Characterization of the molecular defects in the mouse E beta f and E beta q genes. Implications for the origin of MHC polymorphism

The E beta f and E beta q genes have been isolated and sequenced to investigate the molecular basis for their defective expression. A previous study from this laboratory, which characterized the expression of these genes at the RNA level, showed both genes to have defects in posttranscriptional RNA processing. In this paper, the defect in the E beta q gene from the inbred mouse strain B10.G (Mus musculus domesticus) is shown to be a single base insertion in the RNA donor splice site of the first intron. This identical mutation was described previously for the E beta gene of the H-2w17 haplotype, which was derived from the Asian house mouse subspecies Mus musculus castaneus. Although it has been estimated that M. m. domesticus and M. m. castaneus separated from each other more than one million years ago, comparisons of genomic sequences reveal that the nonexpressed E beta q and E beta w17 alleles have not diverged significantly from one another; they are identical in their protein coding regions and have only minor differences elsewhere. In contrast, sequence comparisons of A beta q and A beta w17 show that these two expressed alleles differ by multiple amino acids. These findings provide evidence that selection, acting on expressed MHC proteins, plays a role in accumulation and maintenance of MHC polymorphism. The defective E beta f gene from the inbred strain B10.M has also been isolated. Sequence analysis has identified a mutation in the same RNA donor splice site as E beta q and E beta w17; however, in this gene the mutation is a single base substitution at position 5.

Characterization of the molecular defects in the mouse E beta f and E beta q genes. Implications for the origin of MHC polymorphism

The E beta f and E beta q genes have been isolated and sequenced to investigate the molecular basis for their defective expression. A previous study from this laboratory, which characterized the expression of these genes at the RNA level, showed both genes to have defects in posttranscriptional RNA processing. In this paper, the defect in the E beta q gene from the inbred mouse strain B10.G (Mus musculus domesticus) is shown to be a single base insertion in the RNA donor splice site of the first intron. This identical mutation was described previously for the E beta gene of the H-2w17 haplotype, which was derived from the Asian house mouse subspecies Mus musculus castaneus. Although it has been estimated that M. m. domesticus and M. m. castaneus separated from each other more than one million years ago, comparisons of genomic sequences reveal that the nonexpressed E beta q and E beta w17 alleles have not diverged significantly from one another; they are identical in their protein coding regions and have only minor differences elsewhere. In contrast, sequence comparisons of A beta q and A beta w17 show that these two expressed alleles differ by multiple amino acids. These findings provide evidence that selection, acting on expressed MHC proteins, plays a role in accumulation and maintenance of MHC polymorphism. The defective E beta f gene from the inbred strain B10.M has also been isolated. Sequence analysis has identified a mutation in the same RNA donor splice site as E beta q and E beta w17; however, in this gene the mutation is a single base substitution at position 5.

Sequence elements required for activity of a murine major histocompatibility complex class II promoter bind common and cell-type-specific nuclear factors

We have examined the sequence elements and corresponding DNA-binding factors required for transient expression of the A alpha d promoter fused to the bacterial chloramphenicol acetyltransferase reporter gene in a variety of cultured cell lines. Deletion analysis demonstrated that only about 110 nucleotides of sequence 5' of the transcription start site are required for constitutive expression in the murine B-lymphoma cell line A20 or for gamma interferon-induced expression in the murine monocytic cell line WEHI-3. Linker-scanner mutation of this region indicated that at least three sequence elements are required for promoter activity. These elements correspond to the conserved sequence elements found in other human and mouse class II genes, the X box, the Y box, and the H box. Analysis of DNA-binding activity showed that the three most predominant factors present in extracts from WEHI-3, A20, or L cells (which do not express the class II genes) are actually a family of factors that bind to a fourth sequence element, overlapping the 3' end of the X-box sequence, that is homologous to the cyclic AMP-responsive enhancer element. A single common factor that binds to the Y box was detected in extracts from all cells tested, as has been seen with the Y-box elements of other class II genes. Another common factor was found that binds to the more conserved 5' region of the X-box element, although A20 extracts contained a second, distinct binding activity for this region. A common binding factor for the H-box element was detected in extracts from WEHI-3 and L cells. However, this activity was absent in A20 cell extracts. Instead, two different H-box-binding activities were detected, suggesting that different components are involved in class II gene expression in B cells and macrophages. Finally, gamma interferon treatment did not significantly alter the DNA-binding activity in WEHI-3 cells for any of the sequence elements shown to be required for induced chloramphenicol acetyltransferase expression.

Polymorphic residues on the I-A beta chain modulate the stimulation of T cell clones specific for the N-terminal peptide of the autoantigen myelin basic protein

The effect of polymorphic residues on the A alpha A beta molecule on T cell recognition of the N-terminal nonapeptide of myelin basic protein (R1-9) was determined. Ak-restricted T cell clones recognizing R1-9 were isolated. The peptide-Ia specificities of these clones were determined by testing the response to 1) a panel of peptide analogs of R1-11, 2) splenic APC from mice expressing MHC molecules from serologically distinct haplotypes, and 3) L cell transfectants expressing mutant/recombinant A beta cDNA containing combinations of polymorphic nucleotide sequences from the k and u alleles. Comparisons were made between the Ak-restricted clones and a previously characterized panel of Au-restricted clones. Certain Ak-restricted clones were able to recognize MBP peptide analogs that were not recognized by any of the Au-restricted clones. The Au-restricted T cell clones did not cross-react with R1-9 presented in the context of Ak, whereas the majority of the Ak-restricted clones responded to R1-9 presented in the context of Au. This nonreciprocal cross-reactivity was also reflected in the relative responses of the two sets of T cell clones to the interchange of u- and k-derived residues in the A beta chain. Residues in regions corresponding both the alpha-helical or beta-sheet portions of the hypothetical Ia three-dimensional structure were involved. The results suggest that overall specificity of the T cell clones is the summation of numerous distinct subspecificities for different regions of the peptide-Ia ligand. These results indicate that there can be striking differences in T cell specificity for an autoantigenic epitope, even in the context of A alpha A beta molecules from very closely related haplotypes.

