Some suffering patients have taken this idea to the extreme, and have deliberately infected themselves with parasites to try to alleviate their symptoms. Although this treatment is highly experimental and there is not a lot of clinical data demonstrating efficacy, anecdotal evidence suggests that some patients have observed marked reduction in symptoms following “helminthic therapy.” Obviously, there are many problems associated with introducing parasitic organisms into people, so if scientists can determine how helminthic infection tamps down inflammatory responses, it could potentially lead to novel anti-inflammatory treatments that don’t involve the parasites themselves.
At the core of this issue are two major categories of immune responses, named “Th1” and “Th2” responses for the types of helper T cells that facilitate them. Th1 responses are inflammatory in nature, and involve the activation of macrophages and killer (CD8+) T cells. These responses are elicited by the secreted cytokine interferon-γ (IFN-γ), and typically target intracellular pathogens such as viruses. Th2 responses are mediated largely by B cells, which make antibodies, and are elicited by the cytokine interleukin-4 (IL-4). Th2 responses target extracellular pathogens, such as parasitic worms, and the rationale behind the hygiene hypothesis is that by eliminating many of these Th2 pathogens, the immune system “skews” towards Th1 responses. This, then, leads to increased inflammation and the associated autoinflammatory diseases. However, other responses caused by helminth infection, including the generation of regulatory T cells (TRegs) and the secretion of immunomodulatory cytokines such as IL-10 and transforming growth factor-β (TGF-β), might also mitigate autoinflammatory disease. A recent paper by Hübner et al attempted to distinguish which of these mechanisms was responsible for the protection from autoimmunity accorded by helminth infection. They found that generation of a Th2 response was not required, but that the production of TGF-β was largely responsible for protection.
The system that Hübner and colleagues used was the non-obese diabetic (NOD) mouse, a model for type 1 (autoimmune) diabetes. These mice spontaneously develop diabetes between 4-6 months of age. Hübner et al had previously demonstrated that infecting NOD mice with a helminthic worm, Litomosoides sigmodontis, prevents onset of diabetes (2), and now examined the mechanism for this protection. To do this, they crossed NOD mice with mice that genetically lacked IL-4, the key cytokine for development of a Th2 response. The IL-4-deficient NOD mice did not exhibit a Th2 skewing following L. sigmodontis infection, either in cytokine production or the type of antibodies produced. However, despite the lack of a Th2 response, none of these mice developed diabetes, indicating that Th2-skewing is not the primary mechanism of protection.
The authors then examined other possible mechanisms of protection. They examined levels of TRegs in IL-4-competent and –deficient NOD mice following L. sigmodontis infection, and found that while infection did increase the number and activity of these cells, there was no difference in the presence or absence of IL-4. Using antibody depletion and adoptive transfer experiments, the authors did not observe a strong role for TRegs in the helminth-mediated suppression of autoimmunity, although this data was not as robust. Another possibility is that helminth infection decreases a Th17 response, a third “flavor” of immune response that has been linked to autoimmunity. However, the authors did not observe a decrease in the production of IL-17, a hallmark of the Th17 response, upon L. sigmodontis infection, suggesting that this, too, was not a factor. Finally, the authors examined production of the immunomodulatory cytokines IL-10 and TGF-β. Helminth infection did not lead to increased levels of IL-10, although lower levels were observed in the absence of IL-4. However, L. sigmodontis did induce higher levels of bioactive (blood-borne) TGF-β, and depletion of TGF-β led to an increase in diabetes onset in the presence of helminth infection. These results suggest that the protection against autoimmunity accorded by helminth infection is not due to a Th2 skewing, but to an increase in the production of TGF-β These results are intriguing, but a lot more work must be done to fully understand how helminths might alter autoimmune disease. If similar effects are observed in other models of autoimmune/autoinflammatory disease, and with other infectious parasites, it could indicate that the same benefits of helminthic therapy could be achieved merely by treating patients with TGF-β. Intriguingly, TGF-β has been shown to play a role in suppressing type 1 diabetes in another mouse model (3), indicating that it may be a central anti-inflammatory factor. It is known that TGF-β can help maintain TReg populations, and can counteract the effects of the inflammatory cytokines tumor necrosis factor-alpha (TNF-α) and lymphotoxin (LT) (reviewed in 4). However, TGF-β can also facilitate the production of inflammatory Th17 cells and lead to fibrotic diseases, and is also associated with some forms of cancer, so it may not be the ideal therapeutic. Nevertheless, further research into the functions of TGF-β may provide some hope for those who suffer from autoinflammatory diseases, without their needing to be infected with parasites!
Hübner MP, Shi Y, Torrero MN, Mueller E, Larson D, Soloviova K, Gondorf F, Hoerauf A, Killoran KE, Stocker JT, Davies SJ, Tarbell KV, Mitre E. 2012. Helminth Protection against Autoimmune Diabetes in Nonobese Diabetic Mice Is Independent of a Type 2 Immune Shift and Requires TGF-β. J Immunol. 188:559-68.
(3) Burton, O. T., P. Zaccone, J. M. Phillips, H. De La Peña, Z. Fehérvári,
M. Azuma, S. Gibbs, B. Stockinger, and A. Cooke. 2010. Roles for TGF-beta and programmed cell death 1 ligand 1 in regulatory T cell expansion and diabetes
(4) Mirshafiey A, and Mohsenzadegan M. 2009 TGF-beta as a promising option in the treatment of multiple sclerosis. Neuropharmacology. 56:929-36.