i Immunol Res 1990;9:8-19 I -" 1990S. KargerAG, Basel 0257-277X/90/0091-000852.75/0 Genetic Organization of the Chicken MHC Guido Kroemer, Frangois Guillemot. Charles Auffray Instilut d'Emb~ologie Moleculaire et Cellulaire du CNRS et du Coll6ge de France, Nogent-sur-Marne, France In immunology, the chicken probably re- presents the most intensively investigated nonmammalian species. This can be attrib- uted to historical reasons and, more impor- tantly, to a variety of methodological advan- tages offered by Galfus domesticus, e.g. the anatomical separation of the two lymphoid organ primordia, the extramaternal develop- ment facilitating embryological manipula- tions, the possibility of obtaining a large number of decendants from one pair of par- ents and the availability of close-bred or inbred lines, including models for virus- induced tumorigenesis and organ-specific autoimmunity. In addition, since birds have evolved separately from mammals for ap- proximately 270 million years, the compari- son of the cellular and molecular organiza- tion of the immune systems in these two classes of vertebrates could provide new in- sights into immune function. As in mammals, basic immunological mechanisms of the chicken are MHC-re- stricted, including T helper [1] and effector functions [2], cooperation between immuno- competent cells [3] and intrathymic [4] (but not intrabursal [5]) lymphocyte differentia- tion, fundamental for the induction of self- tolerance. Genetically conferred resistance and susceptibility to several lethal infections (including Marek's disease [6], lymphoid leukosis [6], cecal coccidiosis [7] and fowl cholera [8]), sarcoma induction by an onco- genic retrovirus (Rous Sarcoma virus [6]) and development of spontaneous autoim- mune thyroiditis of Obese strain (OS) chick- ens [9] are known to be associated with cer- tain MHC haplotypes. In the last two cases, furthermore, interstrain differences in the level of expression of class II genes on the target cells subject to (auto)immune surveil- lance appear to be directly implicated in pathogenetic processes [ l 0, 11 ]. The chicken MHC, designated the B lo- cus, comprises functional as well as bio- chemical equivalents of the mammalian class I (B-F determinants), class II (B-L im- mune response genes) and in addition eryth- rocyte-specific class IV (B-G) alloantigens, for which a human or murine counterpart is not known. Extensive studies involving the characterization of B alloantigens in crosses of inbred chicken lines failed to detect ge- netic segregation of the B-F and B-L alleles due to meiotic crossing-over [ 12, 13]. In con- trast, a rather low frequency of recombina- tion between the B-F and B-G loci (0.04%) was observed [ 14, 15]. These data led to the