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

Multiple mutations contribute to the oncogenicity of the retroviral oncoprotein v-Rel

The avian Rev-T retrovirus encodes the v-Rel oncoprotein, which is a member of the Rel/NF-kappaB transcription factor family. v-Rel induces a rapidly fatal lymphoma/leukemia in young birds, and v-Rel can transform and immortalize a variety of avian cell types in vitro. Although Rel/NF-kappaB transcription factors have been associated with oncogenesis in mammals, v-Rel is the only member of this family that is frankly oncogenic in animal model systems. The potent oncogenicity of v-Rel is the consequence of a number of mutations that have altered its activity and regulation: for example, certain mutations decrease its ability to be regulated by IkappaBalpha, change its DNA-binding site specificity, and endow it with new transactivation properties. The study of v-Rel will continue to increase our knowledge of how cellular Rel proteins contribute to oncogenesis by affecting cell growth, altering cell-cycle regulation, and blocking apoptosis. This review will discuss biological and molecular activities of v-Rel, with particular attention to how these activities relate to structure - function aspects of the Rel/NF-kappaB transcription factors.

Mutations in the DNA-binding and dimerization domains of v-Rel are responsible for altered kappa B DNA-binding complexes in transformed cells

The c-rel proto-oncogene encodes a member of the Rel/NF-kappa B family of transcription factors. The oncogenic viral form, v-rel, transduced by avian reticuloendotheliosis virus T, induces lymphoid tumors. v-Rel transformation may be mediated directly by binding of v-Rel to cognate DNA sites, resulting in altered gene expression, and/or indirectly by releasing Rel/NF-kappa B transcription factors from cytoplasmic retention molecules, resulting in their translocation to the nucleus and the inappropriate expression of genes under kappa B control. v-Rel-transformed cell lines of different phenotypes contained v-Rel as well as endogenous kappa B DNA-binding proteins in nuclear extracts. Kinetic analysis with avian leukosis virus-transformed B-cell lines expressing v-Rel or c-Rel indicated that the presence of endogenous kappa B DNA-binding proteins in the nucleus is temporally correlated with the relocalization of v-Rel to the cytoplasm. Supershift analysis of these DNA-binding complexes revealed that v-Rel was present in all of the nuclear DNA-binding complexes heterodimerized with c-Rel, NF-kappa B1, and other proteins. In contrast, c-Rel-transformed cells exhibited a less-complex pattern of nuclear kappa B DNA-binding complexes, and the nuclear appearance of these endogenous complexes was not observed. Studies with c-/v-Rel hybrids suggest that the induction of the endogenous kappa B DNA-binding complexes is the result of the mutations in the C-terminal region of the Rel homology (RH) domain of v-Rel. Moreover, v-Rel differed from c-Rel in its DNA-binding specificity. The altered DNA-binding specificity of v-Rel was associated with mutations located in the N-terminal part of the RH domain of v-Rel. These results suggest that two different regions of v-Rel (both located in the RH domain) influence the formation of kappa B DNA-binding complexes differently.

v-rel Induces ectopic expression of an adhesion molecule, DM-GRASP, during B-lymphoma development

In an effort to identify aberrantly expressed genes in v-rel-induced tumors, monoclonal antibodies were developed that reacted selectively with avian B-cell tumors. One antibody, HY78, immunoprecipitated a 120-kDa glycoprotein (p120) from cells that express v-rel. N-terminal amino acid sequencing of p120 identified a 27-amino-acid sequence that is also present in DM-GRASP, an adhesion molecule belonging to the immunoglobulin superfamily. Evidence from tissue distribution, immunological cross-reaction, PCR amplification, cDNA cloning, and DNA sequence shows that p120 is indeed DM-GRASP. Northern (RNA) analysis using a probe from the DM-GRASP gene identified a 5.3-kb transcript in mRNA from bursa, thymus, and brain as well as from v-rel-induced B-cell lymphomas but not from bursal B cells. The induction of this protein by v-rel during the development of bursal B-cell lymphomas appears, therefore, to be ectopic in nature. Overexpression of v-rel or c-rel in chicken embryonic fibroblasts, B-cell lines, and spleen mononuclear cells induces the expression of DM-GRASP. The ratio of DM-GRASP to v-Rel was fivefold higher than that of DM-GRASP/c-Rel in a B-cell line, DT95. Interestingly, the presence of HY78 antibody inhibits the in vitro proliferation of v-rel-transformed cells but not cells that immortalized by myc. These data suggest that DM-GRASP is one of the genes induced during v-rel-mediated tumor development and that DM-GRASP may be involved in the growth of v-rel tumor cells.

