Somatically generated mouse myeloma variants synthesizing IgA half-molecules.

Whereas mouse myelomas that secrete IgA half-molecules have been shown to arise in vivo, their origin has not been definitely established. We show that somatic variants secreting phenotypically similar molecules can arise directly from the normal IgA-secreting myelomas S107 and W3082. In addition to being improperly assembled, the variant proteins have distinct carboxy-terminal deletions and an aberrant heavy-light chain disulfide bond. For at least one of the variants, variable region serology and affinity for hapten are both unaffected by these changes. Southern and Northern blot analyses indicate normal size DNA restriction fragments and mRNA, suggesting premature termination as the mechanism of deletion. These results are discussed in relation to possible mutational hot spots and long-range interdomain interactions.

Thymocyte subsets transformed by Abelson murine leukemia virus.

The infectious complex of Abelson murine leukemia virus was altered by replacing its usual helper virus, Moloney leukemia virus, with radiation leukemia virus (RadLV). After intrathymic injection of the Abelson-RadLV complex, thymomas arose rapidly, as described previously for injection of the Abelson-Moloney complex. Cell lines were derived from thymomas induced by each Abelson virus complex and were classified according to normal thymus cell phenotypes. Each virus complex induced some cell lines which were like a 0.7% subpopulation of murine thymocytes in that they failed to express the Thy-1 cell-surface antigen. These lines are thus far indistinguishable from some Abelson-derived bone marrow transformants classified as pre-B cells. However, the Abelson-Moloney complex induced some cell lines which expressed low levels of Thy-1 and which shared most markers with immature blast cells of the thymic medulla, whereas the Abelson-RadLV complex induced some lines which were clearly like thymic cortex blast cells. Thus, Abelson virus can induce thymoma cell lines of at least two, and possibly three, distinct phenotypes corresponding to normal thymocyte blast subsets, the determination of which can be influenced by helper virus sequences.

T-cell receptor and immunoglobulin genes are rearranged together in Abelson virus-transformed pre-B and pre-T cells.

We have assessed the state of rearrangement and expression of B- and T-cell antigen receptor genes in cells of Abelson murine leukemia virus-transformed thymomas and other tumors. We found that unrearranged TcR gamma genes are expressed, as are unrearranged C mu genes, in pre-T, pre-B, and myeloid cells. We also found TcR gamma genes rearranged and expressed in putative pre-T cells and in cells apparently committed to the B-cell lineage. This is in contrast to the data from more mature T- and B-cell tumors. We conclude that in immature lymphoid cells both immunoglobulin and TcR gamma genes are accessible for rearrangement. We discuss the implications of these observations for an understanding of the B-T lymphoid differentiation event.

Abelson virus transformation of an interleukin 2-dependent antigen-specific T-cell line.

Abelson murine leukemia virus (A-MuLV) carries the gene v-abl, one of a group of oncogenes with structural and functional (tyrosine kinase) homology to three growth factor receptors. Work in this and other laboratories has shown that A-MuLV infection can render myeloid and lymphoid cells independent of the growth factors interleukin 3 and granulocyte-macrophage colony-stimulating factor. We have now shown that v-abl can also relieve interleukin 2 (IL-2) dependence in T cells. We infected a cloned IL-2-dependent antigen-specific cell line. Transformed cells were generated which were factor independent and tumorigenic. The transformants each bore unique v-abl DNA inserts and expressed v-abl mRNA. No elevation of expression of either IL-2 or its receptor could be detected in these cells. Thus, A-MuLV can short-circuit the dependence of hematopoietic cells on IL-2, IL-3, and possibly granulocyte-macrophage colony-stimulating factor, none of whose receptors are known to be of the tyrosine kinase type.

SCL, the gene implicated in human T-cell leukaemia, is oncogenic in a murine T-lymphocyte cell line.

SCL (TAL-1) is implicated in the generation of human T-cell acute lymphoblastic leukaemia. To directly examine the role of this putative oncogene, an SCL retrovirus was constructed and used to infect a v-ABL transformed T-lymphocyte cell line. Thirteen independent SCL-infected and four control cell lines were established and injected subcutaneously into syngeneic mice. Mice injected with SCL-infected clonal cell lines died significantly more rapidly than control animals. By day 200 46% (40/87) of animals injected with SCL-infected cell lines had died due to disseminated transplantable lymphoid tumours. In contrast only 22% of control mice were dead by day 200 (P < 0.0015). Of possible relevance to the enhanced tumourigenesis, some SCL-infected cell lines displayed increased clonogenicity in agar. Increased cell growth was even more striking when ex-vivo tumour-derived cell lines were studied. Thus, SCL can co-operate with v-ABL to hasten T-cell tumourigenesis. This is the first direct evidence demonstrating that SCL can behave as an oncogene.

Phenotypic effect correlating with loss of a novel tumor suppressor gene: towards cloning by complementation.

