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Immunogenomic Approaches to Study Host Immunity to Enteric Pathogens¹

H. S. Lillehoj^*,2, C. H. Kim^*, C. L. Keeler, Jr. and S. Zhang

^* Animal Parasitic Diseases Laboratory, Animal and Natural Resources Institute, Agricultural Research Service, USDA, Beltsville, MD 20705; Department of Animal and Food Sciences, College of Agriculture and Natural Resources, University of Delaware, Newark 19716; and Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Jackson 39762

² Corresponding author: hlilleho{at}anri.barc.usda.gov

	ABSTRACT

TOP ABSTRACT CHICKEN IMMUNE RESPONSES TO... CHICKEN IMMUNE RESPONSES TO... APPLICATION OF FUNCTIONAL... APPLICATION OF FUNCTIONAL... CONCLUSIONS REFERENCES

 
With increasing consumer demands for safe poultry products, effective control of disease-causing pathogens is becoming a major challenge to the poultry industry. Many chicken pathogens enter the host through the gastrointestinal tract, and over the past few decades, in-feed antibiotics and active vaccination have been the 2 main mechanisms of disease control. However, increasing public concerns are prompting government regulations on the use of growth-promoting drugs in animal production, and the ability of current vaccines to protect against emerging hypervirulent strains of pathogens is becoming an issue. Therefore, there is a need to develop alternative control strategies against poultry pathogens of economic importance as well as to carry out basic research to enhance understanding of host-pathogen interactions at local sites of infection. Effective control strategies against pathogens can only be accomplished by comprehensive analysis of the basic immunobiology of host-pathogen interactions. Recent sequencing of the poultry genome and the availability of several tissue-specific cDNA microarrays are facilitating the rapid application of functional immunogenomic technologies to poultry disease research. Studies using functional genomic, immunology, and bioinformatic approaches have provided novel insights into disease processes and protective immunity to chicken pathogens. In this review, we summarize recent published literature concerning the host response to Eimeria and Salmonella infections with emphasis on our studies using immunogenomic tools to investigate and characterize the mechanisms of avian immunity to these mucosal pathogens. The results clearly indicate that this immunogenomic approach will lead to increased understanding of immune responses to infectious agents that will enable the development of effective prevention strategies against mucosal pathogens.

Key Words: pathogen • immunogenomic tool • Eimeria • Salmonella

	CHICKEN IMMUNE RESPONSES TO EIMERIA

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Coccidiosis is a poultry disease of substantial economic importance, estimated to cost the US industry greater than $700 million annually. In the absence of efficient vaccines to control coccidiosis and the emergence of new antigenic variants of field stains of Eimeria, the broiler industry has relied upon prophylactic medication. However, anticoccidial drugs are expensive, and their effectiveness is hindered by widespread incidence of drug resistance and the high cost of new drug development (Chapman, 1998). Moreover, there are increasing government regulations on the use of in-feed antibiotic growth promoters, and most nontherapeutic antibacterial feed additives have been banned in the European Union since 1999 (European Union legislation, Council Regulation 2821/98). Therefore, it is of interest to develop alternative control strategies for coccidiosis (Dalloul and Lillehoj, 2005). In this regard, understanding the complex nature of the interaction among various factors controlling protective immunity to Eimeria is critical (Lillehoj et al., 2004; Dalloul and Lillehoj, 2005).

Examination of the patterns of disease resistance following experimental coccidiosis has suggested 2 separate mechanisms of protective immunity: innate immunity during initial pathogen exposure (primary infection) and acquired immunity following secondary infection (Lillehoj et al., 1999). In immune chickens, parasites enter the gut early after infection but are prevented from further development, indicating that acquired immunity to coccidiosis may involve mechanisms that inhibit the natural progression of parasite development (Trout and Lillehoj, 1996). Although a direct role of immune effector lymphocytes in inhibiting parasite development has not been demonstrated, CD8⁺ cytotoxic T cells and interferon- (IFN-) have been identified as key components of the host protective immune response (Lillehoj and Choi, 1998; Lillehoj et al., 2004). At the genetic level, both MHC-linked genes and non-MHC genes have been implicated in controlling host immunity to coccidiosis (Lillehoj et al., 1989), and our recent study demonstrated the existence of a quantitative trait locus controlling disease resistance located on chicken chromosome 1 (Zhu et al., 2003; Kim et al., 2006).