A mutant v-rel with increased ability to transform B lymphocytes

We observed that two strains of REV-T differ in the ability to transform bursal cells in vitro. REV-TW, with v-rel derived from a well-characterized clone and considered the prototype of the wild type, fails to generate colonies in soft agar. In contrast, REV-S2A3, derived from the S2A3 cell line, readily transforms bursal cells. With PCR, a 1,591-bp fragment containing v-rel from the REV-S2A3 provirus was cloned into plasmid pREV-0. Except for the absence of v-rel, pREV-0 is identical to pREV-TW. Five clones of pREV-PCR, each produced by an independent amplification, were obtained. The REV-PCR viruses displayed the strong transforming phenotype of REV-S2A3. Two mutations were identified in the 5' region of v-rel from REV-PCR1 to REV-PCR5: a silent mutation and a G-to-T transversion, changing the alanine at position 40 to serine. To confirm the relevance of this amino acid substitution, a 478-bp fragment containing the mutations was exchanged between REV-TW and REV-PCR1. Only the mutant viruses were able to form large colonies of bursal cells in liquid culture and to generate bursal cell colonies in soft agar. When tested on splenocytes, the wild-type viruses induced predominantly non-B-cell colonies while the mutant viruses gave origin mainly to B-cell colonies. The above results indicate that the substitution of serine for alanine at position 40 of v-Rel enhances the ability of REV-T to transform B lymphocytes in vitro. This mutation is close to the DNA-binding region, and the variant v-Rel oncoprotein shows increased kappa B-binding activity, thus confirming the relevance of this property for transformation.

Evolution of the oncogenic potential of v-rel: rel-induced expression of immunoregulatory receptors correlates with tumor development and in vitro transformation

v-rel is a viral oncogene that evolved from turkey c-rel, an NF-kappa B-related transcription factor. Numerous structural alterations record the evolutionary selection of v-rel and distinguish it from c-rel. To evaluate the biological significance of these alterations, we constructed a set of five c/v-rel hybrids in which three mutation clusters (c-Rel amino acids 1 to 97,222 to 302, and 328 to 598) were differentially distributed. These constructs, in addition to parental v-rel and c-rel and two C-terminal deletion mutants of c-rel, were expressed from a retroviral vector. An analysis of cells infected with each of the nine viruses revealed that mutations in all three domains contributed to the ability of v-rel to induce two endogenous c-rel target genes, major histocompatibility complex (MHC) class I and class II, in the B-cell line DT95 as well as MHC class II in normal splenocytes. The analysis revealed a strong nonlinear correlation between the ability of a Rel protein to induce expression of MHC proteins and its capacity to produce splenic tumors and establish in vitro transformation. This correlation is consistent with the hypothesis that v-rel transforms by constitutively altering expression of genes regulated by c-rel and in this way simulates events associated with immune response-linked proliferation of cells of hematopoietic origin. Further, the 16 carboxy-terminal amino acids of c-Rel were identified as a domain responsible for producing a cytotoxic and/or cytostatic effect in DT95. Because this effect is likely to differentially influence induction of MHC expression and tumorigenesis/transformation, it may represent one factor that contributes to the nonlinearity of their correlation.

