Biology of viral satellites and their role in pathogenesis.

Extraviral components that can influence the accumulation and pathogenesis of their associated helper viruses are known as 'satellites'. The maintenance of satellites requires their ability to associate with their helper viruses. Satellites can be categorized as either satellite viruses or satellite nucleic acids based on their ability to encode capsid proteins. Understanding the biology of satellites is important since they are pathogenic to a wide range of plant, animal, and yeast organisms. Most satellites influence the pathogenesis of their helper viruses by altering the interaction between the host and helper virus. However, the molecular mechanism that governs the trilateral interaction between host, satellites, and helper virus remains largely unexplored. This review comprehensively describes details of the association and interaction of helper viruses with satellite viruses, satellite RNAs, and satellite DNAs, and their implications for pathogenesis.

Misfolding of CasBrE SU is reversed by interactions with 4070A Env: implications for gammaretroviral neuropathogenesis.

BACKGROUND: CasBrE is a neurovirulent murine leukemia virus (MLV) capable of inducing paralytic disease with associated spongiform neurodegeneration. The neurovirulence of this virus has been genetically mapped to the surface expressed subunit (SU) of the env gene. However, CasBrE SU synthesized in the absence of the transmembrane subunit (TM) does not retain ecotropic receptor binding activity, indicating that folding of the receptor binding domain (RBD) requires this domain. Using a neural stem cell (NSC) based viral trans complementation approach to examine whether misfolded CasBrE SU retained neurovirulence, we observed CasBrE SU interaction with the "non-neurovirulent" amphotropic helper virus, 4070A which restored functional activity of CasBrE SU. RESULTS: Herein, we show that infection of NSCs expressing CasBrE SU with 4070A (CasES+4070A-NSCs) resulted in the redistribution of CasBrE SU from a strictly secreted product to include retention on the plasma membrane. Cell surface cross-linking analysis suggested that CasBrE SU membrane localization was due to interactions with 4070A Env. Viral particles produced from CasES+4070A-NSCS contained both CasBrE and 4070A gp70 Env proteins. These particles displayed ecotropic receptor-mediated infection, but were still 100-fold less efficient than CasE+4070A-NSC virus. Infectious center analysis showed CasBrE SU ecotropic transduction efficiencies approaching those of NSCs expressing full length CasBrE Env (CasE; SU+TM). In addition, CasBrE SU-4070A Env interactions resulted in robust ecotropic superinfection interference indicating near native intracellular SU interaction with its receptor, mCAT-1. CONCLUSIONS: In this report we provided evidence that 4070A Env and CasBrE SU physically interact within NSCs leading to CasBrE SU retention on the plasma membrane, incorporation into viral particles, restoration of mCAT-1 binding, and capacity for initiation of TM-mediated fusion events. Thus, heterotropic Env-SU interactions facilitates CasBrE SU folding events that restore Env activity. These findings are consistent with the idea that one protein conformation acts as a folding scaffold or nucleus for a second protein of similar primary structure, a process reminiscent of prion formation. The implication is that template-based protein folding may represent an inherent feature of neuropathogenic proteins that extends to retroviral Envs.

Impact of a defective RNA 3 from cucumber mosaic virus on helper virus infection dynamics.

D RNA 3Yalpha (D3Yalpha), a defective (D) RNA 3 derived from the Y strain of cucumber mosaic virus (CMV-Y), was further characterized in combination with different helper viruses in the genus Cucumovirus. Interestingly, Nicotiana benthamiana plants inoculated with CMV-D8 and D3Yalpha developed systemic symptoms which were different from those induced by CMV-D8. To elucidate the potential effects of D RNA 3 on virus infection on the basis of the original combination of CMV-Y and D3Yalpha, a point mutation was made in the coat protein gene, which determined symptoms, of either CMV-Y RNA 3 (Y3) or D3Yalpha. Symptoms induced on N. benthamiana and N. tabacum plants, and semi-quantitative RT-PCR revealed that the ratio of RNA 3 to D RNA 3 was associated with the differences of symptoms in the leaf tissues. Furthermore, analysis of in situ hybridization suggested that there were spatial effects between coat proteins of Y3 and D3Yalpha in the infected leaves.

Adeno-associated virus and human papillomavirus types in cervical samples of pregnant and non-pregnant women.

