**1. Introduction**

#### **1.1 Helminths and parasitosis**

Helminthiases are parasitic diseases caused by helminths, which are colloquially called "parasitic worms". Although a great biodiversity of helminths exists, the most relevant in public and veterinary health are the cestodes (tapeworms), trematodes (flukeworms) and nematodes (roundworms). Helminthiases affect more than 2 billion people worldwide, which can become chronic infections when untreated and persist for the rest of their host's life [1–3]. The population affected by helminthiases is mostly found in tropical and subtropical areas, where precarious health systems and poor sanitation prevail. This situation contributes to an increase in their prevalence due to global climate change that has caused parasites to undergo evolutionary changes to adapt over time, which in turn generate resistance to antiparasitic treatments [3]. Although the immune system could eliminate the helminth from the host's body, parasites can often evade the immune response and, in the worst-case scenario, the host suffers collateral damage, consequence of the immunopathology caused by the immune attack against helminths, as an attenuated immune response can trigger a tolerance towards the helminth [2].

#### **1.2 T-helper immune response**

In vertebrate animals, the immune response is divided in innate- and adaptive immunity. The first one acts in a non-specific but immediate manner, while in the second is antigen-specific and can be classified as cellular (T cells) or humoral (B cells). The adaptive immune response develops antigen-specific immunological memory and drives both inflammation and tissue repair. The cellular immune response play a key role in the development and progression of chronic inflammatory diseases [4]. T cells are divided into several groups, the two most important are the T helper cell and the cytotoxic T cells, both respectively distinguished by CD4+ and CD8+ cell surface markers. The helper (Th) immune response initiates with the interaction of a CD4+ T cell and an antigen-presenting cell. The T-cell receptor, which is in the surface of the CD4 + T cells, bounds with the major histocompatibility complex type II, which is on the cell membrane of an antigen-presenting cell (B cells, dendritic cells, among others). This interaction leads to the differentiation of B cells into plasma cells that produce antibodies. Various cytokines and other co-stimulatory molecules can stimulate the CD4+ T cell population to divide itself into cell subsets, which elicit a different antigen-specific response, as Th1, Th2, Th9, Th17, and Treg [5]. Each subset has specific characteristics and specialized properties. The Th1 response express pro-inflammatory cytokines, as interferon-gamma (IFN-γ), interleukin 2 (IL-2) and tumor necrosis factor beta (TNF-β). Th1 cells respond against intracellular parasites including protozoa, bacteria, viruses, and fungi. Overexpression of Th1 cells can lead to the development of autoimmune diseases such as hypersensitivity, arthritis, and type 1 diabetes. The Th2 express anti-inflammatory cytokines, as IL-4, IL-5 and IL-13; the Th2 is induced as response to helminth parasites, but its over-activation leads to systemic autoimmune inflammatory diseases, such as allergies and atopic dermatitis. Overexpression of Th9 and Th17 cells is also involved in the development of autoimmune and inflammatory disorders. In the case of Treg cells, they regulate the differentiation and proliferation of effector T cells and promote tolerance, thereby limiting the development of autoimmune diseases [5].

#### **1.3 Hygiene hypothesis**

In humans, the immune system has adapted to recurrent infections caused by numerous non-pathogenic organisms. Through exposure to environments rich in microorganisms, this adaptation leads to a more effective immune response to invasion by pathogens. However, the elimination of these "microbial allies" from environments in industrialized cities because of advances in medical care, and improvements in hygiene and urbanization, has been associated with a dramatic increase in autoimmune,

#### *Therapeutic Properties of* Trichinella spiralis *(Nematoda) in Chronic Degenerative Diseases DOI: http://dx.doi.org/10.5772/intechopen.103055*

allergic, and chronic degenerative conditions of inflammatory origin [6, 7]. In 1989, at the London School of Hygiene and Tropical Medicine, the research group led by Dr. Strachan found an inverse relationship between the number of children in British families, their quality of life, and the rate of hay fever in their children. In other words, the incidence of this disease is higher in families with fewer children and better hygienic conditions; thus, family members have a more limited exposure to the various antigens found in the environment, and this probably leads to a lack of stimulation of the immune system at an early age [6]. However, it was not the first study with these observations; previously, in 1968, a study showed that the Swedish urban population was more susceptible to developing bronchial asthma and chronic bronchitis compared to Swedes living in rural areas [7]. In 1976, another study performed in Canada reported that the prevalence of some atopic diseases, as asthma, eczema, and urticarial was higher in the white community respect to a native community called Metis, which showed an elevated serum IgE level and a higher prevalence of helminthiases, in addition to untreated viral and bacterial diseases [8]. These studies comprise the origin of the so-called "hygiene hypothesis" as we know it today.

