Naturally occurring leukemia viruses in H-2 congenic C57BL mice. III. Characterization of C-type viruses isolated from lymphomas induced by milk transmission of B-ecotropic virus

The prevalence of different host-range classes of murine leukemia virus (MuLV) was studied in C57BL mice with (V+) and without (V-) milk transmission of a naturally occurring B-tropic ecotropic MuLV. Virus isolates were studied with respect to growth properties, XC-plaque formation, antigen profiles of their envelope proteins (gp70 and P15(E)), gp70 tryptic-peptide maps, and their potential to induce lymphomas after inoculation into newborn mice. B-tropic ecotropic MuLV with the capacity to cause plaques in XC cells was isolated from almost all lymphomas of both V+ and V- sublines. The reaction patterns of these ecotropic isolates with monoclonal antibodies reactive with MuLV-env proteins and the tryptic-peptide maps of the gp70 molecule indicate that they are similar to each other and differ only slightly from the ecotropic MuLV in the spleens of young V+ animals, which is identical to the milk-transmitted virus. XC-, B-tropic dualtropic mink cell focus-inducing (MCF) viruses were isolated from the majority of different types of lymphoma (B cell, T cell, or neither B nor T cell derived), but not from the spleens or milk of young V+ or V- animals. The env proteins of the MCF isolates are highly heterogeneous, but most isolates originating from B10.AV + T-cell lymphomas share MCF-related epitopes in their gp85 envelope precursor with AKR MCF-247 virus. Most MCF viruses isolated from non-T lymphomas do not possess these epitopes. The results indicate that also in this model the generation of dualtropic MCF viruses might be important in lymphoma induction, although only some of the cloned MCF viruses show enhanced oncogenic properties in comparison with ecotropic isolates. A cloned oncogenic MCF virus induced different lymphoma types in C57BL/10 (= B10, H-2b) and B10.A (H-2a) mice, similar to what was found earlier with the milk-transmitted virus. Hence, the lymphoma-type differences are not due to differences in the B-tropic ecotropic viruses transmitted through the milk in these strains, but reflect an influence of the H-2 complex on the phenotype of the virus-induced lymphomas.     

Different murine cell lines manifest unique patterns of interference to superinfection by murine leukemia viruses

Interference to superinfection by murine leukemia viruses (MuLV) was analyzed in cells chronically infected with other MuLVs. A new sensitive focal immunofluorescence assay employing monoclonal antibodies was used to detect foci of virus infection in live cell monolayers. Monoclonal antibodies were chosen which reacted with the challenge virus but not with the interfering virus. The results obtained confirmed some of the findings of previous workers using Moloney sarcoma virus pseudotypes as challenge viruses on mouse and nonmouse cells. In addition, SC-1 mouse cells nonproductively infected with defective spleen focus-forming virus were found to be resistant to superinfection by recombinant dual-tropic viruses. Furthermore, results indicated that interference patterns between some pairs of viruses differed in different cell types. Thus, xenotropic MuLV blocked superinfection by recombinant dual-tropic viruses in SC-1 feral mouse cells, but not in two lines of NZB mouse cells. Also, in a Mus dunii tail fibroblast cell line some unique patterns of interference were observed. One ecotropic MuLV blocked infection by two xenotropic viruses and three recombinant dual-tropic viruses. Two other ecotropic viruses blocked infection by only one of the two xenotropic viruses tested. These two ecotropic viruses also differed from each other in their ability to block the three recombinant viruses. In addition, two strains of amphotropic MuLV also differed in their interference capacity. As expected, strain 1504A did not block any viruses tested, whereas strain 4070A surprisingly blocked one xenotropic and one ecotropic MuLV. The lack of homogeneity in interference patterns seen in the Mus dunii cells suggested either that a large number of heterogeneous virus receptors were present on this cell line or that interference in these cells might operate through a mechanism other than blocking of virus receptors by the envelope protein of the interfering virus.     

A thymus-dependent human serum factor induces a decrease of terminal deoxynucleotidyl transferase in thymocytes

Previously, we have shown that a thymus-dependent human serum factor (SF) selectively acts on hydrocortisone-sensitive, immunologically immature thymocytes, inducing hydrocortisone resistance and immunological maturation as evaluated by graft-vs-host reaction. Human and mouse thymocytes were incubated with SF for different times and the activity of terminal deoxynucleotidyl transferase (TdT) was measured hereafter. A marked decrease of TdT activity was found in thymocytes exposed to SF for 240 min. These data indicate that the maturation of thymocytes induced by SF is accompanied by a decrease of TdT activity.     

