Tissue-specific expression of an unusual H-2 (class I)-related gene

Ribosylation of the CD8αβ heterodimer permits binding of the nonclassical major histocompatibility molecule, H2-Q10

The CD8αβ heterodimer plays a crucial role in the stabilization between major histocompatibility complex class I molecules (MHC-I) and the T cell receptor (TCR). The interaction between CD8 and MHC-I can be regulated by posttranslational modifications, which are proposed to play an important role in the development of CD8 T cells. One modification that has been proposed to control CD8 coreceptor function is ribosylation. Utilizing NAD⁺, the ecto-enzyme adenosine diphosphate (ADP) ribosyl transferase 2.2 (ART2.2) catalyzes the addition of ADP-ribosyl groups onto arginine residues of CD8α or β chains and alters the interaction between the MHC and TCR complexes. To date, only interactions between modified CD8 and classical MHC-I (MHC-Ia), have been investigated and the interaction with non-classical MHC (MHC-Ib) has not been explored. Here, we show that ADP-ribosylation of CD8 facilitates the binding of the liver-restricted nonclassical MHC, H2-Q10, independent of the associated TCR or presented peptide, and propose that this highly regulated binding imposes an additional inhibitory leash on the activation of CD8-expressing cells in the presence of NAD⁺. These findings highlight additional important roles for nonclassical MHC-I in the regulation of immune responses.

The murine CD94/NKG2 ligand, Qa-1b, is a high-affinity, functional ligand for the CD8αα homodimer

The immune co-receptor CD8 molecule (CD8) has two subunits, CD8α and CD8β, which can assemble into homo or heterodimers. Nonclassical (class-Ib) major histocompatibility complex (MHC) molecules (MHC-Ibs) have recently been identified as ligands for the CD8αα homodimer. This was demonstrated by the observation that histocompatibility 2, Q region locus 10 (H2-Q10) is a high-affinity ligand for CD8αα which also binds the MHC-Ib molecule H2-TL. This suggests that MHC-Ib proteins may be an extended source of CD8αα ligands. Expression of H2-T3/TL and H2-Q10 is restricted to the small intestine and liver, respectively, yet CD8αα-containing lymphocytes are present more broadly. Therefore, here we sought to determine whether murine CD8αα binds only to tissue-specific MHC-Ib molecules or also to ubiquitously expressed MHC-Ib molecules. Using recombinant proteins and surface plasmon resonance-based binding assays, we show that the MHC-Ib family furnishes multiple binding partners for murine CD8αα, including H2-T22 and the CD94/NKG2-A/B-activating NK receptor (NKG2) ligand Qa-1b We also demonstrate a hierarchy among MHC-Ib proteins with respect to CD8αα binding, in which Qa-1b > H2-Q10 > TL. Finally, we provide evidence that Qa-1b is a functional ligand for CD8αα, distinguishing it from its human homologue MHC class I antigen E (HLA-E). These findings provide additional clues as to how CD8αα-expressing cells are controlled in different tissues. They also highlight an unexpected immunological divergence of Qa-1b/HLA-E function, indicating the need for more robust studies of murine MHC-Ib proteins as models for human disease.

Multiple receptors converge on H2-Q10 to regulate NK and γδT-cell development

Class Ib major histocompatibility complex (MHC) is an extended family of molecules, which demonstrate tissue-specific expression and presentation of monomorphic antigens. These characteristics tend to imbue class Ib MHC with unique functions. H2-Q10 is potentially one such molecule that is overexpressed in the liver but its immunological function is not known. We have previously shown that H2-Q10 is a ligand for the natural killer cell receptor Ly49C and now, using H2-Q10-deficient mice, we demonstrate that H2-Q10 can also stabilize the expression of Qa-1b. In the absence of H2-Q10, the development and maturation of conventional hepatic natural killer cells is disrupted. We also provide evidence that H2-Q10 is a new high affinity ligand for CD8αα and controls the development of liver-resident CD8αα γδT cells. These data demonstrate that H2-Q10 has multiple roles in the development of immune subsets and identify an overlap of recognition within the class Ib MHC that is likely to be relevant to the regulation of immunity.

The expanding role of murine class Ib MHC in the development and activation of Natural Killer cells

Major Histocompatibility Complex-I (MHC-I) molecules can be divided into class Ia and class Ib, with three distinct class Ib families found in the mouse. These families are designated as Q, T and M and are largely unexplored in terms of their immunological function. Among the class Ib MHC, H2-T23 (Qa-1b) has been a significant target for Natural Killer (NK) cell research, owing to its homology with the human class Ib human leukocyte antigen (HLA)-E. However, recent data has indicated that members of the Q and M family of class Ib MHC also play a critical role in the development and regulation NK cells. Here we discuss the recent advances in the control of NK cells by murine class Ib MHC as a means to stimulate further exploration of these molecules.

