Neuroimmunoendocrine Interactions in Murine Cysticercosis: From the Lab Bench Work to Its Possible Applications in Controlling Porcine Cysticercosis and Human Neurocysticercosis

1. Introduction

The immune and neuroendocrine systems are interconnected by a network in which hormones, antigens, receptors, cytokines, antibodies and neuropeptides modulate immune response in connection with neuroendocrine changes while maintaining homeostasis (Besedovsky & del Rey, 1996). Two of the main components of this network are the hypothalamic-pituitary-adrenocortical (HPA) and the hypothalamic-pituitary-gonadal axes (HPG) (Besedovsky & del Rey, 2000; Rivier & Rivest, 1993).

Interactions between the immune system and both the HPA and HPG axes are characterized by their activation and starting of the stress response, which, in turn, has immunomodulating activities that are important in preventing excessive immune responses (Chikanza & Grossman, 2000; Morales-Montor et al., 2004b). Furthermore, the important functions of both axes have been shown for adaptation and maintenance of homeostasis during critical illness and viral, bacterial, parasitic and autoimmune diseases (Besedovsky & del Rey, 1996). An important aspect of cell communication that has emerged as a result of studying neuro-endocrine-immune interactions is the redundancy of the use of some chemical messengers to communicate among them. As an example, neurotrophins are chemical messengers first identified and characterized in the nervous system. Members of this protein family are also expressed and secreted by immune and endocrine cells, and have immunological and endocrinological functions (Haddad et al., 2002).

Thus, the lack ubiquitous use of some cellular messengers by different organic systems might be a rule, rather than an exception. Although strong evidence supports that neurons, endocrine and immune cells produce hormones, while neural, endocrine and immune cells synthesize and secrete neuroactive messengers (Ferone et al., 2006), it remains somewhat controversial whether this network is involved in the final outcome of parasitic diseases, and, particularly in cysticercosis (natural and experimental) (Morales-Montor & Larralde, 2005).

Taeniids, particularly Taenia solium, the porcine cysticercosis (CC) and human neurocysticercosis (NCC) causal agent; and Taenia crassiceps, murine cysticercosis causal agent, are highly evolved parasites that have developed diverse mechanisms of survival that facilitate their establishment in the host. These mechanisms can be roughly grouped into two types: 1) evasion of the immune response by molecular mimicry or by inactivating effector immune processes (i.e, complement inhibition) (Baig et al., 2005; Laclette et al., 1989); and 2) by exploiting the host system to allow its establishment, growth or reproduction (Escobedo et al., 2004). This exploitation mechanism provides parasites with a dual benefit: first, obtaining amino acids for metabolism, and second preventing the surface-bound antibody from interfering with cytotoxic cells interacting with the parasite (Damian, 1989; Locksley, 1997). A striking example of host’s molecules exploitation is the ability of several taeniids to use host-synthesized cytokines as indirect growth factors for themselves and also exploit the hormonal microenvironment within the host in their favor (Escobedo et al., 2005). Taeniids have evolved structures similar to the steroid and protein hormone receptors expressed in upper vertebrates, with binding properties and terminal effects similar to the hormonal metabolites synthesized by the host (Gomez et al., 2000). Such cross regulation from host to parasite has been described in at least eight parasitic infections that are caused by parasitic cestodes, like schistosomiasis (LoVerde et al., 1985), filariasis (Rajan et al., 2005), hydatid disease (Brehm et al., 2003) and murine cysticercosis (Escobedo et al., 2004).

Understanding how the host’s endocrine system can favor the establishment of taeniids under certain circumstances, led us to explore the parasite’s hormone receptors that might be involved, designing hormonal analogs and drugs to specifically affect the parasite (Escobedo et al., 2004). This review is focused on the literature concerning sex-steroids, adrenal steroids and other hormones and neurohormones that have been studied in regard to control parasite loads in experimental murine cysticercosis.

