Spontaneous exocrine pancreas hypoplasia in specific pathogen-free C3HeB/FeJ and 101/H mouse pups causes steatorrhea and runting

Under specific pathogen-free conditions, 1.3% to 1.8% of litters born in our inbred 101/H and C3HeB/FeJ mouse colonies had pups with steatorrhea and runting. Clinically affected male and female pups were first identified when they were from 14 to 25 d old. Unaffected littermates were healthy and were weaned successfully. Postmortem findings in 8 clinically affected mice included a small, poorly differentiated exocrine pancreas comprising cytokeratin-negative duct-like structures but lacking recognizable acinar cells with their normal carboxypeptidase B-positive zymogen granules. Endocrine pancreas islets were unremarkable and contained insulin-positive beta cells and glucagon-positive alpha cells. There was mild inflammation of the hindgut but no evidence of intestinal pathogens or marked inflammation or necrosis of pancreas, either alone or as part of a multisystemic inflammatory condition. Sera from pups in 4 affected litters did not contain antibodies to reovirus 3, mouse coronavirus, rotavirus, or mouse adenovirus 2. Furthermore, 4 sets of parental mice and sentinel mice from the facility were negative for 13 viruses, bacteria, and parasites. C3HeB/FeJ and 101/H inbred strains may be genetically predisposed because the steatorrhea and runting was absent in 13 other mouse strains and subspecies bred in the specific pathogen-free facility. This condition resembles exocrine pancreas hypoplasia, but the inheritance is complex. A wider implication is that runting coupled with steatorrhea are phenotypic criteria to suspect pancreatic disease that could be used in the context of a mouse N-ethyl-N-nitrosourea-mutagenesis program to identify potential mutants with defects in pancreas development.

Experimental dental caries on gnotobiotic inbred mice

The purpose of this study in mono-infected gnotobiotic BALB/cA and C3H/HeN mice was to evaluate the cariogenicity of Enterococcus faecalis. The caries incidence and mean caries score in the BALB/cA mice were significantly higher than those in the C3H/HeN. In both of the mouse strains, the mean number of E. faecalis isolated from the cecum content was almost the same, however, the mean number of E. faecalis from the maxilla of BALB/cA was significantly higher than that of C3H/HeN. These results indicate that C3H/HeN has some factors that prevent E. faecalis from attaching to the tooth surfaces.

Endogenous nonecotropic proviruses mapped with oligonucleotide probes from the long terminal repeat region

We characterized endogenous proviruses in C57BL/6J, DBA/2J, and C3H/HeJ mouse strains with oligonucleotide probes derived from long terminal repeat (LTR) sequences of three classes of nonecotropic murine leukemia virus. The segregation of proviral-host DNA junction fragments was followed in BXH and BXD recombinant inbred (RI) strain sets, and most fragments mapped readily to defined chromosomal regions. Most of the LTR fragments appear to correspond to proviruses mapped previously with oligonucleotide env region probes of the same viral class. At least 22 elements represent new proviral loci, no more than half of which may be solo LTRs, and an additional six may correspond to proviruses identified previously with less specific hybridization probes. Together with proviruses identified previously with env probes, the LTR probe-reactive elements represent the majority of endogenous murine leukemia proviruses in the mouse genome.

The virulence plasmid does not contribute to growth of Salmonella in cultured murine macrophages

The virulence plasmid, characteristic of many serovars of Salmonella sp., and specifically its spv genes, promote intracellular growth of the bacteria in the liver and spleen and are essential for the virulence of these Salmonella serovars in the mouse. In an attempt to establish an in vitro model for studying its function, we evaluated its effect on the intracellular growth of the bacteria in macrophages in culture. We used a number of different macrophage-like cell lines (J774-A.1, IC-21 and PU5-1.8), as well as peritoneal or splenic macrophages from genetically Salmonella-sensitive (Itys, BALB/c) or resistant (Ityr, C3H/HeN) mice, and at different states of activation, stimulated in vivo or in vitro with lipopolysaccharide and/or recombinant gamma interferon. These were found to differ in their ability to suppress or sustain intracellular growth of several Salmonella serovars, but in all cases the growth was independent of the spv genes.

