Richard K. Grencis

Impaired Development of Th2 Cells in IL-13-Deficient Mice

We report that Th2 cell cultures generated using T cells or splenocytes from IL-13-deficient mice produce significantly reduced levels of IL-4, IL-5, and IL-10 compared with wild-type. In contrast, IL-4 and IL-5 production by mast cells stimulated in vitro with PMA, ionomycin, or IgE cross-linking are unaffected. In vitro Th2 cell differentiation cannot be rescued by the addition of exogenous factors, but in vivo antigen challenge and administration of IL-13 can increase Th2-like cytokine responses as can infection with the parasitic nematode Nippostrongylus brasiliensis. IL-13-deficient mice also have lower basal levels of serum IgE and biased antigen-specific immunoglobulin responses. Thus, IL-13 is an important regulator of Th2 commitment and may therefore play a central role in atopy and infectious diseases.

IL-33, a Potent Inducer of Adaptive Immunity to Intestinal Nematodes

IL-33 (IL-1F11) binds ST2 (IL-1R4), both of which are associated with optimal CD4+ Th2 polarization. Exogenous IL-33 drives induction of Th2-associated cytokines and associated pathological changes within the gut mucosa. Th2 polarization is also a prerequisite to expulsion of the intestinal-dwelling nematode Trichuris muris. In this study, we demonstrate that IL-33 mRNA is expressed early during parasite infection and susceptible mice can be induced to expel the parasite by a regime of exogenous IL-33 administration. IL-33 prevents an inappropriate parasite-specific Th1-polarized response and induces IL-4, IL-9, and IL-13. This redirection requires the presence of T cells and must occur at the initiation of the response to the pathogen. Interestingly, exogenous IL-33 also induced thymic stromal lymphopoietin mRNA within the infected caecum, an epithelial cell-restricted cytokine essential for the generation of Th2-driven parasite immunity. IL-33 also acts independently of T cells, altering intestinal pathology in chronically infected SCID mice, leading to an increased crypt length and intestinal epithelial cell proliferation, but reducing goblet cell hyperplasia. Thus, the ability of IL-33 to induce Th2 responses has functional relevance in the context of intestinal helminth infection, particularly during the initiation of the response.

A distinct role for interleukin-13 in Th2-cell-mediated immune responses

Immune responses elicited by allergic reactions and parasitic worm infections are characterised by the induction of T helper 2 (Th2) cells. These cells secrete cytokines such as interleukin-4 (IL-4), IL-5 and IL-13, which induce the production of immunoglobulin E (IgE) and eosinophils [1,2]. Previous studies using gastrointestinal nematodes to elucidate the role of Th2-cell-mediated immune responses have demonstrated a causal relationship between T cells and worm expulsion (reviewed in [3]). Although it has been proposed that IL-4 played a central role in these responses, recent studies demonstrated that IL-4-/- mice expel the parasitic gastrointestinal nematode Nippostrongylus brasiliensis normally [4], suggesting that another T-cell mediator is required for efficient worm clearance. Using IL-13-/- mice, we have demonstrated that, unlike wild-type and IL-4-/- mice, the IL-13-/- animals failed to clear N. brasiliensis infections efficiently, despite developing a robust Th2-like cytokine response to infection. Furthermore, treatment of the IL-13-/- mice with exogenous IL-13 resulted in a reduction in the numbers of worms recovered. The IL-13-/- animals also failed to generate the goblet cell hyperplasia that normally occurs coincident with worm expulsion. This observation may link IL-13 with the production of intestinal mucus which is believed to facilitate worm expulsion. These data support a unique role for IL-13 in Th2-cell-mediated immune responses and demonstrate that IL-13 and IL-4 are not redundant.

Mast cells disrupt epithelial barrier function during enteric nematode infection.

We have investigated the influence of mast cells on the barrier function of intestinal epithelium during nematode infection. Trichinella spiralis infection induces a strong type 2 cytokine-mediated inflammation, resulting in a critical mucosal mastocytosis that is known to mediate expulsion of the parasites from the intestine. The host response to infection is also characterized by an increase in mucosal leakiness. We show here that intestinal epithelial permeability is markedly elevated during infection, with kinetics that mirror the adaptive immune response to primary and secondary infection. Furthermore, we have identified degradation of the tight junction protein, occludin, thereby providing a mechanism for increased paracellular permeability during helminth infection. We further demonstrate by using anti-c-kit antibody and IL-9 transgenic mice that mast cells are directly responsible for increasing epithelial paracellular permeability and that mice deficient in a mast cell-specific protease fail to increase intestinal permeability and fail to expel their parasite burden. These results provide the mechanism whereby mucosal mast cells mediate parasite expulsion from the intestine.

Muc5ac: a critical component mediating the rejection of enteric nematodes.

The mucin Muc5ac is essential for the expulsion of Trichuris muris and other gut-dwelling nematodes.

Th2 Cytokines Are Associated with Reduced Worm Burdens in a Human Intestinal Helminth Infection

Although T helper 2 (Th2) cytokines are known to be critical in the generation of protective immunity against intestinal helminths in mouse models, it is unclear whether they are important in natural immunity against gut helminthiases in humans. Therefore, we investigated cytokine production in ex vivo whole-blood cultures in response to Ascaris lumbricoides antigen and mitogen in a cross-section of a community where the parasite is hyperendemic. The intensity of A. lumbricoides infection was significantly reduced after age 11 years. Levels of cytokines associated with Th2 lymphocytes (interleukin [IL]-4, IL-9, IL-10, and IL-13) demonstrated an inverse relationship with intensity of A. lumbricoides infection only in individuals aged >11 years. Furthermore, the IL-9, IL-10, and IL-13 produced in response to parasite antigen were of primary importance in this relationship. These findings promote a role for Th2-mediated responses in the age-dependent reduction of intestinal helminth infections in humans.

