presentations of infection responded in a similar pattern to the fractionated antigens. Marked proliferation was observed in response to antigen fractions with molecular masses of 43 to 36, 33 to 27, and less than 22 kDa. The induction of relatively high levels of gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) was also observed in responses to these same three antigen fractions. In contrast, the proliferative, IFN-gamma, and TNF-alpha responses of patient lymphocytes to antigens with a molecular mass greater than 60 kDa were uniformly low. The results of this study suggest that the antigens of Leishmania parasites, which are recognized by T cells in patients with active cutaneous leishmaniasis, may be partitioned in the lower-molecular-mass antigenic determinants associated with whole-parasite preparations. The observed association between antigen-induced proliferation and IFN-gamma and TNF-alpha production may be indicative of potential disease-limiting immune effector activities which have developed during infection.

Identification of antigens recognized by T cells in human leishmaniasis: analysis of T-cell clones by immunoblotting

Immunity in human leishmaniasis is mediated by sensitized T lymphocytes; however, the antigens involved in eliciting this immunity have not been defined. We describe the generation of human T-lymphocyte clones derived from two patients with healed leishmaniasis. By use of one- and two-dimensional cellular immunoblotting techniques, we directly identified the parasite antigens recognized by these clones. To our knowledge, these are the first leishmanial antigens identified to which CD4+, gamma interferon-producing T cells from immune individuals have been shown to respond, the strategy may be of general use for the identification of antigens involved in immunity in this disease.

Profile of human T cell response to leishmanial antigens. Analysis by immunoblotting

Control and resolution of leishmanial infection depends primarily on T cell-mediated immune mechanisms. The nature of the leishmanial antigens involved in eliciting T cell immunity is unknown. We have examined the pattern of peripheral blood lymphocyte responses in patients with active, healed, or subclinical leishmanial infection to fractionated leishmanial antigens using a T cell immunoblotting method in which nitrocellulose-bound leishmanial antigen, resolved by one or two dimensional electrophoresis, are incorporated into lymphocyte cultures. The proliferative and IFN-gamma responses of cells from patients with healed mucosal or cutaneous leishmaniasis were remarkably heterogeneous and occurred to as many as 50-70 distinct antigens. In contrast, responses from subjects with active, nonhealing, diffuse cutaneous leishmaniasis were either absent or present to only a small number of antigens. Control and resolution of leishmaniasis, and resistance to reinfection, is therefore associated with a T cell response to a large and diverse pool of parasite antigens. The method of T cell immunoblotting appears to offer a powerful, rapid, and relatively simple approach to the identification of antigens involved in eliciting a T cell response in human leishmaniasis.

Mechanisms of resistance/susceptibility to murine encephalitozoonosis

Mechanisms of resistance/susceptibility to the obligate intracellular protozoan Encephalitozoon cuniculi were studied in resistant BALB/c and susceptible C57BL/6 mice. Three immunological functions were examined: the production of lymphokine(s) (LK) by T-lymphocytes, the proliferative response of spleen cells to parasite spore fragments, and the ability of splenic and thioglycollate-induced peritoneal macrophages to act as accessory cells in antigen-induced T-cell proliferation. The two strains showed differences in the time required for LK production in vitro but not in their ability to generate LK. Spore fragment-induced lymphoblastogenesis was found in spleen cells of infected BALB/c and C57BL/6 mice. There was no difference between the two strains in dose response and time of maximal response, but the magnitude of maximal response was significantly less in C57BL/6 mice. Indomethacin was found to augment the lymphoproliferative response of C57BL/6 but not BALB/c mice, suggesting that prostaglandin production may be involved in immunosuppression in C57BL/6 mice. C57BL/6 mice required more splenic adherent cells to achieve the same proliferative response as found in BALB/c mice. The ability of thioglycollate-induced peritoneal macrophages to act as accessory cells in antigen-induced T-cell proliferation was less in C57BL/6 mice than in BALB/c mice. Thus, it appeared that the relative susceptibility of C57BL/6 mice to encephalitozoonosis may be due to defective accessory cell function of splenic and peritoneal macrophages, depressed lymphoproliferation against spore fragments (possibly due to prostaglandin-mediated suppression) and a delay in LK production. There was no significant difference between the survival times of BALB/c-nu and C57BL/6-nu mice, suggesting that non-immune mediated resistance did not play a role in determining resistance/susceptibility.

