Qa gene expression: biosynthesis and secretion of Qa-2 molecules in activated T cells

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.

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.

A physical map of the Q region of B1O.P

The murine class I MHC Q region is part of a large complex multigene family whose members have various peptide binding functions. The structure of the Q region is complex, varying extensively in the b, d, k, and q haplotypes so far examined. To better understand the structural heterogeneity, we examined the Q region of B 10.P, a strain whose immunological characteristics are distinct from other haplotypes. A total of 89 cosmids were isolated from genomic DNA. The B 10.P Q region was found to contain seven genes in a 190-kb cluster linked to DP and two additional Q genes in a separate 55-kb cluster. The gene arrangement in this haplotype was unique and did not correspond to any other haplotype; this underscores the complexity of chromosomal structure in this region. In addition to the Q region clusters, Tla region was tentatively aligned in five clusters spanning approximately 300 kb. One 37-kb M region cosmid was also identified.

Dimerization of soluble major histocompatibility complex-peptide complexes is sufficient for activation of T cell hybridoma and induction of unresponsiveness

Major histocompatibility complex (MHC) class I molecules are cell-surface proteins that present peptides to CD8+ T cells. These peptides are mostly derived from endogenously synthesized protein. Recombinant, soluble MHC class I molecules were produced, purified, and loaded homogeneously with synthetic peptide. These MHC-peptide complexes were used to activate a T cell hybridoma. While monomers of MHC-peptide bound to the T cell, they showed no stimulatory activity. Dimers fully triggered the T cell hybridoma to secrete interleukin 2. This response was followed by a state in which the T cell was refractory to restimulation as a result of defective signal transduction through the T cell receptor.

Alternative splicing of class Ib major histocompatibility complex transcripts in vivo leads to the expression of soluble Qa-2 molecules in murine blood

Class Ib Qa-2 molecules are expressed in tissue culture cells as approximately 40-kDa membrane-bound, glycophosphatidylinositol-linked antigens and as approximately 39-kDa soluble polypeptides. Recently, alternative splicing events which delete exon 5 from a portion of Qa-2 transcripts were demonstrated to give rise to truncated secreted Qa-2 molecules in transfected cell lines. To determine whether this mechanism operates in vivo and to find out whether Qa-2 can be detected in soluble form in circulation, murine blood samples were analyzed. Critical to these experiments was preparation of an anti-peptide antiserum against an epitope encoded by a junction of exon 4 and exon 6. We find that supernatants of splenocytes cultured in vitro as well as serum or plasma contain two forms of soluble Qa-2 molecules. One form corresponds to a secreted molecule translated from transcripts from which exon 5 has been deleted; the other is derived from membrane-bound antigens or their precursors. The levels of both soluble forms of Qa-2 are inducible upon stimulation of the immune system, suggesting an immunoregulatory role for these molecules or for the mechanism leading to the reduction of cell-associated Qa-2 antigens in vivo.

Antigenic heterogeneity of Qa-2 antigens in C57BL/6

The Qa-2 antigens are class I-like molecules encoded by genes mapped telomeric to the H-2D region on chromosome 17 in the mouse. A panel of 8 new monoclonal anti-Qa-2 antibodies derived from a C3H.KBR anti-C3H. SW immunization was studied. Immunoprecipitation of 125I-labeled C57BL/6 splenocyte antigens showed that all of these antibodies precipitated 40 kDa molecules which could be completely precleared by the monoclonal antibody 20-8-4, which had previously been shown to crossreact with Qa-2. One of the monoclonal antibodies (1-12-1), however, was found not to completely preclear Qa-2 antigens precipitable by the other 7 antibodies or by 20-8-4, suggesting the existence of at least two different species of Qa-2 molecules. Cell lines transfected with Q7 or Q9 genes were reactive with all 9 antibodies and the Qa-2 antigens expressed on surface membranes of these cells were completely precleared by both 20-8-4 and 1-12-1. Therefore, the observed heterogeneity of these molecules cannot be explained by an antigenic difference between the Q7 and Q9 gene products. 2D gel analyses showed identical pI spectra between Qa-2 molecules precipitated with 20-8-4 and 1-12-1. In addition, all of the monoclonal antibodies reacted with labeled antigen preparations following treatment with Endo F or neuraminidase, indicating that carbohydrate moieties are probably not responsible for the antigenic difference between the two species of Qa-2 antigen.