Polymorphic residues on the I-A beta chain modulate the stimulation of T cell clones specific for the N-terminal peptide of the autoantigen myelin basic protein

The effect of polymorphic residues on the A alpha A beta molecule on T cell recognition of the N-terminal nonapeptide of myelin basic protein (R1-9) was determined. Ak-restricted T cell clones recognizing R1-9 were isolated. The peptide-Ia specificities of these clones were determined by testing the response to 1) a panel of peptide analogs of R1-11, 2) splenic APC from mice expressing MHC molecules from serologically distinct haplotypes, and 3) L cell transfectants expressing mutant/recombinant A beta cDNA containing combinations of polymorphic nucleotide sequences from the k and u alleles. Comparisons were made between the Ak-restricted clones and a previously characterized panel of Au-restricted clones. Certain Ak-restricted clones were able to recognize MBP peptide analogs that were not recognized by any of the Au-restricted clones. The Au-restricted T cell clones did not cross-react with R1-9 presented in the context of Ak, whereas the majority of the Ak-restricted clones responded to R1-9 presented in the context of Au. This nonreciprocal cross-reactivity was also reflected in the relative responses of the two sets of T cell clones to the interchange of u- and k-derived residues in the A beta chain. Residues in regions corresponding both the alpha-helical or beta-sheet portions of the hypothetical Ia three-dimensional structure were involved. The results suggest that overall specificity of the T cell clones is the summation of numerous distinct subspecificities for different regions of the peptide-Ia ligand. These results indicate that there can be striking differences in T cell specificity for an autoantigenic epitope, even in the context of A alpha A beta molecules from very closely related haplotypes.

Arsonate-specific murine T cell clones. V. Antigen presentation by L cells transfected with normal and mutant class II genes

Class II-restricted murine T cell clones specific for the immunogenic determinant L-tyrosine-p-azobenzenearsonate failed to proliferate to Ag presented by L cell lines transfected with and expressing the appropriate class II genes, but are activated to kill the APC in an Ag-dependent, MHC-restricted manner. Inhibition of APC proliferation was used as an assay to determine the relative contributions of polymorphic sites on the class II alpha- and beta-chains to MHC-restricted activation of I-A beta k-restricted cloned T cells. Transfectants expressing A beta k in conjunction with the alpha chain of k, u, or d were equally effective APCs, whereas transfectants expressing A beta u were completely ineffective, implicating the beta-chain as more critical for the presentation of L-tyrosine-p-azobenzenearsonate. Site-directed mutagenesis of polymorphic positions in the beta chain revealed a remarkable stringency for the k haplotype, in contrast to the relaxed alpha-chain requirement. These results, in conjunction with others, indicate that the relative contribution of polymorphic sites on class II alpha- and beta-chains to T cell Ag recognition can differ markedly, and, furthermore, may vary as a function of the Ag.

Arsonate-specific murine T cell clones. V. Antigen presentation by L cells transfected with normal and mutant class II genes

Class II-restricted murine T cell clones specific for the immunogenic determinant L-tyrosine-p-azobenzenearsonate failed to proliferate to Ag presented by L cell lines transfected with and expressing the appropriate class II genes, but are activated to kill the APC in an Ag-dependent, MHC-restricted manner. Inhibition of APC proliferation was used as an assay to determine the relative contributions of polymorphic sites on the class II alpha- and beta-chains to MHC-restricted activation of I-A beta k-restricted cloned T cells. Transfectants expressing A beta k in conjunction with the alpha chain of k, u, or d were equally effective APCs, whereas transfectants expressing A beta u were completely ineffective, implicating the beta-chain as more critical for the presentation of L-tyrosine-p-azobenzenearsonate. Site-directed mutagenesis of polymorphic positions in the beta chain revealed a remarkable stringency for the k haplotype, in contrast to the relaxed alpha-chain requirement. These results, in conjunction with others, indicate that the relative contribution of polymorphic sites on class II alpha- and beta-chains to T cell Ag recognition can differ markedly, and, furthermore, may vary as a function of the Ag.

The role of polymorphic I-Ak beta chain residues in presentation of a peptide from myelin basic protein

Proteins encoded by genes in the MHC are highly polymorphic. For class II proteins the highest level of polymorphism is found in distinct regions of variability, notably in the membrane-distal domains. To investigate the role of such residues in antigen presentation, we have tested cells transfected with wild-type or mutant I-Ak beta chains for their ability to present the NH2-terminal peptide of myelin basic protein to a panel of T cell clones. We were unable to detect a gross effect on peptide binding, in that all of the mutant cell lines presented antigen to at least one of the cloned T cells. However, the results imply that the more NH2-terminal residues, particularly 12 and 14, are involved in peptide interactions. Mutations at these residues presented antigen only at high antigen concentrations. Furthermore, residues of the more COOH-terminal regions appear to determine TCR interactions. Mutations in the predicted alpha-helical regions of the beta chain affected antigen presentation without abolishing peptide binding.

Molecular defects in the non-expressed H-2 E alpha genes of the f and q haplotypes

The expression and sequences of the mouse MHC class II genes, E alpha f and E alpha q, have been studied to determine the molecular basis for their defective expression. Previous work in our laboratory showed that H-2f and H-2q mice produce no detectable E alpha protein. Northern blot analysis confirms previous results showing normal amounts of E alpha f message and a 100-fold reduction in E alpha q mRNA. Despite that observation, the rates of transcription of both defective E alpha genes, measured by nuclear run-on transcription assays, are comparable to that of the normally expressed E alpha k gene. The nucleotide sequences of the E alpha f and E alpha q genes reveal mutations generating premature translation stop codons in both genes. A single base substitution has created the stop codon TGA at amino acid -2 in the E alpha f leader sequence. A nucleotide insertion at codon 64 in the second exon of the E alpha q gene results in a frame shift and a premature stop codon at amino acid residue 69 of a mature E alpha protein. The low steady state level of E alpha q mRNA may be correlated with the unusual size distribution of the RNA, possibly due to altered RNA processing.

