Modulation of innate cells by helminth infection and helminth derived products

Abstract

Helminth infection affects around a quarter of people worldwide, with no effective vaccines available. Future vaccines against helminth infection will require a more precise understanding of the cellular and molecular basis of protective immunity. In addition, it is notable that the prevalence of allergic and autoimmune diseases has increased, whilst that of helminths infections has reduced. This suggested that immune responses are dampened through direct immunomodulation by helminths infections or their excretory secretory products.

Based on initial observations that Heligmosomoides polygyrus excretory secretory products (HES) can improve disease scores in a chronic T cell induced colitis, we explored the role of (HES) in an innate RAG-/- CD40 colitis. We found that HES did not affect inflammatory scores and disease activity in this model of colitis, however reduced the infiltration of inflammatory cells into the peritoneum.

Immunity to intestinal helminth Nippostrongylus brasiliensis and H. polygyrus requires innate and adaptive mechanisms co-ordinated through the Type 2 IL-4R/STAT6-dependent pathway. We have now found that macrophage migration inhibitory factor (MIF) is also essential for development of immunity to infection. MIF-deficient mice are slower to expel N. brasiliensis, while in wildtype animals, the expression of MIF is upregulated in macrophages in response to infection.

Cellular analyses in the MIF-deficient mice demonstrate reduced recruitment of innate lymphoid cells, eosinophils and alternatively activated macrophages. Type 2 epithelial responses were reduced in the mice showing reduced tuft cell hyperplasia and almost absent RELM-ß protein in goblet cells.

In order to assess if this was a developmental abnormality, we administered 4-IPP, an inhibitor of MIF to infected wild type mice. Mice receiving 4-IPP were unable to expel parasites and demonstrated similar cellular and epithelial responses as the MIF-deficient mice. IL-25 has been shown to accelerate expulsion of N.brasiliensis via the recruitment of ILC2s.

Administration of rIL-25 is able to completely rescue the MIF-deficient cellular and epithelial cell phenotype. The ligands for MIF are hypothesised to be CXCR2, CXCR4 and CD74. We demonstrate that ILCs and macrophages express CXCR4. CXCR2-deficiency did not result in the epithelial cell phenotype, therefore it is unlikely that MIF is acting via CXCR2 in the gut. A deficiency of CXCR2 however, altered the immune response to N. brasiliensis in the lung with reduced alternative activation of macrophages.

In parallel, we assessed the immune responses in H. polygyrus. From previous work, we know that MIF-deficient mice are less able to expel H. polygyrus primary infection, and in addition, do not mount protective secondary immune responses or protective responses to immunisation with HES. We found no difference in the percentage of Foxp3 positive T regulatory cells or HES specific antibody levels. As in the N. brasiliensis model, MIF-deficient mice produced fewer alternatively activated macrophages confirming a defect in the innate immune compartment. A microarray had previously been performed comparing BALB/c and MIF5 deficient duodenum, finding genes arl2bp, phc2 and s100a8 being downregulated in the MIF-deficient mice. In order to assess the role of S100A8 deficiency in helminths infections, we infected s100a9-/- mice in which the A8/A9 complex cannot form. We found no difference in the primary or secondary clearance of H. polygyrus suggesting that S100A8 is not important in the pathogenesis of helminths infection. ARL2BP is known to be important for STAT3 nuclear retention. We assessed STAT6 and STAT3 phosphorylation and found no difference between the BALB/c and MIF-deficient mice in phosphorylation of STAT3/6.

We conclude that in Type 2 infection, MIF plays an important role in the protective Type 2 response, potentially at two levels: firstly in activation of ILCs in a manner which is upstream of, and rescued by, IL-25; and secondly in promoting alternative activation of macrophages in synergy with IL-4.