A cloned cytotoxic T-lymphocyte (CTL) line recognizing a subtype of HLA B27

The lymphoblastoid cell-line JY (HLA-A2,2; B7,7; C-; DR4,w6) was used to stimulate T cells from donor HG (HLA-A2, w23; B40,w44; Cw4; DRw6,7). Cloned CTL line were obtained by limiting dilution after tertiary stimulation. Strong cytotoxic activity on stimulator cells was found with all CTL clones obtained. One of the clones (HG-31 recognized a subtype of the HLA-B7 antigen. In this paper, we describe another long-term cloned CTL line (HG-61). This line, when tested on a panel of 107 target cells from unrelated individuals, recognized a subtype of HLA-B27 (B27 “K”). There was no significant association with any other HLA antigen. The cloned CTLs were T8⁺ and their cytotoxic activity could be blocked by the monoclonal antibody W6/32 which recognizes a framework determinant on HLA-A, -B, and -C molecules. In families, reactivity with cells of the CTL line (HG-61) segregated with HLA. It is concluded that the CTL line interacts with an antigenic determinant shared between the HLA-B7 antigen of JT and the subtype of HLA-B27 (B27 “K”), or detects products of a gene closely linked to HLA-B, not revealed by present-day serology.     

Effect of murine host genotype on MCF virus expression, latency, and leukemia cell type of leukemias induced by Friend murine leukemia helper virus

Leukemias induced by neonatal inoculations of several mouse strains with different strains of Friend murine leukemia helper virus (F-MuLV) were followed for time of disease onset, cytochemical analysis of predominant cell types in leukemic organs, and expression of infectious mink cell focus-inducing (MCF) viruses detected by mink cell foci or MCF-specific monoclonal antibodies. Most BALB.B and IRW mice had a rapidly appearing, severe anemia and hepatosplenomegaly consisting of erythroid cells. MCF viruses were usually isolated from enlarged spleens of IRW mice. In contrast, C57BL/10 mice had a lower incidence of disease and much slower course. Splenomegaly and lymphadenopathy with mild anemia were seen, and the predominant cell types were either myeloid (chloroleukemia) or lymphoid. MCF viruses were never isolated from this mouse strain. (C57BL/10 X IRW)F1 mice were intermediate in latency, but all mice had disease by 8 months. Myeloid, lymphoid, and some mixed leukemias with an erythroid component were observed, but in no case did we see the severe anemia or pure erythroid involvement typical of IRW and BALB.B mice. MCF viruses were, however, isolated from 22% of these mice regardless of leukemia cell type. DBA/2 mice had a disease pattern similar to the (C57BL/10 X IRW)F1 mice, and MCF viruses were isolated from three of six mice tested. Inoculation of IRW mice with the low virulence B3 strain of F-MuLV produced disease with a longer latency than F-MuLV 57, but similar cell types were transformed by both viruses. In vitro cell lines were derived from 14 mice, and most were tumorigenic in vivo. Three lines released infectious MCF virus, and three others expressed MCF-specific cell surface antigens but did not release virus. Eight lines expressed no MCF infectious virus or viral antigens. Several lines released infectious xenotropic viruses and/or expressed xenotropic MuLV cell surface antigens recognized by monoclonal antibodies reactive with xenotropic viruses. The lack of MCF expression in many primary leukemic tissues as well as in in vitro derived leukemia cell lines of C57BL/10 and (B10 X IRW)F1 mice suggested that MCF virus generation and expression may not be required for leukemogenesis in some mouse strains or in some hemopoietic lineages.     

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

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 D₃₇ inactivation dose of 2550 ergs/mm², 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 $\text{NIH}\text{3T3}$ cells with a B-tropic virus, followed by treatment with iodoeoxyuridine, failed to induce productive infection by endogenous or exogenous virus. Cells of F₁ 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.     

p65 of Gazdar murine sarcoma viruses contains antigenic determinants from all four of the murine leukemia virus (MuLV) gag polypeptides (p15, p12, p30, and p10) and can be cleaved in vitro by the MuLV proteolytic activity

