**5. Intestinal immunity against coccidiosis**

GALT is a master tissue in the immune response against coccidiosis because of three crucial functions: acquired immunity development in both cellular and humoral immune responses (including antigen processing and presentation), antibody production and cytokine production [37]. Cellular immunity seems to be the most important effector mechanism against coccidial infection [38]. It is orchestrated by subsets of lym bearing either αβ or γδ T cell receptor (TCR) [39]. Natural infections of epithelial cells such as *Eimeria* infections, for while TCR γδ cells are scarce in systemic circulation, they are commonly represented among IEL [40, 41]. Taking into account that *Eimeria* initiate the first contact with epithelial cells, it is tempting to speculate that IEL may be the first line of defense in response to *Eimeria* antigens which were processed and presented by major histocompatibility complex (MHC) expressed by epithelial cells [42]. Adaptative immune response against coccidiosis requires the involvement of these two pathways enabling proteins of MHC to be loaded with *Eimeria* epitopes. Only liganded expressed on the surface of antigen-presenting cells (APC) can activate T lym, which then execute effector functions, such as cytotoxicity, provision of help to B cells, and cytokine production [43]. In chickens, as in mammals, there are two subsets of lym classified by the system of cluster of differentiation (CD). These are CD4+ (known as T helper) and CD8+ (cytotoxic T cells). Adaptative immunity is highly dependent on T helper cells, and its activation is determined by MHC antigens [44]. Whereas CD8+ only recognizes peptides presented in the context of MHC-I molecules, T lym CD4+ recognizes peptides in the context of MHC-II molecules, and supports for co-stimulatory signals and other molecules. These molecular interactions underlying the regulation of the immune response between T lym and APC are known as immunological synapses [45].

This process is critical during the anticoccidial immune response in chickens. During the infection, the immune system inhibits parasitic development at three key stages in the *Eimeria* life cycle. The first is the sporozoite's search for binding sites in the epithelium cell, which allows it to penetrate the epithelium. While this is relevant, it is not particularly significant. Immune selection against life-cycle stages after the sporozoite stage may be more significant. Sporozoites are usually mentioned because immunity is so effective, but there are several studies that have studied later lifecycle stages and revealed that immunity can also inhibit multiple stages later in the life cycle.

The second stage is when sporozoites are placed within intraepithelial lymphocytes in the villus (IEL). Finally, sporozoite migrate from lamina propria to the crypt [46]. T cells are undoubtedly the protagonist in modulating anticoccidial immunity. Cytotoxic lym has been observed after a primary *Eimeria* challenge with the subsequent increase of interferon gamma (IFNγ) activating proinflammatory pathways to inhibit intracellular *Eimeria* parasite development in host cells [47]. Natural killer (NK) cells are also an important component of the intestinal immune response against coccidiosis [48]. Some subpopulations of NK mediate spontaneous cytotoxicity in chicken intestinal IEL underlying the statement that they are crucial for

### *From Understanding the Immune Response against Coccidiosis to the Use of Coccidia Vaccines DOI: http://dx.doi.org/10.5772/intechopen.110611*

intestinal immunity. NK cell activity depends on the infection stage, which decreases during early stages of infection, and recovers to normal levels 1 week after primary infection as well as in the early stages of secondary infection [49].

There are several cytokines and chemokines reported that play a predominant role during coccidiosis infection including IL-1b, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12, IL-15, IL-17, IL-18, IFNγ, transforming growth factor (TGF)-β1, and tumor necrosis factor, among others [38, 50]. Despite the high number of cytokines described in the pathogenesis of the disease, IFNγ and IL-10 are the key cytokines for host protection and susceptibility against parasitic infections, respectively [51, 52]. Detrimental effects on the parasite have been reported as a result of IFNγ release. This is because of the inhibition of parasite invasion and survival in the host cell as well as the promotion of local inflammation [53], free radical production [54, 55], activation of antibody-dependent cell-mediated cytotoxicity [56] and/or the promotion of the release of cytoplasmic granules containing perforin and proteases [57]. IL-10 has an inhibitory role in the intestinal immune response due to the interference with Th1 response and this decreases the ability of the host to eliminate the parasite [58]. Therefore, IL-10 is a proposed mechanism of host evasion by *Eimeria*. IL-10 have different functions such as the inhibition of nuclear factor kappa B and the suppression of proinflammatory cytokines enrolled in parasite cleaning from the intestinal cells [59]. In the end, the balance between Thl and Th2 responses is crucial to the outcome of the infection, and the cytokine network involved in the control of the immune response needs to be elucidated.

