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BACKGROUND & AIMS: Helminths induce strong regulatory and T helper 2-type responses by targeting host cells. In lymphatic filariasis, the host response to this, together with a multitude of other factors, determines whether a person remains infection and disease free or develops a successful infection. Infection results in either asymptomatic infection, which benefits transmission of the parasite, or chronic pathology, which is responsible of the high levels of morbidity seen in patients. Monocytes and macrophages contribute to helminthinduced dysfunction of the immune response through modulation by microfilariae in the blood and tissues. During patent infection monocytes encounter microfilariae in the blood, an event that occurs in asymptomatically infected patients who are immunologically hyporeactive. Furthermore helminths induce regulatory antibody responses that may impact on disease outcome. Other disease models have shown that altered glycosylation of the IgG Fc region correlates with pathology, whereby decreased galactosylation is associated with inflammation and increased sialylation is associated with anti-inflammatory responses. The aim of this project was to determine whether microfilariae act on blood monocytes and macrophages to induce a regulatory phenotype that interferes with innate and adaptive responses. Furthermore the IgG glycosylation profile of the different disease outcomes was compared with determine a role for glycosylation in lymphatic filariasis.
PRINCIPAL FINDINGS: Monocytes and in vitro generated macrophages from filaria nonendemic normal donors stimulated with Brugia malayi microfilarial (Mf) lysate but not adult female lysate show a drastically altered phenotype. Monocytes stimulated with Mf lysate develop a defined regulatory phenotype, characterised by expression of IL-10 and PD-L1. Importantly, this regulatory phenotype was recapitulated in monocytes from Wuchereria bancrofti asymptomatically infected individuals 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. CD4+ T cell 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 macrophage differentiation in vitro selectively interfered with macrophage abilities to respond to LPS stimulation. Additionally, Fc region N-linked glycans of total IgG from W. bancrofti-exposed donors were analysed. Using capillary electrophoresis it was found that there was no difference in galactosylation of total IgG between the different disease outcomes, however, asymptomatically infected patients had significantly lower levels of disialylated IgG compared with endemic normals and patients with pathology.
CONCLUSIONS & SIGNIFICANCE: Conclusively, this study demonstrates that Mf lysate stimulation of monocytes from healthy donors in vitro induces a regulatory phenotype, able to interfere with CD4+ T cell responses. This phenotype is directly reflected in monocytes from filarial patients with asymptomatic infection but not patients with pathology or endemic normals. The results suggest that suppression of T cell functions typically seen in lymphatic filariasis is caused by microfilaria-modulated monocytes in an IL-10- or PD-1-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.