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IMMUNOLOGY, HEALTH AND DISEASE |
* Department of Poultry Science, Auburn University, 260 Lem Morrison Drive, Auburn, AL 36849-5416; and USDA-ARS, Egg Safety & Quality Research Unit, Russell Research Center, 950 College Station Road, Athens, GA 30605
1 Corresponding author: fasinyo{at}auburn.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, and antiinflammatory interleukin (IL)-10 were determined at 5 and 10 d postchallenge (PC). Intestinal flushes were also collected from each treatment at 7 d PC to estimate IgA and IgG. Results showed an upregulation in IL-1β mRNA in STC chicks at 5 d PC. By 10 d PC, the expression of IL-1β was further increased and accompanied by an upregulation of IL-6 and interferon- mRNA, whereas IL-10 mRNA expression decreased. It was concluded that ST induced an intestinal mucosal inflammatory response in commercial source broiler chicks less than 2 wk of age.
Key Words: cytokine • Salmonella Typhimurium • intestine • commercial-type broiler chick
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Typically, intestinal pathogen colonization is controlled through the administration of broad-spectrum antibiotics. However, misuse of these medications has lead to the emergence of drug-resistant strains of bacteria. In recent years, increasing numbers of antibiotic-resistant isolates of Salmonella have been found (Low et al., 1997; Kiessling et al., 2002; CDC, 2006). In addition, there is increasing consumer and governmental pressure to reduce or remove drugs from feed manufactured for food animals (Young and Craig, 2001). These factors have promoted the exploration of a variety of antimicrobial feed additives and immunotherapies such as vaccination and cytokine therapies for use as alternative control strategies (Barrow et al., 2003; Asif et al., 2004; Lowry et al., 2005; Doyle and Erickson, 2006). Specifically, vaccines (live, attenuated, and killed) developed against Salmonella spp. appear attractive, but their use has met limited success (Lowenthal et al., 2000). Although killed vaccines are preferred because they do not pose the risk of reverting to live (Okamura et al., 2004), they are generally less immunogenic and require more potent adjuvants for long-term protective immune response (Lowenthal et al., 2000). Cytokines function in mediating immune responses to infections, and they have been proposed as natural powerful adjuvants for vaccines (Grimble, 1998; Lowenthal et al., 2000; Takehara et al., 2003; Wigley and Kaiser, 2003; Barouch et al., 2004; Asif et al., 2004).
The gastrointestinal tract is the first site encountered by orally ingested pathogens, with the cecum being the major site of colonization in chickens (Barrow et al., 1988). Identification of cytokines that mediate intestinal immune response during Salmonella infection could perhaps increase the repertoire of bioactive compounds available for evaluation as adjuvants for mucosal (or orally administered) vaccines. In mammals, binding of ST to intestinal cells is known to trigger the production of pro-inflammatory cytokines such as interleukin (IL)-1 and IL-6, chemokines such as IL-8, and type 1 T helper (Th1) cell cytokines such as IL-2, interferon- (IFN-), tumor necrosis factor-
(TNF-), IL-12, IL-15, and IL-18, but downregulate Type 2 T helper (Th2) cell cytokines such as IL-4 and IL-10 (Eckmann and Kagnoff, 2001; Coburn et al., 2007). The Th1 cytokines predominantly mediate cellular immunity against intracellular bacteria and viruses, thereby activating cellular mechanisms that culminate in phagocyte-dependent inflammation (Romagnani, 2000; Rodríguez-Sáinz et al., 2002; Corthay, 2006). On the other hand, Th2 cytokines inhibit several functions of phagocytic cells, regulate humoral immunity, regulate or suppress inflammation, and promote phagocyte-independent inflammation (Romagnani, 2000; Rodríguez-Sáinz et al., 2002; Corthay, 2006).