Molecular defects in the non-expressed H-2 E alpha genes of the f and q haplotypes

The expression and sequences of the mouse MHC class II genes, E alpha f and E alpha q, have been studied to determine the molecular basis for their defective expression. Previous work in our laboratory showed that H-2f and H-2q mice produce no detectable E alpha protein. Northern blot analysis confirms previous results showing normal amounts of E alpha f message and a 100-fold reduction in E alpha q mRNA. Despite that observation, the rates of transcription of both defective E alpha genes, measured by nuclear run-on transcription assays, are comparable to that of the normally expressed E alpha k gene. The nucleotide sequences of the E alpha f and E alpha q genes reveal mutations generating premature translation stop codons in both genes. A single base substitution has created the stop codon TGA at amino acid -2 in the E alpha f leader sequence. A nucleotide insertion at codon 64 in the second exon of the E alpha q gene results in a frame shift and a premature stop codon at amino acid residue 69 of a mature E alpha protein. The low steady state level of E alpha q mRNA may be correlated with the unusual size distribution of the RNA, possibly due to altered RNA processing.

Defective E beta expression in three mouse H-2 haplotypes results from aberrant RNA splicing

The molecular basis for the defective expression of the mouse class II E beta genes in the H-2w17, H-2q, and H-2f haplotypes has been examined. The results of nuclear run-on transcription and S1 nuclease digestion assays demonstrate that E beta transcription is normal in these haplotypes. Northern blot analyses reveal reduced amounts of E beta RNA of both normal and aberrant size in the w17 and q haplotypes; an even more reduced level of E beta RNA of normal size was detected in the f haplotype. In the preceding study, we reported that the only defect detected in the E beta w17 gene is a single nucleotide insertion in the 5' RNA splice site of the first intervening sequence. S1 nuclease analysis of E beta w17 RNA indicates that splicing at this site is aberrant. One major cryptic RNA splice site is used, leading to reduced amounts of aberrantly processed RNA. Limited use of the mutated splice site and of a second cryptic site also is detected. In all three cases, stop codons in the resulting RNA would prevent their translation. The molecular defect in E beta q appears identical to that of E beta w17. In the f haplotype, even more reduced levels of E beta RNA of both normal and aberrant sizes are found. We thus show that in the three defective E beta alleles, two distinct defects are responsible for the absence of E beta protein synthesis; both of these defects affect RNA processing.

Defective E beta expression in three mouse H-2 haplotypes results from aberrant RNA splicing

The molecular basis for the defective expression of the mouse class II E beta genes in the H-2w17, H-2q, and H-2f haplotypes has been examined. The results of nuclear run-on transcription and S1 nuclease digestion assays demonstrate that E beta transcription is normal in these haplotypes. Northern blot analyses reveal reduced amounts of E beta RNA of both normal and aberrant size in the w17 and q haplotypes; an even more reduced level of E beta RNA of normal size was detected in the f haplotype. In the preceding study, we reported that the only defect detected in the E beta w17 gene is a single nucleotide insertion in the 5' RNA splice site of the first intervening sequence. S1 nuclease analysis of E beta w17 RNA indicates that splicing at this site is aberrant. One major cryptic RNA splice site is used, leading to reduced amounts of aberrantly processed RNA. Limited use of the mutated splice site and of a second cryptic site also is detected. In all three cases, stop codons in the resulting RNA would prevent their translation. The molecular defect in E beta q appears identical to that of E beta w17. In the f haplotype, even more reduced levels of E beta RNA of both normal and aberrant sizes are found. We thus show that in the three defective E beta alleles, two distinct defects are responsible for the absence of E beta protein synthesis; both of these defects affect RNA processing.

Molecular basis for the defective expression of the mouse Ew17 beta gene

Four of the eleven independent H-2 haplotypes of inbred mouse strains and approximately 15% of wild mouse chromosomes 17 fail to express the E alpha E beta class II histocompatibility (Ia) Ag. These E- haplotypes are defective in the expression of the E alpha and/or the E beta chain. None of the E beta defects has previously been described at the molecular level. In this study, we report the molecular basis for the defective expression of the E beta gene from the w17 haplotype of the H-2 congenic strain B10.CAS2, derived from wild Mus musculus castaneus. Comparison of the Ew17 beta genomic sequence to those of the functional Eb beta and Ed beta genes reveals a single base insertion in the RNA donor splice site of the first intron. By DNA shuffling, we have corrected the single base mutation, and we show by FACS analysis and 2-D PAGE of immunoprecipitates that the corrected Ew17 beta is expressed in L cells when co-transfected with an Ed alpha gene. Conversely, an Eb beta gene construct containing the mutant RNA splice site from Ew17 beta is not expressed. We conclude that the single base insertion in the first RNA splice donor site is the sole molecular defect in the Ew17 beta gene.

Molecular basis for the defective expression of the mouse Ew17 beta gene

Four of the eleven independent H-2 haplotypes of inbred mouse strains and approximately 15% of wild mouse chromosomes 17 fail to express the E alpha E beta class II histocompatibility (Ia) Ag. These E- haplotypes are defective in the expression of the E alpha and/or the E beta chain. None of the E beta defects has previously been described at the molecular level. In this study, we report the molecular basis for the defective expression of the E beta gene from the w17 haplotype of the H-2 congenic strain B10.CAS2, derived from wild Mus musculus castaneus. Comparison of the Ew17 beta genomic sequence to those of the functional Eb beta and Ed beta genes reveals a single base insertion in the RNA donor splice site of the first intron. By DNA shuffling, we have corrected the single base mutation, and we show by FACS analysis and 2-D PAGE of immunoprecipitates that the corrected Ew17 beta is expressed in L cells when co-transfected with an Ed alpha gene. Conversely, an Eb beta gene construct containing the mutant RNA splice site from Ew17 beta is not expressed. We conclude that the single base insertion in the first RNA splice donor site is the sole molecular defect in the Ew17 beta gene.

Sequence analysis and structure-function correlations of murine q, k, u, s, and f haplotype I-A beta cDNA clones

I-A beta-chain cDNA clones from mice of the q, k, u, s, and f haplotypes have been isolated and sequenced. Nucleotide sequence comparisons among these five A beta chains show considerable allelic variation in the region encoding the first external (beta 1) domain of the mature A beta protein. The beta 1 domain variability is clustered into three discrete regions, two of which divide the A beta chains into subgroups, suggesting an evolutionary history for the separation of alleles in inbred strains of mice. The amino acid sequences of these five chains are compared to each other and to previously published I-A beta chains. Correlations are made between the primary structural differences and the serologic and immune response characteristics mapping to the I-A subregion.