Several studies have documented the existence of an inverse relationship between the increased incidence of inflammatory and metabolic diseases and a decreased prevalence of parasitic helminthiases, such as filariasis, where helminths mildly immunosuppress the host in a chronic and non-specific manner [9]. This modulation is associated with the development of a particular immune mechanism referred to as Th2 (T helper 2) response. Derived from the hypothesis that helminths have evolved in parallel with their hosts, it is possible to think that helminths can survive and perpetuate their life cycle because they "control" the host's immune response. Helminths can live for prolonged periods by maintaining their hosts as asymptomatic carriers. It is likely that their surface proteins, as well as those secreted, excreted, and shed from the parasite, play a significant role in immunomodulation, which, collaterally, can benefit the host by reducing the consequences of exacerbated inflammatory responses from a Th1 response. This mechanism is a normally occurring part of many autoimmune disorders. Parasitic infections have also been observed to have beneficial effects on clinical outcomes of allergy patients [1, 10, 11]. The relationship between Th1 and Th2 immune response mechanisms can be understood, considering the immune system as a dynamic but regulated entity within a balance between these Th1 and Th2 antagonistic responses. Naturally, there are certain cells, such as T regulatory lymphocytes (Tregs), which upon receiving certain stimuli can suppress competing responses and maintain the system balance [5].

## **2. Use of helminths in experimental therapies**

Data from tropical and subtropical countries have shown that inflammatory and autoimmune diseases are rare, but helminthiases are very abundant there. However, in those regions, the anthelmintic treatment is associated with an increased rate of chronic degenerative diseases [1]. These findings have suggested that helminths or their products may be useful to control inflammatory diseases amending the host immune response from Th1 to Th2. The Th2 response is characterized by the production of anti-inflammatory cytokines (IL-4, IL-5, IL-10, and IL-13), non-specific and parasite-specific IgEs, as well as the mobilization of mast cells, basophils, and eosinophils. During infection, Treg cells release cytokines (Il-10 and TGF-β) that negatively regulate the Th1 cell subset [3, 12].

Helminths secrete enzymes and hormones, along with their debris, these molecules are the excretory and secretory products (ESP). ESP are the main mechanism of immune response evasion due to their high antigenicity and ability to migrate away from the helminth, "distracting" the immune response and ensuring parasite survival [3, 13]. Studies on helminth immunomodulation derived from this observation have raised interest in the use of total extracts, ESP or even, recombinant proteins as immunomodulatory treatment in animal models and human clinical trials. Most of these studies have reported clinical improvement, but do not address the molecular mechanisms involved in the process [1, 12].

A vast variety of parasites and their ESP have been used in studies that seek to find emerging therapies for many diseases, for example the findings on *Trichuris suis* ova (TSO). This approach has shown therapeutic effects in diseases such as rheumatoid arthritis, inflammatory bowel disease, or multiple sclerosis, with phase 1 and 2 clinical studies being carried out. Patients received a controlled treatment of 2500 TSO units every 2 weeks for 12 months and a low clinical efficacy was obtained, with just small variations in the immune response of the patients receiving the parasite [14]. Another trial used the nematodes *Trichuris vulpis* and *Uncinaria stenocephala* as a treatment in a model of atopic dermatitis in dogs, which were infected with larval eggs of *T. vulpis* (two groups with 500, and 2500 eggs respectively) or *U. stenocephala* (three groups with 100, 500, and 2500 eggs respectively). The results showed that all dogs improved their lesions; however, there was no change in the inflammation caused by subcutaneous infiltrates. In a subsequent randomized study with *T. vulpis*, no difference was found between parasitized dogs and those receiving a placebo, and it was concluded that *T. vulpis* did not generate significant changes [15]. There are few examples of parasites used as disease modulators and how parasites or molecules derived from them can induce an anti-inflammatory response through Th2/regulatory responses directly associated with the established helminth response.