In vivo evolution of c-rel oncogenic potential

The c-rel proto-oncogene belongs to the NF-kappa B/rel and I kappa B gene families, which regulate several inducible processes, including self-defense/repair and embryogenesis. Transduction of the c-rel transcription factor by the avian retrovirus resulted in the formation of a highly oncogenic virus, reticuloendotheliosis virus strain T (REV-T), that encodes the oncogene v-rel. To examine the oncogenic potential of c-rel, we inserted it into a REV-T-based retroviral vector, rescued virus [REV-C(CSV)], and infected 1-day-old chicks. All birds developed tumors, and all cell lines established from REV-C-induced tumors expressed c-rel proteins that lacked C-terminal sequences. These proteins, responsible for both in vivo and in vitro cell proliferation, were apparently selected for their oncogenic potential. In order to examine the cooperation of C-terminal deletions with other oncogenic alterations in vivo, point mutations present in the N-terminal and middle regions of v-rel were analyzed by a similar protocol. The data obtained support four conclusions. (i) c-rel proteins bearing any of three single-amino-acid mutations present in the N-terminal portion of v-rel were sufficiently oncogenic to induce tumor development in the absence of additional mutations. (ii) Combining a mutation from the N-terminal region of v-rel with a deletion of the C-terminal sequences of c-rel increases the oncogenicity of the protein in an additive manner. (iii) Mutations present in the middle of v-rel cooperated synergistically with C-terminal deletions to produce highly transforming viruses. (iv) Deletion of c-rel produced a variety of transforming rel proteins with sizes that extended from 42 to 65 kDa. The most frequently isolated rel deletion was 62 kDa in size. To examine the basis for the selection of different rel mutants, their ability to induce immunoregulatory surface receptors was analyzed. The data revealed a correlation between the induction capacity of these mutants and their corresponding contribution to in vivo tumorigenic potential. Moreover, an analysis of the subcellular localization of different rel proteins revealed an inverse correlation between the size of the protein and the proportion in the nucleus of lymphoid cells.

v-rel induces expression of three avian immunoregulatory surface receptors more efficiently than c-rel

The c-rel gene is a member of NF-kappa B/rel family of transcription factors that regulate expression of a variety of immunoregulatory molecules. The viral oncogene, v-rel, is a truncated and mutated form of the turkey c-rel gene expressed by reticuloendotheliosis virus, strain T. In this study, we demonstrated that three avian immunoregulatory receptors, major histocompatibility (MHC) antigens class I and class II as well as the interleukin-2 receptor (IL-2R), were induced on the surface of splenic tumor cells isolated from chickens infected with reticuloendotheliosis virus, strain T. All cell lines derived from splenic tumors expressed these three proteins. Their expression also correlated with the appearance of endogenous c-rel during a graft-versus-host reaction. In vitro, both c-rel and v-rel induced MHC class I, MHC class II, and IL-2R on an avian B-lymphoid cell line, DT95, and a T-lymphoid cell line, MSB-1. Quantitative kinetic analysis demonstrated both the accumulation of MHC class II mRNA and the appearance of surface MHC class II protein in response to the synthesis of either v-rel or c-rel. We show that v-rel induced the expression of MHC class II in the avian B-cell lines DT40 and DT95 more rapidly than c-rel and that, several weeks after infection, v-rel induced MHC class II as much as 50-fold more efficiently than c-rel. Finally, in vitro infection of splenocytes with retroviruses that express v-rel or c-rel induced MHC class I, MHC class II, and IL-2R expression. Quantitative analysis confirmed that p59v-rel was consistently more efficient at inducing expression of all three immunoregulatory receptors than exogenous p68c-rel. These data suggest that during tumor development, v-rel functions to induce (or suppress) the expression of genes similarly induced (or suppressed) by c-rel. The observations reported in this study are not in agreement with a model in which v-rel promotes tumor development by functioning as a dominant negative mutant of c-rel. In contrast, these findings support the hypothesis that lymphocyte immortalization and tumor development are the result, at least in part, of the capacity of v-rel to function as a dominant positive mutant that induces expression of genes normally regulated by c-rel.

High levels of c-rel expression are associated with programmed cell death in the developing avian embryo and in bone marrow cells in vitro

To determine the physiological processes in which the transcription factor c-Rel may act, we have examined its pattern of expression in the avian embryo by in situ hybridization. These studies showed that c-rel is expressed ubiquitously at low levels and at high levels in isolated cells undergoing programmed cell death by apoptosis or autophagocytosis. To further establish a functional link between expression of c-rel and cell death, we examined the biological consequences of c-rel overexpression in vitro. In primary avian fibroblasts, overexpression of c-rel leads to transformation and dramatic life span extension. In contrast, bone marrow cells expressing high levels of c-rel undergo a process of programmed cell death displaying features of both apoptosis and autophagocytic cell death. Thus, these experiments suggest a critical role for c-rel not only in the control of cell proliferation, but also in the induction of cell death.