OBJECTIVE: To investigate the prevalence of AAV and HPV DNA and their types in cervical secretion from pregnant and non-pregnant women. STUDY DESIGN: The samples were obtained from 40 pregnant and 62 non-pregnant women who were attended at the outpatient clinic of the Federal University Hospital of Espírito Santo, Southeastern Brazil. AAV and HPV were investigated by PCR and typed by PCR and/or RFLP. RESULTS: The occurrence of AAV in all samples was 25.5% (26/102): 81% (21/26) and 19% (5/26) for AAV2/3 and AAV5 species, respectively. AAV were observed in 35% (14/40) and 19% (12/62) of pregnant and non-pregnant women, respectively. HPV occurred in 22% of all samples; 25% (10/40) in pregnant and 20% (12/60) in non-pregnant women. HPV types were determined for 72.7% of the strains, most of which classified as high-risk. AAV-HPV co-infection was observed in 15.4% (4/26), mostly in pregnant women. CONCLUSIONS: There was a greater prevalence of AAV and HPV in pregnant than in non-pregnant women, which suggests that the gestational state may play a role in reactivating the viruses.

Adenovirus-facilitated nuclear translocation of adeno-associated virus type 2.

We examined cytoplasmic trafficking and nuclear translocation of adeno-associated virus type 2 (AAV) by using Alexa Fluor 488-conjugated wild-type AAV, A20 monoclonal antibody immunocytochemistry, and subcellular fractionation techniques followed by DNA hybridization. Our results indicated that in the absence of adenovirus (Ad), AAV enters the cell rapidly and escapes from early endosomes with a t(1/2) of about 10 min postinfection. Cytoplasmically distributed AAV accumulated around the nucleus and persisted perinuclearly for 16 to 24 h. Viral uncoating occurred before or during nuclear entry beginning about 12 h postinfection, when viral protein and DNA were readily detected in the nucleus. Few, if any, intact AAV capsids were found in the nucleus. In the presence of Ad, however, cytoplasmic AAV quickly translocated into the nucleus as intact particles as early as 40 min after coinfection, and this facilitated nuclear translocation of AAV was not blocked by the nuclear pore complex inhibitor thapsigargan. The rapid nuclear translocation of intact AAV capsids in the presence of Ad suggested that one or more Ad capsid proteins might be altering trafficking. Indeed, coinfection with empty Ad capsids also resulted in the appearance of AAV DNA in nuclei within 40 min. Escape from early endosomes did not seem to be affected by Ad coinfection.

Early dissemination rates of Friend murine leukaemia virus variants correlate with late pathogenesis.

FIS-2, a less oncogenic, immunosuppressive variant of the Friend murine leukaemia virus (F-MuLV), was used to explore whether the differences in biological features were related to early virus dissemination rates or sites of replication. We found that erythroblasts were the primary target cells for both F-MuLV and FIS-2, while B- and T-cells were infected later in the infection. Although FIS-2 replicated to similar titres as F-MuLV, we observed a delay in peak viraemia titre and in the number of virus-positive cells in bone marrow and spleen. Studies including the chimeric viruses RE3 (FIS-2LTR with a F-MuLV background) and RE4 (F-MuLV LTR with a FIS-2 background) indicated that the delay in dissemination was due to mutations in FIS-2 LTR. The kinetics for early virus replication correlated with previously reported mean latency time for virus-induced erythroleukaemia in mice inoculated as newborns and with the onset of immunosuppression in adult mice. In addition, F-MuLV-induced late immunosuppression coincided with signs of erythroleukaemia and persistent viraemia. FIS-2 induced a more moderate late immunosuppression without persistent viraemia or signs of erythroleukaemia. Overall, our findings indicated that early viral replication is a prognostic factor in murine retrovirus-induced pathogenesis.

The helper virus envelope glycoprotein affects the disease specificity of a recombinant murine leukemia virus carrying a v-myc oncogene.