Interferon-gamma binds to high and low affinity receptor components on murine macrophages

Interferon-gamma (IFN-gamma), a glycoprotein secreted by antigen-or mitogen-activated lymphocytes, modulates the activities of lymphocytes and macrophages. For mouse macrophages, murine IFN-gamma (MuIFN-gamma) stimulates several functions, including phagocytosis, tumoricidal activity, and the increased expression of Ia and H-2 antigens of the major histocompatibility complex. Recent reports have suggested that IFN-gamma specifically binds to cell surface receptors corresponding to a single class of binding sites with a Kd of 10(-8) to 10(-10) M. We present evidence that, on the murine macrophage cell line WEHI-3, MuIFN-gamma specifically binds to two classes of binding sites with Kd of 9.1 X 10(-11) M (500 sites/cell) and 2.7 X 10(-9) M (4400 sites/cell). The higher affinity sites most likely represent the physiologically relevant receptors that mediate at least some of the actions of MuIFN-gamma.

Interferon-gamma binds to high and low affinity receptor components on murine macrophages

Interferon-gamma (IFN-gamma), a glycoprotein secreted by antigen-or mitogen-activated lymphocytes, modulates the activities of lymphocytes and macrophages. For mouse macrophages, murine IFN-gamma (MuIFN-gamma) stimulates several functions, including phagocytosis, tumoricidal activity, and the increased expression of Ia and H-2 antigens of the major histocompatibility complex. Recent reports have suggested that IFN-gamma specifically binds to cell surface receptors corresponding to a single class of binding sites with a Kd of 10(-8) to 10(-10) M. We present evidence that, on the murine macrophage cell line WEHI-3, MuIFN-gamma specifically binds to two classes of binding sites with Kd of 9.1 X 10(-11) M (500 sites/cell) and 2.7 X 10(-9) M (4400 sites/cell). The higher affinity sites most likely represent the physiologically relevant receptors that mediate at least some of the actions of MuIFN-gamma.

Independent regulation of IgM, IgD, and Ia antigen expression in cultured immature B lymphocytes

Long-term cultured bone marrow cells were characterized with respect to a number of B and pre-B cell markers. Cells expressing ThB, B-220, and IgM were found within cultures set up according to the procedure of Whitlock and Witte. This culture system was modified by placing sorted pre-B cells (ThB+, IgM-) from bone marrow in culture with previously-established bone marrow adherent layers. These cultures commenced growth without the lag associated with the Whitlock cultures. These cultured nonadherent cells show a high frequency of IgM+ cells, but do not express either IgD or Ia, and we refer to them as immature B cells. Cells with a similar phenotype (IgM+, Ia-, IgD-) are found within the spleens of young but not adult mice. The phorbol ester PMA induces expression of IgD on the cultured immature B cells, but has no effect on Ia expression. This suggests that the processing of H chain RNA transcripts may be affected by protein kinase C. These results demonstrate that the appearance of IgM, IgD, and Ia are independently controlled in long-term cultured B-lineage cells.

Free Ia E alpha chain expression in the E+ alpha : E- beta recombinant strain A.TFR5

Molecular and biochemical techniques have been used to explore the reasons behind low E alpha chain expression in the E+ alpha E- beta I-region recombinant strain, A.TFR5. A.TFR5 (Af Ek, ap5), a recombinant between A.CA (Af Ef) and A.TL (AkEk), carries the Ek subregion. Previous results have shown that it expresses the E alpha chain, but at reduced levels relative to E+ alpha E+ beta strains. No E beta chains were detected, which is consistent with the A.TFR5E beta gene being derived from the A.CA parent, which carries the null Ef beta allele. In this paper, the defect in E alpha-chain expression is explored. Restriction fragment length polymorphism analysis has localized the recombination event in A.TFR5 approximately 30 kb upstream of E alpha, in the region of the large intervening sequence of E beta. Northern blot analysis of total RNA from A.TFR5 shows normal amounts of the E alpha message, but no E beta message. Two-dimensional gel analysis of 15 min pulse-labeled A.TFR5, A.CA, and A.TL E immunoprecipitates shows decreased levels of the intracellular E alpha chain in A.TFR5 relative to A.TL. However, analysis of total cell extracts shows normal levels of this protein. A glycoprotein fraction isolated from total cell extracts of 5 h labeled cells contains normal amounts of intracellular E alpha, but decreased amounts of the mature cell-surface protein. These data suggest that in the absence of E beta, the E alpha chain (1) takes on an altered conformation that is not as efficiently recognized by alloantibodies, and (2) is found in normal levels as the partially glycosylated intracellular precursor, but is not processed and/or transported efficiently to the cell surface.

Coordinate induction of Ia alpha, beta, and Ii mRNA in a macrophage cell line

We demonstrated a tightly coordinated timing in the appearance of mRNA for the four class II (Ia) MHC chains, A alpha, A beta, E alpha, and E beta, and the Ia-associated invariant chain in a murine macrophage cell line after the addition of immune interferon (IFN-gamma) or of IFN-gamma-containing supernatants from Con A-stimulated spleen cells. The marked increase in mRNA levels for these molecules at approximately 8 hr after IFN-gamma addition contrasts sharply with the earlier, more gradual kinetics observed for class I (H-2) and beta 2-microglobulin mRNA. The difference in kinetics of IFN-gamma induction of class I and class II mRNA suggests differential regulation of the expression of Ia and H-2 antigens. The long lag period preceding detection of Ia mRNA raises the possibility that IFN-gamma may not directly mediate the increase in mRNA expression, but may act through an additional cellular intermediate.

