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