Histopathologic observations in weanling B6C3F1 mice and F344/N rats and their adult parental strains

Weanling Fischer 344/N (F344) rats and the first filial hybrid of C57BL/6 x C3H (B6C3F1) mice and retired breeders from the parental stocks of these strains were monitored over a 5-yr-period by examining the histopathology of selected organs and comparing those results to viral and mycoplasmal serology and the intestinal tract bacterial flora of each animal on an individual basis. Serology gave no evidence of viral infection, but Mycoplasma arthriditis antibodies were detected. Reactivity of serum of adult C57BL/6 female mice with control cells or media (tissue culture, TC) was seen in a significant number of mice. TC reactivity correlated positively with lymphoid perivascular infiltrates, predominantly of the lungs, suggesting an allergic response in development of the lesions. Other lesions of note consisted of Harderian gland inflammation of rats, focal necrotizing lesions of the liver of both species, and thickening of the pleura and adjacent pulmonary interstitium of weanling rats. Embolization of bacteria from the gastrointestinal tract to the liver was considered a possible cause of the liver necrosis in both species. Although lesions of the lung and Harderian gland of the rats are similar to those caused by known viral agents, the cause of the latter could not be determined as these animals were negative for viral antibodies and the former was considered to be related to incomplete pulmonary development in the young rat. Features differentiating the lesions observed in animals of this survey from those caused by viral infection are discussed.

The cutaneous microbiology of haired and hairless mice

The cutaneous microflora of the mid-dorsal area of hairless and haired mice was studied by processing skin biopsies. In both C3H and CBA hairless genotype animals the prevalence of colonization and the bacterial density were significantly greater than in the haired animals. The dominant bacteria were staphylococci and aerobic coryneforms. No propionibacteria were isolated. Temporal studies with C3H mice showed that from 0 to 9 days after birth the cutaneous microflora reduced and from then on the haired genotype animals maintained a low cutaneous microflora, whilst hairless genotype animals gradually lost hair from head to tail and the microflora density increased. Reciprocal skin grafting between haired and hairless animals showed that the donor skin acquired the microflora characteristics of the recipient animal after 15 d post-grafting even though the donor skin remained morphologically true to genotype.

Coxsackievirus B3 myocarditis in C3H/HeJ mice: description of an inbred model and the effect of exercise on virulence

The effect of forced exercise on the development of coxsackievirus B3 myocarditis in inbred C3H/HeJ mice was studied. Four groups of mice (30 per group) were formed: infected-exercised (Group I); infected-unexercised (Group II); uninfected-exercised (Group III); and uninfected-unexercised (Group IV). Infected mice were inoculated intraperitoneally with 1.0 x 10(2.1) TCID50 coxsackievirus B3. Exercised animals were swum daily for 60 minutes on days 1-9. Myocardial viral titers were acutely elevated on day 3 of infection and were augmented significantly by exercise on days 6 and 9. Exercise increased the overall mortality from 0-10% to 20-40%; significantly increased heart: body weight ratios on days 6, 9 and 13; and increased the extent of myocardial fiber necrosis. We have reproduced the acceleration of CB3 myocarditis by exercise in the inbred C3H model.

Clearance of different strains of Mycoplasma pulmonis from the respiratory tract of C3H/HeN mice

Pathogen-free C3H/HeN mice were exposed by aerosol to Mycoplasma pulmonis PG34(ASH), UAB 5782C, M1, UAB T, or UAB CT, and clearance of mycoplasmas from the nasal passages, trachea, and lungs was determined during the first 72 h postinoculation (PI). There were differences among strains of mycoplasmas in physical removal of organisms and in killing by nonspecific factors in the nasal passages and trachea. The avirulent strain, PG34(ASH), was quickly removed from the nasal passages and trachea. Physical removal of the other mycoplasmal strains occurred slowly, with 60 to 89% of the radioactive label remaining in the nasal passages and trachea even after 72 h. There were significant differences in killing among mycoplasmal strains by nonspecific host mechanisms in the nasal passages, trachea, and lungs. Strain UAB T was quickly killed at all levels of the respiratory tract. Strains UAB 5782C and M1 were killed at all three sites by 2 to 4 h PI. The most virulent strain, UAB CT, was killed much more slowly than the other strains. However, there was no statistical difference in the relative numbers of mycoplasmas present in the lungs at 72 h PI among strains UAB CT, UAB 5782C, and M1. These studies showed that the different mycoplasmal strains were cleared from the respiratory tract by different mechanisms and suggest that the differences in virulence among the mycoplasma strains can be explained, in part, by the differences in elimination of the organisms from the respiratory tract by nonspecific host defense mechanisms.

