Antibody responses and morbidity following infection with infectious bronchitis virus and challenge with Escherichia coli, in lines divergently selected on antibody response

Genetic resistance to avian pathogenic Escherichia coli (APEC): current status and opportunities

Infections with avian pathogenic Escherichia coli (APEC) can be extremely detrimental to poultry health and production. Investigating host genetic variation could identify the biological mechanisms that control resistance to this pathogen and allow selection for improved resistance in experimental and commercial poultry populations. In this review, the current knowledge of how host genetics contributes to APEC resistance and future opportunities that would benefit the understanding or application of genetic resistance are discussed. Phenotypes, such as antibody responses, lesion scores, and mortality, revealed that genetic background impacts APEC resistance and interacts with other factors including the environment and challenge conditions. Experiments have used divergent selection for APEC-specific antibody levels to facilitate genetic studies, estimated heritabilities in relevant traits, detected quantitative trait loci using microsatellites, and made associations with sequence variation in the major histocompatibility complex, which collectively suggest that improving APEC resistance through selection is feasible, although genetic control is partial, complex, and highly polygenic. Additionally, functional genomics techniques have identified antimicrobial responses, toll-like receptor and cytokine signalling, and the cell cycle as central pathways in the host response to APEC challenge. Opportunities for future research are discussed, including the expansion of existing lines of research and the application of new technologies that are relevant to the study of host genetics and APEC. This review closes with prospective strategies for improvement of host genetic resistance to APEC.

Evaluation of protection conferred by a vaccination program based on the H120 and CR88 commercial vaccines against a field variant of avian infectious bronchitis virus

Due to serotype variations among different avian infectious bronchitis viruses isolated in Tunisia since 2000, protection of chicks, especially broiler flocks, with Mass H120 vaccine often fails. Therefore, association of CR88 (793B type) with H120 vaccines was used for better response. Challenge experiments were then conducted to evaluate tracheal and renal cross-protection in chickens immunized via nasal and eye drops. Conferred protection was measured by clinical signs and macroscopic lesions observed, based on scores attributed according to their severities. The results showed a low protection conferred by H120 alone, as vaccination did not reduce tracheal and kidney lesions (70% scored as 3) after TN20/00 virus challenge, which also led to 10% mortality. Conversely, the challenge results indicated that the combination of the 2 strains (H120/CR88) allow high protection. Based on the results of the challenge experiments, a vaccination protocol coupling CR88 to H120 was applied for industrial broiler flocks. Clinical observations and serological results confirmed that association of heterologous serotypes (H120 and CR88 vaccines) increased the levels of protection against infectious bronchitis viruses compared with the H120 vaccine given alone.

Selective breeding for high natural antibody level increases resistance to avian pathogenic Escherichia coli (APEC) in chickens

Keyhole limpet hemocyanin (KLH)-binding natural antibody (NAb) titers in chickens are heritable, and higher levels have previously been associated with a higher survival. This suggests that selective breeding for higher NAb levels might increase survival by means of improved general disease resistance. Chickens were divergently selected and bred for total NAb levels binding KLH at 16 weeks of age for six generations, resulting in a High NAb selection line and a Low NAb selection line. To for test differences in disease resistance, chickens were challenged with avian pathogenic Escherichia coli (APEC) in two separate experiments. Chickens at 8 days of age received one of four intratracheal inoculations of 0.2 mL phosphate buffered saline (PBS): 1) mock inoculate, 2) with 0.2 mL PBS containing 10^8.20 colony-forming units (CFU)/mL APEC, 3) with 0.2 mL PBS containing 10^6.64 CFU/mL APEC, and 4) with 0.2 mL PBS containing 10^7.55 CFU/mL APEC. Mortality was recorded during 7 days post inoculation. Overall, 50-60% reduced mortality was observed in the High line compared to the Low line for all APEC doses. In addition, morbidity was determined of the surviving chickens at 15 days of age. The High line had lower morbidity scores compared to the Low line. We conclude that selective breeding for high KLH-binding NAb levels at 16 weeks of age increase APEC resistance in early life. This study and previous studies support the hypothesis that KLH-binding NAb might be used as an indicator trait for to selective breed for general disease resistance in an antigen non-specific fashion.

