The beta 1 domain of the mouse E beta chain is important for restricted antigen presentation to helper T-cell hybridomas

Protective role of major histocompatibility complex class II Ebd transgene on collagen-induced arthritis

Collagen-induced arthritis (CIA) is an animal model of autoimmune inflammatory polyarthritis that has features similar to rheumatoid arthritis (RA). Much like RA, susceptibility to mouse CIA is influenced by the major histocompatibility complex (MHC), H-2, and restricted to the H-2q and H-2r haplotypes. Whereas the role of the H-2A molecule in susceptibility to CIA is well established, little is known about the role of H-2E molecule in the disease. In this study, we analyzed the effect of a transgenic E beta d molecule on CIA susceptibility in a recombinant mouse B10.RQB3, which expresses the CIA susceptible Aq genes and an Eak gene, but does not produce an E molecule since Ebq is nonfunctional. In the presence of an Ebd transgene, a viable E molecule is generated. Whereas B10.RQB3 were susceptible to CIA, B10.RQB3-E beta d+ showed a dramatic reduction in the incidence of arthritis as well as a decrease in the level of anti-mouse and anti-bovine CII antibodies in their serum. No clear cut differences in the expression of T cell receptor (TCR) V beta was observed between E beta d+ and E beta d- transgenic mice. Mechanisms underlying the protective effect of E beta d transgenic molecule on CIA may shed light on how HLA-DR molecules influence human RA.

Recruitment of multiple alleles within the Eb recombinational hotspot in murine MHC

Genetic recombination has been proposed to have played a major role in generating the extensive polymorphism that distinguishes the genes of the major histocompatibility complex (MHC). The proximal region of the murine H-2 represents a unique segment of DNA encompassing at least four hotspots for meiotic recombination. One of these hotspots lies within the second intron of the class II Eb gene and has been defined at the nucleotide level for a number of simple two-allele crosses. In this report we studied two crosses in which one or both parents in themselves were H2Eb recombinants and three alleles were present within the hotspots of each pair of the parental haplotypes. Nucleotide analysis indicated that the break points in these secondary recombinants, like those in the primary recombinants, were also discrete and clustered within the H2Eb second intron. Thus, in one instance two and in the other instance three alleles were present within the hotspots of these recombinants. These observations strongly suggest that meiotic recombination could be an important mechanism contributing to MHC polymorphism.

Immune deficiency due to high copy numbers of an Ak beta transgene

Because allelic polymorphism of the major histocompatibility complex class II antigens affects the immune response at several levels, we wished to characterize the contribution of a particular alpha or beta chain in vivo using transgenic mice. We have established and characterized 12 lines of H-2s/s mice carrying from 1 to 65 copies of an Ak beta transgene. The transgene was coexpressed with the endogenous allele in a tissue-specific manner, and Ak beta mRNA expression correlated well with transgene copy number. High copy number (extreme overexpression) of the transgene was associated with a variety of defects, including a significant reduction in Ia cell-surface expression, a severe decrease in B-cell number, abnormal extramedullary granulopoiesis, and an increased susceptibility to infection. In this paper we describe in detail the phenotype associated with high copy numbers of the Ak beta transgene. The defects we have observed may be relevant to similar phenomena seen in other transgenic mice. In addition, these mice have fortuitously provided a system in which to assess the effect of various levels of class II cell-surface expression in the thymus on selection of the T-cell repertoire.

Structure of class II genes in wild mouse Mus saxicola: functional and evolutionary implications

Class II beta genes have been characterized from Mus saxicola, a wild mouse which diverged five millions years ago from the house mouse. The nucleotide sequences of the exons 2 and 3 of E beta 1, A beta 2 and E beta 2 genes have been determined in order to define the molecular mechanisms of interspecies variability. The E beta 1 and A beta 1 (J. X. She, personal communication) genes from Mus saxicola display extensive sequence variability in exon 2 when compared to those of the house mouse, in association with the variability of residues probably involved in antigen binding. On the other hand, most putative T cell contact residues from the I-E beta chain, and to a lesser extent from the I-A beta chain, were found conserved in mice. T cell contact residues from Mus saxicola I-A beta and I-E beta chains are identical to those of the b and q, and of the k haplotypes of the house mouse, respectively. Comparisons performed with A beta 2 and E beta 2 genes revealed that both are highly conserved and, in contrast to A beta 1 and E beta 1 genes, that a strong selective pressure for conservation occurs in the A beta 2 second exon. The different aspects of A beta 2 and E beta 2 genes evolution may correlate with the original function of their products. Comparisons of the intervening sequences revealed that the repeated motifs of the hotspot region present in the E beta 1b gene are also conserved in Mus saxicola. Recombinations may have occurred, in some cases in association with chi-like sequences.