A single point mutation in the envelope gene is responsible for replication and XC fusion deficiency of the endogenous ecotropic C3H/He murine leukemia virus and for its repair in culture

The molecular basis has been determined for differences in infectivity and XC phenotype of endogenous ecotropic murine leukemia virus of the low-leukemia mouse strain C3H/He, its relative in the high-leukemia mouse strain AKR, and highly infectious, XC-positive C3H virus variants selected in vitro. Endogenous ecotropic type C virus induced by iododeoxyuridine from the nontransformed C3H/10T1/2 cell line is XC negative and replication deficient. In contrast, viruses produced late after iododeoxyuridine induction in chemically transformed C3H/10T1/2 cells (MCA5) are XC positive and infectious. XC-negative viruses can be converted to XC-positive viruses by being grown in certain transformed cell lines. We have cloned the endogenous ecotropic provirus of C3H/He from MCA5 cells, which is XC negative and replication deficient, as well as two XC-positive C3H proviruses derived by in vitro conversion. Fragment exchange between the XC-negative molecular clone p110 and the XC-positive AKR virus clone p623 revealed that the defect in p110 lies 3' of the SalI site located in the pol region. Nucleotide sequencing established that the C3H p110 provirus was integrated within the R region of an endogenous VL30 long terminal repeat (LTR) in reverse orientation and that the virus differed from the infectious AKR p623 provirus by a point mutation, substituting Lys for Arg at the potential precursor cleavage site for gp70 and p15E. In vitro-converted XC-positive C3H proviral clones 3211 and 4211 are identical to XC-negative C3H p110, except that they have Arg at this site and the normal cleavage site is thus regenerated in these clones. The XC-negative C3H p110 was blocked in processing of Pr85env, whereas clones 3211 and 4211 had normal cleavage of the env precursor into gp70. Both the XC-negative C3H provirus and the in vitro-converted XC-positive C3H proviruses had a single copy of a 99-base-pair enhancer element in the LTR, whereas two copies of this sequence are present in the AKR proviral LTR. Substitution of Arg for Lys at the envelope precursor processing site of C3H p110 by site-directed mutagenesis is sufficient by itself to convert the virus to the XC-positive replication-competent phenotype. Thus, we have established that a single point mutation at the processing site of the envelope precursor protein Pr85 is responsible for the difference in the infectivity and XC phenotype of endogenous ecotropic murine leukemia virus from C3H/He and AKR mice and that the basis for in vitro conversion is a mutation at this site.

Macrophages from endotoxin-hyporesponsive (Lpsd) C3H/HeJ mice are permissive for vesicular stomatitis virus because of reduced levels of endogenous interferon: possible mechanism for natural resistance to virus infection

The C3H/HeJ mouse strain bears an autosomal gene defect, Lpsd, which results in a greatly diminished capacity to respond to endotoxin, the ubiquitous lipopolysaccharide derived from the cell walls of gram-negative bacteria. These mice also exhibit greater susceptibility to a variety of viral and bacterial infections than syngeneic, fully lipopolysaccharide-responsive (Lpsn) mouse strains and possess macrophages with defects in differentiation which are reversed by treatment with exogenous interferon (IFN). To test directly the hypothesis that C3H/HeJ macrophages are deficient in endogenous IFN levels, macrophages from C3H/HeJ (Lpsd) and C3H/OuJ (Lpsn) mice were compared for sensitivity to vesicular stomatitis virus. At a multiplicity of infection of 0.1, C3H/OuJ macrophages were completely refractory to infection, whereas C3H/HeJ macrophages were permissive for replication, and infection resulted in 100% cytopathic effect. These findings were confirmed with a second inbred Lpsn and Lpsd strain pair. Levels of 2',5'-oligoadenylate synthetase were significantly higher in Lpsn cells. C3H/HeJ macrophages, derived from bone marrow precursors under the influence of macrophage colony-stimulating factor, shown previously to induce IFN in macrophages, were as refractory as C3H/OuJ macrophages. Exposure of nonpermissive macrophages to anti-IFN-alpha/beta antibody prior to infection rendered cells permissive. Our findings suggest that endotoxin provides a primary stimulus for the maintenance of normal macrophage differentiation and innate resistance via the induction of endogenous IFN by macrophages.

