IL-33-binding HpARI family homologues with divergent effects in suppressing or enhancing type 2 immune responses

HpARI is an immunomodulatory protein secreted by the intestinal nematode Heligmosomoides polygyrus bakeri, which binds and blocks IL-33. Here, we find that the H. polygyrus bakeri genome contains three HpARI family members and that these have different effects on IL-33-dependent responses in vitro and in vivo, with HpARI1+2 suppressing and HpARI3 amplifying these responses. All HpARIs have sub-nanomolar affinity for mouse IL-33; however, HpARI3 does not block IL-33-ST2 interactions. Instead, HpARI3 stabilizes IL-33, increasing the half-life of the cytokine and amplifying responses to it in vivo. Together, these data show that H. polygyrus bakeri secretes a family of HpARI proteins with both overlapping and distinct functions, comprising a complex immunomodulatory arsenal of host-targeted proteins.

A Truncated Form of HpARI Stabilizes IL-33, Amplifying Responses to the Cytokine

The murine intestinal nematode Heligmosomoides polygyrus releases the H. polygyrus Alarmin Release Inhibitor (HpARI) - a protein which binds to IL-33 and to DNA, effectively tethering the cytokine in the nucleus of necrotic cells. Previous work showed that a non-natural truncation consisting of the first 2 domains of HpARI (HpARI_CCP1/2) retains binding to both DNA and IL-33, and inhibited IL-33 release in vivo. Here, we show that the affinity of HpARI_CCP1/2 for IL-33 is significantly lower than that of the full-length protein, and that HpARI_CCP1/2 lacks the ability to prevent interaction of IL-33 with its receptor. When HpARI_CCP1/2 was applied in vivo it potently amplified IL-33-dependent immune responses to Alternaria alternata allergen, Nippostrongylus brasiliensis infection and recombinant IL-33 injection, in direct contrast to the IL-33-suppressive effects of full-length HpARI. Mechanistically, we found that HpARI_CCP1/2 is able to bind to and stabilize IL-33, preventing its degradation and maintaining the cytokine in its active form. This study highlights the importance of IL-33 inactivation, the potential for IL-33 stabilization in vivo, and describes a new tool for IL-33 research.

A helminth-derived suppressor of ST2 blocks allergic responses

The IL-33-ST2 pathway is an important initiator of type 2 immune responses. We previously characterised the HpARI protein secreted by the model intestinal nematode Heligmosomoides polygyrus, which binds and blocks IL-33. Here, we identify H. polygyrus Binds Alarmin Receptor and Inhibits (HpBARI) and HpBARI_Hom2, both of which consist of complement control protein (CCP) domains, similarly to the immunomodulatory HpARI and Hp-TGM proteins. HpBARI binds murine ST2, inhibiting cell surface detection of ST2, preventing IL-33-ST2 interactions, and inhibiting IL-33 responses in vitro and in an in vivo mouse model of asthma. In H. polygyrus infection, ST2 detection is abrogated in the peritoneal cavity and lung, consistent with systemic effects of HpBARI. HpBARI_Hom2 also binds human ST2 with high affinity, and effectively blocks human PBMC responses to IL-33. Thus, we show that H. polygyrus blocks the IL-33 pathway via both HpARI which blocks the cytokine, and also HpBARI which blocks the receptor.