Endotoxin-like effects in acute phase response to Trypanosoma brucei brucei infection are not due to gastrointestinal leakage

The relationship of endotoxaemia to peripheral and central nervous system inflammatory responses in Human African Trypanosomiasis

Endotoxaemia has been described in cases of Human African trypanosomiasis (HAT), but it is unclear if this phenomenon influences inflammatory pathology either in the periphery or central nervous system (CNS). We studied endotoxin concentrations in the plasma and cerebrospinal fluid (CSF) of Trypanosoma brucei rhodesiense patients using the chromogenic Limulus Amoebocyte lysate assay. The relationship of endotoxin concentration to the presentation of gross signs of inflammation and the inflammatory/counter-inflammatory cytokine profile of the relevant compartments were analysed. We demonstrate that HAT patients exhibit parasitaemia-independent plasma endotoxaemia, and that this is associated with splenomegaly and lymphadenopathy. Endotoxin concentrations normalize rapidly after treatment. There was no evidence of endotoxin release in the CNS. A rapid normalization of endotoxin levels after treatment and lack of association with parasitaemia suggest that gut leakage is the main source of endotoxin in the circulation. Low CSF endotoxin concentrations and a lack of any association with neuroinflammatory markers or neurological sequelae suggest that endotoxin does not play a role in the pathogenesis of the disease in the CNS.

Tip-DC development during parasitic infection is regulated by IL-10 and requires CCL2/CCR2, IFN-gamma and MyD88 signaling

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-II^low) which is IFN-γ and MyD88 signaling independent; and (iii) a maturation step of inflammatory DCs to functional (CD80/CD86/MHC-II^high) 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.

Most infections are associated with host inflammatory responses that can result in multiple organ failure and death. It is therefore essential to understand the mechanisms balancing host immune response and tissue damage. Mouse models of African trypanosome infection represent valuable tools to study the mechanisms contributing to the inflammatory (pathogenic) or anti-inflammatory (anti-pathogenic) immune response. We recently identified TNF and NO producing DCs (Tip-DCs) as major contributors to liver pathogenicity in Trypanosoma brucei infected mice. Herein, the role of different chemokine and cytokines in the generation of Tip-DCs was investigated. Tip-DCs originated from bone marrow derived monocytes that egressed to the blood in a CCR2 chemokine receptor dependent manner. Then, monocytes extravasated to inflamed liver where IFN-γ and MyD88 signaling promoted their maturation to Tip-DCs. Both the egression of monocytes from bone marrow and their IFN-γ/MyD88 dependent maturation to Tip-DCs was counteracted by IL-10, hereby reducing liver pathogenicity. Liver injury, affecting millions of persons worldwide with often lethal consequences, frequently results from uncontrolled activation of recruited monocyte-derived cells that can be modulated by IL-10. Thus, the mechanisms regulating liver immunopathogenicity during parasitic infection identified herein could lead to new therapeutic policies in the field of hepatic inflammation.