Immunomodulation by helminthic parasites and worm therapy

Abstract

The helminthic parasites have largely been looked upon as chronic infections in developing countries causing morbidity. The helminthic parasites, unlike other microbial pathogens, are unique in the way they interact with the host’s immune system. Their size, complexity, and movement within the host trigger the host’s immune response toward a relative state of hyporesponsiveness, favoring cohabitation. This immunomodulation has been a topic of much debate in the last decade. This review explicitly explains how helminthic parasites are capable of modulating the host’s immune system and how this immunomodulation is brought about at different stages of immune activation. The proven and postulated mechanisms of altered antigen presentation and activation of both arms of the immune system, cell-mediated and humoral, are presented. The review further summarizes what effect this immunomodulation has on bystander immune responses and how the presence of helminthic parasites can contribute to alleviating immune-dysregulation conditions in the host. An updated account of the current usage of “worm therapy” in different autoimmune diseases, allergic conditions, and even cancer therapy is presented.

INTRODUCTION

Helminth is a nonphylogenetic term that refers to multicellular organisms (or metazoans) that have adopted a parasitic lifestyle in mammalian hosts.[1] The helminthic parasites infect more than one-third of the world’s population and several world health programs are going on to curtail the morbidity associated with these infections. The flip side of this infection, which has gained much popularity in recent times, is the role of these helminths in modulating the immune system of the host to their advantage, and, in turn, causing several beneficial effects on the host.[2] The helminths, owing to their sheer size, complexity, and unique inhabitation within the host, are capable of tweaking the immune response of the host toward a state of hypo-responsiveness.[3,4] This milieu, neither makes the host immunosuppressed and vulnerable to more infections nor causes exaggerated immune response leading to collateral damage.[4] Rather, this specific immune modulation brought about by helminthic parasites causes a state of continuous stimulation by low-dose antigens, keeping the immune system rather alert. Such responses, in recent times, have found use in alleviating immune dysregulation conditions such as allergies, autoimmune diseases, and even cancers.[4] The current review summarizes the current concepts in the mechanism of immunomodulation caused by helminthic parasites and the role of worms as therapeutic agents for immune dysregulation conditions. The literature search was done for all articles in the English language available as full-text on the public database, and studies pertaining to “helminthic parasites” and “immune regulation” or “immune modulation” were used to extract relevant information.

UNIQUE PROPERTIES OF HELMINTHS

The helminths, unlike other pathogens, are unique in being macroparasites. They not only have greater size but also greater structural complexity.[1] They have the ability to move actively through various tissues of the human host and undergo developmental changes within the host, thus exposing various host tissues to several helminthic antigens, structural or excretory-secretory, dynamically. Further, the bodies of these helminthic parasites are covered with multiple layers, making them more resistant to immune surveillance and attack.

BASIS OF IMMUNOMODULATION

The helminthic parasites and human hosts have developed ways to coevolve with each other by modifying the immunological responses.[5] Since the helminths cannot complete their life cycle inside the human host, except Strongyloides stercolaris, they have evolved methods to coexist with the host by activating immune regulatory networks. Similarly, the human immune system goes into a state of “disease tolerance” by minimizing the virulence of helminths, without necessarily reducing the worm burden.[6]

The human host can be exposed to two distinct types of pathogen insults; the first type is by rapidly replicating microorganisms such as bacteria, viruses, protozoa, and fungi. These agents have the potential to overcome the host defenses by their sheer numbers, and hence, the host must develop a rapid and robust immunological response against them. The typical response is a type 1 immunological response characterized by T helper 1 (TH-1) cell differentiation leading to the production of interleukin-12 (IL-12), IL-17, and interferon-gamma (IFNγ) and activation of T_H-17 cells, classically-activated macrophages, and neutrophils.[7] This powerful immune response, though necessary to control the rapidly multiplying pathogen, carries the risk of initiating harmful pro-inflammatory cytokines that could lead to collateral tissue damage to the host. The second type of pathogen insult is when the agent breaks through the protective barriers of the body while migrating inside the host, causing physical trauma, as done by the helminths when they enter or exit or migrate through the host.[8] Since these pathogens do not complete their life cycle inside the host, the danger of rapid expansion and dissemination is absent. In such a situation, mounting a type 1 immune response would be either ineffective or too self-damaging for the host. Hence, the host mounts a tipping balance toward a type 2 immune response that is aimed at wound healing rather than helminth killing.[3,9] This type 2 response is characterized by TH-2 cell differentiation leading to a state of hypo-responsiveness by the production of IL-4, IL-5, IL-13, and IL-10 and activation of innate lymphoid cells group 2 (ILC2) via alternatively-activated macrophages.[10]

