Restriction fragment length polymorphism analysis of the chicken B-F and B-L genes and their association with serologically defined B haplotypes

The Chicken MHC: Insights into Genetic Resistance, Immunity, and Inflammation Following Infectious Bronchitis Virus Infections

The chicken immune system has provided an immense contribution to basic immunology knowledge by establishing major landmarks and discoveries that defined concepts widely used today. One of many special features on chickens is the presence of a compact and simple major histocompatibility complex (MHC). Despite its simplicity, the chicken MHC maintains the essential counterpart genes of the mammalian MHC, allowing for a strong association to be detected between the MHC and resistance or susceptibility to infectious diseases. This association has been widely studied for several poultry infectious diseases, including infectious bronchitis. In addition to the MHC and its linked genes, other non-MHC loci may play a role in the mechanisms underlying such resistance. It has been reported that innate immune responses, such as macrophage function and inflammation, might be some of the factors driving resistance or susceptibility, consequently influencing the disease outcome in an individual or a population. Information about innate immunity and genetic resistance can be helpful in developing effective preventative measures for diseases such as infectious bronchitis, to which a systemic antibody response is often not associated with disease protection. In this review, we summarize the importance of the chicken MHC in poultry disease resistance, particularly to infectious bronchitis virus (IBV) infections and the role played by innate immunity and inflammation on disease outcome. We highlight how future studies focusing on the MHC and non-MHC genes can potentially bring clarity to observed resistance in some chicken B haplotype lines.

Advances in methodologies for detecting MHC-B variability in chickens

The chicken major histocompatibility B complex (MHC-B) region is of great interest owing to its very strong association with resistance to many diseases. Variation in the MHC-B was initially identified by hemagglutination of red blood cells with specific alloantisera. New technologies, developed to identify variation in biological materials, have been applied to the chicken MHC. Protein variation encoded by the MHC genes was examined by immunoprecipitation and 2-dimensional gel electrophoresis. Increased availability of DNA probes, PCR, and sequencing resulted in the application of DNA-based methods for MHC detection. The chicken reference genome, completed in 2004, allowed further refinements in DNA methods that enabled more rapid examination of MHC variation and extended such analyses to include very diverse chicken populations. This review progresses from the inception of MHC-B identification to the present, describing multiple methods, plus their advantages and disadvantages.

Establishment of six homozygous MHC-B haplotype populations associated with susceptibility to Marek's disease in Chinese specific pathogen-free BWEL chickens

The highly polymorphic chicken major histocompatibility complex (MHC) is associated with different levels of immunologic responses to certain avian pathogens. MHC-B haplotype chickens are an important genetic resource for studying the genetic determination of pathogen resistance and susceptibility. The BWEL chicken population is the only specific pathogen-free (SPF) chickens bred and developed by the State Center of Poultry Genetic Resources of Laboratory Animals in China. In this study, we successfully established six homozygous MHC-B haplotype populations from the BWEL chickens using microsatellite marker technology, named as BW/G(1, 2, 3, 5, 6, 7) lines, and their molecular genotypes were matched to six serologically defined MHC-B haplotypes, B13, B15, B2, B5, B21 and B19, respectively. The sequences of BF genes exons 2 and 3 from four successive generations (F1-F4) of the BW/G(n) lines were completely consistent with those of serologically defined MHC-B haplotypes. Subsequently, six BW/G(n) line specific allo-antisera were prepared by immunization with red blood cells (RBCs) and hemagglutination tests results showed the BW/G(n) SPF chickens could be serologically differentiated. Additionally, susceptibility to Marek's disease (MD) in the BW/G3 (B2 haplotype) and BW/G7 (B19 haplotype) lines were determined by comparing mortality, macroscopic and histopathological lesions, and viral loads in feather pulp. The BW/G7 line showed greater genetic susceptibility to the very virulent MD virus (MDV) strain than the BW/G3 line. The establishment of MHC-B haplotype chicken populations associated with susceptibility to MD will be helpful for studying host immune responses and further developing the more effective vaccines in the context of MHC specificities, and they are also very useful for an understanding of MHC genes architecture and function.