1.1. Experimental murine cysticercosis

Due to the intrinsic difficulties in working with the natural hosts (pigs and humans) of T. solium and the high costs of enough pigs plus the slowness in data retrieval, we have used an experimental cysticercosis approach to gain knowledge of the complex host (H) parasite (P) relationship in CC.Murine intraperitoneal cysticercosis is caused by the taeniid T. crassiceps and it has been useful to explore the physiological host factors associated with porcine cysticercosis, and to some degree, with human NCC(Larralde et al., 1990). Intraperitoneal T. crassiceps cysticercosis of mice (Huerta et al., 1992; Larralde et al., 1990; Smith et al., 1972) lends itself well to controlled and reproducible experimentation, generating numerical data of parasite loads in individual mice in a matter of weeks after infection. Its general representation of other forms of cysticercosis has later been strengthened by similar results in other mouse and parasite strains (Larralde et al., 1986), by the parasite’s extensive sharing of antigens with other taeniids and cestodes and by the DNA homology between T. crassiceps and T.solium(Larralde et al., 1986; Larralde et al., 1990). These characteristics have made murine cysticercosis a convenient instrument to test vaccine candidates (Sciutto et al., 1990) and new drugs or treatments against CC. Several features of natural cysticercotic disease have been found by extrapolation from experimental murine cysticercosis (Morales-Montor et al., 2002a; Sciutto et al., 2007).

1.2. Immuno-endocrine interactions in the host

In order to figure out the effect of infection on the immune system, on the feminized male mice, thymic cell analysis performed by flow cytometry showed a diminution in the content of CD3+, CD4+, and CD8+ subpopulations in the infected mice. This suggest that the increase in estradiol levels of the host could change the expression pattern of several genes that participate in apoptosis regulation in the thymus of male mice during chronic infection with T. crassicepscysticerci, by inhibiting the inhibit the specific cellular immune response to the parasite (Morales-Montor et al., 1998). Previous immunological experiments had led to suspect that estradiol positively regulates parasite reproduction in hosts of both genders, presumably by interfering with the thymus-dependent cellular immune mechanisms that obstruct parasite growth (Th₁) and favoring those that facilitate it (Th₂) (Bojalil et al., 1993; Terrazas et al., 1994). A specific shift from Th₁ to Th₂ immune response in the course of infection was found that coincided with the initial low rate of reproduction that accelerates at later times of infection. The shift is characterized by a marked decrease of IL-2 and IFN-γ in both genders, while the secretion of cytokines involved in the specific humoral response (IL-6, IL-10 and IL-4) is enhanced (Terrazas et al., 1998; Terrazas et al., 1999). Thus, striking differences in susceptibility to cysticercosis between male and female mice may involve the joint action of the immune system and the gonads, both driven by a parasite, which is able to change the parasite´s restrictive male normal hormonal milieu during chronic infection to a more parasite´s permissive female environment.

To strengthen the above notions and in an effort to identify the sex steroids involved we studied the effects of testosterone (T₄), dihydrotestosterone (DHT), and 17β-estradiol (E₂) in castrated mice of both genders infected with T. crassiceps cysticerci (Morales-Montor et al., 2002a). In this study, we found that castration and treatment with either T₄ or DHT before infection markedly decreased parasite loads in both gender mice, while the treatment with E₂ increased it in both genders (Morales-Montor et al., 2002a). The specific splenocyte cell proliferation and IL-2 and IFN-γ production were depressed in infected-castrated mice of both genders, while treatment with T₄ or DHT produced a significant cell proliferation recovery and enhanced production of IL-2 and IFN-γ (Morales-Montor et al., 2002a). An opposite effect of the same sex steroids was found on the humoral response: it was unaffected with T₄ or DHT restitution, while the treatment with E₂ in both genders augmented the levels of anti-cysticerci IgG, as well as IL-6 and IL-10 production. These results suggest androgens mediate immune functions, which protect mice from cysticercosis, possibly through the stimulation of the specific cellular immunity of the host (Morales-Montor et al., 2002a).

