Viral gene inhibition of class I major histocompatibility antigen expression: not a general mechanism governing the tumorigenicity of adenovirus type 2-, adenovirus type 12-, and simian virus 40-transformed Syrian hamster cells

The association between the level of class I major histocompatibility (MHC) antigen expression and the tumorigenic phenotype was determined for cells from a series of 15 lines of adenovirus type 2 (Ad2)-, Ad12-, and simian virus 40 (SV40)-transformed hamster cells and 16 lines of cells established from hamster tumors induced by SV40 mutants. These cells range from nontumorigenic to highly tumorigenic in both syngeneic and allogeneic adult hamsters. The Ad2-transformed cells--cells that were nontumorigenic in syngeneic adult hamsters--expressed either high levels or low levels of class I MHC antigens. The SV40-transformed cells--cells transformed in vitro that produced tumors with equal efficiency in both syngeneic and allogeneic adult hamsters--or cells derived from SV40-induced tumors expressed very high levels of class I MHC antigens. The Ad12-transformed cells uniformly expressed low levels of class I MHC antigens; these cells produced tumors 200- to 1,000-fold less efficiently in allogeneic adult hamsters than in syngeneic adult hamsters and produced tumors with about the same efficiency in immunoimmature newborns and immunocompetent syngeneic adult hamsters. We conclude that the expression of either high levels or low levels of class I MHC antigens is, at most, a minor factor in the differences observed among these adenovirus- and SV40-transformed cells in their tumor-inducing capacity in naive, immunocompetent hamsters.     

Oncogenic transformation by by equine herpesviruses. II. Coestablishment of persistent infection and oncogenic transformation of hamster embryo cells by equine herpesvirus type 1 preparations enriched for defective interfering particles.

Infection of permissive hamster embryo cells with virus preparations enriched for defective interfering (DI) particles of equine herpesvirus type 1 (EHV-1) resulted in persistent infection and oncogenic transformation. Six cell lines, designated DI-5 to -10, exhibited biological properties (immortality, increased saturation density, growth in soft agar, etc.) inherent to transformed cells, but 2 to 18% of the total cells in these cell lines were shown to release virus as judged by electron microscope studies and infectious center assays. The released virus was shown to be standard EHV-1 and not to contain DI particles as determined by density measurements of the viral DNA in the analytical ultracentrifuge and by interference assays using the released virus. Tumorigenicity studies revealed that inoculation of these persistently infected cells into newborn LSH inbred hamsters resulted in a lethal, fulminating hepatitis, whereas inoculation into older immunocompetent hamsters (+4 weeks) led to the development of metastatic fibrous sarcomas. Tumor cell lines (DI-5T to -10T) established from these sarcomas were shown to be transplantable and virus nonproducers. Hybridization analyses of cellular DNAs from DI transformed and tumor cell lines using 32P-labeled genomic EHV-1 DNA as probes indicated that the whole virus genome was detectable in multiple copies (23 to 45) in the transformed cells and that DNA sequences representing only 43.5 to 56.6% of the virus genome were present in amounts of 2 to 4 copies per cell in the DI tumor cells. Expression of these viral DNA sequences as demonstrated by the detection of virus-neutralizing antibodies, 50% neutralizing dose titers ranging from 1:50 to 1:1,000, in the sera of animals inoculated with either the virus-producing transformed cells or the virus-nonproducing tumor cells. Further, EHV-1-specific proteins were detected in the membrane and the perinuclear region of bothDI transformed and tumor cells by indirect immunofluorescent assays using antisera against EHV-1 structural antigens, EHV-1 nonstructural antigens, or preparations of EHV-1 DI particles. The roles of DI particles in mediating persistent infection and cellular transformation are discussed.     

Adenovirus Transformation of Hamster Embryo Cells

Inoculation of hamster embryo cell cultures with human adenovirus type 12 (Ad12) or simian adenovirus (SA7) resulted in the formation of foci of morphologically transformed cells within 12 days. The rapid appearance of well-defined foci was dependent upon the transfer of cells into new plates, with sufficient dilution after virus adsorption, and was independent of virus dose. Dose-response studies showed linearity of focus formation with dilution of Ad12 or SA7. Results averaged from several experiments show plaque-forming unit to focus-forming unit ratios of approximately 1.8 x 10(6) for Ad12 and 2.6 x 10(5) for SA7. Other experiments showed that most of the adenovirus involved in transformation was adsorbed by 3 hr. Cell lines derived from SA7 transformed cells produced tumors within 19 days when inoculated intradermally into young adult hamsters. Such cell-induced tumors histologically resembled SA7 virus-induced hamster tumors. Formation of tumors with SA7 transformed cells was inhibited by prior immunization of test animals with SA7 or Ad12 virus.     