In chickens, it has been reported that binding of Salmonella to intestinal epithelium induces the recruitment of heterophils and macrophages to the site of infection in the intestine (Henderson et al., 1999; Kogut et al., 2002, 2003; Okamura et al., 2005). Also, several in vitro and in vivo studies have established an inflammatory immune response to Salmonella in chickens by demonstrating an upregulation in the expression of inflammatory cytokine IL-1β and IL-6, chemokine IL-8, Th1 cytokines IL-2, IFN-, IL-12, and antiinflammatory transforming growth factor-β4 (TGF-β4) in immune cells (heterophils and macrophages) isolated from peripheral blood or lymphoid tissues such as spleen and liver (Leshchinsky and Klasing, 2001; Ferro et al., 2004; Okamura et al., 2005; Swaggerty et al., 2006; Kogut et al., 2006). Whereas these studies have characterized the systemic immune response to Salmonella enterica, especially serovar Enteritidis in layers, the experimental models often bypassed the intestine without evaluating the immune response in this tissue. Furthermore, we cannot extrapolate any data obtained with Salmonella Enteritidis to ST because differences exist in cytokine responses to different strains of Salmonella. For instance, Kaiser et al. (2000) observed that invasion of avian cells by Salmonella Gallinarum did not induce the production of any of the inflammatory cytokines (IL-1β, IL-2, or IL-6) assessed, whereas invasion of Salmonella Enteritidis downregulated IL-1β and IL-2.
To the best of our knowledge, only few studies have evaluated cytokine responses to ST infection in the intestine and cecum of meat-type chickens. For instance, Withanage et al. (2004) challenged specific-pathogen-free (SPF) Rhode Island Red chicks with 108 cfu of ST at 1 d of age and found that IL-1β mRNA was upregulated within 12 to 48 h postchallenge (PC) in the ileum and cecum. On the other hand, Withanage et al. (2005) found no change in expression of IL-1β in the intestine of infected chicks when the challenge (with same dose of 108 cfu of ST) was delayed to 7 d of age. Similarly, Sadeyen et al. (2004) observed that cecal IL-1 was not significantly upregulated until 2 wk PC when they inoculated SPF chicks with 104 cfu of Salmonella Enteritidis at 7 d of age. They also found that IL-8 and IFN- were significantly upregulated after 2 and 5 wk PC, respectively. In another study, Beal et al. (2004a) found no significant change in the expression of IFN-, IL-1, and TGF- β4 when they inoculated SPF White Leghorn Line N chickens with 1.6 x 108 cfu of ST at 6 wk of age. Collectively, these studies were done with SPF chickens, and we cannot extrapolate these findings to commercial source broiler chickens because the genetic background of these 2 types of chickens differ (Leshchinsky and Klasing, 2001; Barrow et al., 2003; Sadeyen et al., 2004; van Hemert et al., 2006; Cheeseman et al., 2007). In addition, the data obtained are inconclusive and somewhat varied with the dose of Salmonella used for inoculation and the age at which chicks were inoculated or challenged.
Thus, we conducted an experiment to evaluate intestinal and cecal cytokine responses to ST infection in commercial source broiler chicks at 4 d of age. The expression of inflammatory cytokines IL-1β and IL-6, Th1 cytokine IFN-, and Th2 antiinflammatory cytokine IL-10 were measured in the jejunum, ileum, and cecum at 5 and 10 d PC. Luminal concentrations of Salmonella-specific IgA and IgG in the intestine were also measured on d 6 PC.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Procedures used in this study were approved by the Auburn University Institutional Animal Care and Use Committee. A spontaneous nalidixic acid-resistant mutant of an ST strain of poultry origin (Haroldo Toro, Department of Pathobiology, Auburn University, Auburn, AL) was used to challenge chicks in this study. The frozen ST culture was thawed and 10 µL was inoculated into 10 mL of sterile tryptic soy broth (Acumedia Manufacturers Inc., Baltimore, MD). Inoculated broth was incubated overnight at 37°C (Precision 30M High Capacity Incubator, Precision Scientific, Winchester, VA), then streaked onto xylose lysine tergitol 4 (XLT4) (Acumedia Manufacturers Inc.) plates containing Tergitol 4 supplement (Remel, Lenexa, KS) and 0.1% nalidixic acid (Sigma Chemical Co., St Louis, MO). Streaked plates were incubated for 48 h at 37°C. Black presumptive ST colonies were inoculated into tubes containing 10 mL of fresh sterile tryptic soy broth at one colony per tube. Tubes were subsequently incubated for 24 h, and the resulting ST cultures were pooled for inoculum preparation. Inoculum was diluted to contain 1 x 107 cfu/mL using sterile buffered peptone water (BPW; Acumedia Manufacturers Inc.). Estimation of viable and nonviable ST cell concentration in the inoculum was done spectrophotometrically at 687 nm (SP-830 Plus Visible Spectrophotometer, Metertech Inc., Taipei, Taiwan) relative to a standard curve. Concentration of viable ST in the inoculum was then determined by streaking 10 µL onto an XLT4 plate and counting black colonies after incubating the plate overnight at 37°C. Results showed that the inoculum contained 7.8 x 106 cfu of ST/mL.