A genome-wide association study identifies major loci affecting the immune response against infectious bronchitis virus in chicken

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The genetic basis of host responses to infectious bronchitis virus is unclear.

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We detected 20 significant markers for the antibody response to infectious bronchitis virus in chicken.

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Loci on chromosomes 1 and 5 explained 12% and 13% of phenotypic variation.

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The host immune response cluster had 13 beta-defensin and interleukin-17F genes.

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Our results will contribute to the control of outbreaks of infectious bronchitis.

Coronaviruses are a hot research topic because they can cause severe diseases in humans and animals. Infectious bronchitis virus (IBV), belonging to gamma-coronaviruses, causes a highly infectious respiratory viral disease and can result in catastrophic economic losses to the poultry industry worldwide. Unfortunately, the genetic basis of the host immune responses against IBV is poorly understood. In the present study, the antibody levels against IBV post-immunization were measured by an enzyme-linked immunosorbent assay in the serum of 511 individuals from a commercial chicken (Gallus gallus) population. A genome-wide association study using 43,211 single nucleotide polymorphism markers was performed to identify the major loci affecting the immune response against IBV. This study detected 20 significant (P < 1.16 × 10⁻⁶) effect single nucleotide polymorphisms for the antibody level against IBV. These single nucleotide polymorphisms were distributed on five chicken chromosomes (GGA), involving GGA1, GGA3, GGA5, GGA8, and GGA9. The genes in the 1-Mb windows surrounding each single nucleotide polymorphism with significant effect for the antibody level against IBV were associated with many biological processes or pathways related to immunity, such as the defense response and mTOR signaling pathway. A genomic region containing a cluster of 13 beta-defensin (GAL1–13) and interleukin-17F genes on GGA3 probably plays an important role in the immune response against IBV. In addition, the major loci significantly associated with the antibody level against IBV on GGA1 and GGA5 could explain about 12% and 13% of the phenotypic variation, respectively. This study suggested that the chicken genome has several important loci affecting the immune response against IBV, and increases our knowledge of how to control outbreaks of infectious bronchitis.

Breeding to reduce susceptibility to Escherichia coli in layers

Colibacillosis is a bacterial disease of great concern in the layer industry causing substantial animal and economic losses worldwide. Breeding for resistance to colibacillosis is an important control strategy that would complement traditional management strategies such as vaccinations and therapeutic treatments. Because antibiotic use in animal production is expected to substantially decrease in the future to meet not only consumer demands but also regulations, it is expected that bacterial diseases such as colibacillosis will become even more important. A challenge test was carried out on 353 pedigreed White Leghorn female chickens of 1 pure line. The birds were inoculated with Escherichia coli at an age of 93 to 95 wk (3 hatches) and symptoms of disease and mortality were constantly monitored for up to 6 d after inoculation. The cumulative mortality was 64%, the majority of which took place in the first 2 d. In the present study, genetic parameters of resistance to colibacillosis were estimated to judge the value of genetic selection. The heritability for survival rate was 0.17+/-0.07. Both for quantitative as well as for qualitative traits of economic interest, there was no remarkable difference between survivors and hens that died. Data showed that it is in principle possible to breed for colibacillosis resistance without penalizing the improvement in other key traits. The B21 marker for MHC in this study was associated with a lower colibacillosis susceptibility. In this study, it was demonstrated that it is possible to breed for Escherichia coli resistance. However, due to the inconveniences of a challenge test, it would be very interesting to make use of MAS.