Immunoendocrine interactions during cysticercosis are the cornerstone of the feminization of male mice. When the infected male mice have an intact immune system, there is an increase in serum estradiol levels and a decrease of those of T₄ and DHT. However, when the immune system is knocked down by total irradiation or neonatal thymectomy, there is no change in the levels of serum steroids in chronically infected male mice, and the levels remain steady between infected and uninfected male mice (Morales-Montor et al., 2001). Interleukin-6 (IL-6) was demonstrated a key factor in this puzzle: IL-6^-/- (KO) infected mice do not develop the feminization process, while the restitution with IL-6 again allows the feminization. The expression of IL-6 gene in the testes of parasitized mice was enhanced, a fact that can explain the primordial role of the testes in the feminization process produced by cysticercosis (Morales-Montor et al., 2002a; Morales-Montor et al., 2001). Thus, IL-6 activates aromatase expression in the testes of the cysticercotic mice and produces active aromatization from androgens to estrogens. The increased serum levels of follicle stimulating hormone (FSH), the natural activator of aromatase expression, found in the chronically infected mice, supports the notion that FSH could be also a factor involved in the feminization process in the male mouse (Morales-Montor et al., 2001). Actually, macrophage-migration inhibitory factor (MIF) is known to be involved in immunoendocrinological processes during sex-associated susceptibility in cysticercosis. Thus, to determine the role of IL-6 and MIF during infection, knockout (KO) mice were used, and the number of parasites and serum sex-steroid levels were measured. It was found that IL-6 and MIF KO mice of both genders infected with T. crassiceps cysticerci harbor similar numbers of parasites, with no change in sex-hormone levels. However, in wild-type strains, the sex-associated susceptibility to infection is observed, concomitantly with the feminization process in the chronically infected mice (Morales-Montor et al., 2002b). These results suggest a role for both IL-6 and MIF genes in sex-associated susceptibility in murine T. crassiceps cysticercosis.

The importance of sex hormones driving the specific immune response during cysticercosis was assessed by administration of fadrozole (a P450-aromatase inhibitor) in male and female mice to suppress the production of E₂(Morales-Montor et al., 2002c). A reduction was found in parasite loads (~ 70%) in infected mice treated with fadrozole. The protective effect of the P450-aromatase inhibitor was associated in male mice with a recovery of the specific cellular immune response. IL-6 serum levels, and its production by splenocytes were dramatically augmented, together with an increase in its expression in the testes of infected male mice. Fadrozole treatment returned these levels to baseline values. These results suggest that P450-ase and IL-6 are key molecules in feminization undergone by infected male mice and in regulating parasite loads. Fadrozole treatment appears as a possible new therapeutic approach to control murine cysticercosis (Morales-Montor et al., 2002c) and perhaps other parasites with active asexual reproduction in intermediate hosts.

Progesterone (P₄) was recently tested and implicated in the regulation of the parasite loads during murine cysticercosis. P₄ treatment has a dichotomic effect: in non-gonadectomized (intact) of both genders, P₄ treatment increased parasite loads, while gonadectomized mice, P₄ completely decreases parasite loads, an impressive and unprecedented cysticidal effect. In the first case, the effect of P₄ is possibly associated to the manipulation of the specific cellular immune response, besides the steroid’s promotion of parasite reproduction (Vargas-Villavicencio et al., 2006). In a second analysis, it was demonstrated that infected mice that received P₄ treatment increased estrogen levels two-fold compared to infected control mice. (Vargas-Villavicencio et al., 2005; Vargas-Villavicencio et al., 2006). A flow chart of the main immunoendocrinological effects of T. crassiceps infection in male and female mice is depicted in Figure 1.

Figure 1.

Early immune events during murine cysticercosis. During murine cysticercosis, there is a marked sexual dimorfism in parasite loads, having females, higher parasite loads than males, because of the effect of E₂ and T₄. These hormones also regulate immune responses. During the second week post infection, there is a clear Th1 response characterized by the expression of STAT4, high levels of IFN-γ and reduced levels of IL-4. After 4 weeks post-infection, immune response polarizes to a Th2 profile, in which STAT6, high levels of IL-4 and reduced levels of IFN-γ are observed. All these changes correlate to an overproduction of E₂. IL-6 levels are also increased as infection takes place. High levels of this cytokine are associated to a feminization process, as observed by the low levels of T₄. Doted line: Female parasitic load. Solid line: Male parasitic load. Signs ($\uparrow$and $\downarrow$) denote increase or decrease phenomena.

