Qa antigens and their differential distribution on lymphoid, myeloid and stem cells

Biochemical characterization of activation-associated bovine class I major histocompatibility complex antigens

Utilizing a 'sandwich' ELISA assay we have been able to demonstrate that mAb W6/32, B1G6 and IL-A19 are reactive with three different monomorphic determinants on bovine class I major histocompatibility complex (MHC) molecules. Sequential immunoprecipitations performed with the mAb revealed that class I molecules on PBM comprise a single population with respect to reactivity with the mAb in that the beta 2m-associated proteins bear all three epitopes. By contrast, TCGF-driven lymphoblasts and cells transformed by Theileria parva (Tp) additionally express molecules of Mr 45000 bound to beta 2m which are recognized by mAb B1G6 and IL-A19 but not by W6/32. These two subclasses of molecules were further distinguished on the basis that, when tunicamycin was added to cultures in the preparation of cells for analysis, mAb W6/32 precipitated class I heavy chains of Mr 39000 while the extra molecules detected only by mAb B1G6 and IL-A19 were of Mr 37000 and 39000. On thymocytes, the mAb W6/32-non-reactive class I molecules are present in low amounts and are expressed by cells in the medulla area, unlike BoT1 (analogous to human CD1) molecules which are expressed by the cortical cells. Our studies also revealed that the supposed beta 2m-specific mAb B1G6 does not recognize the beta 2m-associated molecules (BoT1) precipitated by mAb TH97A and thus the specificity of mAb B1G6 in cattle is for an epitope on bovine beta 2m which is strongly influenced by the nature of the heavy chain with which the beta 2m is associated.

Expression of a nonclassical MHC class Ib molecule in the eye

BACKGROUND

MHC class Ia molecules are absent, or expressed at low levels, on cells lining the anterior chamber of the eye, an immune-privileged site. Although the scarcity of class Ia MHC antigens may protect cells from T cell-mediated tissue injury, it may also render them vulnerable to natural killer cell-mediated cytolysis. There is growing evidence that MHC class Ib molecules share similar functions to class Ia. In this study, we examine the expression and distribution of Qa-2, one of the best-characterized murine MHC class Ib molecules in the eye.

METHODS

The transcription of Qa-2 mRNA in whole eye and eye-derived cells was analyzed by sensitive and specific RNase protection and reverse transcription-polymerase chain reaction assays. Immunohistochemistry, flow cytometry, and ELISA were used to determine whether Qa-2 was expressed as cell surface proteins. Expression levels of Qa-2 were monitored in resting cells and cells stimulated with interferon-gamma.

RESULTS

Expression of membrane-bound and soluble Qa-2 isoforms was detected in various tissues of the eye, including cell subsets lining the anterior chamber. Immunohistological staining revealed Qa-2 expression on corneal epithelium as well as endothelium, iris ciliary bodies, and retina. These observations were confirmed by analysis of cultured, eye-derived cells. Qa-2 expression was inducible by interferon-gamma. Qa-2 was not detected in lens cells.

CONCLUSIONS

The results demonstrate that membrane-bound and soluble MHC class Ib molecules are expressed by eye cells. Expression of Qa-2 in the corneal endothelium and other substructures lining the anterior chamber suggests that this class Ib protein may contribute to the immune-privileged status of the anterior chamber.

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.

Unusual expression pattern of the MHC class I Q5k gene in the AKR mouse: possible role in embryo development

Analysis of the tissue-specific expression of the Q5k gene in the AKR mouse reveals an unusual expression pattern. The Q5k mRNA is present in embryos from day 12, but expression is switched off in most tissues except thymus and testis shortly after birth. Late in pregnancy the gene is again transcribed in females. Analysis at the epitope level, with a Qa-2 specific monoclonal antibody revealed that in most cases the Q5k product is confined to the cytoplasm. These results suggest that Q5k has a most unusual tissue distribution and timing of expression among all the H-2 class I and Q genes so far described.

The MHC class I associated beta 2-microglobulin (beta 2m) light chain is expressed in a molar excess over HLA-ABC and CD1 on the membrane of leukaemic B cells but not leukaemic T cells: evidence for further beta 2m-associated molecules

Beta 2-microglobulin (beta 2m) constitutes the common light chain of both the MHC-encoded HLA-ABC molecules and a group of structurally related glycoproteins recognized by antibodies of the first cluster of differentiation (CD1a, CD1b and CD1c). These CD1 antigens appear similar to murine T1 and Qa molecules in terms of structure and tissue distribution, although the question of inter-species homology is controversial. A further group of alloantigens expressed predominantly on T cells has been reported however, with immunogenetic characteristics more closely analogous to the murine T1/Qa system than the CD1 antigens, although their precise identity remains ill-defined. Having previously shown that malignant B cells may express membrane CD1c, we examined leukaemic B-cells corresponding to early lymphoblastic differentiation (null- and common acute lymphoblastic leukaemia) through to the terminal plasma cell stage for the expression of other non-HLA class I beta 2m-associated molecules. It was found that leukaemic B-cells at intermediate/late stages of differentiation, represented by non-Hodgkin's lymphoma (B-NHL) and 'hairy-cell' leukaemia (HCL), had significantly higher beta 2m:HLA-ABC ratios than did the cells from other types of B-cell malignancy. Although leukaemic B cells with a demonstrable non HLA-ABC-associated beta 2m component expressed detectable levels of CD1c, and insignificant levels of CD1a and CD1b, the antigen density was insufficient to account for the excess beta 2m. In vitro stimulation of leukaemic B cells by phorbol ester substantially increased the expression of HLA-ABC and CD1c, but also accentuated further the difference between the expression of these molecules and that of beta 2m. There was no detectable beta 2m other than that associated with HLA-ABC and CD1 on the surface of malignant T cells by contrast. Our findings strongly support the existence, at certain stages of leukaemic B-cell differentiation, of an additional beta 2m component(s) other than that associated with HLA-ABC and CD1.

The regulation and expression of MHC class I genes

The expression of mouse MHC class I genes and their products in vivo reveals complex patterns of regulation. Different promoter elements, which are required for gene activation or modulation in response to various external stimuli, have now been characterized as well as the proteins that bind to them. As described here by Brigitte David-Watine and colleagues, the picture that has gradually emerged from these in vitro studies is of an intricate interplay of transacting factors that ultimately lead to the fine tuning of MHC class I expression in vivo.

The RT1.G locus in the rat encodes a Qa/TL-like antigen

A new antigenic system in the rat homologous to the Qa/TL antigen system in the mouse has been characterized. It was detected by antibodies raised in donor-recipient combinations that were matched for the RT1.A, B, D, E loci in the major histocompatibility complex (MHC): (R11 X BN)F1 anti-BN.1L(LEW), (R18 X BN)F1 anti-BN.1L, and BN.1LV1(F344) anti-BN.1L. Absorption analyses using these antisera and a variety of inbred, congenic, and recombinant strains identified three alleles, RT1.Ga, Gb, Gc, of which Gc is a null allele. The strain distribution of these alleles was determined using 37 strains of rats representative of all of the prototypic haplotypes and a number of congenic and recombinant strains. The use of the congenic and recombinant strains showed that the RT1.G locus was linked to the MHC and that the most probable gene order was A-E-G. Testcross analysis showed that the map distance between A and G was 1.4 cM (4/285 recombinants). The RT1.G antigen has a heavy chain of Mr 46,000 and is present on both T and B cells.

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.

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.