Sexual differentiation of the steroid feedback mechanism regulating follicle-stimulating hormone secretion in the Syrian hamster

The ability of gonadal steroid hormones to influence tonic follicle-stimulating hormone (FSH) secretion was investigated in Syrian hamsters. In Experiment 1, males were castrated as adults, and administered testosterone in 20-, 30-, 40-, and 50-mm silastic capsules (s.c.) at 67, 74, 81, and 88 days, respectively. Circulating FSH was reduced by testosterone in a dose-dependent manner. A similar FSH response to testosterone in adulthood was evident in neonatally androgenized hamsters given testosterone proprionate (TP) on Days 0 and 1 of life. By contrast, the absence of gonadal androgens during the neonatal period (females ovariectomized at 60 days of age and males orchidectomized at birth) resulted in only a partial suppression of circulating FSH by even the highest dose of testosterone during adulthood. Treatment with estradiol benzoate at birth failed to produce a masculine response to androgen in adulthood. In Experiment 2, using a similar protocol, the nonaromatizable androgen, dihydrotestosterone, produced a dose-dependent suppression in serum FSH in males castrated in adulthood (30-, 60-, 90-mm capsules). However, dihydrotestosterone failed to alter the hypersecretion of FSH produced by orchidectomy at birth in males or in females ovariectomized at 60 days of age and treated neonatally with either vehicle or TP. In Experiment 3, treatment with estradiol (10-, 20-, 30-mm capsules) decreased serum FSH in gonadectomized hamsters in a dose-dependent manner; males and females treated neonatally with TP were more responsive to estradiol as adults compared to neonatally orchidectomized males or females treated with vehicle at birth.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antisperm antibodies in human immunodeficiency virus infection: effects on fertilization and embryonic development

Sera from human immunodeficiency virus (HIV)-infected males (n = 10) and females (n = 5) were analyzed for the presence of antisperm antibodies reacting against sperm-specific antigens. Of the HIV-positive males tested, sera of 40% were positive for human sperm extract (HSE), 70% for protamine, and 70% for fertilization antigen (FA-1) for at least one class of antibodies, compared to sera from HIV-negative males. Of the HIV-positive females tested, sera of 40% were positive for HSE, 30% for protamine, and 30% for FA-1 compared to sera from HIV-negative females. The majority of the sperm antigen-reactive antibodies belonged to the IgG class. The reactions observed with FA-1 were weaker than those with other antigens. Ninety percent of HIV-positive male sera and 80% of the HIV-negative female sera were found to contain immune complexes, 20% of which showed the presence of FA-1. HIV-positive male or female sera did not bind to any specific protein on the Western blot of HSE. The minimal amount of free anti-FA-1 antibodies present in sera did not bind to live sperm in the sperm immobilization technique, sperm agglutination technique, or immunobead binding technique and thus were incapable of affecting human sperm penetration of zona-free hamster ova (SPA). Nor did HIV-positive sera induce any apparent abnormality in the development of 2-cell embryos to blastocysts in vitro in murine bioassay. In conclusion, these results indicate that HIV-infected patients have sperm-specific antibodies in their sera that do not adversely affect SPA and murine embryo bioassay. There was a high incidence of immune complex formation after HIV infection. These data will provide the basis for exploring further the role of sperm antigens in altering the immunoregulatory mechanisms after HIV infection.     

Asymmetric learning to avoid heterospecific males in Mesocricetus hamsters

If a female mates with a male of a closely related species, her fitness is likely to decline. Consequently, females may develop behavioral mechanisms to avoid mating with heterospecific males. In some species, one such mechanism is for adult females to learn to discriminate against heterospecific males after exposure to such males. We have previously shown that adult, female Syrian hamsters (Mesocricetus auratus) learn to discriminate against male Turkish hamsters (Mesocricetus brandti) after exposure to a single heterospecific male during 8 days across a wire-mesh barrier. Here we repeated that experiment but this time we exposed female Turkish hamsters to a male Syrian hamster for 8 days and then measured sexual and aggressive behaviors towards that heterospecific male and towards a conspecific male. In contrast to female Syrian hamsters, female Turkish hamsters did not differ in their latency to go into lordosis or in any measure of aggression towards either type of male. Female Turkish hamsters spent less time in lordosis with the heterospecific male, but the percentage of trials in which females copulated with conspecific and heterospecific males did not differ. When comparing females from both species that had been exposed to a heterospecific male for 8days, female Syrian hamsters copulated less and were more aggressive towards the heterospecific male compared to the behavior of female Turkish hamsters. We discuss how this asymmetric response between females of the two species may be due to the much larger geographical range of Turkish hamsters compared to Syrian hamsters.     

