Allelic forms of beta 2-microglobulin in the mouse

Molecular cloning and characterization of sea bass (Dicentrarchus labrax, L.) MHC class I heavy chain and β2-microglobulin

In this work, the gene and cDNA of sea bass (Dicentrarchus labrax) β2-microglobulin (Dila-β2m) and several cDNAs of MHC class I heavy chain (Dila-UA) were characterized. While Dila-β2m is single-copy, numerous Dila-UA transcripts were identified per individual with variability at the peptide-binding domain (PBD), but also with unexpected diversity from the connective peptide (CP) through the 3' untranslated region (UTR). Phylogenetic analysis segregates Dila-β2m and Dila-UA into each subfamily cluster, placing them in the fish class and branching Dila-MHC-I with lineage U. The α1 domains resemble those of the recently proposed L1 trans-species lineage. Although no Dila-specific α1, α2 or α3 sub-lineages could be observed, two highly distinct sub-lineages were identified at the CP/TM/CYT regions. The three-dimensional homology model of sea bass MHC-I complex is consistent with other characterized vertebrate structures. Furthermore, basal tissue-specific expression profiles were determined for both molecules, and expression of β2m was evaluated after poly I:C stimulus. Results suggest these molecules are orthologues of other β2m and teleost classical MHC-I and their basic structure is evolutionarily conserved, providing relevant information for further studies on antigen presentation in this fish species.

The beta 2-microglobulin locus of rainbow trout (Oncorhynchus mykiss) contains three polymorphic genes

Beta2-microglobulin (beta2m) associates with MHC and related class I H chains to form cell surface glycoproteins that mediate a variety of functions in defense. In humans, monomorphism of a single beta2m gene contrasts with the diversity and polymorphism of the class I H chain genes, and a similar picture was seen in almost all other species examined. In this regard, rainbow trout (Oncorhynchus mykiss) appeared unusual: trout beta2m genes gave a complicated and polymorphic pattern in Southern blots, and a minimum of 10 different mRNA encoding two distinct types of beta2m were expressed by a single fish. Characterization of genomic clones from the same fish now shows that the rainbow trout beta2m locus consists of two expressed genes and one partial gene that are closely linked. Four copies of the locus were identified and allelic variants of each gene defined, largely through comparison of the noncoding regions. A dramatic variation in the lengths of introns is caused by variable repetitive elements and accounts for the complex pattern seen in Southern blots. By comparison to noncoding sequences, the coding regions are conserved but the three loci differ within a cluster of codons that encode residues of beta2m that do not interact with class I H chains. Additional diversity in the trout beta2m genes appears to be due to somatic mutation that might be facilitated by the abundance of repetitive DNA elements within the 12 beta2m genes of an individual rainbow trout.

Purification of bovine urinary beta2-microglobulin and its biochemical characteristics

In this study, bovine beta2-m was purified from urine by ion-exchange chromatography and gel chromatography, and the characteristics were compared with those of colostral beta2-m by the immunological reactivity, isoelectric points, peptide map, and amino acid sequence. The characteristics of purified urinary beta2-m were consistent with those of the colostral beta2-m. The urinary and colostral beta2-m possessed the same polypeptide chain consisting of 98 amino acids, and its molecular weight is 11.8 kDa. Furthermore, four isoforms of beta2-m were found. The isoelectric points were different from each other.

Clearance of Sendai virus by CD8+ T cells requires direct targeting to virus-infected epithelium

Minimal numbers of CD8+ T cells are found in bronchoalveolar lavage (BAL) populations recovered from Sendai virus-infected mice that are homozygous (-/-) for a beta 2-microglobulin (beta 2-m) gene disruption. The prevalence of the CD8+ set was substantially increased in the pneumonic lungs of 8-12-week radiation chimeras made using substantially class I major histocompatibility complex (MHC) glycoprotein-negative beta 2-m (-/-) recipients and normal beta 2-m (+/+) bone marrow. Even so, the CD8+ (but not the CD4+) lymphocyte counts were still much lower than in the (+/+)-->(+/+) controls. The (+/+)-->(+/+) and (+/+)-->(-/-) chimeras cleared Sendai virus and potent virus-immune CD8+ cytotoxic T lymphocytes (CTL) specific for H-2Kb+viral nucleoprotein peptide were found in the BAL from both groups. However, following in vivo depletion of the CD4+ population, only the (+/+)-->(+/+) mice were able to deal with the infection. Similarly, adoptively transferred, H-2Kb-restricted CD8+ T cells from previously-primed (+/+) mice also failed to clear virus from the lungs of (+/+)-->(-/-) chimeras infected within 2 weeks of reconstitution with bone marrow, though they were effective in the (+/+)-->(+/+) controls. Sendai virus-immune CD8+ T cells are thus unable to eliminate virus-infected beta 2-m (-/-) lung epithelial cells that might be thought to be expressing very small amounts of either isolated class I heavy chain, or class I MHC glycoprotein that has bound beta 2-m derived from beta 2-m (+/+) T cells or macrophages present in the pneumonic lung. Furthermore, the CD8+ CTL that are being exposed to beta 2-m (+/+) stimulators in the BAL population cannot operate in some bystander mode to clear virus from respiratory epithelium.

