Helminths are master regulators of host immune responses utilising complex mechanisms to dampen host protective Th2-type responses and favour long-term persistence. Such evasion mechanisms ensure mutual survival of both the parasite and the host. In this paper, we present recent findings on the cells that are targeted by helminths and the molecules and mechanisms that are induced during infection. We discuss the impact of these factors on the host response as well as their effect in preventing the development of aberrant allergic inflammation. We also examine recent findings on helminth-derived molecules that can be used as tools to pinpoint the underlying mechanisms of immune regulation or to determine new anti-inflammatory therapeutics.

BackgroundMonocytes and macrophages contribute to the dysfunction of immune responses in human filariasis. During patent infection monocytes encounter microfilariae in the blood, an event that occurs in asymptomatically infected filariasis patients that are immunologically hyporeactive.AimTo determine whether blood microfilariae directly act on blood monocytes and in vitro generated macrophages to induce a regulatory phenotype that interferes with innate and adaptive responses.Methodology and principal findingsMonocytes and in vitro generated macrophages from filaria non-endemic normal donors were stimulated in vitro with Brugia malayi microfilarial (Mf) lysate. We could show that monocytes stimulated with Mf lysate develop a defined regulatory phenotype, characterised by expression of the immunoregulatory markers IL-10 and PD-L1. Significantly, this regulatory phenotype was recapitulated in monocytes from Wuchereria bancrofti asymptomatically infected patients but not patients with pathology or endemic normals. Monocytes from non-endemic donors stimulated with Mf lysate directly inhibited CD4+ T cell proliferation and cytokine production (IFN-γ, IL-13 and IL-10). IFN-γ responses were restored by neutralising IL-10 or PD-1. Furthermore, macrophages stimulated with Mf lysate expressed high levels of IL-10 and had suppressed phagocytic abilities. Finally Mf lysate applied during the differentiation of macrophages in vitro interfered with macrophage abilities to respond to subsequent LPS stimulation in a selective manner.Conclusions and significanceConclusively, our study demonstrates that Mf lysate stimulation of monocytes from healthy donors in vitro induces a regulatory phenotype, characterized by expression of PD-L1 and IL-10. This phenotype is directly reflected in monocytes from filarial patients with asymptomatic infection but not patients with pathology or endemic normals. We suggest that suppression of T cell functions typically seen in lymphatic filariasis is caused by microfilaria-modulated monocytes in an IL-10-dependent manner. Together with suppression of macrophage innate responses, this may contribute to the overall down-regulation of immune responses observed in asymptomatically infected patients.

d Interleukin-27 (IL-27) is known to control primary CD4 ؉ T cell responses during a variety of different infections, but its role in regulating memory CD4؉ T responses has not been investigated in any model. In this study, we have examined the functional importance of IL-27 receptor (IL-27R) signaling in regulating the formation and maintenance of memory CD4 ؉ T cells following malaria infection and in controlling their subsequent reactivation during secondary parasite challenge. We demonstrate that although the primary effector/memory CD4 ؉ T cell response was greater in IL-27R-deficient (WSX-1 ؊/؊ ) mice following Plasmodium berghei NK65 infection than in wild-type (WT) mice, there were no significant differences in the size of the maintained memory CD4؉ T population(s) at 20 weeks postinfection in the spleen, liver, or bone marrow of WSX-1 ؊/؊ mice compared with WT mice. However, the composition of the memory CD4 ؉ T cell pool was slightly altered in WSX-1 ؊/؊ mice following clearance of primary malaria infection, with elevated numbers of late effector memory CD4 ؉ T cells in the spleen and liver and increased production of IL-2 in the spleen. Crucially, WSX-1 ؊/؊ mice displayed significantly enhanced parasite control compared with WT mice following rechallenge with homologous malaria parasites. Improved parasite control in WSX-1 ؊/؊ mice during secondary infection was associated with elevated systemic production of multiple inflammatory innate and adaptive cytokines and extremely rapid proliferation of antigen-experienced T cells in the liver. These data are the first to demonstrate that IL-27R signaling plays a role in regulating the magnitude and quality of secondary immune responses during rechallenge infections.