Relationship between susceptibility and immune response to staphylococcal exfoliative toxin A in mammalian species

Staphylococcal exfoliative toxin A (ETA) had a splitting effect at the granular layer of skin in humans and neonatal mice, but not in rabbits, guinea pigs, golden hamsters, or rats. Besides its splitting effect, ETA could stimulate productions of neutralizing antibody to ETA in rabbits, rats and B10D2 mice, but not in golden hamsters, guinea pigs, or ICR, HRS/J, and C57BL/10 mice. In our epidemiological investigation of human sera, the percentage of antibody to ETA in sera obtained from patients with impetigo (8%) was lower than those in sera of healthy males (23%) and females (29%). The relationship between susceptibility and immune response to ETA in these mammalians could be divided into three groups: the possession of resistant skin and high production of antibody to ETA; the possession of resistant skin and low production of antibody to ETA; the possession of sensitive skin and various titers of antibody to ETA.     

Transformation of Hamster Embryo Cells and Tumor Induction in Newborn Hamsters by Simian Adenovirus SV11

Simian adenovirus, SV11, readily transformed hamster embryo cell cultures in vitro and produced tumors in vivo when inoculated into newborn hamsters. Foci consisting of small, loosely attached, rounded cells could be seen as early as 7 days postinoculation. Many of these cells contained several nuclei or the nucleus was multilobed. The cells grew without extensive cell to cell contact or formed small chains or clusters when passaged in vitro. This pattern of cell morphology and growth has not been reported with other simian or human adenovirus-transformed cells. Linearity of foci formation with virus dilution was observed when the virus multiplicity was less than 3 plaque-forming units (PFU)/cell. The PFU to focus-forming units ratio for SV11 was found to be 2 x 10(4) to 4 x 10(4), which is approximately 5- to 10-fold and 50- to 100-fold lower than those reported for simian adenovirus, SA7, and human adenovirus type 12, respectively. Cells transformed by SV11: (i) produced tumors when inoculated into young hamsters, (ii) contained tumor antigen which reacts with serum obtained from hamsters bearing SV11 passaged tumors, and (iii) could be propagated in vitro through an indefinite number of generations.     

Studies on Nondefective Adenovirus-Simian Virus 40 Hybrid Viruses I. A Newly Characterized Simian Virus 40 Antigen Induced by the Ad2+ND1 Virus

The nondefective adenovirus 2 (Ad2)-simian virus 40 (SV40) hybrid virus, Ad2(+)ND(1), does not induce heat-labile SV40 T antigen but does induce a previously uncharacterized heat-stable SV40 antigen-the SV40 "U" antigen. This antigen is detectable by both immunofluorescence and complement fixation by using sera from hamsters with SV40 tumors. Sera from hamsters bearing SV40 tumors can be divided into two groups, those that react with both SV40 T and U antigens (T(+)U(+) sera) and those that react with SV40 T antigen only (T(+)U(-) sera). SV40 U-specific sera from monkeys immunized with Ad2(+)ND(1)-infected cells do not react with SV40 T antigen by immunofluorescence but do react with an antigen in the nucleus of SV40-transformed cells and with an early, cytosine arabinoside-resistant antigen present in the nucleus of SV40-infected cells. A heat-stable SV40 antigen detectable by complement fixation with T(+)U(+) hamster sera is present in extracts of SV40-induced hamster tumors and in cell packs of SV40-infected or -transformed cells. SV40 U-antigen synthesis by Ad2(+)ND(1) virus is partially sensitive to inhibitors of deoxyribonucleic acid synthesis, whereas U-antigen synthesis by SV40 virus is an early cytosine arabinoside-resistant event. As an early SV40 antigen differing from SV40 T antigen, U antigen may play a role in malignant transformation mediated by SV40.     