Stochastic rearrangement of immunoglobulin variable-region genes in chicken B-cell development

The molecular mechanism by which immunoglobulin (Ig) gene rearrangement occurs is highly conserved between mammalian and avian species. However, in avian species, an equivalent to the mammalian pre-B cell, which has undergone Ig heavy-chain gene rearrangement and expresses mu heavy chains in the absence of Ig light-chain rearrangement, has not been convincingly demonstrated. It is consequently unclear whether an ordered progression of gene rearrangement events leading to functional Ig expression occurs in avian species. To examine the sequence of Ig gene rearrangement events in chicken B-cell development, we transformed day 12 embryo bursal cells with the REV-T(CSV) retrovirus. More than 100 clones were analyzed by Southern blotting and polymerase chain reaction for the presence of Ig gene rearrangements. The majority of these clones contained only germline Ig sequences. Several clones contained complete heavy- and light-chain rearrangements and 13 clones contained only heavy-chain rearrangements analogous to stages of mammalian B-cell development. However, 5 clones contained rearrangements of light-chain genes in the absence of complete heavy-chain rearrangement. Consequently, we conclude that rearrangement of chicken Ig light-chain genes does not require heavy-chain variable-region rearrangement. This observation suggests that chicken Ig gene rearrangement events required for Ig expression occur stochastically rather than sequentially.

Activation of the c-myb locus is insufficient for the rapid induction of disseminated avian B-cell lymphoma

We have previously reported that infection of 9- to 13-day-old chicken embryos with RAV-1 results in rapid development of a novel B-cell lymphoma in which proviral insertion has activated expression of the c-myb gene (E. Pizer and E. H. Humphries, J. Virol. 63:1630-1640, 1989). The biological properties of these B-cell lymphomas are distinct from those associated with the B-cell lymphomas that develop following avian leukosis virus proviral insertion within the c-myc locus. In an extension of this study, more than 200 chickens, infected as 10- to 11-day-old embryos, were examined for development of lymphomas that possess disrupted c-myb loci. Fourteen percent developed disseminated B-cell lymphoma. In the majority of these tumors, the RAV-1 provirus had inserted between the first and second exons that code for p75c-myb. However, insertions between the second and third exons and between the third and fourth exons were also detected. In situ analysis of myb protein expression in tumor tissue revealed morphological features suggesting that the tumor originates in the bursa. Within the bursa, the lymphoma appeared to spread from follicle to follicle without compromising the structural integrity of the organ. Tumor masses in liver demonstrated heterogeneous levels of myb protein suggestive of biologically distinct subpopulations. In contrast to the morbidity data, immunohistological analysis of bursae from 4- to 6-week-old chickens at risk of developing lymphomas bearing altered c-myb loci revealed lesions expressing elevated levels of myb in 16 of 19 birds. The activated myb lymphoma displayed very poor capacity to proliferate outside its original host. Only 1 of 33 in vivo transfers of tumor to recipient hosts established a transplantable tumor. None of the primary tumor tissue nor the transplantable tumor exhibited the capacity for in vitro proliferation. Similar experimental manipulation has yielded in vitro lines established from avian B-cell lymphomas expressing elevated levels of c-myc or v-rel. The dependence on embryonic infection for development of activated-myb lymphoma suggests a requirement for a specific target cell in which c-myb is activated by proviral insertion. It is likely, moreover, that continued tumor development requires elevated expression of myb proteins within a specific cell population in a restricted stage of differentiation.

An avian retrovirus expressing chicken pp59c-myc possesses weak transforming activity distinct from v-myc that may be modulated by adjacent normal cell neighbors

We demonstrate that EF168, an avian retrovirus that expresses the chicken pp59c-myc proto-oncogene, transforms quail embryo fibroblasts in vitro. An EF168-transformed quail clone, EF168-28, containing a single provirus, synthesizes several hundred copies of c-myc RNA and expresses elevated levels of the pp59c-myc gene product. The EF168 provirus in EF168-28 was isolated as a molecular clone, and the nucleotide sequence of its c-myc allele was confirmed as identical to that of exons 2 and 3 of the chicken c-myc proto-oncogene. Extended infection of quail embryo fibroblast cultures with EF168 induced a number of in vitro transformation-associated parameters similar to those elicited by the oncogenic v-myc-encoding retrovirus MC29, including alteration of cellular morphology, anchorage-independent growth, and induction of immortalized cell lines. Despite the fact that EF168 and MC29 shared these biological activities, further analysis revealed that EF168 initiated transformation in quail embryo fibroblasts, bone marrow, or adherent peripheral blood cultures 100- to 1,000-fold less efficiently than did MC29. Further, in contrast to MC29-induced foci, EF168 foci were smaller, morphologically diffuse, and less prominent. Analysis of newly infected cells demonstrated efficient expression of EF168 viral RNA in the absence of transformation. These differences suggest that while the pp59v-myc gene product can exert dominant transforming activity on quail embryo fibroblasts, its ability to initiate transformation is distinct from that of the pp110gag-v-myc gene product encoded by MC29 and may be suppressed by adjacent nontransformed cell neighbors.

