Antiparasite responses are associated with the recruitment of monocytes that differentiate to macrophages and dendritic cells at the site of infection. Although classically activated monocytic cells are assumed to be the major source of TNF and NO during Trypanosoma brucei brucei infection, their cellular origin remains unclear. In this study, we show that bone marrow-derived monocytes accumulate and differentiate to TNF/inducible NO synthase-producing dendritic cells (TIP-DCs) in the spleen, liver, and lymph nodes of T. brucei brucei-infected mice. Although TIP-DCs have been shown to play a beneficial role in the elimination of several intracellular pathogens, we report that TIP-DCs, as a major source of TNF and NO in inflamed organs, could contribute actively to tissue damage during the chronic stage of T. brucei brucei infection. In addition, the absence of IL-10 leads to enhanced differentiation of monocytes to TIP-DCs, resulting in exacerbated pathogenicity and early death of the host. Finally, we demonstrate that sustained production of IL-10 following IL-10 gene delivery treatment with an adeno-associated viral vector to chronically infected mice limits the differentiation of monocytes to TIP-DCs and protects the host from tissue damage.

The development of classically activated monocytic cells (M1) is a prerequisite for effective elimination of parasites, including African trypanosomes. However, persistent activation of M1 that produce pathogenic molecules such as TNF and NO contributes to the development of trypanosome infection-associated tissue injury including liver cell necrosis in experimental mouse models. Aiming to identify mechanisms involved in regulation of M1 activity, we have recently documented that during Trypanosoma brucei infection, CD11b+Ly6C+CD11c+ TNF and iNOS producing DCs (Tip-DCs) represent the major pathogenic M1 liver subpopulation. By using gene expression analyses, KO mice and cytokine neutralizing antibodies, we show here that the conversion of CD11b+Ly6C+ monocytic cells to pathogenic Tip-DCs in the liver of T. brucei infected mice consists of a three-step process including (i) a CCR2-dependent but CCR5- and Mif-independent step crucial for emigration of CD11b+Ly6C+ monocytic cells from the bone marrow but dispensable for their blood to liver migration; (ii) a differentiation step of liver CD11b+Ly6C+ monocytic cells to immature inflammatory DCs (CD11c+ but CD80/CD86/MHC-IIlow) which is IFN-γ and MyD88 signaling independent; and (iii) a maturation step of inflammatory DCs to functional (CD80/CD86/MHC-IIhigh) TNF and NO producing Tip-DCs which is IFN-γ and MyD88 signaling dependent. Moreover, IL-10 could limit CCR2-mediated egression of CD11b+Ly6C+ monocytic cells from the bone marrow by limiting Ccl2 expression by liver monocytic cells, as well as their differentiation and maturation to Tip-DCs in the liver, showing that IL-10 works at multiple levels to dampen Tip-DC mediated pathogenicity during T. brucei infection. A wide spectrum of liver diseases associates with alteration of monocyte recruitment, phenotype or function, which could be modulated by IL-10. Therefore, investigating the contribution of recruited monocytes to African trypanosome induced liver injury could potentially identify new targets to treat hepatic inflammation in general, and during parasite infection in particular.

Uncontrolled inflammation is a major cause of tissue injury/pathogenicity often resulting in death of a host infected with African trypanosomes. Thus, comparing the immune response in hosts that develop different degrees of disease severity represents a promising approach to discover processes contributing to trypanosomiasis control. It is known that limitation of pathogenicity requires a transition in the course of infection, from an IFN-γ-dependent response resulting in the development of classically activated myeloid cells (M1), to a counterbalancing IL-10-dependent response associated with alternatively activated myeloid cells (M2). Herein, mechanisms and downstream effectors by which M2 contribute to lower the pathogenicity and the associated susceptibility to African trypanosomiasis have been explored. Gene expression analysis in IL-10 knockout and wild-type mice, that are susceptible and relatively resistant to Trypanosoma congolense infection, respectively, revealed a number of IL-10-inducible genes expressed by M2, including Sepp1 coding for selenoprotein P. Functional analyses confirm that selenoprotein P contributes to limit disease severity through anti-oxidant activity. Indeed, Sepp1 knockout mice, but not Sepp1Δ240-361 mice retaining the anti-oxidant motif but lacking the selenium transporter domain of selenoprotein P, exhibited increased tissue injury that associated with increased production of reactive oxygen species and increased apoptosis in the liver immune cells, reduced parasite clearance capacity of myeloid cells, and decreased survival. These data validate M2-associated molecules as functioning in reducing the impact of parasite infection on the host.

Inflammatory responses mounted to eliminate parasites can be lethal if not counterbalanced by regulatory responses protecting the host from collateral tissue damage. Here, we show that the maintained inflammation associated with tissue damage, anemia, and reduced survival of Trypanosoma brucei-infected mice correlates with the absence of the expansion of the regulatory T (T(reg)) cell population. Induction of T(reg) cell expansion via CD28 superagonist antibody treatment in these mice down-regulated interferon-gamma production by T cells and tumor necrosis factor-alpha and reactive oxygen species production by classically activated macrophages, triggered the development of alternatively activated macrophages, delayed the onset of liver injury, diminished the anemia burden, and prolonged the survival of infected animals. Thus, triggering the expansion of the T(reg) cell population coupled with the induction of alternatively activated macrophages can restore the balance between pro- and anti-inflammatory signals and thereby limit the pathogenicity of African trypanosomiasis.