The major steroid produced by the adrenal gland is the androgen dehydroepiandrosterone (DHEA). So, another set of experiments showed DHEA effect on male and female infected mice. DHEA treatment reduced parasite loads by 70 and 80% respectively. In contrast with the common assumption of DHEA as an immune-stimulatory hormone, the immune responses of our mice, characterized by the expression of IL-2, IFN-γ, IL-4 or IL-10, was not affected by DHEA treatment (Vargas-Villavicencio et al., 2008). In vitro, treatment of T. crassiceps cysticerci with DHEA induced 80% reduction in parasite reproduction, which may partially explain the reduction of parasite loads observed in vivo a partial effect suggesting the involvement of other unknown factors in the in vivo regulation of parasite loads (Vargas-Villavicencio et al., 2008). In addition, the use of 16α-bromoepiandrosterone (EpiBr), a DHEA analog without significant androgenic activity, inhibited proliferation of cysticerci in vitro and decreased the T. crassiceps cysticerci load up to 50% in the peritoneal cavity of Balb/c mice (Carrero et al., 2010).

1.3. Behavioral changes in the infected host

The hormonal changes in cysticercotic mice, profoundly affect their behavior, such as sexual activity (Morales et al., 1996), aggressiveness (Gourbal et al., 2001), social status (Gourbal et al., 2002) prey/predator defense responses, and also cognitive functions. Male mice infected with T. crassiceps show remarkable changes in sexual behavior, characterized by a complete loss of the ejaculation response early at the infection (six weeks), followed by a gradual decrease in the number of mounts and intromissions, and their latencies increased, until none of the parasitized mice showed any sexual response toward female mice (Morales et al., 1996). Moreover, it was demonstrated that changes in sexual behavior were due to the change in the normal production of sex-steroids by the mouse, since there was a complete restoration of their sexual behavior after T₄ or DHT restitution of the infected male mice (Gourbal et al., 2001). Since c-fos and progesterone receptor gene (PGR), both are key E₂-regulated genes involved in the regulation of sexual behavior, we studied possible changes of c-fos and PGR expression in the central nervous system (CNS) of infected male mice.Indeed, c-fos and PGR expression oscillated with time of infection and to different magnitudes in hypothalamus, brain cortex and preoptic area but neither in other areas of the brain nor in several other organs of the host (Morales-Montor et al., 2004a; Rodriguez-Dorantes et al., 2007).

Furthermore, infection disrupts the dominant-subordinate status (Gourbal et al., 2002). In infected male mice strong perturbations in territorial behavior and aggressiveness were found. In addition, during confrontation between naive infected and healthy mice, infected animals more often assumed a subordinate status than healthy ones. The effects of the infection by T. crassiceps were more likely to prevent adult male mice from becoming behaviorally dominant than to reverse existing dominance relationships (Gourbal et al., 2001).

Significant CNS changes in c-fos, and PRexpression during infection signifies the brain senses the infection episode and may be involved in the ensuing behavioral changes of the infected mice, as well as, through its connectivity, extend the effects of infection to other physiological systems under its influence. That these changes in CNS are beneficial to the host or parasite remains speculative (Klein, 2000). One could argue that feminization of male hosts favors the parasite by allowing reproduction in otherwise restrictive male mice, but equally arguable would be to consider feminization of the male host as deleterious to the parasite’s completion of its cycle by reducing male exposure to its predators, the definitive hosts. Other similar mutually conflicting statements may be elaborated with the above premises, the true ones remain to be identified and could perhaps vary with each different host-parasite relationship.

Figure 2.

Neuroimmunoendocrine changes in host during murine cysticercosis. Infection with Taenia sp, leads to a Th2 response with high levels of IL-6 activating the P450-aromatase, which convert T₄ into E₂ increasing its level in the host. Estrogens are permissive for the infection while androgens (T₄, DHT) inhibit parasite reproduction and reduce parasite load. E₂ overproduction induces behavioral changes in male and female. Signs (+ and -) means induction and inhibition respectively; signs ($\uparrow$and $\downarrow$) denote increase or decrease phenomena.

