Specialized functions of major histocompatibility complex class I molecules. II. Hmt binds N-formylated peptides of mitochondrial and prokaryotic origin

The physiological functions of the mouse telomeric major histocompatibility complex (MHC) class I molecules, including Hmt, are unknown. Hmt presents a polymorphic, N-formylated peptide encoded by the mitochondrial gene ND1 forming the cell surface maternally transmitted antigen (Mta). Because the N-formyl moiety is required for Hmt binding, we proposed that Hmt may function generally in presentation of N-formylated antigens. This hypothesis was validated by a competitive binding assay, demonstrating that synthetic N-formyl peptides from other mitochondrial genes also bound Hmt. Bacteria similarly initiate protein synthesis with N-formylmethionine; indeed, we established that Hmt can also present prokaryotic peptides in an N-formyl-dependent manner. These results indicate biochemical specialization of this MHC-peptide interaction and suggest a unique role for Hmt in prokaryotic host defenses.

Mutational analysis of regulation of MHC and anti-viral genes

CTL-mediated selection for loss of expression of Mta by H-2-heterozygous SV40-transformed mouse fibroblasts (line 24SV) produced an unusual phenotypic class of maternally transmitted Ag negative mutants defective in both MHC expression and in anti-viral activity. Severely reduced surface expression of class I MHC Ag from multiple loci of both haplotypes correlated with low levels of MHC H chain and beta 2-microglobulin mRNA. Inasmuch as IFN can up-regulate class I expression and some fibroblasts elaborate autocrine IFN-beta, we examined whether IFN could restore wild-type expression of class I MHC Ag. However, IFN could not restore wild-type expression. Moreover, the fold-increases in class I Ag and mRNA expression were significantly reduced in mutant cells compared to wild-type cells. These results suggested that the mutants might have generalized defects in IFN response. Inasmuch as the induction of an anti-viral state is a hallmark of IFN responses, we exposed cells to IFN-alpha, -beta, or -gamma and challenged with virus. 24SV cells, exposed to any of the three IFNs, were completely protected from destruction by vesicular stomatitis, mengovirus or respiratory syncytial viruses. In contrast, MHC and anti-viral defective mutants could not be protected from virus-induced lysis by any IFN. Somatic cell hybridization analyses indicated that both basal MHC and IFN-inducible phenotypes were recessive to wild-type, and that a trans-acting regulatory factor required for basal MHC expression is defectively expressed in the mutants. Such a factor may integrate the organismal response to virus infection, encompassing both immune and nonimmune anti-viral responses.

Mutational analysis of regulation of MHC and anti-viral genes

CTL-mediated selection for loss of expression of Mta by H-2-heterozygous SV40-transformed mouse fibroblasts (line 24SV) produced an unusual phenotypic class of maternally transmitted Ag negative mutants defective in both MHC expression and in anti-viral activity. Severely reduced surface expression of class I MHC Ag from multiple loci of both haplotypes correlated with low levels of MHC H chain and beta 2-microglobulin mRNA. Inasmuch as IFN can up-regulate class I expression and some fibroblasts elaborate autocrine IFN-beta, we examined whether IFN could restore wild-type expression of class I MHC Ag. However, IFN could not restore wild-type expression. Moreover, the fold-increases in class I Ag and mRNA expression were significantly reduced in mutant cells compared to wild-type cells. These results suggested that the mutants might have generalized defects in IFN response. Inasmuch as the induction of an anti-viral state is a hallmark of IFN responses, we exposed cells to IFN-alpha, -beta, or -gamma and challenged with virus. 24SV cells, exposed to any of the three IFNs, were completely protected from destruction by vesicular stomatitis, mengovirus or respiratory syncytial viruses. In contrast, MHC and anti-viral defective mutants could not be protected from virus-induced lysis by any IFN. Somatic cell hybridization analyses indicated that both basal MHC and IFN-inducible phenotypes were recessive to wild-type, and that a trans-acting regulatory factor required for basal MHC expression is defectively expressed in the mutants. Such a factor may integrate the organismal response to virus infection, encompassing both immune and nonimmune anti-viral responses.