Coordinate induction of Ia alpha, beta, and Ii mRNA in a macrophage cell line

We demonstrated a tightly coordinated timing in the appearance of mRNA for the four class II (Ia) MHC chains, A alpha, A beta, E alpha, and E beta, and the Ia-associated invariant chain in a murine macrophage cell line after the addition of immune interferon (IFN-gamma) or of IFN-gamma-containing supernatants from Con A-stimulated spleen cells. The marked increase in mRNA levels for these molecules at approximately 8 hr after IFN-gamma addition contrasts sharply with the earlier, more gradual kinetics observed for class I (H-2) and beta 2-microglobulin mRNA. The difference in kinetics of IFN-gamma induction of class I and class II mRNA suggests differential regulation of the expression of Ia and H-2 antigens. The long lag period preceding detection of Ia mRNA raises the possibility that IFN-gamma may not directly mediate the increase in mRNA expression, but may act through an additional cellular intermediate.

Induction of Ia and H-2 antigens on a macrophage cell line by immune interferon

Recent work from a number of laboratories has suggested that Ia antigen expression on macrophages can be modulated by soluble factors released by T cells after antigen or mitogen activation. The murine macrophage tumor cell line, WEHI-3, provides an in vitro system for the study of this type of lymphokine regulation of Ia antigen expression. Previous work from our laboratory has shown that supernates from Concanavalin A-activated rat or mouse spleen cells (Con A sup) can stimulate increased levels of biosynthesis and cell surface expression of both Ia and H-2K,D antigens on this cell line. The experiments described in this report were designed to identify the regulatory factor responsible for this inducing activity, and in particular to determine whether the inducing factor present in crude supernatant fluids is immune interferon (IFN-gamma). These experiments show that: 1) murine IFN-gamma, produced free of other lymphokines by recombinant DNA technology and DNA-mediated gene transfer, can induce Ia antigen expression and increase H-2 antigen expression on WEHI-3 cells; 2) the biochemical characteristics of partially purified inducing factor from crude supernate fluids are similar to those previously described for murine IFN-gamma by other investigators; and 3) both crude Con A sup and IFN-gamma-containing supernates have inducing activity and antiviral activity which exhibit very similar dose-related responses. These results support the conclusion that the previously described ability of Con A sup to regulate MHC antigen expression on WEHI-3 cells is due to the activity of immune interferon in those supernates.

Induction of Ia and H-2 antigens on a macrophage cell line by immune interferon

Recent work from a number of laboratories has suggested that Ia antigen expression on macrophages can be modulated by soluble factors released by T cells after antigen or mitogen activation. The murine macrophage tumor cell line, WEHI-3, provides an in vitro system for the study of this type of lymphokine regulation of Ia antigen expression. Previous work from our laboratory has shown that supernates from Concanavalin A-activated rat or mouse spleen cells (Con A sup) can stimulate increased levels of biosynthesis and cell surface expression of both Ia and H-2K,D antigens on this cell line. The experiments described in this report were designed to identify the regulatory factor responsible for this inducing activity, and in particular to determine whether the inducing factor present in crude supernatant fluids is immune interferon (IFN-gamma). These experiments show that: 1) murine IFN-gamma, produced free of other lymphokines by recombinant DNA technology and DNA-mediated gene transfer, can induce Ia antigen expression and increase H-2 antigen expression on WEHI-3 cells; 2) the biochemical characteristics of partially purified inducing factor from crude supernate fluids are similar to those previously described for murine IFN-gamma by other investigators; and 3) both crude Con A sup and IFN-gamma-containing supernates have inducing activity and antiviral activity which exhibit very similar dose-related responses. These results support the conclusion that the previously described ability of Con A sup to regulate MHC antigen expression on WEHI-3 cells is due to the activity of immune interferon in those supernates.

The gene encoding the Ia antigen-associated invariant chain (Ii) is not linked to the H-2 complex

The invariant (Ii) chain of murine Ia antigens is associated with the intracellular but not the cell-surface forms of the A alpha:A beta and E alpha:E beta Ia complexes. Due to its unique subcellular localization, Ii has been postulated to play a part in the assembly or intracellular transport of the Ia alpha:beta complexes, which function in immune recognition. A more general role for Ii in the transport of other cell proteins has also been suggested. Because of the unusual subunit composition of Ia antigens and because the synthesis of alpha, beta and Ii chains is coordinately regulated, it was of interest to determine whether, like the alpha and beta chains, Ii is encoded by a gene in the I region of the H-2 histocompatibility complex. We report here the use of an Ii chain polymorphism present in Mus spretus to demonstrate that the gene for Ii is not linked to the H-2 complex. Thus, intracellular Ia antigens consist of the products of two linked genes and one unlinked gene.

Identification of a second class I antigen controlled by the K end of the H-2 complex and its selective cellular expression

Immunoprecipitates obtained from [35S]methionine-labeled spleen cells by using monoclonal antibodies specific for H-2Kd and H-2Dd have been separated by two-dimensional polyacrylamide gel electrophoresis. Analysis of these gel patterns revealed the presence of an additional product of the K end of the H-2d complex, designated here as H-2K'. To determine whether H-2K' is a unique protein or a differentially glycosylated form of the previously characterized H-2Kd histocompatibility antigen, nonglycosylated molecules labeled in the presence of tunicamycin were examined. The results showed that both H-2K and H-2K' have distinct nonglycosylated polypeptide precursor forms. The approximate molecular weight differences between the fully glycosylated and nonglycosylated molecules also indicated the presence of three oligosaccharide side chains on H-2K', as is the case with H-2Kd, whereas H-2Dd has only two oligosaccharide units. The cellular expression of H-2K' was also investigated. Comparison of H-2 antigens immunoprecipitated from normal spleen cells and from thioglycollate-induced adherent peritoneal exudate cells cultured in the presence or absence of supernatant fluids from concanavalin A-stimulated spleen cells revealed that H-2K' was not expressed on the adherent peritoneal cells. This indicates that H-2K' is expressed in a tissue-specific manner, unlike the classical histocompatibility antigens H-2K and H-2D.