The development of novel strategies to control coccidiosis will only be realized after completion of a systematic and detailed analysis of local host-parasite interactions at the molecular and cellular levels. In particular, there is a need to advance fundamental knowledge of the basic immunobiology of the events associated with parasite invasion and intracellular development, as well as parasite biology and metabolism. Immune responses to coccidia are extremely complex, with different effector mechanisms playing roles at various stages of parasite development. The level of susceptibility to coccidiosis depends on a variety of factors including the particular Eimeria species, prior host exposure, the nutritional status of infected chickens, and the genetic makeup of the host (Lillehoj et al., 2004), and in many of these areas, we are only beginning to scratch the surface of elucidating their roles in Eimeria infection.

One area that has received considerable attention in recent years is cell-mediated immunity to coccidiosis, particularly with respect to the cloning and analysis of chicken cytokines and chemokines. Cytokines mediate intercellular signals during normal immune responses and have been investigated as potential vaccine immunopotentiators for avian coccidiosis (Lillehoj and Lillehoj, 2000; Lillehoj et al., 2004). Most chicken cytokines homologous to their mammalian counterparts have been described (Kaiser et al., 2000; Hong et al., 2006b), of which IFN- has shown promise as an immunomodulator (Yun et al., 2000). The chicken IFN- gene has been cloned (Song et al., 2000), transfected into chicken fibroblast cells, and shown to inhibit intracellular development of Eimeria tenella following in vitro infection (Lillehoj and Choi, 1998). An identical effect on E. tenella development in vivo has been observed after administration of recombinant IFN- protein to chickens before challenge with virulent parasites. Recently, a 19-kDa recombinant Eimeria acervulina protein (3-1E) stimulating IFN- production by chicken spleen cells was identified as profilin. Recombinant Eimeria profilin expressed in bacterial and eukaryotic vectors has been shown to induce protective immunity against live challenge infection when injected into young chickens (Lillehoj et al., 2000). Coadministration of recombinant profilin with cDNA encoding chicken IFN- or interleukin- (IL) 15 has been shown to lead to further enhancement of Eimeria-specific immunity.

At the level of the host response to coccidiosis, a novel peptide secreted by T and natural killer (NK) cells with antiparasitic activity has been identified as NK lysin (Hong et al., 2006a). Natural killer lysin is an antimicrobial and antitumor protein, and a full-length cDNA encoding chicken NK lysin has been isolated from a library prepared from chicken intestinal intraepithelial lymphocytes (IEL). Quantitative reverse transcription-PCR (RT-PCR) analysis of NK lysin gene expression has revealed high levels of transcripts in the intestinal IEL and spleen cells but low levels in thymic lymphocytes. Following infection with Eimeria maxima, NK lysin transcript levels have been shown to increase 3- to 4-fold in CD4⁺ and CD8⁺ intestinal IEL. Recombinant chicken NK lysin expressed in COS7 cells have been shown to exhibit antitumor cell activity against LSCC-RP9 tumor targets and have been directly cytotoxic for sporozoites of E. acervulina.

Recent development of functional genomic technologies and the availability of tissue-specific chicken cDNA microarrays have facilitated a more comprehensive investigation of host-pathogen interactions during avian coccidiosis (Min et al., 2003; Hong et al., 2006b,d). New candidate genes that influence host immune response to Eimeria have been identified using global gene expression analyses of chicken macrophage and intestinal IEL cDNA microarrays (Min et al., 2005; Hong et al., 2006a,c). These new developments in our understanding of host-pathogen immunobiology in avian coccidiosis raise the exciting possibility of developing novel control strategies against coccidiosis.

	CHICKEN IMMUNE RESPONSES TO SALMONELLA

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Salmonella is a gram-negative, aerobic, noncapsulated, nonsporulating, motile bacillus that infects amphibian, avian, and mammalian hosts. Salmonellosis is one of the most important foodborne zoonotic diseases throughout the world, and poultry represent an important source of infection in man. Among the 2,200 serovars of Salmonella, Salmonella enterica serovar Enteritidis (Salmonella Enteritidis) and Typhimurium (Salmonella Typhimurium) are responsible for the majority of foodborne enteritis in humans. Approximately 4% of the US population suffers from at least 1 case of food poisoning each year, with an estimated cost of almost $6 billion. During the last decade, many strategies have been applied in an effort to reduce Salmonella contamination on commercial poultry farms. These include use of antibiotics and other chemical feed supplements that inhibit Salmonella adhesion in the intestine, competitive exclusion by nonpathogenic bacteria, genetic selection of chicken strains for improved immune responses, and development of Salmonella vaccines (McHan et al., 1991; Bailey et al., 1998; Kaiser et al., 2000).

