General aspects of the avian enteric immune system

As in mammalian species, the avian immune system includes well developed mucosa-associated lymphoid tissue (MALT), which is the first line of defence on mucosal surfaces (LILLEHOJ u. LILLEHOJ 2000; YUN et al. 2000b; BAR-SHIRA et al. 2003). MALT represents the largest lymphoid organ in the body and consists of antigen-presenting cells, immunoregulatory cells and effector cells, which are mainly located in the lamina propria (LP) mucosae and tela submucosa. The lymphoid tissue of avian MALT is organized in lymphoid follicles, as well as scattered or aggregated lymphoid cells (LILLEHOJ u. TROUT 1996; DAVISON 2008; CASTELEYN et al. 2010). A major component of MALT, which is located in the intestinal tract, is called gut-associated lymphoid tissue (GALT). It contains

more than half of the total lymphocyte pool of the MALT (YUN et al. 2000b) and mounts immune responses against various parasitic, viral and bacterial enteral pathogens (ROTHWELL et al. 1995; MAST u. GODDEERIS 1999; MUIR et al. 2000).

Morphologically, GALT consists of two layers, which are separated by a basal membrane. In the outer layer are located intraepithelial lymphocytes (IEL), which are scattered between epithelial cells, and beneath the basal membrane are the lamina propria, which is rich in lymphocytes and submucosa (LILLEHOJ u. LILLEHOJ 2000; DAVISON 2008).

In comparison to mammals, the avian immune system does not possess structured peripheral lymph nodes (BAR-SHIRA et al. 2003; DAVISON 2008; CASTELEYN et al. 2010). This emphasises the role of the avian GALT as the major secondary lymphoid organ for the defence against avian intestinal infections (OLAH et al. 1984; LILLEHOJ u. TROUT 1996;

MUIR et al. 2000).

Avian GALT contains unique lymphoid structures along the gut (YUN et al. 2000b;

CASTELEYN et al. 2010), such as the cecal tonsils (DEL CACHO et al. 1993; KITAGAWA et al. 1998; JANARDHANA et al. 2009), Meckels diverticulum (OLAH u. GLICK 1984;

BESOLUK et al. 2002) and the bursa cloacalis (RATCLIFFE 2006; CASTELEYN et al.

2010), which have not been described for mammalian species. In addition, birds possess, analogue to mammals, Payer Patches (BEFUS et al. 1980; BURNS 1982), lymphoid follicles within the lamina propria, with varying degrees of organisation, and single lymphoid cells scattered throughout the epithelium and lamina propria of the GALT (YUN et al. 2000b).

Antigen stimulation in the gut of chicken usually leads to the development of diffuse lymphoid tissue in the GALT (DAVISON 2008).

Avian GALT consists of a diverse set of lymphoid cell subsets. Heterophils, eosinophils, macrophages, natural killer cells, dendritic cells and T and B lymphocytes are present in different proportions along the gut, dependent on age, location and antigen stimulation (LILLEHOJ u. CHUNG 1992; LILLEHOJ 1993; GÖBEL et al. 2001; BAR-SHIRA et al.

2003).

IEL are a special cell population of the GALT. Avian IEL mainly consist of TCRαβ + and TCRγδ+ T cells and natural killer cells (NK) (GÖBEL et al. 2001; DAVISON 2008). Most of the avian IEL T cells express a CD8α co-receptor, whereas TCRγδ+CD8α+ IEL are more dominant than TCRαβ+CD8α+ IEL (BUCY et al. 1988; COOPER et al. 1991; LILLEHOJ et al. 2004). The population of IEL CD4+ T cells is very small and B cells are almost absent among those (LILLEHOJ 1993; VERVELDE u. JEURISSEN 1993). IEL have been shown to release several cytokines, such as different interleukins and IFN-γ and influence the activities of intestinal epithelial cells (YUN et al. 2000b).

In the lamina propria various leukocytes, such as granulocytes, macrophages, dendritic cells and B- and T lymphocytes are present. B and T cells compound about 90% of the LP lymphocyte pool, the rest are NK cells (DAVISON 2008). In contrast to IEL, CD4+ T cells are more numerous among the LP lymphocytes than CD8α+ T cell subsets, and TCRαβ+ T cells are more dominant than TCRγδ+ lymphocytes (ROTHWELL et al. 1995). Most of the B lymphocytes in the LP express the secretory IgA isotype (YUN et al. 2000b).

In comparison to mammals, chicken lack some components of the anthelmintic worm responses that are controlled by the Th2 cytokines and are important in the immune reactions following parasitic infections in mammalian species. Chicken have a reduced repertoire of polymorphonuclear cells, neutrophils, eosinophils, and basophils. They are replaced by heterophils, which are predominant cell type in the innate inflammatory reactions. Recently it has been shown that the chicken orthologue of the gene for the Th2 cytokine IL-5, which is important in the mobilization of the bone marrow eosinophil pool in mammals is a pseudogene (KAISER et al. 2005). IgE, which is produced by B cells and play an essential role in nematode resistance in mammals, has not been described for birds. It is suggested that avian IgG partly fulfils the functions of mammalian IgE (DAVISON 2008).

2.4.1. Immunity to enteric parasitic infections in birds

At present, the knowledge about immunity to enteric parasites in birds is mainly based on studies with protozoan parasites in chicken, such as Eimeria. It has been shown that mechanisms of resistance can vary between different Eimeria species (spp.), and the level of

immunity to Eimeria is highly influenced by the genetics of the host (LILLEHOJ u. RUFF 1987; ROSE 1987; BUMSTEAD et al. 1995; TROUT u. LILLEHOJ 1996).

T lymphocytes have been shown to play a crucial role in immunity to coccidia in chicken (LILLEHOJ u. TROUT 1993; TROUT u. LILLEHOJ 1996). The protective immunity to Eimeria has been shown to be TCRαβ+ T cell dependent, in which both CD4+ and CD8+

cells are involved (LILLEHOJ u. TROUT 1996; TROUT u. LILLEHOJ 1996; DAVISON 2008). Partial depletion of CD4+ cells generated by intra-peritoneal injections of anti-CD4 monoclonal antibodies resulted in an increased oocyst shedding rate following primary Eimeria tenella infection in chicken (TROUT u. LILLEHOJ 1996). The mRNA expression of numerous cytokines in the intestinal tissue was upregulated due to Eimeria infections in chicken, but only the T helper (Th) 1 type cytokine IFN-γ induced a protective effect (LILLEHOJ u. CHOI 1998; YUN et al. 2000a; HONG et al. 2006).

The role of cell-mediated immunity in intestinal protozoan infections has also been demonstrated in other parasite models. Studies on thymectomized and bursectomized chicken, which were infected with Cryptosporidium baileyi, indicated a primary role of T cells in the resistance to the infection. Thymectomized chicken showed an increase in the total parasite oocyst shedding rate and failed to resist challenge infection (SRETER et al. 1996).

Not much work has been published so far on the specific immune reactions following helmintic infections in birds. Recently, it has been demonstrated, that Th2 polarisation of the immune response and induction of systemic circulating specific IgG antibodies in the course of nematode infection also exists in avian species. The studies, which were performed on Ascaridia galli-infected chicken, demonstrated systemic and local increase in IL-4 and IL-13 mRNA expression in splenic and ileal tissues (DEGEN et al. 2005; KAISER 2007). It has also been shown that chicken develop circulating IgG antibodies against A. galli soluble somatic antigen and embryonated egg extract starting two to three weeks after infection (MARCOS-ATXUTEGI et al. 2009). However, there is a lack of information on the local cell-mediated immunity in nematode infection in chicken.