Identification of two independent MHC class II antigens in a bovine lymphoblastoid cell line

Loss of MHC I transcription trans-activator in the bovine B-LCL, BL3.1

BL3.1, a variant derived from the BLV infected bovine B cell line, BL3, is distinguished by a loss of expression of MHC class I. All surface MHC I products were down-regulated in BL3.1 compared with BL3 correlating with a diminution in MHC I heavy chain transcription. Class I genes, including regulatory elements, showed no aberrations. The variant, BL3.1, did not differ from the parent cell line in expression of Bovine Leukemia Virus (BLV) or oncogene, c-myc. Transient transfection experiments determined the defect was trans rather than cis, and was due to loss of a trans-activator rather than gain of a trans-suppressor as evidenced by transient heterokaryon fusions. Southwestern blot analysis indicated that two DNA binding proteins associated with the MHC class I promoter were missing in BL3.1 cells. The specific response elements for these proteins in BL3 did not appear to be within the enhancerA region, the major enhancer region of the MHC I promoter.

CD8+ lymphocytes that kill allogeneic and xenogeneic major histocompatibility complex class I targets

CD8+ CTLs generated in a two-way MLR should lyse target cells only if these targets share a class I MHC allele with the original stimulators. Using cattle PBMCs in a two-way MLR, we generated CD8+ CTLs that kill allogeneic and xenogeneic cell lines. We have named these cells MLK cells. PBMCs isolated from two unrelated animals were cultured together. After 14 days microfluorimetry analysis was performed on the MLK cells with results showing > 90% CD8+ cells. RFLP analysis revealed these cells to be predominately of one animal. MLK cells were then used as effector cells in cytotoxicity assays with syngeneic, allogeneic, and xenogeneic target cells. MLK cells were able to kill all targets. Incubating MLK cells with mAbs to CD8 markedly reduced killing, suggesting a TCR-mediated cytolytic pathway. Effective cytolysis of these targets by MLK cells was dependent on class I expression. MHC class I expression-impaired mutants of allogeneic and xenogeneic targets were not susceptible to cytolysis. Comparisons to LAK cells revealed similarities in phenotype and function to the NK1.1-, CD8+ subset.

Reduction of class I expression in a bovine B-lymphoblastoid cell line is locus specific

A bovine B-lymphoblastoid cell line, BL3.1.2, defective in MHC class I expression was isolated following gamma irradiation and immunoselection of a previously mutated cell, BL3.1. Flow microfluorimetry demonstrated that BL3.1.2 cells expresed reduced amount of class I molecules on the cell surface when compared with the parent cells. 2D gel electrophoresis showed that two less class I gene products were synthesized by BL3.1.2 cells than the parent cells. However, no detectable RFLP difference was revealed between this mutant and the parent cell lines as analyzed by Southern blots using bovine class I cDNA and locus-specific oligonucleotide probes, suggesting no major deletions, insertions, or translocations within class I genes. Northern blot analysis, using class I cDNA probes, revealed a marked reduction of class I mRNA in BL3.1.2 cells. RNA unblots, probed with loci-specific oligonucleotides, and slot blot analysis demonstrated that absence of class I gene transcription was locus specific. This finding indicates there is separate regulation of each class I locus in bovine cells. Furthermore, recombinant IFN-gamma upregulated class I gene expression on the BL3.1.2 cells, as indicated by flow cytometry and slot blot analysis. These findings indicate that the locus-specific MHC class I gene was functionally downregulated in its expression. The silent class I gene could be activated by cytokines known to activate the cis-interferon-responsive element for subsequent transcription and translation of the newly synthesized class I RNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Bovine major histocompatibility complex class I specific monoclonal antibodies characterized by flow cytometry, one- and two-dimensional electrophoresis, western blot and inhibition of cytotoxic T lymphocyte function

To identify and characterize the bovine major histocompatibility complex (MHC) class I molecules, a panel of 11 monoclonal antibodies (mAbs) were analyzed. The mAbs reacted with bovine MHC class I antigens, as assessed by flow cytometry and immunoprecipitation followed by one- and two-dimensional gel electrophoresis. Analysis by flow cytometry revealed that class I molecules were expressed less on a class I mutant B-lymphoblastoid cell line than on the parent cell line. The relative molecular weights of the proteins identified by these mAbs were similar to those reported previously for cattle and humans. Nonequilibrium pH gradient two-dimensional gel electrophoresis showed that RH16C recognized four different class I gene products, indicating this mAb reacts with a conserved epitope present on different class I molecules. These mAbs effectively blocked cytotoxic T lymphocyte killing of allogeneic lymphoblasts, suggesting the functional importance of beta-2m in this process. These mAbs should be useful reagents for studying bovine MHC class I molecules.

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.

CD5dim peripheral blood lymphocytes proliferate to exogenous IL-2 in the absence of antigen and kill in an NK-like manner

Bovine CD5 lymphocytes can be categorized into "bright" and "dim" populations on the basis of CD5-specific monoclonal antibody (mAb) binding. CD5dim lymphocytes enriched by negative selection using mAbs to CD2 and MHC class II were examined phenotypically and functionally. CD5dim-enriched lymphocytes were 50-74% T19+ and 5-36% CD8+. Fewer than 13% expressed CD4, IgM, or MHC class II on their surface, and 6-20% expressed low levels of CD2. These cells did not proliferate to alloantigen or uv-inactivated BHV-1, but did proliferate to Con A and IL-2 in the absence of antigenic or mitogenic stimulation. Following IL-2 stimulation, the T19+ CD5dim lymphocytes proliferated and CD11c expression was induced. Freshly isolated CD5dim lymphocytes failed to kill in a bovine NK-like assay; however, following culture CD5dim-enriched lymphocytes exhibited non-MHC restricted cytotoxicity. We suggest that the CD5dim-enriched lymphocyte population contains the precursors of bovine LAK killing; however, a more extensive phenotype of these precursors in the bovine is yet to be determined.