Feline glucose-6-phosphate dehydrogenase cellular mosaicism. Application to the study of retrovirus-induced pure red cell aplasia

CellDestiny: A RShiny application for the visualization and analysis of single-cell lineage tracing data

Single-cell lineage tracing permits the labeling of individual cells with a heritable marker to follow the fate of each cell's progeny. Over the last twenty years, several single-cell lineage tracing methods have emerged, enabling major discoveries in developmental biology, oncology and gene therapies. Analytical tools are needed to draw meaningful conclusions from lineage tracing measurements, which are characterized by high variability, sparsity and technical noise. However, the single cell lineage tracing field lacks versatile and easy-to-use tools for standardized and reproducible analyses, in particular tools accessible to biologists. Here we present CellDestiny, a RShiny app and associated web application developed for experimentalists without coding skills to perform visualization and analysis of single cell lineage-tracing datasets through a graphical user interface. We demonstrate the functionality of CellDestiny through the analysis of (i) lentiviral barcoding datasets of murine hematopoietic progenitors; (ii) published integration site data from Wiskott-Aldrich Symdrome patients undergoing gene-therapy treatment; and (iii) simultaneous barcoding and transcriptomic analysis of murine hematopoietic progenitor differentiation in vitro. In summary, CellDestiny is an easy-to-use and versatile toolkit that enables biologists to visualize and analyze single-cell lineage tracing data.

Sex and life expectancy

BACKGROUND

A sexual dimorphism in human life expectancy has existed in almost every country for as long as records have been kept. Although human life expectancy has increased each year, females still live longer, on average, than males. Undoubtedly, the reasons for the sex gap in life expectancy are multifaceted, and it has been discussed from both sociological and biological perspectives. However, even if biological factors make up only a small percentage of the determinants of the sex difference in this phenomenon, parity in average life expectancy should not be anticipated.

OBJECTIVE

The aim of this review is to highlight biological mechanisms that may underlie the sexual dimorphism in life expectancy.

METHODS

Using PubMed, ISI Web of Knowledge, and Google Scholar, as well as cited and citing reference histories of articles through August 2012, English-language articles were identified, read, and synthesized into categories that could account for biological sex differences in human life expectancy.

RESULTS

The examination of biological mechanisms accounting for the female-based advantage in human life expectancy has been an active area of inquiry; however, it is still difficult to prove the relative importance of any 1 factor. Nonetheless, biological differences between the sexes do exist and include differences in genetic and physiological factors such as progressive skewing of X chromosome inactivation, telomere attrition, mitochondrial inheritance, hormonal and cellular responses to stress, immune function, and metabolic substrate handling among others. These factors may account for at least a part of the female advantage in human life expectancy.

CONCLUSIONS

Despite noted gaps in sex equality, higher body fat percentages and lower physical activity levels globally at all ages, a sex-based gap in life expectancy exists in nearly every country for which data exist. There are several biological mechanisms that may contribute to explaining why females live longer than men on average, but the complexity of the human life experience makes research examining the contribution of any single factor for the female advantage difficult. However, this information may still prove important to the development of strategies for healthy aging in both sexes.

Quantification of endogenous and exogenous feline leukemia virus sequences by real-time PCR assays

Endogenous retroviruses are integrated in the genome of most vertebrates. They represent footprints of ancient retroviral infection and are vertically transmitted from parents to their offspring. In the genome of all domestic cats, sequences closely related to exogenous FeLV known as endogenous feline leukemia virus (enFeLV), are present. enFeLV are incapable of giving rise to infectious virus particles. However, transcription and translation of enFeLV have been demonstrated in tissues of healthy cats and in feline cell lines. The presence of enFeLV-env has been shown in specific embryonic tissues and adult thymic cells. In addition, the enFeLV-env region recombines with FeLV subgroup A giving rise to an infectious FeLV-B virus. enFeLV envelope protein, FeLIX (FeLV infectivity X-essory protein) is also involved in mediating FeLV-T infection. In order to test the hypothesis that the enFeLV loads play a role in exogenous FeLV-A infection and pathogenesis, quantitative real-time PCR and RT-PCR assays were developed. An assay, specific to U3 region of all different subtypes of exogenous FeLV, was designed and applied to quantify exogenous FeLV proviral or viral load in cats, while three real-time PCR assays were designed to quantify U3 and env enFeLV loads (two within U3 amplifying different sequences; one within env). enFeLV loads were investigated in blood samples derived from Swiss privately owned domestic cats, specific pathogen-free (SPF) cats and European wildcats (Felis silvestris silvestris). Significant differences in enFeLV loads were observed between privately owned cats and SPF cats as well as among SPF cats originating from different catteries and among domestic cats of different breeds. When privately owned cats were compared, FeLV-infected cats had higher loads than uninfected cats. In addition, wildcats had higher enFeLV loads than domestic cats. In conclusion, the quantitative real-time PCR assays described herein are important prerequisites to quantify enFeLV proviral loads in felids and thus are important tools to investigate the role of enFeLV loads in FeLV infection.

