Blocked B cell differentiation and emigration support the early growth of Myc-induced lymphomas

Endogenous Avian Leukosis Virus in Combination with Serotype 2 Marek's Disease Virus Significantly Boosted the Incidence of Lymphoid Leukosis-Like Bursal Lymphomas in Susceptible Chickens

Lymphoid leukosis (LL)-like lymphoma is a low-incidence yet costly and poorly understood disease of domestic chickens. The observed unique characteristics of LL-like lymphomas are that the incidence of the disease is chicken line dependent; pathologically, it appeared to mimic avian leukosis but is free of exogenous ALV infection; inoculation of the nonpathogenic ALV-E or MDV-2 (SB-1) boosts the incidence of the disease; and inoculation of both the nonpathogenic ALV-E and SB-1 escalates it to much higher levels. This study was designed to test the impact of two new ALV-E isolates, recently derived from commercial broiler breeder flocks, in combination with the nonpathogenic SB-1 on LL-like lymphoma incidences in both an experimental egg layer line of chickens and a commercial broiler breeder line of chickens under a controlled condition. Data from this study provided an additional piece of experimental evidence on the potency of nonpathogenic ALV-E, MDV-2, and ALV-E plus MDV-2 in boosting the incidence of LL-like lymphomas in susceptible chickens. This study also generated the first piece of genomic evidence that suggests host transcriptomic variation plays an important role in modulating LL-like lymphoma formation.

In 2010, sporadic cases of avian leukosis virus (ALV)-like bursal lymphoma, also known as spontaneous lymphoid leukosis (LL)-like tumors, were identified in two commercial broiler breeder flocks in the absence of exogenous ALV infection. Two individual ALV subgroup E (ALV-E) field strains, designated AF227 and AF229, were isolated from two different breeder farms. The role of these ALV-E field isolates in development of and the potential joint impact in conjunction with a Marek’s disease virus (MDV) vaccine (SB-1) were further characterized in chickens of an experimental line and commercial broiler breeders. The experimental line 0.TVB*S1, commonly known as the rapid feathering-susceptible (RFS) line, of chickens lacks all endogenous ALV and is fully susceptible to all subgroups of ALV, including ALV-E. Spontaneous LL-like tumors occurred following infection with AF227, AF229, and a reference ALV-E strain, RAV60, in RFS chickens. Vaccination with serotype 2 MDV, SB-1, in addition to AF227 or AF229 inoculation, significantly enhanced the spontaneous LL-like tumor incidence in the RFS chickens. The spontaneous LL-like tumor incidence jumped from 14% by AF227 alone to 42 to 43% by AF227 in combination with SB-1 in the RFS chickens under controlled conditions. RNA-sequencing analysis of the LL-like lymphomas and nonmalignant bursa tissues of the RFS line of birds identified hundreds of differentially expressed genes that are reportedly involved in key biological processes and pathways, including signaling and signal transduction pathways. The data from this study suggested that both ALV-E and MDV-2 play an important role in enhancement of the spontaneous LL-like tumors in susceptible chickens. The underlying mechanism may be complex and involved in many chicken genes and pathways, including signal transduction pathways and immune system processes, in addition to reported viral genes.

IMPORTANCE Lymphoid leukosis (LL)-like lymphoma is a low-incidence yet costly and poorly understood disease of domestic chickens. The observed unique characteristics of LL-like lymphomas are that the incidence of the disease is chicken line dependent; pathologically, it appeared to mimic avian leukosis but is free of exogenous ALV infection; inoculation of the nonpathogenic ALV-E or MDV-2 (SB-1) boosts the incidence of the disease; and inoculation of both the nonpathogenic ALV-E and SB-1 escalates it to much higher levels. This study was designed to test the impact of two new ALV-E isolates, recently derived from commercial broiler breeder flocks, in combination with the nonpathogenic SB-1 on LL-like lymphoma incidences in both an experimental egg layer line of chickens and a commercial broiler breeder line of chickens under a controlled condition. Data from this study provided an additional piece of experimental evidence on the potency of nonpathogenic ALV-E, MDV-2, and ALV-E plus MDV-2 in boosting the incidence of LL-like lymphomas in susceptible chickens. This study also generated the first piece of genomic evidence that suggests host transcriptomic variation plays an important role in modulating LL-like lymphoma formation.