Signals delivered via the Qa-2 molecule can synergize with limiting anti-CD3-induced signals to cause T lymphocyte activation

Qa-2 is a glycolipid anchored, MHC encoded class I molecule expressed at high levels on all murine peripheral T lymphocytes. Anti-Qa-2 antibodies have previously been found to stimulate T cells to proliferate in the presence of crosslinking antibody and PMA. We have examined the effect of anti-Qa-2 antibodies on T cells stimulated with a suboptimal concentration of immobilized anti-CD3. When anti-Qa-2 antibodies were co-immobilized with limiting anti-CD3, in the absence of PMA, a clear augmentation of T cell proliferation was seen. Interestingly, the co-stimulatory anti-Qa-2 antibodies could be directed against epitopes mapped to either the alpha 3 or the alpha 1/alpha 2 Qa-2 domains. As was the case with activation induced by soluble/crosslinked anti-Qa-2 antibodies plus PMA, CD8+ T cells were less able to be costimulated with anti-Qa-2 antibodies than CD4+ cells. Surprisingly, Ca2+ mobilization was only seen when two anti-Qa-2 antibodies reactive to separate structural domains were co-crosslinked on the surface of Indo-1 loaded T cells with a suboptimal concentration of anti-CD3. Collectively these results raise questions regarding the mechanism of Qa-2 mediated signaling and its potential role in T cell activation.

Heat shock proteins can regulate expression of the Tla region-encoded class Ib molecule Qa-1

T cells recognize foreign antigens in association with the highly polymorphic class I and class II molecules encoded in the major histocompatibility complex (MHC). In addition to these highly polymorphic molecules, the murine MHC also encodes, in the Qa/Tla region, several less polymorphic structures referred to as class I-like or class Ib molecules. Although no specific function has been assigned to these molecules, their overall structural similarities to the classical class I molecules and their association with beta 2-microglobulin suggest a role in antigen recognition. Recent data have suggested that the class Ib molecule Qa-1 may be involved in antigen presentation to T cells expressing gamma delta receptors. In addition, several reports have demonstrated that gamma delta T cells can respond to mycobacterial heat shock proteins. We report that transfection of a mouse fibroblast line with gene T23b leads to the surface expression of a molecule that is structurally identical to lymphocyte Qa-1b. In the transfected cells the predominant Qa-1 species was present in an immature intracellular form. The expression of mature cell surface Qa-1 was dramatically and selectively increased following heat shock. Furthermore, the addition of a tryptic digest of Mycobacterium bovis 65-kDa heat shock protein stabilized the surface expression of Qa-1b. These observations suggest that the Qa-1 molecule may be involved in the presentation of heat shock protein-derived peptides to the immune system.

Expression and regulation of Q8b in a transfected cell line

RNase protection experiments showed that Q8b was actively transcribed in a stably transfected cell line. Moreover, Q8b responded to interferon-gamma (IFN-gamma) treatment with increased levels of mRNA expression. Thus Q8b demonstrates a regulatory response to IFN-gamma characteristic of many other class I genes. Cell surface expression of a Q8b product could also be detected by flow cytometric analysis with the Qa-2-specific monoclonal antibody D3.262. The expression of the Q8b cell surface product increased only slightly after cells were treated with IFN-gamma. The Q8b cell surface product was not sensitive to cleavage by phosphatidylinositol-phospholipase C. These results suggest that the Q8b product, unlike the predominant forms of Qa-2-bearing molecules, is not anchored via phosphatidylinositol to the cell membrane. These results also suggest that Q8b has the potential to contribute to the Qa-2 phenotype in vivo.

MHC gene Q8/9d of the BALB/cJ mouse strain cannot encode a Qa-2,3 class I antigen

We have determined the nucleotide sequence of Q8/9d gene of the BALB/c strain of mice, isolated from Steinmetz's cosmid library. As for all other class I genes of the Qa region, the Q8/9d gene spans approximately 4.7 kilobases (kb) and consists of seven exons and six introns. A seven bases deletion in exon 3 results in the occurrence of an early termination codon. Thus the Q8/9d gene cannot encode a normal class I protein. Comparison of the nucleotide sequence of the Q8/9d gene with that of other class I MHC genes revealed a stronger homology to Q7 and Q8 than to K, D, L, TL, and other Q genes. However, the gene cannot originate from a mere fusion between Q8 and Q9 genes except if the ancestor to putative Q8d was markedly different from the present Q8b gene. Using polymerase chain reaction (PCR) technology, we have confirmed the presence of a Q8/9 gene, identical to that present in cosmid 46.1, in the genome of BALB/cJ (Qa-2low). Finally, it has been reported that cDNA clone 94-A, which codes for a Qa-2 antigen, could derive from a transcript of gene Q8/9d. The nucleotide sequences of gene Q8/9d and of cDNA clone 94-A are distinctly different in their 5' regions, in spite of an almost perfect matching in their 3' regions. Thus, clone 94-A cannot derive from an mRNA transcribed from the Q8/9d gene.