Clonality analysis of various hematopoietic disorders in cats naturally infected with feline leukemia virus

The clonality analysis of the bone marrow cells was carried out by detecting the integrated proviruses of feline leukemia virus (FeLV) to understand the pathogenesis of FeLV-associated hematopoietic disorders in cats. Bone marrow cells from 4 cases with acute myeloid leukemia (AML), 9 cases with myelodysplastic syndromes (MDS), 2 cases with pure red cell aplasia (PRCA) and 3 healthy carriers infected with FeLV were subjected to Southern blot analyses using an exogenous FeLV probe. Clonal hematopoiesis was found in all the cases with AML and in 6 of the 9 cases with MDS, but not in the cases with both PRCA and healthy carriers infected with FeLV. In the 2 cases with MDS, it was thought that the same clones of the hematopoietic cells might proliferate before and after the progression of the disease irrespective of the changes of the hematological diagnoses by cytological examination. This study indicates that MDS in cats is a disease manifestation as a result of clonal proliferation of hematopoietic cells and can be recognized as a pre-leukemic state of AML.

Transduction of feline hematopoietic cells by oncoretroviral vectors pseudotyped with the subgroup A feline leukemia virus (FeLV-A)

The domestic cat is an outbred species with many identified analogues of human genetic diseases. Therefore, it has the potential to serve as a large animal model for evaluating the feasibility of hematopoietic stem cell gene therapy. This study compared gene transfer rates into feline hematopoietic progenitors by oncoretroviral vectors pseudotyped with the subgroup A feline leukemia virus (FeLV-A), the gibbon ape leukemia virus (GALV), and the murine amphotropic virus. Gene transfer rates were superior with the FeLV-A pseudotypes, which were then tested for their ability to transduce a cat hematopoietic repopulating cell. At more than 1 year posttransplantation, persistent marking was seen in both lymphoid and myeloid lineages of a myeloablated domestic cat that had received autologous marrow cells transduced with an FeLV-A pseudotyped vector.

Cloning of the cellular receptor for feline leukemia virus subgroup C (FeLV-C), a retrovirus that induces red cell aplasia

Feline leukemia virus-C (FeLV-C) causes red cell aplasia in cats, likely through its interaction with its cell surface receptor. We identified this receptor by the functional screening of a library of complementary DNAs (cDNA) from feline T cells. The library, which was cloned into a retroviral vector, was introduced into FeLV-C–resistant murine (NIH 3T3) cells. The gene conferring susceptibility to FeLV-C was isolated and reintroduced into the same cell type, as well as into FeLV-C–resistant rat (NRK 52E) cells, to verify its role in viral infection. The receptor cDNA is predicted to encode a protein of 560 amino acids with 12 membrane-spanning domains, termed FLVCR. FLVCR has significant amino acid sequence homology with members of the major facilitator superfamily and especially D-glucarate transporters described in bacteria and in C. elegans. As FeLV-C impairs the in vivo differentiation of burst-forming unit–erythroid to colony-forming unit–erythroid, we hypothesize that this transporter system could have an essential role in early erythropoiesis. In further studies, a 6-kb fragment of the human FLVCR gene was amplified by polymerase chain reaction from genomic DNA, using homologous cDNA sequences identified in the human Expressed Sequence Tags database. By radiation hybrid mapping, the human gene was localized to a 0.5-centiMorgan region on the long arm of chromosome 1 at q31.3.

