Heterogeneity of EBV-transformable human B lymphocyte populations

Although most human B cells express receptors for Epstein Barr virus (EBV), few (usually less than 1%) are readily transformed into B lymphoblastoid cell lines after exposure to EBV. Transformable cells previously have been found to be mostly resting B lymphocytes. We recently developed a limiting dilution culture system which permits the growth of EBV-transformed B lymphocytes with high efficiency. Because in this system up to over 30% of peripheral blood- or tonsil-derived B cells respond to EBV, we re-examined the properties of EBV-transformable cells. Frequencies of transformable lymphocytes were determined by Poisson analysis. EBV-susceptible B cells committed to IgM, IgG, or IgA secretion were found to occur in the range of 3 to 27, 0.1 to 6, and 0.1 to 5 per 100 B cells, respectively. Under our culture conditions, a major proportion of the IgM-committed cells derived from large lymphocytes which appeared to have entered the cell cycle. This population contains most of the EBV-responsive cells detected and, therefore, most of the additional cells responding in our culture system. In contrast, precursors of IgG- or IgA-producing lymphoblast lines were small cells with DNA contents typical for the G0/G1 phases of the cell cycle. Anti-immunoglobulin antibodies were used in panning experiments to separate B cell subpopulations which expressed different immunoglobulin isotypes on their surface. In limiting dilution cultures of these purified B lymphocytes subsets, it was found that virtually all precursors of IgM-producing cell lines expressed surface IgM (sIgM) before their infection and transformation by EBV. The "cloning efficiency" of positively selected, large sIgM+ cells approached 100%. In contrast, sIgG or sIgA were found only on cells committed to the production of IgG or IgA, respectively. The expression of sIgD was examined by using sequential panning procedures. Virtually all IgM-committed lymphocytes and a subset of cells committed to IgA secretion were found among the sIgD+ cells. The majority of cells committed to IgA production and all IgG-committed cells were found in the sIgD- B cell population. Our findings indicate that the EBV-susceptible B cell subset of normal lymphocytes is heterogeneous with respect to cell size, cell cycle, sIg determinants, and efficiency of transformation. On the basis of our findings and previous reports, we propose a model in which transformability is a B cell-inherent property. Factors unrelated to the virus but present in our culture system appear responsible for the enhanced vulnerability to viral transformation in some cells which entered into the cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)     

Chemical and spectroscopic evidence for the formation of a ferryl Fea3 intermediate during turnover of cytochrome c oxidase

When partially reduced cytochrome c oxidase samples are reoxidized with dioxygen, an EPR-silent dioxygen intermediate, which is at the three-electron level of dioxygen reduction, is trapped at the dioxygen reduction site. The intermediate has novel spectral features at 580 and 537 nm. Combined optical and EPR results reveal that this intermediate reacts rapidly with CO at 277-298 K causing the abolition of the 580/537 mm features and the appearance of a rhombic CuB EPR signal. A ferryl Fea3, or an intermediate at the same formal level of oxidation, is proposed to oxidize CO to CO2 producing an EPR-detectable CuB adjacent to a low-spin ferrous Fea3-dioxygen (or carbon monoxide) adduct.     

Characterization of the phosphotyrosyl protein phosphatase activity of calmodulin-dependent protein phosphatase

Calmodulin-dependent protein phosphatase from bovine brain and heart was assayed for phosphotyrosine and phosphoserine phosphatase activity using several substrates: 1) smooth muscle myosin light chain (LC20) phosphorylated on tyrosine or serine residues, 2) angiotensin I phosphorylated on tyrosine, and 3) synthetic phosphotyrosine- or phosphoserine-containing peptides with amino acid sequences patterned after the autophosphorylation site in Type II regulatory subunit of the cAMP-dependent protein kinase. The phosphatase was activated by Ni2+ and Mn2+, and stimulated further by calmodulin. In the presence of Ni2+ and calmodulin, it exhibited similar kinetic constants for the dephosphorylation of phosphotyrosyl LC20 (Km = 0.9 microM, and Vmax = 350 nmol/min/mg) and phosphoseryl LC20 (Km = 2.6 microM, Vmax = 690 nmol/min/mg). Dephosphorylation of phosphotyrosyl LC20 was inhibited by phosphoseryl LC20 with an apparent Ki of 2 microM. Compared to the reactions with phosphotyrosyl LC20 as the substrate, reactions with phosphotyrosine-containing oligopeptides exhibited slightly higher Km and lower Vmax values. The reaction with the phosphoseryl peptide based on the Type II regulatory subunit sequence exhibited a slightly higher Km (23 microM), but a much higher Vmax (4400 nmol/min/mg) than that with its phosphotyrosine-containing counterpart. Micromolar concentrations of Zn2+ inhibited the phosphatase activity; vanadate was less potent, and 25 mM NaF was ineffective. The study provides quantitative data to serve as a basis for comparing the ability of the calmodulin-dependent protein phosphatase to act on phosphotyrosine- and phosphoserine-containing substrates.     