Difference in susceptibility to gram-negative urinary tract infection between C3H/HeJ and C3H/HeN mice

The difference in susceptibility to urinary tract infection between C3H/HeJ and C3H/HeN mice was tested for with gram-negative strains differing in lipopolysaccharide composition. Recently, impaired clearance of Escherichia coli from the kidney of C3H/HeJ compared to C3H/HeN mice was shown to be correlated with the LPS low responsiveness. In this study, a difference in clearance from the kidneys of C3H/HeJ and C3H/HeN mice was found only with lipopolysaccharide-containing bacteria. Gram-positive bacteria, e.g., Staphylococcus saprophyticus and Streptococcus agalactiae, were recovered in essentially equal numbers from the kidneys of mice of both strains. In contrast, of the lipopolysaccharide-containing strains used, all persisted in higher numbers in the kidneys of C3H/HeJ mice than in the kidneys of C3H/HeN mice. Variations in the O side chain did not eliminate this difference. E. coli Hu734 O75+K5+ and the rfb- mutant O75-K5+ remained in similar numbers in C3H/HeJ mice, although O75-K5+ was eliminated more rapidly in C3H/HeN mice. The core structure did not affect the differential persistence in the two mouse strains. The rfb mutants with R1-R4 cores were eliminated after 24 h from the C3H/HeN mice, but remained in significant numbers in the kidneys of C3H/HeJ mice. Even the Re mutant of Salmonella minnesota persisted in low numbers in C3H/HeJ mice. The relative bacterial recovery from either mouse strain was related to the overall virulence of the infecting bacterial strain, but the difference between C3H/HeJ and C3H/HeN mice was associated with responsiveness to parts of lipopolysaccharide common to the bacterial strains tested.

Recovery of aerobic and anaerobic bacteria from irradiated mice

Anaerobic and aerobic bacteria were often found as mixed infections in 225 lethally irradiated mice. Of a total of 57 mice that were sacrificed, aerobic bacteria were recovered exclusively in 9 (27%) of the 34 culture-positive mice, anaerobic bacteria were recovered exclusively in 15 (44%), and mixed aerobic and anaerobic flora were recovered in 10 (29%). The predominant organisms were anaerobic cocci Escherichia coli, Proteus mirabilis, Staphylococcus spp., and Bacteroides spp.

Distribution and effects of a defined six-member murine-derived microflora in gnotobiotic gerbils

The gnotobiotic gerbil was selected as a model with which to study the effects of colonization with a defined microflora on organ morphology, histology, and selected blood biochemical parameters. Gerbils were maintained germfree for 13 months but failed to reproduce, presumably because of the enlarged cecum. A colony of gnotobiotic gerbils that was associated with a bacterial flora consisting of Lactobacillus brevis, Streptococcus faecalis, Staphylococcus epidermidis, Bacteroides vulgatus, Enterobacter aerogenes, and a Fusobacterium sp. was established. These gnotobiotic gerbils had smaller ceca than germfree gerbils and proved capable of reproduction. Except for the presence of large numbers of Bacteroides organisms in the stomach and greater numbers of S. epidermidis in gnotobiotic gerbils, the number and location of gastrointestinal bacteria were similar in conventional and gnotobiotic gerbils. Bacteroides sp. was the second most predominant microorganism present in gnotobiotic gerbils, whereas clostridia were reported to be the second most predominant microorganism in conventional gerbils. Microscopic examination of direct-impression smears indicated that fusobacteria were present on mucosal surfaces. Intestines of gnotobiotic gerbils weighed twice as much as the intestines of conventional gerbils. Intestinal tissue water weight values from conventional and gnotobiotic gerbils were similar. Histological examination of gerbil intestinal tissue revealed no cellular hypertrophy and no evidence of inflammation in gnotobiotic gerbil intestines. Spleens of gnotobiotic gerbils showed no germinal center stimulation. Statistical differences in total serum glucose, serum protein, and hematocrit levels were found between conventional and gnotobiotic gerbils.

