Reconstitution of the trypanolytic factor from components of a subspecies of human high-density lipoproteins

Natural human immunity to trypanosomes

Complement-dependent destruction of invading micro-organisms is a crucial first-line defense against infection, yet both African and American trypanosomes are able to resist attack by complement. African trypanosomes resist non-specific complement attack by virtue of a thick glycoprotein surface coat, and the host range of certain African trypanosomes is believed to be defined by their susceptibility to a subclass of human high density lipoprotein (HDL) and/or a high molecular weight protein complex present in human serum. In the first part of this review, Stephen Tomlinson and Jayne Raper look at the properties and mechanisms of action of these trypanolytic factors on African trypanosomes, and discuss briefly the possible mechanisms whereby these human pathogens resist lysis by human serum. The mechanisms that enable the American trypanosome Trypanosoma cruzi to resist complement attack are reviewed in the second part of this article.

Serum resistance-associated protein blocks lysosomal targeting of trypanosome lytic factor in Trypanosoma brucei

Trypanosoma brucei brucei is the causative agent of nagana in cattle and can infect a wide range of mammals but is unable to infect humans because it is susceptible to the innate cytotoxic activity of normal human serum. A minor subfraction of human high-density lipoprotein (HDL) containing apolipoprotein A-I (apoA-I), apolipoprotein L-I (apoL-I), and haptoglobin-related protein (Hpr) provides this innate protection against T. b. brucei infection. This HDL subfraction, called trypanosome lytic factor (TLF), kills T. b. brucei following receptor binding, endocytosis, and lysosomal localization. Trypanosoma brucei rhodesiense, which is morphologically and physiologically indistinguishable from T. b. brucei, is resistant to TLF-mediated killing and causes human African sleeping sickness. Human infectivity by T. b. rhodesiense correlates with the evolution of a resistance-associated protein (SRA) that is able to ablate TLF killing. To examine the mechanism of TLF resistance, we transfected T. b. brucei with an epitope-tagged SRA gene. Transfected T. b. brucei expressed SRA mRNA at levels comparable to those in T. b. rhodesiense and was highly resistant to TLF. In the SRA-transfected cells, intracellular trafficking of TLF was altered, with TLF being mainly localized to a subset of SRA-containing cytoplasmic vesicles but not to the lysosome. These results indicate that the cellular distribution of TLF is influenced by SRA expression and may directly determine the organism's susceptibility to TLF.

Human High Density Lipoproteins Are Platforms for the Assembly of Multi-component Innate Immune Complexes

Human innate immunity to non-pathogenic species of African trypanosomes is provided by human high density lipoprotein (HDL) particles. Here we show that native human HDLs containing haptoglobin-related protein (Hpr), apolipoprotein L-I (apoL-I) and apolipoprotein A-I (apoA-I) are the principle antimicrobial molecules providing protection from trypanosome infection. Other HDL subclasses containing either apoA-I and apoL-I or apoA-I and Hpr have reduced trypanolytic activity, whereas HDL subclasses lacking apoL-I and Hpr are non-toxic to trypanosomes. Highly purified, lipid-free Hpr and apoL-I were both toxic to Trypanosoma brucei brucei but with specific activities at least 500-fold less than those of native HDLs, suggesting that association of these apolipoproteins within the HDL particle was necessary for optimal cytotoxicity. These studies show that HDLs can serve as platforms for the assembly of multiple synergistic proteins and that these assemblies may play a critical role in the evolution of primate-specific innate immunity to trypanosome infection.

The kinetoplastida endocytic apparatus. Part I: a dynamic system for nutrition and evasion of host defences

The endocytic system of kinetoplastid parasites is a highly polarized membrane network focused on the flagellar pocket localized at one end of the cell. When first characterized, the endosomal network was envisioned as a simple system for uptake of extracellular material by fluid-phase or receptor-mediated mechanisms. Subsequently, it has become clear that the kinetoplastid endosomal system has an active and vital role in avoiding the host immune system and virulence, as well as providing the basic functions to fulfil cellular nutritional requirements. In two reviews, recent advances in the definition and comprehension of kinetoplastida endocytosis are discussed and, in Trypanosoma brucei in particular as the more developed experimental system. In Part 1, the endocytic system is considered in context of the surface molecules and their potential roles in virulence.