Thin-section electron micrographs of Gazdar murine sarcoma virus (Gz-MSV) particles showed that 100% of the particles possessed an immature morphology. Correspondingly, p65 (the major 65,000-dalton protein observed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis for Gz-MSV particles) possessed antigenic determinants from all four of the murine leukemia virus (MuLV) Pr65^gag polypeptides—that is, p30, p15, p12, and p10. This result is in contrast to earlier observations (A. Pinter and E. deHarven (1979), Virology, 99, 103–110) which reported that p65 lacked antigenic determinants of MuLV p10. It is consistent with the recent finding of Maxwell and Arlinghaus ((1981), J. Virol., 39, 963–967) that Gz-MSV p65, when cleaved in vitro, gives rise to a polypeptide with the size and antigenic determinant of MuLV p10. Thus, we suggest that Gz-MSV p65 should be designated as Gz-MSV Pr65^gag. We also found that Gz-MSV Pr65^gag could be cleaved in vitro by using a partially purified proteolytic factor that had been derived from Moloney murine leukemia virus (M-MuLV) by Sephadex G-75 column chromatography (Y. Yoshinaka and R. B. Luftig (1980), J. Gen. Virol., 48, 329–340). Protein bands were produced that migrated on gels and had the same antigenic determinants as the MuLV intermediates Pr40^gag (p30, p10) and Pr27^gag (p15, p12). Pr55^gag (p15, p12, p30), a minor component, was also produced. Additional incubation of Gz-MSV Pr65^gag led to a breakdown of the intermediate polyproteins into the four MuLV gag polypeptides p30, p10, p15, and p12. The final processing of Pr55^gag and Pr40^gag occurred more rapidly than that of Pr27^gag. It thus seems that in vitro sequentially different processing events are involved in production of the four internal gag antigens from Gz-MSV Pr65^gag.     

Use of a focal immunofluorescence assay on live cells for quantitation of retroviruses: distinction of host range classes in virus mixtures and biological cloning of dual-tropic murine leukemia viruses

A rapid and sensitive focal immunofluorescence assay (FIA) using monoclonal antibodies or heterologous antisera was employed for detection and biological cloning of viruses capable of inducing viral antigens on cell surfaces. The FIA was performed directly on a variety of live cells in tissue culture dishes and was used successfully with C-type murine leukemia viruses (MuLV) of different tropism including ecotropic, xenotropic, amphotropic, and dual-tropic recombinant mink cell focus-inducing (MCF) viruses. With the FIA, we were able to titrate and distinguish ecotropic Friend-MuLV and Friend-MCF viruses present in mixtures. Dual-tropic MCF viruses could be specifically detected directly in mouse cells by using MCF-specific monoclonal antibodies. These antibodies replaced the requirement for production of typical MCF cytopathic effect in mink cells for MCF virus detection, and also allowed efficient titration in mouse cells of MCF virions pseudotyped with ecotropic envelope proteins. Furthermore, by picking foci of fluorescent cells and using their cell-free viral progeny, MCF viruses were cloned from complex pseudotypic mixtures. This allowed the cloning of viruses present at low frequency in heterogeneous mixtures obtained from leukemic tissues.     

Immunoglobulin E antibodies that crossreact with vegetable foods,pollen, and Hymenoptera venom

IgE in some human sera reacted with an antigen present in a large number of unrelated foods: potato, spinach, wheat, buckwheat, peanut, honey, and others. The antigen, which was periodate-sensitive and heat-stable, was also found in pollen. Even more surprisingly, these antibodies often reacted in vitro with bee and vespid venom and were sometimes apparently induced by Hymenoptera stings.     

Rapid microassays for the measurement of superoxide and hydrogen peroxide production by macrophages in culture using an automatic enzyme immunoassay reader

Two simple semiautomated microassays for the measurement of superoxide (O-2) and hydrogen peroxide (H2O2) production by cultured macrophages (MPs) are described. The measurement of O-2 is based on the reduction of ferricytochrome c as assayed by the increase in its absorbance at 550 nm. Quantitation of H2O2 is based on the horseradish peroxidase (HRPO)-dependent oxidation of phenol red which is assayed by its increased absorbance at 600 nm. MPs are cultured in monolayers in 96-well flat-bottom tissue culture plates and covered with 100 mul amounts per well of either a ferricytochrome c solution containing phenol red and HRPO. Following the addition of an agent eliciting an oxidative burst (OB) and incubation of the plates at 37 degrees C for various time intervals, the changes in the absorbance of ferricytochrome c and phenol red, respectively, are measured directly in the wells of the tissue culture plates with the cells in situ, by using an automatic 8-channel photometer which reads absorbances vertically through individual wells. This instrument, which was originally designed for reading enzyme immunoassays in microtitration plates, can be easily adapted for use in the above test, when fitted with interference filters with wave lengths of 550 nm (for the assay of O-2) and 600 nm (for the assay of H2O2). The principal advantages of this techniques are: the ability to perform the assays directly in the culture plates with cells in situ; the small amounts of cells and reagents needed; its sensitivity and reproducibility; the ease with which kinetic experiments can be done; the large number of samples which can be tested in parallel, and especially the speed and convenience offered by the automated reading and printout of absorbance values.     