Biochemical nature of an Fc mu receptor on human B-lineage cells

An IgM-binding protein of approximately 60 kD has been identified on activated B cells, but not on resting and activated T cells, monocytes, or granulocytes. Here, we characterize this IgM-binding protein as a receptor for the Fc portion (CH3 and/or CH4 domains) of IgM molecules (Fc microR). The Fc microR can be expressed as a cell surface activation antigen throughout the pre-B and B cell stages in differentiation. Receptor expression is not directly linked with IgM production, as both mu- pre-B cells and isotype-switched B cells may express the Fc microR. The receptor molecules produced by both pre-B and B cells are identical in size and are characterized as an acidic sialoglycoprotein with O-linked, but no N-linked, oligosaccharide. The Fc microR is anchored to the surface of B-lineage cells via a glycosyl phosphatidylinositol linkage. The Fc microR is thus the third member of a family of Fc receptors expressed on B-lineage cells, and its preferential expression on activated B cells suggests a potential role in the response to antigens.

Biochemical differences in Qa-2 antigens expressed by Qa-2+,6+ and Qa-2+,6- strains. Evidence for differential expression of the Q7 and Q9 genes

Cell surface forms of Qa-6 class I molecules are biochemically indistinguishable from Qa-2 although Qa-6 maps telomeric to Qa-2 with the recombinant strain B6.K2. Analysis of appropriate F1 strains did not demonstrate the presence of a trans acting factor that could modify the Qa-2 molecule to produce the Qa-6 determinant. Also, neither a neighboring cell surface molecule nor oligosaccharides were found to block the recognition of the Qa-6 determinant in Qa-2+,6- strains. The 2-D gel profiles of neuraminidase or endoglycosidase treated anti-Qa-2 immunoprecipitates from lysates of cell surface iodinated Qa-2+,6+ strains revealed an additional basic polypeptide which was absent from that of Qa-2+,6- strains. Thus, differential sialylation/glycosylation of Qa-2 molecules masks detection of Qa-2 antigen heterogeneity when cell surface forms are analyzed. Qa-6+ phenotype associated polypeptides were also found at various stages of post-translational processing in cells metabolically labeled in the presence and absence of tunicamycin. Northern analyses using Q7 and Q9 specific oligonucleotide probes revealed appropriate sized transcripts for both genes in the Qa-2+,6+ strain B6 but only Q9 in the Qa-2+,6- strain B6.K2. These data demonstrate that there is structural heterogeneity in Qa-2 antigens expressed by Qa-2+,6+ and Qa-2+,6- strains which results from differential expression of the Q7 and Q9 genes.

Genetic polymorphisms of Q region genes from wild-derived mice: implications for Q region evolution

Both serological and DNA sequence analyses were performed to determine the extent of genetic polymorphism in Q region genes. A panel of Qa-2-specific monoclonal antibodies (mAbs) was tested on 35 wild-derived and inbred mouse strains. Members of this reagent panel recognize multiple and distinct epitopes on the Qa-2-bearing molecule(s). Although quantitative variations in Qa-2 levels were observed, no structural polymorphisms were detected. All strains were either entirely positive or entirely negative with the complete set of reagents. Moreover, cell surface Qa-2 expression was not significantly affected by differences in age or sex of the mouse or cell cycle status. To confirm this apparent lack of genetic polymorphism, the polymerase chain reaction (PCR) technique was used to amplify a portion of the 3' end of the Q region genes, Q4 to Q9, from several independent wild-derived strains of mice. Sequence analysis of the amplified material revealed very little evidence of nucleotide divergence. All strains tested had a Q even DNA sequence identical to that of Q6/Q8 in the B10 strain. Likewise, all tested strains had a Q odd DNA sequence identical to Q7/Q9 in the B10 strain. Two strains showed additional Q even sequences, while all strains tested possessed additional Q odd sequences. The observed lack of polymorphism suggests that the Q genes have evolved in a different manner from H-2K and H-2D. Moreover, duplications of these genes appear to have arisen prior to nucleotide sequence divergence.

