A critical role for IL-10 in limiting inflammation during toxoplasmic encephalitis

https://doi.org/10.1016/j.jneuroim.2005.04.018Get rights and content

Abstract

IL-10 plays a vital role in controlling the inflammatory response during acute Toxoplasma gondii infection, however the production of IL-10 during the chronic phase of toxoplasmosis has been associated with parasite persistence. To address this paradox, the production and effect of IL-10 in the brain during toxoplasmic encephalitis (TE) was investigated. Analysis of brain mononuclear cells (BMNC) from chronically infected mice revealed that infiltrating macrophages and CD4+ T cells were the major sources of IL-10. Endogenous levels of IL-10 inhibited the production of IL-12, IFN-γ, TNF-α, and IL-6 from both hematopoetic and non-hematopoetic cells in the brain, as well as anti-microbial activity of astrocytes. Furthermore, IL-10-/- mice that progressed to the chronic phase of infection had equivalent parasite burden to WT mice but developed a lethal inflammatory response within the brain characterized by increased numbers of CD4+ T cells and macrophages, and elevated production of inflammatory cytokines. Finally, partial depletion of CD4+ T cells decreased the severity of the inflammation in the brain and allowed IL-10-/- mice to survive infection. Together these results point to a vital role for IL-10 in the control of CD4+ T cell mediated inflammation in the brain during TE.

Introduction

IL-10 is a pleiotropic cytokine produced by many cells including T and B lymphocytes, macrophages as well as certain subsets of dendritic cells. Since this cytokine can directly limit accessory cell production of pro-inflammatory cytokines and prevent the upregulation of co-stimulatory and class II molecules, it is an important modulator of innate and adaptive elements of cell mediated immunity (for review see Moore et al., 2001, de Waal Malefyt et al., 1991a, de Waal Malefyt et al., 1991b, Fiorentino et al., 1991, Berkman et al., 1995). The critical role of IL-10 as a negative regulator of inflammation is illustrated by studies which revealed that IL-10-/- mice spontaneously develop inflammatory bowel disease (IBD) in response to commensal bacteria (Kuhn et al., 1993). Within the context of infectious disease, IL-10 plays an important role in the balance between protective immunity and the development of immune pathology. For example, infection with the protozoan parasite Toxoplasma gondii is dominated by a strong type 1 response characterized by systemic levels of IL-12 and IFN-γ required for resistance to this pathogen. In the absence of IL-10, mice acutely infected with T. gondii develop a lethal CD4+ T cell mediated response distinguished by excessive levels of IL-12 and IFN-γ, large areas of necrosis and severe cellular infiltrate into multiple organs (Gazzinelli et al., 1996, Neyer et al., 1997, Suzuki et al., 2000, Wille et al., 2001).

While IL-10 has a prominent role in the prevention of immune hyperactivity, there are other situations in which this cytokine interferes with efficient effector responses and can promote susceptibility to certain intracellular pathogens. Thus, IL-10 can inhibit macrophage production of the anti-microbial effector molecule nitric oxide (NO) and antagonize the ability of activated macrophages to kill Leishmania major, Trypanosoma cruzi and T. gondii (Bogdan et al., 1991, Gazzinelli et al., 1992a, Oswald et al., 1992). In contrast, during infection with Leishmania spp. (Belkaid et al., 2001, Murphy et al., 2001, Padigel et al., 2003), T. cruzi (Silva et al., 1992, Reed et al., 1994) or Pneumocystis carinii (Qureshi et al., 2003), IL-10-/- mice have increased CD4+ T cell production of IFN-γ and improved macrophage activation that leads to an enhanced ability to control these pathogens. Strikingly, recent studies with L. major demonstrated that neutralization of IL-10 did not significantly alter Th1 responses but did result in sterile cure (Belkaid et al., 2001). These latter studies indicate that, in the context of chronic infection, the production of IL-10 is an important factor that can promote parasite persistence.

While much is known about the role of IL-10 during the acute stage of toxoplasmosis (Gazzinelli et al., 1992b, Gazzinelli et al., 1996, Candolfi et al., 1995, Neyer et al., 1997, Villegas et al., 2000, Wille et al., 2001, Wille et al., 2002) relatively little is known about its function during the chronic phase of infection. At this point there is a constant requirement for IFN-γ from both CD4+ and CD8+ T cells to prevent parasite reactivation, replication in the brain and the development of toxoplasmic encephalitis (TE) (Suzuki et al., 1989, Gazzinelli et al., 1992c). However, in certain mouse strains that are susceptible to TE, these animals develop protective T cell responses but parasite replication is not fully controlled which results in chronic inflammatory disease (Hunter et al., 1992, Brown et al., 1995). Previous studies from this laboratory and others have found an increase in levels of IL-10 in the brain during TE and associated the production of IL-10 in the CNS with susceptibility to TE (Hunter and Remington, 1994, Fischer et al., 1999). Furthermore, Deckert-Schulter and colleagues demonstrated that transient neutralization of IL-10 during chronic toxoplasmosis resulted in an increase in the number of T cells and the production of IFN-γ in the brain coupled with a decrease in parasite burden (Deckert-Schluter et al., 1997). These results suggested that during TE, the local production of IL-10 contributes to the continued presence of parasites. However, the studies presented here reveal that the absence or presence of IL-10 does not affect parasite burden. Instead, the major role for IL-10 during toxoplasmosis is to prevent T cell mediated immunopathology in the brains of chronically infected mice.

Section snippets

Mice and parasites

CBA/CaJ, Swiss Webster and C57BL/6 mice were obtained from Jackson laboratories (Bar Harbor, ME) and IL-10 -/- BL/6 mice were originally provided by DNAX (Palo Alto, CA). Mice were bred and maintained in specific pathogen free facilities in the Department of Pathobiology at the University of Pennsylvania. Experiments were performed on 6–8 week old female mice. The Me49 strain of T. gondii was maintained in infected Swiss Webster and CBA/CaJ mice and cysts were prepared from brains of donor mice

Production of IL-10 during TE

Previous studies have shown that IL-10 is produced in the brain during chronic Toxoplasma infection (Gazzinelli et al., 1993, Hunter and Remington, 1994, Burke et al., 1994, Deckert-Schluter et al., 1995, Schluter et al., 1997). To determine the source of IL-10 in the brain, BMNC from chronically infected C57BL/6 mice, 6 weeks p.i., were stained directly ex vivo for intracellular IL-10 (Fig. 1a–c). There was a distinct population of IL-10+ Mac-3+ cells (Fig. 1a) that likely contains both

Discussion

While several studies have identified a critical role for IL-10 in limiting infection-induced pathology during acute toxoplasmosis, other work indicates that the production of this cytokine during TE contributes to parasite persistence. In those latter reports, blockade of IL-10 resulted in a decrease in the number of cysts found in the brain (Deckert-Schluter et al., 1997) and treatment with IL-10 on days 10–20 post-infection led to increased cyst burden (Aliberti et al., 2002). Indeed, the

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    This work was supported by NIH grants AI41158 and the State of Pennsylvania.

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