Sequence divergence of B2m alleles of wild Mus musculus and Mus spretus implies positive selection

Mouse beta 2-microglobulin (beta 2m) is polymorphic. Sequences of five allelic wild mouse B2m genes have been determined from the large exons of genomic DNA using the polymerase chain reaction. Relative to the standard B2m(a) allele, the products of four alleles of Mus musculus origin (w2, w3, w4, and w5), differ by only one or two amino acids. w5 has a single nucleotide change, Asp85-->Val, and is identical to the c allele. w3 has two changes, Val(-13)-->Ile and Lys44-->Glu. w2 differs at Arg81-->Thr and w4 at His34-->Gln, and they share the Asp85-->Val change with B2mc and B2mw5. w5 and c cells are lysed by S19.8, a monoclonal antibody specific for beta 2mb (Ala85), in a complement-mediated cytotoxicity assay, whereas w4 cells are not. Thus, distant changes appear to introduce subtle conformational effects on beta 2m structure. Five independent isolates of Mus spretus (w1) differ the most from B2m(a), with 12 amino acid changes and only one silent substitution. Replacements predicted from the nucleotide sequence occur in loops of the molecule facing away from the class I heavy chain and not in regions where beta 2m associates with class I alpha 3 domains. Concordantly, the w1-5 allelic forms of beta 2m associate well with H-2 heavy chains. The many amino acid changes in the spretus sequence and the paucity of silent substitutions suggest that B2m has been subject to positive selection.

High-resolution one-dimensional isoelectric focusing of mouse MHC class I antigens. Identification of natural and experimentally induced class I antigens

A method is described for a biochemical comparison of mouse class I antigens utilizing antisera with a monomorphic pattern of reaction and high-resolution one-dimensional isoelectric focusing (1D-IEF). The most commonly occurring and studied H-2K and D alleles were identified in a comparison of over 40 mouse strains. By comparing H-2 mutant mouse strains, cell lines transfected with defined class I genes, or mice transgenic for a mouse class I gene and H-2 recombinant mouse strains, unambiguous identification of class I alleles was possible. The complex pattern presented by H-2-heterozygous mice was readily resolved into the contribution by the individual parental alleles. The H-2b bm series of mutants was analyzed, and for those mutants where a charge difference was predicted based on their known sequence, a change in isoelectric point (IEP) was indeed observed. Based on analysis by IEF, the bm8 mutant may contain (an) amino acid substitution(s) in addition to those reported. The present method further appears useful in elucidating defects in class I antigen synthesis and post-translational modifications, as these cannot be easily characterized when using surface staining with monoclonal antibodies (mAbs).

Restricted recognition of beta 2-microglobulin by cytotoxic T lymphocytes

Recognition of foreign antigen by cytotoxic T lymphocytes (CTL) is restricted by class I major histocompatibility complex (MHC) products. Class I heavy chains (relative molecular mass (Mr) 45,000-48,000) are reversibly and noncovalently associated with beta 2-microglobulin (beta 2M, Mr = 12,000). Cells expressing human or murine class I heavy chains can exchange their native beta 2M for exogenously added free beta 2M, which is present in serum. Two allelic forms of beta 2M exist among the common laboratory mouse strains, beta 2M-A and beta 2M-B, which are represented in BALB and C57BL mice, respectively. The two forms differ at a single amino acid at position 85, the gene (beta 2m) is located on chromosome 2 linked to a minor histocompatibility (H) region, H-3. It has been proposed that one of the H-3 loci is identical with beta 2m, and that CTL raised across certain H-3 incompatibilities are actually specific for beta 2M. Here we describe CTL raised in such a combination which recognize endogenous as well as exogenous beta 2M-B in the context of H-2Kb. This represents a unique case of CTL recognition, as CTL usually recognize antigens inserted into the membrane, and it is the first molecular identification of the product of a minor H locus.