Experimental Birds and Treatments
Mixed sex Ross x Ross 708 commercial source chicks totaling 132 were used for experimentation. To confirm that chicks were free of ST, 20 chicks were randomly killed by CO2 asphyxiation and aseptically necropsied for the removal of cecum. Each cecum was placed in a Whirl-Pak filter bag (Nasco, Fort Atkinson, WI) that contained sterile BPW. Bag contents were homogenized at 230 rpm for 60 s (Stomacher 400 Circulator, Seward Limited, London, UK), and the macerate was allowed to enrich overnight at 37°C. Each culture was evaluated for the presence of ST using 0.1 and 0.5 mL that were inoculated into 10 mL of sterile tetrathionate and Rappaport-Vassiliadis broths (Acumedia Manufacturers Inc.), respectively. The inoculated tetrathionate and Rappaport-Vassiliadis broths were incubated at 42°C (Precision 30M High Capacity Incubator, Precision Scientific, Winchester, VA) for 24 h, then 10 µL of each broth was streaked onto XLT4 agar plates containing Tergitol 4 supplement and 0.1% nalidixic acid. After 48 h at 37°C, presumptive ST colonies were isolated and confirmed by transference into triple sugar iron and lysine iron agar (Acumedia Manufacturers Inc.) as described by the USDA Laboratory Guide (2004).
The remaining 112 chicks were randomly divided into control (CN) and the ST challenged (STC) treatments. The STC chicks were challenged with ST at 4 d of age. Each treatment consisted of 4 replicate pens with 14 chicks per pen. All pens were in Petersime raised wire batteries maintained at 30°C ± 2°C during the first week, after which temperature was gradually reduced weekly by 2°C. Continuous lighting and access to feed and water was provided throughout the experiment. Chicks received an un-medicated corn and soybean meal based starter diet (22% CP and 3,080 ME/kg of diet) that met or exceeded the recommendations of the National Research Council (1994).
Recovery of Viable ST from Challenged Chicks
At 4-d of age, chicks in STC treatment were challenged with ST by orally gavaging each chick with 1 mL of inoculum (7.8 x 106 cfu/mL). Chicks in the CN treatment were mock challenged in a similar manner with 1 mL of BPW. To confirm the presence of infection, 3 chicks were randomly taken from each pen on d 4 and 9 PC, killed by CO2 asphyxiation, and aseptically necropsied for the removal of the small intestine (SI) and cecum. Each SI and cecum was placed in separate Whirl-Pak filter bags (Nasco) containing sterile BPW and contents homogenized at 230 rpm for 60 s in a Stomacher before serial dilution (serial 10-fold dilutions up to 105). From each dilution, 1 mL was taken and plated on XLT4 agar. The XLT4 plates were incubated for 48 h at 37°C, and then presumptive ST colonies were counted. Total ST concentrations (log10 cfu) in the intestine and cecum were summed up and then expressed as total gut ST concentration.
Experimental Protocol
Measurement of cytokine mRNA levels from intestine employed the reverse transcription-PCR (RT-PCR). Three chicks were randomly taken from each pen on d 5 and 10 PC. The small intestine was aseptically excised, placed on ice, and divided into duodenum, jejunum, and ileum. The mid-portion of the jejunum, ileum, and one-third of a cecal lobe were cut, rinsed in cold saline (0.9% NaCl) to remove gut contents, placed in separate DNase- and RNase-free containers, snap frozen in liquid N2, and kept at –70°C until time for analysis.
Another 2 chicks per pen were randomly taken on d 7 PC for collection of intestinal flushes for ELISA to estimate the levels of Salmonella-specific IgA and IgG. Chicks were fasted overnight to reduce lumen contents, and then the small intestine was excised whole and placed on ice. The lumen of each intestine was flushed with 10 mL of glycine buffer (0.025% Tween 20 and 0.05 M glycine, pH 7.9) using a 10-mL disposable syringe having a 20-G needle. Perfusate was introduced at the duodenum, and flush was collected at the end of ileum into a sterile 50-mL plastic centrifuge tube. Each flush was kept on ice until centrifuging at 4,891 x g for 5 min (Sorvall Biofuge Stratos with Heraeus Rotor # 3047, Fisher Scientific Company, Suwanee, GA). Supernatant was decanted from pellet and kept frozen at –70°C for analysis.