Expression of immunoglobulin genes in the avian embryo bone marrow revealed by retroviral transformation

Analysis of the early stages of avian B lymphocyte differentiation has been hampered by the low frequency of extra-bursal B lineage cells in sites of hematopoiesis. Consequently, little is known about B lineage precursors prior to their migration into the bursa of Fabricius. Colonization of the bursa typically occurs between about days 8 and 14 of embryonic (e) development, although cells which can colonize the bursa, functionally defined as pre-bursal stem cells, can be demonstrated in embryo bone marrow up until about the time of hatch. As a novel approach to analyzing early stages of avian B lymphocyte development, we show here that transformed B lineage cells can be derived from chick embryo bone marrow after infection in vitro with the replication-defective retrovirus REV-T produced in the context of the non-cytopathic CSV helper virus. Thus, exposure of day 14e-15e chick embryo bone marrow cells to REV-T (CSV) results in the generation of transformed, polyclonal lines of cells. From these lines, cells expressing cell surface immunoglobulin were readily isolated by flow cytometric cell sorting and single cell cloning. Analysis of the phenotype of REV-T(CSV)-transformed clones with a panel of monoclonal antibody reagents demonstrated that transformation by v-rel likely leads to marked changes in cell surface antigen expression. Nonetheless, clones expressing cell surface immunoglobulin expressed apparently normal mRNA for immunoglobulin mu and light chain and contained apparently normal immunoglobulin heavy and light chain gene rearrangements. Furthermore, no evidence for chromosomal deletions or aberrations of the Ig loci was detected among either sIg+ or sIg- REV-T(CSV)-transformed clones.

The v-rel oncogene of avian reticuloendotheliosis virus transforms immature and mature lymphoid cells of the B cell lineage in vitro

Heavy chain gene rearrangements were analyzed in 67 independently derived reticuloendotheliosis virus (REV-T) transformed avian lymphoid cell lines. The status of the heavy chain genes in these REV-T transformed cell lines was determined, in part, by the age of the chicken whose spleen cells were transformed. Cell lines derived by the in vitro transformation of splenic lymphocytes obtained from embryos did not contain heavy chain gene Ig rearrangements. By contrast, splenic lymphocytes transformed by REV-T obtained from birds 1 week or older generally exhibited heavy chain gene rearrangements. The REV-T transformed lymphoid cell lines with heavy chain rearrangements also had light chain gene rearrangements. The Ig gene rearrangements in REV-T transformed cells were functional. The majority of the cells which had heavy chain rearrangements expressed a 2.2-kb mu transcript and synthesized and secreted IgM. An REV-T transformant was also identified which produced IgG, suggesting that v-rel can transform a terminally differentiated cell. Irrespective of their Ig chain gene status the REV-T transformed cell lines expressed variable amounts of some but not all normal B cell-specific markers and failed to express T cell markers. All the cell lines analyzed expressed the B-L (Ia-like) antigen as well as a common leukocyte antigen. Based on the expression of these surface molecules, the transformants with or without Ig gene rearrangements all appear to be committed to the B cell pathway.