The hypo-responsiveness to helminthic parasites is most evident in tissue settings such as lymphatic filariasis and schistosomiasis, where the intensity of infection does not necessarily positively correlate with the pathology.[8] While a host may be completely asymptomatic despite a high parasite burden owing to a strong regulatory response, another host may develop chronic pathology despite lower-level infection owing to immunological hyper-responsiveness. For instance, in lymphatic filariasis,[11] if the host exposed to third-stage larva mounts a dominant Th1 response (high IFNγ), there is more tissue damage leading to typical lymphatic disease and obstructive edema. But if the exposed host mounts a modified Th2 response (low IFNγ, high IL-4, and IL-10), he will remain asymptomatic but infected, i.e., there will be filarial adult worm in lymphatic vessels and microfilariae in peripheral blood but no symptoms. However, if the exposed host mounts a balanced Th1-Th2 immune response (balanced IFNγ/IL-4, low IL-10), he will remain free of infection.[11] Similarly, in schistosomiasis, though the distinction between asymptomatic and pathological infections is less clear, the two are governed by contrasting immune responses.[12] While the infection in previously unexposed travelers leads to acute inflammatory response causing large granulomatous reactions around deposited eggs, reinfection or chronic infection causes Th2 response with blunted inflammation such that only a small proportion of patients progress-to-severe hepatosplenic disease.[13]

MODULATION OF ANTIGENS AND THEIR PRESENTATION

Helminths produce immune-modulating antigens that lead to antigen-specific hypo-responsiveness. A classical example is children who are born to microfilaremic mothers are more likely to remain asymptomatic due to hypo-responsiveness toward microfilarial antigens.[14] The omega-1 glycoprotein, excretory-secretory antigen, and phosphatidylcholine lipids of Schistosoma mansoni condition the dendritic cells to promote TH2 differentiation by increasing production of IL-10 and downregulating IL-12.[13] The heat shock protein HSP70 of several helminthic parasites induces Treg proliferation by increased production of IL-10.[15] The cystatins downregulate overall T-cell proliferation by dampening antigen presentation.

Due to tipping toward TH2 response, the antigen-presenting cells (APCs) also undergo significant changes. In a normal pro-inflammatory milieu, the bacterial/fungal/protozoal pathogens are recognized by various toll-like receptors (TLRs) and processed inside the endoplasmic reticulum of the antigen-presenting cell for presentation to T-cells by interaction between the major histocompatibility complex (MHC) and T-cell receptor (TCR).[7] This is supported by the production of IL-12 and co-stimulatory molecules like CD40 that bind to specific ligands on the TH1 cells. However, in the case of helminths, the antigen recognition by TLRs is blunted, and the antigens escape processing by the endoplasmic reticulum. This means that the APCs do not mature.[8] These “tolerogenic” APCs downregulate TCR signals and TH1-polarizing signals. Further, helminthic cystatins downregulate MHC expression and inhibit IL-12 generation.[4] All these lead to a predominant TH2 response that is characterized by immune hypo-responsiveness against the helminthic antigens. Not only this, the TLR-driven inflammatory responses in the presence of intestinal helminths are suppressed to such a level that the host’s immune system becomes unresponsive even to the lipopolysaccharide (endotoxin) of bacteria and does not mount an immune response despite a leaky gut.[16]

MODULATION OF CELL-MEDIATED IMMUNITY

Some of the cells, such as macrophages, B-cells, and T-cells, involved in the immune response against helminthic infections are the same as those upregulated after an encounter with any other pathogen; however, they are “differently activated” after a helminthic encounter. The macrophages, after encounter with bacterial/fungal/protozoan pathogen, are activated by the classical pathway leading to a typical TH1 response. On the other hand, macrophages exposed to helminthic antigens are activated “alternatively” by the production of IL-4 and IL-13 released by TH2 cells.[17] These alternatively activated macrophages further activate ILC2, which then release several key factors such as arginase 1, chitinases like Ym1, resistin-like molecules, and retinoic acid.[7] While arginase restricts T-cell amino acid reserve, retinoic acid promotes differentiation to Treg cells at the inflammation site. Ym1 and resistin-like molecules promote the deposition of extracellular matrix around the worm or its egg, thus leading to wound healing and granuloma formation.[17] This TH2 response is aimed at plugging back the defects caused in the intestinal lining by the penetration of the helminth and encapsulating the helminth to prevent further damage by it. This response, though favorable at first, can prove counterproductive if left unchecked as excessive TH2 response would lead to large granulomas and excessive fibrosis around helminths or their products. This is prevented by regulatory B-cells and regulatory T-cells that cause activation of Tregs to prevent/halt the deleterious effects of continued TH2 response.[18] The natural Foxp3+ Treg mounts a rapid response to infection, leading to impairment of the ongoing TH2 response, in an IL-10 independent pathway. A more sustained response is generated by the host-induced adaptive Foxp3+ Tregs that are stimulated in a IL-10 dependent pathway, once the helminthic proteins have been processed by the host immune system.[18] Thus, the cell-mediated immunity is modulated first toward TH2 response to contain the helminth movement and repair the epithelial integrity and later switched to Treg response to contain the granuloma size and prevent excessive fibrosis.

