Our laboratory integrates immunology with molecular parasitology to understand how parasites evade and direct host immune systems to their own advantage. Our investigations are both at the reductionist level of individual molecules, and at the system level in terms of the whole-body outcome, and we aim to develop new strategies to intervene and cure disease. This objective requires us to use multidisciplinary expertise encompassing cellular immunology, parasite biology, protein chemistry and genomics, at three levels (see Schematic below)
MOLECULES : how individual parasite genes mediate evasion
CELLS : the subsets of immune system cells which initiate, control and impose immunity
SYSTEMS : the whole-body impact of infection on allergy and autoimmunity and new interventions to inhibit disease.
Our focus is on proteins and glycoconjugates secreted by live parasites which are likely to interact with the host (see our review by Hewitson et al., 2009 Mol Biochem Parasitol 167: 1-11), and the identification of these products requires a combination of genomics, transcriptomics and proteomics.
Nematode parasites have sizeable genomes, at 1-3 x 109 bp which is up to 10% of the size of their mammalian hosts. Several of the major organisms are now the subject of full genome sequencing, by the Wellcome Trust Sanger Institute and Washington University at St Louis. We are collaborating with the Wellcome Trust Sanger Institute in their genome projects on Nippostrongylus brasiliensis and Echinococcus granulosus, and conducting in-house transcriptomic studies of Heligmosomoides polygyrus. These genomic and transcriptomic databases allow us to match data from proteomic analyses, in which we focus on parasite secreted ("ES" : excreted/secreted) antigens such as those we described in Hewitson et al (2008 Mol Biochem Parasitol 160:8-21).
Remarkably, no study has yet established whether filarial nematodes exhibit antigenic diversity or variation. We are addressing this by sequencing certain key genes in different isolates in Indonesia. Isolates of B.malayi from different regions of Indonesia are being studied for the degree of variation both within each strain, and between differing strains.
Further details on molecules which we are studying as candidate "immune evasion" mediators by viewing "Immune Evasion Genes from Parasitic Nematodes".
Parasites which live in mammalian hosts for long periods are successful xenografts, and must survive by altering the host cellular immune response to attenuate or ablate attack. Certainly, parasites have evolved a web of mechanisms for diverting host immune responses. We are dissecting this system by analysing the status, phenotype and specificity of host immune cell populations. We are particuarly interested in two dominant, but competing, host cell populations: the Th2 and Treg subsets.
Th2 responses are virtually universal among helminth parasite infections. Currently, we are analysing the initiation of Th2 responses in vivo, in concert with identifying the parasite molecules which induce this reaction. We are establishing which host receptors and cells recognise Th2-driving ligands from nematodes, and the sequence of events which generates overriding type-2 responses. Cells of both the innate and adaptive immune systems synergise in this regard. Several approaches are being matched for this end: in vitro differentiated or purified cell populations (eg dendritic cells and mast cells), T cells from TCR-transgenic mice, and knockout animals in which defined cytokines, receptors or cell populations are deficient.
We have also defined Treg populations in chronic helminth infection (see "Regulatory T cells in Parasite Infection"), and are testing the hypothesis that these are responsible for the suppression of key T cell functions such as antigen-specific proliferation and secretion of pro-inflammatory cytokines. A combination of flow cytometry (both surface and intracellular staining of lymphocytes), cell fractionation and co-culture, and the use of cytokine knockout mice, is being applied to this study. Depletion of Tregs in vivo is being accomplished by both antibody-mediated and genetic strategies. Through collaborations with Prof Maria Yazdanbakhsh (Leiden University) and Dr B Ravindran (Bhubaneswar) we are also investigating the function of Tregs in human filariasis.
The choice between effector Th2 and regulatory T cell is one of great import for infection outcome. The role of dendritic cells (DCs) in this dichotomy is being studied, using both in vitro-differentiated DCs exposed to various helminth products, and ex vivo purified DCs, taken from tissues exposed to active infection in vivo.
A further important cell population in infection is the B cell. We have discovered a "Breg"-like population in helminth infected mice, and are currently investigating if and how parasites can induce differential antibody isotype switching during infection.
There is now litte doubt that Immune down-regulation effected by helminth infections can spill over to modulate other responses, including allergies, autoimmune diseases, other infections and even responsiveness to vaccination. Allergies and helminth infections share a Th2-dominated immune response, and yet allergic disease is lowest in developing countries with high parasite prevalences. By studying the interaction between parasite infection and susceptibility to allergies, using chronic models of airway allergy, we are testing the “Hygiene Hypothesis”. This will permit us to probe the mechanisms for this important cross-regulatory phenomenon.
In models of animal airway allergy, we have found that helminth infections down-regulate inflammation, and indeed protection from allergy can be conferred by introducing secreted helminth proteins rather than live infection. This result is being pursued with a view to developing new therapies for allergy. The same infections also delay and mitigate autoimmune pathology such as experimental autoimmune encephalitis, a model for human multiple sclerosis. Again, the possibility of applying the natural suppressive effects of parasites (and their products) to critical disease conditions is one we are pursuing.
In addition to manipulating systemic responses to exo-allergens and auto-antigens, a major objective is to use our knowledge of parasite immunomodulatory tactics to reverse their immunosuppression and achieve elimination of disease. Several avenues are open to us: we are studying the benefits of specific intervention to block Treg activity; we plan to modify vaccine formulations (and vaccine antigen epitopes) to exclude Treg stimulating components; and we can vaccinate with the immunomodulatory molecules themselves in order to pre-empt parasite suppression in vivo.