Signaling through Galphai2 protein is required for recruitment of neutrophils for antibody-mediated elimination of larval Strongyloides stercoralis in mice

Strongyloides infection in rodents: immune response and immune regulation

The human pathogenic nematode Strongyloides stercoralis infects approximately 30-100 million people worldwide. Analysis of the adaptive immune response to S. stercoralis beyond descriptive studies is challenging, as no murine model for the complete infection cycle is available. However, the combined employment of different models each capable of modelling some features of S. stercoralis life cycle and pathology has advanced our understanding of the immunological mechanisms involved in host defence. Here we review: (i) studies using S. stercoralis third stage larvae implanted in diffusion chambers in the subcutaneous tissue of mice that allow analysis of the immune response to the human pathogenic Strongyloides species; (ii) studies using Strongyloides ratti and Strongyloides venezuelensis that infect mice and rats to extend the analysis to the parasites intestinal life stage and (iii) studies using S. stercoralis infected gerbils to analyse the hyperinfection syndrome, a severe complication of human strongyloidiasis that is not induced by rodent specific Strongyloides spp. We provide an overview of the information accumulated so far showing that Strongyloides spp. elicits a classical Th2 response that culminates in different, site specific, effector functions leading to either entrapment and killing of larvae in the tissues or expulsion of parasitic adults from the intestine.

Strongyloidiasis with emphasis on human infections and its different clinical forms

Strongyloidiasis (caused by Strongyloides stercoralis, and to a lesser extent by Strongyloides fuelleborni) is one of the most neglected tropical diseases with endemic areas and affecting more than 100 million people worldwide. Chronic infections in endemic areas can be maintained for decades through the autoinfective cycle with the L3 filariform larvae. In these endemic areas, misdiagnosis, inadequate treatment and the facilitation of the hyperinfection syndrome by immunosuppression are frequent and contribute to a high mortality rate. Despite the serious health impact of strongyloidiasis, it is a neglected disease and very little is known about this parasite and the disease when compared to other helminth infections. Control of the disease is difficult because of the many gaps in our knowledge of strongyloidiasis. We examine the recent literature on different aspects of strongyloidiasis with emphasis in those aspects that need further research.

Protection and pathology during parasite infection: IL-10 strikes the balance

The host response to infection requires an immune response to be strong enough to control the pathogen but also restrained, to minimize immune-mediated pathology. The conflicting pressures of immune activation and immune suppression are particularly apparent in parasite infections, where co-evolution of host and pathogen has selected many different compromises between protection and pathology. Cytokine signals are critical determinants of both protective immunity and immunopathology, and, in this review, we focus on the regulatory cytokine IL-10 and its role in protozoan and helminth infections. We discuss the sources and targets of IL-10 during parasite infection, the signals that initiate and reinforce its action, and its impact on the invading parasite, on the host tissue, and on coincident immune responses.

Extracellular traps are associated with human and mouse neutrophil and macrophage mediated killing of larval Strongyloides stercoralis

Neutrophils are multifaceted cells that are often the immune system's first line of defense. Human and murine cells release extracellular DNA traps (ETs) in response to several pathogens and diseases. Neutrophil extracellular trap (NET) formation is crucial to trapping and killing extracellular pathogens. Aside from neutrophils, macrophages and eosinophils also release ETs. We hypothesized that ETs serve as a mechanism of ensnaring the large and highly motile helminth parasite Strongyloides stercoralis thereby providing a static target for the immune response. We demonstrated that S. stercoralis larvae trigger the release of ETs by human neutrophils and macrophages. Analysis of NETs revealed that NETs trapped but did not kill larvae. Induction of NETs was essential for larval killing by human but not murine neutrophils and macrophages in vitro. In mice, extracellular traps were induced following infection with S. stercoralis larvae and were present in the microenvironment of worms being killed in vivo. These findings demonstrate that NETs ensnare the parasite facilitating larval killing by cells of the immune system.

Parasite-antigen driven expansion of IL-5(-) and IL-5(+) Th2 human subpopulations in lymphatic filariasis and their differential dependence on IL-10 and TGFβ

BACKGROUND

Two different Th2 subsets have been defined recently on the basis of IL-5 expression - an IL-5(+)Th2 subset and an IL-5(-)Th2 subset in the setting of allergy. However, the role of these newly described CD4(+) T cells subpopulations has not been explored in other contexts.