MODULATION OF HUMORAL-MEDIATED IMMUNITY

Following infection by helminthic parasites, even the humoral-mediated immunity is modulated to work toward a state of hypo-responsiveness. For instance, the asymptomatic microfilaremic state that develops due to activation of TH2 response following exposure to filariasis leads to the production of immunoglobulin G4 (IgG4).[19] The IgG4 subtype of IgG is produced at exceedingly high levels in response to filarial antigens instead of IgE. This IgG4 isotype, unlike IgE, cannot cross-link receptors on the surface of basophils, mast cells, or eosinophils, hence does not cause their activation. This leads to a blunted humoral immune response in terms of the generation of effective antibody. Further, the IgG4 subtype, unlike other IgG molecules, neither activates complement nor acts as opsonin. Thus, there is a blunting of the effector functions of the humoral immune response.[19] Not only this but it also leads to the dampening of several bystander responses as well. The most serious of them is an inability to mount a protective immune response even after vaccination, leading to vaccination failure. For example, patients with schistosomiasis develop weaker IFN-g response to tetanus vaccination, and children infected with ascariasis have a weakened response to the Bacillus Calmette–Guérin vaccine.[20] The presence of helminthic parasite causes downregulation of several key steps needed for effective vaccination, such as recruitment of APCs, activation of TLRs, inflammasome activation, and MHC presentation for effective T-cell development that would, in turn, cause expansion of plasma cells for specific antibody response.[21]

OTHER EFFECTS OF HELMINTHIC IMMUNO-MODULATION

Other than a compromised vaccine efficacy, helminthic infections can cause downregulation of some other bystander immune responses as well. The helminths can drive an antigenic-specific anergic state, and this suppression is not confined solely to parasite-specific responses. One such effect is the promotion of coinfection.[22] The blunted TH1 and TH17 responses, in the presence of helminthic parasites, lead to coinfection with infections such as HIV, malaria, and tuberculosis. Helminth-HIV coinfection promotes each other by maintaining an environment of hypo-responsiveness by increasing the threshold for immune activation, defective intracellular signaling, decrease in costimulatory molecules, T-cell disbalance, and dysregulation of cytokine secretion.[23] When coinfected with Plasmodium species, the helminthic parasites shift the immune balance toward the TH2 response, and there is minimal pro-inflammatory response.[24] This means that the typical features of over-exaggerated immune response to malarial antigens, as those seen in cerebral malaria, are characteristically absent, while the chronic low-level parasitemia continues to cause anemia by both hemolysis and dyserythropoiesis, leading to exaggerated anemia.[22] The coinfection of helminthic parasites with tubercle bacilli also causes activation of contrasting responses.[25] The TH1 immune response, essentially needed to clear intracellular pathogens like tubercle bacilli, is blunted significantly in the presence of helminthic parasites. Thus, chronic infection, even if asymptomatic, by helminthic parasites promotes intracellular survival of tubercle bacilli.[25] This interaction could be an important cause of challenges being faced in the developing countries in eliminating tuberculosis, as such countries are more often than not endemic and simultaneously coinfected with several helminthic parasites also.

Other bystander immune responses that are affected by the helminthic immune modulation are allergic response and autoimmunity.[26] Garn et al. gave the “hygiene hypothesis” after observing that the lower rates of autoimmune diseases in Africa were linked to higher rates of helminthic parasites.[27] Similarly, Gerrard et al. documented that Canadian Indians had higher rates of asthma, eczema, and urticaria than Indian residents due to inverse parasitic infection prevalence.[28] The development of allergic disease occurs as a consequence of the magnitude of TH1 and TH2 responses and the overall balance between them.[29] For example, in developed countries, the incidence of infectious diseases (TH1 response) as well as infection with worms (TH2) is low. Although both are nearly balanced, their overall magnitude is low, paving the way for the emergence of allergic disease.[30] In developing countries, on the other hand, the incidence of both infectious diseases and worm infections is high, that is, both TH1 and TH2 responses are higher in magnitude and nearly balanced, this milieu prevents the development of allergic disease.[29,31] The relationship between autoimmune disease and worm infection is also inversely related. While a high incidence of autoimmune diseases such as multiple sclerosis and inflammatory bowel disease is encountered in countries with low worm burden, less autoimmune diseases are seen in population cohorts infected with helminths, with reversion after anti-helminthic treatment![16] This idea of “hygiene hypothesis” led to the concept of worm therapy, which is using helminthic parasites as therapy for immune-dysregulatory conditions such as allergy, autoimmunity, and even tumor surveillance.[32]