Recognition of the Major Histocompatibility Complex (MHC) Class Ib Molecule H2-Q10 by the Natural Killer Cell Receptor Ly49C

Murine natural killer (NK) cells are regulated by the interaction of Ly49 receptors with major histocompatibility complex class I molecules (MHC-I). Although the ligands for inhibitory Ly49 were considered to be restricted to classical MHC (MHC-Ia), we have shown that the non-classical MHC molecule (MHC-Ib) H2-M3 was a ligand for the inhibitory Ly49A. Here we establish that another MHC-Ib, H2-Q10, is a bona fide ligand for the inhibitory Ly49C receptor. H2-Q10 bound to Ly49C with a marginally lower affinity (∼5 μm) than that observed between Ly49C and MHC-Ia (H-2K(b)/H-2D(d), both ∼1 μm), and this recognition could be prevented by cis interactions with H-2K in situ To understand the molecular details underpinning Ly49·MHC-Ib recognition, we determined the crystal structures of H2-Q10 and Ly49C bound H2-Q10. Unliganded H2-Q10 adopted a classical MHC-I fold and possessed a peptide-binding groove that exhibited features similar to those found in MHC-Ia, explaining the diverse peptide binding repertoire of H2-Q10. Ly49C bound to H2-Q10 underneath the peptide binding platform to a region that encompassed residues from the α1, α2, and α3 domains, as well as the associated β2-microglobulin subunit. This docking mode was conserved with that previously observed for Ly49C·H-2K(b) Indeed, structure-guided mutation of Ly49C indicated that Ly49C·H2-Q10 and Ly49C·H-2K(b) possess similar energetic footprints focused around residues located within the Ly49C β4-stand and L5 loop, which contact the underside of the peptide-binding platform floor. Our data provide a structural basis for Ly49·MHC-Ib recognition and demonstrate that MHC-Ib represent an extended family of ligands for Ly49 molecules.

Natural polymorphisms in Tap2 influence negative selection and CD4∶CD8 lineage commitment in the rat

Genetic variation in the major histocompatibility complex (MHC) affects CD4∶CD8 lineage commitment and MHC expression. However, the contribution of specific genes in this gene-dense region has not yet been resolved. Nor has it been established whether the same genes regulate MHC expression and T cell selection. Here, we assessed the impact of natural genetic variation on MHC expression and CD4∶CD8 lineage commitment using two genetic models in the rat. First, we mapped Quantitative Trait Loci (QTLs) associated with variation in MHC class I and II protein expression and the CD4∶CD8 T cell ratio in outbred Heterogeneous Stock rats. We identified 10 QTLs across the genome and found that QTLs for the individual traits colocalized within a region spanning the MHC. To identify the genes underlying these overlapping QTLs, we generated a large panel of MHC-recombinant congenic strains, and refined the QTLs to two adjacent intervals of ∼0.25 Mb in the MHC-I and II regions, respectively. An interaction between these intervals affected MHC class I expression as well as negative selection and lineage commitment of CD8 single-positive (SP) thymocytes. We mapped this effect to the transporter associated with antigen processing 2 (Tap2) in the MHC-II region and the classical MHC class I gene(s) (RT1-A) in the MHC-I region. This interaction was revealed by a recombination between RT1-A and Tap2, which occurred in 0.2% of the rats. Variants of Tap2 have previously been shown to influence the antigenicity of MHC class I molecules by altering the MHC class I ligandome. Our results show that a restricted peptide repertoire on MHC class I molecules leads to reduced negative selection of CD8SP cells. To our knowledge, this is the first study showing how a recombination between natural alleles of genes in the MHC influences lineage commitment of T cells.

Peptides from degraded cytoplasmic proteins are transported via TAP into the endoplasmic reticulum for loading onto MHC class I molecules. TAP is encoded by Tap1 and Tap2, which in rodents are located close to the MHC class I genes. In the rat, genetic variation in Tap2 gives rise to two different transporters: a promiscuous A variant (TAP-A) and a more restrictive B variant (TAP-B). It has been proposed that the class I molecule in the DA rat (RT1-A^a) has co-evolved with TAP-A and it has been shown that RT1-A^a antigenicity is changed when co-expressed with TAP-B. To study the contribution of different allelic combinations of RT1-A and Tap2 to the variation in MHC expression and T cell selection, we generated DA rats with either congenic or background alleles in the RT1-A and Tap2 loci. We found increased numbers of mature CD8SP cells in the thymus of rats which co-expressed RT1-A^a and TAP-B. This increase of CD8 cells could be explained by reduced negative selection, but did not correlate with RT1-A^a expression levels on thymic antigen presenting cells. Thus, our results identify a crucial role of the TAP and the quality of the MHC class I repertoire in regulating T cell selection.

Biochemical evidence that equine leucocyte antigens W13, W22 and W23 are present on horse major histocompatibility complex class II molecules

A number of horse alloantisera were characterized biochemically as being directed against MHC class I or class II antigens by immunoprecipitation of the corresponding antigens from lysates of biosynthetically radioactively labelled lymphocytes and determination of their molecular weights by SDS-PAGE and fluorography. Sera recognizing A2 and A3 specificities precipitated antigens of 44,000 Daltons molecular weight (class I heavy chain), whereas sera with specificities W13, W22 and W23 precipitated antigens corresponding to class II dimers (30,000 and 32,000 Daltons). Comparison with antigens precipitated from horse lymphocyte lysates using (cross-reacting) antibodies to human class I and class II MHC molecules confirmed the results obtained.