Detection and Assay of Avian Tumor Virus Group-Specific Antigen and Antibody by the Paired Radioiodine-Labeled Antibody Technique

The paired radioiodine-labeled antibody technique (PRILAT) was applied to the detection and quantitation of avian tumor virus group-specific (gs) antigens and antibody. The technique proved to be specific, repeatable, and appreciably more sensitive than the microcomplement-fixation test for avian leukosis (COFAL). The PRILAT facilitated direct measurement of comparative antigen content of several types of transformed, neoplastic, or virus-infected cells and the magnitude of nonspecific antibody binding by appropriate control cells. The versatility of the technique was illustrated by application to the detection and quantitation of gs antibody content of chicken, turkey, pigeon, and hamster sera. Antibodies were detected in COFAL-negative sera from hamsters bearing tumors induced by the Schmidt-Ruppin strain of Rous sarcoma virus. Sera from chickens bearing similar tumors were not positive for gs antibodies, although sera from turkeys and chickens immunized with avian leukosis virus did contain gs antibodies.     

Sex differences in Seoul virus infection are not related to adult sex steroid concentrations in Norway rats

Field studies of hantavirus infection in rodents report that a higher percentage of infected individuals are males than females. To determine whether males were more susceptible to hantavirus infection than females, adult male and female Long Evans rats (Rattus norvegicus) were inoculated with doses of Seoul virus ranging from 10(-4) to 10(6) PFU. The 50% infective doses (ID(50)) were not significantly different for male and female rats (10(0.05) and 10(0.8) PFU, respectively). To determine whether sex differences in response to infection were related to circulating sex steroid hormones, sex steroid concentrations were manipulated and antibody responses and virus shedding were assessed following inoculation with the ID(90). Regardless of hormone treatment, males had higher anti-Seoul virus immunoglobulin G (IgG) and IgG2a (i.e., Th1) responses than females and IgG1 (i.e., Th2) responses similar to those of females. Males also shed virus in saliva and feces longer than females. Manipulation of sex steroids in adulthood did not alter immune responses or virus shedding, suggesting that sex steroids may organize adult responses to hantavirus earlier during ontogeny.     

Identification of human hookworm in failed-treatment cases using Chinese hamsters (Cricetulus griseus) and scanning electron microscopy

An attempt was made to identify the human hookworm involved in failed-treatment cases using abnormal hosts and scanning electron microscopy. Thirty-seven, 2 to 6 month old Chinese hamsters (Cricetulus griseus) from a closed, outbred, conventional colony, were each given between 20 and 120 filariform larvae per os. The larvae were cultured from faeces from mebendazole (Vermox) 500 mg single-dose, failed-treatment cases living in the lowveld farming area of the Transvaal Province, South Africa. About 60 to 78 days after inoculation, the animals were killed and adult worms were removed from their small intestines. Eleven (30%) of the 37 hamsters harboured a total of 31 adult worms (19 males and 12 females), while 26 hamsters were refractory to infection. The greatest number of worms recovered from a single animal was six. A total of 27 worms (17 males and 10 females) were subjected to examination by scanning electron microscopy. Micrographs showed male and female worms to be morphologically all of the Necator americanus species, as identified by a pair of ventral and dorsal cutting plates, a dorsal tooth and the fused terminus of spicules in the male bursa. The transverse cuticular striations were distinct and smooth. Several points of interest arose from the results of this study and are discussed.     

Antineoplastic activity of N-maleamide homocysteine thiolactone amide encapsulated within liposomes

The antineoplastic activity of N-maleamide homocysteine thiolactone amide (MHTA) encapsulated within liposomes was studied in mice with transplanted tumors. Tumor weight was decreased by 4-5 biweekly intraperitoneal injections of MHTA in liposomes in DBA/2N females with MTG mammary adenocarcinoma (35% of control value, P less than 0.005) and in C57B1/6N males with MUO4 rhabdomyosarcoma (11% of control value, P less than 0.0000001). Tumor incidence was reduced from 84 to 63% (P less than 0.05) and from 100 to 32% (P less than 0.001) in the two systems, respectively. When the compound was administered in dimethyl sulfoxide to A/HeJ females with A10 mammary adenocarcinoma by daily intraperitoneal injection, tumor weight was reduced to 70% of control value (P less than 0.05), and there was no decrease in tumor incidence (100%). No toxicity was observed at the therapeutic dose utilized, 10 mg/kg/day. N-Maleamide homocysteine thiolactone amide is a derivative of the normal biochemical constituents, maleic acid and homocysteine thiolactone. The results show that the N-substituted maleamide derivative of homocysteine thiolactone decreases the growth of murine tumors of two different histological types, when administered encapsulated within liposomes.     