Reticuloendotheliosis virus REV-T(REV-A)-induced neoplasia: development of tumors within the T-lymphoid and myeloid lineages

Infection of 1-day-old chicks with reticuloendotheliosis virus strain T induces a neoplastic disease that kills the chicks 7 to 14 days postinfection. In association with reticuloendotheliosis-associated virus (REV-A), reticuloendotheliosis virus T (REV-T) induces tumors that are predominantly immunoglobulin M (IgM) negative. We examined a variety of REV-T(REV-A)-induced tumors and tumor-derived cell lines and concluded that the principal IgM-negative tumors that develop in REV-T(REV-A)-infected chicks are neither pre-B or pre-B-pre-T but rather mature T lymphoid and myeloid. Without exception, the immunoglobulin heavy- and light-chain loci were in germ line configuration. Furthermore, the cell lines expressed neither sterile transcripts of the heavy- or light-chain immunoglobulin genes nor elevated levels of c-myb, two characteristics associated with murine pre-B lymphomas. Cell lines were also examined by using monoclonal antibodies for expression of a variety of cell surface markers expressed on B lymphocytes and/or T lymphocytes and/or myeloid cells. These reagents defined two types of IgM-negative tumor cell lines, one CIa+ CT-3+ (T lymphoid) and the other CIa+ CT-3-. By using the same approaches, tumor development was examined following REV-T(REV-A) infection at 1 and 3 weeks post-hatching of cyclophosphamide-treated chicks shown to be devoid of B-lymphoid cells. Again, the tumors that developed were either CIa+ CT-3+ (T lymphoid) or CIa+ CT-3-. Furthermore, the frequency and rate with which IgM-negative tumors developed in cyclophosphamide-treated chicks were not different from those observed in normal chicks. In 3-week-old cyclophosphamide-treated chicks, the presence of CIa+ CT-3- tumors bearing hematopoietic lineage markers, such as CLA-3 and 5M19, are most likely to have been derived from cells within the myeloid lineage.

Templated insertions in the rearranged chicken IgL V gene segment arise by intrachromosomal gene conversion

Chickens create a repertoire for their immunoglobulin light-chain gene by a novel process of sequence substitution within a unique rearranged V gene segment (VL1) during B-cell development in the bursa of Fabricius. Sequence analysis has shown that these nucleotide substitutions are not random. Potential donors for observed sequence substitutions are present within the 25 psi VL segments located 5' of the VL1 gene. In this report, we demonstrate that VL1 sequence substitutions: (1) are derived from the psi VL donor segment templates in cis, (2) do not result in reciprocal transfer of VL1 gene sequences to the psi VL segments, and (3) lead to the rapid disappearance of cells with nondiversified rearranged VL1 genes during B-cell development in the bursa of Fabricius. Together, these data provide evidence that VL1 sequence diversity arises as a result of intrachromosomal gene conversion.

Rearrangement and diversification of immunoglobulin light-chain genes in lymphoid cells transformed by reticuloendotheliosis virus

Avian lymphoid cells transformed by reticuloendotheliosis virus (REV-T) serve as a model to analyze the mechanism by which B-cell differentiation and antibody diversification occur in birds. Immunoglobulin light-chain gene rearrangements, diversification, and expression were analyzed in 72 independently derived REV-T-transformed cell lines. Lymphoid cells transformed as the result of expression of the v-rel oncogene were divided into two distinct groups based on light-chain gene rearrangements. The status of the light-chain gene loci in these REV-T-transformed cell lines was determined in part by the ages of the chickens whose spleen cells were transformed. In embryonic spleen cell lines transformed by the v-rel oncogene, rearrangements were not detected, even after prolonged culture in vitro, indicating that these cells are arrested in B-cell differentiation. REV-T transformants derived from spleens obtained from chickens 2 weeks old or older, however, had at least one light-chain allele rearranged. All of the cell lines analyzed which exhibited rearranged light-chain genes contained light-chain transcripts, and most of the REV-T-transformed cells which displayed light-chain rearrangements expressed immunoglobulin protein. REV-T, therefore, transforms B-lymphoid cells at phenotypically different stages of development. Many REV-T-transformed cells undergo immunoglobulin chain gene rearrangements during prolonged propagation in vitro. Most of the cell lines which rearrange their light-chain alleles also undergo diversification during cultivation in vitro. Light-chain diversification occurs during or after the rearrangement event.