Modeling variability in immunocompetence and immunoresponsiveness

The purposes of this paper were to 1) develop a stochastic model that would reflect observed variation between animals and across ages in immunocompetence and responsiveness; and 2) illustrate consequences of this variability for the statistical power of genotype comparisons and selection. A stochastic model of immunocompetence development and responsiveness kinetics was developed. This model enabled variability in immunological variables to be taken into account in the evaluation of challenge and measurement strategies for selection. The characteristics of the variation in model output reflect those observed in the literature, to the extent that variation in the literature shows a consistent pattern; knowledge of true variation and patterns of variation in immunological variables is limited. The model created correlations between immunocompetence and immunoresponsiveness components, as well as correlations within each component across time. These correlations were generally in agreement with literature estimates, where available. The model enabled predictions of the effectiveness of selection for improved health through immunocompetence or immunoresponsiveness. It was predicted that effective selection for increased general immunocompetence to improve health should be done only when baseline immunity has matured. Further, the model implied that selection is unlikely to be successful if it is based only on a single measurement. Problems with low statistical power to detect differences between genotypes can be reduced by increasing challenge age in the experimental design, and one should ensure that the effects of maternal immunity are minimal when the challenge is done. The ability to detect differences between different groups of animals differs substantially with measurement timing because of low repeatabilities of immunocompetence and responsiveness across time. In general, the probability of detecting differences becomes higher when the challenge age is increased. Consequently, both the age at selection and the age at which information is gathered for selection must be considered carefully when designing genetic evaluations.

Modeling Immunocompetence Development and Immunoresponsiveness to Challenge in Chicks

The purpose of this study was 2-fold: 1) to develop a deterministic model that describes the development of immunocompetence and the kinetics of immunoresponsiveness to a pathogenic challenge in chicks and 2) to use this model to illustrate the importance of factors in experimental design, such as type of variable measured, measurement timing, and challenge age. Difficulties in evaluating immunological variables hinder attempts to improve animal health through selection on immunological variables. In young chicks, evaluating immunological variables is additionally complicated by immune system development and maternal immunity. The evaluation of immunocompetence and immunoresponsiveness and the definition of appropriate challenge and measurement strategies may be enabled through a mathematical model that captures the key components of the immune system and its development. Therefore, a model was developed that describes the development of immunocompetence as well as the kinetics of immunoresponsiveness to a pathogenic extracellular bacterial challenge in an individual chick from 0 to 56 d of age. The model consisted of 4 components describing immunocompetence (maternal and baseline immunity) and immunoresponsiveness (acute phase and antibody response). Individual component equations generally fit published data adequately. Four scenarios that represented combinations of challenge age and measurement timing were simulated. In each scenario, the immunoresponsiveness to a particular challenge was compared for 3 different levels of baseline immunity, representing 3 broiler genotypes. It was illustrated that experimental design (type of immunoresponsiveness measured, measurement timing, and challenge age) can have an important effect on the ranking of genotypes, groups, or individuals and on the reliability of extrapolations based on this ranking. It is concluded that this model is a potentially useful tool in the definition of appropriate challenge and measurement strategies when evaluating immunocompetence and immunoresponsiveness. Further, it may be used as a generator of hypotheses on global immunological relationships to be tested experimentally.

Genetic variation among broiler genotypes in susceptibility to colibacillosis

Selection for reduced susceptibility to colibacillosis in broilers may contribute to the prevention of colibacillosis. Such selection should focus on the responses to Escherichia coli rather than the associated primary agent(s). The purpose of the current study was to examine whether genetic variation is present in the susceptibility to colibacillosis. This was achieved through an evaluation of the susceptibility to primary colibacillosis in 5 pure broiler lines, a slow-growing line, and two 2-way crosses of the pure lines (altogether referred to as genotypes). A challenge experiment was executed in 2 trials. Per trial, 24 chicks per genotype were challenged and 20 chicks per genotype were controls. At 7 d of age, challenged chicks were intratracheally inoculated with 0.3 mL of E. coli O78K80 solution, and controls with 0.3 mL of PBS. All chicks were euthanized at 14 or 15 d. Traits measured were mortality, lesion scores (airsacculitis, pericarditis, and perihepatitis) at 14 or 15 d, and BW at 1, 4, 7, 10, 12, and 14 or 15 d. An effect of genotype on mortality, lesion prevalence, and growth retardation was found, indicating the presence of genetic variation in susceptibility to colibacillosis, and suggesting that selection for reduced susceptibility is possible. There were large between-genotype differences in mortality (up to 46%) and in lesion prevalence (up to 41%). Growth retardation was not observed for any genotype in chicks without lesions, whereas genotypes differed from none to 20% growth retardation for chicks with airsacculitis but no systemic lesions, and up to 13% for chicks with systemic lesions. The heterosis in susceptibility and growth retardation was found to be either negative or absent, indicating that crossbreeding would not be an advantage for the selection for reduced susceptibility, and that test crossing is essential.