Inhibition of proliferation without affecting the generation of cytotoxicity in the human mixed lymphocyte reaction

Phosphoramide mustard (PM) is considered to be the major tumoricidal metabolite of cyclophosphamide in vivo. The effects of this metabolite in vitro on several immune functions of human lymphocytes have been investigated. Very low concentrations (10(-7) to 10(-9) M) of PM added to lymphocyte cultures inhibited proliferation of the lymphocytes in response to mitogens and alloantigens. At these concentrations, inhibition of proliferation appeared to be due to a direct action of PM on the proliferative cells. Thus, concanavalin A-stimulated lymphocytes still acquired IL-2 receptors (Tac antigen) normally in the presence of PM (10(-6) to 10(-9) M). Only exceedingly high concentrations of PM (10(-5) M or greater) prevented the acquisition of Tac antigen. Similarly, the inhibition of proliferation was probably not related to endogenous IL-2 levels: addition of exogenous IL-2 to PM-containing cultures did not result in any restoration of proliferation. Further evidence that PM directly affected proliferative cells was that low concentrations of PM inhibited the proliferation of T cells continuously growing in IL-2. The exposure time to PM necessary for inhibition was essentially identical to those for lymphoproliferative responses to mitogens and alloantigens. Paradoxically, however, the generation of cytotoxic lymphocytes in mixed lymphocyte reactions (MLRs) and mixed lymphocyte tumor cell cultures (MLTCs) was very resistant to PM. In parallel MLRs and MLTCs the cytotoxic responses were resistant to approximately 1000-fold more PM than were the proliferative responses. Only at 10(-5) M PM were these inhibited. These data suggest that clonal expansion of cytotoxic lymphocytes or their precursors by proliferation is not an absolute requirement for the generation of cytolytic activity.     

Differential effects of abrin on normal and tumor cells

The effects of the plant toxin abrin on normal mouse embryonic fibroblasts (MEF), an untransformed mouse cell line (NIH 3T3), and two mouse tumor cell lines (LMTK- and S-180) were studied. Measurements of cell growth and colony formation showed that MEF and S-180 cells were more sensitive to abrin intoxication than NIH 3T3 and LMTK- cells. Also, the effects of abrin on the inhibition of [3H]leucine and [3H]thymidine incorporation were more evident in MEF and S-180 cells. The basis for these varying responses to abrin by the four different cells was examined. The number of abrin binding sites per cell was determined from [125I]abrin binding studies: NIH 3T3 and LMTK- cells had significantly fewer abrin binding sites than MEF and S-180 cells. The fate of the [125I]abrin after internalization was examined by gel electrophoresis and autoradiography. A pattern of time-dependent degradation was observed, degradation being more rapid in NIH 3T3 and S-180 cells than in LMTK- and MEF cells. We conclude that the varying responses of different cells to the toxin abrin may be due to several factors, including the relative number of abrin binding sites on the cell surface and the rate of degradation of the toxin once internalized. The results also show that the sensitivities of the cells to abrin do not necessarily correlate with their normal or neoplastic state.     

Influence of accessory cell and T cell surface antigens on mitogen-induced IL 2 receptor expression

We have assessed the inhibitory effects of various monoclonal antibodies on the expression of the IL 2 receptor. Anti-LFA-1, but not anti-Ly-2, markedly inhibited the induction of the IL 2 receptor on the Ly-2+ subset. T-depleted spleen cells, L cells, and B lymphoma cells all functioned as potent accessory cells (AC) for the induction of the IL 2 receptor on L3T4+ T cells. Anti-LFA-1 inhibited the induction of the IL 2 receptor irrespective of the type of AC used. Anti-L3T4 only inhibited the induction of IL 2 receptor expression when L cells were the source of AC. The inhibitory capacity of anti-L3T4 was not related to the expression of Ia on the AC population, because the magnitude of inhibition was comparable in cultures containing either Ia+ or Ia- L cells, whereas no inhibition was seen with either Ia+ or Ia-B lymphoma cells. We conclude from these studies that LFA-1 plays a critical role in mitogen-induced activation of both T cell subsets by promoting both T-AC and T-T interactions. Although anti-L3T4 can inhibit T cell activation in the absence of the recognition of Ia, the mechanism of inhibition and the proposed target molecule for L3T4 on the AC or the T cell have not been determined in our studies. A number of different models for the function of this cell surface antigen are discussed.     

Alterations in immunolocalization of the phosphoprotein B23 in HeLa cells during serum starvation

Bright nucleolar immunofluorescence was observed in HeLa S3 cells by immunostaining with a monoclonal antibody to the nucleolar phosphoprotein B23 (MW 37 kD/pI 5.1). After 48 h of incubation in a serum-free medium, the nucleolar fluorescence was diminished and a general nuclear immunofluorescence was observed. This change in localization of fluorescence indicated that protein B23 had migrated out of the nucleoli. No gross morphological change in nucleoli was observed by light microscopy and the immunolocalization of another nucleolar phosphoprotein, C23, was unaffected by serum deprivation. Relocation of protein B23 in nucleoli was observed after refeeding with serum-containing medium. This re-entry process was not observed after treatment with actinomycin D (50 ng/ml-5 micrograms/ml), but the process was unaffected by cycloheximide (0.2 mM). Quantitation of protein B23 in the nucleoli of the control (fed) or starved HeLa cells was done by ELISA immunoassay. A marked decrease in the amount of protein B23 occurred in the nucleoli of the starved cells (11.8 micrograms B23/mgDNA) as compared with the control nucleoli (20.8 micrograms B23/mgDNA). The amount of protein B23 in the nucleoplasm (excluding nucleoli) was 70% higher in the starved cells. Protein B23 was analysed by one- and two-dimensional PAGE. Three components of protein B23 with slightly different molecular weights and pIs (37 kD/5.1, 35 kD/5.1 and 35 kD/5.3) were observed in nucleoli. The lower molecular weight components were predominantly found in the nucleoplasm.     