Quantitative Analysis of Cytokine mRNA by Real-Time RT-PCR
Jejunal, ileal, and cecal tissues (30 µg each) were homogenized with a MiniBeadbeater-96 (Biospec Products Inc., Bartlesville, OK). Total RNA was isolated from homogenates using an RNeasy mini kit (Qiagen, Waltham, MA) following the manufacturer’s protocol. Ribonucleic acid was then eluted in 100 µL of RNase-free water and assessed for quality by spectrophotometric measurement of nucleic acid concentration at 260 nm and 280 nm. The eluted RNA was then stored at –70°C until use. Expression levels of IL-1β, IL-6, IFN-, and IL-10 in purified RNA samples from both CN and STC treatments were determined using a real-time quantitative RT-PCR method described by Kaiser et al. (2000) and Smith et al. (2005). All primers and probes have been described previously (Kogut et al., 2003; Smith et al., 2005) and presented in Table 1
for clarity. The quantitative RT-PCR was performed using the Abgene Absolute Max QRT-PCR Mix (Abgene Inc., Rochester, NY). The RT-PCR reaction for each RNA sample was run in triplicate and consisted of 3 x 12 µL of Absolute PCR Mix, 0.1 µL of Absolute Quantitative Reverse Transcriptase (including RNase inhibitor), 5 µM forward primer, 5 µM reverse primer, 10 µM probe, 1.25 µL of Quantitative Reverse Transcriptase Enhancer, 0.5 µL of ROX passive reference dye, 0.9 µL of RNase-free water, and 5 µL of diluted RNA sample (1:250 for 28S RNA and 1:10 for all others). Cytokine RNA standards (prepared as log 10 dilution series; 10–1 to 10–4) and no-template controls were included in the assay along with the RNA samples. Amplification and detection of specific products were performed with the Applied Biosystems 7500 Real-Time PCR System (Applied Biosystems, Foster City, CA) with the following program: one cycle of 47°C for 30 min, 95°C for 15 min followed by 40 cycles of 95°C for 15 s and 60°C for 1 min. Data generated by the Applied Biosystems 7500 included threshold cycle (Ct) values and standard curve for each cytokine. The Ct values of each gene were normalized against that of 28S RNA (housekeeping gene). Fold change in the expression of each gene was then calculated by the Pfaffl (2001) method as a change in the expression of a gene in STC samples relative to the mock-infected CN samples.
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The IgA and IgG specific to ST were determined in the frozen supernatants from intestinal flushes by ELISA. Costar medium-binding ELISA plates (Corning Life Sciences, Acton, MA) were coated overnight at 4°C with 10 µg/mL of ST lipopolysaccharide (Sigma Chemical Co., St. Louis, MO) in carbonate buffer (100 µL/well). After washing with phosphate buffered saline (PBS) containing 0.05% Tween 20 (PBS-T; wash buffer), plates were blocked with a buffer consisting of PBS-T supplemented with 3% polyvinylpyrrolidone (Sigma Chemical Co.) for 1 h at room temperature (RT). Plates were washed twice with PBS-T and incubated for 90 min at RT with intestinal flush supernatants that have undergone serial doubling dilutions in sample buffer containing 1% BSA. After incubation and 3 subsequent washes, plates were incubated with primary antibodies (1:1,000 heavy chain-specific mouse anti-chicken IgA or IgG; Southern Biotech, Birmingham, AL) for 1 h at RT and washed 3 times. To facilitate colorimetric detection of ST-specific antibodies, alkaline phosphatase-conjugated goat anti-mouse IgG (100 µL/well; 1:1,000; Calbiochem, San Diego, CA) was added and plates were incubated for 1 h at RT. After washing, p-nitrophenyl phosphate in diethanolamine buffer was added as substrate and plates were incubated in the dark to allow reaction between alkaline phosphatase and substrate. After 15 min reaction, color development was evaluated at 405 nm in a Bio-Tek EL311sx Auto Reader (Bio-Tek Instruments, Winooski, VT).