## **3.** *Trichinella spiralis*

*Trichinella* is a nematode genus comprised of 12 species and 3 genotypes. *Trichinella spiralis*, *T. nativa*, *T. murrelli*, *T. britovi*, *T. patagoniensis*, *T. nelsoni* species and T6, T8 and T9 genotypes are distinguished by encapsulation in the host muscle tissues, while *T. pseudospiralis*, *T. papuae* and *T. zimbabwensis* species do not induce capsule formation. The genus *Trichinella* is cosmopolitan and parasitizes more than 150 species of domestic and wild vertebrates, mostly carnivorous mammals. All *Trichinella* species can be transmitted zoonotically, although the one most frequently related to human disease is *T. spiralis*. The parasite load is correlated with the severity of the disease and is the cause host death [13, 16–19].

#### **3.1 Life cycle, physiopathology and diagnosis**

The adult worm settles in the small intestine, while the larvae inhabits the skeletal muscle, this is the muscle larva (ML) which lives inside of a myocyte surrounded by a collagen capsule. In general, there is one ML per myocyte but, sometimes two or more larvae are found [20]. The enteric phase occurs during the first week after infection and is associated with gastroenteritis, diarrhea, and abdominal pain. The life cycle begins when the host ingests raw or undercooked meat with viable ML. In the stomach, the ML is released from the collagen capsule. In the duodenum, the ML

#### *Therapeutic Properties of* Trichinella spiralis *(Nematoda) in Chronic Degenerative Diseases DOI: http://dx.doi.org/10.5772/intechopen.103055*

invade the epithelial columnar cells and molt four times to become adult. At 90 hours after copulation, females deposit the first larval stage, called newborn larva (NBL), which enters the bloodstream. The migration and invasion phase continues during 3 to 7 days and at the end of 30 days post infection, the NBL matured into ML, and the invaded myocyte was repaired, but does not recovered its contractile functions; on the contrary, the glycocalyx hypertrophies, generating a collagen capsule and surrounding itself with a network of new blood capillaries. This new structure is the nurse cell (NC), which allows to the ML remains in hypobiosis for months or years to be transmitted by ingestion to a new host to complete the life cycle. During this last phase, fever, myalgia, and arthralgia are observed; however, individuals with a low parasite load may remain asymptomatic [4]. The diagnostic methods are (1) trichinoscopy, where the ML and the NC are sought by microscopic examination of striated muscle; this technique is used in *post-mortem* studies and food safety protocols. (2) The artificial enzymatic digestion allows isolate ML from meat samples; this method is used in food safety. (3) Antigen detection seeks parasite proteins as biomarkers, mainly in experimental issues (4). Nowadays ELISA and Western blotting are used to determine parasite-specific antibodies in the host serum; this is the gold standard to corroborate clinical suspicion. (5) The molecular diagnosis uses diagnostic probes specific to unique DNA sequences of the parasite for taxonomic purposes [21, 22].

#### **3.2 Immunobiology**

ESP from ML and intestinal larval stages as well as from adult helminths play an important role in a successful infection and trigger an early immune response in the host. Many of these proteins are glycosylated and have an N-terminal signal peptide indicating that they are secreted proteins. The high immunogenicity is due to these glycosylations being formed by repetitive chains of oligosaccharides, such as tyvelose and fucose, that confer them modulating properties of the host immune response. Tyvelose is the main antigenic component of ESP and is part of the ML immunodominant antigens [13, 23, 24]. Proteomics and immunoproteomics analyses have shown that some of these proteins are serine proteases, a family of proteolytic enzymes with varied biological functions during a parasite infection. These functions involve host tissue invasion, migration, and proteolysis by helminths. Serine proteases purified from ESP participate in the degradation of host intestinal tissues. They also allow the penetration of a wide range of tissues for acquiring nutrients, and mediate apoptosislike cell death and phagocytosis, which contributes to a higher parasite-mediated immunosuppression. ESP may play an important regulatory activity by controlling host immune reaction and recognition. In addition to serine proteases, different studies have found other functional proteins involved in the interactions between *T. spiralis* and its host, such as multiple DNase II isoforms that could function as immunomodulators [25].