Unusual association of beta 2-microglobulin with certain class I heavy chains of the murine major histocompatibility complex

Class I products of the major histocompatibility complex (MHC) comprise a heavy chain of about 45 kDa noncovalently linked to a 12-kDa beta 2-microglobulin (beta 2m) light chain encoded on a different chromosome. We find that class I products of some mouse strains include an additional 62-kDa molecule which on the following evidence consists of a heavy chain linked covalently with beta 2m. Production of the 62-kDa protein invariably accorded with the occurrence of cysteine at position 121 of the heavy chain (Kb,Kbm1,Kbm3,Dd, and Ld). Substitution of arginine at position 121 invariably accorded with absence of the 62-kDa protein (Kbm6,Kbm7,Kbm9,Kd, and Db). On the basis of observed production versus nonproduction of the 62-kDa molecule, predictions are made regarding residue 121 in class I products for which this is not yet known; namely, Kk, Ks, and Dk, which produce the 62-kDa molecule, as compared with Kj, Qa-2, and TL, which do not. Reported differences in immunologic reactivity between Kb mutant strains with Arg-121 in place of Cys-121 imply that the occurrence of 62-kDa class I products in mice of Cys-121 genotype has functional consequences.

Beta-2 microglobulin types in mice of wild origin

To determine the distribution of beta-2 microglobulin (B2m) alleles in wild mice we have typed mice derived from natural populations in Europe, North Africa, South America, and East Asia. Mus musculus domesticus mice from Germany, France, Italy, and Peru were all B2ma as were most from the United Kingdom. M.m. musculus mice from Denmark and Czechoslovakia, several stocks of M.m. molossinus from Japan, and M.m. castaneus from China, Thailand, and the Philippines were of B2mb type. This is consistent with the notion that C57BL/6 may have obtained some of its genes, including B2m, from Eastern mice. A BglI restriction site characteristic of B2mb was also found in mice from Czechoslovakia and Japan, confirming that B2mb is a naturally occurring allele of B2m. A new type of beta 2m (beta 2mwl) was found in four stocks of M. spretus from Portugal, Spain, and Morocco. This molecule differs in apparent size and charge from the a and b types. Beta 2mw2 was found together with beta 2ma in one stock of M.m. domesticus (brevirostris) from Morocco. Beta 2mw3 and beta 2mw4 were found in a few M.m. bactrianus from Pakistan. In all cases tested, these new beta 2m molecules associate with class I histocompatibility antigens.

Linkage of beta 2-microglobulin and ly-m11 by molecular cloning and DNA-mediated gene transfer

beta 2-Microglobulin (beta 2m) is expressed on the cell surface after introduction of a beta 2mb (C57BL/6N) genomic clone into thymidine kinase-deficient mouse L cells by cotransformation using the calcium phosphate precipitate method. Stable transformant cell lines were identified that express the beta 2mb allele, as determined by reaction of the cells with appropriate monoclonal antibodies and by two-dimensional gel electrophoresis of endogenously labeled immunoprecipitates of cell extracts. These beta 2mb transformants now express ly-m11.2, as detected by an indirect radioimmunoassay. A plasmid subclone of the beta 2mb gene that contains an 8.4-kilobase insert, after introduction into mouse L cells, similarly directs the synthesis of both the beta 2mb and the ly-m11.2 antigens. Thus, the beta 2mb and ly-m11.2 determinants most likely represent sites on the same protein structure.

Evidence for a major cluster of lymphocyte differentiation antigens on murine chromosome 2

The region of chromosome 2 between H-13 and H-3 has been shown to contain loci coding for a variety of other alloantigens, including Ly-4 and the locus coding for beta 2-microglobulin. Herein we show that Ly-6 and Ly-11 are coded for by genes in a segment of chromosome 2 adjacent to the H-3-H-13 region and that this segment of chromosome also contains the tightly linked loci coding for antigens Ala-1, DAG, H9/25, H-30, Ly-8, and ThB. In addition, at least one locus (and probably more) affecting susceptibility to leukemia induction is found within this gene cluster.