Characterization of a nonclassical class I MHC gene in a reptile, the Galápagos marine iguana (Amblyrhynchus cristatus)

Squamates are a diverse order of vertebrates, representing more than 7,000 species. Yet, descriptions of full-length major histocompatibility complex (MHC) genes in this group are nearly absent from the literature, while the number of MHC studies continues to rise in other vertebrate taxa. The lack of basic information about MHC organization in squamates inhibits investigation into the relationship between MHC polymorphism and disease, and leaves a large taxonomic gap in our understanding of amniote MHC evolution. Here, we use both cDNA and genomic sequence data to characterize a class I MHC gene (Amcr-UA) from the Galápagos marine iguana, a member of the squamate subfamily Iguaninae. Amcr-UA appears to be functional since it is expressed in the blood and contains many of the conserved peptide-binding residues that are found in classical class I genes of other vertebrates. In addition, comparison of Amcr-UA to homologous sequences from other iguanine species shows that the antigen-binding portion of this gene is under purifying selection, rather than balancing selection, and therefore may have a conserved function. A striking feature of Amcr-UA is that both the cDNA and genomic sequences lack the transmembrane and cytoplasmic domains that are necessary to anchor the class I receptor molecule into the cell membrane, suggesting that the product of this gene is secreted and consequently not involved in classical class I antigen-presentation. The truncated and conserved character of Amcr-UA lead us to define it as a nonclassical gene that is related to the few available squamate class I sequences. However, phylogenetic analysis placed Amcr-UA in a basal position relative to other published classical MHC genes from squamates, suggesting that this gene diverged near the beginning of squamate diversification.

Expression of nonclassical MHC class Ib genes: comparison of regulatory elements

Peptide binding proteins of the major histocompatibility complex consist of the "classical" class Ia and "nonclassical" class Ib genes. The gene organization and structure/function relationship of the various exons comprising class I proteins are very similar among the class Ia and class Ib genes. Although the tissue-specific patterns of expression of these two gene families are overlapping, many class Ib genes are distinguished by relative low abundance and/or limited tissue distribution. Further, many of the class Ib genes serve specialized roles in immune responses. Given that the coding sequences of the class Ia and class Ib genes are highly homologous we sought to examine the promoter regions of the various class Ib genes by comparison to the well characterized promoter elements regulating expression of the class Ia genes. This analysis revealed a surprising complexity of promoter structures among all class I genes and few instances of conservation of class Ia promoter regulatory elements among the class Ib genes.

Listing, location, binding motifs, and expression of nonclassical class I and related genes and molecules

The tables presented in this appendix list nonclassical class I or related genes/molecules arranged by the chromosomal region where they are encoded. This includes genes that fall into the Ib region of the murine major histocompatibility complex (MHC) which includes H2-Q, -T, and -M, as well as CD1, which lies outside the MHC region. A final table includes genes/molecules that are encoded in diverse regions. They are included in this section because they are either class I related in that their heavy chain is related to classical class I and/or they are associated with ion given is for the C57BL/6 (B6) strain unless otherwise noted.

Repression of the nonclassical MHC class I gene H2-M1 by cis-acting silencer DNA elements

H2-M1 is a non-classical major histocompatibility complex (MHC) class I gene that is highly divergent from classical class I genes; M1 was the first gene in the recently classified M region of the mouse MHC to be cloned. Although the M1 DNA sequence contains normal splice sites, open reading frames within its exons, and a recognizable promoter, no M1 transcripts were detected in various healthy mouse tissues. However, M1 transcripts were detected in transfected L cells and in vivo in brains of M1 transgenic mice, albeit at very low levels, and the level of expression is correlated with transgene copy number. Analysis of the M1 promoter region identified a competent promoter capable of directing transcription, but whose expression is repressed by two strong upstream silencer elements, one mapping between -184 base pairs (bp) and -266 bp and the other between -1149 bp and -1702 bp. These studies suggest that M1 expression is highly regulated and restricted either temporally or to a very limited number of cell types.

Biochemical and functional characteristics of soluble MHC molecules derived from H-2Ld/Q10d chimeric gene

We have constructed a chimeric class I gene in which the 5' half of the H-2Ld gene is linked to the 3' half of Q10d. The resulting H-2Ld/Q10d protein is homologous to the native H-2Ld heavy chain for the three external domains except for an Arg to His substitution at position 260. The transmembrane and intracytoplasmic domains of the H-2Ld chain are replaced by the short low hydrophobic transmembrane-like domain of the Q10d chain. Following DNA-mediated gene transfer into mouse L cells, transformants were selected for the presence of specific mRNA. Radiolabelling and immunoprecipitation analysis revealed secretion of a 48-46 kd chain weakly associated with beta 2-microglobulin. This molecule reacts with H-2Ld-specific mAb that identify determinants on the first and second domains as well as with an anti-Q10 carboxyl-terminal peptide antiserum, but is not recognized by a mAb specific for a determinant of H-2Ld third domain. The integrity of antibody reactivity of the first and second domains together with beta 2-microglobulin association suggest that our molecule may be considered a good soluble counterpart of the native membrane H-2Ld molecule with which to perform functional studies. In order to analyze the immunogenic capacities and T-cell recognition of the soluble H-2Ld molecules, T-cell lines were produced from mice of various inbred strains immunized with supernatant from H-2Ld/Q10d-transfected fibroblasts. Characterization of these T cells revealed that they expressed a CD4+CD8- phenotype, and recognized H-2Ld/Q10d products in a class II-restricted manner.

HBx gene of hepatitis B virus induces liver cancer in transgenic mice

The exact role of hepatitis B virus in the development of liver cancer is not known. The recent identification of a viral regulatory gene HBx suggests a possible direct involvement of the virus whereby the HBx protein, acting as a transcriptional transactivator of viral genes, may alter host gene expression and lead to the development of hepatocellular carcinoma. We have tested this possibility of placing the entire HBx gene under its own regulatory elements directly into the germline of mice. Transgenic animals harbouring this viral gene succumbed to progressive histopathological changes specifically in the liver, beginning with multifocal areas of altered hepatocytes, followed by the appearance of benign adenomas, and proceeding to the development of malignant carcinomas. Male mice developed disease and died much earlier than females. This transgenic animal model appears ideal for defining the molecular events that follow the expression of the viral HBx gene and are responsible for the development of liver cancer.

