Concepts of Immune Regulation in Chronic Filarial Infections

Abstract

Despite worldwide efforts to reduce filariasis, over 100 million individuals still suffer from filarial infections. Filarial infections continue to be a substantial drain at both economic and health levels, especially in rural endemic communities. Part of the problem is the longevity of filariae in man, the different life-stages that the filariae pass through in the host and the number of organs and tissues that are affected. Thus, researchers continue to unravel the complex host:filarial interactions that allow such chronic manifestations. These interactions resulted from an evolutionary process since it is important for the filariae´s own survival to evade the host´s immune responses without killing the host. Using the rodent model of filariasis <em>Litomosoides sigmodontis</em>, immune reactions of the host towards the parasite can be analysed in a controlled setting. Interestingly, this nematode can only develop a patent (release of microfilariae (Mf)) infection in immune-component BALB/c mice since in C57BL/6 mice infections are cleared 40 days post infection. These models have been used to generate a plethora of data showing that control of Mf and worm burden depends on CD4⁺ T and B1 cells, granulocytes, chemokines, granzymes, alternatively activated macrophages, IL-10, Th1 and Th2 cytokines as well as Th17 cytokines indicating that both innate and adaptive immune responses interact to mediate helminth control. Experiments in this thesis work addressed three questions using filarial/ host-specific interactions. The first experiments expanded on previous data showing unique filarial-specific responses from patent and latent <em>L. sigmodontis</em>-infected BALB/c mice. Here, cytokine release was monitored by ELISA in cultures of isolated CD4⁺ T cells from early and late infected mice using soluble or pellet-derived antigens prepared from total, female or male adult filariae. Interestingly, IL-13, IFN-&gamma; and IL-10 responses were absent in CD4⁺ T cell cultures of early-infected mice regardless of antigen source. Developing adaptive immune responses are strongly influenced by innate signalling pathways since earlier studies have noted toll-like receptor (TLR)-triggering by filarial-derived components and infections in TLR-deficient BALB/c strains presented changed parasitology and host immunity. Investigations in the second part of this work addressed whether lack of TLR-signalling, especially central adaptor molecules Myd88 and TRIF, had an impact on < <em>L. sigmodontis</em> infections in C57BL/6 mice. Therefore, C57BL/6 mice deficient for TLR3, TLR4, TRIF, MyD88, IRF1 and IRF3 were analysed at different time points of infection. Interestingly, worm numbers in mice deficient for TRIF were significantly increased at day 30 post infection and the recruitment of CD4⁺ T cells to the site of infection was reduced. Therefore, our results indicate that the adaptor molecule TRIF plays a key role in worm development. <br /> In the final part of this thesis the RAG2/IL-2R&gamma;-deficient mouse strain (lacks T, B and NK cells) was used to analyse the impact of T cells on worm clearance during <em>L. sigmodontis</em> infection and additionally characterise the phenotype of infection-induced T cells using advanced flow cytometry. Previous studies showed that in contrast to WT mice <em>L. sigmodontis</em> infections in RAG2/IL-2R&gamma;-deficient mice resulted in patent infections with extremely high worm and Mf numbers. Expanding on those data, adoptive transfers of CD4⁺ or CD8⁺ T cells from WT C57BL/6 donor mice into RAG2/IL-2R&gamma;-deficient recipients did not alter worm burden per se but did result in reduced adult worm length and in the case of CD4⁺ T cells reduced fecundity as well. Preliminary findings show that these effects are not mediated by IL-4 or IL-10. However, the transfer of CD4⁺ T cells from <em>L. sigmodontis</em>-infected donor C57BL/6 mice additionally resulted in significantly reduced worm numbers in RAG2/IL-2R&gamma;-deficient recipients. Thus, these data enhance our understanding of immune networks and interplay during infection since although "educated" CD4⁺ T cells control worm fecundity, further host components are required to restore the phenotype of <em>L. sigmodontis</em>-infected WT C57BL/6 mice.