Regulatory T cells in parasite infections: susceptibility, specificity and specialisation

Regulatory T cells (Tregs) are essential to control immune system responses to innocuous self-specificities, intestinal and environmental antigens. However, they may also interfere with immunity to parasites, particularly in chronic infection. Susceptibility to many parasite infections is, to a greater or lesser extent, controlled by Tregs, but often they play a more prominent role in moderating the immunopathological consequences of parasitism, and dampening bystander reactions in an antigen-nonspecific manner. More recently, Treg subtypes have been defined which may preferentially act in different contexts; we also discuss the degree to which this specialisation is now being mapped onto how Tregs maintain the delicate balance between tolerance, immunity, and pathology in infection.

Helminth-derived cystatins: the immunomodulatory properties of an Ascaris lumbricoides cystatin

Helminth infections such as ascariasis elicit a type 2 immune response resembling that involved in allergic inflammation, but differing to allergy, they are also accompanied with strong immunomodulation. This has stimulated an increasing number of investigations, not only to better understand the mechanisms of allergy and helminth immunity but to find parasite-derived anti-inflammatory products that could improve the current treatments of chronic non-communicable inflammatory diseases such as asthma. A great number of helminth-derived immunomodulators have been discovered and some of them extensively analysed, showing their potential use as anti-inflammatory drugs in clinical settings. Since Ascaris lumbricoides is one of the most successful parasites, several groups have focused on the immunomodulatory properties of this helminth. As a result, several excretory/secretory components and purified molecules have been analysed, revealing interesting anti-inflammatory activities potentially useful as therapeutic tools. One of these molecules is A. lumbricoides cystatin, whose genomic, cellular, molecular, and immunomodulatory properties are described in this review.

Whipworm and roundworm infections

Trichuriasis and ascariasis are neglected tropical diseases caused by the gastrointestinal dwelling nematodes Trichuris trichiura (a whipworm) and Ascaris lumbricoides (a roundworm), respectively. Both parasites are staggeringly prevalent, particularly in tropical and subtropical areas, and are associated with substantial morbidity. Infection is initiated by ingestion of infective eggs, which hatch in the intestine. Thereafter, T. trichiura larvae moult within intestinal epithelial cells, with adult worms embedded in a partially intracellular niche in the large intestine, whereas A. lumbricoides larvae penetrate the gut mucosa and migrate through the liver and lungs before returning to the lumen of the small intestine, where adult worms dwell. Both species elicit type 2 anti-parasite immunity. Diagnosis is typically based on clinical presentation (gastrointestinal symptoms and inflammation) and the detection of eggs or parasite DNA in the faeces. Prevention and treatment strategies rely on periodic mass drug administration (generally with albendazole or mebendazole) to at-risk populations and improvements in water, sanitation and hygiene. The effectiveness of drug treatment is very high for A. lumbricoides infections, whereas cure rates for T. trichiura infections are low. Novel anthelminthic drugs are needed, together with vaccine development and tools for diagnosis and assessment of parasite control in the field.

Infection with Toxocara canis Inhibits the Production of IgE Antibodies to α-Gal in Humans: Towards a Conceptual Framework of the Hygiene Hypothesis?

α-Gal syndrome (AGS) is a type of anaphylactic reaction to mammalian meat characterized by an immunoglobulin (Ig)E immune response to the oligosaccharide α-Gal (Galα1-3Galβ1-4GlcNAc-R). Tick bites seems to be a prerequisite for the onset of the allergic disease in humans, but the implication of non-tick parasites in α-Gal sensitization has also been deliberated. In the present study, we therefore evaluated the capacity of helminths (Toxocara canis, Ascaris suum, Schistosoma mansoni), protozoa (Toxoplasma gondii), and parasitic fungi (Aspergillus fumigatus) to induce an immune response to α-Gal. For this, different developmental stages of the infectious agents were tested for the presence of α-Gal. Next, the potential correlation between immune responses to α-Gal and the parasite infections was investigated by testing sera collected from patients with AGS and those infected with the parasites. Our results showed that S. mansoni and A. fumigatus produce the terminal α-Gal moieties, but they were not able to induce the production of specific antibodies. By contrast, T. canis, A. suum and T. gondii lack the α-Gal epitope. Furthermore, the patients with T. canis infection had significantly decreased anti-α-Gal IgE levels when compared to the healthy controls, suggesting the potential role of this nematode parasite in suppressing the allergic response to the glycan molecule. This rather intriguing observation is discussed in the context of the 'hygiene hypothesis'. Taken together, our study provides new insights into the relationships between immune responses to α-Gal and parasitic infections. However, further investigations should be undertaken to identify T. canis components with potent immunomodulatory properties and to assess their potential to be used in immunotherapy and control of AGS.