Properties of reticulum cell sarcomas in SJL/J mice. VIII. Prominent role of RCS cell I-A antigens in the stimulation of syngeneic T cells

While T cells from SJL and from F1 hybrids of SJL that do not express I-E antigens give strong proliferative responses to RCS, T cells from F1 hybrids expressing surface I-E do not. The nature of the stimulating antigen on the RCS cell surface was examined using monoclonal antibodies. Complete inhibition of the T-cell proliferative response was obtained with antibodies to I-A antigens, whereas antibodies to I-E antigens did not inhibit at all. This inhibition was mediated via an effect of the antibodies on the stimulating cells. Biochemical characterization of immunoprecipitated 125I-and 35S-labeled RCS antigens was performed using two-dimensional gel electrophoresis. Using this technique, I-A antigens were readily detected. However, neither Ia.7-specific antibodies nor antibodies specific for E alpha: E beta complexes precipitated any E alpha or E beta chains. Comparison of I-A antigens from RCS and normal SJL spleen cells revealed minor mobility differences in the gels, possibly due to differences in glycosylation, the significance of which needs to be further evaluated. Examination of RNA extracted from RCS, using E alpha and A alpha cDNA probes showed that RCS cells do not transcribe the E alpha gene as has been shown previously for normal H-2s cells. Furthermore, DNA from RCS cells showed a defect in the E alpha gene similar to that known to exist in normal H-2s cells. Our findings exclude the presence of E alpha on RCS cells and suggest a major role for I-A, either alone or in conjunction with another as yet unidentified cell surface antigen, in the stimulation of T cells.

Differential expression of Ia glycoprotein complexes in F1 hybrid mice detected with alloreactive cloned T cell lines

T cell lines alloreactive to Aeb:E alpha Ia antigen complexes have been prepared and used to determine the relative amount of Aeb:E alpha expressed by stimulator cells in homozygous recombinant and F1 mice expressing these complexes. We find that homozygous cells stimulate most strongly and therefore probably express the highest density of Aeb:E alpha. Among heterozygotes, H-2bxd F1 mice preferentially express Aeb:E alpha d complexes, H-2bxa F1 mice express relatively fewer Aeb:E alpha k complexes, and H-2bxu F1 mice express preferentially Aeu:E alpha u complexes. This differential association of Aeb chains with E alpha chains thus influences the biologic activity of these Ia antigens in this and other functional assays. The functional data are supported by biochemical analysis of Aeb:E alpha complex expression. These findings suggest that a reanalysis of HLA-DR associations with human diseases should be undertaken in which both HLA-DR alleles are included, with the prediction that certain combinations would show greater susceptibility, whereas others would show less susceptibility than predicted from the susceptibility associated with presence of a single allele at HLA-DR.

Differential expression of Ia glycoprotein complexes in F1 hybrid mice detected with alloreactive cloned T cell lines

T cell lines alloreactive to Aeb:E alpha Ia antigen complexes have been prepared and used to determine the relative amount of Aeb:E alpha expressed by stimulator cells in homozygous recombinant and F1 mice expressing these complexes. We find that homozygous cells stimulate most strongly and therefore probably express the highest density of Aeb:E alpha. Among heterozygotes, H-2bxd F1 mice preferentially express Aeb:E alpha d complexes, H-2bxa F1 mice express relatively fewer Aeb:E alpha k complexes, and H-2bxu F1 mice express preferentially Aeu:E alpha u complexes. This differential association of Aeb chains with E alpha chains thus influences the biologic activity of these Ia antigens in this and other functional assays. The functional data are supported by biochemical analysis of Aeb:E alpha complex expression. These findings suggest that a reanalysis of HLA-DR associations with human diseases should be undertaken in which both HLA-DR alleles are included, with the prediction that certain combinations would show greater susceptibility, whereas others would show less susceptibility than predicted from the susceptibility associated with presence of a single allele at HLA-DR.

The selective solubilization of different murine splenocyte membrane fractions with lubrol WX and triton X-100 distinguishes two forms of Ia antigens. A cell surface (alpha, beta) and an intracellular (alpha, Ii, beta)

The selective solubilization of different murine lymphocyte membrane compartments with several nonionic detergents was used to study the subcellular distribution of two distinct forms of lymphocyte cell recognition structures (Ia antigens). Ia antigens were isolated with a monoclonal anti-Ia immunoadsorbent from murine splenocytes that had been solubilized with four different nonionic detergents. Analyses of the immunoprecipitates indicated that Lubrol WX was selectively solubilizing a subpopulation of Ia consisting of mature highly glycosylated alpha and beta polypeptides which were not associated with Ii polypeptide. A second Ia subpopulation consisting of less glycosylated cytoplasmic precursor alpha and beta polypeptides associated with Ii polypeptide was immunoprecipitated from the Lubrol WX-insoluble material after solubilizing this material with Triton X-100. Comparable results were obtained when HLA-DR antigens were similarly isolated from cultured human lymphoblastoid cells. This selective solubilization phenomenon was not unique to Ia antigens. Only mature highly glycosylated H-2K molecules were immunoprecipitated from the Lubrol WX-soluble material while the less glycosylated precursor H-2K molecules were immunoprecipitated from the Triton X-100-solubilized Lubrol-insoluble material. These data directly demonstrate that the Ii polypeptide is exclusively associated with the intracellular Ia antigen cytoplasmic precursor molecules. These data also indicate that, under the conditions used in these experiments, Lubrol WX does not completely solubilize integral membrane proteins that have previously been shown to be associated with the rough endoplasmic reticulum.

Immune response gene function correlates with the expression of an Ia antigen. I. Preferential association of certain Ae and E alpha chains results in a quantitative deficiency in expression of an Ae:E alpha complex

These studies were stimulated by the observation, reported in the accompanying paper (19), that IEu failed to interact with I-Ak or I-As in F1 mice to allow a response to the antigen, pigeon cytochrome c, unlike I-E subregions derived from other Ia.7+ haplotypes. Serological and biochemical analyses were performed to determine whether or not cells from these F1 mice express the Ak,se:E alpha complexes that should function as restriction elements for T cell recognition of pigeon cytochrome c on antigen-presenting cells. Using the Y-17 monoclonal antibody, which recognizes the combinatorial or conformational determinant Ia.m44 on certain Ae:E alpha complexes, we were able to distinguish between Aue:Eu alpha and Ab,k,se:Eu alpha complexes on cell surfaces. Although complement-dependent microcytotoxicity with Y-17 failed to detect Ab,k,se:Eu alpha complexes on cells from appropriate F1 mice, these molecules were detected by both quantitative absorption and quantitative immunofluorescence studies. However, Ab,k,se:Eu alpha complexes were found to be present at levels only one-seventh to one-eighth the levels expressed by homozygous I-Ab, I-Ek; I-Ak, I-Ek; and I-As, I-Ek cells. The results of two-dimensional polyacrylamide gel electrophoresis analyses suggest that the low levels of expression of Ab,k,se:Eu alpha complexes are a consequence of the preferential association of Aue and Eu alpha chains with each other in the F1 cells. As will be shown in the following paper (19), the quantitative deficiency in the expression of Ake:Eu alpha and Ase:Eu alpha complexes results in a corresponding defect in antigen-presenting cell function, thus providing strong evidence that Ia antigens represent products of Ir genes.