New mammalian transforming retrovirus: demonstration of a polyprotein gene product

A new acute transforming type C retrovirus was isolated from mice inoculated with a virus stock obtained by iododeoxyuridine induction of methylcholanthrene-transformed C3H/10T1/2 mouse cells. This virus, designated 3611-MSV, transforms embryo fibroblasts and epithelial cells in culture and induces fibrosarcomas in vivo. 3611-MSV is replication defective, requiring a type C helper virus for propagation both in vitro and in vivo. By using endpoint transmission of 3611-MSV to MMCE C17 mouse and FRE 3A rat cells, several nonproductively transformed clonal cell lines have been derived. Pseudotype virus stocks obtained from such clones transform cells in vitro, are highly oncogenic in vivo, and exhibit host range and serological properties that are characteristic of their helper virus component. Analysis of viral antigen expression in 3611-MSV-transformed cells has led to the demonstration of a 90,000-molecular-weight (Mr) polyprotein and a 75,000-Mr probable cleavage product, both containing the amino-terminal murine leukemia virus gag gene proteins p15 and p12. In contrast to gene products of many previously described mammalian transforming viruses, 3611-MSV-encoded polyproteins lack detectable protein kinase activity, and 3611-MSV-transformed cells resemble chemically transformed cell line C3H/MCA-5, from which 3611-MuLV was originally derived, in that they do not exhibit elevated levels of phosphotyrosine. By using molecular hybridization the 3611-MSV transforming gene was found to be distinct from previously described mammalian cellular oncogenic sequences, including c-ras, c-abl, c-fes, c-fms, c-sis, and c-mos.

Genome structure of mink cell focus-forming murine leukemia virus in epithelial mink lung cells transformed vitro by iododeoxyuridine-induced C3H/MuLV cells

We characterized mink cell focus-forming murine leukemia viruses that were isolated from C3H/MCA-5 cells after induction with 5-iododeoxyuridine in culture. Mink lung epithelial cells malignantly transformed in vitro by induced virus were the source of four molecular clones of mink cell focus-forming virus. CI-1, CI-2, CI-3, and CI-4. Three clones, CI-1, CI-2, and CI-3, had full-length mink cell focus-forming viral genomes, one of which (CI-3) was infectious. In addition, we obtained a defective viral genome (CI-4) which had a deletion in the envelope gene. A comparison between the envelope genes of CI-4 and those of spleen focus-forming virus by heteroduplex mapping showed close homology in the substitution region and defined the deletion as being identical to the p15E deletion of spleen focus-forming virus. The recombinant mink cell focus-forming genomes are not endogenous in C3H/MCA-5 cells and therefore must have been formed in culture after induction by 5-iododeoxyuridine. CI-3, the infectious clone of mink cell focus-forming murine leukemia virus, was dualtropic, and mink cells infected with CI-3 were altered in their response to epidermal growth factor. In the presence of epidermal growth factor at 10 ng/ml, uninfected mink cells retained their epithelial morphology in monolayer culture and did not form colonies in soft agar. In contrast, CI-3 virus-infected mink cells grew with fibroblastic morphology in monolayer culture and showed an increased growth rate in soft agar in the presence of epidermal growth factor.

Kinetics of expression of infectious ecotropic, xenotropic, and mink cell focus-forming murine leukemia virus after 5-iododeoxyuridine induction of cells from high- and low-leukemia mouse strains

Endogenous murine leukemia virus (MuLV) was induced with 5-iododeoxyuridine (IdUrd) from the high-leukemia mouse strain AKR and from two low-leukemia strains, C3H/He and BALB/c. A virus-free cell line from strain AKR readily gave rise to infectious, XC-positive MuLV upon treatment with IdUrd, whereas cells from strains C3H/He and BALB/c produced replication-deficient, XC-negative MuLV. IdUrd-induced cells also produced xenotropic and mink cell focus-forming MuLV. Xenotropic virus emerged at a higher titer from both AKR and BALB/c cells during two discrete time intervals, first at day 3 after induction and a second time during spread of the induced ecotropic MuLV. Xenotropic and mink cell focus-forming MuLVs were also produced by IdUrd-induced C3H/He cells but required another round of infection in Sc-1 cells for detection. The in vitro infectivity of endogenous ecotropic MuLV isolated by IdUrd induction from C3H/He cells correlated with pathogenicity in the Fv-1-compatible, leukemia-negative mouse strain NFS/N. Thus, the virulence of endogenous ecotropic MuLV may be an important factor in determining the leukemia incidence in these inbred strains of mice.