Assignment of the gene for beta 2-microglobulin (B2m) to mouse chromosome 2

We have assigned the gene (B2m) coding for murine beta 2-microglobulin (B2M) to mouse chromosome 2 by using a novel panel of Chinese hamster-mouse somatic cell hybrid clones. Because of 35 independent primary hybrids used in this study were derived from two types of feral mice, each with a different combination of Robertsonian translocation chromosomes, as well as from mice with a normal complement of acrocentric chromosomes, analysis of 16 selected mouse enzyme markers provided data on the segregation of all 20 mouse chromosomes in these hybrids. Mouse B2M was identified in cell hybrids by immunoprecipitation with a species-specific anti-mouse B2M antiserum followed by two-dimensional polyacrylamide gel electrophoresis of the immunoprecipitated polypeptides. Enzyme analysis of the segregant clones excluded all chromosomes for B2m assignment except mouse chromosome 2, and karyotype analysis of nine informative hybrid clones confirmed the assignment of B2m to this chromosome. These results demonstrate that, in the mouse, as in man, B2m is not linked to the major histocompatibility or immunoglobulin loci.

Gene for a major cell surface glycoprotein of mouse macrophages and other phagocytic cells is on chromosome 2

A gene controlling the expression of a polymorphic 92,000-dalton glycoprotein of mouse macrophages and granulocytes has been identified. This glycoprotein was previously shown to be the major iodinated, trypsin-sensitive component of the murine phagocyte cell surface. The gene has been provisionally designated Pgp-1 for phagocyte glycoprotein 1. Expression of the glycoprotein was measured by monoclonal antibody binding to a polymorphic antigenic determinant. Antibody binding to cells of positive strains of mice was proportional to cell number, whereas binding to cells of negative strains was insignificant. The concentration of the antigen in cells of heterozygous mice was approximately 50% of that in homozygous mice. Thirteen of 33 recombinant inbred strains of mice were positive, with binding values 100-fold over background, suggesting that a single gene controlled expression of the antigen. Segregation of the antigen correlated with markers on chromosome 2. The segregation of Pgp-1, with nonagouti coat color (a) and hemolytic complement (Hc) activity among progeny of (C3H/HeJ x DBA/2J)F1 x DBA/2J mice confirmed the single gene control and the chromosomal assignment. Another gene on chromosome 2, Ly-m11, was also typed by using (BALB/cJ x C57BL/6J)F1 x C57BL/6J mice. The data from both of these crosses indicated the following gene order: Hc--Pgp-1--Ly-m11--a.

Subunit structure of cell surface proteins: disulfide bonding in antigen receptors, Ly-2/3 antigens, and transferrin receptors of murine T and B lymphocytes

The surface proteins of lymphocytes from spleen and thymus and several cultured lymphoid tumor lines were radioiodinated in situ, solubilized with Triton X-100, and examined for the presence of disulfide-bonded subunits by two-dimensional (intact, reduced) NaDodSO4/polyacrylamide gel electrophoresis. [Hynes, R. O. & Destree, A. (1977) Proc. Natl. Acad. Sci. USA 74, 2855-2859]. Few lymphocyte surface proteins were found to consist of disulfide-bonded subunits, and the most prominent of these could be identified. In normal B lymphocytes and B-lymphoma cells, IgD or IgM (or both) were the major disulfide-bonded proteins, and these were easily detectable, even without immunoprecipitation. In contrast, analysis of thymocytes and T-lymphoma cells did not reveal any protein resembling immunoglobulin in its chain structure. The major labeled thymocyte membrane protein consisting of disulfide-bonded subunits was identified as the Ly-2/3 antigen. It appeared to contain disulfide-bonded homodimers of Mr 35,000 (alpha 2) noncovalently associated with a second pair of homodimers of Mr 30,000 (beta 2). Peptide mapping showed these polypeptides to be homologous. A third disulfide-bonded homodimer, which was heterogeneous in apparent Mr, appeared to be part of the Ly-2/3 complex. All cultured T- and B-lymphoma lines examined were found to possess a major surface protein that appeared to be a disulfide-bonded homodimer of a polypeptide of Mr 95,000. This protein was identified as the receptor for transferrin. It is suggested that the presence of two or more subunits in cell surface receptors renders their ligand functionally bivalent, making ligand-induced receptor aggregation possible.

Structural evidence that the small subunit found associated with the TL antigen is beta 2-microglobulin

Comparative tryptic peptide mapping and partial amino-terminal primary sequence analysis of the light chain component associated with the TL antigens showed that the small subunit of TL was identical to the beta 2m light chain associated with the H-2K or D product of the same strain. Peptide comparison of the beta 2m from the Tla products of an A strain X-ray induced leukemia RADA1 (Tlaa) and of a C57BL/6 strain X-ray induced leukemia ERLD (Tlab) showed differences to the extent of 25-35% in their peptides. This is consistent with previous results showing beta 2m allelic variations between these mouse strains. The data prove the structural identity of the beta 2m molecules from TL and H-2K, D antigens as well as reveal the strain specific polymorphism of the beta 2m associated with these products.