Genetic variation in major histocompatibility complex class I alpha2 gene among broilers divergently selected for high or low early antibody response to Escherichia coli

The MHC genes have a profound effect on animal abilities to respond to specific antigens because they play a role in presenting foreign antigens to T cells during the course of the humoral or cellular immune response. In the current study, polymorphism in the MHC class I alpha2 domain was compared in 2 lines divergently selected for high (HH) or low (LL) antibody response to Escherichia coli vaccine. These lines also differ markedly in their antibody response to natural E. coli exposure and to vaccination with Newcastle disease virus, infectious bronchitis virus, and infectious bursa disease virus. Recent trials have shown that the LL chicks exhibit a significantly higher percentage of CD8+ T lymphocytes in their peripheral blood lymphocytes and spleen than HH chicks. Despite symmetrical selection intensity in both lines, polymorphism of the alpha2-domain gene was higher in the LL line than in the HH line. Among 29 single-nucleotide polymorphism positions found, 3 were unique to the HH line, 15 were unique to the LL line, and 11 were polymorphic in both lines. These single nucleotide polymorphism positions were not 100% line specific and were in agreement with the genetic variation in antibody level or cellular response still found within the selection lines. Five amino acid positions showed significant differences in polymorphism between the selection lines. These were located within the antigen-binding cleft, suggesting that these positions might influence the ability of MHC class I to bind foreign antigens and leading to differences in immunocompetence between the lines.

Stress response differences and disease susceptibility reflected by heterophil to lymphocyte ratio in turkeys selected for increased body weight

Three genetic lines of turkeys were compared for their responses to Escherichia coli challenge following dexamethasone injection (Dex) or E. coli challenge preceding transport stress (TS). The turkey lines were a slow growing line selected for increased egg production (Egg line), a fast growing line selected for increased 16-wk BW (F line), and a commercial line (Comm line). At 14 wk of age, the Dex group was treated with 3 injections of 2 mg of Dex/kg of BW followed by airsac challenge with 100 cfu of E. coli. The TS group was given the same E. coli challenge at 1 x 10(4) cfu/bird without Dex treatment, and was subjected to transport stress, including 12 h of holding time in a transport vehicle, 8 d after the challenge. All treated birds and untreated control birds were bled at the same time, which was 1 d after transport and 9 d after challenge with E. coli. The main effect mean (MEM) total leukocyte counts (WBC) and the percentages of eosinophils (Eos) and basophils (Baso) were the same for all 3 lines; however, the MEM percentages of heterophils (Het) and monocytes (Mono) and the heterophil/lymphocyte ratio (H/L) were lower and the percentage of lymphocytes (Lym) was higher in the Egg line compared with the 2 fast-growing lines. Both stress treatments increased WBC, Het, and H/L and decreased Lym in all 3 lines; however, these effects were significantly greater in both fast growing lines compared with the Egg line. Sixteen-week BW was unaffected by either treatment in the Egg line and was decreased by both treatments in the Comm line and by the Dex treatment in the F line. Main effect mean airsacculitis score (AS) was not affected by line and was significantly increased by TS and Dex treatments. Neither treatment affected AS of the Egg line birds, whereas Dex treatment increased AS of the F line, and both Dex and TS increased AS of the Comm line. Mortality was significantly higher in the Comm line compared with the Egg line and was intermediate in the F line. The differences between these lines in their disease resistance and physiological response to stress in 2 stress models suggests that increasing selection for BW of turkeys is accompanied by changes in the stress response resulting in increased susceptibility to opportunistic bacterial infection.

