Down-Regulated Lymphoproliferation Coincides with Parasite Maturation and with the Collapse of Both Gamma Interferon and Interleukin-4 Responses in a Bovine Model of Onchocerciasis

105

Both T cells and APC have been strongly implicated in the immune suppression observed during filarial nematode infections, but their relative roles are poorly understood, particularly in regard to timing and locality of action. Using Litomosoides sigmodontis infection of susceptible BALB/c mice, we have studied the progression of filarial immunosuppression leading to patent infection with blood microfilaremia. Patent infection is associated with decreased immune responsiveness in the draining thoracic lymph nodes (tLN) and intrinsically hyporesponsive CD4+ T cells at the infection site. We now show that we are able to separate, both in time and space, different suppressive mechanisms and cell populations that contribute to filarial hyporesponsiveness. L. sigmodontis infection recruited a F4/80+ population of alternatively activated macrophages that potently inhibited Ag-specific CD4+ T cell proliferative responses even in the presence of competent naive APC. T cell responsiveness was partially restored by neutralizing TGF-β, but not by blocking IL-10 or CTLA-4 signaling. During prepatent infection, the macrophage population was restricted to the infection site. However, once infection became patent with systemic release of microfilariae, the suppressive macrophage activity extended peripherally into the tLN. In contrast, the hyporesponsive CD4+ T cell phenotype remained localized at the infection site, and the tLN CD4+ T cell population recovered full Ag responsiveness in the absence of suppressive macrophages. Filarial immunosuppression, therefore, evolves over time at sites increasingly distal to infection, and the mechanisms of filarial down-regulation are dependent on proximity to the infection site.

supporting

confidence: 66%

F4/80+ Alternatively Activated Macrophages Control CD4+ T Cell Hyporesponsiveness at Sites Peripheral to Filarial Infection

Taylor

Harris

Nair

et al. 2006

105

“…It is known from kinetic studies in natural hostpathogen relationships that cytokine responses tend to fluctuate over time, resulting in periods of polarization and nonpolarization (36). In bovine onchocerciasis, the initial cytokine response in cattle is the elevation of IFN-␥ preceding the elevation of IL-4 (37). Similarly, in an experimental model of loiasis, induction of IL-12 was shown to precede induction of IL-10 (38).…”

Section: Discussionmentioning

confidence: 99%

Proinflammatory Cytokines Dominate the Early Immune Response to Filarial Parasites

Babu

Nutman

2003

Although the early human immune response to the infective-stage larvae (L3) of Brugia malayi has not been well-characterized in vivo (because of the inability to determine the precise time of infection), the consensus has been that it must involve a predominant Th2 environment. We have set up an in vitro system to study this early immune response by culturing PBMC from unexposed individuals with live L3 of B. malayi. After 24 h of culture, T cell responses were examined by flow cytometry and by quantitative real-time RT-PCR for multiple cytokines. T cells were activated early following exposure to L3 as indicated by up-regulation of surface markers CD69 and CD71. The frequency of T cells expressing proinflammatory Th1 cytokines (IFN-γ, TNF-α, GM-CSF, IL-1α, and IL-8) but not Th2 cytokines (IL-4, IL-5, IL-6, IL-10, and IL-13) was significantly increased in response to L3. This T cell response occurred in both the CD4 and CD8 T cell compartment and was restricted to the effector/memory pool (CD45RO+). This T cell response was not due to LPS activity from the parasite or from its endosymbiont, Wolbachia; moreover, it required the presence of APC as well as direct contact with live L3. Real-time RT-PCR analysis of multiple cytokines in the T cells confirmed the increased expression of proinflammatory Th1 cytokines. Up-regulation of these cytokines suggests that the primary immune response to the live infective stage of the parasite is not predominantly Th2 in nature but rather dominated by a proinflammatory response.

“…In onchocerciasis or "river blindness," caused by O. volvulus, inflammatory responses elicited by the death of larval microfilaria passing through the cornea decreases visual acuity that is manifest in its most severe form by blindness. The mechanisms underlying the inflammation induced by filarial infection are poorly understood, but studies of humans with natural infection and experimental animals exposed to live filaria or soluble extracts of the organisms have implicated a role for both adaptive and innate immunity (1)(2)(3). The remarkable observation that W. bancrofti, B. malayi, and O. volvulus harbor endosymbiotic Wolbachia bacteria (4 -6) has raised the possibility that infected individuals respond to these organisms in addition to nematode ligands and that Wolbachia can initiate or promote inflammation (7)(8)(9)(10)(11)(12).…”

mentioning

confidence: 99%

“…The remarkable observation that W. bancrofti, B. malayi, and O. volvulus harbor endosymbiotic Wolbachia bacteria (4 -6) has raised the possibility that infected individuals respond to these organisms in addition to nematode ligands and that Wolbachia can initiate or promote inflammation (7)(8)(9)(10)(11)(12). Despite remarkable longevity in the human host (13), adult filarial worms themselves are not strong inducers of inflammation (3,14). Wolbachia bacteria are in the family Rickettsiacea and are obligatory intracellular bacteria that infect a wide variety of invertebrates, in which they have an important role in sex determination and speciation (15).…”

mentioning

confidence: 99%

Innate Immune Responses to Endosymbiotic Wolbachia Bacteria in Brugia malayi and Onchocerca volvulus Are Dependent on TLR2, TLR6, MyD88, and Mal, but Not TLR4, TRIF, or TRAM

Hise

Daehnel

Gillette‐Ferguson

et al. 2007

Self Cite

106

The discovery that endosymbiotic Wolbachia bacteria play an important role in the pathophysiology of diseases caused by filarial nematodes, including lymphatic filariasis and onchocerciasis (river blindness) has transformed our approach to these disabling diseases. Because these parasites infect hundreds of millions of individuals worldwide, understanding host factors involved in the pathogenesis of filarial-induced diseases is paramount. However, the role of early innate responses to filarial and Wolbachia ligands in the development of filarial diseases has not been fully elucidated. To determine the role of TLRs, we used cell lines transfected with human TLRs and macrophages from TLR and adaptor molecule-deficient mice and evaluated macrophage recruitment in vivo. Extracts of Brugia malayi and Onchocerca volvulus, which contain Wolbachia, directly stimulated human embryonic kidney cells expressing TLR2, but not TLR3 or TLR4. Wolbachia containing filarial extracts stimulated cytokine production in macrophages from C57BL/6 and TLR4−/− mice, but not from TLR2−/− or TLR6−/− mice. Similarly, macrophages from mice deficient in adaptor molecules Toll/IL-1R domain-containing adaptor-inducing IFN-β and Toll/IL-1R domain-containing adaptor-inducing IFN-β-related adaptor molecule produced equivalent cytokines as wild-type cells, whereas responses were absent in macrophages from MyD88−/− and Toll/IL-1R domain-containing adaptor protein (TIRAP)/MyD88 adaptor-like (Mal) deficient mice. Isolated Wolbachia bacteria demonstrated similar TLR and adaptor molecule requirements. In vivo, macrophage migration to the cornea in response to filarial extracts containing Wolbachia was dependent on TLR2 but not TLR4. These results establish that the innate inflammatory pathways activated by endosymbiotic Wolbachia in B. malayi and O. volvulus filaria are dependent on TLR2-TLR6 interactions and are mediated by adaptor molecules MyD88 and TIRAP/Mal.