Helminths have evolved numerous pathways to prevent their expulsion or elimination from the host to ensure long-term survival. During infection, they target numerous host cells, including macrophages, to induce an alternatively activated phenotype, which aids elimination of infection, tissue repair, and wound healing. Multiple animal-based studies have demonstrated a significant reduction or complete reversal of disease by helminth infection, treatment with helminth products, or helminth-modulated macrophages in models of allergy, autoimmunity, and sepsis. Experimental studies of macrophage and helminth therapies are being translated into clinical benefits for patients undergoing transplantation and those with multiple sclerosis. Thus, helminths or helminth-modulated macrophages present great possibilities as therapeutic applications for inflammatory diseases in humans. Macrophage-based helminth therapies and the underlying mechanisms of their therapeutic or curative effects represent an under-researched area with the potential to open new avenues of treatment. This review explores the application of helminth-modulated macrophages as a new therapy for inflammatory diseases.

Helminths induce strong regulatory and T helper 2-type responses, whereby antibody-derived host protection and regulation are essential components. Lymphatic filariasis is an immune-mediated spectral disease that manifests as two main clinical outcomes: chronic pathology or asymptomatic infection. These outcomes depend on a multitude of factors, including parasite-induced immunoregulation and host genetic background; antibody responses contribute to this outcome. N-glycosylation of the Fc region of antibodies is a post-translational modification required for the structure and molecular function, influencing host inflammatory and regulatory responses. Altered IgG glycosylation correlates with disease, whereby decreased galactosylation is associated with inflammation while increased sialylation is associated with anti-inflammatory responses. We purified N-linked glycans from the Fc region of total IgG from Wuchereria bancrofti-infected patients characterizing the two clinical manifestations (chronic pathology and asymptomatic infection) and compared them to infection-free endemic normals. Using capillary electrophoresis, we found that there was no difference in galactosylation of total IgG between the three groups; however, asymptomatically infected patients had significantly lower levels of disialylated IgG compared to endemic normals and patients with pathology. These data suggest that while galactosylation does not contribute to disease outcome, sialylation may be involved in asymptomatic infection.

Lymphatic filariasis (LF) is a parasitic infection, caused by three closely related nematodes, namely Wuchereria bancrofti, Brugia malayi, and Brugia timori. Previously, we have shown that lysate from B. malayi microfilariae induces the expression of interleukin (IL)-10 and programmed death-ligand (PD-L) 1 on monocytes, which lead to inhibition of CD4 + T-cell responses. In this study, we investigated associations of IL-10 and programmed cell death (PD)-1 pathway gene polymorphisms with clinical manifestation in LF. We evaluated the frequency of alleles and genotypes of IL-10 (rs3024496, rs1800872), IL-10RA (rs3135932), IL-10RB (rs2834167), PD-1 (rs2227982, rs10204525), PD-L1 (rs4143815), PD-L2 (rs7854413), and single-nucleotide polymorphisms (SNPs) in 103 patients with chronic pathology (CP), such as elephantiasis or hydrocele and 106 endemic normal (EN) individuals from a South Indian population living in an area endemic for LF. Deviations from the Hardy-Weinberg equilibrium were tested, and we found a significant difference between the frequency of polymorphisms in PD-L2 (rs7854413; P < 0.001) and IL-10RB (rs2834167; P = 0.012) between the CP and the EN group, whereas there were no significant differences found among IL-10, IL-10RA, PD-1, and PD-L1 SNPs. A multivariate analysis showed that the existence of a CC genotype in PD-L2 SNP rs7854413 is associated with a higher risk of developing CP (OR: 2.942; 95% confidence interval [CI]: 0.957-9.046; P = 0.06). Altogether, these data indicate that a genetically determined individual difference in a non-synonymous missense SNP of PD-L2 might influence the susceptibility to CP.

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