Dramatic differences in the response of macrophages from B2 and B19 MHC-defined haplotypes to interferon gamma and polyinosinic:polycytidylic acid stimulation

The chicken MHC has been associated with disease resistance, though the mechanisms are not understood. The functions of macrophages, critical to both innate and acquired immunity, were compared between the more infectious bronchitis virus-resistant B2 and the more infectious bronchitis virus-susceptible B19 lines. In vivo peripheral blood concentrations of monocytes were similar in B2 or B19 homozygous haplotypes. Peripheral blood-derived macrophages were stimulated with poly I:C, simulating an RNA virus, or IFNγ, a cytokine at the interface of innate and adaptive immunity. Not only did B2-derived peripheral monocytes differentiate into macrophages more readily than the B19 monocytes, but as determined by NO production, macrophages from B2 and B2 on B19 genetic background chicks were also significantly more responsive to either stimulant. In conclusion, the correlation with resistance to illness following viral infection may be directly linked to a more vigorous innate immune response.

Abundant polymorphisms at the microsatellite locus LEI0258 in indigenous chickens

The chicken major histocompatibility complex (MHC) has abundant SNP and indels, and is closely related with host genetic resistance or susceptibility to disease. The LEI0258 locus is the most variable in the MHC region, and is a useful marker in reflecting the variability of MHC. In this study, we applied the LEI0258 microsatellite marker to investigate polymorphism of MHC in Chinese indigenous chickens. The size of LEI0258 fragments in 1,617 individuals from 33 Chinese chicken breeds was detected by capillary electrophoresis, and 213 samples with different fragment sizes were further sequenced. A total of 69 alleles ranging from 193 to 489 bp were found, including 21 novel alleles and 28 private alleles that existed in only one breed. Three alleles, 249 bp (7.04%), 489 bp (6.57%), and 309 bp (6.10%), were the most frequent in the indigenous chickens. A 489-bp novel allele was unique in Chinese local chicken breeds. Three indels and 4 SNP of upstream/downstream of 2 repeat regions (R13/R12) were found. Abundant variations indicate high genetic diversity at the MHC region in indigenous chickens. Rare alleles are vulnerable to genetic drift in small populations, and can be used as molecular markers for monitoring the dynamic conservation of many indigenous breeds.

Chicken mannose-binding lectin (MBL) gene variants with influence on MBL serum concentrations

Mannose-binding lectin (MBL) plays a major role in the innate immune defence by activating the lectin complement pathway or by acting as an opsonin. Two forms of MBL have been characterised from several species, but for humans and chickens, only one form of functional MBL has been described. The human MBL2 gene is highly polymorphic, and it causes varying MBL serum levels. Several of the single-nucleotide polymorphisms (SNPs) have been associated with the severity of diseases of bacterial, viral or parasitic origin. Association between various diseases and different MBL serum levels has also been identified in chickens. In this study, two inbred chicken lines (L10L and L10H) which have been selected for low and high MBL levels in serum and four other experimental chicken lines were analysed for polymorphism in the MBL gene. The presence of polymorphisms in the MBL gene was revealed by southern blot analyses, and the differences in the serum concentrations of MBL were found to be of transcriptional origin according to real-time quantitative reverse transcription PCR analysis. Several SNPs were discovered in the promoter and the 5' untranslated region of the chicken MBL gene which resulted in the identification of six different alleles. Mapping of regulatory elements in the promoter region was performed, and SNPs that could affect the MBL serum concentration were identified. One SNP that was found to be located in a TATA box was altered in one of the six alleles only. This allele was associated with low MBL serum concentration.

LEI0258 microsatellite variability in Khorasan, Marandi, and Arian chickens

Microsatellite LEI0258 is a genetic marker for chicken MHC haplotypes and can be used as an indicator of the influence of population genetics on immune responses. LEI0258 microsatellite variability in three Iranian indigenous chicken populations (Khorasan, Marandi, and Arian) was investigated. In total, 142 Khorasan, 42 Marandi, and 58 Arian chickens were examined. Collectively, 25 different alleles and 79 genotypes could be found. The observed levels of heterozygosity were 81% in Khorasan and Marandi and 34% in Arian chickens. Our results indicate that LEI0258 diversity in Marandi chickens is higher than in the other populations. Allelic diversity in Iranian chickens is relatively higher than in the local chicken breeds reported for Brazil and Vietnam.