In vivo effects of monoclonal antibodies to distinct epitopes of Qa-2 antigens

The effects of in vivo treatment with anti-Qa-2 mAbs on in vivo and in vitro parameters of T cell immunity have been examined. Two anti-Qa-2 mAbs of the same isotype and with similar avidities but directed against distinct epitopes of the same Qa-2 molecules were studied. mAb 1-1-2 was found to induce rapid T cell depletion, with maximal effect observed within 2-3 d, while administration of mAb 1-9-9 caused little or no depletion in the first few days, and reached maximal effect only by day 8. Surprisingly, administration of both antibodies resulted in a depletion pattern similar to that of the nondepleting antibody 1-9-9. Consistent with these effects on T cell depletion, treatment with 1-1-2 caused significant prolongation of survival of allogeneic skin grafts placed 1 d after antibody administration, while treatment with 1-9-9 or with the combination of both antibodies caused no prolongation. In an attempt to determine the mechanism of this phenomenon, we examined Qa-2 expression on the cell surface by flow microfluorometry after treatment with each of the two mAbs. Our data indicate that mAb 1-9-9 mediates significantly greater modulation of Qa-2 expression from the surface of peripheral T cells within 1 d than does mAb 1-1-2. Apparently, therefore, modulation occurs more rapidly than cell clearance, and the efficiency of T cell depletion and consequent immune suppression is correlated inversely with the ability of each mAb to cause modulation. The ability of 1-9-9 to cause Qa-2 modulation suggests that it may react with a determinant on this molecule of physiological relevance to the natural ligand interactions of Qa-2 antigens.

Expression of the Q8/9d gene by T cells of the mouse

The Q genes, specifying Qa antigens and situated in the extended part of the major histocompatibility complex (MHC) of the mouse, comprise a subgroup of MHC class I genes whose significance and function are still largely unknown. In screening a cDNA library made from the BALB/c inducer T-cell line Cl.Ly1-T1, we isolated 11 clones representing Q8/9, but none representing Q6 or Q7. Confirmatory evidence is given that the Q8/9 gene originated from fusion of the 5' region of the Q8 gene with the 3' region of the Q9 gene at a recombination site or hot spot in the vicinity of intron 4. Contrary to previous impressions that Q8/9 is an inert pseudogene, we find that the Q8/9 gene can be functional and encode a Qa-2, 3 antigen. One variety of the 11 Q8/9 clones isolated lacked exon 5, which encodes the transmembrane domain of class I glycoproteins, and thus may account for secretion of a soluble form of Qa-2, 3 antigen thought to be released by activated T cells.

Qa-12--a novel determinant of activated T and B lymphocytes

Investigation of the antigenic phenotype of activated lymphocytes using the broadly cross-reactive mAb 6.3.4 revealed two phenotypic alterations as compared with the resting lymphocytes. The Qa specificity Qa-m208 disappears after lectin activation of Qa-m208-positive T lymphocytes. Analysis of Q7 and Q9 transfectants expressing the Qa-2 polypeptides shows that Qa-m208 is an epitope of the Qa-2 antigen. Because the Qa-2 antigens are still expressed on T lymphoblasts which have lost Qa-m208, changes of the Qa-2 molecules occur and result in the loss of certain epitopes. The second phenotypic change that we observed is the appearance of a novel specificity, Qa-12. Its expression is induced by lymphocyte activation and it is expressed on lymphoblasts of both T and B cell origin. The presence of this novel non-ubiquitous antigenic specificity is determined by the Tla region.

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).

Regulation of Qa-1 expression and determinant modification by an H-2D-linked gene, Qdm

We examined the regulation of cell surface expression of the Qa-1 alloantigens using a panel of monoclonal anti-Qa-1 cytotoxic T-lymphocyte (mCTL) lines. In contrast to previous reports of tissue-specific expression, we found that Qa-1 was widely expressed, resembling the prototypical class I H-2K/D molecules. We further found that an H-2D-linked gene, which we termed Qdm for Qa-1 determinant modifier, controlled expression of certain CTL-defined Qa-1 antigenic determinants. H-2Dk homozygous haplotypes expressed a recessive allele of the modifier, Qdmk, whereas all other H-2 haplotypes tested expressed a dominant allele, Qdm+. The Qdm+ allele regulated in trans Qa-1 epitope expression from a Qdmk chromosome, modifying expression of particular CTL-defined Qa-1 antigenic determinants rather than affecting levels of cell surface expression. Mechanisms of Qdm function may include either a novel protein modification system or an unprecedented case of antigen recognition restricted by a nonclassical major histocompatibility complex molecule.