The role of physical forces on cytotoxic T cell-target cell conjugate stability

Theoretical considerations suggest that external forces play a significant role in cell-cell conjugate formation and may lead to the misinterpretation of adhesion data. To test this, the stability of conjugates formed between CTL and fibroblast target cells (TC) was examined in the controlled shear environment of a parallel plate flow chamber. Murine fibroblast targets expressing class I maternally transmitted Ag Mtaa or Mtab were grown on a glass slide that formed one wall of the flow chamber and were used in conjunction with anti-Mtaa and anti-Mtab specific mouse CTL clones to establish a panel of Ag-reciprocal targets and lymphocytes. Although cytolysis assays indicated that lymphocytes recognized and destroyed appropriate but not inappropriate targets, the stability of some CTL/TC conjugates was Ag independent. In all cases, the conjugate stability was shear dependent over a 100-fold range (0.04 to 4.0 dynes/cm2). For some clones, the ratio of the stabilities of Ag-specific CTL/TC conjugates to nonspecific conjugates was significantly enhanced with increasing shear. This implies that the role of Ag specificity in CTL/TC adhesion may be misinterpreted if the shear environment of CTL/TC conjugates is unknown or uncontrolled. Kinetic analysis revealed that conjugate stability was dependent on the exposure time to external forces and that there existed two populations of conjugates; weak associations that disengaged within the first 30 s of flow, and strong associations that remained attached even after a 5-min exposure to a steady shear stress. The stability of Ag-specific CTL/TC conjugates at 0.04 dynes/cm2 was enhanced by 50% as the temperature was increased from 25 to 37 degrees C, whereas the stability of nonspecific CTL/TC associations was not affected. This result indicates that significant Ag-specific strengthening may occur at physiologic temperatures. This work suggests the importance of attention to role of fluid mechanical shear stress in standard adhesion assays.

The role of physical forces on cytotoxic T cell-target cell conjugate stability

Theoretical considerations suggest that external forces play a significant role in cell-cell conjugate formation and may lead to the misinterpretation of adhesion data. To test this, the stability of conjugates formed between CTL and fibroblast target cells (TC) was examined in the controlled shear environment of a parallel plate flow chamber. Murine fibroblast targets expressing class I maternally transmitted Ag Mtaa or Mtab were grown on a glass slide that formed one wall of the flow chamber and were used in conjunction with anti-Mtaa and anti-Mtab specific mouse CTL clones to establish a panel of Ag-reciprocal targets and lymphocytes. Although cytolysis assays indicated that lymphocytes recognized and destroyed appropriate but not inappropriate targets, the stability of some CTL/TC conjugates was Ag independent. In all cases, the conjugate stability was shear dependent over a 100-fold range (0.04 to 4.0 dynes/cm2). For some clones, the ratio of the stabilities of Ag-specific CTL/TC conjugates to nonspecific conjugates was significantly enhanced with increasing shear. This implies that the role of Ag specificity in CTL/TC adhesion may be misinterpreted if the shear environment of CTL/TC conjugates is unknown or uncontrolled. Kinetic analysis revealed that conjugate stability was dependent on the exposure time to external forces and that there existed two populations of conjugates; weak associations that disengaged within the first 30 s of flow, and strong associations that remained attached even after a 5-min exposure to a steady shear stress. The stability of Ag-specific CTL/TC conjugates at 0.04 dynes/cm2 was enhanced by 50% as the temperature was increased from 25 to 37 degrees C, whereas the stability of nonspecific CTL/TC associations was not affected. This result indicates that significant Ag-specific strengthening may occur at physiologic temperatures. This work suggests the importance of attention to role of fluid mechanical shear stress in standard adhesion assays.

