Polymorphism in chicken immune response genes and resistance to disease

Abstract

Genetic resistance to a number of viral pathogens has been linked to polymorphisms in genes of the immune system. Among these are the genes of the major histocompatibility complex (MHC) and those induced by the type I interferons (IFN). The chicken MHC contains two classical loci, BF1 and BF2, with BF2 reported as the dominantly expressed gene that functions to present antigen to cytotoxic T lymphocytes (CTL). The BF1 molecule has recently been shown to function as negative regulator of Natural Killer (NK)-like cells. In mammals, the level of expression of MHC molecules influences their function. Furthermore selective pressures exerted by pathogens are thought to drive diversification of MHC expression. Little is known in chickens about the effects of allelic differences among BF1 molecules on expression or in regulatory elements that control transcription.

The first two objectives during this investigation were to quantify the level of expression of the two alleles of the MHC class I in the chicken in several haplotypes and then to attempt to provide a molecular explanation for the differences in expression by sequencing of the promoter and signal peptide regions and also to sequence the entire gene of one BF1 allele with very low transcript levels. This study confirmed by quantitative reverse transcription PCR the lower level of expression of BF1 relative to the BF2 gene. In addition, nucleotide sequences were obtained that showed extensive polymorphism in promoter regions between alleles with conservation in identified regulatory elements. One BF1 allele had very low transcript abundance, presumably caused by a mutation that abolished a 3’ splice site in intron 7. Interferon induced antiviral activity is an important line of defense against viral pathogens. Among the genes induced by interferon, the GTPase Mx is one of the best characterized in mammals, and polymorphism at residue 631 determines antiviral activity in chickens. The role of chicken Mx has been assessed against vesicular stomatitis virus and influenza virus in transfection experiments. However, the role of chicken Mx against infectious bursal disease virus, an important pathogen of chicken, has not been examined. In the third part of this investigation the antiviral activity of interferon and the IFN-induced protein Mx were examined against infectious bursal disease virus in cell cultures differing in Mx genotype. This study confirmed that IFN has antiviral activity against IBDV infection in this system in a dose dependent manner. Collectively, the results presented in this dissertation may contribute to understanding the immune mechanisms involved in response to disease and genetic factors that determine susceptibility or resistance.