MHC haplotype and susceptibility to experimental infections (Salmonella Enteritidis, Pasteurella multocida or Ascaridia galli) in a commercial and an indigenous chicken breed

In three independent experimental infection studies, the susceptibility and course of infection of three pathogens considered of importance in most poultry production systems, Ascaridia galli, Salmonella Enteritidis and Pasteurella multocida were compared in two chicken breeds, the indigenous Vietnamese Ri and the commercial Luong Phuong. Furthermore, the association of the Major Histocompatibility Complex (MHC) with disease-related parameters was evaluated, using alleles of the LEI0258 microsatellite as markers for MHC haplotypes. The Ri chickens were found to be more resistant to A. galli and S. Enteritidis than commercial Luong Phuong chickens. In contrast, the Ri chickens were more susceptible to P. multocida, although production parameters were more affected in the Luong Phuong chickens. Furthermore, it was shown that the individual variations observed in response to the infections were influenced by the MHC. Using marker alleles of the microsatellite LEI0258, which is located within the MHC region, several MHC haplotypes were identified as being associated with infection intensity of A. galli. An association of the MHC with the specific antibody response to S. Enteritidis was also found where four MHC haplotypes were shown to be associated with high specific antibody response. Finally, one MHC haplotype was identified as being associated with pathological lesions and mortality in the P. multocida experiment. Although not statistically significant, our analysis suggested that this haplotype might be associated with resistance. These results demonstrate the presence of local genetic resources in Vietnamese chickens, which could be utilized in breeding programmes aiming at improving disease resistance.

Gastrointestinal helminths in indigenous and exotic chickens in Vietnam: association of the intensity of infection with the Major Histocompatibility Complex

This study compared the prevalence and intensity of infections of helminths in 2 chicken breeds in Vietnam, the indigenous Ri and the exotic Luong Phuong. Also, possible correlations with the Major Histocompatibility Complex (MHC) were tested. The most prevalent helminths were Ascaridia galli, Heterakis beramporia, Tetrameres mothedai, Capillaria obsignata, Raillietina echinobothrida and Raillietina tetragona. Differences in prevalence and intensity of infection were found between the 2 breeds. Comparing the 2 groups of adult birds, Ri chickens were observed to have higher prevalence and infection intensities of several species of helminths, as well as a higher mean number of helminth species. In contrast, A. galli and C. obsignata were shown to be more prevalent in Luong Phuong chickens. Furthermore, an age-dependent difference was indicated in the group of Ri chickens in which the prevalence and the intensity of infection was higher for the adult than the young chickens for most helminths. The most notable exception was the significantly lower prevalence and intensities of A. galli in the group of adult chickens. In contrast, the prevalence and intensity were very similar in both age groups of Luong Phuong chickens. Using a genetic marker located in the MHC, a statistically significant correlation between several MHC haplotypes and the infection intensity of different helminth species was inferred. This is the first report of an association of MHC haplotype with the intensity of parasite infections in chickens.

Immune response to a killed infectious bursal disease virus vaccine in inbred chicken lines with different major histocompatibility complex haplotypes

The influence of MHC on antibody responses to killed infectious bursal disease virus (IBDV) vaccine was investigated in several MHC inbred chicken lines. We found a notable MHC haplotype effect on the specific antibody response against IBDV as measured by ELISA. Some MHC haplotypes were high responders (B201, B4, and BR5), whereas other MHC haplotypes were low responders (B19, B12 and BW3). The humoral response of 1 pair of recombinants isolated from a Red Jungle Fowl (BW3 and BW4) being identical on BF and BG, but different on BL, indicated that part of the primary vaccine response was an MHC II restricted T-cell dependent response. The humoral response in another pair of recombinant haplotypes originating in 2 different White Leghorn chickens being BF21, BL21, BG15 (BR4) and BF15, BL15, BG21 (BR5) on the MHC locus indicated that the BG locus may perform an adjuvant effect on the antibody response as well. Vaccination of chickens at different ages and in lines with different origin indicated that age and background genes also influence the specific antibody response against inactivated IBDV vaccine.

Molecular genotype identification of the Gallus gallus major histocompatibility complex

The chicken major histocompatibility complex (MHC) is commonly defined by serologic reactions of erythrocytes with antibodies specific to the highly polymorphic MHC class I (BF) and MHC class IV (BG) antigens. The microsatellite marker LEI0258 is known to be physically located within the MHC, between the BG and BF regions. DNA from various serologically defined MHC haplotypes was amplified by polymerase chain reaction with primers surrounding this marker. Twenty-six distinctive allele sizes were identified. Some serologically well-defined MHC haplotypes shared a common LEI0258 allele size but could be distinguished either by the addition of information from another nearby marker (MCW0371) or by small indels or single nucleotide polymorphism (SNP) differences between the alleles. The association between LEI0258 allele and serologically defined MHC haplotype was very consistent for the same haplotype from multiple sources. Sequence information for the region defined by LEI0258 was obtained for 51 different haplotypes. Two internal repeats whose lengths were 13 and 12 bp, respectively, are the primary basis for allelic variability. Allele size variation ranges from 182 to 552 bp. Four indels and five SNPs in the surrounding sequence provide additional means for distinguishing alleles. Typing with LEI0258 and MCW0371 will be useful in identifying MHC haplotypes in outbred populations of chickens particularly for the initial development of serological reagents.