Specialized functions of MHC class I molecules. I. An N-formyl peptide receptor is required for construction of the class I antigen Mta

Maternally transmitted factor (Mtf) is a mitochondrial gene that controls the antigenic polymorphism of the MHC class I maternally transmitted antigen (Mta). Synthetic peptides from the NH2 terminus of the mitochondrially encoded NADH dehydrogenase subunit 1 (ND1) mimic Mtf peptide activity in an allele-specific manner. We show that the minimal ND1-alpha peptide length recognized by Mtaa-specific polyclonal CTLs was between 8 and 12 amino acids, while some Mtaa-specific CTL clones recognized a six amino acid peptide. The N-formyl group at the NH2 terminus of ND1 was essential for Mta activity. Competition experiments using N-substituted ND1-alpha peptides showed that an N-formyl peptide receptor on the target cell, which differs from the chemotactic peptide receptor, was required for Mta expression. The specificity of this receptor can account for the distinct immune restriction of Mta in which Mtf peptides are uniquely restricted by Hmt. It is possible that the Hmt gene product is the N-formyl peptide receptor itself and that it represents a class I antigen presentation molecule specialized for binding, transport, and immune presentation of N-formyl-peptide antigens of mitochondrial and prokaryotic origin.

Retrovirus-mediated insertional mutagenesis: phagemid rescue of flanking DNA by selecting plasmid ori

A method for screening recombinant lambda libraries was devised to select phage containing genomic regions containing provirus insertions of retroviruses that carry the kanamycin and G418 resistance factor neo and the origin of replication derived from pBR322 (oripBR). Such recombinants are phagemids, able to replicate as bacteriophages or as plasmids under lambda repressor control. lambda repressor was cloned into a plasmid derived from pSC101 that is compatible with pBR322-derived phagemids. A strain carrying this plasmid may be used to select phagemids derived from a single proviral insertion with 100% efficiency from complex recombinant libraries. Homologous recombination between proviral long terminal repeats was observed at a rate of 10(-4)/plaque-forming unit in recABC+ strains. Despite this frequency, intact phagemids are easily recovered as phage after temperature shift to 42 degrees C. Since oripBR itself is a selectable marker in this system, the method could be applied to recover any sequence carrying the ori sequence from pBR322.

An unexpectedly labile mitochondrially encoded protein is required for Mta expression

Maternally transmitted antigen (Mta) is a mouse major histocompatibility antigen recognized by cytotoxic T lymphocytes. A role for mitochondria in expression of this class I-like cell surface antigen has been previously established. We now show that a labile product of mitochondrial protein synthesis is required for Mta expression. Reexpression of Mta determinants after enzymatic removal occurred within 24 h, and the regeneration process was sensitive to chloramphenicol (CAP), a selective inhibitor of mitochondrial protein synthesis. Additionally, target cells treated with CAP for as little as 18 h showed diminished expression of Mta. The estimated half-life for Mtf products ranged from 6 to 15 h, less than the half-lives of known mitochondrial translation products. This suggests that the Mtf product is not generated by the normal turnover of stable mitochondrial respiratory proteins. Instead, these results indicate the existence of either labile unknown mitochondrially encoded peptides or a rapid turnover pathway for known mitochondrial products.

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.

Unglycosylated Mtaa expresses an Mtab-like determinant

Antigenic polymorphism of the class I-like maternally transmitted antigen (Mta) is controlled by a maternally transmitted factor (Mtf) thought to reside in mitochondria. However, the mechanisms by which Mtf generates antigenic polymorphism are not known. To address this issue, we investigated a possible role of posttranslational oligosaccharide addition in the formation of Mta determinants. We examined the expression of Mta on cytotoxic T lymphocyte (CTL) target cells cultured in tunicamycin (TM), a known inhibitor of asparagine(N)-linked glycosylation. Of 18 Mtab-specific CTL lines, 8 lysed TM-treated Mtaa targets. Furthermore, a subclone of one of these eight lines, 17D5.G2, lysed TM-treated targets from all Mtaa strains tested, regardless of H-2K/D haplotype. On the other hand, this CTL clone did not lyse TM-treated target cells from the Mta null, but H-2 expressing strain B10.CAS2. Therefore expression of this Mtab-like determinant is concordant with the expression of Mtaa and seems unlikely to represent a cross-reactive H-2K/D epitope. Our data suggest that an Mtab-like determinant is expressed on unglycosylated Mtaa molecules. Thus, N-linked oligosaccharides probably prevent the expression of an Mtab-like determinant on the Mtaa molecule. We discuss how Mtf may contribute to Mta polymorphism through glycosylation.

