Filarial infection and filarial antigen administration promotes glucose tolerance in diet-induced obese mice

Abstract

Excess of energy intake combined with reduced physical activity leads to accumulation and expansion of adipose tissue. Imbalance between adipose tissue expansion and oxygenation during a high fat diet results in adipocytes stress and defects to store excessive energy. Pro-inflammatory mediators produced by stressed adipocytes and infiltrated classically activated macrophages eventually trigger low grade and chronic inflammation. Several studies highlighted that obesity-induced chronic inflammation is a critical factor that triggers insulin resistance and alters the cellular composition within the adipose tissue.<br /> Given that parasitic helminths are well known immunoregulators of host immune responses which induce a suppressive, regulatory immune response via the induction of regulatory T cells, AAM, anti-inflammatory cytokines, and induce a type 2 immune response, the aim of this thesis was to investigate whether the tissue-invasive rodent filarial nematode <i>Litomosoides sigmodontis</i> (<i>L.s.</i>) mediates protection against insulin-resistance in diet-induced obese (DIO) mice by counter-regulating inflammatory immune responses during a high fat diet.<br /> In order to study whether <i>L.s.</i> infection has a beneficial impact on high fat diet-induced insulin resistance, 6 week old male BALB/c mice were fed with a high fat diet and a subgroup was infected 2-4 weeks later with <i>L.s.</i>. Following 8-10 weeks on high fat diet, mice were evaluated for glucose tolerance and immune responses. In separate experiments, daily injections of LsAg for 2 weeks were performed in male DIO C57BL/6 mice after 7-12 weeks of high fat diet feeding. DIO mice were evaluated for glucose tolerance and immunological studies afterwards.<br /> This thesis demonstrates that both <i>L.s.</i> infection and LsAg administration improved glucose tolerance in DIO mice. This improvement was associated with increased eosinophil and AAM frequencies within the stromal vascular fraction of the epididymal adipose tissue (EAT) during <i>L.s.</i> infection and LsAg administration. Absence of eosinophils abrogated the beneficial impact of <i>L.s.</i> infection as was shown with eosinophil deficient dblGATA mice, suggesting that improved glucose tolerance by <i>L.s.</i> infection was dependent on eosinophils.<br /> Further analysis showed reduced total numbers of B cells, but an increased frequency of the B1 subset in the adipose tissue of <i>L.s.</i>-infected DIO mice compared to uninfected DIO controls. Accordingly, pathogenic IgG2a/b levels were lower in <i>L.s.</i>-infected animals compared to uninfected DIO controls. qPCR array analysis of EAT further revealed an induction of genes related to insulin signaling, cell migration, suppressive immune responses as well as a reduced expression of genes related to adipogenesis in <i>L.s.</i>-infected DIO mice. Our in vitro experiments using the 3T3-L1 pre-adipose cell line confirmed that LsAg treatment suppressed the differentiation to mature adipocytes.<br /> Multiple gene expression analysis of EAT from DIO mice that obtained LsAg administrations further revealed an induction of type 2 immune responses, as well as an upregulated expression of genes-related to insulin signalling and genes-related to fatty acid uptake in LsAg-treated DIO mice. Two weeks of daily LsAg administration in DIO mice further improved body temperature tolerance under cold exposure, which was accompanied by an increased expression of <i>Ucp1</i> in EAT, suggesting that LsAg administration promotes browning of white adipose tissue and increased energy expenditure.<br /> In conclusion, this thesis demonstrates that both L.s. infection and LsAg administration reduces diet-induced EAT inflammation, improves insulin signaling, and glucose tolerance. The findings of this thesis suggest that helminth-derived products may offer a new strategy to ameliorate diet-induced insulin resistance.