Cell surface interactions between Trypanosoma congolense and macrophages during phagocytosis in vitro

Host Intracellular Signaling Events and Pro-inflammatory Cytokine Production in African Trypanosomiasis

Pathogens, such as bacteria, viruses, and parasites, possess specific molecules or proteins that are recognized by several host innate immune receptors, leading to the activation of several intracellular signaling molecules and pathways. The magnitude and quality of these events significantly affect the outcome of infection. African trypanosomes, including Trypanosoma congolense, are capable of manipulating the host immune response, including the activity of macrophages, which are the key immune cells that contribute to the immunopathogenesis of African trypanosomiasis. Although it is known that immune hyperactivation and excessive pro-inflammatory cytokine production are the hallmarks of African trypanosomiasis, the mechanisms through which these events are triggered are poorly defined. However, it is known that macrophages may play a significant role in these processes, because phagocytosis of trypanosomes by macrophages initiates intracellular signal transduction cascades that lead to the release of pro-inflammatory cytokines and alteration in cell function. This review highlights recent progress in our understanding of the innate immune receptors, signaling pathways, and transcription factors involved in T. congolense-induced pro-inflammatory cytokine production in macrophages. It will reveal the existence of complex signaling events through which the parasite modulates the host immune response, thus identifying novel targets that could aid in designing strategies to effectively control the disease.

Intradermal infections of mice by low numbers of african trypanosomes are controlled by innate resistance but enhance susceptibility to reinfection

Antibodies are required to control blood-stage forms of African trypanosomes in humans and animals. Here, we report that intradermal infections by low numbers of African trypanosomes are controlled by innate resistance but prime the adaptive immune response to increase susceptibility to a subsequent challenge. Mice were found 100 times more resistant to intradermal infections by Trypanosoma congolense or Trypanosoma brucei than to intraperitoneal infections. B cell-deficient and RAG2(-/-) mice are as resistant as wild-type mice to intradermal infections, whereas inducible nitric oxide synthase (iNOS)(-/-) mice and wild-type mice treated with antibody to tumor necrosis factor (TNF) α are more susceptible. We conclude that primary intradermal infections with low numbers of parasites are controlled by innate defense mediated by induced nitric oxide (NO). CD1d(-/-) and major histocompatibility complex (MHC) class II(-/-) mice are more resistant than wild-type mice to primary intradermal infections. Trypanosome-specific spleen cells, as shown by cytokine production, are primed as early as 24 h after intradermal infection. Infecting mice intradermally with low numbers of parasites, or injecting them intradermally with a trypanosomal lysate, makes mice more susceptible to an intradermal challenge. We suggest that intradermal infections with low numbers of trypanosomes or injections with trypanosomal lysates prime the adaptive immune system to suppress protective immunity to an intradermal challenge.

Trypanosoma (Duttonella) vivax: its biology, epidemiology, pathogenesis, and introduction in the New World--a review

The biology, epidemiology, pathogenesis, diagnostic techniques, and history of the introduction of Trypanosoma (Duttonella) vivax in the New World are reviewed. The two main immunological responses of trypanosome-infected animals - antibody production and immunodepression - are discussed in the context of how these responses play a role in disease tolerance or susceptibility. Isolation and purification of T. vivax are briefly discussed. The recent reports of bovine trypanosomiasis diagnosed in cattle on farms located in the Pantanal region of the states of Mato Grosso do Sul and Mato Grosso, Brazil, are also discussed.

CR3 (CD11b/CD18) is the major macrophage receptor for IgM antibody-mediated phagocytosis of African trypanosomes: Diverse effect on subsequent synthesis of tumor necrosis factor α and nitric oxide

Immunoglobulin M (IgM) antibodies to the variant surface glycoproteins (VSG) of African trypanosomes are the first and predominant class of anti-trypanosomal antibodies in the infected host. They are a major factor in controlling waves of parasitemia, but not in long-term survival. The macrophage receptor(s) that enables phagocytosis of IgM anti-VSG-coated African trypanosomes is unknown. We assessed whether complement receptor CR3 (CD11b/CD18) might be involved in mediating phagocytosis of Trypanosoma congolense. We show that murine complement C3 fragments are deposited onto T. congolense when the trypanosomes are incubated with IgM anti-VSG and fresh mouse serum. In the presence of fresh mouse serum, there is significantly and markedly less phagocytosis of IgM-opsonized T. congolense by CD11b-deficient macrophages compared to phagocytosis by wild-type macrophages (78% fewer T. congolense are ingested per macrophage). Significantly less tumor necrosis factor (TNF)-alpha (38% less), but significantly more nitric oxide (NO) (63% more) are released by CD11b-deficient macrophages that have engulfed trypanosomes than by equally treated wild-type macrophages. We conclude that CR3 is the major, but not the only, receptor involved in IgM anti-VSG-mediated phagocytosis of T. congolense by macrophages. We further conclude that IgM anti-VSG-mediated phagocytosis of T. congolense enhances synthesis of disease-producing TNF-alpha and inhibits synthesis of parasite-controlling NO. We suggest that signaling of inhibition of NO synthesis is mediated via CR3.