Maternally transmitted antigens are codominantly expressed by mouse cells containing two kinds of mitochondrial DNA

Mtf, a cytoplasmic, probably mitochondrial factor, controls Mta polymorphism. We tested for dominance between two forms of Mtf to determine whether Mta is controlled by positive or negative genetic mechanisms. We fused Mtf-disparate cells containing distinct mtDNA markers and selected for hybrids containing both. Such mtDNA heteroplasmons codominantly and stably express alternative Mta antigens. Stable codominance excludes negative genetic mechanisms as well as a model of induced nuclear compensation, and implies Mtf controls Mta expression through a positive genetic mechanism.

Cyclopentadienyl-ruthenium and -osmium chemistry. XIX. Some complexes containing η2-olefin, η2-allene and η2-alkyne ligands

Reactions between RuCl(Pme3)2(η-C5H5) and unsaturated hydrocarbons in the presence of NH4PF6 have given [Ru(η2-un)(PMe3)2(η-C5H5)] PF6[un = C2H4, (E)-CH(CN)=CH(CN),CH2=CHCH=CH2, CH2=C=CH2, CH2=C=CMe2, C2Ph2, PhC2C2Ph, PhC2CO2Et, C2(CO2Me)2 and C2(CF3)2]. The molecular structures of the buta-1,3-diene (3) and allene (4) complexes have been determined. In both compounds, the unsaturated hydrocarbon is bonded to ruthenium in an asymmetric η2 mode: Ru-C 2.185, 2.242(5) [for (3)], 2.084, 2.172(6) [for (4)l; coordinated C=C 1.386(8), 1.394(8) A, respectively. The allene skeleton is bent by 35� at the central carbon. Crystal data: (3) monoclinic, P21/n, a 17.343(2), b 13.692(4), c 17.997(3) �, β 94.91(1)�, Z 8; (4) orthorhombic, Pca21, a 18.999(3), b 9.253(3), c 23.016(6) �, Z 8. The structures were refined to residuals of R 0.026, R, 0.028 for 4076 data with 1 > 2.5σ(I) [for (3)], and R 0.022, Rw 0.024 for 3054 data [for (4)].

Hydroxy sulfoxides derived from norbornene: determination of stereochemistry, and synthesis by stereoselective oxidation

The structure (5) of one of the hydroxy sulfoxides formed by thiol-oxygen cooxidation of norbornene has been determined by X-ray diffraction. Other products have been related to it by appropriate chemical transformations. The stereospecific, or highly stereoselective, conversion of suitable hydroxy sulfides into one of the two possible diastereoisomeric hydroxy sulfoxides demonstrates the directive influence of the hydroxyl group on oxidation of vicinal sulfur.

Construction and preliminary characterization of the rat casein and alpha-lactalbumin cDNA clones

We have constructed a double-stranded cDNA library using total poly(A)-containing RNA extracted from 8-day lactating rat mammary gland and have utilized this library to isolate clones for each of the four major milk proteins. These four cDNA clones, representing the three major rat caseins and alpha-lactalbumin, were initially identified by colony hybridization with labeled cDNA probes synthesized from individual mRNA fractions purified by preparative gel electrophoresis. Additional characterization was accomplished by hybridizing individual clones labeled with 32P by nick translation to a Northern gel blot of an enriched fraction of the four major milk protein mRNAs. The individual mRNAs were clearly resolved by electrophoresis on fully denaturing methylmercury hydroxide agarose gels. The identity of each milk protein clone was further established by the location of unique restriction enzyme sites within each clone. Final identification of each clone was performed by hybrid-arrested cell-free translation. The sizes of the milk protein cDNA clones ranged frm 70% for the alpha-lactalbumin gene to essentially full length for the gamma-casein gene, in comparison to their respective mRNAs. This represents the first isolation of a family of peptide hormone-responsive genes.