Diversity of the class II (I-Ak/I-Ek)-restricted T cell repertoire for influenza hemagglutinin and antigenic drift. Six nonoverlapping epitopes on the HA1 subunit are defined by synthetic peptides

H-2k-restricted T cell clones derived from CBA mice infected with X31 (H3N2) influenza virus, were shown to recognize distinct, nonoverlapping sequences within the HA1 subunit of the viral hemagglutinin (HA) using synthetic peptides. Three I-Ak-restricted T cell sequences were identified within HA1 68-83, 120-139, and 269-288, and two recognition sites presented in the context of the I-Ek molecule were mapped to HA1 sequences 226-245 and 246-265. T cell clones specific for these regions of HA1 demonstrated varying abilities to differentiate between natural variant viruses that had accumulated substitutions within their HA molecules as a result of antigenic drift. Clones that recognized sequences HA1 226-245 and HA1 246-265 failed to discriminate between natural variants and focused on conserved sequences within these epitopes. A majority of T cell clones were sensitive to amino acid substitutions that have featured in antigenic drift occurring within three major antigenic sites of the HA1 subunit; substitutions at HA1 residues 78 (V)/83(K) and 275(D)/278(I) within the HA1 subunit of mutant viruses correlated with a 75% reduction in the proliferative response for T cell clones specific for sequences HA1 68-83 and HA1 269-288, respectively. Furthermore, a clone that recognized HA1 120-139 was nonresponsive to a mutant virus HK/71, implicating amino acids at HA1 position 129(G) and/or 132(Q) within this sequence as crucial for recognition. Our data, together with the previous finding that sequence HA1 53-63 is also a major I-Ak-restricted T cell recognition site, demonstrate a level of diversity in the T cell recognition of influenza HA, within a single mouse haplotype hitherto unrecognized, and imply that the T cell repertoire diversity against foreign antigens may be greater than previously assumed. Furthermore, the frequency at which HA-specific T cells have been identified that focus on amino acids within the HA1 subunit of HA also featuring in antigenic drift, suggests that a failure of MHC class II-restricted T cells to recognize specific epitopes within mutant HA molecules may contribute significantly to the capacity of variant influenza viruses to evade immune recognition.

Nucleotide substitution at major histocompatibility complex class II loci: evidence for overdominant selection

To study the mechanism of maintenance of polymorphism at major histocompatibility complex (MHC) loci, synonymous and nonsynonymous (amino acid-altering) nucleotide substitutions in the putative antigen-recognition site (included in the first domain of the MHC molecule) and other regions of human and mouse class II genes were examined. In the putative antigen-recognition site, the rate of nonsynonymous substitution was found to exceed that of synonymous substitution, whereas in the second domain the former was significantly lower than the latter. In light of a previous theoretical study and parallel findings in class I MHC loci, we conclude that the unusually high degree of polymorphism at class II MHC loci is caused mainly by overdominant selection (heterozygote advantage) operating in the antigen-recognition site.

