Macrophage type 3 complement receptors mediate serum-independent binding of Leishmania donovani. Detection of macrophage-derived complement on the parasite surface by immunoelectron microscopy.

In this study, direct visual evidence for local opsonization of L. donovani by macrophage (M phi)-derived complement components was obtained using immunoelectron microscopy. C3 deposition was detected on the surface of both promastigotes and amastigotes after 20 min serum-free incubation with murine resident peritoneal M phi (RPM), followed by fixation and incubation first with specific antibody directed against C3 and then with gold-labelled protein A. Gold deposition was not observed around either form of the parasite if the anti-C3 antibody was omitted. For promastigotes, the degree of C3 deposition under serum-free conditions was comparable with that observed in the presence of an exogenous (serum) source of C3, but did not result in the same severe damage to the parasite as did the latter. Addition of sodium salicyl hydroxamate, which prevents covalent binding of C3 to activator surfaces, abrogated promastigote binding. Hence, although the anti-C3 antibody did not distinguish between native C3 and its breakdown product iC3b, these data support our earlier conclusion that promastigote binding to the CR3 of murine RPM is complement dependent. For amastigotes, gold deposition and binding to murine RPM were not eliminated by sodium salicyl hydroxamate. The presence of normal mouse serum resulted in increased gold deposition, but did not mediate either enhanced binding to M phi or damage to the amastigote. These data suggest that a proportion of C3 binding to the amastigote surface may be via noncovalent linkages, and that the C3 bound may not be in the correct form to mediate binding to CR3.

Damage to malaria-infected erythrocytes following exposure to oxidant-generating systems.

A study has been made of the damage incurred by normal and Plasmodium falciparum-infected human erythrocytes following exposure to a variety of oxidant-generating systems. Hydrogen peroxide, produced by the glucose-glucose oxidase system, increased methaemoglobin formation within normal erythrocytes while normal levels of oxyhaemoglobin were maintained. Exposure to products of the xanthine-xanthine oxidase interaction did not have the same effect. Malondialdehyde measurements indicated that the host cell membranes of parasitized cells had undergone lipid peroxidation even before exposure to the oxidant-generating systems. Lipid peroxidation of normal and parasitized cell membranes was increased upon exposure to reagent-grade hydrogen peroxide and alloxan: this increase was not observed following exposure to the two enzyme-substrate systems that generated reactive oxygen intermediates. In addition, the effects of parasitism on intracellular levels of catalase and superoxide dismutase were assessed. Normal and parasitized erythrocytes were found to possess similar levels of these enzymes, which protect against oxidant-induced damage. It was therefore concluded that the increased susceptibility of infected cells to oxidant damage was probably not related to any decrease in the function of these enzymes.

DNA-binding antibodies and parasitic diseases.

DNA-binding antibodies are produced during the course of many parasitic infections, including malaria, leprosy and schistosomiasis. These antibodies are also present in certain autoimmune diseases, such as systemic lupus erythematosus, and much information is available about their properties and contribution to related disease processes Here, Anne Wozencraft and Norman Staines consider how DNA-binding antibodies might arise during parasitic infection and discuss how an increased knowledge of their properties and functions could lead to a greater understanding of mechanisms of immuno-pathology in these diseases.

Increased infectivity of stationary-phase promastigotes of Leishmania donovani: correlation with enhanced C3 binding capacity and CR3-mediated attachment to host macrophages.

This study demonstrated that the greater infectivity of stationary-phase promastigotes of Leishmania donovani is related to increased complement fixation on the parasite surface, resulting in increased binding to host mononuclear phagocytes (MPs) via complement type 3 receptors (CR3). The in vivo infectivity of log- and stationary-phase promastigotes was compared by measuring parasite loads in the livers of BALB/c mice 14 days after i.v. inoculation. The same populations were tested for their ability to bind to resident murine peritoneal macrophages (RPM) in vitro during a 20-min serum-free incubation period. Stationary-phase parasites displayed both higher in vivo infectivity and increased in vitro binding. However, following uptake by RPM, no significant difference in the 72 hr survival of the two populations could be detected. The in vitro binding of log and stationary parasites was uniformly inhibited in the presence of a mAb (M1/70) specific for CR3, confirming that the interaction of this receptor with its ligand, iC3b, plays a vital role in initial attachment of both promastigote populations. Following incubation with a human serum source, the amount of ligand appeared to be greater on the surface of stationary-phase promastigotes, as indicated by their ability to trigger the alternative complement pathway and by solid-phase ELISA measurements using antiserum specific for human C3. Collectively, these findings suggest that the infectivity of L. donovani promastigotes is influenced by the extent of initial attachment to host MPs, as determined by the levels of complement deposition and subsequent CR3-mediated binding.