Natural immunity to human African trypanosomiasis: trypanosome lytic factors and the blood incubation infectivity test

This review focuses on the epidemiology of human African trypanosomiasis: why it occurs in humans, the current methods of surveillance, and the drugs available to treat it. Emphasis is placed on the identification of human-infective trypanosomes by the blood incubation infectivity test. This test distinguishes between trypanosomes that are non-infective for humans and those that are potentially infective. Currently the test requires incubation of parasites with human serum before injection into mice; any surviving parasites are considered human-infective. The factors in serum that kill all non-human-infective parasites are known as trypanosome lytic factors. The paper details the biochemistry of these factors and recommends standardization of the test based on current knowledge. This test can be used to screen animals with trypanosomiasis, in order to evaluate their role during endemic and epidemic human African trypanosomiasis.

Activity of human trypanosome lytic factor in mice

The inability of the cattle pathogen Trypanosoma brucei brucei to infect humans is due to an innate factor in human serum termed Trypanosome Lytic Factor (TLF). Human haptoglobin-related protein is the proposed toxin in TLF and can exist either as a component of a minor subclass of high-density lipoprotein (TLF-1) or as a lipid free, high molecular weight protein complex (TLF-2). The trypanolytic activity of both TLF-1 and TLF-2 has been studied in vitro but their relative contributions to protection against T. b. brucei infection in vivo has not been established. In the present studies we show that treatment of T. b. brucei infected mice with TLF-1 resulted in a dose dependent decrease in parasite numbers but did not affect parasite numbers in mice infected with Trypanosoma brucei rhodesiense, the causative agent of the human sleeping sickness. Similarly, pretreatment of mice with TLF-1 resulted in protection against a challenge by T. b. brucei but had no effect on T. b. rhodesiense challenge. Induction of the acute phase protein haptoglobin, a natural antagonist of TLF-1, diminished but did not abolish the protection against trypanosome challenge. In addition, haptoglobin knockout mice showed higher levels of TLF-1 mediated protection against a T. b. brucei challenge. These results suggest that while TLF-1 is active in vivo, even in the presence of elevated levels of haptoglobin, its activity is modulated in a dose dependent fashion by haptoglobin in the circulation.

Structural models of human apolipoprotein A-I: a critical analysis and review

Human apolipoprotein (apo) A-I has been the subject of intense investigation because of its well-documented anti-atherogenic properties. About 70% of the protein found in high density lipoprotein complexes is apo A-I, a molecule that contains a series of highly homologous amphipathic alpha-helices. A number of significant experimental observations have allowed increasing sophisticated structural models for both the lipid-bound and the lipid-free forms of the apo A-I molecule to be tested critically. It seems clear, for example, that interactions between amphipathic domains in apo A-I may be crucial to understanding the dynamic nature of the molecule and the pathways by which the lipid-free molecule binds to lipid, both in a discoidal and a spherical particle. The state of the art of these structural studies is discussed and placed in context with current models and concepts of the physiological role of apo A-I and high-density lipoprotein in atherosclerosis and lipid metabolism.

Innate and acquired resistance to African trypanosomiasis

The review discusses the current field status of human and bovine trypanosomiases, and focuses on the molecular basis of innate and acquired control of African trypanosomes in people, cattle, and Cape buffalo.

Mechanism of resistance of African trypanosomes to cytotoxic human HDL

Trypanosoma brucei brucei, the causative agent of ngana in cattle, is non-infectious to humans because of its sensitivity to the cytolytic activity of normal human serum. The toxin in normal human serum is human haptoglobin-related protein (Hpr) which is found either as an apolipoprotein associated with a minor subclass of high-density lipoprotein (HDL), named trypanosome lytic factor (TLF1), or as an unstable, high-molecular-mass protein complex known as TLF2 (refs 5, 9-12). TLF-mediated lysis of T. b. brucei requires binding, internalization and lysosomal targeting. The human sleeping-sickness trypanosome, Trypanosoma brucei rhodesiense is resistant to TLF. Our studies reveal that resistant trypanosomes fail to endocytose TLF yet continue to bind TLF through cell-surface receptors. On the basis of these results, we conclude that one mechanism of resistance of human sleeping-sickness trypanosomes to human serum is decreased internalization of receptor-bound TLF.