However, not only male mice are behaviorally affected by CC, female mice also suffer perturbations in their sexual behavior. Infected females were less receptive to the male at 6 weeks post infection (p.i.) and until 12 weeks p.i.; and there was also an interruption in the estrous cycle that was clearly starting at 12 weeks of infection and remained until the 16th weeks. These changes were associated to the low levels of serum E₂ that were comparable to those found during the diestrus stage of control mice (Arteaga-Silva et al., 2009). That sex steroids, particularly E₂ is very important to the host-parasite relationship in CC, was recently demonstrated by Guzman, et al (Guzman et al., 2009). A single injection of E₂ administered to 4-day-oldmale and female mice increased the cellular immune response and induced resistance to T. crassiceps cysticercosis. At the same time, there are changes in the expression pattern of PR and estrogen receptor (ER) isoforms in the brain and splenocytes. Regardless of gender, when treated mice reached adulthood, they were highly resistant to infection. Female mice presented early vaginal opening and altered estrous cycles. In male and female mice, the expression of the PR and ER isoforms in the brain was differentially regulated after neonatal exposure to estradiol. Moreover, an increase in the expression of IL-4 and IFN-γ was found in the serum of experimentally infected neonatally estrogenized animals, which correlated with the observed protection against T. crassiceps infection. In conclusion, early exposure to E₂ permanently modifies immune system activity and sex steroid hormone receptors in the brain, and causes profound changes in sex-associated susceptibility, leading to resistance to helminthic infection (Guzman et al., 2009). A chart of all neuroimmunoendocrine changes during murine cysticercosis is depicted in Figure 2.

1.4.Taenia. solium cysticercosis, neurocysticercosis and taeniosis

As described for the mouse model of CC, host’s biological factors such as genetic background, the innate and acquired immunity and gender can lead to resistance and/or susceptibility to cysticercosis/taeniosis by Taenia solium. It has been shown that sexual hormones play an important role in porcine CC. For instance, castrated male pigs increased by 30% the prevalence of naturally acquired CC, while pregnant sows doubled the prevalence, compared to non castrated males and non-pregnant sows (Morales et al., 2002). Another interesting finding, was found when in experiments designed to explore the hormonal profiles of sex steroids, as well as DHEA in serum of boars that had naturally acquired CC, it was observed a significant reduction in the T₄ levels, change that was independent of the heterogeneous genetic background and large range of ages. These findings suggest that sex steroids can be associated to T. solium susceptibility and transmission dynamics of this parasite in natural conditions (Pena et al., 2007).

Interestingly, sex steroids are not the unique hormones that can be useful to this parasite. It has been recently described that the in vitro culture of T. crassiceps with insulin stimulates parasite reproduction by two-fold and increased the average of the bud diameter. However, this hormone had no effect on T. solium cysticerci culture (Escobedo et al., 2009). On the other hand, hamsters treated with P₄ and experimentally infected with T. solium showed an 80% reduction of anchored-T. solium tapeworms, a reduced growth of these parasites and a down-regulation of the PR expression in 50% duodenum associated tapeworms. Immune response of P₄ treated hamsters showed an increase in IL-4, IL-6 and TNF-α cytokines and an exacerbated inflammatory infiltrate located along the lamina propria, compared to infected non-progesterone treated controls (Escobedo et al., 2011). These results clearly support the fact that P₄ protects against the T. solium adult tapeworm establishment by improving the immune system.

The studies in mice and pigs have already been scaled to the natural human disease. A recent study on human NCC, caused by larvae of T. solium infiltration in the CNS demonstrated a strong correlation between the endocrine, immune systems, gender and age during the clinical manifestation and susceptibility to the disease. Cardenas, et. al, designed a clinical study to evaluate the hormonal changes associated with NCC and the relationships between disease heterogeneity, endocrine and immunological status. Since the proper diagnostic of the disease was fundamental in this study, a precise clinical and radiological description of the disease and a complete hormonal and immunological profile was performed to 50 patients and 22 healthy subjects. Patients had lower DHEA levels compared to healthy subjects. Concerning to gender, male patients showed a clear decrease in 17β-estradiol serum levels and high levels of Luteinizing Hormone (LH), while female patients with clinically severe disease showed lower levels of progesterone and androstenedione. Higher concentrations of follicle stimulating hormone (FSH) and lower concentrations of testosterone (similar to those reported previously during murine cysticercosis) were found in male patients with severe disease, when compared with the less clinically severe patients. Significant correlations were found between E₂ and IL-10 in male patients, and between DHEA and IL-1β, and androstenedione and IL-17 in female patients, suggesting a possible immunoendocrine interaction. The study by Cardenas, et. al. in neurocysticercotic patients constitutes the first demonstration that the presence of T. solium larvae in the central nervous system, can modify the host environment by the induction of endocrine and immunological changes. As the authors point out, these results provide a stimulating background to analyze the repercussions of the hormonal changes during the course of disease and on patient reproductive health (Cardenas et al., 2012). Taken together, the above-presented evidence open up the possibility of the use analogues of hormones, as novel anti-cysticercotic agents.