METHODS

To study the role of the Th2 subpopulation in a chronic, tissue invasive parasitic infection (lymphatic filariasis), we examined the frequency of IL-5(+)IL-4(+)IL-13(+) CD4(+) T cells and IL-5(-)IL-4 IL-13(+) CD4(+) T cells in asymptomatic, infected individuals (INF) and compared them to frequencies (Fo) in filarial-uninfected (UN) individuals and to those with filarial lymphedema (CP).

RESULTS

INF individuals exhibited a significant increase in the spontaneously expressed and antigen-induced Fo of both Th2 subpopulations compared to the UN and CP. Interestingly, there was a positive correlation between the Fo of IL-5(+)Th2 cells and the absolute eosinophil and neutrophil counts; in addition there was a positive correlation between the frequency of the CD4(+)IL-5(-)Th2 subpopulation and the levels of parasite antigen - specific IgE and IgG4 in INF individuals. Moreover, blockade of IL-10 and/or TGFβ demonstrated that each of these 2 regulatory cytokines exert opposite effects on the different Th2 subsets. Finally, in those INF individuals cured of infection by anti-filarial therapy, there was a significantly decreased Fo of both Th2 subsets.

CONCLUSIONS

Our findings suggest that both IL-5(+) and IL-5(-)Th2 cells play an important role in the regulation of immune responses in filarial infection and that these two Th2 subpopulations may be regulated by different cytokine-receptor mediated processes.

Human and mouse macrophages collaborate with neutrophils to kill larval Strongyloides stercoralis

Macrophages are multifunctional cells that are active in TH1- and TH2-mediated responses. In this study, we demonstrate that human and mouse macrophages collaborate with neutrophils and complement to kill the parasite Strongyloides stercoralis in vitro. Infection of mice with worms resulted in the induction of alternatively activated macrophages (AAM) within the peritoneal cavity. These cells killed the worms in vivo and collaborated with neutrophils and complement during the in vitro killing process. AAM generated in vitro killed larvae more rapidly than naive macrophages, which killed larvae after a longer time period. In contrast, classically activated macrophages were unable to kill larvae either in vitro or in vivo. This study adds macrophages to the armamentarium of immune components that function in elimination of parasitic helminths and demonstrate a novel function by which AAM control large extracellular parasites.

The regulating function of heterotrimeric G proteins in the immune system

Heterotrimeric guanine nucleotide-binding proteins (G proteins), which consist of an α-, a β- and a γ-subunit, have crucial roles as molecular switches in the regulation of the downstream effector molecules of multiple G protein-coupled receptor signalling pathways, such as phospholipase C and adenylyl cyclase. According to the structural and functional similarities of their α-subunits, G proteins can be divided into four subfamilies: Gαs, Gαi/o, Gαq/11 and Gα12/13. Most of the α- and the βγ-subunits are abundantly expressed on the surface of immune cells. Recent studies have demonstrated that G proteins are a group of important immunomodulatory factors that regulate the migration, activation, survival, proliferation, differentiation and cytokine secretion of immune cells. In this review, we summarise the recent findings on the functions of G proteins in immune regulation and autoimmunity.

Innate and adaptive immunity to the nematode Strongyloides stercoralis in a mouse model

Mice have been used to the study the mechanisms of protective innate and adaptive immunity to larval Strongyloides stercoralis. During primary infection, neutrophils and eosinophils are attracted by parasite components and kill the larvae by release of granule products. Eosinophils also function as antigen-presenting cells for the induction of a Th2 response. B cells produce both IgM and IgG that collaborate with neutrophils to kill worms in the adaptive immune response. Vaccine studies have identified a recombinant diagnostic antigen that induced high levels of immunity to infection with S. stercoralis in mice. These studies demonstrate that there are redundancies in the mechanisms used by the immune response to kill the parasite and that a vaccine with a single antigen may be suitable as a prophylactic vaccine to prevent human strongyloidiasis.