Statistical Analysis
The experimental unit for all data was pen average. All analyses were conducted using the GLM procedure of SAS (SAS Institute Inc., 2004). Fold change values obtained from quantitative RT-PCR were subjected to 3-way ANOVA to determine the effects of treatment (i.e., uninfected CN versus ST-infected STC), intestinal site (i.e., jejunum, ileum, and cecum), and day of cytokine analysis (d 5 versus d 10) PC and their interactions on cytokine response. Because the effect of treatment was significant for all cytokines assessed, a one-way ANOVA for completely randomized designs was used to determine differences in fold change values between the uninfected CN and STC treatments at each day of sampling PC. Results of ELISA were also subjected to one-way ANOVA to determine differences in antibody (IgA and IgG) levels between the CN and STC treatments. Significant differences among means were determined with the Duncan option of the GLM procedure (Waller and Duncan, 1969; SAS Institute Inc., 2004). Data are presented as means ± SD. Statements of statistical significance were based upon P < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Changes in cytokine expression in response to treatment, day of cytokine analysis, and intestinal segment in STC chicks relative to CN chicks were calculated as fold change in standardized mRNA levels and presented in Table 2 and Figures 1 to 4. Fold change values represent the amount by which a cytokine mRNA level in STC chicks is increased or decreased compared with its corresponding level in CN chicks. As such, CN chicks automatically had a fold change value of "1" while values for STC chicks were either >1 when cytokine expression was higher (or upregulated) compared with the level in CN chicks or <1 when cytokine expression was lower (or downregulated) compared with that in CN chicks. Thus, to enhance easy comprehension of cytokine fold change data, Figures 1 to 4 were constructed such that the x-axis crosses the y-axis at 1.
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). At d 5 PC, only the expression of IL-1β and IL-10 were affected by ST infection. IL-1β was upregulated, whereas IL-10 was downregulated in the cecum and jejunum of STC chicks, respectively (Figures 1 and 4). By d 10 PC, ST infection affected the expression of all the cytokines evaluated. The fold change of IL-1β was higher in all intestinal segments of STC chicks compared with CN chicks (Figure 1), fold change of IL-6 was higher in jejunum and ileum of STC chicks (Figure 2), fold change of IFN- was higher in the ileum of STC chicks (Figure 3), and the fold change of IL-10 was lower in the ileum of STC chicks (Figure 4).
The day of cytokine analysis affected only IL-6 and IFN- (Table 2). This is emphasized by the fact that ST infection showed no effect of the expression levels of IL-6 and IFN- at 5 d PC, but resulted in an increased expression (P < 0.05) of these cytokines in the jejunum and ileum by d 10 PC (Figures 2 and 3). On the other hand, the expression of IL-1β was already upregulated (P < 0.05) at d 5 PC in the cecum, and this increase in expression became more prominent and robust in all intestinal segments assessed by d 10 PC (Figure 1). Furthermore, IL-10 was already downregulated (P < 0.05) in the jejunum at d 5 PC and also downregulated (P < 0.05) in the ileum at d 10 PC (Figure 4).
The interactions among the independent variables (treatment, day of cytokine analysis, and intestinal segment) in the experimental model treatment are presented in Table 2. There was a significant interaction between treatment, day of cytokine analysis, and intestinal segment for IL-6, IFN- and IL-10. In addition, the Coefficient of determination values ranged from approximately 0.50 to 0.90, indicating that the statistical analysis employed was appropriate.
The IgA and IgG levels in intestinal flushes of chicks in the CN and STC treatments are presented in Figure 5. There was no difference in the estimated levels of IgA (P = 0.3741) or IgG (P = 0.2745) in the intestinal flushes obtained at 7 d PC (Figure 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In mammals, binding of Salmonella to the intestinal epithelium is known to initiate immune responses, which partly culminates in the production of cytokines and chemokines that activate processes aimed at eradicating the pathogen (McCormick et al., 1995; Gulig, 1996). Cytokines may act to reduce luminal colonization of pathogens in the gut because of their capability to alter gut motility in mammals (Collins, 1996; Tjwa et al., 2003; Akiho et al., 2007). For instance, cytokine-induced increases in gut motility during an infection are likely to enhance clearing and fecal shedding of pathogens. In this study, the 1.5-fold reduction in "total gut ST concentration" between d 4 and 9 PC was not significant. Another important function of pro-inflammatory cytokines is in preventing the translocation of bacteria invading the intestinal mucosa into systemic compartments of the organism by recruiting bacteria-phagocytosing cells (neutrophils, macrophages and dendritic cells) to the site of infection (Van Ginkel et al., 2000). Bacterial invasion of the intestinal mucosa causes rapid inflammation of the intestinal mucosa, followed by infiltration of heterophils (the avian counterpart of mammalian neutrophils), and subsequently macrophages (Kaiser et al., 2000). Light and electron microscopy studies have revealed an influx of heterophils and macrophages to the luminal surface of the intestine of chickens infected with Salmonella spp. (Turnbull and Snoeyenbos, 1973; Barrow et al., 1987b; Henderson et al., 1999). Heterophils modulate acute immune response through phagocytosis of invading microbes and foreign particles, production of oxygen intermediates (such as nitric oxide), and releasing proteolytic enzymes (Swaggerty et al., 2006). Avian macrophages secret 30-fold more nitric oxide (a mediator of macrophage bactericidal activities) and may therefore have higher bactericidal capacity than heterophils (Crippen et al., 2003).
