Immunogenetics (2000) 51 : 138–147 Q Springer-Verlag 2000 ORIGINAL PAPER Jansen P. Jacob 7 Sarah Milne 7 Stephan Beck Jim Kaufman The major and a minor class II b-chain (B-LB ) gene flank the Tapasin gene in the B-F /B-L region of the chicken major histocompatibility complex Received: 29 July 1999 / Revised: 27 September 1999 The nucleotide sequence data reported in this paper have been submitted to the EMBL/GenBank nucleotide sequence databases and have been assigned the accession numbers AJ248572 to AJ248586 J.P. Jacob 7 J. Kaufman (Y) Institute of Animal Health, Compton, Berkshire, RG20 7NN, UK e-mail: jim.kaufman6bbsrc.ac.uk Tel.: c44-1635-577234 Fax: c44-1635-577263 S. Milne 7 S. Beck Sanger Center, Wellcome Trust Genome Campus, Hinxton, Cambridgeshire, CB10 1SA, UK Abstract We have identified the major histocompati- bility complex class II b-chain (B-LB) genes present in the B-F/B-L region of the B complex of nine well-char- acterized lines of chickens and have cleared up much of the confusion concerning numbers and location of B- LB genes in this region. By amplifying DNA sequences between adjacent genes, we found two B-LB genes that lie on either side of Tapasin. The dominantly expressed ‘major’ B-LB gene in all haplotypes lies between Tapa- sin and RING-3, and belongs to the B-LBII family of class II b-chain genes. The poorly expressed ‘minor’ B- LB gene in all haplotypes lies between B-lec1 and Ta- pasin, and belongs either to the B-LBII family or to the previously unmapped B-LBVI family of class II b-chain genes. The data suggest that the B-LBII and B-LBVI genes are two lineages of B-LB genes and we propose that they all be termed B-LB genes. The location of a third B-LB gene in the B12 haplotype (and possibly other haplotypes as well) has yet to be determined. The structural organization and expression of the class II b- chain genes in the B-F/B-L region is similar to that of chicken class I (B-F) genes, one functional result of which is differential resistance to disease and response to vaccines. Key words Class II 7 Major histocompatibility complex 7 B locus 7 Tapasin 7 Chicken Introduction The chicken major histocompatibility complex (MHC) is located on a microchromosome (chromosome 16) which contains at least two regions encoding MHC class I a- and class II b-chain genes, the B locus, and the Y locus. These two regions are apparently sepa- rated by the nucleolar organizer region (NOR), which encodes rRNAs (Bloom and Bacon 1985; Fillon et al. 1996; Miller et al. 1996). These regions are represented in cosmid clusters isolated from chickens of the CB strain (B12 haplotype) and were partially characterized (Guillemot et al. 1988), with most of one cluster corre- sponding to part of the B locus (the B-F/B-L region) now sequenced (deposited in EMBL/GenBank under accession number AL023516; Kaufman et al. 1999a, 1999b). While the chicken B locus was discovered as a sero- logical blood group (Briles et al. 1950), it is identified as the MHC by the fact that it determines rapid allo- graft rejection, a strong mixed-lymphocyte response, and cellular control of antibody production (Briles and Briles 1982; Crone and Simonsen 1987; Pink et al. 1977; Schierman and Nordskog 1961; Vainio et al. 1984, 1988), evidently because it contains highly polymorphic class I and class II genes which encode classical MHC molecules (Hunt and Fulton 1998; Kaufman et al. 1992; Pink et al. 1977; Zoorob et al. 1993). The chicken B locus is also strongly associated with responses to cer- tain infectious pathogens and to some vaccines with, unlike the situation in well-studied mammals, large dif- ferences between MHC haplotypes (reviewed in Cal- neck 1985; Dietert et al. 1990; Kaufman and Lamont 1996; Plachy et al. 1992; Schat 1987). In contrast, the Y locus does not have strong effects on graft rejection, the mixed-lymphocyte reaction, or disease resistance (Bumstead 1998; Juul-Madsen et al. 1997; Lakshmanan and Lamont 1998; Pharr et al. 1996; Vajello et al. 1997; but see Wakenell et al. 1996), and contains what appear to be nonclassical class II b-chain genes based on low polymorphism, low sequence divergence, and low ex-