Identification of haptoglobin as a natural inhibitor of trypanocidal activity in human serum

Trypanosomes are protozoan parasites of medical and veterinary importance. Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense infect humans, causing African sleeping sickness. However, Trypanosoma brucei brucei can only infect animals, causing the disease Nagana in cattle. Man is protected from this subspecies of trypanosomes by a toxic subtype of high density lipoproteins (HDLs) called the trypanosome lytic factor (TLF). The toxic molecule in TLF is believed to be the haptoglobin-related protein that when bound to hemoglobin kills the trypanosome via oxidative damage initiated by its peroxidase activity. The amount of lytic activity in serum varies widely between different individuals with up to a 60-fold difference in activity. In addition, an increase in the total amount of lytic activity occurs during the purification of TLF, suggesting that an inhibitor of TLF (ITLF) exists in human serum. We now show that the individual variation in trypanosome lytic activity in serum correlates to variations in the amount of ITLF. Immunoblots of ITLF probed with antiserum against haptoglobin recognize a 120-kDa protein, indicating that haptoglobin is present in partially purified ITLF. Haptoglobin involvement is further shown in that it inhibits TLF in a manner similar to ITLF. Using an anti-haptoglobin column to remove haptoglobin from ITLF, we show that the loss of haptoglobin coincides with the loss of inhibitor activity. Addition of purified haptoglobin restores inhibitor activity. This indicates that haptoglobin is the molecule responsible for inhibition and therefore causing the individual variation in serum lytic activity.

Trypanosoma brucei brucei and high-density lipoproteins: Old and new thoughts on the identity and mechanism of the trypanocidal factor in human serum

Nature has provided humans with a surprising means of protection against the African trypanosome Trypanosoma brucei brucei There is consensus, in that this singular trypanocidal factor is serum high-density lipoproteins (HDL). which the trypanosomes engulf through a physiological, receptor-mediated pathway for delivery to acidic intracellular vesicles. There is also controversy, however, in that the active particles and their essential cytotoxic elements are disputed, in part reflecting the ill-defined mechanism by which the parasites are finally killed. Here Patrick Lorenz, Bruno Betschart and Jim Owen discuss the possibilities for resolving these discrepancies and speculate on the prospects of exploiting this unexpected property of human HDL for protecting livestock.

Killing of trypanosomes by the human haptoglobin-related protein

African trypanosomes cause disease in humans and animals. Trypanosoma brucei brucei affects cattle but not humans because of its sensitivity to a subclass of human high density lipoproteins (HDLs) called trypanosome lytic factor (TLF). TLF contains two apolipoproteins that are sufficient to cause lysis of T. b. brucei in vitro. These proteins were identified as the human haptoglobin-related protein and paraoxonase-arylesterase. An antibody to haptoglobin inhibited TLF activity. TLF was shown to exhibit peroxidase activity and to be inhibited by catalase. These results suggest that TLF kills trypanosomes by oxidative damage initiated by its peroxidase activity.

High-density-lipoprotein-independent killing of Trypanosoma brucei by human serum

The cattle pathogen Trypanosoma brucei brucei is morphologically indistinguishable from the human pathogens T.b. rhodesiense and T.b. gambiense. However, unlike the human pathogens, T.b. brucei is lysed by normal human serum (NHS). The trypanolytic factor in NHS co-purifies with high-density lipoproteins (HDL), but its precise nature is unknown. Using a new fluorescence-based viability assay to assess T.b. brucei killing, we find that the HDL-deficient sera from two patients with Tangier disease are as trypanolytic as NHS. Fractionation of the Tangier sera by density ultracentrifugation revealed that the activity resides only in lipoprotein-depleted fractions. Tangier and NHS were also subjected to molecular sieving chromatography, and the activity profiles were identical. Lytic fractions to T. brucei (but not to T. rhodesiense) appeared under two distinct peaks of 100-600 kDa and > 1000 kDa. Neither peak coincided with the position of the major serum lipoproteins, as determined by cholesterol titrations. The high-molecular-mass peak did not contain the HDL-associated apolipoprotein-A1. Further, we did not find that purified apolipoproteins A1 or J are lytic for the trypanosomes. We conclude that the killing of T. brucei by human serum can be independent of HDL.