WORM THERAPY

Several researchers have investigated the association of the presence of worms with improvement in immune dysregulation conditions, with variable results.[33] In the earliest of such studies, a parasitic regimen of 2500 ova of Trichuris suis every 3 weeks for 12–24 weeks showed a significant reduction in the pathology of Crohn’s disease and ulcerative colitis, with more than 70% achieving remission.[34] Contrasting results, however, were documented in a similar regimen and another involving Necator americanus was not able to cause any change in disease pathology in patients of allergic rhinitis and allergic rhino-conjunctivitis.[35] It was then thought that probably the difference in the success rates was due to the different target sites, while “worms” were able to modulate the immune system better in the gut, their usual site of infection, the same was not achieved at sites such as nose and conjunctiva, where “worms” do not routinely wander. However, this belief was put to rest when helminthic regimen consisting of predominantly gut worms – Hymenolepis nana, Trichuris trichiura, and Ascaris lumbricoides were used to successfully manage multiple sclerosis,[36] a disease of the nervous system, well outside the usual worm trail. The recent success of Schistosomal antigens in relieving symptoms of several autoimmune diseases has earned Schistosoma the tag of “future magic bullet” for autoimmune diseases.[12]

Worm therapy has been harnessed as a tool for tumor immune surveillance as well. The presence of helminthic worms shifts the immunological predominance toward a TH2 response, causing activation of alternatively-activated macrophages and Treg cells.[17,18] These cells cause several immunological changes in the tumor milieu, leading to cancer cell apoptosis, cell cycle arrest, and inhibition of angiogenesis.[37] The activities have revived hope in cancer therapy without painstaking side effects, as experienced with chemotherapy. For instance, the surface antigen gp82 of Trypanosoma cruzi is able to induce apoptosis in melanoma cells without causing any collateral damage to normal melanocytes.[38] The excretory-secretory antigen of Trichinella spiralis and others stimulate mitochondrial apoptotic and death-receptor pathways that have an antineoplastic effect.[39] Another unique feature of parasites that has been found useful in cancer therapy is the phenomenon of molecular mimicry. The mucin-rich O-glycan antigens of cancer cells, which are a hallmark of invasion and metastasis, have been found to share epitopes with some helminthic parasites, culminating in robust antineoplastic activity by sharing of common epitopes.[40] The antiparasite antibodies formed against these shared antigens selectively react with certain cancer cells without affecting the normal host cells. The breast cancer mice model has shown favorable results in this regard.[41] Several in vitro and in vivo studies have shown effective antineoplastic activity triggered by helminthic parasites in several cancers such as gastrointestinal, lung, gynecological, sarcomas, leukemia, lymphoma, and bone tumors.[37]

Worm therapy presents unique challenges as well. Although the therapeutic role of helminthic parasites has been shown in several in vitro and in vivo studies, and some standardization of the dosing and frequency has also been documented, it is too early to comment on their overall benefit.[32] A thorough follow-up, over several years, will be able to shed more light on their actual benefit in different disease conditions. Another concern is the ethical consideration of using worms as therapy. Many trials would be using an asymptomatic dose for ethical reasons, and there are apprehensions that such doses may not be efficacious in suppressing pathology. The third concern is whether the available literature can be extrapolated to different geographical areas or not. This is because the endemicity of helminthic infections varies drastically worldwide, and hence, the immune-modulation brought about by them would vary in different geographical regions. That is, individuals who live in areas where helminth diseases are endemic are already exposed to parasitic infections from an early age, before the development of immune dysregulation disease; hence, it is not clear whether worms will still be effective as therapeutic agents in such populations.[31] Further, it is yet unknown what bearing will the ongoing “de-worming campaigns” in endemic countries have on the potential role of worms as therapeutic agents.

To conclude, the helminths are indeed unique in modulating the host’s immune system to their advantage, aiming for a relatively immune hypo-responsiveness that allows peaceful co-existence. If collateral damages such as malabsorption and anemia are taken care of, the presence of worms seems more beneficial than harmful in patients having immune dysregulation diseases such as allergies and autoimmune disorders. The contemporary studies documenting the successful usage of worms as anti-neoplastic agents, without damaging normal host cells have given a new hope for an effective, broad-spectrum anticancer therapy that is free of all side effects of chemotherapeutic agents. The future of worm therapy, as of now, seems promising; however, the true picture with emerge once a thorough follow-up of ongoing clinical trials is assessed and the variable effects of preexposures to helminths in endemic areas are taken into account. Future studies could explore important research areas like an association of a specific helminthic parasite on specific immune dysregulation conditions, standardizing optimal dosage for therapy, utilizing artificial intelligence to ascertain site-specific delivery of target antigen for specific action, constructing geospatial maps depicting changing trends in the incidence of helminthic parasites and their consequent effects on incidence of allergic/autoimmune diseases.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.