African Trypanosomiasis-Associated Anemia: The Contribution of the Interplay between Parasites and the Mononuclear Phagocyte System

Abstract

African trypanosomosis (AT) is a chronically debilitating parasitic disease of medical and economic importance for the development of sub-Saharan Africa. The trypanosomes that cause this disease are extracellular protozoan parasites that have developed efficient immune escape mechanisms to manipulate the entire host immune response to allow parasite survival and transmission. During the early stage of infection, a profound pro-inflammatory type 1 activation of the mononuclear phagocyte system (MPS), involving classically activated macrophages (i.e., M1), is required for initial parasite control. Yet, the persistence of this M1-type MPS activation in trypanosusceptible animals causes immunopathology with anemia as the most prominent pathological feature. By contrast, in trypanotolerant animals, there is an induction of IL-10 that promotes the induction of alternatively activated macrophages (M2) and collectively dampens tissue damage. A comparative gene expression analysis between M1 and M2 cells identified galectin-3 (Gal-3) and macrophage migration inhibitory factor (MIF) as novel M1-promoting factors, possibly acting synergistically and in concert with TNF-α during anemia development. While Gal-3 enhances erythrophagocytosis, MIF promotes both myeloid cell recruitment and iron retention within the MPS, thereby depriving iron for erythropoiesis. Hence, the enhanced erythrophagocytosis and suppressed erythropoiesis lead to anemia. Moreover, a thorough investigation using MIF-deficient mice revealed that the underlying mechanisms in AT-associated anemia development in trypanosusceptible and tolerant animals are quite distinct. In trypanosusceptible animals, anemia resembles anemia of inflammation, while in trypanotolerant animals' hemodilution, mainly caused by hepatosplenomegaly, is an additional factor contributing to anemia. In this review, we give an overview of how trypanosome- and host-derived factors can contribute to trypanosomosis-associated anemia development with a focus on the MPS system. Finally, we will discuss potential intervention strategies to alleviate AT-associated anemia that might also have therapeutic potential.

Keywords: IFN-γ; IL-10; MIF; MPS; anemia; erythrophagocytosis; hemodilution; inflammation.

Figure 1

The activation state of myeloid cells correlates with anemia development during trypanosome infections in trypanosusceptible and trypanotolerant animals. (A) Anemia development in trypanosusceptible (green) and trypanotolerant (blue) animals during the course of infection. Anemia progression can be divided into (i) an acute phase characterized by a rapid drop in red blood cell (RBC) numbers (i.e., consumptive anemia) followed by a partial recovery phase, (ii) a late stage characterized in the susceptible model by a progressive decline in RBC numbers (below that of the acute phase) and host death. In the tolerant model, this decline is less pronounced and leads to (iii) a chronic phase (i.e., progressive anemia), whereby RBC numbers keep on declining till finally reaching levels of that of the acute phase. (B) Throughout the course of infection progressive hepatosplenomegaly occurs, whereby the onset of splenomegaly precedes hepatomegaly. At the late/chronic stage of infection, splenomegaly is more pronounced than hepatomegaly. (C) During the different stages of infection, the host produces different mononuclear phagocyte system (MPS) polarizing molecules. During the early/acute stage, both trypanosusceptible (upper, green) and trypanotolerant (lower, blue) animals produce IFN-γ (green line) required to trigger the induction of classically activated macrophages (M1), which is followed by a moderate induction of IL-10 (blue line) to dampen the pathogenic effects of the M1. Only in the trypanotolerant model, there is a second progressive increase in IL-10 during the late/chronic phase of infection, which is required to induce alternatively activated macrophages (M2). (D) Occurrence of M1 and M2 during the course of aggressive Trypanosoma brucei or T. brucei attenuating strategies (GPI-based strategy or AAV-10/anti-CD28) or less virulent T. brucei PLC^−/−/Trypanosoma congolense infection.

Figure 2

Proposed model for acute anemia development during trypanosome infections in trypanosusceptible animals (i.e., Trypanosoma brucei). During the early stage of infection, saliva components in concert with parasite-derived factors such as KHC [fueling parasite nutrient production (i.e., polyamines)] and AdC (dampening the potential of M1), temporarily disable/attenuate the M1-mediated innate immune response, thereby allowing parasite initiation/establishment. In addition, parasites release TLTF to trigger IFN-γ production by NK/NKT and CD8⁺ T cells, which will promote M1 cell activation. At the peak of parasitemia, these IFN-γ-primed M1 in concert with parasite-derived factors such as GPI-VSG (GIP/DMG) and CpG fuel M1 to produce pro-inflammatory molecules such as TNF, MIF, and Gal-3. In turn, TNF in concert with parasite-released extracellular vesicles (EVs) and/or sialidase trigger/enhance RBC senescence and hence erythrophagocytosis by M1 cells, leading to the development of acute anemia (i.e., hemophagocytic/consumptive anemia). Both MIF and Gal-3 promote M1 polarization and at this stage, there is an amplification of the iron homeostasis [i.e., increased iron storage (ferritin) and export (FPN-1)], which triggers extramedullary erythropoiesis. MIF also promotes recruitment/retention of mononuclear phagocytes and neutrophils, which in turn fuel the pathogenic effects of M1. At this stage, the host transiently produces IL-10, which is required to dampen the pathogenic effects of the M1 and in concert with the extramedullary erythropoiesis results in a partial recovery from acute anemia. Abbreviations: TLTF, T-lymphocyte triggering factor; KHC, kinesin heavy chain; AdC, adenylate cyclase; GPI, glycosylphosphatidylinositol; GIP, glycosylinosytolphosphate; RBC, red blood cell; MIF, macrophage migration inhibitory factor; Gal-3, galectin-3; FPN-1, ferroportin-1; Hemato, hematopoietic; Non-Hemato, non-hematopoietic; DMG, dimyristoylglycerol; VSG, variant surface glycoprotein.

Figure 3

Proposed model for pathogenicity development during the late/chronic stage of infection in trypanosusceptible and trypanotolerant animals. (Left panel) Following recovery from acute anemia (see Figure 2), trypanosusceptible animals are unable to trigger a second wave of IL-10 production resulting in persistence of M1, which is most likely promoted via macrophage migration inhibitory factor (MIF) and galectin-3. These M1 produce TNF that further promotes erythrophagocytosis and exhibit iron accumulation [increased ferritin and reduced ferroportin-1 (FPN-1) expression], resulting in iron deprivation from erythropoiesis and collectively leading to progressive anemia development. MIF also promotes myeloid cell recruitment/retention leading to liver injury. By contrast, in trypanotolerant animals, following the acute stage of infection, hematopoietic (hemato) or non-hematopoietic (non-hemato) cells produce IL-10, which dampens the pathogenic effects of the M1 by triggering the induction of M2. These M2 exhibit reduced erythrophagocytosis and reduced iron retention, leading to a more efficient erythropoiesis and in turn to reduced progressive anemia development. IL-10 also reduces liver injury by dampening the pathogenic effects of the M1. (Right panel) However, the chronic production of MIF by M1 and persistent IL-10 production can collectively promote progressive hemodilution, which is most likely due to enhanced (hepato)-splenomegaly. In turn, the hemodilution will cause an apparent anemia development in trypanotolerant animals, resulting in thrombocytopenia and impaired coagulation in infected mice. The heme catabolism following the chronically induced erythrophagocytosis leads to iron accumulation in myeloid cells and to hyperbilirubinemia. The combination of liver injury and hemodilution leads to declined serum albumin levels, thereby preventing efficient removal of toxic molecules from the circulation, including bilirubin, and cause multiple organ failure that culminates in reduced survival of the infected host.