The kinetic equation for the chloride transport cycle of band 3. A 35Cl and 37Cl NMR study

The nature of a transmembrane transport process depends largely on the identity of the reaction that is rate-limiting in the transport cycle. The one-for-one exchange of two chloride ions across the red cell membrane by band 3 can be decomposed into two component reactions: 1) the binding and dissociation of chloride at the transport site, and 2) the translocation of bound chloride across the membrane. The present work utilizes 35 Cl NMR and 37 Cl NMR to set lower limits on the rates of chloride binding and dissociation at the saturated inward- and outward-facing band 3 transport sites (greater than or equal to 10(5) events site-1 s-1 in all cases). At both 0-3 and 37 degrees C, the NMR data specify that chloride binding and dissociation at the saturated transport sites are not rate-limiting, indicating that translocation of bound chloride across the membrane is the slowest step in the overall transport cycle. Using these results, it is now possible to describe many features of the kinetic equation for the ping-pong transport cycle of band 3. This transport cycle can be decomposed into two half-reactions associated with the transport of two chloride ions in opposite directions across the membrane, where each half-reaction is composed of sequential binding, translocation, and dissociation events. One half-reaction contains the rate-limiting translocation event that controls the turnover of the transport cycle; in this half-reaction, translocation must be slower than binding and dissociation. The other half-reaction contains the non-rate-limiting translocation event that in principle could be faster than binding or dissociation. However, when the following sufficient (but not necessary) condition is satisfied, both translocation events are slower than binding and dissociation: if the non-rate-limiting translocation rate is within a factor of 10(2) (0-3 degrees C) or 2 (37 degrees C) of the overall turnover rate, then translocation is rate-limiting in each saturated half-reaction. Thus, even though chloride appears to migrate through a channel that leads from the transport site to solution, the results support a picture in which the binding, dissociation, and channel migration events are rapid compared to the translocation of bound chloride across the membrane. In this case, chloride binding to the transport site can be described by a simple dissociation constant (KD = kappa OFF/kappa ON) rather than by a Michaelis-Menten constant (KM = (kappa OFF + kappa TRANSLOCATION)/KAPPA ON).     

Nucleotide sequence and gene organization of ColE1 DNA

The primary structure of the plasmid ColE1 DNA has been determined. The plasmid DNA consists of 6646 base pairs (molecular mass of 4.43 MDa) and is 48.46% in GC content. The phi 80 trp insert of the composite plasmid of ColE1, pVH51, has also been determined. The determination of the nucleotide sequence of ColE1 DNA provides the basis for examining the relationships between the DNA sequence and the gene organization of the plasmid. The focus of this paper is to use this sequence data coupled with a review of the literature and our own work to examine the nine known functional regions of ColE1: imm (colicin E1 immunity), rep (replication function), inc (plasmid incompatibility and copy number control), bom (basis of mobility), rom (modulator of inhibition of primer formation by RNA I), mob (plasmid mobilization), cer (determinant for conversion of plasmid multimers to monomers), exc (plasmid entry exclusion), cea (structural gene for colicin E1), and kil (structural gene for the Kil protein).     

The effect of total parenteral nutrition (TPN) on the enteroinsular axis in the rat

The effect of 6 days of total parenteral nutrition (TPN) on the enteroinsular axis was studied in vivo and in vitro in the rat. During the TPN period, blood samples were taken from control and TPN animals to determine the comparative pattern of GIP release. Glucose, insulin and GIP responses to oral glucose (OGTT) were compared in TPN and control rats. The effect of glucose and GIP on insulin release from the isolated perfused pancreas of the same animals was investigated to determine if TPN altered the sensitivity of the beta cell. In conjunction with these studies the number and distribution of GIP-containing cells were compared in control and TPN animals. TPN resulted in no change in basal levels of glucose, insulin and IR-GIP. An exaggerated insulin response to OGTT occurred after TPN whereas the glucose response was reduced. The IR-GIP response to glucose was normal following TPN. The isolated perfused pancreas showed a 30% increase in insulin release in response to GIP after TPN. The insulin response to glucose appeared normal as did the number and distribution of GIP cells. Fluctuations in GIP and insulin levels in control animals were diurnal in nature, whereas IR-GIP levels in TPN animals remained near fasting levels. It was hypothesized that the increase in beta cell sensitivity to GIP may be causally connected to the exposure of the pancreas to chronically low levels of GIP during TPN.