1.5. Direct effect of sex steroids on Taenia crassiceps and Taenia solium cysticercy

As described before, hormones play a significant role in murine T. crassiceps cysticercosis and participate in the susceptibility to T. solium cysticercosis. However, this effect is not only due to the regulation of the immune response by sex steroids, but also by acting directly upon cysticerci reproduction. For instance, it has been shown that in vitro treatment with E₂ and P₄ stimulates T. crassiceps reproduction. In contrast, T₄ or DHT inhibit and even exert a toxic effect on the parasite (Escobedo et al., 2004). The possible molecular mechanisms by which sex-steroids affect T. crassiceps reproduction imply the presence of sex hormone receptors, since the use of tamoxifen, an anti-estrogenic compound (Escobedo et al., 2005), decreases parasite reproduction in vitro and parasite loads in vivo, as described before (Vargas-Villavicencio et al., 2007), we look for a estrogen receptor expressed by parasitic cells. As described by Ibarra, et. al. there is a molecule similar to the estrogen receptor alpha (ERα) identified by diverse methods such as western blot and PCR. This receptor is expressed exclusively by parasitic cells, as identified by the expression of a parasite-specific protein, paramyosin (Ibarra-Coronado et al., 2011). Parasite cells expressing the ER-like protein were located by confocal microscopy in the subtegumental tissue exclusively. Sequencing of the spot produced a small fragment of protein similar to the mammalian nuclear ER. In addition to, once host’s E₂ has bound to its parasite estrogen receptor, the dimeric complex would activate the transcription of several T. crassiceps proliferative genes, such as c-fos, c-jun and cyclin D1, directly or through the activation of the MAPK family of serine/threonine kinase, such as ERK, which has been also recently described in this parasite (Escobedo et al., 2010). Since this hypothetical molecular mechanism could be in vitro interrupted by using tamoxifen, we suggest a genomic action mechanism for E₂ on the parasite.

On the other hand, the androgen mechanism might be different from the found for estrogens and progesterone. T₄ and DHT directly affect parasitic DNA integrity probably by activating apoptotic mechanism in the cysticercus cells. This experimental finding is not dependent of a nuclear receptor because flutamide (a well studied and used anti-androgen) did not have effects upon parasite reproduction in vitro(Escobedo et al., 2005). These results demonstrate that sex steroids act directly upon parasite reproduction perhaps by binding to receptors closely resembling classic and specific sex-steroid vertebrate receptors (Escobedo et al., 2005).

Furthermore, it has been shown that T. crassiceps cysticerci are able to metabolize sex steroids in vitro and, and that it possibly uses hosts hormones as positive or negative factors for its own reproduction (Gomez et al., 2000). T. solium cysticercosis is a health problem in underdeveloped and developed countries. Sex hormones are involved in cysticercosis prevalence in female and male pigs. Interestingly, progesterone increased T. solium scolex evagination and worm growth, in a concentration-independent pattern. This effect could be mediated by the expression of a novel T. solium progesterone receptor (TsPR) since RU486 inhibits both scolex evagination and worm development induced by progesterone (Figure 3).

Figure 3.

Effect of progesterone (P₄) and RU486 (progesterone analogue) in cultured cysts of T. solium. Progesterone promotes scolex evagination and worm growth while RU486 inhibit scolex evagination.

Using RT-PCR and western blot, sequences related to progesterone receptor were detected in the parasite. A phylogenetic analysis reveals that TsPR is highly related to fish and amphibian progesterone receptors, whereas it has a distant relation with birds and mammals. Conclusively, progesterone directly acts upon T. solium cysticerci, possibly by binding to a progesterone receptor synthesized by the parasite. The strong effects of sex steroids upon cysticercosis open new avenues for its control. In Table 1, there is a list of some hormone receptors reported in helminth parasites to date.