Immune response gene function correlates with the expression of an Ia antigen. II. A quantitative deficiency in Ae:E alpha complex expression causes a corresponding defect in antigen-presenting cell function

A series of experiments were performed to explore the role of complementing major histocompatability complex (MHC)-linked immune response Ir genes in the murine T cell proliferative response to the globular protein antigen pigeon cytochrome c. The functional equivalence of I-E-subregion-encoded, structurally homologous E(a) chains from different haplotypes bearing the serologic specificity Ia.7 was demonstrated by the complementation for high responsiveness to pigeon cytochrome c of F(1) hybrids between low responder B 10.A(4R) (I-A (k)) or B 10.S (I-A(8)) mice and four low responder E(a)- bearing haplotypes. Moreover, this Ir gene function correlated directly with both the ability of antigen-pulsed spleen cells from these same F(1) strains to stimulate pigeon cytochrome c-primed T cells from B10.A or B10.S(9R) mice, and with the cell surface expression of the two-chain Ia antigenic complex, A(e):E(a), bearing the conformational or combinatorial determinant recognized by the monoclonal anti-Ia antibody, Y-17. The B 10.PL strain (H-2(u)), which expresses an Ia.7-positive I-E- subregion-encoded E(a) chain, failed to complement with B10.A(4R) or B10.S mice in the response to pigeon cytochrome c. However, (B10.A(4R) x B10.PL)F(1) and (B10.S x B10.PL)F(1) mice do express A(k)(e):E(u)(a) and A(8)(e):E(u)(a) on their cell surface, although in reduced amounts relative to A(k,s)(e):E(k,d,p,r)(a) complexes found in corresponding F(1) strains. This quantitative difference in Ia antigen expression correlated with a difference in the ability to present pigeon cytochrome c to B 10.A and B 10.S(9R) long-term T cell lines. Thus, (B10.A(4R) x B10.PL)F(1) spleen cells required a 10-fold higher antigen dose to induce the same stimulation as (B10.A(4R) x B10.D2)F(1) spleen cells. In addition, the monoclonal antibody, Y-17, which reacts with A(e):E(a) molecules of several strains, had a greater inhibitory effect on the proliferative response to pigeon cytochrome c of B10.A T cells in the presence of (B10.A(4R) X B10.PL)F(1) spleen cells than in the presence of (B10.A(4R) X B10.D2)F(1) spleen cells. These functional data, in concert with the biochemical and serological data in the accompanying report, are consistent with the molecular model for Ir gene complementation in which appropriate two-chain Ia molecules function at the antigen-presenting cell (APC) surface as restriction elements. Moreover, they clearly demonstrate that the magnitude of the T cell proliferative response is a function of both the concentration of nominal antigen and of the amount of Ia antigen expressed on the APC. Finally, the direct correlation of a quantitative deficiency in cell surface expression of an Ia antigen with a corresponding relative defect in antigen-presenting function provides strong independent evidence that the I-region-encoded Ia antigens are the products of the MHC-linked Ir genes.

Detection of two distinct class II alpha:beta:Ii complexes in the Syrian hamster

Hamsters alloantisera and a monoclonal antibody originally generated against antigens controlled by the murine I-Ek subregion, which cross-reacts with hamster cell surface antigens, have been used to define two distinct Ia-like complexes in the Syrian hamster. These complexes have been named alpha 1:beta 1 and alpha 2:beta 2 and are detected by hamster alloantisera or monoclonal antibody 14-4-4, respectively. For the three strains studied, alpha 1:beta 1 appears to be polymorphic in both alpha and beta chains, while the alpha 2:beta 2 complex is nonpolymorphic, as revealed by 2-D PAGE analyses. A third nonpolymorphic glycoprotein that appears to be the hamster's equivalent of the murine invariant chain (Ii) is associated with both the alpha 1:beta 1 and alpha 2:beta 2 complexes. In addition, we report the first biochemical detection of polymorphism between the closely related CB and MHA Syrian hamster strains.

Variable synthesis and expression of E alpha and Ae (E beta) Ia polypeptide chains in mice of different H-2 haplotypes

The genetic and molecular requirements for cell-surface expression of Ia antigens precipitated by anti-I-E subregion sera have been examined. Inbred mice f the d, k, p, and r haplotypes synthesize and express on their lymphocytes the two I-region products normally found in anti-I-E-subregion immunoprecipitates, E alpha and Ae (E beta). Cells from mice of the b and s haplotypes fail to synthesize E alpha chains but do synthesize Ae chains, which remain in the cytoplasm as partially glycosylated precursors. Cells of the f and q haplotypes fail to synthesize either the Ae or E alpha polypeptide chains, as shown by both genetic complementation tests and analyses of total cell proteins by two-dimensional polyacrylamide gel electrophoresis. The patterns of expression of the intact E alpha: Ae complex are consistent with the theory that both the Ae and E alpha polypeptide chains must be present in the cells for either chain to be expressed in normal amounts on the cell surface. The implications of these observations for the genetics of I-region-controlled functions are discussed.