Class I major histocompatibility complex cDNA clones from sheep thymus: alternative splicing could make a long cytoplasmic tail

To investigate the class I major histocompatibility complex (MHC) genes expressed in the young sheep thymus, a cDNA library was screened with a human HLA-B7 cDNA probe under conditions of relaxed stringency. Thirteen clones were isolated and found by partial sequences to fall into five classes, requiring the expression of at least three loci. One sequence was found six times, almost half of the total, and may thus represent the major message expressed in the young sheep thymus. One of the clones was found to have failed to excise the intron between cytoplasmic exons 7 and 8, leading to the predicted synthesis of a cytoplasmic domain 23 amino acids longer than the other sheep sequences, and 15 amino acids longer than any cytoplasmic domain previously described. The sequences of all the clones were found to be most similar to bovine, and least similar to mouse class I MHC sequences.

Comparison of exon 5 sequences from 35 class I genes of the BALB/c mouse

DNA sequences of the fifth exon, which encodes the transmembrane domain, were determined for the BALB/c mouse class I MHC genes and used to study the relationships between them. Based on nucleotide sequence similarity, the exon 5 sequences can be divided into seven groups. Although most members within each group are at least 80% similar to each other, comparison between groups reveals that the groups share little similarity. However, in spite of the extensive variation of the fifth exon sequences, analysis of their predicted amino acid translations reveals that only four class I gene fifth exons have frameshifts or stop codons that terminate their translation and prevent them from encoding a domain that is both hydrophobic and long enough to span a lipid bilayer. Exactly 27 of the remaining fifth exons could encode a domain that is similar to those of the transplantation antigens in that it consists of a proline-rich connecting peptide, a transmembrane segment, and a cytoplasmic portion with membrane-anchoring basic residues. The conservation of this motif in the majority of the fifth exon translations in spite of extensive variation suggests that selective pressure exists for these exons to maintain their ability to encode a functional transmembrane domain, raising the possibility that many of the nonclassical class I genes encode functionally important products.

Analysis of a new class I gene mapping to the Hmt region of the mouse

The major histocompatibility complex (MHC) of the BALB/c mouse contains three genes encoding classical class I molecules, as well as at least 32 nonclassical class I genes. Although much is known about the genes encoding the classical class I molecules, the majority of the nonclassical genes have not been characterized. This report describes a newly identified nonclassical class I gene, Thy19.4, which contains an open reading frame and resembles in several regards the genes encoding classical class I molecules. The similarities include shared amino acid sequence motifs which suggest that the putative Thy19.4 molecule may assume a tertiary structure similar to that of the classical class I molecules, as well as widespread transcription in a variety of tissues. However, unlike the classical class I genes, the Thy19.4 gene maps approximately 1 cm distal to the Tla region of the MHC, in the same region as the gene encoding the Hmt element of the maternally transmitted antigen.

Soluble class I antigens in serum and CSF of patients with varicella-zoster virus meningitis

Soluble class I antigens (sHLA) are secreted by lymphocytes upon activation in vitro. The intrathecal synthesis (ITS) of these molecules has been studied in patients with the varicella-zoster virus (VZV) meningitis. In this paper we describe a sHLA index IH = (CSF sHLA/serum sHLA): (CSF albumin/serum albumin) which is expected to increase only when sHLA is synthesised within the central nervous system (CNS). The IH is elevated in the first week of meningitis, when antibody synthesis is still low, and decreases thereafter. We think IH is an index of early lymphocyte activation within the CNS. The relation of these findings with previous in vitro studies is also discussed.

Genetics and expression of the Q6 and Q8 genes. An LTR-like sequence in the 3' untranslated region

A unique 2.2-kb mRNA is transcribed from the Q6 and Q8 genes of the mouse major histocompatibility complex. RNase protection experiments and DNA sequence analysis have mapped the 3' terminus to a site located 1110 bp downstream from exon 8. Comparison of the 3' sequence of Q8 to that of Q7 revealed that the two genes diverge from one another at a point located 200 bp into the untranslated domain. This finding explains the increased size of the transcript. RNase protection experiments involving twelve different strains of mice have revealed few sequence polymorphisms. A tissue distribution of the 2.2-kb transcript in B6 mice revealed that the highest quantities of message were present in the spleen with decreasingly lower amounts in the thymus, liver, kidney, testis, and brain. The H-2r haplotype appeared novel; it is phenotypically Qa-2-, yet expressed a 2.2-kb transcript which hybridizes to a Q8 probe. A comparison of a Qa-2hi expressor, B6, and a Qa-2low expressor, BALB/cFla, has established that these quantitative cell surface differences are reflected in mRNA differences. A homology search of the Genbank database has revealed that the 3' portion of Q8 contains extensive homology to a retrovirus-like long terminal repeat sequence that is characteristic of an embryonic-specific transposon (ETn).