Cell-mediated pathology during murine malaria-associated nephritis.

We have studied the cellular mechanisms involved in the development of nephritis during acute and chronic murine malaria infections induced by Plasmodium vinckei petteri and P. berghei respectively. Albuminuria and uraemia were observed during the early stages of both types of infection, and were associated with glomerular and interstitial hypercellularity. There was a gradual increase in numbers of CD45+ cells from the early stages of both infections onwards. These infiltrates contained CD4+ and CD8+ cells and mononuclear phagocytes. The interstitial and glomerular hypercellularity was due to an influx of inflammatory cells rather than an increase in renal cell division. These findings indicate the importance of cell-mediated immune mechanisms in the development of nephritis during murine malaria and illustrate an example of naturally occurring infection-induced nephritis.

Human monocyte infection by Leishmania (Viannia) panamensis. Role of complement receptors and correlation of susceptibility in vitro with clinical phenotype.

Peripheral blood monocytes (PBMs) from healthy individuals who had experienced distinctive clinical outcomes after natural infection with Leishmania (Viannia) were evaluated in vitro with respect to susceptibility to infection by stationary phase promastigotes of L. (V). panamensis. Concomitantly, the role of complement receptors (CR) CR1 and CR3 in the attachment and entry of L. (V). panamensis into human monocytes was analyzed using mAbs to CR1 (CD35) and CR3 (CD11b) to inhibit competitively these early events in the host-parasite interaction. Cell adherence to fibronectin was examined to determine how modulation of CR activity affected the attachment and uptake of this parasite species. The human monocyte cell line U-937 was also evaluated and found to provide a reproducible control for L. (V). panamensis infection in vitro. Opsonization with fresh AB+ serum markedly enhanced uptake by both PBMs and U-937 cells, and the fluid phase blocking of CR1 and CR3 resulted in partial inhibition of attachment and/or internalization. Uptake rather than attachment was abrogated by antireceptor antibodies in PBMs from previously infected individuals, whereas attachment was diminished in PBMs from unexposed controls. Adherence of PBMs to fibronectin resulted in decreased infection. PBMs from persons who had experienced chronic disease 5 to 8.4 yr before these studies were significantly more susceptible to in vitro infection by L. (V). panamensis than PBMs from asymptomatically infected or control individuals based on the percentage of cells infected, the number of parasites per cell, and viability of intracellular parasites at 48 h postinfection. Neither blocking of CR nor modulation by fibronectin altered the pattern of susceptibility of PBMs from the different clinical groups. These findings provide evidence for the participation of CR in the infection of human monocytes by L. (V). panamensis and demonstrate a correlation between clinical phenotype and in vitro infection of PBMs cultured in the presence of autologous plasma before experimental infection.

Oxygen radical release by adherent cell populations during the initial stages of a lethal rodent malarial infection.

A series of experiments was carried out to assess the levels of reactive oxygen intermediates (ROI) released by macrophages and monocytes during an acute malarial infection, and to consider the importance of oxidant-induced parasite killing in host protection. Adherent cell populations were removed from the peritoneum and spleen of BALB/c and B10/D2/n mice between Days 0-5 of a Plasmodium yoelii nigeriensis infection. These cell populations were quantified, characterized and their ROI-releasing capacity was measured by following ferricytochrome c reduction upon stimulation with phorbol myristate acetate (PMA). Both strains of mice displayed higher numbers of macrophages and macrophage precursors as the infection progressed; this rise was more marked and accompanied by splenomegaly in BALB/c mice. A concurrent decrease in peritoneal cell numbers was observed. Splenic adherent cell populations released much lower levels of ROI than peritoneal macrophages upon triggering. The levels of ROI released from BALB/c splenic adherent cells rose gradually until Day 3, when the parasitaemia was slightly decreased. In contrast, splenic populations from B10 mice had a decreased capacity to release ROI, particularly after Day 3, when the parasitaemia rose sharply. In further studies, electron microscopy was used to detect H2O2 release during the in vitro interaction of peritoneal macrophages and parasitized erythrocytes. Cerium chloride staining techniques demonstrated that H2O production was not dependent on phagocytosis or the presence of immune serum, although levels were increased by the presence of the latter.