Qa-region class I gene expression: identification of a second class I gene, Q9, encoding a Qa-2 polypeptide

A feature of the expression of the tissue-specific class I antigen Qa-2 is the quantitative variation among mouse strains. Recently, the class I gene Q7 has been shown to encode a protein product that is biochemically indistinguishable from the lymphocyte-bound Qa-2 molecule. Utilizing gene transfection, we have identified a second Qa-2 subregion class I gene (Q9), in H-2b mice, which encodes a polypeptide biochemically similar to the Q7 and the Qa-2 polypeptides. Furthermore, we have observed that cell lines transfected with the allelic forms of the Q7 gene from C57BL/10 (Qa-2hi) or BALB/C (Qa-2low) display quantitative differences in cell-surface expression. Based on these studies, we suggest that gene dosage and allele-specific variation in cell-surface expression contribute to the strain-specific variation in the levels of Qa-2 antigen expression.

Sequence of the mouse Q4 class I gene and characterization of the gene product

The Q4 class I gene has been shown to participate in gene conversion events within the mouse major histocompatibility complex. Its complete genomic nucleotide sequence has been determined. The 5' half of Q4 resembles H-2 genes more strongly than other Q genes. Its 3' end, in contrast, is Q-like and contains a translational stop signal in exon 5 which predicts a polypeptide with an incomplete membrane spanning segment. The presence of two inverted B1 repeats suggests that part of the Q4 gene may be mobile within the genome. Gene transfer experiments have shown that the Q4 gene encodes a beta 2-microglobulin associated polypeptide of Mr 41,000. A similar protein was found in activated mouse spleen cells. The Q4 polypeptide was found to be secreted both by spleen cells and by transfected fibroblasts and was not detectable on the cell surface. Antibody binding and two-dimensional gel electrophoresis indicate that the Q4 molecule is identical to a mouse class I polypeptide, Qb-1, which has been previously described.

Structural and functional roles of glycosyl-phosphatidylinositol in membranes

Glycosylated forms of phosphatidylinositol, which have only recently been described in eukaryotic organisms, are now known to play important roles in biological membrane function. These molecules can serve as the sole means by which particular cell-surface proteins are anchored to the membrane. Lipids with similar structures may also be involved in signal transduction mechanisms for the hormone insulin. The utilization of this novel class of lipid molecules for these two distinct functions suggests new mechanisms for the regulation of proteins in biological membranes.

Molecular mapping of signals in the Qa-2 antigen required for attachment of the phosphatidylinositol membrane anchor

Proteins anchored in the membrane by covalent linkage to phosphatidylinositol (PtdIns) can be released by treatment with purified PtdIns-specific phospholipase C (Ptd-Ins-PLC). A recent survey of leukocyte antigens using flow cytometry has shown that staining of certain Qa antigens was diminished after PtdIns-PLC treatment, but staining of structurally related H-2 antigens was not affected. Therefore, in this study, the sensitivity of cell-surface Qa-2, H-2Kb, and H-2Db to hydrolysis by PtdIns-PLC was investigated biochemically by immunoprecipitation of radioiodinated molecules from cell lysates or supernatants. Qa-2, but not H-2Kb, was released from the surface of PtdIns-PLC-treated C57BL/10 mouse spleen cells and recovered in the cell supernatants. Similar analysis of thymoma cells transfected with cloned C57BL/10 genes showed that cell-surface Qa-2 molecules encoded by a Q7b cDNA and the Q7b or Q9b gene were sensitive to hydrolysis by PtdIns-PLC, whereas the H-2Kb and H-2Db gene products were resistant. Using thymoma cells transfected with hybrid genes constructed by exchanging exons between Q7b and H-2Db, the signals for PtdIns modification were localized to a defined region of Qa-2. This region differs from H-2Db most significantly by the presence of a central aspartate residue in the transmembrane segment and in the length of the cytoplasmic portion.

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.