Hormone Receptor	Parasite	Ref/AC
Epidermal Growth Factor	Schistosoma mansonii; Echinococcus multilocularis	(Ramachandran et al., 1996); (Spiliotis et al., 2003)
Insulin Receptor	Schistosoma mansonii; Schistosoma japonicum; Echinococcus multilocularis	(Khayath et al., 2007); (You et al., 2010); (Konrad et al., 2003)
Retinoid X receptor	Schistosomamansonii; Brugia malayi	(Freebern et al., 1999); (Tzertzinis et al., 2010)
Transforming growth factor-β	Brugia pahangi	(Gomez-Escobar et al., 1997)
Retinoic acid-binding protein	Onchocerca volvulus; Onchocerca gibsoni, Dipetalonemaviteae; Brugia pahangi; Dirofilaria immitis	(Sani et al., 1985)
Estrogen Receptor	Taenia crassiceps	(Ibarra-Coronado et al., 2011)
Constitutive androstane Receptor	Schistosoma mansonii	(Hu et al., 2006)
Estrogen-related receptor-β	Schistosoma japonicum	(Wu et al., 2012)
Putative Follicle-stimulating hormone receptor	Trichinella spiralis	XP_003375811
Ecdysone receptor	Brugia malayi; Dirofilaria immitis	(Tzertzinis et al., 2010); (Shea et al., 2010)
Nuclear Hormone Receptors	Brugia malayi; Dirofilaria immitis; Onchocerca volvulus; Echinococcus multilocularis	(Crossgrove et al., 2002); (Crossgrove et al., 2008); (Unnasch et al., 1999); (Forster et al., 2011)

Table 1.

Principal hormonal receptors in parasites reported to date.

In vitro effects of human chorionic gonadotropin (hCG) on the larval stages of T. crassiceps (WFU strain) and T. solium have also been reported. As a matter of fact,these effects were correlated to the presence of receptors for hCG in different developmental phases of both cultured parasites. Authors argued that direct effect of hCG can be recognized as a factor contributing to the growth and development of T. crassiceps and T. solium cysticerci (Castellanos-Sanchez et al., 2009). T. crassiceps and T. soliumcysticerci have the ability to metabolize exogenous androstenedione to T₄, using different hormonal precursors (Jimenez et al., 2006). A chart of all the immunoendocrine effects during T. solium cysticercosis is presented in Figure 2.

Finally, both T. crassiceps and T. solium expressed the insulin receptor, although insulin had effects only on T. crassiceps. Thus, insulin may be recognized by T. crassiceps cysticercus cells as a mitogenic factor, and contribute to parasite proliferation inside the host. This phenomenon has not been reported for cysticercosis caused by T. solium, which could, in part, be related to the poor effect of insulin upon the human parasite (Escobedo et al., 2009).

1.6. Therapeutic applications of hormones in cysticercosis/taeniosis

The study and evaluation of new substances of hormonal nature and its analogues has emerged with great importance to develop a new generation of anti-parasitic drugs. In the case of T. crassiceps, the reconstitution and substitution (depending the host and hormone used) with sex steroids showed that T₄ or DHT treatment before infection decreased parasite loads by 50 and 70% respectively, suggesting a protective role for androgens (Morales-Montor et al., 2002a). In contrast the treatment with E₂ increased it by three times in both genders as compared with controls. Even more, castrated mice of both genders treated with progesterone before infection, decreased parasite loads by 100% compared with intact uninfected mice. However the negative effects in the establishment, growth and reproduction of T. crassiceps was mediated by the metabolism of progesterone to DHEA in the adrenal gland of castrated animals (Vargas-Villavicencio et al., 2006). To strength out the notion of DHEA as a protective agent, we designed experiments to test this hypothesis, by incubating in vitroT. crassiceps cysticerci with increasing concentrations of DHEA, and by in vivo treating male and female mice with DHEA and infecting them with T. crassiceps cysticerci. In vitro, DHEA has a strong cysticidal effect, being dose-dependent in physiological and pharmacological conditions. In vivo assays showed that DHEA reduce the viability of cysticerci in both male and female mice (Carrero et al., 2006; Carrero et al., 2010).