Salmonella enterica infection in chickens triggers both humoral and cell-mediated immune responses. Immunoglobulin G, IgA, and IgM produced in the intestinal mucosa and serum, CD4⁺ T and IgG⁺ B cells in cecal tonsils, and IgA⁺ and IgM⁺ B cells and CD4⁺ T cells in spleen are all increased during experimental salmonellosis in chickens (Hassan et al., 1991; Brito et al., 1993; Sasai et al., 1997, 2000). Interestingly, CD8⁺ T cells are decreased in the cecal tonsils and spleen after primary infection but not following secondary challenge infection with Salmonella (Sasai et al., 1997; Babu et al., 2003). Additional changes in immune cells have been observed in the ovary and oviduct following primary and secondary infections with Salmonella Enteritidis. The number of macrophages and T cells in ovary and oviduct epithelium have been shown to increase significantly by d 7 postinoculation, peak at d 10, and return to preinoculation levels by d 21. The increases in macrophages and T-cell proliferation have been shown to immediately precede a decline in the number of Salmonella Enteritidis-positive tissues (Withanage et al., 1998, 2003). The correlation of elevated lymphocytes and macrophages with decreased Salmonella Enteritidis recovery suggests that local cell-mediated immunity is involved in controlling Salmonella Enteritidis colonization. Interleukin-1, IL-6, IL-8, and transforming growth factor-ß4 are highly induced, whereas IL-18 and IFN- are downregulated in chicken heterophils or in immune organs with Salmonella Enteritidis infection (Beal et al., 2004). A CXC chemokine, K60, and macrophage inflammatory protein-1ß (MIP-1ß) have been upregulated in the intestine and liver (Withanage et al., 2004), and increased levels of other cytokines such as CCLi2, CXCLi2, IL-10, IL-12, and IL-12ß also have been demonstrated in Salmonella Enteritidis-infected chickens (Cheeseman et al., 2007).

In mammals, macrophage recognition of Salmonella is carried out through the cell surface pathogen-associated molecular patterns such as toll-like receptors (TLR) and CD14. Toll-like receptor 4 binds to the lipopolysaccharide (LPS) of gram-negative bacteria (Takeuchi et al., 1999) and activates macrophages through an intracellular signaling pathway that results in the induction of various proinflammatory cytokines (e.g., IL-6) and cytotoxic molecules (e.g., NO). In chickens, macrophages express TLR4 and CD14 (Dil and Qureshi, 2002), and TLR4 has been linked to a Salmonella Enteritidis-resistant phenotype in broilers (Leveque et al., 2003). Toll-like receptor 5, which recognizes bacterial flagella, has also been described in chickens (Hayashi et al., 2001), and its expression has been associated with IL-1ß production (Iqbal et al., 2005). Chicken TLR15 is a newly identified pathogen-associated molecular patterns receptor, and its expression in the cecum of Salmonella Typhimurium-infected chickens suggests a role in host defense against bacteria (Higgs et al., 2006).