The ex vivo expansion of feline marrow cells leads to increased numbers of BFU-E and CFU-GM but a loss of reconstituting ability

Some studies in mice suggest that hematopoietic stem cells can be maintained and possibly expanded ex vivo. As there is a paucity of data from larger animals, we have studied hematologic reconstitution following autologous marrow transplantation in cats. Transplantation of very low density marrow cells (<1.050 g/ml), termed "1050 cells," at 2 x 10(5) cells/kg leads to rapid hematopoietic recovery (granulocytes >200/microl by day 20+/-2 and platelets >50 x 10(3)/microl by day 21+/-3). Recovery rates are comparable when 1-2 x 10(7) nucleated marrow cells/kg are infused, suggesting that reconstituting cells are enriched 50- to 100-fold in the 1050 cell preparation. To explore if the numbers of reconstituting cells could be expanded ex vivo, 1050 cells were cultured in the presence of 5 ng/ml recombinant human interleukin 1beta, 10 ng/ml recombinant canine (rc)G-CSF, 2 U/ml rHu erythropoietin, and 5 ng/ml rc stem cell factor. Maximum numbers of BFU-E and colony-forming units-granulocyte/macrophage (CFU-GM) were generated at day 6. However, when 10(6) 1050 cells/kg (5x that needed for hematologic recovery) were cultured for six days and all resulting cells infused into irradiated donor animals, two of nine (22%) engrafted. Even when flt3 ligand (100 ng/ml) was added to cultures, only two of five animals (40%) engrafted (p = NS versus studies without flt3 ligand). These data confirm that BFU-E and CFU-GM provide inaccurate estimates of reconstituting cells and demonstrate that the number or function of feline reconstituting cells is impaired by in vitro culture with cytokines.

An X chromosome gene regulates hematopoietic stem cell kinetics

Females are natural mosaics for X chromosome-linked genes. As X chromosome inactivation occurs randomly, the ratio of parental phenotypes among blood cells is approximately 1:1. Recently, however, ratios of greater than 3:1 have been observed in 38-56% of women over age 60. This could result from a depletion of hematopoietic stem cells (HSCs) with aging (and the maintenance of hematopoiesis by a few residual clones) or from myelodysplasia (the dominance of a neoplastic clone). Each possibility has major implications for chemotherapy and for transplantation in elderly patients. We report similar findings in longitudinal studies of female Safari cats and demonstrate that the excessive skewing that develops with aging results from a third mechanism that has no pathologic consequence, hemizygous selection. We show that there is a competitive advantage for all HSCs with a specific X chromosome phenotype and, thus, demonstrate that an X chromosome gene (or genes) regulates HSC replication, differentiation, and/or survival.

Evidence that hematopoiesis may be a stochastic process in vivo

To study the behavior of hematopoietic stem cells in vivo, hematopoiesis was simulated by assuming that all stem cell decisions (that is, replication, apoptosis, initiation of a differentiation/maturation program) were determined by chance. Predicted outcomes from simulated experiments were compared with data obtained in autologous marrow transplantation studies of glucose 6-phosphate dehydrogenase (G6PD) heterozygous female Safari cats. With this approach, we prove that stochastic differentiation can result in the wide spectrum of discrete outcomes observed in vivo, and that clonal dominance can occur by chance. As the analyses also suggest that the frequency of feline hematopoietic stem cells is only 6 per 10(7) nucleated marrow cells, and that sem cells do not replicate on average more frequently than once every three weeks, these large-animal data challenge clinical strategies for marrow transplantation and gene therapy.

Cytopathic feline leukemia viruses cause apoptosis in hemolymphatic cells

Certain isolates of the oncoretrovirus feline leukemia virus (FeLV) are strongly cytopathic for hemolymphatic cells. A major cytopathicity determinant is encoded by the SU envelope glucoprotein gp70. Isolates with subgroup C SU gp70 genes specifically induce apoptosis in hemolymphatic cells but not fibroblasts. In vitro exposure of feline T-cells to FeLV-C leads first to productive viral replication, next to virus-induced cell agglutination, and lastly to apogenesis. This in vitro phenomenon may explain the severe progressive thymic atrophy and erythroid aplasia which follow viremic FeLV-C infection in vivo. Inappropriate apoptosis induction has also been hypothesized to explain the severe thymico-lymphoid atrophy and progressive immune deterioration associated with isolates of FeLV containing variant envelope genes. The influence of envelope hypervariability (variable regions 1 [Vr1] and 5 [Vr5] on virus tropism, viremia induction, neutralizing antibody development and cytopathicity is discussed. Certain potentially cytopathic elements in FeLV SU gp70 Vr5 may derive from replication-defective, poorly expressed, endogenous FeLVs. Other more highly conserved regions in FeLV TM envelope p15E may also influence apoptosis induction.

