Alain Declève

Replication kinetics of N- and B-tropic murine leukemia viruses on permissive and nonpermissive cells in vitro.

Abstract The kinetics of infection by a B-tropic, an N-tropic, and an NB-tropic murine leukemia virus (MuLV) have been studied on the cells of several mouse strains of permissive, restrictive, and F 1 hybrid genotype. The titration curves reveal that host cell restriction is measured by two independent parameters: (1) the slope of the curve (“hitness”); and (2) the level of sensitivity at comparable multiplicities of infection. Once established, infection is propagated from cell to cell at similar rates in both permissive and restrictive cell cultures. Anomalous results were obtained with the cells of strain NZB mice. Rat cells propagated these MuLV with 1-hit kinetics at low levels of sensitivity.

Physicochemical, biological and serological properties of a leukemogenic virus isolated from cultured radl-V-induced lymphomas of C57BL/Ka mice

Abstract RadLV/VL 3 , a virus spontaneously and continuously produced at high titer by BL/VL 3 cells, a line established in culture from a RadLV-induced C57BL/Ka mouse thymic lymphoma, has been extensively characterized. Evidence is presented indicating that it is a C-type RNA virus with highly thymotropic and leukemogenic biologic properties identical with those of its RadLV parent. It was not significantly neutralized by antibodies prepared against other ecotropic or xenotropic murine C-type viruses, used individually or in combination, whereas these antibodies effectively neutralized MCF-247 virus. However, antisera prepared against RadLV/VL 3 in rabbits and mice readily neutralized RadLV/VL 3 . Curiously, the rabbit antiserum also had strong neutralizing activity against three other C-type endogenous viruses of the C57BL/Ka strain, BL/Ka(B), BL/Ka(N), and BL/Ka(X), all of which are fibrotropic (replicate readily in fibroblasts of appropriate tropism) and lack thymotropic and leukemogenic activity. In interference assays, these viruses and RadLV/VL 3 exhibited little or no capacity for mutual or self-interference, nor did RadLV/VL 3 interfere with the replication of MCF-247 virus in mink cells. However, the addition to cultures of increasing concentrations of the purified envelope glycoprotein (gp71) of RadLV/VL 3 , though devoid of effect on BL/Ka(B) and BL/Ka(N), strongly inhibited the replication of RadLV and RadLV/VL 3 both in vitro and in vivo , and partially inhibited the xenotropic replication of BL/Ka(X). Competition radioimmunoassays with purified virion polypeptides demonstrated that the gp71 of RadLV/VL 3 is distinctively different from that of all other viruses tested; that the p12s of RadLV/VL 3 and BL/Ka(B) are similar or identical, whereas that of BL/Ka(N) competes in the AKR-MuLV p12 assay; and that the p30s of all of the viruses studied are indistinguishable by this method. Finally, RadLV/VL 3 failed to elicit cytopathic changes or transformed cell foci after serial passage in mink lung cells, even after co-infection with the xenotropic BL/Ka(X) virus, indicating that it is apparently devoid of MCF activity. Thus, MCF activity is apparently not a prerequisite for thymotropic and leukemogenic activity, at least in the C57BL/Ka strain.

An improved murine leukemia virus immunofluorescence assay

Abstract The use of DEAE-dextran and polybrene during infection of mouse embryo fibroblast cell lines of Fv-1bb origin with the radiation leukemia virus strikingly increased the proportion of cells in which virus-induced cytoplasmic antigens were detectable by indirect immunofluorescence and significantly accelerated the time course of their detection, thus making possible the development of a rapid, sensitive, and practical new assay based upon the percentage of immunofluorescence-positive cells. Direct comparisons with several other in vitro assays are presented. The assay was also tested successfully with Gross leukemia virus grown on cells of Fv-1nn origin, and should be applicable to other murine leukemia viruses and, perhaps with minor modifications, to other mammalian leukemia viruses as well.