Different serovars of Salmonella use different survival strategies to circumvent host immunity, but the underlying mechanisms of immune evasion in avian salmonellosis are not well understood. For instance, Salmonella Typhimurium infection of chickens induces lymphocyte depletion and atrophy of lymphoid organs that may have a critical role in Salmonella pathogenesis (Hassan and Curtiss, 1994). Furthermore, Salmonella Enteritidis invasion downregulates IL-1 and IL-2 production, whereas Salmonella Gallinarum infection does not induce any inflammatory cytokine response including IL-1, IL-2, or IL-6 (Kaiser et al., 2000). Although the underlying mechanisms that Salmonella utilizes to evade protective host immune responses need to be better identified, Salmonella vaccines have shown promise in controlling the infection in poultry. For example, following vaccination of chickens with killed Salmonella Enteritidis, increased splenic lymphocyte proliferation and serum IL-2 and IL-6 levels were observed by Okamura et al. (2003, 2004). Protective immune response to vaccination depends on several variables, including age: Four-week-old chickens have shown higher lymphoproliferation response to Salmonella Enteritidis LPS and flagella compared with 8-mo-old birds. Younger chickens have also produced higher levels of IFN- and IL-2 by antigen-stimulated splenocytes after vaccination with an inactivated Salmonella vaccine compared with older animals, but the vaccination has been shown to induce higher cytokine production regardless of age. Furthermore, splenocytes from vaccinated chickens have shown significantly increased numbers of TCR⁺ cells at 7 d postvaccination compared with nonvaccinated controls, although no significant differences in the number of CD4⁺, CD8⁺, or TCRß⁺ cells was observed. Higher levels of Salmonella Enteritidis flagella-induced NO production have been observed at 4, 7, 11, and 14 d postvaccination, and serum IFN-, IL-1, IL-6, and IL-8 have been shown to be elevated at 7 d following Salmonella vaccination. These results indicate that younger chickens demonstrate a more robust antigen-specific immune response to the Salmonella Enteritidis vaccine compared with older birds, and vaccination induces a T-cell–mediated proinflammatory response. However, further studies are necessary to obtain a more comprehensive understanding on host-bacteria interaction in avian salmonellosis.

	APPLICATION OF FUNCTIONAL GENOMICS TO COCCIDIOSIS

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Complete sequencing of its genome together with the availability of various tissue-specific microarrays has opened the field of large-scale functional genomic analyses of chickens. Among these approaches, DNA microarray technology, which allows high throughput measurement of global gene transcription on a whole-genome or tissue-specific basis, now enables the investigation of the transcription status of complex biological systems. Specifically, genomics technology combined with immunology (immunogenomics) allow in-depth analysis of complex immunological processes based on large-scale genomic approaches.

Deoxyribonucleic acid arrays have become popular, because they are generally considered easier to use compared with the other gene expression profiling methods, and they allow the parallel quantification of thousands of genes from multiple samples. Gene expression analysis using microarrays has become a powerful tool to evaluate the complexity of host-pathogen immunobiology. Conceptually, DNA array technologies rely on nucleic acid hybridization between labeled free targets derived from a biologic sample and an array of many DNA fragments (the probes, representing genes of interest), tethered to a solid surface. The targets, often produced by reverse transcription of mRNA and simultaneous labeling of the corresponding cDNA, form a complex mixture of fragments that hybridize with their cognate probes during the assay. The signal generated on each probe reflects the mRNA expression level of the corresponding gene in the sample. After detection, quantification, and integration of signals with specialized software, intensities are normalized for technical deviations, providing a gene expression profile for each sample comparable to profiles from other samples.

Although a variety of large-scale commercial microarrays are available for human and other mammalian species, there are few such tools available for agricultural species. In chickens, a limited number of low- and high-density cDNA microarrays have been developed (Morgan et al., 2001; Min et al., 2003; Neiman et al., 2003; Cogburn et al., 2004; Bliss et al., 2005). More recently, a consortium of research groups developed a 13,000-element chicken cDNA microarray, and there is a commercially available whole-genome chicken oligonucleotide array (Affymetrix Corp., Sunnyvale, CA) for use by the avian research community. Our own studies have focused on the development of a chicken intestinal IEL array (CIELA) that contains >10,000 EST selected from a normalized chicken cDNA library. Included in this array are chemokine, cytokine, and lymphokine elements, TLR, IFN-antiviral response pathway genes, genes involved in the oxidative burst response, apoptosis-related genes (including perforin and granzyme B), and a variety of genes encoding surface markers for monocytes, macrophages, heterophils, dendritic cells, and T cells. In addition, 2 tissue-specific chicken cDNA microarrays with about 5,000 and 10,000 preselected EST from macrophages (Bliss et al., 2005) and intestine IEL (Min et al., 2005), respectively, have been used to investigate local gene expression profiles and host innate and adaptive immune responses of broiler chickens infected with Eimeria.