Properties of mouse leukemia viruses. XIX. Effective antibody therapy of AKR leukemia occurs independently of virus neutralization and produces long-term changes in the virus status of the thymus

Administration of high-titered goat anti-FLV gp71 IgG to AKR mice during a narrow neonatal therapy "window" suppresses the early development of MuLV infectious cell centers (ICC) in spleen, thymus, and bone marrow. By 4-5 weeks of age ICC appear in spleen and marrow cell populations, rapidly increasing to plateau levels within 2 weeks in the absence of viremia. The thymus of treated animals remains devoid of detectable ICC throughout the 4-month period of testing. This pattern of ICC suppression occurs independently of virus neutralization as shown by the inability of F(ab')2 preparations, which retained neutralizing activity, to prevent early establishment of ICC. Immune IgG significantly decreases, but does not eliminate gp71 expression in all tissues tested. In control animals, ICC reside within a minor subpopulation of cortical, thymic T cells, whereas peripheral (i.e., splenic) ICC are totally devoid of conventional T cell, B cell, and macrophage phenotypic markers. Although thymocytes appear to be a major target of this antibody therapy, T-cell reactivity is not compromised.     

Assembly of a temperature-sensitive mutant of Rauscher murine leukemia virus at the cell surface induced by low temperature and by ligands.

NIH/3T3 mouse fibroblasts, infected with a Rauscher murine leukemia virus temperature-sensitive mutant (ts25) defective in assembly of budding particles at 39°, produce virus at the cell surface when the temperature is shifted rapidly to 0°. Virus buds are not assembled within the first 10 min at 0° and gradually increase in number and degree of development over a 2-hr period. However, release of infectious virus could not be demonstrated at 0° and might, therefore, be an energy-dependent process. Significant budding activity was also induced at the nonpermissive temperature by incubating cells with 0.25% glutaraldehyde or with antiserum to the major virus envelope glycoprotein, gp70. Anti-gp70 serum may induce budding by promoting aggregation of gp70-containing molecular assemblies and, consequently, association of core components in some transmembrane fashion. Induction of virus assembly with glutaraldehyde might occur as a results of nonspecific cross-linking of membrane proteins. These results suggest that procedures commonly used to minimize ligand-induced redistribution of cell surface molecules, i.e., labeling at low temperatures or after mild aldehyde fixation, may not immobilize certain membrane-associated molecules.     

A restriction map and analysis of the terminal redundancy in the group a streptococcal bacteriophage SP24

The DNA isolated from the group A streptococcal bacteriophage SP24 is a linear double-stranded molecule 42.0 kb in length. The DNA has been characterized by electron microscopy, and by agarose gel electrophoresis after cleavage with the restriction endonucleases SalI, BglII, XbaI, PvuI, HindIII, and BamHI. Analysis of SalI digests indicates that two fragments are present in submolar amounts and exist as a subset of sequences present in another SalI fragment. Moreover, overlapping endonuclease fragments suggested that the physical map is circular. This was confirmed when homoduplex phage DNA revealed circular structures with single-stranded tails that were 7.7% of the circumference of the genome length molecule. Tails were observed to be separated by as much as 42% of the circular homoduplex structure. These results indicate that the phage SP24 genome is terminally redundant and circularly permuted; and the data are consistent with a model in which DNA packaging into phage heads is initiated at a specific site on concatermeric DNA and proceeds sequentially to package up to five "headfuls" of DNA per concatemer.     

Inhibition of HSV-transformed murine cells by nucleoside analogs, 2'-NDG and 2'-nor-cGMP: mechanisms of inhibition and reversal by exogenous nucleosides

A murine cell line transformed with HSV TK (LH-1) exhibits a greatly enhanced cytotoxicity to the nucleoside analog 9-[(2-hydroxy-1-(hydroxymethyl)ethoxy) methyl]guanine (2'-NDG) as compared to the parental LM cell line (I50 LH-1 = 0.4 microM; I50 LM = 44.4 microM). Toxicity of 2'-NDG for LH-1 and LM is reversed only by the addition of 100 microM thymidine (dThd), indicating that 2'-NDG is a substrate for the viral and cellular TK. In LM(TK-) cells--murine cells expressing no TK activity, 2'-NDG cytotoxicity is partially reversed only with dGuo. A cyclic phosphate derivative of 2'-NDG, 2'-nor-cGMP, contains a phosphodiester bond, is also taken up by cells, and does not depend on viral TK for activation. LH-1 cells and LM(TK-) cells are inhibited by similar concentrations of this analog (5.1 and 4.1 microM, respectively). In all three cell lines (LM, LH-1, LM(TK-], the toxicity of 2'-nor-cGMP is significantly reversed with dGuo or cyclic dGMP. This pattern of reversal differs significantly from that observed with 2'-NDG, suggesting that 2'-nor-cGMP is metabolized as a guanosine analog, similar to acyclovir, in LM and LM(TK-) cells. These results indicate that a cyclic monophosphate analog of 2'-NDG can be activated independently of viral TK expression and that cellular metabolic pathways resulting in elevated dGTP concentrations are important for reversal of toxicity induced by guanosine-like nucleoside analogs.