Ivermectin, levamisole and thymic extract for chemotherapy and immunostimulation of visceral leishmaniasis in hamsters and mice

The anti-leishmanial activity of ivermectin, pentostam or combination of pentostam with either levamisole or thymic extract was tested against Leishmania donovani infection in hamsters and mice. In vitro peritoneal macrophage-parasite interaction, the effect of splenic cells on the interaction of macrophages and parasites, spleen weight, parasite burden in spleen tissue as well as the antibody titers using micro-ELISA were used as parameters for evaluating the efficacy of these chemotherapeutic regimens. Treatment with ivermectin and immunoenhancement with pentostam combined with levamisole gave best results in both animal models. Furthermore, regimens used in this work were all active in reducing phagocytosis of promastigotes by macrophages in vitro.

Preparation and use of liposomes in the treatment of microbial infections

The potential application of liposomes to drug delivery has been apparent since 1965, when these phospholipid vesicles were first described by Bangham. Since then, experiments on animals have shown that liposome encapsulation can dramatically alter the distribution of drugs in the body and their rate of clearance. These pharmacokinetic differences, as well as other less well-understood effects, can result in reduced toxicity and enhanced efficacy of the encapsulated drug. The vast majority of studies on the therapeutic use of liposomes have involved the delivery of drugs used in cancer chemotherapy and metabolic storage diseases, but there is now more literature on the use of liposomes for the delivery of antimicrobial drugs and immunomodulating agents. This review briefly discusses the general properties of liposomes and the rationale for their use in antimicrobial drug delivery and immunomodulation, as well as the encapsulation of specific agents and the effect of encapsulation on the treatment of infectious diseases.

Cutaneous leishmaniasis. The defect in T cell influx in BALB/c mice

Local cellular responses to cutaneous infection with Leishmania mexicana amazonensis were examined in susceptible (BALB/c) and resistant (C57BL/6) mouse strains by immunocytochemical and electron microscopic studies. Infection during the first 8 wk in both animal strains was characterized by progressively enlarging lesions, epidermal thickening and ulceration, and accumulation of eosinophils and Ia+ infected macrophages. Healing of C57BL/6 mouse lesions began after 12 wk of infection and was associated with local influx of both Th (L3T4+) and T cytotoxic/suppressor (Lyt-2+) cells into the dermis, and Ia antigen expression on epidermal keratinocytes. T lymphocyte infiltration was marked and intracellular parasites were scarce by 21 wk of C57BL/6 infection. Similarly, granulomas in C57BL/6 livers contained L3T4+ and Lyt-2+ T lymphocytes and no visible intracellular parasites by 21 wk of infection. In contrast, BALB/c mouse lesions continued to enlarge and never healed. Throughout the entire course of infection, T lymphocyte influx into the heavily infected dermis was minimal. Keratinocyte Ia expression was absent in BALB/c lesions. BALB/c livers were heavily infected by 18 wk of cutaneous infection, with few demonstrable T lymphocytes. A systemic absence of T cells could not be demonstrated in BALB/c mice. Both L3T4+ and Lyt-2+ T cells were found in the peripheral blood in normal numbers in both mouse strains. Our results support the role of T cells as important local effector cells in the healing response of murine cutaneous leishmaniasis. We suggest that local T lymphocyte infiltration may provide lymphokines, particularly IFN-gamma, that can activate infected macrophages to destroy the intracellular parasites. Alternatively, T cells may play a cytotoxic role, killing infected macrophages and allowing local humoral factors to destroy released extracellular parasites.

Patogenia da leishmaniose cutânea experimental: a importância da necrose na eliminação dos parasitos das lesões

Um estudo histopatológico e ultraestrutural das lesões da leishmaniose cutânea causada pela Leishmania mexicana amazonensis em duas cepas isogênicas de camundongo, uma susceptível (Balb/c) e outra resistente (A/J), demonstrou que os amastigotas ficavam bem preservados nos vacúolos parasitóforos dos macrófagos, igualmente em ambas as cepas. A reação de imunofluorescência revelou antigenos parasitários no interior e na membrana dos macrófagos de maneira idêntica para ambas as cepas. A diferença ocorria quando os macrófagos apareciam destruídos e as leishmanias ficavam livres ou fagocitadas por polimorfonucleares, neutrófilos e eosinófilos. Estes parasitos exibiam então graus variáveis de nítidas alterações degenerativas. No camundongo resistence, a necrose, de tipo caseoso ou fibrinóide, era mais disseminada e mais freqüente que no animal susceptível. Os achados observados indicaram que as leishmanias não são destruídas no interior dos macrófagos e sim fora deles, especialmente quando fagocitadas por leucócitos polimorfonucleares. A necrose apareceu como o mecanismo mais saliente através do qual o hospedeiro elimina os parasitos das lesões, sendo a mesma um aspecto importante da reação de hipersensibilidade tardia que ocorre nos animais resistentes.