Many retroviruses that carry oncogenes (acute transforming viruses) are generally replication-defective and therefore require co-infection with a replication competent 'helper' retrovirus for infectivity. The helper virus provides the retroviral proteins necessary for particle production and infection. These include the envelope glycoproteins that specifically bind to cell surface receptors and mediate viral adsorption and entry. Thus, a particular helper virus may influence the nature of disease induced by an oncogene-containing retrovirus due to tissue tropism of the helper. In a previous study, a replication-defective recombinant Moloney murine leukemia virus containing the v-myc oncogene was generated (M-MuLV(myc); Brightman B.K., Pattengale P.K., and Fan H., J Virol 60: 68-81, 1986). When M-MuLV(myc) was inoculated into mice using the non-pathogenic amphotropic murine leukemia virus (Am-MuLV 4070) as a helper, T- and B-lymphoblastic lymphomas resulted with the following two surface phenotypes, namely, (1) Thy 1.2+, B220- and (2) Thy 1.2-, B220+. Thy 1.2 surface antigen is characteristic of cells of the lymphoid lineage, whereas B220 surface antigen is characteristic of cells of the B-lymphoid lineage. In these experiments, to assess the influence of the helper virus on the disease specificity of M-MuLV(myc), two weakly pathogenic ecotropic helper MuLVs that interact with different cell surface receptors than Am-MuLV (Mo+PyF101 and AKV MuLV) were used to pseudotype M-MuLV(myc). In both cases, when inoculated into mice, these pseudotypes induced only T-lymphoblastic lymphoma. These results indicate that for M-MuLV(myc) the types of the tumors induced are influenced by the helper virus utilized, and they suggest that different lymphoid cells may express different levels of retroviral receptors.

Simian sarcoma-associated virus fails to infect Chinese hamster cells despite the presence of functional gibbon ape leukemia virus receptors.

We have sequenced the envelope genes from each of the five members of the gibbon ape leukemia virus (GALV) family of type C retroviruses. Four of the GALVs, including GALV strain SEATO (GALV-S), were originally isolated from gibbon apes, whereas the fifth member of this family, simian sarcoma-associated virus (SSAV), was isolated from a woolly monkey and shares 78% amino acid identity with GALV-S. To determine whether these viruses have identical host ranges, we evaluated the susceptibility of several cell lines to either GALV-S or SSAV infection. GALV-S and SSAV have the same host range with the exception of Chinese hamster lung E36 cells, which are susceptible to GALV-S but not SSAV. We used retroviral vectors that differ only in their envelope composition (e.g., they contain either SSAV or GALV-S envelope protein) to show that the envelope of SSAV restricts entry into E36 cells. Although unable to infect E36 cells, SSAV infects GALV-resistant murine cells expressing the E36-derived viral receptor, HaPit2. These results suggest that the receptors present on E36 cells function for SSAV. We have constructed several vectors containing GALV-S/SSAV chimeric envelope proteins to map the region of the SSAV envelope that blocks infection of E36 cells. Vectors bearing chimeric envelopes comprised of the N-terminal region of the GALV-S SU protein and the C-terminal region of SSAV infect E36 cells, whereas vectors containing the N-terminal portion of the SSAV SU protein and C-terminal portion of GALV-S fail to infect E36 cells. This finding indicates that the region of the SSAV envelope protein responsible for restricting SSAV infection of E36 cells lies within its amino-terminal region.

Detection of infectious adeno-associated virus particles in human cervical biopsies.

Recently we reported that DNA of the human oncogenic papillomaviruses (HPV) and the tumor suppressive human helper virus-dependent parvoviruses, adeno-associated viruses type 2 (AAV-2), colocalize in cervical epithelium. To analyze whether infectious AAV particles are present in cervical tissue, we examined cervical biopsies from 36 patients with HPV-related lesions (squamous intraepithelial lesions) for the presence of AAV DNA and of infectious AAV. From each patient specimens from the lesion and from adjacent normal epithelium were analyzed. After PCR analysis AAV DNA-containing samples were purified by CsCl gradient centrifugation. The presence of AAV virions in CsCl gradients was analyzed and infectivity of AAV was determined. In addition, the biopsies were tested for the presence of HPV DNA. AAV DNA could be detected in biopsies from 23 of 36 patients. AAV particles were found in 11 AAV DNA-positive biopsies from 7 patients (lesions and/or normal tissue, respectively). AAV particles were found to be infectious virions in 10 of the 11 cases. These results demonstrate for the first time that infectious AAV can be isolated from human cervical biopsies, indicating a possible sexual transmission of AAV.

[SV40 as a possible cofactor in the etiopathogenesis of mesothelioma and other human tumors]. / SV40 come possible cofattore nell'eziopatogenesi del mesotelioma e di altri tumori umani.