The role of humoral immunity against coccidiosis is still controversial, and there is more consideration paid to cellular immunity responses. Humoral immunity appears to play a minor role in resistance against infection. In one of the classical studies, in which the BF was removed, chickens were not affected after a secondary infection despite their ability to produce immunoglobulins [60]. During *Eimeria* infection, specific antibodies are produced, but they do not seem to be involved in controlling the infection [61] and immunoglobulin levels are not correlated with disease susceptibility [62]. IgA was also considered important as humoral protection against parasite invasion in earlier studies [63]. In a chicken kidney cell line model of *Eimeria* infection, caecal content from immunized chickens was co-cultured. Sporozoite invasion did reduce. However, there was no correlation in either antibody levels or the neutralization of sporozoites [64]. One of the major challenges has been to replicate *in vivo* results from the *in vitro* findings regarding the humoral immune response against Coccidiosis.

Immunoglobulins, therefore, do not appear to play an important role in protective immunity against Coccidiosis and cell immunity seems to be more crucial. Manuscripts underlying the key role of antibodies and humoral immunity as a protective mechanism against coccidiosis have been published, however [65]. It was determined that IgY antibodies injected systemically are capable of reaching the site of infection and effectively blocking parasite development in the intestine [66]. A positive association between antibody titers and protection [67] was also shown. In other studies, it was established that egg IgY from hens immunized with live infections of *Eimeria acervulina*, *Eimeria maxima*, and *Eimeria tenella* could be used as a feed additive to passively protect young chicks against all three species [68, 69]. The results described above support the concept that providing large amounts of protective antibodies to young chicks, through passive or maternal immunization, can interrupt the growth, development, and replication of *Eimeria*. Although antibody production is a mechanism to limit the propagation of several pathogens [65], T-cell

mediated response is the major criterion for the control of intracellular parasites such as *Eimeria* [39, 70].

## **6. Vaccines as a strategy to control coccidiosis**

Vaccines provide an effective strategy for the control of coccidiosis in chickens and benefit the sustainability of the poultry industry worldwide [71]. The first vaccine against coccidia utilized a sporulated oocyst of a live *Eimeria tenella* wild type strain, and it was initially launched in 1950. This vaccine was based on the concept that low doses of oocysts over a number of days induced protective immunity against a homologous challenge [72, 73]. Current *Eimeria* vaccines are marketed and consist of live wild-type (virulent) parasites or live attenuated vaccines (precocious lines). Thus, up till now, there are more than 25 commercial anticoccidial vaccines utilized in poultry (reviewed in [73, 74]).

In breeders, vaccination programs based on live vaccines are tremendously useful and have been very successful. There are, however, some hurdles such as homogenous mass application to the flock. If the application is not done correctly, it may lead to suboptimal immunization and insufficient protection against the different *Eimeria* species. Even with homogenous mass application to the flock, there are additional hurdles which can lead to uneven application, triggering outbreaks [75].

A recent report showing the vaccine-induced immune response was published [76]. Briefly, three important findings were reported. First, *Eimeria* species can elicit an innate immune response by expressing TLR21 in macrophages through the recognition of pathogen-associated molecular patterns (PAMPs). Next, Coccidia vaccine induced a Th1 pattern characterized by proinflammatory cytokines and cell subsets in both systemic and local lymphoid organs. Second, *Eimeria tenella* induced the strongest activation of macrophages. Cellular analysis showed that vaccination led to an increase in macrophages and activated T cells (immunophenotypes CD8 + CD44+ and CD4 + CD44+). Other important effects were reported, including a decrease in fecal oocyst shedding as well as an improvement in body weight gain. However, this was not statistically different.

Precocious lines are defined as lines of *Eimeria* selected from a population that complete their endogenous life cycle in the host more quickly than wild-type parent strains. They are not only different because of an abbreviated life cycle but also by significant attenuation of virulence [77, 78]. Therefore, precocious lines are proposed as a successful strategy to control coccidiosis because they are less pathogenic than their parents, no adverse effects are observed in vaccinated birds and, despite their reduced multiplication within the intestine are able to stimulate protective immunity which is virtually as good as that induced by their pathogenic parent strains [75].

## **7. Conclusions**

For more than 70 years, the main tools for the prevention and control of coccidia were performed using coccidiostats. As the number of available products is limited and no new molecules have been introduced in the last 30 years, it is a challenge to keep coccidiostats as effective as they were at their introduction to the poultry industry. Parallel to the advances in our knowledge of the avian immune system and the study of avian coccidiosis immune responses, strategies which can protect the birds

*From Understanding the Immune Response against Coccidiosis to the Use of Coccidia Vaccines DOI: http://dx.doi.org/10.5772/intechopen.110611*

against different species of *Eimeria* are being considered to overcome health issues caused by coccidiosis.

The application of both types of vaccines (wild-type live strains and attenuated or precocious vaccines) are still a challenge due to mass application. Advantages and disadvantages of each vaccine exist. Therefore, it deserves continuous research and field work in different scenarios and facilities to identify effective control strategies for avian coccidiosis which will ultimately benefit the sustainability of the global poultry industry.