Impact of anti-sandfly saliva antibodies on biological aspects of Phlebotomus papatasi (Diptera: Psychodidae), vector of cutaneous leishmaniasis

Sandflies are the main vectors of Leishmania parasites in tropical and subtropical areas. The immunization of vertebrate hosts with vector components through repeated bites may offer an alternative method for sandfly control. Aliquots of female Phlebotomus papatasi (Scopoli) (Diptera: Psychodidae) were weekly blood fed on 12 individual hamsters throughout 18 successive weeks. Significant biological and biochemical changes resulting from antibodies developed by immunized host sera against repeated biting were observed in sandfly females. Blood feeding and fertility rates of females significantly gradually declined to the end of the study period. No appreciable difference was observed in mortality rates among flies repeatedly fed on individual hamsters throughout weeks 9 and 18, compared to flies fed on naïve hamsters. Total salivary gland proteins of female sandflies were compared to proteins in sera of sensitized hamsters. SDS-page revealed bands common to both flies and hosts, indicating the development of anti-saliva antibodies in hamster sera. The importance of anti-sandfly saliva antibodies as a potential tool for vector control leading to the interruption of leishmaniasis is discussed.     

Photoperiodic adjustments in immune function protect Siberian hamsters from lethal endotoxemia

Seasonal changes in day length enhance or suppress components of immune function in individuals of several mammalian species. Siberian hamsters (Phodopus sungorus) exhibit multiple changes in neuroendocrine, reproductive, and immune function after exposure to short days. The manner in which these changes are integrated into the host response to pathogens is not well understood. The present experiments tested the hypothesis that short-day changes in immune function alter the pathogenesis of septic shock and survival after challenge with endotoxin. Male and female Siberian hamsters raised in long-day photoperiods were transferred as adults to short days or remained in their natal photoperiod. Six to 8 weeks later, hamsters were injected i.p. with 0, 1, 2.5, 10, 25, or 50 mg/kg bacterial lipopolysaccharide (LPS) (the biologically active constituent of endotoxin), and survival was monitored for 96 h. Short days significantly improved survival of male hamsters treated with 10 or 25 mg/kg LPS and improved survival in females treated with 50 mg/kg LPS. Transfer from long to short days shifted the LD50 in males by approximately 90%, from 5.3 to 9.9 mg/kg, and in females from 11.1 to 15.0 mg/kg (+35%). Long-day females were more resistant than were males to lethal endotoxemia. In vitro production of the proinflammatory cytokine TNFalpha in response to LPS stimulation was significantly lower in macrophages extracted from short-day relative to long-day hamsters, as were circulating concentrations of TNFalpha in vivo after i.p. administration of LPS, suggesting that diminished cytokine responses to LPS in short days may mitigate the lethality of endotoxemia. Adaptation to short days induces changes in immune parameters that affect survival in the face of immune challenges.     

Thiazide-induced hypercholesterolemia: sex differences

Thiazide diuretics are used commonly to treat hypertension. Unfortunately, they also are known to elevate serum cholesterol levels. Because serum lipid fraction levels differ between the sexes, possible sex-related differences in thiazide-induced changes in serum total cholesterol (TC), triglycerides (TG) and high density lipoprotein cholesterol (HDL-C) levels were examined. Four groups of male and female hamsters were treated for a minimum of 3 months with hydrochlorothiazide (HCTZ) at zero, 1, 2 or 4 mg/kg/day. At zero dose, there was no difference in TG levels between the sexes; however, females had significantly higher TG concentrations than did males at 1, 2 and 4 mg HCTZ (all p less than 0.05). Females demonstrate a significant dose response with HCL-C levels increasing with increasing doses of HCTZ, (r = 0.983; p less than 0.02); in contrast males had a similar increase in HDL-C at all dose levels (all p less than 0.05) thus there was no demonstrable dose response (r = 0.539). Total cholesterol concentrations were significantly higher in the females than in males (p less than 0.05) at all 3 dose levels as well as at zero dose. Further, the females demonstrated a direct dose response in TC levels (r = 0.986; p less than 0.02) while the males showed no such dose response (r = 0.824; p less than 0.01). Based on these findings we conclude that: 1) HCTZ increases TG, TC and HDL-C levels in both male and female hamsters; 2) TC levels are higher in females than in males regardless of HCTZ dose; 3) only females show a dose-dependent increase in HCL-C and TC in response to HCTZ. These sex-related changes in lipid fractions occurring with HCTZ treatment, if they occur in humans, may contribute to sex-related differences in rates and severity of atherosclerosis in HCTZ-treated populations.     