Trypanosoma congolenseinfections: antibody-mediated phagocytosis by Kupffer cells

Immunohistochemical double-label technique was used to detect trypanosomal antigen in macrophages. Immunoglobulin (Ig)M as well as IgG2a monoclonal antibodies (mAb) specific for the variant surface glycoprotein (VSG) mediated phagocytosis of Trypanosoma congolense variant antigenic type (VAT) TC13 by macrophages [bone marrow-derived macrophage cell line from BALB/c (BALB.BM)] in vitro. Administration of these IgM or IgG2a antibodies to BALB/c mice 30 min after injection of 3 x 10(8) T. congolense mediated phagocytosis of trypanosomes by Kupffer cells of the liver within 1 h. Plasma levels of the monokines interleukin (IL)-1beta, IL-10, and IL-12p40 were significantly increased 6-48 h after phagocytosis. In BALB/c mice infected with 10(3) T. congolense, a small degree of phagocytosis of trypanosomes by Kupffer cells, mediated by actively synthesized antibodies, was detected as early as 5 days after infection. Phagocytosis of trypanosomes was dramatically enhanced on day 6. Concomitantly, the Kupffer cells trippled in size. In BALB/c mice infected for 6 days, treatment with IgM or IgG2a mAb specific for T. congolense VSG led to clearance of VAT TC13 parasitemia but did not prevent death at the second parasitemia of a different VAT. We conclude that IgM as well as IgG antibody mediate phagocytosis of trypanosomes by Kupffer cells.

Susceptibility and resistance to Trypanosoma congolense infections

We have put emphasis on recent findings in experimental Trypanosoma congolense infections in highly susceptible BALB/c and relatively resistant C57Bl/6 mice. Based on various analyses, it has been shown that a major difference in resistance to T. congolense infections is expressed early in infection at the macrophage level. A novel plastic-adherent Thy1.2(+) suppressor lymphocyte, which in absolute synergy with a Thy 1.2(-) cell exerts its suppression via interleukin-10 and interferon-gamma opens up an exciting new field of research.

Cell lysis induces redistribution of the GPI-anchored variant surface glycoprotein on both faces of the plasma membrane of Trypanosoma brucei

African trypanosomes are coated by 10 million copies of a single variant specific glycoprotein (VSG) which are anchored in the plasma membrane by glycosylphosphatidylinositol (GPI). A GPI-specific phospholipase C (GPI-PLC) triggers fast VSG release upon cell lysis but in vivo it is safely controlled and topologically concealed from its substrate by being intracellular. One enigmatic aspect of GPI-PLC action therefore consists of how it could gain access to the VSG in the exoplasmic leaflet of the membrane. The data presented herewith disclose an unexpected possible solution for this puzzle: upon cell rupture the VSG invades the cytoplasmic face of the plasma membrane which thus becomes double coated. This unusual VSG rearrangement was stable in ruptured plasma membrane from GPI-PLC null mutant trypanosomes but transiently preceded VSG release in wild-type parasites. The formation of double coat membrane (DCM) was independent of the presence or activation of GPI-PLC, occurred both at 4 degrees C and 30 degrees C and was unaffected by the classical inhibitor of VSG release, p-choromercuryphenylsulfonic acid (PCM). DCMs conserved the same coat thickness and association with subpellicular microtubules as in intact cells and were prone to form vesicles following gradual detachment of the latter. Our data also demonstrate that: (i) GPI-PLC expressed by one trypanosome only targets its own plasma membrane, being unable to release VSG of another parasite; (ii) DCMs concomitantly formed from trypanosomes expressing different VSGs do not intermix, an indication that DCM might be refractory to membrane fusion.

Cell surface glycosaminoglycans are not obligatory for Plasmodium berghei sporozoite invasion in vitro

The malaria circumsporozoite (CS) protein binds to glycosaminoglycan chains from heparan sulfate proteoglycans present on the basolateral surface of hepatocytes and hepatoma cells in vitro. When injected into mice, CS protein is rapidly cleared from the blood circulation by hepatocytes. The binding region for the HSPGs is the evolutionarily conserved region II-plus of the CS protein. Here we have asked whether the presence of glycosaminoglycans on the plasma membrane of target cells is required for sporozoite invasion in vitro. Two types of target cells were used: HepG2 cells, which are permissive for Plasmodium berghei sporozoite development into mature exoerythrocytic forms, and CHO cells, in which the intracellular development of the parasites is arrested early after penetration. The invasion of mutant CHO cells expressing undersulfated glycosaminoglycans or no glycosaminoglycans was only inhibited 41-49% or 24-32%, respectively, in comparison to invasion of CHO-K1 cells. Previous cleavage of HepG2 surface membrane glycosaminoglycans with heparinase or heparitinase had no significant inhibitory effect on subsequent P. berghei sporozoite invasion and EEF development in these cells, although the glycosaminoglycan lyase treatments removed over 80% of CS binding sites from the cell surface. These results suggest that although the presence of glycosaminoglycans on the target cell surface enhances sporozoite invasion, glycosaminoglycans are not required for sporozoite penetration or the development of exoerythrocytic forms in vitro.

Complement resistance of parasites

The complement system is a first-line defence mechanism against parasites. All parasites causing deep infections and getting into contact with human plasma must, in one way or another, avoid the destructive effect of this powerful defence system. Several specific strategies of complement resistance of parasites have been reported, and this rather large spectrum of regulatory mechanisms covers the whole cascade of complement activation. Analysis of the known and elucidation of the yet unknown mechanisms will probably help in the development of new therapeutic and preventive approaches to control the different parasitic diseases. This paper will review the complement resistance mechanisms reported and their utilization by various parasites.