Growth-inhibitory effects of vitamin E succinate on retrovirus-transformed tumor cells in vitro

Vitamin E succinate inhibited proliferation of C4#1 cells, an established avian retrovirus [reticuloendotheliosis virus (REV)]-transformed immature lymphoid tumor cell line, in a dose-dependent manner. The cytostatic effects of vitamin E succinate were reversible in that treated cells regained their ability to divide after vitamin E succinate removal. Possible mechanism(s) for the antiproliferative actions of vitamin E succinate were investigated. Analyses of C4#1 cell surface membrane antigen profiles and morphology indicated that vitamin E succinate was not inducing differentiation of the tumor cells to a more mature, differentiated, nonproliferative state. Five antioxidants, including a synthetic analogue of vitamin E, Trolox, as well as the active vitamin form, DL-alpha-tocopherol, were incapable of inhibiting C4#1 tumor cell growth, indicating that a mechanism of action other than or in addition to functions as an antioxidant may be operating. Cell cycle analyses suggested that C4#1 tumor cells treated with vitamin E succinate were blocked in the G0G1/early S phases of the cell cycle. Tumor growth arrested by vitamin E succinate did not affect the expression of the REV-encoded oncogene, v-rel, at either the RNA or protein level. These studies demonstrated that vitamin E, in the form of vitamin E succinate, inhibited the growth of retrovirus-transformed tumor cells in vitro and suggested that the antiproliferative effects of vitamin E succinate did not involve antioxidant properties but rather, as yet, unidentified mechanisms leading to cell cycle blockage.

Rearrangement and diversification of immunoglobulin light-chain genes in lymphoid cells transformed by reticuloendotheliosis virus

Avian lymphoid cells transformed by reticuloendotheliosis virus (REV-T) serve as a model to analyze the mechanism by which B-cell differentiation and antibody diversification occur in birds. Immunoglobulin light-chain gene rearrangements, diversification, and expression were analyzed in 72 independently derived REV-T-transformed cell lines. Lymphoid cells transformed as the result of expression of the v-rel oncogene were divided into two distinct groups based on light-chain gene rearrangements. The status of the light-chain gene loci in these REV-T-transformed cell lines was determined in part by the ages of the chickens whose spleen cells were transformed. In embryonic spleen cell lines transformed by the v-rel oncogene, rearrangements were not detected, even after prolonged culture in vitro, indicating that these cells are arrested in B-cell differentiation. REV-T transformants derived from spleens obtained from chickens 2 weeks old or older, however, had at least one light-chain allele rearranged. All of the cell lines analyzed which exhibited rearranged light-chain genes contained light-chain transcripts, and most of the REV-T-transformed cells which displayed light-chain rearrangements expressed immunoglobulin protein. REV-T, therefore, transforms B-lymphoid cells at phenotypically different stages of development. Many REV-T-transformed cells undergo immunoglobulin chain gene rearrangements during prolonged propagation in vitro. Most of the cell lines which rearrange their light-chain alleles also undergo diversification during cultivation in vitro. Light-chain diversification occurs during or after the rearrangement event.

Selection for B cells with productive IgL gene rearrangements occurs in the bursa of Fabricius during chicken embryonic development

The vast majority of immunoglobulin-expressing mature chicken B lymphocytes contain one functionally rearranged and one unrearranged allele of the immunoglobulin light chain (IgL) gene. Therefore, nearly all IgL V-J rearrangements present in mature chickens are in-frame. In contrast, the Ig genes of mature mammalian B cells contain a high proportion of out-of-frame V-J joints. To investigate the basis for this difference, gene rearrangement at the chicken IgL locus was characterized during embryonic development and in mature B-cell lines. Joining of the single functional variable (VL) segment with the single joining (JL) segment occurs in cells in multiple tissues during a transient period of chicken embryogenesis. Only one-third of the V-J joints cloned from days 10-12 of development are in-frame. An increasing proportion of in-frame V-J joints is observed within the bursa of Fabricius at successively later stages of development. Our data suggest that the bursa of Fabricius serves during embryonic development as a site of selective amplification of cells that have undergone productive V-J joining, such that nearly all V-J joints present in postembryonic B cells are in-frame. The high frequency of rearranged alleles joined in-frame that is found in posthatching bursal cells and mature B-cell lines appears to result from a low frequency with which cells undergo IgL rearrangement at both alleles, rather than from an increase in the precision of V-J joining in avian species.