Rejection of B16 melanoma induced by expression of a transfected major histocompatibility complex class I gene

Transfection of a functional major histocompatibility complex class I gene into certain tumor cells, induced by oncogenic viruses or chemical carcinogens, can effectively abrogate their tumorigenic activity. Since experimentally induced tumors possess strong tumor-specific transplantation antigens, expression of cell surface class I antigens may present the tumor cells to appropriate immune effector cells. Most spontaneously arising tumors do not possess tumor-specific transplantation antigens, and their tumorigenicity may not be affected by the expression of a transfected class I gene. We demonstrate that the poorly immunogenic B16-BL6 melanoma can be rendered nontumorigenic in syngeneic mice by the expression of the class I H-2K antigen but not the class II I-A antigen. Furthermore, the poorly tumorigenic, class I-expressing B16-BL6-transfected cells can effectively immunize syngeneic C57BL/6 mice against the highly tumorigenic, class I-deficient B16-BL6 parental cells. Our success in experimentally manipulating the tumorigenicity of a spontaneously derived neoplasm offers hope for a potential modality for the effective treatment of human cancer.

Class I transplantation antigens in solution in body fluids and in the urine. Individuality signals to the environment

Classical class I transplantation antigens present in solution in the body fluids have been studied. These antigens have been found in a monomeric, soluble form in blood, lymph, and urine, and a major source is the hemopoietic system which gives rise to cells that secrete these molecules into the blood. The cell types most probably involved in their secretion are of the macrophage/dendritic cell lineage. The serum molecule is a heterodimer with a heavy chain of 39,000 mol wt associated noncovalently with beta 2-microglobulin and is present in serum at a concentration between 350 and 390 ng/ml. These molecules have a short half-life of 2.7 h and are excreted into the environment via the kidneys in the urine. In the urine, greater than 90% of the molecules are degraded into smaller fragments. This finding that normal metabolic processes lead to the excretion of classical highly polymorphic class I molecules in the urine provides a direct explanation in molecular terms of the ability of animals to identify individuals on the basis of urinary odor. Since intact class I molecules are unlikely to be the odoriferous component in the urine, two hypotheses have been suggested. Either small fragments of class I molecules are detected or the molecule acts as a carrier that transports volatiles from the serum into the urine where they are released, giving rise to the class I-associated odor.

Extension of the H-2 TLb molecular map. Isolation and characterization of T13, T14, and T15 from the C57BL/6 mouse

A region of the TLb locus encompassing T11 to T13 contains retroviral sequences TLev1 and TLev2. As part of a study to determine whether the retroviral elements are involved in the expression of TL genes, the genomic organization of this region was reexamined in greater detail. A result of these investigations is the extension of the H-2 TLb molecular map. Two additional TL genes have been isolated from C57BL/6 mice, T14 and T15. The genomic organization of T9 through T15 is presented. The nucleotide sequence has been determined for exons 4, 5, and 6 of T13. As a result of a C to T conversion, a termination codon is introduced into exon 4, indicating that T13 either encodes a secreted protein or is a pseudogene. T13 was found to be more homologous to the H-2 genes outside the TL region. T14 has been physically disrupted by the integration of TLev1, and the H-2 sequences appear to have diverged greatly. The relationship of the TL regions of the b and c haplotypes has been investigated using numerous low copy probes. The genome of BALB/c (TLc) is shown to lack a counterpart of the T13-T15b region. Homologous regions exist in the two haplotypes; yet considerable polymorphism is observed. TLb mice do not express TLa on the cell surface of normal thymocytes while TLc mice do; TLa expression is activated in many TLb leukemias. The diversity seen in the T13-T15 region may provide insights into the phenotypic expression or regulatory mechanisms of TL expression in these two haplotypes.

Rejection of B16 melanoma induced by expression of a transfected major histocompatibility complex class I gene

Transfection of a functional major histocompatibility complex class I gene into certain tumor cells, induced by oncogenic viruses or chemical carcinogens, can effectively abrogate their tumorigenic activity. Since experimentally induced tumors possess strong tumor-specific transplantation antigens, expression of cell surface class I antigens may present the tumor cells to appropriate immune effector cells. Most spontaneously arising tumors do not possess tumor-specific transplantation antigens, and their tumorigenicity may not be affected by the expression of a transfected class I gene. We demonstrate that the poorly immunogenic B16-BL6 melanoma can be rendered nontumorigenic in syngeneic mice by the expression of the class I H-2K antigen but not the class II I-A antigen. Furthermore, the poorly tumorigenic, class I-expressing B16-BL6-transfected cells can effectively immunize syngeneic C57BL/6 mice against the highly tumorigenic, class I-deficient B16-BL6 parental cells. Our success in experimentally manipulating the tumorigenicity of a spontaneously derived neoplasm offers hope for a potential modality for the effective treatment of human cancer.

Biochemical complexity of serum HLA class I molecules

Human serum was found to contain a variety of class I-like molecules by Western blotting with anti-class I heavy chain reagents: major bands usually are observed around Mr 44,000, 40,000, and 35,000-37,000. HLA-A24-positive individuals are distinguished by higher serum levels of Mr 44,000 and 40,000 class I-like molecules than those found in HLA-A24-negative individuals. The Mr 44,000 serum molecules are probably intact class I molecules that have been shed from the cell membrane, because they contain both a transmembrane segment (TM), as deduced from detergent-binding experiments, and a cytoplasmic tail (CT), as inferred from reactivity with an antipeptide serum specific for the cytoplasmic domain of class I antigens (RaCT). The Mr 35,000 and 37,000 molecules contain neither a TM nor a CT region and therefore are probably proteolytic breakdown products of cellular and/or serum Mr 44,000 molecules, although the existence of Q10-like molecules in man cannot be ruled out. The Mr 40,000 molecules do not contain a TM region. Mr 40,000 molecules reactive with the RaCT serum were found in the minority (2/13) of sera tested. We conclude that alternative splicing resulting in a precise excision of the TM exon plays a minor role in the generation of serum HLA class I antigens.

