NonMHClinked Ir genes

A number of non-MHC-linked genes have also been shown to affect the magnitude of the immune response to particular antigens, and therefore fulfill the requirements for Ir genes. However, unlike the MHC genes, the mechanisms of action are poorly understood. The Ir-2 locus controls the antibody response to the blood group antigen Ka-1. lr-2 appears to be a single locus, mapping to chromosome 2 of the mouse, for which nonresponse is dominant. The Ir-3 locus controls the response to the Pro-Lys backbone of the synthetic copolymer (Phe,Ci)-Pro—L. This antigen is also under MHC-linked Ir gene control, making Ir-3 difficult to study. The lr-4 locus controls the antibody response to the H2.2 specificity of the H2Db molecules. The Ir-5 locus controls the antibody response to the mouse Thy-1 antigen and maps approximately 17 cM telomerically from H2. As with Ir-3, Ir-5 interacts with H2 in the response. The Ir-7 locus controls the response to the cell wall polysaccharide of group A streptococci. Three alleles have been described that interact in a complex fashion.

Genetic analysis of Ir phenotypes indicates that there are many additional loci that can manifest Ir gene effects. In fact, any polymorphic self protein, in theory, can influence the immune response to foreign antigen through the process of negative selection in the thymus. This is most dramatically seen in the case of the endogenous retroviral superantigens. Although not strictly a self molecule, these integrated defective retroviruses show mendelian inheritance and produce protein products that are indistinguishable from other self proteins. Mouse mammary tumor virus 7 (MMTV-7), previously known as Mls-1", produces a superantigen that interacts with all T cells bearing Vp 6, 7, 8.1 and 9 T cell receptors. Consequently, mice carrying MMTV-7 clonally delete most, if not all T cells bearing these particular Vp T cell receptors (TCR), resulting in a large 'hole' in the T cell repertoire. Finally, the genes encoding the immunoglobulin and T cell receptor V regions are technically Ir genes since the presence of particular V genes varies between haplotypes. Many haplotypes have large deletions of TCR or Ig V genes, significantly altering the T or B cell repertoire in these mice.

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