Clonal anergy of CD117+chB6+ B cell progenitors induced by avian leukosis virus subgroup J is associated with immunological tolerance

BACKGROUND

The pathogenesis of immunological tolerance caused by avian leukosis virus subgroup J (ALV-J), an oncogenic retrovirus, is largely unknown.

RESULTS

In this study, the development, differentiation, and immunological capability of B cells and their progenitors infected with ALV-J were studied both morphologically and functionally by using a model of ALV-J congenital infection. Compared with posthatch infection, congenital infection of ALV-J resulted in severe immunological tolerance, which was identified as the absence of detectable specific antivirus antibodies. In congenitally infected chickens, immune organs, particularly the bursa of Fabricius, were poorly developed. Moreover, IgM-and IgG-positive cells and total immunoglobulin levels were significantly decreased in these chickens. Large numbers of bursa follicles with no differentiation into cortex and medulla indicated that B cell development was arrested at the early stage. Flow cytometry analysis further confirmed that ALV-J blocked the differentiation of CD117+chB6+ B cell progenitors in the bursa of Fabricius. Furthermore, both the humoral immunity and the immunological capability of B cells and their progenitors were significantly suppressed, as assessed by (a) the antibody titres against sheep red blood cells and the Marek's disease virus attenuated serotype 1 vaccine; (b) the proliferative response of B cells against thymus-independent antigen lipopolysaccharide (LPS) in the spleen germinal centres; and (c) the capacities for proliferation, differentiation and immunoglobulin gene class-switch recombination of B cell progenitors in response to LPS and interleukin-4(IL-4) in vitro.

CONCLUSIONS

These findings suggested that the anergy of B cells in congenitally infected chickens is caused by the developmental arrest and dysfunction of B cell progenitors, which is an important factor for the immunological tolerance induced by ALV-J.

Wnt5a is essential for intestinal elongation in mice

Morphogenesis of the mammalian small intestine entails extensive elongation and folding of the primitive gut into a tightly coiled digestive tube. Surprisingly, little is known about the cellular and molecular mechanisms that mediate the morphological aspects of small intestine formation. Here, we demonstrate that Wnt5a, a member of the Wnt family of secreted proteins, is essential for the development and elongation of the small intestine from the midgut region. We found that the small intestine in mice lacking Wnt5a was dramatically shortened and duplicated, forming a bifurcated lumen instead of a single tube. In addition, cell proliferation was reduced and re-intercalation of post-mitotic cells into the elongating gut tube epithelium was disrupted. Thus, our study demonstrates that Wnt5a functions as a critical regulator of midgut formation and morphogenesis in mammals.

Tumor-induced sentinel lymph node lymphangiogenesis and increased lymph flow precede melanoma metastasis

Lymphangiogenesis is associated with human and murine cancer metastasis, suggesting that lymphatic vessels are important for tumor dissemination. Lymphatic vessel alterations were examined using B16-F10 melanoma cells implanted in syngeneic C57Bl/6 mice, which form tumors metastasizing to draining lymph nodes and subsequently to the lungs. Footpad tumors showed no lymphatic or blood vessel growth; however, the tumor-draining popliteal lymph node featured greatly increased lymphatic sinuses. Lymph node lymphangiogenesis began before melanoma cells reached draining lymph nodes, indicating that primary tumors induce these alterations at a distance. Lymph flow imaging revealed that nanoparticle transit was greatly increased through tumor-draining relative to nondraining lymph nodes. Lymph node lymphatic sinuses and lymph flow were increased in mice implanted with unmarked or with foreign antigen-expressing melanomas, indicating that these effects are not due to foreign antigen expression. However, tumor-derived immune signaling could promote lymph node alterations, as macrophages infiltrated footpad tumors, whereas lymphocytes accumulated in tumor-draining lymph nodes. B lymphocytes are required for lymphangiogenesis and increased lymph flow through tumor-draining lymph nodes, as these alterations were not observed in mice deficient for B cells. Lymph node lymphangiogenesis and increased lymph flow through tumor-draining lymph nodes may actively promote metastasis via the lymphatics.

The protooncogene MYC can break B cell tolerance

The protooncogene MYC has been implicated in both the proliferation and programmed cell death of lymphoid cells, and in the genesis of lymphoid tumors. Here, we report that overexpression of MYC, as found in many lymphomas, can break immune tolerance. Mice that would otherwise be tolerant to a transgenic autoantigen mounted an immune response to the antigen if MYC was vigorously expressed in the B cell lineage. The responsive B cells converted to an activated phenotype and produced copious amounts of autoantibody that engendered immune complex disease of the kidney. MYC was required to both establish and maintain the breach of tolerance. These effects may be due to the ability of MYC to serve as a surrogate for cytokines. We found that the gene could mimic the effects of cytokines on both B cell proliferation and survival and, indeed, was required for those effects. These findings demonstrate a critical role for MYC in the response of B cells to antigen and expand the potential contributions of MYC to the genesis of lymphomas.

