Trypanosoma cruzi: endocytosis and degradation of specific antibodies by parasite forms

The Trypanosoma cruzi Diamine Transporter Is Essential for Robust Infection of Mammalian Cells

Trypanosoma cruzi is incapable of synthesizing putrescine or cadaverine de novo, and, therefore, salvage of polyamines from the host milieu is an obligatory nutritional function for the parasite. A high-affinity diamine transporter (TcPOT1) from T. cruzi has been identified previously that recognizes both putrescine and cadaverine as ligands. In order to assess the functional role of TcPOT1 in intact parasites, a Δtcpot1 null mutant was constructed by targeted gene replacement and characterized. The Δtcpot1 mutant lacked high-affinity putrescine-cadaverine transport capability but retained the capacity to transport diamines via a non-saturable, low-affinity mechanism. Transport of spermidine and arginine was not impacted by the Δtcpot1 lesion. The Δtcpot1 cell line exhibited a significant but not total defect in its ability to subsist in Vero cells, although initial infection rates were not affected by the lesion. These findings reveal that TcPOT1 is the sole high-affinity diamine permease in T. cruzi, that genetic obliteration of TcPOT1 impairs the ability of the parasite to maintain a robust infection in mammalian cells, and that a secondary low-affinity uptake mechanism for this key parasite nutrient is operative but insufficient for optimal infection.

Specific cleavage sites on human IgG subclasses by cruzipain, the major cysteine proteinase from Trypanosoma cruzi

Cruzipain, the major cysteine proteinase of Trypanosoma cruzi, might have other biological roles than its metabolic functions. In this report, we have explored the interaction of cruzipain with molecules of the immune system. The enzyme was used to digest all human IgG subclasses at different pH values and lengths of time. At pH 7.3, all subclasses were readily split at the hinge region. Immunoblot and amino acid sequence analysis showed fragments of IgG1 and IgG3 to be compatible with Fab and Fc, whereas IgG2 and IgG4 rendered Fab2 and Fc. In all cases the fragments produced might impair the binding capacities and the effector functions of specific IgG. At these cleavage sites cruzipain displays cathepsin L and/or cathepsin B activities and shows a clear preference for Pro at the P'2 position and polar residues at P1. Despite the activity of cruzipain within the hinge, the enzyme also cleaved all heavy chains between the CH2 and CH3 domains; producing Fc'-like-fragments of 14 kDa. These fragments are potential candidates to block or saturate Fc receptors on immunocompetent cells. At mild acidic pH cruzipain produced further degradation of the Fc of all subclasses, the Fd of IgG4 and partially the Fd of IgG1, with the consistent loss of any antibody activity. The L chains apparently were not affected. Thus, cruzipain should be able to modulate, depending on the subclass selected and the pH of the environment, the production and the length of different biologically active/inactive IgG fragments.

How protozoan parasites evade the immune response

Protozoan pathogens such as Plasmodium, Leishmania, Trypanosoma and Entamoeba are responsible for several of the most widespread and lethal human diseases. Their successful survival depends mainly on evading the host immune system by, for example, penetrating and multiplying within cells, varying their surface antigens, eliminating their protein coat, and modulating the host immune response. Immunosuppression is sometimes caused directly by parasite products and sometimes involves antigenic mimicry, which often appears in association with parasitic diseases. However, one of the most sophisticated mechanisms of evasion is the selective activation of a subset of T helper cells.

The surface glycoconjugates of parasitic protozoa: potential targets for new drugs

Protozoan parasites are the cause of many disease in humans and their domestic livestock. Glycoconjugates (i.e. glycoproteins, glycolipids) on the cell surface of these extremely diverse and very primative eukaryotes play a crucial role in determining the specificity of the host-parasite interaction and in protecting the parasites within their respective hosts. These molecules frequently share a common structural feature in that they are attached to the plasma membrane via a glycosylphosphatidylinositol (GPI) glycolipid. While GPI protein-membrane anchors are ubiquitous among the eukaryotes, they are used with exceptionally high frequency in the protozoa. Some kinetopastid parasites also synthesise very high levels of GPI-related glycolipids that are not linked to protein. Thus GPI-anchored molecules or free GPI glycolipids send to dominate the cell surface molecular architecture of these organisms. The highly elevated levels and specialised nature of GPI metabolism in the kinetoplastid and other parasites suggests that the GPI biosynthetic pathway might be a good target for the development of new chemotherapeutic agents. This article reviews the wide range of functions that GPI protein anchors and GPI-related glycolipids are thought to perform in these organisms and some aspects of their biosynthesis.

Endocytosis of gold-labeled proteins and LDL by Trypanosoma cruzi

Endocytosis was studied at the ultrastructural level in different developmental forms of Trypanosoma cruzi after incubation of the parasites in the presence of gold-labeled proteins (albumin-Au, peroxidase-Au and transferrin-Au) and low-density lipoprotein (LDL-Au). Epimastigote (culture) forms actively ingested LDL and proteins. Initially, gold particles were seen adhering only to the cytostome and inside the flagellar pocket. In parasites incubated at 4 degrees C with transferrin-Au or peroxidase-Au, labeling was found only at these two sites, showing that receptor-mediated endocytosis occurs in both regions. In the cytoplasm, gold particles were seen only inside two different compartments: membrane-bound vesicles and reservosomes. Incubation of epimastigotes with acridine orange followed by fluorescence microscopy revealed intense orange staining, indicating that the reservosomes have an acidic pH. This staining was abolished after incubation of the parasites in the presence of ammonium chloride. These data confirm that this compartment is the site of accumulation of ingested lipids and proteins. Little intracellular labeling with transferrin-Au was found in in vitro-derived amastigotes and trypomastigotes (both lack reservosomes). However, although in amastigotes very few gold particles were seen bound to the cells, in trypomastigotes they were observed bound to the membrane that encloses the cell body, the flagellar pocket, and the flagellum, suggesting that the receptors are more abundant in this form.

Digestion of human immunoglobulin G by the major cysteine proteinase (cruzipain) from Trypanosoma cruzi

The major cysteine proteinase (cruzipain) from Trypanosoma cruzi was able to digest human IgG, as shown by polyacrylamide gel electrophoresis in the presence of SDS, and by gel filtration on a Superose 12 column, in a FPLC system. The Fab fragment of IgG was only slightly degraded, but Fc was extensively hydrolyzed to small peptides. The results suggest that cruzipain might be involved in the defense mechanisms of the parasite against the immune response of the host.