Molecular Characterization of Major Histocompatibility Complex Class I (B-F) mRNA Variants from Chickens Differing in Resistance to Marek's Disease

In this study, the relative distributions of two alternatively polyadenylated chicken major histocompatibility complex (MHC) mRNA isoforms of approximately 1.5 and 1.9 kb were analysed in spleen cells from chickens homozygous for the MHC haplotypes B21 and B19v1 as well as in heterozygous B19v1/B21 birds. Both isoforms are likely to encode classical MHC class I (B-F) alpha chains. The B19v1 and B21 MHC haplotypes confer different levels of protection against Marek's disease (MD), which is caused by infection with MD virus (MDV). In spleen cells, MD-resistant B21 birds were shown to have the highest percentage of the 1.5 kb variant relative to the total MHC class I expression, MD-susceptible B19v1 birds the lowest and B19v1/B21 birds an intermediate percentage. Infection of 4-week-old chickens with the GA strain of MDV was shown to cause a significant increase in the relative amount of 1.5 kb transcripts in B21 birds 32 days postinfection (dpi). Alternatively polyadenylated mRNA isoforms may encode identical proteins, but differences in the 3' untranslated region (UTR) can influence polyadenylation, mRNA stability, intracellular localization and translation efficiency. It was shown that the increased 1.5 kb percentage in B21 birds 32 days postinfection may be a result of a change in the choice of poly(A) site rather than a locus-specific upregulated transcription of the BF1 gene that preferentially expresses the 1.5 kb variant. Furthermore, the 3' end of the 1.5 kb mRNA variants deriving from B19v1 and B21 chickens was characterized by Rapid Amplification of cDNA Ends (RACE) and sequencing. No potentially functional elements were identified in the 3' UTR of the RACE products corresponding to this short isoform. However, variation in polyadenylation site was observed between the BF1 and BF2 mRNA transcripts and alternative splicing-out of the sequence (exon 7) encoding the second segment of the cytoplasmic part of the mature BF2*19 molecules. This alternative exon 7 splice variant was also detected in other MD-susceptible haplotypes, but not in the MD-resistant B21 and B21-like haplotypes, suggesting a potential role of exon 7 in MHC-related MD resistance.

Differences in chicken major histocompatibility complex (MHC) class Ialpha gene expression between Marek's disease-resistant and -susceptible MHC haplotypes

The expression of chicken major histocompatibility complex (MHC) class Ialpha genes was investigated in spleen cells from a panel of chickens with well-defined MHC haplotypes, and two class Ialpha transcripts of 1.9 and 1.5 kb were detected in various amounts. In BW1, B130 and B21, the two transcripts were almost equally expressed. In B2, B6, B12 and B19, the ratio between the two transcripts was 4 : 1, with the 1.9 kb transcript having the strongest expression. In B14 and B15, the 1.5 kb transcript was undetectable and the 1.9 kb transcript appeared to be exclusively expressed. Thus, haplotypes considered to have an MHC-determined resistance to Marek's disease (MD) had the highest relative amount of the 1.5 kb transcript, whereas haplotypes considered to be MD-susceptible had the lowest. In order to address a possible correlation between MHC-Ialpha transcriptional patterns and MD resistance, a larger animal material experimentally infected with MD virus (MDV) was examined. The expression of MHC class Ialpha genes was investigated in spleens as well as in other organs, 9 weeks post-infection (p.i.), from animals of the two MD-resistant haplotypes B21 and BW1 as well as from the MD-susceptible haplotype B19. In the spleen cells of infected animals, the relative amount of the 1.5 kb transcript in the haplotypes BW1 and B21 was shown to be significantly higher than that in B19. Interestingly, in infected BW1 and B21 animals, the relative amount of the 1.5 kb transcript was also significantly higher than that in healthy MHC-matched controls. In B19, no differences were detected between uninfected and infected animals. Furthermore, it was shown in BW1 and B21 that the two classical MHC-Ialpha genes located in the MHC region were both able to produce both mRNA transcripts. Hybridization experiments, using specific probes upstream and downstream of the polyadenylation signals in the 3' end of the MHC-Ialpha genes, demonstrated that alternate use of these signals is probably involved in the production of the two mRNA transcripts.