Simian virus 40 (SV40) has been introduced into the human population with contaminated polio vaccines between 1955 and 1963. Previous research conducted by southern blot hybridization and recent analysis by PCR have shown the presence of SW0 sequences in human brain tumors, mesotheliomas and osteosarcomas as well as in normal tissues such as blood and sperm fluids. SV40 RNA and T antigen were detected in the same tissues. All the samples were coinfected by BK Virus (BKV), suggesting that BKV may have a helper function for SV40 replication in human cells. The presence of SV40 in human tumors suggests that the virus may be a cofactor in the etiopathogenesis of human neoplasia. In addition, blood and semen may represent the vectors for transmission of SV40 by horizontal infection in the human population.

Defective viral vectors as agents for gene transfer in the nervous system.

Viral vectors have attracted great interest as vehicles for gene therapy. Due to concerns regarding continued viral gene expression in several systems, new approaches have been sought for gene transfer in the nervous system. This article reviews the general concepts and basic biology of defective viral vectors. These are vectors which can package into a viral coat but contain no viral genes, thereby allowing efficient gene transfer in the absence of viral gene expression in target cells. The defective herpes simplex virus (HSV) vector has been applied to numerous interesting questions in neurobiology. The inability to completely eliminate helper viruses has raised concern regarding the application of this vector to human disease. The adeno-associated virus (AAV) vector has recently been introduced into the nervous system. This vector harbors no viral genes, however helper viruses can also be completely eliminated from the system. Although the smaller size may limit the range of applications for this vector, it has received great interest as a potential agent for gene therapy in the nervous system. Potential future directions are discussed as well.

Quinolinic acid levels in a murine retrovirus-induced immunodeficiency syndrome.

Mice infected with the retrovirus mixture designated LP-BM5 murine leukemia virus (MuLV) develop an immunosuppressive disease. Quinolinic acid (QUIN) is an endogenous neurotoxic N-methyl-D-aspartate agonist that may contribute to the pathogenesis of HIV-associated neurologic disease. In the present study, the levels of QUIN in brain and blood were measured in mice infected with LP-BM5 MuLV and compared with those in uninfected mice and mice infected with the nonpathogenic strain of ecotropic MuLV (helper component of LP-BM5 MuLV). Infection with LP-BM5 MuLV resulted in progressive increases in blood QUIN levels beginning 2 weeks after inoculation that peaked by 16 weeks postinfection. QUIN levels were also increased in cerebral cortex, hippocampus, and striatum. In systemic tissues, QUIN levels were increased in lung, liver, and spleen. In contrast, infection with the ecotropic viral component of the LP-BM5 MuLV mixture was not associated with any changes in brain, blood, or systemic tissue QUIN levels, even though helper virus burdens were comparable to those in mice infected with LP-BM5 MuLV. Treatment of LP-BM5 MuLV-infected mice with the antiretroviral agent zidovudine (azidothymidine) significantly reduced blood and brain QUIN levels in association with reductions in viral load in brain and spleen. These observations suggest that elevated QUIN production is not attributable to productive infection with retrovirus per se but occurs in response to an agent or agents, such as cytokines, that are produced by the host in response to virus infection.

Complex interactions among several nucleotide positions determine phenotypes defective for long-distance transport in the satellite RNA of cucumber mosaic virus.

Ix-satRNA is a CMV-satRNA necrogenic on tomato that is defective for long-distance systemic movement when coinoculated in tobacco with V-TAV. To analyze the determinants for this defective phenotype, full-length cDNA clones of Ix-satRNA and of the closely related, nondefective I17N-satRNA were obtained. Infectious transcripts from these clones (Ix5-satRNA and I17N1-satRNA) had the same phenotypes as the original satRNAs and differed only in four positions: positions 20 (Ix5, C; I17N1, U), 102 (Ix5, C; I17N1, U), 224 (Ix5, deleted; I17N1, A), 327 (Ix5, G; I17N1, A). By site-directed mutagenesis at positions 224 and 327 and by recombination using two common restriction sites, satRNAs in which the bases at these four positions were changed from the composition at Ix5-satRNA to the composition at I17N1-satRNA were obtained. A comparison of the phenotypes of the 13 single, double, and triple mutants (respective to Ix5-satRNA) showed that the defective phenotype of Ix5-satRNA is determined by the nature of the four positions analyzed; mutants at any of the positions 102, 224, and 327 accumulated as efficiently as I17N1-satRNA in systemically infected tobacco leaves when coinoculated with V-TAV. The change C20-->U also restored systemic movement, albeit imperfectly, giving rise to a phenotype that moved systemically less efficiently than I17N1-satRNA. This phenotype determined by U20 is expressed irrespective of the base at position 102, indicating an epistatic interaction between determinants 20 and 102; this interaction does not occur with position 224 or 327. That differences in the analyzed satRNAs are due to their being able, or not, to move systemically is shown by the fact that all of them (including Ix5-satRNA) accumulated to the same high level in directly inoculated leaves. The similarity in the sequences of the analyzed satRNAs and the complexity of the interactions among the effects of base changes at four positions scattered over the satRNA molecule suggest that the observed movement phenotypes may depend on conformational changes in the satRNAs.