Health evaluation of Galapagos Hawks (Buteo galapagoensis) on Santiago Island, Galapagos

Galapagos Hawks (Buteo galapagoensis), the only endemic, diurnal raptor species in Galapagos, are currently distributed on eight Galapagos Islands having been extirpated from three of the human-inhabited islands. In January 2009, we performed health assessments of 89 Galapagos Hawks on Santiago Island, Galapagos. Four of the 89 Galapagos Hawks (4%) evaluated had physical abnormalities. Blood parameters did not differ between males and females, except for aspartate transaminase values, which were significantly higher in females than males. No Galapagos Hawks tested positive for antibodies to avian encephalitis virus, Marek virus, and paramyxovirus-1 or to haemosporidian antigen. Chlamydophila psittaci antigen was detected in 2 of 86 Galapagos Hawks (2%), with 24 of 43 Galapagos Hawks (56%) antibody-positive for avian adenovirus-1 and 1 of 48 Galapagos Hawks (2%) antibody positive for Toxoplasma gondii. There were no significant differences in infectious disease results based on sex. This study contributes to the understanding of the health status of the Galapagos Hawk and to the establishment of baseline information for the species.     

Characterization of the Tumorlike (T) Antigen Induced by Type 12 Adenovirus : I. Purification of the Antigen from Infected KB Cells and a Hamster Tumor Cell Line

The T antigen induced by type 12 adenovirus was purified from KB cells infected in the presence of 10(-6)m 5-fluoro-2-deoxyuridine to inhibit synthesis of viral capsid antigens. The antigen was purified approximately 200-fold, and the purified product contained only negligible amounts of host-cell contaminants, as judged by the residual radioactivity from (14)C-labeled uninfected cells which had been added to infected cells at the initiation of the purification. Immunoelectrophoresis indicated that the purified T-antigen preparation contained a single antigenic species. The T antigen from a hamster cell line (HT-1) derived from a type 12 adenovirus-induced tumor was purified by the same procedure. The T antigens from the two different sources were shown to be immunologically similar by use of a rabbit antiserum prepared against the purified T antigen from infected KB cells and sera from hamsters bearing tumors induced by type 12 adenovirus.     

Compensatory adrenal growth in aldosterone-treated male and female hamsters

The aim of the study was to investigate the compensatory adrenal growth in aldosterone-treated male and female hamsters. Hemiadrenalectomised and sham-operated animals were treated for 5 days with a daily d-aldosterone dose of 25 micrograms/animal. In both male and female aldosterone-treated hamsters monoadrenalectomy did not change the relative adrenal weight if compared with sham-operated groups. The fasciculata zonae of monoadrenalectomised aldosterone-treated males was larger and contained more parenchymal cells than in appropriate control group. There was no difference in the volume of adrenocortical zones, average cell volume and in cell number between sham-operated and unilaterally adrenalectomised females. In vitro 3H-thymidine incorporation per adrenal was markedly higher in monoadrenalectomised than in sham-operated aldosterone-treated males while the opposite was true for female hamsters. Thus, the action of aldosterone on CAG in the hamster seems to depend on sex, with no effect in males and inhibitory action in females.     

Influence of the viral regulatory region on tumor induction by simian virus 40 in hamsters

Most of the simian virus 40 (SV40) genome is conserved among isolates, but the noncoding regulatory region and the genomic region encoding the large T-antigen C terminus (T-ag-C) may exhibit considerable variation. We demonstrate here that SV40 isolates differ in their oncogenic potentials in Syrian golden hamsters. Experimental animals were inoculated intraperitoneally with 10⁷ PFU of parental or recombinant SV40 viruses and were observed for 12 months to identify genetic determinants of oncogenicity. The viral regulatory region was found to exert a statistically significant influence on tumor incidence, whereas the T-ag-C played a minor role. Viruses with a single enhancer (1E) were more oncogenic than those with a two-enhancer (2E) structure. Rearrangements in the 1E viral regulatory region were detected in 4 of 60 (6.7%) tumors. Viral loads in tumors varied, with a median of 5.4 SV40 genome copies per cell. Infectious SV40 was rescued from 15 of 37 (40%) cell lines established from tumors. Most hamsters with tumors and many without tumors produced antibodies to T antigen. All viruses displayed similar transforming frequencies in vitro, suggesting that differences in oncogenic potential in vivo were due to host responses to viral infection. This study shows that SV40 strains differ in their biological properties, suggests that SV40 replicates to some level in hamsters, and indicates that the outcome of an SV40 infection may depend on the viral strain present.