Diversity and dialogue in immunity to helminths

The vertebrate immune system has evolved in concert with a broad range of infectious agents, including ubiquitous helminth (worm) parasites. The constant pressure of helminth infections has been a powerful force in shaping not only how immunity is initiated and maintained, but also how the body self-regulates and controls untoward immune responses to minimize overall harm. In this Review, we discuss recent advances in defining the immune cell types and molecules that are mobilized in response to helminth infection. Finally, we more broadly consider how these immunological players are blended and regulated in order to accommodate persistent infection or to mount a vigorous protective response and achieve sterile immunity.

Major basic protein from eosinophils and myeloperoxidase from neutrophils are required for protective immunity to Strongyloides stercoralis in mice

Eosinophils and neutrophils contribute to larval killing during the primary immune response, and neutrophils are effector cells in the secondary response to Strongyloides stercoralis in mice. The objective of this study was to determine the molecular mechanisms used by eosinophils and neutrophils to control infections with S. stercoralis. Using mice deficient in the eosinophil granule products major basic protein (MBP) and eosinophil peroxidase (EPO), it was determined that eosinophils kill the larvae through an MBP-dependent mechanism in the primary immune response if other effector cells are absent. Infecting PHIL mice, which are eosinophil deficient, with S. stercoralis resulted in development of primary and secondary immune responses that were similar to those of wild-type mice, suggesting that eosinophils are not an absolute requirement for larval killing or development of secondary immunity. Treating PHIL mice with a neutrophil-depleting antibody resulted in a significant impairment in larval killing. Naïve and immunized mice with neutrophils deficient in myeloperoxidase (MPO) infected with S. stercoralis had significantly decreased larval killing. It was concluded that there is redundancy in the primary immune response, with eosinophils killing the larvae through an MBP-dependent mechanism and neutrophils killing the worms through an MPO-dependent mechanism. Eosinophils are not required for the development or function of secondary immunity, but MPO from neutrophils is required for protective secondary immunity.

Soluble extract from the nematode Strongyloides stercoralis induces CXCR2 dependent/IL-17 independent neutrophil recruitment

Neutrophil recruitment via CXCR2 is required for innate and adaptive protective immunity to the larvae of Strongyloides stercoralis in mice. The goal of the present study was to determine the mechanism of CXCR2-mediated neutrophil recruitment to S. stercoralis. Mice deficient in the receptor for IL-17A and IL-17F, upstream mediators of CXCR2 ligand production, were infected with S. stercoralis larvae; there was no difference in larval survival, neutrophil recruitment, or production of CXCR2 ligands compared with wild type mice. In vivo and in vitro stimulation of neutrophils with S. stercoralis soluble extract resulted in significant neutrophil recruitment. In vitro assays demonstrated that the recruitment functioned through both chemokinesis and chemotaxis, was specific for CXCR2, and was a G protein-coupled response involving tyrosine kinase and PI3K. Finally, neutrophil stimulation with S. stercoralis soluble extract induced release of the CXCR2 ligands MIP-2 and KC from neutrophils, thereby potentially enhancing neutrophil recruitment.

Granulocytes: effector cells or immunomodulators in the immune response to helminth infection?

Granulocytes are effector cells in defence against helminth infections. We review the current evidence for the role of granulocytes in protective immunity against different helminth infections and note that for each parasite species the role of granulocytes as effector cells can vary. Emerging evidence also points to granulocytes as immunomodulatory cells able to produce many cytokines, chemokines and modulatory factors which can bias the immune response in a particular direction. Thus, the role of granulocytes in an immunomodulatory context is discussed including the most recent data that points to an important role for basophils under this guise.

Characterisation of effector mechanisms at the host:parasite interface during the immune response to tissue-dwelling intestinal nematode parasites

The protective immune response that develops following infection with many tissue-dwelling intestinal nematode parasites is characterised by elevations in IL-4 and IL-13 and increased numbers of CD4+ T cells, granulocytes and macrophages. These cells accumulate at the site of infection and in many cases can mediate resistance to these large multicellular pathogens. Recent studies suggest novel potential mechanisms mediated by these immune cell populations through their differential activation and ability to stimulate production of novel effector molecules. These newly discovered protective mechanisms may provide novel strategies to develop immunotherapies and vaccines against this group of pathogens. In this review, we will examine recent studies elucidating mechanisms of host protection against three widely-used experimental murine models of tissue-dwelling intestinal nematode parasites: Heligmosomoides polygyrus, Trichuris muris and Trichinella spiralis.