In Vivo interaction between RNA viruses isolated from the C57BL/Ka strain of mice

Abstract Virus replication can be initiated in the C57BL/Ka thymus by inoculation of three virus preparations isolated from mice of this strain, designated RadLV, RadLV-LTC, and BL/Ka(6), but not by three others, designated BL/Ka(B), BL/Ka(N), and BL/Ka(X). Coinfection by various mixtures of the three nonthymotropic viruses does not result in productive infection of the thymus. The addition of these nonthymotropic viruses to serial dilutions of RadLV, RadLV-LTC, or BL/Ka(6) does not significantly alter the replication of the wild-type RadLV preparations tested, whereas it significantly facilitates the replication of RadLV-LTC and BL/Ka(6). It is postulated that RadLV-LTC and BL/Ka(6) contain an attenuated or defective thymotropic and leukemogenic particle, the intrathymic replication of which is facilitated by coinfection with the nonthymotropic viruses.

Conversion of restrictive mouse cells to permissiveness during sequential and mixed double infection by murine leukemia viruses

Abstract Conversion from restrictive (two-hit) to permissive (one-hit) kinetics was observed when N- or B-tropic murine leukemia viruses were titrated on mouse embryo fibroblasts of nonpermissive Fv-1 genotype that had previously been nonproductively infected with the same virus at an average multiplicity of one. The effect was transient, disappearing within about 24 hr after the first infection, and was abrogated by exposure of the first virus preparation to increasing doses of ultraviolet light, with a D 37 inactivation dose of 2550 ergs/mm 2 , about three times that for inactivation of viral replication. Prior infection of nonpermissive mouse cells with the NZB xenotropic virus did not alter their restrictive response to later infection with ecotropic viruses. Low multiplicity infection of NIH 3T3 cells with a B-tropic virus, followed by treatment with iodoeoxyuridine, failed to induce productive infection by endogenous or exogenous virus. Cells of F 1 hybrid Fv-1 genotype, which are restrictive for both N- and B-tropic viruses, were converted to permissiveness for virus of either tropism after low multiplicity infection with virus of the opposite tropism. No evidence of NB-tropic recombinant progeny could be detected under these experimental conditions. The implications of these experimental observations with respect to the mechanism of restriction governed by the Fv-1 gene are discussed.

Kinetics of propagation of B-tropic murine leukemia virus on Fv-1b cell lines: Requirement for multiple cycles of cell replication for transformation and viral antigen expression by RadLV

The kinetics of intracellular viral antigen production in UC1-B, BALB3T3 and BL-5 cells (all Fv-1b) and of focus formation in UC1-B cells have been studied following infection with free RadLV∗, a naturally occurring, B-tropic murine leukemia virus adapted to continuous propagation in vitro, and with infective centers of BL-5(RadLV) cells chronically infected with RadLV∗. Whereas such infective centers induced focus formation in UC1-B cells with high efficiency within a single culture passage, the development of infective center activity in BL-5 cells freshly infected with RadLV∗ and the direct induction of focus formation in UC1-B cells infected with free RadLV∗ both required one or more subcultures and several rounds of cell replication. The end-point dilution titer was not detected until four to five subcultures and about 15 rounds of cell cycle replication had been completed. Viral antigen content, measured by indirect immunofluorescence (IF), followed a triphasic pattern with respect to time and viral input concentration. Yet, both the IF assay at 3 days and the focus formation assay at 7 days (one subculture) were characterized by single-hit kinetics, excluding the possibility that mutual complementation of defective RadLV∗ particles might account for the delayed expression of viral infection. When viral replication was related to the number of rounds of cell replication, rather than to elapsed time, no significant differences among the three cell lines were observed. As in previous studies with NB-tropic viruses, RadLV∗ failed to rescue a putative sarcoma virus from UC1-B cells. It is concluded that RadLV∗ is not defective and can replicate to high titer in several Fv-1b cell lines in the absence of a detectable murine sarcoma virus (MSV) genome. However, the kinetics of its replication suggest that certain expressions of infection may be dependent on the attainment of a threshold level of some viral product(s), the synthesis of which is dependent on sustained cell proliferation.