We intend to use a gene complementation approach to clone a tumor suppressor gene on mouse chromosome 2, the loss of which contributes to myeloid leukemia. An in vitro model system has been generated using a clonal cell line, in which tumorigenic chromosomal lesions have been selected along with myeloid differentiation. Among these lesions are deletions of chromosome 2. Comparison of subclones with deleted vs intact chromosomes 2 has allowed the identification of a growth related phenotypic pattern which correlates with the deletion, viz the retention of a marker of immature cells, resistance to inhibition by lipopolysaccharide (LPS), even in the presence of markers of mature myeloid cells, such as resistance to killing by apoptosis-inducing agents. The phenotype is shared by chromosome 2-deleted cell lines derived from conventional tumors. We have begun to investigate the mechanism of the phenotype. The LPS resistance does not correlate with lack of mRNA for CD14, a known cell surface receptor for this agent, or with failure to induce TNF alpha or nitric oxide synthase in response to its binding. The system should allow cloning of the gene using complementation of this phenotype in transfected cell lines.

Gene deletion explains both in vivo and in vitro generated chromosome 2 aberrations associated with murine myeloid leukemia.

Ninety-five percent of radiation-induced murine myeloid leukemias contain chromosome 2 aberrations. A dominant molecular defect has not yet been identified: both deletions and breakpoint-specific events have been postulated. We have generated a model in which chromosome 2 lesions have been generated in vitro in a clonal tumor cell line. In this study cytogenetic and molecular comparisons are made between two of these in vitro generated lesions and eight derived in vivo: seven by the conventional radiation protocol, and one by infection with Moloney leukemia virus. All 10 lines consistently exhibited hemizygous loss of an 18 cM region between Hoxd-4 and II-1 alpha, with variable breakpoints at both ends. These results are consistent with deletion of a gene in common rather than breakpoint-specific events, for lesions resulting from all three protocols. This will allow a novel approach to the identification of a putative tumor suppressor gene, ie to describe the biological effect of the in vitro generated deletion, and to clone the gene by complementation. In preparation for this approach, we have further narrowed the region to approximately 6.5 cM by microsatellite mapping of 22 radiation-induced F1 tumors. In addition, we have eliminated the possibility that imprinting ablates expression from the remaining undeleted chromosome.

Dental polyelectrolyte cements. III. Effect of additives on their rheology.

As part of a series of investigations of dental polyelectrolyte cements, the influence of additives was studied by viscometry. At low concentrations, tartaric acid was found to exert an accelerating influence on glass ionomer cements but to retard the reaction by complex formation at high concentrations. When dissolved with the liquid component, NaF accelerated some glass ionomer cements and retarded others. NaCl was found to accelerate the zinc polycarboxylate cements, possibly through its reduction in the electrical potential between powder and liquid phases. Neutralization of the polyacids retarded the reactions but not enough to indicate that hydrogen ions were directly involved in the rate determining step.

Dental polyelectrolyte cements: II: effect of powder/liquid ratio on their rheology.

The rheological behaviour during the setting of a range of zinc polycarboxylate and glass ionomer dental cements has been studied. The influence of the powder/liquid ratio was found to alter the rate of reaction without altering the basic form of the kinetics. Two models were advanced to explain the rheological and chemical differences between the two types of polyelectrolyte cement. The setting of the glass ionomer cements was consistent with the development of a homogeneous polymer network whereas the zinc polycarboxylate cements were viewed as setting by an inhomogeneous core-growth reaction.

Dental polyelectrolyte cements. I. Chemistry of the early stages of the setting reaction.

As part of an investigation of the setting of dental polyelectrolyte cements, the chemistry of a selection of glass ionomer and zinc polycarboxylate cements was studied by pH, conductivity and I.R. measurements. The zinc polycarboxylate cements were found to react at a greater rate than the glass ionomer cements. The effect of reducing the powder/liquid (P/L) ratio is to decrease the surface area available to attack and hence the reaction rate. The basic form of the kinetics appears to be unaffected except for low P/L ratios where there is a deficiency of available metal cations.

Curing efficiency and ocular hazards of dental photopolymerization sources.

The spectral distributions of a range of dental photocuring sources were measured at the exit window and at a distance of 10 cm. The former enabled the evaluation of a newly proposed photocuring efficiency index which correlates well with the depth of cure of the photopolymerized resins, thus providing a basis for the comparison of different photocuring sources. The spectral irradiance of the sources obeyed the inverse-square law, allowing a comparison with the ACGIH threshold limit values. According to these criteria, no ocular hazard is posed to the patient or clinician by u.v.-A or u.v.-B radiation nor to the patient by the visible light when momentarily exposed to the sources. Similarly the ACGIH criterion indicates that the clinician does not risk chorioretinal injury provided the exposure is restricted to less than 140 s in a 3 h period.

Colour stability and visual perception of dimethacrylate based dental composite resins.

The spectrophotometric method of colour measurement was applied to an investigation of the colour stability of dimethacrylate based composite resins exposed to thermal and photochemical aging. A good correlation was observed between visual perception of colour change and the spectrophotometric colour difference [delta E (FMC-2)] of unexposed and aged specimens, but 50% of the observers considered the colour match of two specimens to be clinically unacceptable when the colour difference exceeded 10.6. Kinetic studies of thermally and photochemically induced colour change showed a sigmoidal dependence of colour change on the logarithm of exposure time. Because the direction and magnitude of the colour change differed for the two procedures, it would appear that different mechanisms are involved.