In this study, expression of proinflammatory cytokines IL-1β and IL-6, Th1 cytokine IFN-, and Th2 antiinflammatory cytokine IL-10 was compared in jejunum, ileum, and cecum of control (CN) and infected (STC) chicks. Comparing cytokine responses between intestinal segments, only the ileum demonstrated significant changes in all the cytokines assessed (Figures 1 to 4). This may be due to the presence of Peyer’s patch in the distal ileum. The Peyer’s patch is the major inductive site for intestinal mucosal immune responses (Monteleone et al., 2006). The cecum gave the least response and showed a change only in IL-1β. The lower cytokine activity in the cecum may be due to the fact that we did not cut the cecal tonsil with the cecum during tissue collection. The cecal tonsil is the main lymphoid nodule for the cecum (Kitagawa et al., 1996).
Interleukin-1 is a major mediator of inflammation in mammals and birds and is produced by monocytes, tissue macrophages, enterocytes, and other cells (Radema et al., 1991; Ogle et al., 1994; Staeheli et al., 2001; Bird et al., 2002; Staeheli, 2002; Bar-Shira and Friedman, 2006). Compared with our results, Withanage et al. (2004) who challenged day-old SPF Rhode Island Red chicks with 108 cfu of ST observed approximately 60-fold upregulation within 12 to 48 h PC in ileal IL-1β mRNA of infected chicks (compared with nonchallenged chicks), whereas we observed a 1.3-fold upregulation at 10 d PC (Figure 1). Clearly, both studies observed upregulation in ileal IL-1β mRNA. In contrast, Withanage et al. (2005) found no change in intestinal expression of IL-1β between 1 and 7 d PC when he challenged 7 d-old SPF Rhode Island Red chicks with 108 cfu of ST. The lack of IL-1β mRNA upregulation in the study by Withanage et al. (2005) who challenged chicks at 7 d of age, compared with the 60-fold IL-1β mRNA upregulation reported by Withanage et al. (2004) who challenged chicks at 1-d-old, and the 1.3-fold IL-1β mRNA upregulation observed in this study when we challenged chicks at 4-d-old, suggests that the age at which chicks become infected with ST influences cytokine response. It has been well documented that chickens become more resistant to Salmonella infection with increasing age because of increasing gut and immune system development (Holt et al., 1999; Bjerrum et al., 2003; Beal et al., 2004a,b). Furthermore, it is well documented that chickens that are of different species, breed, strain, or age respond differently immunologically to Salmonella infection (Bailey, 1988; Leshchinsky and Klasing, 2001; Koenen et al., 2002; Bjerrum et al., 2003; Barrow et al., 2003; Sadeyen et al., 2004; van Hemert et al., 2006; Cheeseman et al., 2007).
In mammals, IL-6 is a multifunctional cytokine that helps to resolve innate immunity and develop acquired immunity by stimulating neutrophils to degranulate, activating macrophages, promoting the differentiation of normal B lymphocytes to antibody-producing cells, and activating T lymphocytes (Borish et al., 1989; Hideshima et al., 2005; Jones, 2005). However, in chickens, in vitro studies have shown that IL-6 has no direct effect on heterophils (which are counterparts of mammalian neutrophils) and may require the presence of other cytokines to stimulate heterophils and other cells of the immune system (Ferro et al., 2005). Because IL-6 and IL-1β are proinflammatory cytokines, upregulation of IL-1β mRNA at 5 d PC, followed by subsequent upregulation of IL-6 at 10 d PC along with further increases in IL-1β expression in this study (Figures 1 and 2) implies that ST infection induced inflammatory response in STC chicks.