The avian macrophage microarray (AMM) and CIELA have been successfully applied to different avian cell or tissue types as well as a variety of pathogens, and both have proven to be useful tools to identify global gene expression changes. For example, changes in expression of genes involved in innate immunity have been observed in vivo from experimentally infected chickens and in vitro using peripheral blood monocytes, heterophils, nonadherent blood lymphocytes, and multiple avian macrophage cell lines. In addition, innate immune responses have been elucidated following stimulation with Salmonella, Escherichia coli, Mycoplasma, influenza viruses, Eimeria, cellular components (LPS), and immune modulators (IFN-) using these arrays. Use of the AMM has led to better understanding of the innate immune response mediated by avian macrophages in response to 3 antigenically distinct species of Eimeria, E. acervulina, E. maxima, and E. tenella (Dalloul et al., 2007). In a study by Dalloul et al. (2007), a set of core response elements was identified comprising 25 genes, including many immune-related genes, whereas 60 to 67 elements were uniquely induced or repressed by the individual species. Such differential responses may be attributed to the species-specific immunity induced by different Eimeria species, and a deeper look into the functional aspects of these elements may lead to the elucidation of the pathogenicity, immunogenicity, or both, of each species and better design of a coccidiosis vaccine.

One may ask, "Why is there a need to determine the similarities and differences in host immune responses induced by Eimeria species?" Eimeria acervulina, E. maxima, and E. tenella are the most common coccidia encountered in the field, and each infects a unique site in the chicken intestine. Infections, when not deadly, induce protective immunity against subsequent challenges; however, such immunity remains confined to homologous species with no cross-species protection (Lillehoj et al., 2004). Among the 3, E. maxima infection is characterized by high immunogenicity, in which priming infection with few oocysts induces full protective immunity to subsequent homologous challenge. Conversely, far more E. acervulina and E. tenella oocysts are required to induce comparable protective immunity. For these reasons, identification of the early host responses at the gene transcription level provides a molecular immune profile of the events that occur during and immediately following infection with Eimeria.

The total number of unique elements exhibiting significant expression changes using the AMM is shown in Table 1. Table 2 summarizes the response of several immune-related genes following infection of chickens with E. acervulina, E. maxima, and E. tenella. Tables 3 and 4 list the 20 genes demonstrating the greatest increases or decreases in expression following infection with E. maxima. Of the 25 core response elements that have been induced by all 3 Eimeria are several important immune effector genes, such as the proinflammatory cytokine IL-1ß, the chemokines ah221 and MIP-1ß, and osteopontin (Table 2). By quantitative RT-PCR, IL-1ß has been shown to be highly induced (>5-fold) by the 3 Eimeria parasites. Rodenburg et al. (1998) showed that IL-1ß is secreted by macrophages and other cells upon activation by a variety of different stimuli. In turn, IL-1ß upregulates the production of other chemokines including MIP-1ß, K203, and ah221 and cytokines such as osteopontin, thereby amplifying the immune response. Furthermore, MIP-1ß and K203 belong to the CC chemokine family, normally involved in the recruitment of macrophages. Using IFN--stimulated HD11 chicken macrophages, Laurent et al. (2001) observed similar results, suggesting that macrophages are the main effector inflammatory cells at Eimeria infection sites. Osteopontin has been described as an important component of early cellular immune responses (Patarca et al., 1993). It is known to directly induce chemotaxis and indirectly facilitate macrophage migration to other chemoattractants and has been characterized as an early protein expressed by activated macrophages and NK cells (O’Regan et al., 2000). Osteopontin enhances T helper (Th) 1 and inhibits Th2 cytokine expression. In mice, it directly induces macrophages to produce IL-12 and inhibits IL-10 expression by LPS-stimulated macrophages (Ashkar et al., 2000).

Following treatments with different Eimeria species, IL-18 and MIP-1ß have also shown differential gene expression; IL-18 has been induced at 18 h in E. acervulina-and E. tenella-treated macrophages but only after 48 h in response to E. maxima exposure. Expression of MIP-1ß has been shown to peak at 18 h in response to E. acervulina and E. tenella sporozoites, but it has been shown to induce at its highest level at 4 h postinfection with E. maxima. Further, although little is known about the chemokine ah221, it is noteworthy that it has been upregulated very early on after infection with all 3 Eimeria infections, albeit at a much higher level in E. acervulina, and the transcript levels have been shown to decrease progressively with time. In summary, although a shared similarity in transcript quality exists among the 3 Eimeria parasites using the AMM, a difference remains obvious in the magnitude, direction, and timing of the immune responses to each individual species. Preliminary studies using the CIELA to assess local transcriptional changes following coccidiosis have revealed that 1,120 elements have been significantly modulated, confirming the results from the AMM. These studies using functional genomics clearly indicate the power of applying new molecular tools to investigate complex interactions between host and pathogen.