Expression of v-rel induces mature B-cell lines that reflect the diversity of avian immunoglobulin heavy- and light-chain rearrangements

The infection of newly hatched chickens with reticuloendotheliosis virus strain T (REV-T) and a nonimmunosuppressive helper virus, chicken syncytial virus, induces rapidly metastatic B-cell lymphomas. In vivo analysis of these tumors with monoclonal antibodies detected the expression of the B-cell surface markers immunoglobulin M (IgM), CIa, Bu2, and CLA-1, but not IgG, Bu1, or a T-cell surface marker, CT-1. Cell lines derived from tumors exhibited the same pattern of staining, suggesting that expression of cell surface markers does not change during in vitro cell line development. All cell lines examined synthesized IgM in varying amounts. Northern (RNA blot) analysis confirmed abundant expression of v-rel mRNA, and Southern analysis revealed rearrangement of both heavy- and light-chain immunoglobulin loci. Analysis of the light-chain locus demonstrated that 20 of 22 lines contained a single rearranged allele. With respect to specific restriction enzyme sites within the V lambda 1 gene, the active allele in any given clone was either diversified or nondiversified. In contrast, examination of the heavy-chain loci within these lines demonstrated that 16 of the 22 had both alleles rearranged. Further diversification of the V lambda 1 locus did not occur after prolonged in vitro passage of the cell lines. We propose that v-rel expression arrests diversification of the light-chain locus in these lymphoid cells, allowing the production of stable, clonal B-cell populations. The development of these and similar cell lines will make it possible to identify specific stages of avian lymphoid ontogeny and to study the mechanism of rearrangement and diversification in the avian B lymphocyte.

RAV-1 insertional mutagenesis: disruption of the c-myb locus and development of avian B-cell lymphomas

Infection of young chickens with RAV-1, a subgroup A isolate of avian leukosis virus, results in the development of lymphoid leukosis, a B-cell lymphoma characterized by provirus insertion into the c-myc locus. We report here that when 12- to 13-day-old embryos rather than 1-day-old chickens were infected with RAV-1, a novel B-cell lymphoma developed in which proviral insertions had activated expression of the c-myb gene. These tumors expressed elevated levels of a 4.5-kilobase myb-containing mRNA transcript that contained c-myb sequences not found in v-myb. The c-myc locus in these tumors appeared normal. The biological properties of the activated myb lymphoma were distinct from those of lymphoid leukosis. Metastatic disease developed within 7 weeks of infection. Distinct intermediate pathogenic stages with preneoplastic and primary neoplastic lesions were not detected. Although bursal tissues appeared to be nonmalignant on gross examination, Southern analyses of bursal DNA revealed the presence of tumor with the same clonal origin as abdominal lymphoma masses. The dependence on embryonic infection for development of activated myb lymphoma suggests that the target cells in which c-myb is activated are found only in embryos and are distinct from those cells that give rise to lymphoid leukosis.

Expression of v-rel induces mature B-cell lines that reflect the diversity of avian immunoglobulin heavy- and light-chain rearrangements

The infection of newly hatched chickens with reticuloendotheliosis virus strain T (REV-T) and a nonimmunosuppressive helper virus, chicken syncytial virus, induces rapidly metastatic B-cell lymphomas. In vivo analysis of these tumors with monoclonal antibodies detected the expression of the B-cell surface markers immunoglobulin M (IgM), CIa, Bu2, and CLA-1, but not IgG, Bu1, or a T-cell surface marker, CT-1. Cell lines derived from tumors exhibited the same pattern of staining, suggesting that expression of cell surface markers does not change during in vitro cell line development. All cell lines examined synthesized IgM in varying amounts. Northern (RNA blot) analysis confirmed abundant expression of v-rel mRNA, and Southern analysis revealed rearrangement of both heavy- and light-chain immunoglobulin loci. Analysis of the light-chain locus demonstrated that 20 of 22 lines contained a single rearranged allele. With respect to specific restriction enzyme sites within the V lambda 1 gene, the active allele in any given clone was either diversified or nondiversified. In contrast, examination of the heavy-chain loci within these lines demonstrated that 16 of the 22 had both alleles rearranged. Further diversification of the V lambda 1 locus did not occur after prolonged in vitro passage of the cell lines. We propose that v-rel expression arrests diversification of the light-chain locus in these lymphoid cells, allowing the production of stable, clonal B-cell populations. The development of these and similar cell lines will make it possible to identify specific stages of avian lymphoid ontogeny and to study the mechanism of rearrangement and diversification in the avian B lymphocyte.