Widespread transcription of a Qa region gene in adult mice

The mouse MHC class I family includes genes encoded in four regions: H-2K, H-2D, Qa and Tla. While K/D genes are well characterized, relatively little is known about Qa or Tla genes. We have studied the transcription of a B10.P Qa region gene. DNA sequence comparisons of the transmembrane region, supported by Southern blot analysis of cosmid and genomic DNAs from BALB/c and C57BL/10, demonstrate the lambda 3a gene corresponds to Q4p. In both Northern blots and RNA protection experiments using probes derived from the 3' noncoding region, we found that Q4, like the H-2K and H-2D genes, is widely transcribed in B10.P tissues. These data demonstrate for the first time widespread transcription of a Qa gene.

Identification in rat liver and serum of water-soluble class I MHC molecules possibly homologous to the murine Q10 gene product

We have identified large quantities of a water-soluble, non-RT1.A class I MHC molecule in the serum of the DA rat strain, with a similar molecule being found in aqueous extracts of DA liver. The non-RT1.A class I molecules have heavy chains of 41 kD, which is smaller than RT1.A class I molecules isolated from liver membranes (45 kD) but larger than water-soluble RT1.A class I molecules previously identified in serum and aqueous extracts of liver and kidney (40 kD). NH3-terminal amino acid sequencing of bulk-purified RT1.A class I molecules and of this novel non-RT1.A class I molecule revealed two substitutions, in the first 25 amino acids, Tyr----His at position 9, and Ala----Ser at position 24. The non-RT1.A class I molecule did not react with any of the well-characterized polymorphic and monomorphic antibodies directed against RT1.Aa class I molecules, but did react with the MRC OX18 antibody. A similar class I molecule could not be identified on liver membranes. The non-RT1.A class I molecule was found in large quantities (approximately 20 micrograms/ml) in the serum of the DA rat strain, and similarly large quantities appeared to be present in the sera of BN, PVG, and LEW.RT1a rats. WAG and LEW.RT1u rats had readily detectable but lower amounts of this molecule in their serum, while LEW and SHR rats had little if any present. This molecule probably represents the rat homologue of the murine Q10 gene product, and is the major class I product in the serum of the DA rat strain.

A transcriptional enhancer and an interferon-responsive sequence in major histocompatibility complex class I genes

The major histocompatibility complex class I antigens play an indispensable role in cell-cell interactions. Perturbation of their expression has been shown to have deleterious physiological consequences, including the escape of transformed cells from immune detection. In an attempt to understand how class I genes are regulated, we dissected the Ld gene to identify potential control regions. By using a test vector containing the simian virus 40 early promoter placed upstream of the bacterial chloramphenicol acetyltransferase (cat) gene, we demonstrated the presence of a transcriptional enhancer within the 5'-flanking region. The sequence is functional in both orientations and has been mapped within 350 base pairs upstream of the Ld transcriptional start site. Although human adenovirus 12 can suppress endogenous class I genes, it cannot down-regulate the activity of the transiently transfected cat gene which has been placed under the control of the Ld enhancer and promoter. Our results suggested that if the human adenovirus 12-induced function regulates the expression of class I genes by a trans mechanism, then its target site must not be within 1.9 kilobases of the 5'-flanking region. Treatment of cells with interferon increases the accumulation of class I transcripts. Expression of the cat gene under the control of the Ld enhancer and promoter also can be up-regulated by interferon. Our study shows that the target sequence required for this enhancement resides, at least in part, within the same 350-base pair segment which contains the transcriptional enhancer.

Expression of a secreted transplantation antigen gene during murine embryogenesis

We examined the midgestation mouse embryo for transcripts related to the secreted transplantation antigen Q10 and show here that this gene is transcribed in the endoderm of the visceral yolk sac. Its level of expression is highest at day 14 and then declines as development proceeds. Concurrently with the decrease in yolk sac expression, the amount of transcripts accumulating in the fetal liver increases during late embryogenesis.

Water-soluble form of RT1.A class I MHC molecules in the kidney and liver of the rat

The RT1.A (H-2K,D type) class I major histocompatibility complex (MHC) antigens of the rat are well recognized as membrane-bound glycoproteins. In this report, we demonstrate that liver and kidney in the DA rat strain contain large amounts of a water-soluble RT1.A class I molecule with a discrete heavy chain approximately 5 kd smaller than the membrane-bound form. An identical molecule could be identified in DA rat serum. This small class I molecule carries all of the polymorphic antigenic determinants of the RT1.A av1 class I molecule. The water-soluble molecule is readily denatured in its pure form when frozen and thawed, but this does not occur when it is mixed with serum, presumably because of a stabilizing interaction with one or more carrier proteins. The half-life of the class I molecule in serum was measured to be approximately 1.5 h. The LEW rat strain produced detectable but substantially smaller amounts of water-soluble RT1.A molecules. Our studies indicate that RT1.A class I MHC antigens are synthesized and presumably secreted in a smaller water-soluble form by liver, kidney, and possibly other tissues under physiological conditions, a point of considerable interest in view of the immunoregulatory functions of the membrane-bound forms of these molecules.