Quantitative analysis of integrated Ed alpha gene expression in C57BL/6 transgenic mice

We performed quantitative analysis of Ed alpha gene expression in the transgenic mice, created by microinjecting cloned Ed alpha gene fragments into C57BL/6 fertilized eggs. DNA dot-blot analysis revealed that Ed alpha gene-introduced transgenic mice (B6Ed alpha transgenic mice) contain 20 copies per cell of the Ed alpha gene in their genome. RNA dot-blot analysis revealed that the amount of Ed alpha mRNAs in B6Ed alpha transgenic spleen cells is 20-40-fold higher than those in normal BALB/c or (BALB x C57BL/6)F1 (CBF1) spleen cells. However, the amount of Ed alpha molecules expressed on B6Ed alpha transgenic spleen cells was similar to that expressed on normal BALB/c of CBF1 spleen cells on a gene-dose basis. The amount of endogenous Ed alpha mRNA in the B6Ed alpha transgenic spleen cells was almost equal to that of normal B6 spleen cells. Since the cell surface I-E molecule is formed by non-covalent association of E alpha and E beta chain, these results suggest that, in spite of the high expression of integrated Ed alpha gene in the cytoplasm of B6Ed alpha transgenic mice, the amount of Ed alpha gene expression on the cell surface is limited by the amount of endogenous Eb beta gene products.

Cytoplasmic domain affects membrane expression and function of an Ia molecule

The association of foreign antigen with Ia molecules on the surface of antigen-presenting cells is necessary for the interaction with the clonally distributed antigen receptor on T cells and is therefore critical in the initiation and regulation of immune responses. Ia polypeptides (alpha and beta) are composed of two extracellular domains, a transmembrane domain and a cytoplasmic domain. Although exon-shuffling experiments have demonstrated that antigen associates with the NH2-terminal alpha 1 and beta 1 domains, the roles that the other domains play in Ia function are still poorly understood. The B-hybridoma cell line 2B1 was selected in a series of positive and negative immunoselection steps for a mutation in the Ek alpha polypeptide. It was found to fortuitously contain a mutation in the Ak alpha polypeptide as well. Sequence analysis of the Ak alpha gene showed that a single base transition (C----T) resulted in a stop codon at amino acid residue 222. This caused the loss of 12 amino acids from the cytoplasmic domain of the mature polypeptide. This mutation results in a decreased level of Ak alpha polypeptide expression on the cell surface (50% of wild-type levels), an increased half-life of Ak alpha polypeptide in the cell, and a specific limited defect in antigen presentation.

A hypothetical model of the foreign antigen binding site of class II histocompatibility molecules

Class II and class I histocompatibility molecules allow T cells to recognize 'processed' polypeptide antigens. The two polypeptide chains of class II molecules, alpha and beta, are each composed of two domains (for review see ref. 6); the N-terminal domains of each, alpha 1 and beta 1, are highly polymorphic and appear responsible for binding peptides at what appears to be a single site and for being recognized by MHC-restricted antigen-specific T cells. Recently, the three-dimensional structure of the foreign antigen binding site of a class I histocompatibility antigen has been described. Because a crystal structure of a class II molecule is not available, we have sought evidence in class II molecules for the structural features observed in the class I binding site by comparing the patterns of conserved and polymorphic residues of twenty-six class I and fifty-four class II amino acid sequences. The hypothetical class II foreign-antigen binding site we present is consistent with mutation experiments and provides a structural framework for proposing peptide binding models to help understand recent peptide binding data.

Structural mutation affecting intracellular transport and cell surface expression of murine class II molecules

We have selected Ia variants from the Ia+ (H-2d) M12.4.1 B cell lymphoma that are negative on the cell surface for one or both Ia isotypes. The molecular analysis of two such independently selected cell lines, M12.A2 and M12.C3, is reported here. This analysis revealed that the genes encoding Ad beta (M12.A2) and Ed beta (M12.C3) contained identical single-nucleotide transitions that resulted in the substitution of Ser (mutant) for Asn (wild-type) at residue 82/83 of the extracellular NH2-terminal (membrane distal) beta 1 domain. This conservative substitution caused a cytoplasmic accumulation of I-A or I-E molecules in the respective cell line although predicted secondary-structure analysis suggests a minimal effect on protein conformation. Thus, the mutation appears to have either created a negative signal that stops transport or eliminated a positive signal that is required for transport and targeting to the cell surface.