Influence of chemical structure on the fracture behaviour of dimethacrylate composite resins.

Previous studies have shown that the fracture resistance of dimethacrylate-based dental composite resins is enhanced by post-curing the matrix. Here, the influence of the chemical nature of the resin matrix is examined by a study of the fracture properties of composite resins formulated from 15 homologous dimethacrylate monomers and filled to 75 wt% with treated silica. The fracture toughness was determined via the double torsion technique and the elastic modulus and flexural strength were measured in flexure. The fracture energy calculated from the fracture toughness and elastic modulus, varied between 60 and 300 J/m2 while the fracture toughness ranged from 0.2 to 2.0 MN/m3/2 and the flexural strength varied from 17 to 111 MPa. The use of a blend of monomers was found to have a synergistic effect on the fracture resistance. Increasing the length of flexible spacer units (methylene or oxyethylene) between the methacrylate groups initially improved the fracture properties; however, beyond a certain length, these properties were impaired.

Structure and properties of methacrylate based dental restorative materials.

The chemistry and structure of the dimethacrylate resins and the nature of the filler systems in dental composite resins are reviewed in relation to their influence on the setting behaviour, dimensional stability, aesthetics, fracture behaviour and adhesive potential. It is clear that a deeper understanding of the structure of the polymeric matrix and the mechanism of clinical wear is required. As a result of ongoing research in this area and with the development of dentine adhesives, the future prospects of composite resins are encouraging.

Tear and rupture of elastomeric dental impression materials.

The curing time dependence of the tear energy, tensile strength and ultimate extension ratio of a range of characterized elastomeric impression materials was investigated. This variation of tensile strength and ultimate extension ratio was correlated with the development of the network structure. The tear energy showed only minor changes as the curing proceeded.

Factors affecting the depth of cure of UV-polymerized composites.

Knoop hardness measurements of UV-polymerized dental composites showed that, at a certain depth below the surface, the degree of cure rapidly decreases. The depth of cure was found to be critically dependent on the UV source and exposure time used. A theoretical model was derived which was in quantitative accord with the results.

The oscillating rheometer--what does it measure?

The motion involved in the operation of an oscillating rheometer was analyzed, and revealed that the instrument measured a complex mixture of an instrument-dependent parameter, the spring constant, and two material-dependent parameters--the dynamic viscosity and the shear storage modulus. The significance of the working and setting time parameters measured by the rheometer are discussed.

A simple method for the measurement of polymerization shrinkage in dental composites.

OBJECTIVE: In this study a simple non-contact method was developed to measure the polymerization shrinkage of dental composites. METHODS: A gas pycnometer was used to determine the volumes of specimens prior to and after photopolymerization and from which the total volumetric shrinkage could be determined. RESULTS: Four commercial composites were studied and were found to have polymerization shrinkages varying from 1.6 to 2.5%. The method was found to be labour efficient and produced reproducible results with a standard deviation of approximately 10%. SIGNIFICANCE: This method is appropriate for shrinkage measurements where only the total amount shrinkage is required and in particular for the measurement of shrinkage of photocured materials which are sensitive to water absorption.

Fracture toughness of water-aged resin composite restorative materials.

OBJECTIVES: The purpose of this study was to assess the effects of aging experimental dimethacrylate resin composites in water at 37 degrees C for periods up to 6 wk by measuring the variations in fracture toughness (K(c)), elastic modulus (E), fracture energy (G(c)), and water sorption. METHODS: Six experimental resins were formulated from dimethacrylate resins, and were filled to 86 wt% (ca. 70 vol%) with treated inorganic filler to form six experimental composites. The fracture toughness was determined using a double torsion technique, the elastic modulus was measured in flexure, and the fracture energy was calculated from the fracture toughness and elastic modulus. RESULTS: As a result of aging in water, K(c) and the G(c) increased, and the elastic modulus decreased, but all values approached a plateau near 6 wk. Water sorption also occurred during this period, mainly during the first 2 wk. SIGNIFICANCE: Variations in the mechanical properties are interpreted as being due to plasticization of the resin matrix by water, which appears to lower the yield stress and increase in the size of the plastic zone ahead of the crack, thereby causing the observed increase in G(c) and K(c). After approximately 6 wk, no further changes in properties occurred.

An investigation of the radiopacity of composite restorative materials.

The radiopacity of 33 commercially available composite materials was measured in terms of their equivalent thickness of an aluminum alloy per unit thickness of material. The theoretical significance of this definition was discussed. The radiopacities were found to vary by more than an order of magnitude. Although nearly 30 percent of the materials have radiopacities less than dentine, all materials claimed to be radiopaque were more so than dentine. The influence of operating potential, specimen thickness, X-ray generator type, film type, filtration and X-ray dose were investigated. The former two factors were found to have a large influence on the radiopacity.