	APPLICATION OF FUNCTIONAL GENOMICS TO SALMONELLOSIS

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Salmonellosis is complex disease involving many different components of host immunity (Monack et al., 2004). Because different serovar of Salmonella elicit different cellular and humoral immune responses, understanding the pathogenesis of this microorganism and developing new strategies against salmonellosis will require comprehensive investigation of the cellular and molecular events that occur during the invasion, survival, and induction of protective immunity. Functional genomics offers a powerful tool in this regard to assess changes in immune-related gene expression on a global basis. As of 2006, however, few studies have been reported describing alterations in gene expression during avian salmonellosis. Van Hemert et al. (2006) compared gene expression profiles in the intestines of Salmonella-infected chicken lines with differing growth rates and found that faster-growing broiler chickens showed enhanced expression of T cell activation-related genes, whereas slow-growing broiler chickens demonstrated macrophage activation-related genes at 1 d postinfection. Recently, we investigated transcriptional profiling of avian macrophages following in vitro infection with Salmonella with the goal of characterizing the cellular and molecular events that occur during Salmonella Enteritidis persistence in chickens. Using the AMM, the transcriptional profiles of HD11 cells infected with nalidixic acid-resistant Salmonella Enteritidis were analyzed at 2, 5, and 24 h postinfection. Out of 4,906 array elements, 338 genes exhibited 2-fold increases or decreases in expression (P < 0.001). Table 5 shows the 10 most representative annotated elements that were altered at the 3 time points. The chemokine ah294 consistently showed the highest expression at all time points examined in the Salmonella Enteritidis-infected HD11 cells. Additionally, ah294 is a CC chemokine that activates innate immune responses and prevents the apoptosis of virus-infected mammalian macrophages (Tyner et al., 2005). Also, CLN8 may play a role in protein secretion during immune activation (Winter and Ponting, 2002), and its defect has been associated with neuronal ceroid lipofuscinosis 8 disease, which is characterized by the accumulation of ATP synthase subunit C. Other immune-related genes with enhanced gene expression include immune-responsive protein 1, IL-6, inducible T cell costimulator, antiapoptotic NR13, matrix metalloproteinase 9, and glutamate-Cys ligase. The antiapoptotic NR13 protects cells from apoptosis by counteracting the proapoptotic protein BAX in chickens (Lee et al., 1999). Glutamate-Cys ligase is a rate-limiting enzyme for glutathione synthesis and is associated with oxidative stress reduction as well as NO production (Murphy et al., 1991).

In conclusion, global transcriptional profiling analysis of Salmonella infection in chicken macrophages showed that many different genes are involved in the host response to Salmonella, and further studies are needed to better understand pathogen survival strategy and Salmonella persistence inside the host. Furthermore, integrated analysis of both in vivo local gene expression using the CIELA and in vitro transcription changes of immune-related genes in chicken macrophages following Salmonella infection needs to be carried out to obtain a better understanding of host-pathogen immunobiology in salmonellosis.

	CONCLUSIONS

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In the poultry industry, there are mounting concerns over increasing governmental regulations of the use of in-feed antibiotic growth promoters, which historically have been used to raise poultry under high density housing conditions. In addition, questions concerning the ability of current vaccines to adequately protect against emerging hypervirulent strains of pathogens and a lack of suitable, cost-effective adjuvants are raising new interest in the development of alternative control strategies against poultry pathogens such as Eimeria and Salmonella. Comprehensive investigation of the immunobiology of host-pathogen interactions in coccidiosis and salmonellosis must precede the development of novel effective alternative control strategies for these pathogens. Application of immunogenomic and proteomic tools to enteric disease research is critical not only to enhance our understanding of basic immunological mechanisms of the avian host but also to develop efficient vaccination protocols against pathogens of economic importance to the poultry industry.

	ACKNOWLEDGMENTS

 
This work described in this review has been supported, in part, by a competitive grant from the National Research Initiative, Cooperative State Research, Education, and Extension Service, USDA, grant 2004-35204-14798. We thank Erik P. Lillehoj for critical review.

	FOOTNOTES

 
¹ Presented as part of the Ancillary Scientists Symposium, Functional Genomics: Building the Bridge between the Genome and Phenome, Poultry Science Association Annual Meeting, Sunday, July 16, 2006.

Received for publication February 6, 2007. Accepted for publication February 10, 2007.

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