Structural analysis of a new B-cell-differentiation antigen associated with products of the I-A subregion of the H-2 complex

Ia.W39 is a private specificity of the I-Ab subregion of the H-2 complex. It is selectively expressed on a subset of B lymphocytes that is absent in newborn normal and adult mutant mice carrying the xid gene. Immunoprecipitation and one-dimensional NaDodSO4/polyacrylamide gel electrophoresis showed that the molecule bearing Ia.W39 consists of two noncovalently linked glycoproteins of apparent Mr 33,000 and 28,000. Anti-Ia.W39 serum did not preclear the Iab molecule; however, the conventional allo-anti-I-Ab serum cleared Ia.W39 completely. In view of the identical two-dimensional gel pattern generated by the Ia.W39 and the conventional Iab immunoprecipitates, we believe that all Ia molecules bear the conventional specificities and only a subset would in addition express Ia.W39. Ia.W39 is probably not a carbohydrate antigen, because the antibiotic tunicamycin had no influence on its expression. It may be a conformational determinant on the A alpha and A beta complex induced by the association of an unknown molecule with these chains.

Structure of murine Ia antigens. Two dimensional electrophoretic analyses and high pressure liquid chromatography tryptic peptide maps of products of the I-A and I-E subregions and of an associated invariant polypeptide

We demonstrate that an invariant polypeptide, first described by Jones et al. (21), co-immunoprecipitates with our Ia molecules, that its interaction with Ia polypeptides varies with haplotype, and that it is not a precursor of the Aalpha, Abeta, Ealpha, or Ebeta. polypeptides. We also show that the polypeptides that we have previously characterized are contaminated with very little, if any, invariant protein. Further, we have used our high-pressure liquid chromatography tryptic peptide map technique to formally map the genes encoding Aalpha, Abeta, and Ebeta to the I-A subregion using recombinant and F1 hybrid mice.

Aberrant Ae (E beta) Ia polypeptide chain in H-2g2 haplotype mice. Possible result of an intragenic recombination or mutation

Mice of strains D2.GD and B10.GD, which carry the recombinant haplotype H-2g2 (previously typed as H-2Kd I-Ad / I-Bb I-Jb I-Eb I-Cb Sb H-2Db), have an Ae (E beta) polypeptide chain electrophoretically distinct from the Ae chains of both b and d haplotype mice, including the progenitor strains from which the recombinant H-2g2 chromosome was derived. The evidence presented suggests that the altered molecular properties of the Aeg2 chain may be a consequence of an intragenic recombination event in the I-A subregion within the structural gene for this polypeptide chain. Because the A alpha and A beta chains controlled by H-2g2 appear to be d haplotype in origin, this finding would map the gene for Ae to the right of the loci controlling the A alpha and A beta chains

Monoclonal antibody against an Ir gene product?

Genetic, biochemical, and functional studies have been performed using a monoclonal antibody, Y-17, directed at a conformational or combinatorial determinant formed by certain Ae:E alpha complexes. This determinant appears to be a marker present on a subset of B cells as well as on non-T and non-B spleen cells. Besides Ae and E alpha chains, Y-17 precipitates a third chain that is indistinguishable from the A alpha chain in two-dimensional gels. This results suggests additional combinatorial complexity in the generation of I-region encoded antigens. Y-17 can inhibit the response of T cells to Ae:E alpha determinants in mixed lymphocyte cultures. Furthermore, Y-17 blocks antigen-specific T cell proliferative responses to GLPhe and pigeon cytochrome c which have been shown to require the Ae:E alpha complex as a restriction element for antigen presentation. These results provide strong evidence for the molecular identity of Ia antigens, Ir-gene products and Lad antigens.

Interaction between I region loci influences the expression of a cell surface Ia antigen

Loci clustered in the I region of the murine H-2 gene complex control the capacity to generate an immune response against foreign antigens (Ir loci) and control differentiation antigens which appear to serve as structures used by cells to interact with and regulate one another (Ia loci). Both genetic and functional studies suggest that Ia antigens may be products of Ir loci. Recent studies have shown that interaction between closely linked Ir loci is required for generating immune responses to certain foreign antigens, and that interaction between H-2-linked loci determines the appearance of an Ia glycoprotein (the Ae chain) on lymphocyte cell surfaces. In this report, we show that one Ia locus regulates the quantitative expression of the product (the E alpha chain) of a second Ia locus. This regulatory locus is dominantly expressed and exerts its effects in either the cis or trans chromosomal position. Thus, the quantitative as well as the qualitative expression of some Ia products is dependent on interaction between tightly linked loci. Our results suggest a possible molecular basis for this regulation: the synthesis and intracellular association of Ae and E alpha chains may be an absolute requirement for the expression of normal levels of either polypeptide chain on the lymphocyte cell surface. The implications these findings have for I region control of immune responses and study of human HLA-D antigens are discussed.

Characterization of a spontaneous murine B cell leukemia (BCL1). I. Cell surface expression of IgM, IgD, Ia, and FcR

The surface marker expression of a spontaneous B lymphocyte leukemia discovered in a BALB/c mouse (BCL1) was examined and found to include a subset of markers known to occur on normal B lymphocytes. The tumor cells bore surface Ig that included both mu- and delta-chains associated with the lambda light chain. Alloantigens coded for within the murine MHC, including H-2D, H-2K, and I-region products, were identified on the tumor cells. Although normal B lymphocytes are thought to express products coded for within both the I-A and I-E subregions, the BCL1 expressed only normal amounts of I-E subregion products. In addition, the H-2 and Ia antigens revealed by 2-dimensional gel electrophoresis exhibited an abnormal pattern of post-translational modifications. The Fc, but not the complement-receptor, was present on the surface of tumor cells. The presence of IgD, Ia antigens, and the responsiveness to lipopolysaccharide (see subsequent paper) have led us to postulate that the BCL1 tumor represents a later differentiative stage than murine B lymphocyte tumors previously described.

Expression of MHC antigens by mouse thymic dendritic cells

Thymic epithelial cells express MHC antigens in several different patterns. I-A is present throughout the thymic cortex on dendritic cells. The remainder of the I region and H-2K/D are expressed on dendritic cells apparently only variably in the cortex (at least in some haplotypes). All MHC antigens tested are present in the medulla on epithelial cells; expression on medullary lymphocytes cannot be evaluated. Monoclonal anti-MHC antibodies confirm these results. The significance of these findings to T cell maturation is discussed.