Murine encephalitozoonosis model for studying the host-parasite relationship of a chronic infection

Encephalitozoon cuniculi caused chronic nonlethal infections in euthymic BALB/cAnN mice, whereas athymic BALB/cAnN-nu mice died from infection. Specific, anamnestic, transferable, and acquired responses against E. cuniculi were expressed by infected euthymic mice. Resistance to lethal disease appears to be T-cell dependent. Immune serum failed to protect infected athymic mice, whereas the transfer of T-enriched spleen cells from E. cuniculi-sensitized euthymic donors prevented lethal E. cuniculi infections in athymic mice. These findings indicate that murine encephalitozoonosis may be an excellent system for studies of a chronic infection in an immunologically well-characterized host.

Cell-mediated killing of protozoa

Cell-mediated immunity represents an important host defence mechanism against protozoal infections. The effector cells directly involved are neutrophils, macrophages and, ultimately, activated macrophages. Within this simple scheme there are, however, considerable variations in activity. Effector cells from different animal species, and even from different strains of the same species, may be more or less effective in controlling a certain protozoal infection. Different protozoa differ in their susceptibility to cell-mediated killing according to genus, species, strain and morphological form. The most susceptible morphological form is that which occurs in the insect vector, and which has not yet adapted to protect itself from the vertebrate host. Epimastigotes of Trypanosoma and promastigotes of Leishmania are readily killed by phagocytic cells, while the corresponding trypomastigote and amastigote forms are considerably more resistant. Protozoa which live in macrophages, such as amastigotes of Leishmania, endozoites (tachyzoites) of Toxoplasma and amastigotes of reticulotropic strains of T. cruzi, have developed a remarkable resistance to the microbicidal activity of the host cell. Conversely, amastigotes of myotropic strains of T. cruzi, which live in muscle cells, have not developed this resistance to cell-mediated killing by macrophages. Readily accessible protozoa, such as T. brucei trypomastigotes and Plasmodium merozoites in the bloodstream, while they lack the marked resistance developed by reticulotropic protozoa, have a partial protection since they are attacked by phagocytic cells only when specific antibody is present. Granulocyte-mediated killing can be largely attributed to neutrophils. Eosinophils appear to play only a minor role and compete ineffectually when neutrophils are also present. The only group of protozoal species which may be significantly controlled by eosinophils are the stercorarian species of Trypanosoma. In vitro experiments show that antibody-coated trypomastigotes of T. cruzi can be killed by eosinophils, although there is little evidence that this occurs in vivo. Interestingly, this is the only species that has been reported to be susceptible to the major basic protein of eosinophils, a toxic component of the lysosomal granules which is very active against helminths. Neutrophils are not very active against endozoites of Toxoplasma gondii, Trypanosoma, trypomastigotes of salivarian Trypanosoma, free merozoites of Plasmodium, and promastigotes and amastigotes of Leishmania.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxygen-dependent microbicidal systems of phagocytes and host defense against intracellular protozoa

The role of oxygen-dependent microbicidal systems of leukocytes in the host defense against the major nonerythrocytic intracellular protozoa which infect man--Toxoplasma gondii, Trypanosoma cruzi, and the Leishmania species--is reviewed. The hydrogen peroxide-halide-peroxidase microbicidal system is uniformly cidal to these organisms in vitro. Peroxidase-independent oxygen product(s) toxicity is more variable. Studies to data indicate that phagocytes which contain granule peroxidase and which have the capacity to generate a vigorous respiratory burst; eg, neutrophils and monocytes, possess substantial activity against these protozoa. The absence of granule peroxidase together with the markedly attenuated respiratory burst of resident macrophages leaves these cells with a severe microbicidal defect. These protozoa can enter resident macrophages in the absence of antibody and survive and replicate within the intracellular environment. Enhancement of the antiparasite activity of resident macrophages can be accomplished either by activation of these cells by exposure to sensitized T-cell products, or by the introduction of exogenous peroxidase into the vacuole. Other factors influencing the ability of protozoa to survive intracellularly include the capacity of these organisms to avoid effective triggering of the macrophage respiratory burst and the levels of endogenous scavengers of oxygen products within the parasite.