A balance between parasite elimination and control of infection‐associated pathogenicity is crucial for resistance to African trypanosomiasis. By producing TNF and NO, CD11b+ myeloid cells with a classical activation status (M1) contribute to parasitemia control in experimental Trypanosoma congolense infection in resistant C57BL/6 mice. However, in these mice, IL‐10 is required to regulate M1‐associated inflammation, avoiding tissue/liver damage and ensuring prolonged survival. In an effort to dissect the mechanisms behind the anti‐inflammatory activity of IL‐10 in T. congolense‐infected C57BL/6 mice, we show, using an antibody blocking the IL‐10 receptor, that IL‐10 impairs the accumulation and M1 activation of TNF/iNOS‐producing CD11b+Ly6C+ cells in the liver. Using infected IL‐10flox/floxLysM‐Cre+/+ mice, we show that myeloid cell‐derived IL‐10 limits M1 activation of CD11b+Ly6C+ cells specifically at the level of TNF production. Moreover, higher production of TNF in infected IL‐10flox/floxLysM‐Cre+/+ mice is associated with reduced nuclear accumulation of the NF‐κB p50 subunit in CD11b+ M1 cells. Furthermore, in infected p50−/− mice, TNF production by CD11b+Ly6C+ cells and liver injury increases. These data suggest that preferential nuclear accumulation of p50 represents an IL‐10‐dependent anti‐inflammatory mechanism in M1‐type CD11b+ myeloid cells that regulates the production of pathogenic TNF during T. congolense infection in resistant C57BL/6 mice.

Bovine African trypanosomiasis causes severe economical problems on the African continent and one of the most prominent immunopathological parameters associated with this parasitic infection is anemia. In this report we review the current knowledge of the mechanisms underlying trypanosomiasis-associated anemia. In first instance, the central role of macrophages and particularly their activation state in determining the outcome of the disease (i.e. trypanosusceptibility versus trypanotolerance) will be discussed. In essence, while persistence of classically activated macrophages (M1) contributes to anemia development, switching towards alternatively activated macrophages (M2) alleviates pathology including anemia. Secondly, while parasite-derived glycolipids such as the glycosylphosphatidylinositol (GPI) induce M1, host-derived IL-10 blocks M1-mediated inflammation, promotes M2 development and prevents anemia development. In this context, strategies aimed at inducing the M1 to M2 switch, such as GPI-based treatment, adenoviral delivery of IL-10 and induction of IL-10 producing regulatory T cells will be discussed. Finally, the crucial role of iron-homeostasis in trypanosomiasis-associated anemia development will be documented to stress the analogy with anemia of chronic disease (ACD), hereby providing new insight that might contribute to the treatment of ACD.

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Recently, we identified the CD20 homolog Ms4a8a as a novel molecule expressed by tumor-associated macrophages that directly enhances tumor growth. Here, we analyzed Ms4a8a + macrophages in M2-associated infectious pathologies. In late-stage Trypanosoma congolense and Taenia crassiceps infections, Ms4a8a expression was detected in hepatic and peritoneal macrophages respectively. Innate immunity in these infections is modulated by Toll-like receptor (TLR) signaling and TLR2/4/7 agonists strongly induced Ms4a8a expression in bone marrow derived macrophages (BMDMs) treated with M2 mediators (glucocorticoids/IL-4). LPS/dexamethasone/IL-4-induced Ms4a8a + BMDMs were characterized by strong expression of mRNA of mannose receptor (Mmr), arginase 1, and CD163, and by decreased iNOS expression. Coinduction of Ms4a8a by M2 mediators and TLR agonists involved the classical TLR signaling cascade via activation of MyD88/TRIF and NF-κB. Forced overexpression of Ms4a8a modulated the TLR4 response of RAW264.7 cells as shown by gene expression profiling. Upregulation of Hdc, Tcfec, and Sla was confirmed both in primary LPS/dexamethasone/IL-4-stimulated Ms4a8a + BMDMs and in peritoneal macrophages from late-stage Taenia crassiceps infection. In conclusion, we show that TLR signaling skews the typical alternative macrophage activation program to induce a special M2-like macrophage subset in vitro that also occurs in immunomodulatory immune reactions in vivo, a process directly involving the CD20 homolog Ms4a8a. Eur. J. Immunol. 2012Immunol. . 42: 2971Immunol. -2982 IntroductionMacrophages have an extraordinary ability to rapidly adapt to new environmental stimuli by altering their gene expression profile and functions [1]. These enormous variations enable macrophages to participate in different biological and pathological processes such as the maintenance of tissue homeostasis, orchestration of defense responses, and healing processes in tissues. The phenotypic plasticity and functional heterogeneity of macrophages renders their classification difficult. Thus, a simplified approach was put forward in analogy to the Th1/Th2 dichotomy [2,3]. Accordingly, M1 or classically activated macrophages differentiate from monocytes under the predominant influence of pro-inflammatory cytokines such as IFN-γ, IL-12, or TNF-α, while M2, or alternative, macrophage activation was primarily described as a response to Th2 cytokines such as IL-4, IL-13, and IL-10, as well as to anti-inflammatory mediators such as glucocorticoids (GCs) [4][5][6][7]. As these macrophage phenotypes influence the tissue environment, for example, via secretion of active mediators, a solid characterization of macrophages, especially in pathological processes such as cancer or chronic inflammation, is needed to uncover unique key cellular regulators [8].In a previous study, the CD20 homolog Ms4a8a was found to be expressed by M2-like tumor-associated macrophages (TAMs) in murine mammary carcinoma and malignant melanoma [9]. For the induction of Ms4a8a expression in these ...