Helminth products bypass the need for TSLP in Th2 immune responses by directly modulating dendritic cell function

Thymic stromal lymphopoietin (TSLP) is an interleukin (IL)-7-like cytokine, mainly expressed by epithelial cells, and key to the development of allergic responses. The well-documented involvement of TSLP in allergy has led to the conviction that TSLP promotes the development of inflammatory Th2 cell responses. However, we now report that the interaction of TSLP with its receptor (TSLPR) has no functional impact on the development of protective Th2 immune responses after infection with 2 helminth pathogens, Heligmosomoides polygyrus and Nippostrongylus brasiliensis. Mice deficient in the TSLP binding chain of the TSLPR (TSLPR−/−) exhibited normal Th2 cell differentiation, protective immunity and memory responses against these two distinct rodent helminths. In contrast TSLP was found to be necessary for the development of protective Th2 responses upon infection with the helminth Trichuris muris (T. muris). TSLP inhibited IL-12p40 production in response to T. muris infection, and treatment of TSLPR−/− animals with neutralizing anti-IL-12p40 monoclonal antibody (mAb) was able to reverse susceptibility and attenuate IFN-γ production. We additionally demonstrated that excretory-secretory (ES) products from H. polygyrus and N. brasiliensis, but not T. muris, were capable of directly suppressing dendritic cell (DC) production of IL-12p40, thus bypassing the need for TSLP. Taken together, our data show that the primary function of TSLP is to directly suppress IL-12 secretion, thus supporting Th2 immune responses.

The Trichuris muris System: a Paradigm of Resistance and Susceptibility to Intestinal Nematode Infection

Gastrointestinal helminths infect over 1 billion people worldwide. Although rarely causing death, such diseases are associated with high levels of morbidity and furthermore bear a large economic burden within areas where infections are endemic. Trichuris muris, a natural intestinal parasite of mice has been extensively utilised as a laboratory model for the study of human whipworm Trichuris trichiura. This has proven to be an invaluable tool in dissecting the different components involved in immunity to trichuris infection. Moreover, it has become a paradigm of cytokine mediated immunity to gastrointestinal nematodes in general. It is well established that resistance and susceptibility to T. muris infection are tightly associated with the generation of a T helper 2 (TH2) or a T helper 1 (TH1) immune response, respectively. This review gives a detailed account of the experimental work which has provided us with this knowledge, and further builds upon this, by focusing upon the most recent developments and important findings from this host-parasite relationship.

Gastrointestinal nematode expulsion in IL-4 knockout mice is IL-13 dependent.

Female IL‐4 knockout (KO) mice on a C57BL / 6 background (F4KOC57) are susceptible to infection with the cecal‐dwelling nematode Trichuris muris whereas wild‐type C57BL / 6 mice are resistant and expel the parasite. In this study we show that in sharp contrast, female IL‐4 KO mice on a BALB / c background (F4KOB / c) are resistant to infection as are wild‐type BALB / c mice. Although susceptible F4KOC57 make negligible levels of all type 2 cytokines, resistant F4KOB / c were capable of producing significant levels of antigen‐specific IL‐13 (a cytokine shown to be critical in resistance to T. muris). To examine if the IL‐13 in F4KOB / c mice was of functional importance, it was neutralized in vivo using a fusion protein, A25 (sIL‐13 R.Fc). The results presented here clearly demonstrate that neutralization of IL‐13 in vivo did indeed prevent T. muris expulsion in normally resistant F4KOB / c mice. In addition, administration of recombinant mouse IL‐13 to normally susceptible male IL‐4KO BALB / c mice (M4KOB / c) caused an 87.85 % reduction in worm burden. Collectively, these data show that IL‐13 is important in the poorly understood effector mechanisms resulting in the expulsion of T. muris from the gut. Moreover, the present data highlight the functional importance of gender and background strain in interpretation of studies using gene‐targeted animals.

Mucin Gene Deficiency in Mice Impairs Host Resistance to an Enteric Parasitic Infection

Background & Aims Hyperplasia of mucin-secreting intestinal goblet cells accompanies a number of enteric infections, including infections by nematode parasites. Nevertheless, the precise role of mucins in host defense in nematode infection is not known. We investigated the role of the mucin (Muc2) in worm expulsion and host immunity in a model of nematode infection. Methods Resistant (BALB/c, C57BL/6), susceptible (AKR), and Muc2-deficient mouse strains were infected with the nematode, Trichuris muris, and worm expulsion, energy status of the whipworms, changes in mucus/mucins, and inflammatory and immune responses were investigated after infection. Results The increase in Muc2 production, observed exclusively in resistant mice, correlated with worm expulsion. Moreover, expulsion of the worms from the intestine was significantly delayed in the Muc2-deficient mice. Although a marked impairment in the development of periodic acid Schiff (PAS)–stained intestinal goblet cells was observed in Muc2-deficient mice, as infection progressed a significant increase in the number of PAS-positive goblet cells was observed in these mice. Surprisingly, an increase in Muc5ac, a mucin normally expressed in the airways and stomach, was observed after infection of only the resistant animals. Overall, the mucus barrier in the resistant mice was less permeable than that of susceptible mice. Furthermore, the worms isolated from the resistant mice had a lower energy status. Conclusions Mucins are an important component of innate defense in enteric infection; this is the first demonstration of the important functional contribution of mucins to host protection from nematode infection.