Major histocompatibility complex-linked immune response of young chickens vaccinated with an attenuated live infectious bursal disease virus vaccine followed by an infection

The influence of the MHC on infectious bursal disease virus (IBDV) vaccine response in chickens was investigated in three different chicken lines containing four different MHC haplotypes. Two MHC haplotypes were present in all three lines with one haplotype (B19) shared between the lines. Line 1 further contains the BW1 haplotype isolated from a Red Jungle Fowl. Line 131 further contains the B131 haplotype isolated from a meat-type chicken. Finally, Line 21 further contains the international B21 haplotype. The chickens were vaccinated with live attenuated commercial IBDV vaccine at 3 wk of age, followed by a challenge with virulent IBDV at 6 wk of age. In this study, we found a notable MHC haplotype effect on the specific antibody response against IBDV, as measured by ELISA. The BW1 haplotype was found to have a significantly higher serum antibody titer against IBDV (7,872) than haplotypes B19 (mean 5,243), B21 (5,570), and B131 (5,333) at 8 d postinfection. However, a virus-neutralizing antibody test did not reflect this result. Nevertheless, the MHC haplotype-associated protective immunity was further supported by the bursa of Fabricius (bursa) recovery from the disease, as measured by histological scorings of the bursa. Chickens carrying the BW1 haplotype had a significantly lower bursa lesion score (1.7) than the haplotypes B19 (mean 3.8), B21 (3.6), and B131 (4.3) 8 d postinfection. Furthermore, multiple line effects were found in other variables when comparing Day 6 with Day 8. Body weight, relative weights of the bursa and the spleen, percentage and relative number of MHC II molecules on MHC II-positive lymphocytes, percentage and relative number of CD4 molecules on CD4-positive lymphocytes, and the specific antibody response all differed significantly among lines. Line 1, with Red Jungle Fowl genes, was clearly differentiated from the other two investigated lines. These results suggest an MHC II restricted T-cell dependent secondary antibody response against IBDV.

Rainbow trout offspring with different resistance to viral haemorrhagic septicaemia

To study immunological and immunogenetical parameters related to resistance against viral haemorrhagic septicaemia (VHS), attempts to make gynogenetic strains of rainbow trout selected for high and low resistance to VHS were initiated in 1988. The first gynogenetic generation of inbreeding resulted in the more resistant offspring E8 and the low resistance offspring K3; the K3 offspring having the same high mortality as the susceptible reference strain of outbred trout in infection trials. A second gynogenetic generation derived from the E8 strain resulted in some low resistance offspring, and two gynogenetic families in which all, or nearly all, fish survived challenge with VHS virus. In this study, an attempt to associate the distribution of different MHC class II genotypes with low and high resistance gynogenetic offspring was performed. Two different MHC haplotypes could be distinguished, and in both low and high resistance families all three genotypes were found, which could be explained by the fact that the mother fish carried the heterozygous genotype. Although no significant differences in MHC II genotypes were found between the high and low resistance offspring, a significantly different distribution of haplotypes in the low resistance offspring was observed, that could not be explained by a one- or two-locus model.

A polymorphic major histocompatibility complex class II-like locus maps outside of both the chicken B-system and Rfp-Y-system

Chickens have two major regions encoding major histocompatibility complex (MHC) class Ialpha genes and MHC class IIss genes, the serological and functional B-system and the Rfp-Y-system. Recently, they have been shown to assort in a genetically independent way although still located on the same microchromosome. Moreover, the monomorphic MHC class IIalpha gene maps at a third locus located 5 cM from the nearest class IIss genes, located in the B-system (Kaufman et al., 1995). A pedigree family was studied in three generations in order to assign MHC class IIss restriction fragments observed in Southern blot analyses to either the B-system, the Rfp-Y-system or the B-Lalpha locus. In this study, we demonstrate by classical genetic testing of chickens within this fully pedigreed family the existence of an MHC class II-like polymorphic restriction fragment that segregates independently of the B-system, the Rfp-Y-system and of the B-Lalpha locus.