Myc regulates VEGF production in B cells by stimulating initiation of VEGF mRNA translation

Deregulated c-myc gene expression is associated with many human and animal cancers. Myc overexpression promotes the growth of blood and lymphatic vessels, which is due in part to induction of growth factors including vascular endothelial growth factor (VEGF). We determined that the P493-6 human B-cell line increases VEGF production 10-fold upon Myc overexpression. Myc overexpression in avian B cells similarly resulted in high level VEGF production. Real-time RT-PCR analyses showed that Myc did not alter the VEGF mRNA content of these cell lines, indicating that a post-transcriptional mechanism regulates VEGF production. VEGF mRNA translation was examined by RT-PCR analysis of monosome and polysome sucrose gradient fractions from Myc-on and Myc-off P493-6 cells. Myc increased VEGF mRNA translation initiation, as VEGF mRNA loading onto polysomes increased 14-fold in Myc-on cells, and the number of ribosomes loaded per VEGF mRNA increased threefold. This translational regulation is specific to VEGF mRNA, as total polysomes show the same sucrose gradient profile in Myc-on and Myc-off cells, with no change in the percent ribosomes in polysomes, or in the number of ribosomes per polysomal mRNA. Myc stimulates VEGF production by a rapamycin- and LY294002-sensitive pathway, which does not involve alteration of eIF4E activity.

Developmental stage determines the effects of MYC in the mammary epithelium

Epidemiological findings suggest that the consequences of a given oncogenic stimulus vary depending upon the developmental state of the target tissue at the time of exposure. This is particularly evident in the mammary gland, where both age at exposure to a carcinogenic stimulus and the timing of a first full-term pregnancy can markedly alter the risk of developing breast cancer. Analogous to this, the biological consequences of activating oncogenes, such as MYC, can be influenced by cellular context both in terms of cell lineage and cellular environment. In light of this, we hypothesized that the consequences of aberrant MYC activation in the mammary gland might be determined by the developmental state of the gland at the time of MYC exposure. To test this hypothesis directly, we have used a doxycycline-inducible transgenic mouse model to overexpress MYC during different stages of mammary gland development. Using this model, we find that the ability of MYC to inhibit postpartum lactation is due entirely to its activation within a specific 72-hour window during mid-pregnancy; by contrast, MYC activation either prior to or following this 72-hour window has little or no effect on postpartum lactation. Surprisingly, we find that MYC does not block postpartum lactation by inhibiting mammary epithelial differentiation, but rather by promoting differentiation and precocious lactation during pregnancy, which in turn leads to premature involution of the gland. We further show that this developmental stage-specific ability of MYC to promote mammary epithelial differentiation is tightly linked to its ability to downregulate caveolin 1 and activate Stat5 in a developmental stage-specific manner. Our findings provide unique in vivo molecular evidence for developmental stage-specific effects of oncogene activation, as well as the first evidence linking MYC with activation of the Jak2-Stat5 signaling pathway.

Reduced Myc overexpression and normal B-cell differentiation mediate resistance to avian leukosis virus lymphomagenesis

Avian leukosis virus (ALV) induces bursal lymphoma in tumor-susceptible chicken strains after proviral integration within the c-myc gene, and subsequent expansion of Myc-overexpressing lymphocytes within transformed follicles. Line 6(3) strain chickens are resistant to ALV tumorigenesis, largely failing to develop Myc-transformed follicles, although they show similar levels of ALV infection and integration as lymphoma-susceptible strains. Immunohistochemical analysis determined that the transformed follicles that do arise in lymphoma-resistant birds show much lower and more variable Myc overexpression than those of susceptible birds. This reduced Myc overexpression fails to block B-cell differentiation in resistant birds, while high Myc consistently blocks development at a late embryo stage in susceptible birds. This failure of Myc to block differentiation results in a normal pattern of posthatching bursal emigration in resistant transformed follicles, while transformed follicles of susceptible birds grow rapidly due to blocked emigration. Forced Myc overexpression produces transformed follicles in resistant birds, indicating that resistant lymphocytes can tolerate high Myc expression. The coding sequence and expression of the endogenous c-myc gene is the same in resistant and susceptible birds, suggesting that genetic resistance is instead mediated by reduced ALV LTR enhancer-driven transcription in the target lymphocytes of resistant birds.