Expression of human class II major histocompatibility complex antigens using retrovirus vectors

Retrovirus vectors [direct orientation (DO) vectors] that permit the simultaneous expression of an inserted protein-coding sequence and a dominant-acting selectable marker have been constructed. In these vectors, an internal simian virus 40 or human metallothionein promoter sequence serves to drive the expression of the bacterial neomycin phosphotransferase or guanine-xanthine phosphoribosyltransferase genes, whereas the viral long terminal repeat sequences are utilized to promote expression of inserted sequences. In some of the vectors, the viral 5' splice site, normally used in the biogenesis of the subgenomic env-encoding mRNA, has been eliminated. These vectors yield high transient and stable titers of virus after transfection of viral packaging cell lines, show little or no depression of virus titer with a variety of inserts, and faithfully transmit recombinant proviral sequences to recipient cells. To characterize the expression potential of these vectors, a variety of inserts encoding the alpha and beta subunits of the human major histocompatibility complex class II antigen HLA-DR have been introduced into these vectors. NIH 3T3 cells infected by viruses containing HLA-DR alpha or beta cDNAs express these proteins as shown by immunoprecipitation of metabolically labeled extracts. In addition, through the sequential infection of cells with retrovirus constructions expressing two different selectable markers, both subunits of the class II antigen have been introduced into NIH 3T3 cells. Such infected cells express HLA-DR molecules at the cell surface.

Molecular analysis of the hotspot of recombination in the murine major histocompatibility complex

Biological and serological assays have been used to define four subregions for the I region of the major histocompatibility complex (MHC) in the order I-A, I-B, I-J, and I-E. The I-J subregion presumably encodes the I-J polypeptide of the elusive T-cell suppressor factors. Restriction enzyme site polymorphisms and DNA sequence analyses of the I region from four recombinant mouse strains were used to localize the putative I-B and I-J subregions to a 1.0-kilobase (kb) region within the E beta gene. Sequencing this region from E beta clones derived from the two mouse strains: B10.A(3R), I-Jb and B10.A(5R), I-Jk initially used to define the I-J subregion revealed that these regions are identical, hence the distinct I-Jb and I-Jk molecules cannot be encoded by this DNA. In addition, the DNA sequence data also refute the earlier mapping of the I-B subregion. Analysis of the DNA sequences of three parental and four I region recombinants reveals that the recombinant events in three of the recombinant strains occurred within a 1-kb region of DNA, supporting the proposition that a hotspot for recombination exists in the I region. The only striking feature of this hotspot is a tetramer repeat (AGGC)n that shows 80 percent homology to the minisatellite sequence which may facilitate recombination in human chromosomes.

Differential expression of RT1 class II genes in fibroblast cell lines: recognition by allogeneic and xenogeneic T lymphocytes

A cosmid (cos a 13.1) containing RT1 class II B alpha and B beta genes was introduced into mouse and rat fibroblast cell lines by transformation. Mouse L cells transformed with cos a 13.1-synthesized cell surface class II molecules that were similar, with respect to apparent molecular weight and binding of xenogeneic- and allogeneic-specific antibodies, to class II molecules on rat B cells. RT1 class II molecules on the surface of mouse L cells were recognized by both allogeneic and xenogeneic T cells. In contrast, rat-2 cells transformed with cos a 13.1 did not synthesize any detectable RT1 class II molecules at the cell surface, and the levels of B alpha and B beta mRNA were generally very low or undetectable. This differential expression of exogenous class II genes was not exclusively due to a trans-acting positive regulatory factor in L cells. Other possible explanations for this difference are discussed.

Molecular biology of the H-2 histocompatibility complex

The H-2 histocompatibility complex of the mouse is a multigene family, some members of which are essential for the immune response to foreign antigens. The structure and organization of these genes have been established by molecular cloning, and their regulation and function is being defined by expression of the cloned genes.

Recognition of exon-shuffled class II molecules by T helper cells

Exon shuffled I-A beta genes transfected into the B lymphoma cell line A20-2J were used to localize the epitope recognized by the monoclonal antibody 10.2.16 to the carboxy terminal portion of the beta 1 domain. In addition, several T helper cell hybrids were tested against these novel I-A molecules and the following observations were made: the beta 1 domain of A beta plays a dominant role in the restricted recognition by T helper cells; there appear to be multiple restriction epitopes on the I-A molecule; these epitopes can consist of conformational epitopes created by specific alpha and beta chains or consist of the polymorphic determinants encoded on the beta chain alone, and these novel I-A molecules serve as restriction elements in the antigen-specific recognition by T cells and in one case stimulate an alloreaction in the absence of antigen.