Rescue of a thymotropic, leukemogenic C-type virus from cultured, nonproducer lymphoma cells of strain C57BL/Ka mice☆

Abstract A permanent cell line, BL/RL12-NP, derived from a radiation-induced C57BL/Ka mouse lymphoid tumor, has remained devoid of MuLV expression, except for the rare, sporadic initiation of virus production in some cultures. It can, however, be stably infected by the radiation leukemia virus (RadLV), and the progeny virus population retains the biological and serological properties of the parental RadLV. The cells can also be infected by a B-ecotropic, nonthymotropic, nonleukemogenic C57BL/Ka virus isolate, BL/Ka (B). In the latter situation, the emerging virus particles may exhibit thymotropic and leukemogenic (T+L+) attributes similar to those of RadLV, while retaining at least some of the envelope determinants of BL/Ka (B). These observations suggest that, following productive infection by a nonleukemogenic helper virus, oncogenic sequences endogenous to the non-producer lymphoma cells may be packaged in infectious progeny virions. The data are interpreted as providing strong support for the existence, in radiogenic lymphomas, of defective T+L+ sequences, designated RadLV-O. Possible mechanisms whereby RadLV-O is later expressed as an infectious leukemogenic virus are discussed.

Biological and biochemical differences among ecotropic C-type RNA viral isolates chemically induced from C57BL/Ka mouse embryo cells in vitro☆

Abstract Several biological properties of the ecotropic viruses isolated after IUdR induction of C57BL/Ka mouse embryo cells in vitro were found to vary depending on the type of cell from which the viruses were recovered and their passage history in culture. The majority of the induced viral isolates were N-tropic. They could be separated into two classes on the basis of their XC plaque-forming ability and their electrophoretic mobility in SDS-polyacrylamide gels. The exogenous transfer to and propagation in C57BL/Ka fibroblasts of one N-tropic parent virus led to the appearance of a progeny virus population with N- and B-tropic activity and high plaque-forming ability. This preparation, which had B- and N-tropic activity, was either a heterogeneous mixture of N- and B-tropic particles or a heterozygote particle which contained N- and B-tropic genomes. After passage of limiting dilutions of that preparation in C57BL fibroblasts, a novel B-tropic virus was recovered which was distinct from other B-tropic fibrotropic viruses isolated previously from strain C57BL/Ka mice. This new isolate is believed to have originated from recombinational events which had taken place between the B-tropical viral genome endogenous to the C57BL/Ka fibroblasts and the exogenous N-tropic virus used to infect these cells.

Potentiating effect of iododeoxyuridine on mulv replication in mouse embryo fibroblasts.

Abstract Treatment of mouse embryo fibroblasts with concentrations of 5-iododeoxyuridine (IUdR) too low to induce detectable replication of endogenous C-type viruses has been found to augment the susceptibility of permissive and nonpermissive cells to exogenous infection by murine leukemia viruses, without changing the kinetics of viral replication. No evidence could be obtained to support the hypothesis that this phenomenon results from induction of endogenous virus followed by complementation or recombination between endogenous and exogenous virus. Instead, the responsible mechanism seems likely to involve IUdR-induced DNA strand breakage, repair, and enhanced recombinational integration of the exogenous viral genome into cellular DNA.

Fractionation and characterization of the blast cell population of the mouse thymus by physical cell separation methods and ultrastructural analysis

Abstract Sedimentation at unit gravity and discontinous density gradients were used either separately or sequentially in an attempt to isolate cell preparations that are significantly enriched in one of the several subtypes of thymocytes present in the C57BL mouse thymus. Electron microscopy analysis of the cellular content of fractions obtained by such methods demonstrates that fractionation by 1g sedimentation allows the isolation of either pure preparations of small cortical lymphocytes or of fractions that are significantly enriched in blast cells, whereas density gradient centrifugation yields to the selection of the low to medium density lymphoblasts. A sequential separation procedure using both sedimentation at unit gravity and density gradient centrifugation was found to be the method of choice to isolate pure preparations of medullary lymphoid cells or to obtain fractions devoid of small lymphocytes and highly enriched in blast cells of the X subtype. These methods may thus serve as a basis for the selective isolation of putative target cells which are believed to be present in the thymus and to play a role in the pathogenesis of thymic lymphomas in C57BL mice.