Helper virus induced T cell lymphoma in nonhuman primates after retroviral mediated gene transfer.

Moloney Murine Leukemia Virus (MoMuLV) causes T cell neoplasms in rodents but is not known to be a pathogen in primates. The core protein and enzyme genes of the MoMuLV genome together with an amphotropic envelope gene are utilized to engineer the cell lines that generate retroviral vectors for use in current human gene therapy applications. We developed a producer clone that generates a very high concentration of retroviral vector particles to optimize conditions for gene insertion into pluripotent hematopoietic stem cells. This producer cell line also generates a much lower concentration of replication-competent virus that arose through recombination. Stem cells from rhesus monkeys were purified by immunoselection with an anti-CD34 antibody, incubated in vitro for 80-86 h in the presence of retroviral vector particles with accompanying replication-competent virus and used to reconstitute recipients whose bone marrow had been ablated by total body irradiation. The retroviral vector genome was detected in circulating cells of five of eight transplant recipients of CD34+ cells and in the circulating cells of two recipients of infected, unfractionated bone marrow mononuclear cells. Three recipients of CD34+ cells had a productive infection with replication-competent virus. Six or seven mo after transplantation, each of these animals developed a rapidly progressive T cell neoplasm involving the thymus, lymph nodes, liver, spleen, and bone marrow. Lymphoma cells contained 10-50 copies of the replication-competent virus, but lacked the retroviral vector genome. We conclude that replication-competent viruses arising from producer cells making retroviral vectors can be pathogenic in primates, which underscores the importance of carefully screening retroviral producer clones used in human trials to exclude contamination with replication-competent virus.

[The enhancement of the penetration of the human immunodeficiency virus into cells with the aid of a helper virus]. / Usilenie proniknoveniia virusa immunodefitsita cheloveka v kletki s pomoshch'iu virusa-pomoshchnika.

Penetration of human immunodeficiency virus (HIV) into the cells of lymphoblastoid T-cell line H9 was studied using 35S-methionine-labeled virus by demonstration of virus-specific proteins in the cytoplasm of the infected cells. Purification of the virus by ultracentrifugation through 30% glycerol was shown to lead to virus aggregation and its partial destruction manifested by the loss of gp120 protein, therefore unlabeled concentrated virus was used mainly with subsequent determination of virus-specific proteins by immune blotting. The addition of Sendai virus inactivated with UV rays to HIV increased the amount of HIV associated with cells as well as the amount of virus-specific proteins in the cytoplasm of the infected cells.

Distinct helper virus requirements for Abelson murine leukemia virus-induced pre-B- and T-cell lymphomas.

Abelson murine leukemia virus (A-MuLV) can induce pre-B- or T-cell lymphomas (thymomas) in mice depending on the route and time of injection. Previous studies have shown that the choice of the helper virus used to rescue A-MuLV greatly influences its ability to induce pre-B-cell lymphomas. In this study, we investigated the role of the helper virus in A-MuLV-induced thymomas. A-MuLV rescued with the helper Moloney MuLV, BALB/c endogenous N-tropic MuLV, and two chimeric MuLVs derived from these two parents were injected intrathymically in young adult NIH Swiss mice. All four A-MuLV pseudotypes were found to be equally efficient in the induction of thymomas, whereas drastic differences were observed in their pre-B-cell lymphomagenic potential. Thymoma induction by A-MuLV was independent of the replication potential of the helper virus in the thymus, and no helper proviral sequences could be detected in the majority of thymomas induced by A-MuLV rescued with parental BALB/c endogenous or chimeric MuLVs. In the thymomas in which helper proviruses were present, none of them were found integrated in the Ahi-1 region, a common proviral integration site found in A-MuLV-induced pre-B-cell lymphomas (Y. Poirer, C. Kozak, and P. Jolicoeur, J. Virol. 62:3985-3992, 1988). In addition, helper-free stocks of A-MuLV were found to be as lymphomagneic as other pseudotypes in inducing thymomas after intrathymic inoculation, in contrast to their inability to induce pre-B-cell lymphomas when injected intraperitoneally in newborn mice. Restriction enzyme analysis revealed one to three A-MuLV proviruses in each thymoma, indicating the oligoclonality of these tumors. Analysis of the immunoglobulin and T-cell receptor loci confirmed that the major population of cells of these primary thymomas belongs to the T-cell lineage. Together, these results indicate that the helper virus has no effect in the induction of A-MuLV-induced T-cell lymphomas, in contrast to its important role in the induction of A-MuLV-induced pre-B-cell lymphomas. Our data also revealed distinct biological requirements for transformation of these two target cells by v-abl.