Extensive deletions in the Q region of the mouse major histocompatibility complex

By use of Southern blot analyses and low copy number probes, the fine structure of the Q region of the mouse major histocompatibility complex was studied in more detail. With a probe recognizing the even-numbered genes Q4, Q6, and Q8, it was evident that Q4 and/or the regions flanking Q4 are polymorphic, whereas Q6 and Q8, and their flanking regions are nonpolymorphic. Perhaps the most noteworthy finding is that at least two strain haplotypes, H-2k and H-2f, possessed extensive deletions in the Q region. The most striking deletion was found in the H-2f haplotype, where the Q1 through Q9 genes appear to be missing. Because of these extensive deletions the functional importance of the Q region is questioned.

Expression of major histocompatibility complex class I antigens as a strategy for the potentiation of immune recognition of tumor cells

Like many primary tumors, human adenovirus type 12 (Ad12)-transformed mouse cells express greatly reduced levels of the major histocompatibility complex (MHC) class I antigens and are highly tumorigenic in immunocompetent hosts. Expression of a transfected class I gene by these cells can abrogate their tumorigenicity. Both the K and the L class I genes can suppress the malignant phenotype. Previous studies showed that interferon can induce class I gene expression in certain Ad12-transformed cells and can suppress their tumorigenic phenotype. We now demonstrate that preimmunization of mice with a nontumorigenic dose of interferon-treated Ad12-transformed tumor cells can afford protection against a subsequent challenge by a tumorigenic dose of untreated Ad12-transformed tumor cells. Similar immunity can also be induced by using cells transfected with the K gene, and the observed protection appears specific to Ad12-transformed cells. Significant protection can be achieved even if immunization is provided subsequent to the tumor challenge. Since increasing numbers of human tumors have been found to have reduced levels of MHC class I antigens, the prospect of therapy by immunization with the parental tumor cells that have been manipulated to induce class I gene expression offers an attractive experimental model.

Expression of a secreted transplantation antigen gene during murine embryogenesis

We examined the midgestation mouse embryo for transcripts related to the secreted transplantation antigen Q10 and show here that this gene is transcribed in the endoderm of the visceral yolk sac. Its level of expression is highest at day 14 and then declines as development proceeds. Concurrently with the decrease in yolk sac expression, the amount of transcripts accumulating in the fetal liver increases during late embryogenesis.

A transcriptional enhancer and an interferon-responsive sequence in major histocompatibility complex class I genes

The major histocompatibility complex class I antigens play an indispensable role in cell-cell interactions. Perturbation of their expression has been shown to have deleterious physiological consequences, including the escape of transformed cells from immune detection. In an attempt to understand how class I genes are regulated, we dissected the Ld gene to identify potential control regions. By using a test vector containing the simian virus 40 early promoter placed upstream of the bacterial chloramphenicol acetyltransferase (cat) gene, we demonstrated the presence of a transcriptional enhancer within the 5'-flanking region. The sequence is functional in both orientations and has been mapped within 350 base pairs upstream of the Ld transcriptional start site. Although human adenovirus 12 can suppress endogenous class I genes, it cannot down-regulate the activity of the transiently transfected cat gene which has been placed under the control of the Ld enhancer and promoter. Our results suggested that if the human adenovirus 12-induced function regulates the expression of class I genes by a trans mechanism, then its target site must not be within 1.9 kilobases of the 5'-flanking region. Treatment of cells with interferon increases the accumulation of class I transcripts. Expression of the cat gene under the control of the Ld enhancer and promoter also can be up-regulated by interferon. Our study shows that the target sequence required for this enhancement resides, at least in part, within the same 350-base pair segment which contains the transcriptional enhancer.

Interactions between liver allografts and lymphocytotoxic alloantibodies in inbred rats

Several clinical and experimental findings suggest that liver allografts are less sensitive than other organ allografts to lymphocytotoxic antibodies. In this experimental study in hypersensitized inbred rat recipients, rejection of liver allografts was delayed compared to that of heart allografts. Furthermore, there was a marked decrease in the level of cytotoxic antibodies after liver allografting but not after heart allografting in these animals. The decrease in the level of antibodies also occurred after donor-specific extracorporeal liver hemoperfusion in hypersensitized recipients. Whether the decrease was caused by a massive absorption of antibodies on the liver or related to excretion of major histocompatibility complex antigens in a soluble form remains to be demonstrated. These results support the hypothesis that the liver has a privileged position in regard to rejection and are consistent with clinical observations made following ABO incompatible or cross-match positive liver transplantations.

A deletion in a rat major histocompatibility complex class I gene is linked to the absence of beta 2-microglobulin-containing serum molecules

Class I major histocompatibility antigens are composed of a heavy chain that is noncovalently associated with beta 2-microglobulin (beta 2m). Most class I molecules are membrane bound, but mouse and rat cDNA clones and genes without a functional code for the transmembrane amino acids have been identified. The membrane-associated class I molecules are important in the control of cell-mediated cytotoxicity, while the function of the soluble molecules remains unclear. Previous studies have shown that beta 2m circulates in rat serum in three different molecular weight classes. The first is free beta 2m (Mr, 12,000), the second is about Mr 70,000, and the third is roughly Mr 200,000. In an inbred subline of immunodeficient, diabetes-prone BioBreeding rats (BioBreeding/Hagedorn), previous work detected two restriction fragment polymorphisms in class I major histocompatibility complex genes, one of them a gene deletion on a 7-kilobase BamHI fragment and the other on a 2-kilobase BamHI fragment. In these rats we have found that the third serum beta 2m-binding size class is absent. Analysis of F1 and F2 individuals following cross-breeding between BioBreeding/Hagedorn rats and genetically related (nondiabetic) control BioBreeding w-subline rats demonstrated that the large-size serum peak of beta 2m was associated with the presence of the class I restriction fragments.