Detection of different quantitative trait loci for antibody responses to keyhole lympet hemocyanin and Mycobacterium butyricum in two unrelated populations of laying hens

Quantitative trait loci involved in the primary antibody response to keyhole lympet hemocyanin (KLH) and Mycobacterium butyricum were detected in two independent populations of laying hens. The first population was an F2 cross (H/L) of lines divergently selected for either high or low primary antibody responses to SRBC, and the second population was an F2 cross between 2 commercial layer lines displaying differences in feather pecking behavior (FP). Both populations were typed with microsatellite markers widely distributed over the genome with similar intervals between markers. Titers of antibodies binding KLH and M. butyricum were measured for all individuals by ELISA. Two genetic models were applied to detect QTL involved in the humoral immune response: a half-sib model and a line-cross model, both using the regression interval method. In the half-sib analysis, 2 QTL (on GGA14 and GGA27) were detected for the antibody response to KLH for the H/L population, and 2 QTL (on GGA14 and GGA18) were detected for the FP population. Only 1 QTL was detected for M. butyricum on GGA14 in the FP population using the half-sib analysis model. Two QTL were detected for the FP population on GGA2 and GGA3 using the line-cross analysis model. A QTL for the primary antibody response to KLH detected on GGA14 was validated in both populations under the half-sib analysis model. The present data suggest differences in the genetic regulation of antibody responses to two different T-cell dependent antigens.

Microsatellite markers associated with quantitative trait loci controlling antibody response to Escherichia coli and Salmonella enteritidis in young broilers

A unique resource population was produced to facilitate detection of microsatellite markers associated with quantitative trait loci controlling antibody (Ab) response in broiler chickens. Three F1 males were produced by mating two lines divergently selected on Ab response to Escherichia coli vaccination. Each F1 male was mated with females from four genetic backgrounds: F1, high-Ab line (HH), low-Ab line and commercial line, producing three resource families, each with four progeny types. About 1700 chicks were immunized with E. coli and Salmonella enteritidis vaccines. Selective genotyping was conducted on the individuals with highest or lowest average Ab to E. coli and S. enteritidis within each progeny type in each sire family. Twelve markers were significantly associated with Ab to E. coli and six of them were also associated with Ab to S. enteritidis, mostly exhibiting a similar low effect (approximately 0.35 phenotypic SD) in all progeny types. Four markers exhibited a highly significant and much larger effect (approximately 1.7 SD), but only in progeny of females from the HH, suggesting that a backcross to the high parental line should be preferred over the commonly used F2 population. Results from two markers suggested a quantitative trait locus on chromosome 2 around 400 cM. The marker MCW0083, significant in two sire families, is closely linked to the bone morphogenetic protein 2 (BMP2) gene, known to be associated with the control of T-cell transformation in humans.

Genetic and phenotypic correlations between antibody responses to Escherichia coli, infectious bursa disease virus (IBDV), and Newcastle disease virus (NDV), in broiler lines selected on antibody response to Escherichia coli

The genetic control of antibody (Ab) response to Escherichia coli (EC), infectious bursa disease virus, and Newcastle disease virus and the genetic and phenotypic correlation between these Ab responses, were evaluated under farm conditions in which chicks were simultaneously exposed to these antigens. The experimental population comprised five groups: two lines divergently selected for high (HH) or low (LL) Ab response to EC vaccination; a commercial broiler dam-line (CC), from which HH and LL had been derived; and the HH x CC and LL x CC hybrid groups (HC and LC, respectively). Lines LL and HH expressed similar symmetric divergence to all three antigens. The ranking of the LL, LC, CC, HC, and HH genetic groups according to their mean Ab responses and their very high linear correlation with the LL vs. HH genomic scale clearly indicate the additive nature of the genetic divergence between these lines. Several estimates of correlation were calculated between Ab responses of each pair of antigens and between BW and Ab to each antigen. The high correlation between group means, the near-zero within-group correlation, and the low phenotypic correlation indicate the strongly positive genetic correlation between Ab responses and no correlation with BW. The results of this study suggest that overall immunocompetence of commercial broilers can be improved by selection for high Ab response of young chicks to controlled immunization with a single antigen, without counteracting further selection for high BW.