I-A-restricted T cell antigen recognition. Analysis of the roles of A alpha and A beta using DNA-mediated gene transfer

The contributions of A alpha and A beta chains, and of subregions of A beta, to Ia-restricted recognition of antigen by Th lymphocytes were analyzed using a panel of L cells transfected with various pairs of A alpha b,d, or k genes and recombinant or wild-type A beta b,d, or k genes. The A beta genes included all possible exchanges of the whole NH2-terminal (beta 1) domain or halves of the beta 1 domain among these three allelic A beta genes. The Ia+ L cells derived from such transfections were used as antigen-presenting cells with a 21 member panel of responding Ia-restricted T hybridoma cells of differing nominal antigen specificity and Ia-restriction. Special care was taken to account for quantitative variation in levels of Ia expression throughout the experiments. The results of this analysis reveal that (a) only 2 of the 21 Th cells recognized Ia molecules involving either a nonparental A alpha or a nonparental A beta chain, and in both cases the degeneracy extended to only one of the two other alleles tested. This suggests that allele specific contributions from both A alpha and A beta chains are important in restricted recognition for most, if not all I-A-restricted Th cells. (b) In no case did substitution of the A beta 2 domain from either of the alternative haplotypes lead to any functionally detectable effects, demonstrating that polymorphisms in the A beta 1 domain can entirely account for the restriction imposed on Th cell responses by the entire A beta chain. (c) For 90% of the cells tested, replacement of the NH2-terminal portion of the beta 1 domain with an allogeneic segment led to Ia molecules unable to elicit Th responses. Furthermore, of all the cells permissive of the substitution of one or other half of the beta 1 domain, only two permitted the substitution of sequence from both alternative haplotypes. Taken together, these data strongly suggest that antigen recognition by most, if not all, I-A-restricted Th cells involves contributions from both halves of the A beta 1 domain. These data suggest that the role of I-A molecules in restricted Th cell recognition of antigen depends on conformational determinants unique to a particular combination of polymorphic alpha and beta chains, and that multiple such sites exist on a single Ia molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of functional regions on the I-Ab molecule by site-directed mutagenesis

Functional analysis of mutant class II major histocompatibility complex molecules has begun to identify regions important for antibody binding and for T-cell activation. By using in vitro mutagenesis directed at the beta 1 domain of the Ab beta gene we have constructed three structurally distinct mutant Ab beta genes. Each of these genes, as well as the wild-type Ab beta gene, was cotransfected together with the wild-type Ab alpha gene into the Ia-negative B-lymphoma cell line M12.C3. Transfection resulted in the successful synthesis and cell surface expression of three mutant class II antigens that showed serological and functional alterations as compared to the I-Ab antigens from the M12.C3 cell transfected with the wild-type gene. The variable patterns of both I-Ab-specific monoclonal antibody binding and activation of I-Ab-specific T-cell hybridomas show that the mutations result in the loss of structural epitopes required for both monoclonal antibody binding and for T-cell recognition. The data suggest that there are multiple sites on a single Ia molecule that are recognized by T helper cells and also that the tertiary conformation of the Ia molecule can be critical in the formation of such sites.

T-cell recognition of Ia molecules selectively altered by a single amino acid substitution

T lymphocytes recognize foreign antigen together with allele-specific determinants on membrane-bound class I and class II (Ia) gene products of the major histocompatibility complex. To identify amino acids of class II molecules critical to this recognition process, the genes encoding the beta chains of the I-Ak molecule were cloned from a wild-type B-cell hybridoma and from an immunoselected variant subline showing distinct serological and T-cell stimulatory properties. Nucleotide sequencing and DNA-mediated gene transfer established that a single base transition (G----A) encoding a change from glutamic acid to lysine at position 67 in the I-Ak beta molecule accounted for all the observed phenotypic changes of the variant cells. These results confirm the importance of residues 62 to 78 in the amino terminal domain of I-A beta for class II-restricted T-cell recognition of antigen and demonstrate the ability of a single substitution in this region to alter this recognition event.