Colonization of tissue surfaces in the gastrointestinal tract of gnotobiotic animals by lactobacillus strains

Eight strains of lactobacilli isolated from conventional mice, rats, fowls, and pigs were tested for their ability to colonize squamous epithelia in the gastrointestinal tracts of gnotobiotic animals. Two of the strains isolated from rodents could colonize the nonsecretory epithelium of the stomach of gnotobiotic rodents, and one strain isolated from a fowl could colonize the crop epithelium of gnotobiotic chickens. The specificity of colonization of squamous epithelia by lactobacilli was demonstrated by these in vivo studies; strains of lactobacilli isolated from rodents could only colonize rodent epithelium; the strain from a fowl could only colonize crop epithelium.

Pulmonary clearance of Haemophilus pleuropneumoniae and Serratia marcescens in mice

Bacterial counts (per lung) were made from Swiss White and/or C3H mice, inoculated intranasally with either Haemophilus pleuropneumoniae or Serratia marcescens and killed at specific times. The percentages of bacterial survival and clearance at each time were determined. Serratia marcescens was cleared progressively, but not completely, from the lungs of C3H and Swiss White mice. The overall pulmonary clearance of S marcescens by Swiss White mice was significantly greater than that of C3H mice (P less than 0.01). The pulmonary clearance pattern of H pleuropneumoniae in C3H mice differed according to the dose inoculated. With a larger H pleuropneumoniae median lethal dose (0.1 LD50), the organism multiplied consistently up to 25 times by 12 hours after the inoculations were done, when clinical signs and lesions appeared in a few of the mice. The clearance rates at 24 and 48 hours after inoculations were 60.65% and 10.3%, respectively. Mice given 0.04 LD50 H pleuropneumoniae showed a 10-fold increase of H pleuropneumoniae in the first 4 hours, with clearances reaching 33%, 66%, and 99% at 8, 12, and 24 hours, respectively.

Molecular cloning and characterization of murine leukemia virus-related DNA sequences from C3H/HeN mouse DNA

Ten murine leukemia virus (MuLV)-related DNA sequences were isolated from C3H/HeN mouse genomic DNA by cloning of EcoRI fragments in a Charon 4A vector. Detailed restriction endonuclease maps of four of the clones were developed by using AKR MuLV [32P]cDNA as a probe. C3H clone 14-9 contains approximately 7 kilobase pairs of MuLV-related DNA, one copy of an MuLV long terminal repeat-like sequence, and a region of flanking mouse DNA. C3H clones 34.2 and 36.1 contain approximately 2 kilobase pairs of MuLV-related DNA, one copy of a MuLV LTR-like sequence, and differing lengths of flanking mouse DNA sequences. C3H clone 8.13 was found to contain an insert of 5.7 kilobase pairs of MuLV-related DNA with two long terminal repeat-like regions and sequences which are partially homologous to AKv-1. Comparison fo the restriction endonuclease cleavage maps of these C3H clones with maps recently developed for ecotropic and xenotropic MuLV DNAs indicates that C3H clone 14-9 corresponds to the 5'-terminal portion of a genomic DNA sequence related to xenotropic MuLVs, whereas C3H clones 34.2 and 36.1 correspond to the 3' terminal portions of genomic DNA sequences related to xenotropic MuLVs. Clone 8.13 represents a deleted, xenotropic MuLV-related provirus. C3H clones 14-9, 34.2, 36.1, and 8.13 provide defined DNA sequence probes with which to characterize the organization and expression of endogenous MuLV-related DNA sequences in the mouse genome.

Continuing germ line integration of AKV proviruses during the breeding of AKR mice and derivative recombinant inbred strains

The gel electrophoresis-hybridization technique of Southern was used to analyze genetically transmitted proviruses coding for the AKV strain of murine leukemia virus. We were able to identify the restriction endonuclease EcoRI fragments containing two previously unidentified, genetically transmitted AKV proviruses of AKR mice. Comparison of different sublines of AKR mice revealed considerable heterogeneity in their complement of germ line proviruses. This heterogeneity provides evidence that the provirus complement of AKR mice is not stable. Rather, the number of genetically transmitted proviruses increases during inbreeding. Examination of a series of sublines of the C3H strain indicated that this amplification is dependent on viremia. We estimate that, in viremic strains of mice, one new provirus becomes fixed in the germ line every 15 to 30 years.

