The MHC of the chicken, genomic structure, gene products, and resistance to oncogenic DNA and RNA viruses

Chickens as a simple system for scientific discovery: The example of the MHC

Chickens have played many roles in human societies over thousands of years, most recently as an important model species for scientific discovery, particularly for embryology, virology and immunology. In the last few decades, biomedical models like mice have become the most important model organism for understanding the mechanisms of disease, but for the study of outbred populations, they have many limitations. Research on humans directly addresses many questions about disease, but frank experiments into mechanisms are limited by practicality and ethics. For research into all levels of disease simultaneously, chickens combine many of the advantages of humans and of mice, and could provide an independent, integrated and overarching system to validate and/or challenge the dogmas that have arisen from current biomedical research. Moreover, some important systems are simpler in chickens than in typical mammals. An example is the major histocompatibility complex (MHC) that encodes the classical MHC molecules, which play crucial roles in the innate and adaptive immune systems. Compared to the large and complex MHCs of typical mammals, the chicken MHC is compact and simple, with single dominantly-expressed MHC molecules that can determine the response to infectious pathogens. As a result, some fundamental principles have been easier to discover in chickens, with the importance of generalist and specialist MHC alleles being the latest example.

Pathogen diversity drives the evolution of generalist MHC-II alleles in human populations

Central players of the adaptive immune system are the groups of proteins encoded in the major histocompatibility complex (MHC), which shape the immune response against pathogens and tolerance to self-peptides. The corresponding genomic region is of particular interest, as it harbors more disease associations than any other region in the human genome, including associations with infectious diseases, autoimmune disorders, cancers, and neuropsychiatric diseases. Certain MHC molecules can bind to a much wider range of epitopes than others, but the functional implication of such an elevated epitope-binding repertoire has remained largely unclear. It has been suggested that by recognizing more peptide segments, such promiscuous MHC molecules promote immune response against a broader range of pathogens. If so, the geographical distribution of MHC promiscuity level should be shaped by pathogen diversity. Three lines of evidence support the hypothesis. First, we found that in pathogen-rich geographical regions, humans are more likely to carry highly promiscuous MHC class II DRB1 alleles. Second, the switch between specialist and generalist antigen presentation has occurred repeatedly and in a rapid manner during human evolution. Third, molecular positions that define promiscuity level of MHC class II molecules are especially diverse and are under positive selection in human populations. Taken together, our work indicates that pathogen load maintains generalist adaptive immune recognition, with implications for medical genetics and epidemiology.

Whereas specialist major histocompatibility complex (MHC) molecules initiate immune response against only relatively few pathogens, generalists provide protection against a broad range. Accordingly, this study shows that the geographical distribution of generalist MHC alleles in human populations reflects exposure to diverse infectious diseases.

Variation in the human genome influences our susceptibility to infectious diseases, but the causal link between disease and underlying mutation often remains enigmatic. Major histocompatibility complex II (MHC class II) molecules shape both our immune response against pathogens and our tolerance of self-peptides. The genomic region that encodes MHC molecules is of particular interest, as it is home to more genetic disease associations than any other region in the human genome, including associations with infectious diseases, autoimmune disorders, cancers, and neuropsychiatric diseases. Here, we propose that MHC class II molecules can be categorized into two major types; specialists initiate effective immune response against only relatively few pathogens, while generalists provide protection against a broad range of pathogens. As support, we demonstrate that generalist MHC class II variants are more prevalent in human populations residing in pathogen-rich areas.

Generalists and Specialists: A New View of How MHC Class I Molecules Fight Infectious Pathogens

In comparison with the major histocompatibility complexes (MHCs) of typical mammals, the chicken MHC is simple and compact with a single dominantly expressed class I molecule that can determine the immune response. In addition to providing useful information for the poultry industry and allowing insights into the evolution of the adaptive immune system, the simplicity of the chicken MHC has allowed the discovery of phenomena that are more difficult to discern in the more complicated mammalian systems. This review discusses the new concept that poorly expressed promiscuous class I alleles act as generalists to protect against a wide variety of infectious pathogens, while highly expressed fastidious class I alleles can act as specialists to protect against new and dangerous pathogens.

A broad overview of classical MHC I expression and bound peptides reveals an inverse correlation between repertoire breadth and cell-surface expression in some chicken and human alleles.

Several chicken class I alleles with wide peptide-binding repertoires (promiscuity) are associated with resistance to a variety of common diseases.

Conversely, a narrow peptide-binding repertoire (fastidiousness) in some human HLA-B alleles is associated with resistance to HIV progression.

Cell-surface expression of some classical class I alleles depends on the regulation of translocation to the cell surface rather than of transcription or translation. MHC translocation is influenced by peptide translocation in chickens and by tapasin interaction in humans.

Molecular characterization of major and minor MHC class I and II genes in B21-like haplotypes in chickens

The major histocompatibility complex (MHC) sequences of three B21-like haplotypes deriving from very different origins including the Red Jungle Fowl Gallus Gallus gallus were compared with the MHC sequences of the standard B21 haplotype from Scandinavian White Leghorn Gallus domesticus. The present analysis reveals two cDNA sequences for B-F and two cDNA sequences for B-LB for every B21-like haplotype, including B21 itself. Contrary to expectation, no sequence polymorphism in the antigen-binding domains of the MHC genes, between the investigated haplotypes, was found. The relative level of MHC class I molecules on the surface of leukocytes measured by flow cytometry was also analysed and found to be low in Marek's Disease (MD)-resistant B haplotypes (B21 and B21-like) and high in MD-susceptible B haplotypes (B15 and B19). However, in heterozygous (resistant/susceptible) animals, the relative level was almost as high as in susceptible haplotypes.

Serological evidence for major histocompatibility complex (B complex) antigens in broilers selected for humoral immune response

Genetic selection for early humoral immune responsiveness against two simultaneous antigens, namely, heat-killed Escherichia coli and Newcastle disease virus vaccine (NDV), was performed in a heterogenic population of broiler chickens. The humoral immune response was measured prior to 24 days of age, depending on the vaccine. Chicks were divided into two populations according to their antibody response: a population of high responders to both antigens and a population of low responders. After four generations of selection, a significant difference was observed in the immune response of the two populations, and therefore, an attempt was made to identify possible differences in the segregation of major histocompatibility complex genes. Using alloantisera prepared in B-congenic White Leghorn, the high responders exhibited a high percentage of chickens having erythrocytes agglutinated with B5 antisera, and the low responders exhibited a high percentage of chicks with erythrocytes agglutinated with B15 and B19 antisera. The B13 antisera agglutinated cells of similar proportions of chickens in the two populations. A random commercial broiler flock was screened with the antisera, and correlation was high between B haplotype and antibody level to E. coli following vaccination. The present work indicated the possibility of a direct association between the genetic characteristics represented in the B complex and humoral antibody response in a broiler population.

Attempt to detect recombination between B-F and B-L genes within the chicken B complex by serological typing, in vitro MLR, and RFLP analyses

In search for recombinants within the chicken major histocompatibility B complex, 1155 animals from crosses between the congenic lines CB (B12) and CC (B4) were tested with alloantibodies and monoclonal antibodies for the B-F (class I), B-L (class II), and B-G (class IV) antigens and by mixed lymphocyte reaction. The absence of detectable recombination was confirmed by restriction fragment length polymorphism analysis with B-L beta and B-F probes. Together with previous reports, this indicates that the distance between the B-F and B-L loci is below 0.01 centimorgan.