The IFN- is a Th1 cytokine that stimulates macrophages to secret oxidants with antimicrobial activities and is produced by natural killer cells and T-lymphocytes (Lowenthal et al., 1995; Alam et al., 2002). In this study, IFN- mRNA was upregulated only in the ileum at 10 d PC (Figure 3). In contrast, Withanage et al. (2005) observed an upregulation of IFN- mRNA from 3 to 14 d PC in the ileum of SPF Rhode Island Red chicks challenged with 108 cfu of ST at 7 d of age. The earlier upregulation of IFN- found by Withanage et al. (2005) may be due to the higher dose (108 cfu) of ST they used in challenging the chicks, compared with the 106 cfu of ST that we used in this study. Upregulation of IFN- between d 5 and 10 PC (Figure 3) suggest a probable increase in the activation of macrophages. Earlier microscopy studies have reported an influx of macrophages to the lumenal surface of the intestine of chickens infected with Salmonella spp. (Turnbull and Snoeyenbos, 1973; Barrow et al., 1987b). Previous studies have reported that the rate of clearance of Salmonella infection correlates with an upregulation of IFN- mRNA and a strong T cell response (Beal et al., 2004a; Kogut et al., 2005).
Interleukin-10 is a cytokine with potent immunoregulatory and antiinflammatory properties and is produced by activated macrophages and T cells (Hammer et al., 2005; Fife et al., 2006). IL-10 suppresses T cell proliferation and the release and function of many proinflammatory cytokines such as IL-1 and IL-6 (De Waal Malefyt et al., 1991; Taylor et al., 2006). Throughout this study, IL-10 mRNA was downregulated in infected (STC) chicks (Figure 4), indicating that the expression and release of proinflammatory cytokines and T cell proliferation were not inhibited in STC chicks.
There were no significant changes in the levels of Salmonella specific-IgA or IgG in response to ST infection in this study. This is not surprising because chickens typically do not elicit antibody responses until after 2 to 3 wk of age, and this has been attributed to the immaturity of T lymphocytes (Bar-Shira et al., 2003). It has been found that chickens are capable of eliciting secretory IgA responses against Salmonella flagella within 2 to 3 wk PC (Holt and Porter, 1993). The lack of antibody response in this study is probably due to the fact that the gut flush samples used for antibody analysis were collected at 6 d PC when the chicks were only 11 d old.
The best defense against enteric pathogens such as Salmonella would probably be the administration of mucosal vaccines that are capable of inducing systemic and mucosal immunity (Van Ginkel et al., 2000). Live Salmonella vaccines are known to induce strong cell-mediated immune response, whereas killed Salmonella vaccines are poor immunogens and enhance humoral immune responses (Babu et al., 2004; Barrow, 2007). It has been established that clearance of ST infection correlates with high cell-mediated responses associated with strong T-cell response, and not with high antibody levels (Beal et al., 2006; Barrow, 2007). Therefore, formulation of live or killed mucosal vaccines that contain adjuvants that can promote strong inflammatory and Th1 cytokine responses for the induction of phagocyte-dependent inflammation and cell-mediated immunity against Salmonella infection in broilers are recommended. From this study, we observed that ST infection induced an intestinal mucosal inflammatory immune response that was characterized by an upregulation in IL-1β, IL-6, and IFN-, whereas IL-10 was downregulated. We propose that IL-1β and IFN- should be evaluated for inclusion as adjuvants in mucosal vaccines formulated against Salmonella. We refrain from recommending IL-6 for evaluation as vaccine adjuvant because its effect on avian immune cells is not yet clear (Ferro et al., 2005).
To the best of our knowledge, results of this study demonstrated for the first time that ST (nalidixic acid-resistant) infection induces an inflammatory response in the intestine of commercial source broiler chicks. We stipulate that IL-1β and IFN- are factors to be considered when developing immunotherapeutic prophylactic measures (such as vaccines or cytokine therapy) for the reduction of Salmonella in commercial broiler flocks.
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ACKNOWLEDGMENTS |
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Received for publication December 28, 2007. Accepted for publication March 6, 2008.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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