Genetic engineering of an H-2Dd/Q10b chimeric histocompatibility antigen: purification of soluble protein from transformant cell supernatants

We have constructed a recombinant class I gene in which 5' sequences of H-2Dd are linked to the 3' half of a Qa subregion gene, Q10b. This hybrid gene would be expected to direct the synthesis of a protein containing the N and C1 domains of H-2Dd covalently linked to the C2 domain of the secreted, nonpolymorphic, Q10b antigen. Following DNA-mediated gene transfer into mouse L cells, transformants were analyzed by radiolabeling and immunoprecipitation. These cells secreted a molecule reactive with anti-H-2Dd monoclonal antibodies that identify epitopes on the N and C1 domains as well as with an anti-Q10 carboxyl-terminal peptide antiserum. The H-2Dd-derived antigen is associated with beta 2-microglobulin and is readily purified in milligram amounts from culture supernatants by immunoaffinity chromatography.

A soluble class I molecule analogous to mouse Q10 in the horse and related species

Horse serum is shown to contain a soluble class I molecule analogous to the secreted Q10 molecule in the mouse. This molecule has several similarities to the recently described mouse Q10 molecule: it is smaller than membrane-bound equine class I molecules; it occurs in a high molecular mass complex of 200-300 kd in serum; and the serum levels of the equine molecule are similar to that of the Q10 molecule (about 30 micrograms/ml). A soluble molecule is also detected in the sera of species related to the horse; it has in fact been found in all the wild members of the order Perissodactyla so far tested. However, it was not detected in the serum of members of the orders Carnivora, Sirenia, Proboscidea, Artiodactyla, and Primates that were tested, nor in the serum of members of the order Rodentia other than in that of the genus Mus.

Organization and evolution of D region class I genes in the mouse major histocompatibility complex

Chromosome walking has been used to study the organization of the class I genes in the D and Qa regions of the MHC of the BALB/c mouse and in the D region of the AKR mouse. Five and eight class I genes are found in the D and Qa regions of the BALB/c mouse, respectively, while the AKR mouse contains only a single class I D region gene that has been identified by transfection as the Dk gene. Restriction map homologies and crosshybridization experiments suggest that the multiple class I genes in the D region of the BALB/c mouse have been generated by unequal crossing-over involving class I genes from the Qa region. The expanded D region of BALB/c and other H-2d haplotype mouse strains appears to be metastable, since evidence for gene contraction in the Dd region has been found in two mutant strains. Thus the D region and also the Qa region class I genes are in a dynamic state, evolving by gene expansion and contraction.

Secretion of HLA-A and -B antigens via an alternative RNA splicing pathway

Human class I major histocompatibility antigens (HLA-A, -B and -C) are integral membrane protein heterodimers, which are anchored in the membrane via a stretch of hydrophobic amino acids near the carboxyl terminus of the heavy chain. It has previously been shown that a mutagenized cell line secretes a water soluble form of the HLA-A2 antigen, due to a pattern of RNA splicing that removes exon 5 (encoding the transmembrane hydrophobic amino acids) from mature, HLA-A2--encoding transcripts. The present study was undertaken to assess whether a similar process might be operative in nonmutagenized cells. It is shown that water soluble class I molecules (primarily HLA-A24) are secreted by the T leukemic cell line HPB-ALL, and that alternative splicing removes exon 5 from a fraction of HLA-A24--encoding transcripts. It is further shown that class I molecules are secreted, possibly in an allele-specific fashion, from a variety of tumor cells and normal cells. The possible relationship between these findings and previous reports of HLA-A and -B antigens in human serum is discussed.

Mitogenic activity of the adenovirus type 12 E1A gene induced by hormones in rat cells

Several lines of rat 3Y1 cells in which expression of the adenovirus type 12 E1A gene can be regulated by dexamethasone were established by introduction of recombinant vector DNA containing the adenovirus type 12 E1A gene placed downstream of the hormone-inducible promoter of mouse mammary tumor virus. These cell lines (gMA cells) produced low basal levels of the E1A transcripts and proteins in normal medium and much higher levels upon addition of dexamethasone to the medium. When dexamethasone was added to density-arrested cells, DNA synthesis was induced in 10 to 40% of the cells, the percentage depending on the cell line. DNA synthesis was increased to up to 60% of the cell population by further addition of epidermal growth factor. Indirect immunofluorescence detection of E1A proteins in gMA cells treated with dexamethasone indicated that the intensity of fluorescence in cells varied and that the proportion of cells synthesizing DNA was correlated with the proportion that exhibited strong fluorescence. These results indicate that the E1A gene has a function to trigger the synthesis of cellular DNA.

Detection of a serum class I molecule in rat with anti-rat liver beta-2-microglobulin

Rat serum was fractionated on a column of Sephacryl-300 and tested with a rabbit anti-rat beta-2-microglobulin (B2m) antiserum. This antiserum was directed against B2m purified from rat liver, and its specificity was confirmed by immunoprecipitation procedures. The antiserum recognized three peaks in the fractionated rat serum: a 200- to 300-kd (kilodalton) fraction, a 40- to 70-kd component, and the free 12-kd B2m. Indirect immunoprecipitation from the 200- to 300-kd fraction led to the identification of a 43-kd polypeptide associated with B2m. A xenoantiserum against RT1 class I antigen also precipitated a similar polypeptide from the same fraction, but this molecule differed in size and antigenic specificity from the one precipitated by anti-rat B2m.