Analysis of hematopoietic and lymphopoietic tissue during a regenerative aplastic crisis induced by avian retrovirus MAV-2(O).

Infection of 10-day-old chickens with an avian osteopetrosis virus resulted in a severe regenerative aplastic crisis. Hematopoietic and lymphopoietic tissues of chickens infected with myeloblastosis-associated virus (of subgroup B, inducing osteopetrosis, MAV-2(O] were analyzed for integrated and unintegrated viral DNA sequences, cell population shifts, weight changes, and morphological alterations. By 6 days postinfection (p.i.), DNA from bone marrow cells and peripheral blood leukocytes (PBL) contained between 0.50 and 0.70 copies of viral DNA per haploid genome. Erythrocytes and splenic leukocytes contained less than 0.10 copies/haploid genome. Granulocytes and precursor mesomyelocytes were absent from bone marrow, but numbers of erythrocytes, erythroblasts, and reticulocytes were normal. By 9 days p.i., bone marrow was severely hypoplastic and both granulopoietic and erythropoietic colonies were depleted. By 12 days p.i., erythrocytes and granulocytes were maximally depressed in peripheral blood and the amount of integrated virus in bone marrow and PBL decreased to less than 0.20 copies/haploid genome. In contrast, erythrocytes contained integrated viral DNA of up to 0.30 copies/haploid genome, indicating infection of erythrocyte precursors. At 18 days p.i., viral DNA was detected only in erythrocytes. Unintegrated viral DNA was not detected in any organs. Anemia was accompanied by splenomegaly and erythrophagocytosis. Viral DNA was never detected in thymus or bursa. Differential counting and flow cytometry of cells from bursa, thymus, and spleen, and of blood lymphocytes did not detect significant population shifts. These results suggest that MAV-2(O) infection of immunocompetent chickens occurs primarily in myelopoietic tissues, and tissues are selectively infected.

A nonimmunosuppressive helper virus allows high efficiency induction of B cell lymphomas by reticuloendotheliosis virus strain T.

We have documented the effect of two nondefective helper viruses, reticuloendotheliosis virus A (REV-A) and chick syncytial virus (CSV) infection on bursal tissue. REV-A infection results in bursal atrophy, destroying both its structural and functional integrity. In contrast, the bursae in CSV-infected chicks, while reduced slightly in size, appear both structurally and functionally normal. REV-A-induced bursal atrophy is not a result of viral replication in the B-lymphocyte as (a) both viruses are capable of inducing, with equal efficiency, the formation of preneoplastic lesions containing proliferating B lymphocytes and (b) it appears that equivalent amounts of viral antigen are expressed in the bursae of chicks infected with either virus. We have examined the phenotype of tumors induced by the replication-defective virus REV-T when replicated by the two different helper viruses, REV-A and CSV. In REV-T(REV-A)-infected chicks, the majority of tumors that develop are negative for IgM expression. In contrast, the majority of tumors induced by REV-T(CSV) infection are IgM+. This finding is confirmed by recovery of IgM- cell lines from REV-T(REV-A)-infected chicks and IgM+ cell lines from REV-T(CSV)-infected chicks. In addition, repopulation studies show that bursal-derived cells that are IgM+ serve as target cells for REV-T(CSV)-induced lymphomas. This study demonstrates, therefore, that REV-T can induce IgM+, B cell lymphomas with high efficiency. We conclude that infections by the helper viruses, REV-A and CSV, differ dramatically in their effects on the composition of the population of cells that serve as targets for REV-T-induced neoplasia.