The use of EpiBr (analogue of DHEA) reduced the parasitic load in a major proportion in both male and female infected with T. crassicepps(Freilich et al., 2000). Tamoxifen is an antiestrogen compound belonging to the group of Selective Estrogen Receptor Moduladors (SERM). Administration of tamoxifen in infected mice of both genders produced an 80% parasite load reduction in female mice, and a weaker effect of 50% in male mice. This protective effect was associated in both genders, with an increase in the mRNA levels of IL-2 (a cytokine associated with protection against cysticerci) and IL-4 (no effect on infection). Tamoxifen treatment modified E₂ production on females, whereas serum testosterone was not affected. However, the expression of the 2 types of ER, i.e., ER-αand ER-β in the spleen of infected mice of both genders, was decreased by tamoxifen treatment. In vitro, treatment of T. crassiceps with tamoxifen reduced reproduction and loss of motility. These results indicate that tamoxifen treatment is a new therapeutic possibility to treat CC, because it can act at both ends of the host–parasite relationship, i.e., by increasing the cellular immune response protective against the parasite and by directly affecting the parasite’s reproduction and survival (Vargas-Villavicencio et al., 2007). Table 2 has a summary of the above-mentioned therapeutic use of hormone analogue in T. crassiceps cysticercosis.

Compound	Infection	Reference
DHEA / DHEAS	Cryptosporidium parvum	(Rasmussen & Healey, 1992)
Plasmodium falciparum	(Kurtis et al., 2001)
Entamoeba histolytica	(Carrero et al., 2006)
Schistosoma mansoni	(Fallon et al., 1998)
Trypanosoma cruzi	(dos Santos et al., 2005)
16α-bromoepiandrosterone (EpiBr or HE2000)	Taenia crassiceps	(Carrero et al., 2010)
Entamoeba histolytica	(Carrero et al., 2010)
AIDS	(Reading et al., 2006)
Feline immunodeficiency virus	(Pedersen et al., 2003)
Plasmodium falciparum; Plasmodium berghei	(Freilich et al., 2000)

Table 2.

Therapeutic use of DHEA or an analogue (EpiBr) during several parasitic infections.

2. Concluding remarks

The evidence presented above illustrates the complexity and importance of neuroimmunoendocrine interactions during CC and provides clues to the many other possible mechanisms of parasite establishment, growth and reproduction in an immunocompetent host. Further, strong neuroimmunoendocrine interactions may have implications in the control of transmission and treatment of this parasitic disease in porcine and humans. In practical importance, the complexity of the cysticerci-host relationship suggests that all physiological factors (i.e., sex, age) should be taken into account in the design of vaccines and new drugs.

The differential response of cysticerci to sex steroids may also be involved in their ability to grow faster in the murine female or feminized male host. Host and parasite sex-associated biases may be combined to favor their evolution towards a mutually acceptable relationship. Moreover, the changes in behavior observed during CC, should not be regarded as simple biological curiosities but more as strong evidence of the plasticity of the host phenotype in response to infection by parasitic helminthes. Furthermore, by changing the reproductive, aggressive and dominant capacity of the host, parasites generate novel questions regarding to the evolution of host-parasite relationships in addition to only the prey/predator interaction.

Finally, we have documented here that a complex interactive network involving the immune, endocrine and nervous systems of the mouse, as well as the reproductive system of the cysticerci, is in control of the parasite load of each infected individual mouse. If such complex a management of the parasite loads, as that we shown here between mice and cysticerci, extends to other parasite diseases of mammals, as current research seems to indicate in a number of infections, their means of exploration, fuller understandings and forms of control must be reviewed and approached with designs matching in complexity and plasticity that of the infections.

Acknowledgement

Financial support: Grant # 40072-Q from Consejo Nacional de Ciencia y Tecnología (CONACYT) de México, and Grant # IN-208103 from Programa de Apoyo a Proyectos de Investigación e Inovación Tecnológica (PAPIIT) from Dirección General de Asuntos del Personal Académico (DGAPA), U.N.A.M., both to J M-M. KNC received a postdoctoral fellowship from Dirección General de Asuntos del Personal Académico (DGAPA), U.N.A.M. and RHB received a postdoctoral fellowship from Instituto de Ciencia y Tecnología del Distrito Federal (ICyTDF).