B lymphocyte-specific c-Myc expression stimulates early and functional expansion of the vasculature and lymphatics during lymphomagenesis

Expression of the c-myc proto-oncogene is deregulated in many human cancers. We examined the role of c-Myc in stimulating angiogenesis and lymphangiogenesis in a highly metastatic murine model of Burkitt's lymphoma (E micro -c-myc), where c-Myc is expressed exclusively in B lymphocytes. Immunohistochemical analysis of bone marrow and lymph nodes from young (preneoplastic) E micro -c-myc transgenic mice revealed increased growth of blood vessels, which are functional by dye flow assay. Lymphatic sinuses also increased in size and number within the lymph nodes, as demonstrated by immunostaining for with a lymphatic endothelial marker 10.1.1. The 10.1.1 antibody recognizes VEGFR-2- and VEGFR-3-positive lymphatic sinuses and vessels within lymph nodes, and also recognizes lymphatic vessels in other tissues. Subcutaneously injected dye traveled more efficiently through draining lymph nodes in E micro -c-myc mice, indicating that these hypertrophic lymphatic sinuses increase lymph flow. Purified B lymphocytes and lymphoid tissues from E micro -c-myc mice expressed increased levels of vascular endothelial growth factor (VEGF) by immunohistochemical or immunoblot assays, which could promote blood and lymphatic vessel growth through interaction with VEGFR-2, which is expressed on the endothelium of both vessel types. These results indicate that constitutive c-Myc expression stimulates angiogenesis and lymphangiogenesis, which may promote the rapid growth and metastasis of c-Myc-expressing cancer cells, respectively.

Functional genomic analysis reveals distinct neoplastic phenotypes associated with c-myb mutation in the bursa of Fabricius

Avian retroviral integration into the c-myb locus is casually associated with the development of lymphomas in the bursa of Farbricius of chickens; these arise with a shorter latency than bursal lymphomas caused by deregulation of c-myc. This study indicates that c-myb mutation in embryonic bursal precursors leads to an oligoclonal population of developing bursal follicles, showing a variable propensity to form a novel lesion, the neoplastic follicle (NF). About half of such bursas rapidly developed lymphomas. Detection of changes in gene expression, during the development of neoplasms, was carried out by cDNA microarray analysis. The transcriptional signature of lymphomas with mutant c-myb was more limited than, and only partially shared with, those of bursal lymphomas caused by Myc or Rel oncogenes. The c-myb-associated lymphomas frequently showed overexpression of c-myc and altered expression of other genes involved in cell cycle control and proliferation-related signal transduction. Oligoclonal, NF-containing bursas lacked detectable c-myc overexpression and demonstrated a pattern of gene expression distinct from that of normal bursa and partially shared with the short-latency lymphomas. This functional genomic analysis uncovered several different pathways of lymphomagenesis by oncogenic transcription factors acting in a B-cell lineage.

Avian bic, a gene isolated from a common retroviral site in avian leukosis virus-induced lymphomas that encodes a noncoding RNA, cooperates with c-myc in lymphomagenesis and erythroleukemogenesis

bic is a novel gene identified at a common retroviral integration site in avian leukosis virus-induced lymphomas and has been implicated as a collaborator with c-myc in B lymphomagenesis. It lacks an extensive open reading frame and is believed to function as an untranslated RNA (W. Tam, Gene 274:157-167, 2001; W. Tam, D. Ben-Yehuda, and W. S. Hayward, Mol. Cell. Biol. 17:1490-1502, 1997). The oncogenic potential of bic, particularly its ability to cooperate with c-myc in oncogenesis, was tested directly by expressing c-myc and bic, either singly or in pairwise combination, in cultured chicken embryo fibroblasts (CEFs) and in chickens using replication-competent retrovirus vectors. Coexpression of c-myc and bic in CEFs caused growth enhancement of cells. Most importantly, chick oncogenicity assays demonstrated that bic can cooperate with c-myc in lymphomagenesis and erythroleukemogenesis. The present study provides direct evidence for the involvement of untranslated RNAs in oncogenesis and provides further support for the role of noncoding RNAs as riboregulators.