Tissue-specific expression of cell-surface Qa-2 antigen from a transfected Q7b gene of C57BL/10 mice

We screened a cDNA library prepared from a BALB.B10 CTL clone that expresses Qa-2 antigen, and isolated four clones derived from Q7b, a Qa region gene of C57BL/10. One of these Q7b cDNAs and the Q7b chromosomal gene were subcloned into expression vectors and transfected into L cells and R1.1 thymoma cells. We found that the chromosomal Q7b gene expresses Qa-2 on the surface of R1.1 cells, but not on L cells while the Q7b cDNA expresses protein on the surface of both cell types. The levels of Qa-2 expression do not correlate with the total levels of Q7b mRNA in these transfectants. Our results suggest that the tissue-specific expression of Qa-2 may be controlled, in part, by mechanisms of alternate RNA splicing. By using hybrid gene constructs, we have mapped the tissue-specific element to the 3' part of the gene, downstream of a site near the middle of exon 4. The hybrid polypeptides differ significantly in their transmembrane and cytoplasmic regions. These portions of the protein also may play a role in the tissue-specific expression of Qa-2.

Molecular complexity of Qa-2 antigens demonstrated by two-dimensional gel electrophoresis

Biosynthetically labelled Qa-2 antigens were isolated from mouse spleen cells by immunoprecipitation with anti-Qa-2 antisera. When newly synthesized Qa-2 molecules from several different inbred strains were analysed by two-dimensional (2D) gel electrophoresis; four different phenotypes were observed that differed in the number of polypeptides present. The ability to distinguish Qa-2+ phenotypes was used to map the recombination points in two congenic strains, B6.Tlaa and A.Tlab. No alternative Qa-2-like polypeptides were detected in B6.K1 (Qa-2-) cells using a polyspecific rabbit antiserum against mouse class I antigens, but a new molecule was detected in BALB/cBy (Qa-2-) cells. Pulse-chase and surface-labelling experiments showed that some, but not all, of the newly synthesized Qa-2 precursor forms are processed to mature cell surface molecules.

Two different biosynthetic pathways for the secretion of Qa region-associated class I antigens by mouse lymphocytes

Treatment of supernates of labeled C57BL/6 mouse lymphocytes with antibodies against beta 2-microglobulin reveals the presence of two different soluble class I molecules. One molecule (Mr, 37,000) is found in supernates of both 125I surface-labeled and [35S]methionine biosynthetically labeled cells and reacts with antibodies against Qa-2 antigens. The other molecule (Mr, 42,000) is found labeled only in supernates of [35S]methionine-labeled cells and reacts with antibodies against Qb-1. Analysis of mutant and recombinant mouse strains demonstrates that both soluble class I molecules are encoded in the Qa region. Pulse-chase experiments show that the Qa-2 molecules are released more slowly than Qb-1. It is proposed that Qb-1 molecules are secreted directly, whereas Qa-2 is first expressed on the cell surface and then processed to a soluble form.

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.

Qa alloantigen expression on functional T lymphocytes from spleen and thymus

The cell-surface expression of the class I alloantigen Qa-2 was analyzed on resting and activated spleen and thymus cells using cytotoxic elimination and immunofluorescence and flow cytometry. Spleen cells activated by mitogens or alloantigen were homogeneously positive for cell surface Qa-2, but activated splenic T cells expressed only about one-third as much Qa-2 per cell as did nonstimulated T cells. These data correlated with the ability to perform cytotoxic elimination with Qa-2-specific monoclonal antibodies (mAbs) in that cytotoxic T lymphocyte (CTL) activity was completely abrogated by pretreatment of spleen cells prior to in vitro culture but was only partially eliminated by treatment of CTL effectors. Qa-2-positive cells constituted only a small subpopulation of fresh normal thymocytes, but were enriched (greater than 40% positive) among cortisone-resistant thymocytes (CRT). These Qa-2-positive CRT contained mature thymocytes as defined by Ly phenotype Ly-2-, Ly-1hi. When normal thymocytes were treated with Qa-2-specific mAb and complement prior to in vitro sensitization for generation of allogeneic CTL, CTL activity was completely abrogated despite the fact that the fraction of cells eliminated were undetectable as assessed by cell recovery. CTL effectors from alloantigen-stimulated thymocytes were also susceptible to cytotoxic elimination with Qa-2-specific mAb. These data suggest that the Qa-2 molecule may serve not only as a marker on resting and activated peripheral T cells, but also as a unique marker for functionally mature T cells in the thymus.