Cis-acting elements determine the locus-specific shutoff of class I major histocompatibility genes in murine S49 lymphoma sublines

Development of a novel monoclonal antibody that binds to most HLA-A allomorphs in a conformation-dependent yet peptide-promiscuous fashion

Specificity analyses of peptide binding to human leukocyte antigen (HLA)-A molecules have been hampered due to a lack of proper monoclonal antibodies (mAbs) for certain allomorphs, such as the prevalent HLA-A1 for Caucasians and HLA-A11 for Asians. We developed a mAb that recognizes a conformational epitope common to most HLA-A allomorphs. The mAb, named A-1, does not discriminate peptides by amino acid sequences, making it suitable for measuring peptide binding. A stabilization assay using TAP-deficient cell lines and A-1 was developed to investigate the specificity of peptide binding to HLA-A molecules. Regarding the evolution of HLA-A genes, the A-1 epitope has been conserved among most HLA-A allomorphs but was lost when the HLA-A gene diversified into the HLA-A*32, HLA-A*31, and HLA-A*33 lineages together with HLA-A*29 after bifurcating from the HLA-A*25 and HLA-A*26 branchs. The establishment of A-1 is expected to help researchers investigate the peptide repertoire and develop computational tools to identify cognate peptides. Since no HLA-A locus-specific mAb has been available, A-1 will also be useful for analyzing the locus-specific regulation of the HLA gene expression.

Transcription of major histocompatibility complex class I (Kb) and transporter associated with antigen processing 1 and 2 genes is up-regulated with age

The transporter associated with antigen processing 1 and 2 (TAP1 and TAP2) genes belong to the ATP-binding cassette family of transporter genes. They provide peptides necessary for the assembly of major histocompatibility complex (MHC) class I molecules by transporting these peptides into the endoplasmic reticulum. As MHC class I protein expression increases with age, we have explored the effect of age on the transcription of MHC class I genes (Kb) and TAP1 and TAP2 genes in C57BL/6 mice. Blood and spleen lymphocytes were isolated from mice aged from 3 months to over 24 months. RNA was extracted and mRNA for Kb, TAP1, TAP2 was quantified using slot-blot hybridization followed by densitometry. There was a parallel age-related increase (1.5-fold) in blood lymphocyte mRNA of these genes from 3 months to 21 months. In mice over 24 months old there was a decrease in Kb and TAP1 mRNA, but an increase in TAP2 mRNA. In spleen lymphocytes an age-related increase in all three mRNA species occurred throughout life. While MHC class I and Tap genes underwent very similar age-related changes, MHC class I mRNA was about 50 times more abundant than either TAP1 or TAP2 mRNA.

Damage in B2m genes and DNA methylation of H-2 genes are involved in loss of expression of class I MHC products on the membrane of LR.4, a cell line derivative of the T-cell lymphoma L5178Y

We have isolated an H-2 deficient cell line (LR.4) from the T-cell lymphoma L5178Y which grew without restrictions in the peritoneal cavity of different inbred strains of mice. The use of polyclonal anti-H-2 antiserum and complement indicated that LR.4 cells did not express class I determinants on the cell membrane. Southern blots of genomic DNA of LR.4 cells showed that B2m genes were severely damaged and that class I H-2 genes were extensively methylated. Consequently, LR.4 cells failed to transcribe mRNAs for both B2m and class I H-2 genes. On the other hand, specific immunity to LR.4 was demonstrated in C57BL/6J mice since, in subsequent challenges with either LR.4 or EL4.4, LR.4 did not grow, whereas EL4.4 grew and killed the mice. In C57BL/6J mice, rejection of LR.4 was accompanied by the production of cytotoxic antibodies. The immune response induced in C57BL/6J mice was determined by non-H-2 antigenic determinants in LR.4 cells.

Impairment of MHC class I transcription in a mutant bovine B cell line

To better define the regulation and expression of bovine major histocompatibility complex (MHC) antigens, the bovine B lymphoblastoid cell line, BL3, was exposed to gamma-irradiation and surviving cells were immunoselected for MHC class I antigen loss. The resulting class I expression loss variant, BL3.1, was characterized at both the protein and genetic levels to ascertain the nature of the defect. Microfluorimetry analysis revealed a 3--5-fold surface density reduction of all class I products on BL3.1 cells relative to the parental BL3 cells. This decreased surface expression was specific for MHC class I and not for MHC class II or the non-MHC-linked gene product, immunoglobulin (Ig). Northern and quantitative slot blot analyses demonstrated a corresponding diminution of class I RNA in BL3.1 suggesting a transcriptional level defect. Nuclear run-off and transcription inhibition experiments confirmed no post-transcriptional changes while Southern blot analysis provided no evidence for alterations within or near the class I genes. To help elucidate the mechanism of altered class I expression, the parent, BL3, and variant, BL3.1, were cultured with factors known to enhance MHC class I transcription. Interferon (IFN)-gamma, lipopolysaccharide (LPS), and activated peripheral blood lymphocyte (PBL) supernatant cultured with both cell lines induced MHC class I transcription and surface expression 2--3-fold greater than the untreated controls. It is likely, therefore, that a genetic alteration outside of the class I genes has occurred within BL3.1 impairing expression of MHC class I.

Identification of locus-specific DNA-binding factors for the regulation of HLA class-I genes in human colorectal cancer

The expression of HLA class-I mRNA in human colorectal cancer cell lines was studied. Locus-specific down-regulation of HLA class-I mRNA could be demonstrated in some of the human colorectal lines. This transcriptional suppression of HLA mRNA, however, was not a result of genetic alterations in the HLA structural genes. The transcription of the HLA class-I genes in the HLA-deficient cell lines could be induced by the addition of human recombinant gamma-IFN. In this report, we have employed these human colorectal cell lines to study locus-specific transcriptional regulation of HLA class-I gene expression. Our results demonstrate that the locus-specific suppression of HLA gene expression in human colorectal cell lines is mediated by the loss of certain DNA-binding transcription factors which act in a locus-specific manner. Our conclusion is further supported by experiments which showed that exogenously introduced HLA-CAT DNA constructs were also regulated in a locus-specific fashion when assayed by in vitro functional assays using these human colorectal cell lines.

Differential up-regulation of H-2D versus H-2K class I major histocompatibility expression by interferon-gamma: evidence against a trans-acting allele-specific factor

Previous investigation described a unique differential phenotype for the murine T-cell tumor AKR SL3 with regard to augmentation of class I major histocompatibility complex antigen expression by interferon-gamma (IFN-gamma). Dk expression was increased by IFN-gamma as expected, but Kk expression remained at constitutive levels, despite treatment with a range of doses and times of exposure to IFN-gamma. Analysis of somatic cell hybrids obtained by fusion of AKR SL3 with an H-2Kb and Db IFN-gamma augmentable partner tumor argued against the involvement of locus-specific, trans-acting factors as the basis for the nonaugmentable nature of the Kk gene in AKR SL3. Here, we provide evidence against the remaining possibility of an allele-specific, negative-acting factor in AKR SL3. Hybrids were constructed between drug-marked sublines of AKR SL3 and the R1.G1 T-cell tumor which carries IFN-gamma augmentable Kk and Dk genes. The uniform ability of IFN-gamma to cause substantial increases in the expression of Kk in hybrid populations and a large number of hybrid clones from three separate fusions indicated that a trans-acting, negative factor was not present in AKR SL3. Rather, these data coupled with Northern analysis were consistent with cis alterations operating at the level of transcription as the basis for Kk nonaugmentation. These results are discussed with regard to the further study of AKR SL3 to better understand IFN-gamma regulation of class I MHC expression.