Role of DNA-binding antibodies in kidney pathology associated with murine malaria infections.

We performed a series of studies to examine the sequential development of nephritis during murine malaria infections and to define the role of DNA-binding antibodies in the associated pathology. Serum levels of these antibodies were assessed throughout acute and chronic malaria infections. Increased levels of double-stranded DNA- and single-stranded DNA-binding antibodies were initially detected in mice infected with Plasmodium vinckei or Plasmodium yoelii nigeriensis during the middle stages of infection, and these levels were maintained until death. Infection with the more chronic organism Plasmodium berghei clone RC also resulted in increased single-stranded DNA-binding antibody titers, which fluctuated as the infection progressed. All three species caused kidney damage and dysfunction, as assessed by changes in morphology, blood urea nitrogen, and excreted albumin; this damage correlated with the extent of parasitemia and was observed before the levels of DNA-binding antibodies were detectably elevated in the serum. However, the results of immunohistochemical studies demonstrated that DNA-binding monoclonal antibodies bound ex vivo to glomeruli within kidneys prepared from mice at late stages of infection, after the initial damage had been incurred. Our findings suggest how DNA-binding antibodies could contribute to the kidney pathology associated with both malaria and certain autoimmune diseases, such as systemic lupus erythematosus.

Killing of human malaria parasites by macrophage secretory products.

The susceptibility of the human malaria parasite, Plasmodium falciparum, to killing in vitro by macrophage secretory products was investigated. The effect of O2 radicals and tumor necrosis factor on parasite viability was assessed both morphologically and by following the uptake of [3H]hypoxanthine. H2O2 produced by the interaction of glucose and glucose oxidase was found to reduce viability; this effect was reversed by the addition of exogenous catalase. Further studies indicated that the catalase level within the erythrocyte was not altered upon parasite invasion. O2 radicals produced during the xanthine-xanthine oxidase interaction also killed P. falciparum. The addition of various O2 radical scavengers (including catalase) did not reverse this effect; therefore, it was not possible to determine which of the O2 radicals were involved in the killing process. Samples from three different sources containing tumor necrosis factor, a nonspecific soluble mediator derived from Mycobacterium bovis BCG-activated macrophages treated with endotoxin, also killed the parasite. There was no evidence that tumor necrosis factor or the products of the xanthine-xanthine oxidase interaction caused damage to the erythrocyte membrane that could be implicated as an important aspect of the killing process. These findings all strongly suggest that such macrophage products play an important role in immunity to malaria.

Characterization and pathological significance of monoclonal DNA-binding antibodies from mice with experimental malaria infection.

Malaria infection is accompanied by the production of a number of autoantibodies, including some that react with DNA. Epidemiological evidence implicates these in the nephritides that arise in human quartan malaria and in experimental malaria infections in mice. Through parallels with the involvement of DNA-reactive antibodies in the autoimmune syndrome systemic lupus erythematosus, a role for DNA-reactive antibodies in forming phlogistic immune deposits in the kidneys is implied. To more fully understand the relationship between antibodies of this specificity made in malaria and systemic lupus erythematosus, we prepared monoclonal DNA-reactive antibodies from BALB/c mice infected with Plasmodium berghei (clone RC) and compared their properties with those of other antibodies previously isolated from lupous MRL/Mp lpr/lpr and (NZB x NZW)F1 mice. Antibodies from malarial mice were all immunoglobulin M class and bound to single-stranded but not double-stranded DNA in an enzyme-linked immunosorbent assay. They also reacted with synthetic polyribonucleotides in the enzyme-linked immunosorbent assay and with parasitized erythrocytes and parasite pigment in kidney sections. None of the antibodies from lupous mice had identical specificities. The potential involvement of the DNA-reactive antibodies in malarial nephritis was demonstrated, by use of immunocytochemical methods, on the basis of their binding to existing immune deposits in kidney sections from malarial mice, a similar property having been previously demonstrated for antibodies from lupous mice. Furthermore, antibodies from malarial mice expressed public idiotypes, notably Id.V-88, which is a member of the Id.16/6 family, commonly found on DNA-reactive antibodies in lupus and other infectious and connective tissue diseases. This study indicates that DNA-reactive antibodies in malaria have immunochemical properties similar but not identical to those of such antibodies in systemic lupus erythematosus and that they have the potential to participate in the formation of immune deposits in nephritic malarial kidneys.