[Detection of MMTV gp52 in C3H and C3Hf strain mice]

Immunofluorescence in paraffine sections demonstrated the difference in expression of the membrane glycoprotein of mouse mammary tumor virus (gp52) in two mouse sublines: C3H and C3Hf. In the C3Hf the glycoprotein becomes detectable in the mammary gland in the second part of pregnancy while in C3H line mature females before pregnancy. No viral glycoprotein could be found in the spleen, liver, kidneys and other organs in males and females of the two sublines or in C3H embryos.

Genetic linkage of C3H/HeJ and BALB/c endogenous ecotropic C-type viruses to phosphoglucomutase-1 on chromosome 5

The genetic linkage of the endogenous C3H/HeJ C-type ecotropic virus to phosphoglucomutase-1 (0.28, recombinant fraction) on chromosome 5 was established by means of serological assays of backcrossed mice. With a combination of serological techniques and DNA-DNA hybridization the BALB/c endogenous ecotropic virus was shown to be either closely linked or allelic with the C3H/HeJ locus.

Generation of oncogenic type C viruses: rapidly leukemogenic viruses derived from C3H mouse cells in vivo and in vitro

A type C virus was isolated from C3H/10T1/2 mouse cells in culture after activation with iododeoxyuridine. This virus was poorly infectious for mouse cells and did not cause tumors upon inoculation into newborn NIH Swiss mice. Variants with increased infectivity for mouse cells were then derived both in vivo and in vitro by selecting for variants able to grow to high titers. The highly infectious variants were found to induce mouse fibroblasts to grow in soft agar. When the viruses were inoculated into newborn NIH Swiss mice, 100% of the animals died of leukemia within 4 months. Solid tumors developed at the injection site. Both mouse-tropic and dualtropic viruses were isolated from the leukemic mice and plaque purified. The first group of viruses produced large syncytial plaques on rat XC cells and did not grow in mink cells. The viruses of the other group replicated well in both mouse and mink cells, producing morphologic changes similar to transformation but not XC syncytia; they, therefore, are members of the newly described MCF class of mouse type C viruses. Isolates from either group were highly leukemogenic on retesting, the mean latent period being 67 days for a mouse-tropic virus and 105 days for one of the dual-tropic viruses. The results led to the conclusion that the better a mouse type C virus grows in cell culture the more effective it is as a leukemogen. Further, it is possible to start with a weakly infectious, nonleukemogenic virus and to convert it to a rapidly replicating, highly leukemogenic virus by passage either in cell culture or in the animal. The availability of a defined series of viruses from a low-leukemia mouse strain that differ greatly in their biologic properties should facilitate studies of the molecular basis for the acquisition of type C virus oncogenicity.

Radioimmunoassay for intact Gross mouse leukemia virus

A radioimmunoassay for intact Gross leukemia virus has been developed using 125I-labeled Gross virus grown in tissue culture and guinea pig antisera to Gross virus grown either in tissue culture or harvested from leukemic C3H(f) mice. Separation of bound from free labeled virus was effected using the double antibody method. The assay can detect fewer than 10(8) virus particles and has been used to measure the viral content of individual organs from inoculated leukemic C3H(f) mice and from Ak mice with spontaneous leukemia. Organs from noninoculated healthy C3H(f) mice crossreacted poorly in the system, virus generally being detectable only in the thymus and spleen and at low concentration. In some of the inoculated C3H(f) leukemic mice the viral content of as little as 0.5 mul of plasma is measurable. That this assay is for intact virus and not for soluble antigens of the viral envelope was proven by the observation that the immunoreactive material of plasma and extracts from thymus and liver of leukemic mice has a buoyant denisty in sucrose of 1.17-1.18 g/ml, corresponding to that of intact virus grown in tissue culture. With this sensitivity it may now be possible to quantitate viral concentrations in tissue and body fluids from the time of inoculation through the development of obvious pathology.