Body fluid from the parasitic worm Ascaris suum inhibits broad-acting pro-inflammatory programs in dendritic cells

Dendritic cells (DCs) are essential for generating T-cell-based immune responses through sensing of potential inflammatory and metabolic cues in the local environment. However, there is still limited insight into the processes defining the resultant DC phenotype, including the type of early transcriptional changes in pro-inflammatory cues towards regulatory or type 2 immune-based cues induced by a variety of exogenous and endogenous molecules. Here we compared the ability of a selected number of molecules to modulate the pro-inflammatory phenotype of lipopolysaccharide (LPS) and interferon-γ (IFN-γ)-stimulated human monocyte-derived DCs towards an anti-inflammatory or regulatory phenotype, including Ascaris suum body fluid [helminth pseudocoelomic fluid (PCF)], the metabolites succinate and butyrate, and the type 2 cytokines thymic stromal lymphopoietin and interleukin-25. Our data show that helminth PCF and butyrate treatment suppress the T helper type 1 (Th1)-inducing pro-inflammatory DC phenotype through induction of different transcriptional programs in DCs. RNA sequencing indicated that helminth PCF treatment strongly inhibited the Th1 and Th17 polarizing ability of LPS + IFN-γ-matured DCs by down-regulating myeloid differentiation primary response gene 88 (MyD88)-dependent and MyD88-independent pathways in Toll-like receptor 4 signaling. By contrast, butyrate treatment had a strong Th1-inhibiting action, and transcripts encoding important gut barrier defending factors such as IL18, IL1B and CXCL8 were up-regulated. Collectively, our results further understanding of how compounds from parasites and gut microbiota-derived butyrate may exert immunomodulatory effects on the host immune system.

Reflections upon immunological mechanisms involved in fertility, pregnancy and parasite infections

During a pregnancy, the mother accepts her semi-allogeneic fetus with no signs of immunological rejection. Therefore, some modulation of the maternal immune system must occur. Similarly, changes in the host's immune system occurs during infections with parasites. In a study conducted in an endemic area in Bolivia, it has been reported that women infected with either the helminthic parasite roundworm or hookworm were estimated to give birth to either two more, or three fewer, children than uninfected, endemic women, respectively. Immune regulation by helminthic parasites is a rather well-researched concept, but there are few reports on the effects on human fecundity. The current review focuses on mechanisms of possible importance for especially the increased fertility rates in women infected with roundworm. The host immune response to roundworm has been hypothesized to be more favourable for a successful pregnancy because it bears resemblance to the anti-inflammatory immunological responses observed in pregnancy, steering the immunological response away from a pro-inflammatory state that seem to suppress fecundity. Further research into parasitic worm interactions, fertility, and the molecular mechanisms that they unfold may widen our understanding of the immunomodulatory pathways in both helminthic infections and in fertility and pregnancy.

Ascariasis as a model to study the helminth/allergy relationships

Ascariasis is the most frequent soil transmitted helminthiasis and, as well as other helminth infections, is expected to influence the clinical presentation of allergic diseases such as asthma. Indeed, several clinical and experimental works have shown an important impact either increasing or suppressing symptoms, and the same effects have been detected on the underlying immune responses. In this review we analyze the work on this field performed in Colombia, a Latin American tropical country, including aspects such as the molecular genetics of the IgE response to Ascaris; the allergenic activity of Ascaris IgE-binding molecular components and the immunological and clinical influences of ascariasis on asthma. The analysis allows us to conclude that the impact of ascariasis on the inception and evolution of allergic diseases such as asthma deserves more investigation, but advances have been made during the last years. The concurrent parasite-induced immunostimulatory and immunosuppressive effects during this helminthiasis do modify the natural history of asthma and some aspects of the practice of allergology in the tropics. Theoretically it can also influence the epidemiological trends of allergic diseases either by its absence or presence in different regions and countries.