The role of T helper 17 cells in the Pathogenesis of Human Onchocerciasis

Abstract

Onchocerciasis, also known as river blindness, is a tropical disease causing health and socioeconomic problems in endemic communities, especially sub-Saharan Africa. The disease is elicited by a filarial parasite called <em>Onchocerca volvulus</em>, which is transmitted by the bite of infected blackflies (<em>Simulium</em>). Characteristic disease symptoms include dermatological disorders and eye lesions that can lead to blindness. Two polar forms of clinical manifestations can occur: generalized onchocerciasis (GEO), presenting mild skin disease or the hyperreactive form (HO) which exhibits severe skin disorders and inflammation. Globally, 37 million individuals are estimated to be infected. The intensity of immune reactions against the parasite is believed to influence the clinical outcomes of the disease and the immune determinants that drive tolerance or pathology are still not fully characterized. Th17 cells are a distinct subpopulation of helper T cells with potent inflammatory properties and are characterized by their predominant production of interleukin (IL)-17 (IL-17A and IL-17F). These cells are implicated in several auto-immune and inflammatory diseases. Commitment into Th17 cells or regulatory T cells (Treg) required almost the same pathway since both need TGF-β. IL-6 is one of the innate cytokines that determine the balance Th17/Treg. The present study, investigated the role of Th17 cells and the way the balance of Th17/Treg influences the outcome of clinical manifestations during human onchocerciasis. Moreover, the mechanisms underlying the plasticity between a regulatory phenotype and an inflammatory Th17 response was studied. Using a flow cytometry approach, this study investigated the expression patterns of Th1, Th2, Th17, Treg markers and other immune cells such as B cells, monocytes, CD8 and NK cells in peripheral blood mononuclear cells (PBMCs) from HO and GEO patients to putative immune/endemic normals (EN). The findings indicated that HO individuals exhibit strong Th2 and Th17 responses, as reflected by higher expression of IL-4, GATA3, and IL-17A, RORC2 expressing CD4 T cells when compared to either GEO or EN. Based on the enhanced numbers of IL-10-expressing CD4^⁺ T cells and CD4⁺CD25⁺Foxp3⁺ Treg, the work also showed that patients with GEO phenotype exhibited a regulatory milieu. EN individuals on the other hand showed prominent Th1 (CD4⁺IFN-γ⁺ T cells). Flow cytometry data was further confirmed using a Th17 based PCR array. Th17-related genes such as IL-17 cytokine family members as well as IL-6, IL-1β and IL-22, Th17 transcription factor and signalling pathway molecules (RORC2 and STAT3) and Th2-related (IL-4, IL-13, STAT6) genes were all significantly up-regulated in HO individuals. Moreover, using <em>in vitro</em> cell cultures of PBMCs, this study further demonstrated that whereas <em>Onchocerca volvulus</em>-specific Th1 responses were increased in cultures from EN, a strong Th2 phenotype was observed in cultures from HO individuals due to elevated levels of filarial-specific IL-5 and IL-13. To further investigate the mechanism underlying the differences between GEO and HO, an <em>in vitro</em> Th17 cell polarization model was performed and provided initial evidence that the commitment to a Th17 lineage could be modulated in PBMCs from GEO individuals by the addition of innate receptor triggering. Interestingly, it was demonstrated that products of innate stimulation, such us IL-6 and IL-1β, could also modify the GEO phenotype into one reflecting that from HO individuals. For the first time, this study provides evidence that elevated frequencies of Th2 and Th17 cells and the balance of Th17/Treg form part of the immune network instigating the development of severe pathology in human onchocerciasis. At the end a mechanism elucidating possible pathways that drive GEO and HO phenotype was proposed.