Behavior of hematopoietic stem cells in a large animal

To study the behavior of hematopoietic stem cells in vivo, we transplanted glucose-6-phosphate dehydrogenase (G6PD) heterozygous (female Safari) cats with small amounts of autologous marrow. The G6PD phenotypes of erythroid burst-forming units and granulocyte/macrophage colony-forming units were repeatedly assayed for 3.5-6 years after transplantation to track contributions of stem cell clones to the progenitor cell compartment. Two phases of stem cell kinetics were observed, which were similar to the pattern reported in comparable murine studies. Initially there were significant fluctuations in contributions of stem cell clones. Later clonal contributions to hematopoiesis stabilized. The initial phase of clonal disequilibrium, however, extended for 1-4.5 years (and not 2-6 months as seen in murine experiments). After this subsided, all progenitor cells from some animals expressed a single parental G6PD phenotype, suggesting that blood cell production could be stably maintained by the progeny of one (or a few) cells. As the hematopoietic demand of a cat (i.e., number of blood cells produced per lifetime) is over 600 times that of a mouse, this provides evidence that an individual hematopoietic stem cell has a vast self-renewal and/or proliferative capacity. The long phase of clonal instability may reflect the time required for stem cells to replicate sufficiently to reconstitute a large stem cell reserve.

Haematological disorders associated with feline retrovirus infections

Feline oncornavirus and lentivirus infections have provided useful models to characterize the virus and host cell factors involved in a variety of marrow suppressive disorders and haematological malignancies. Exciting recent progress has been made in the characterization of the viral genotypic features involved in FeLV-associated diseases. Molecular studies have clearly defined the causal role of variant FeLV env gene determinants in two disorders: the T-lymphocyte cytopathicity and the clinical acute immunosuppression induced by the FeLV-FAIDS variant and the pure red cell aplasia induced by FeLV-C/Sarma. Variant or enFeLV env sequences also appear to play a role in FeLV-associated lymphomas. Additional studies are required to determine the host cell processes that are perturbed by these variant env gene products. In the case of the FeLV-FAIDS variant, the aberrant env gene products appear to impair superinfection interference, resulting in accumulation of unintegrated viral DNA and cell death. In other cases it is likely that the viral env proteins interact with host products that are important in cell viability and/or proliferation. Understanding of these mechanisms will therefore provide insights to factors involved in normal lymphohaematopoiesis. Similarly, studies of FeLV-induced haematological neoplasms should reveal recombination or rearrangement events involving as yet unidentified host gene sequences that encode products involved in normal cell growth regulation. These sequences may include novel protoncogenes or sequences homologous to genes implicated in human haematological malignancies. The haematological consequences of FIV are quite similar to those associated with HIV. As with HIV, FIV does not appear to directly infect myeloid or erythroid precursors, and the mechanisms of marrow suppression likely involve virus, viral antigen, and/or infected accessory cells in the marrow microenvironment. Studies using in vitro experimental models are required to define the effects of each of these microenvironmental elements on haematopoietic progenitors. As little is known about the molecular mechanisms of FIV pathogenesis, additional studies of disease-inducing FIV strains are needed to identify the genotypic features that correlate with virulent phenotypic features. Finally, experimental FIV infection in cats provides the opportunity to correlate in vivo virological and haematological changes with in vitro observations in a large animal model that closely mimics HIV infection in man.

Hematopoietic target cells of anemogenic subgroup C versus nonanemogenic subgroup A feline leukemia virus

Feline leukemia viruses (FeLVs) belonging to interference subgroup C induce fatal anemia resembling human pure red cell aplasia (PRCA). Subgroup A FeLVs, although closely related genetically to FeLVs of subgroup C, do not induce PRCA. The determinants for PRCA induction by a molecularly cloned prototype subgroup C virus (FeLV-Sarma-C [FSC]) have been localized to the N-terminal 241 amino acids of the surface glycoprotein (SU) gp70. To investigate whether the anemogenic activity of FSC reflects a unique capacity to infect erythroid progenitor cells, we used correlative immunogold, immunofluorescence, and cytological staining to study prospectively the hemopoietic cell populations infected by either FSC or FeLV-FAIDS-61E-A (F6A), a prototype of subgroup A virus. The results demonstrated that although only FSC-infected animals developed erythrocyte aplasia, the env SU and the major core protein (p27) were expressed in a surprisingly large fraction of the lymphoid, erythroid, and myeloid lineage marrow cells in both FSC- and F6A-infected cats. Between days 8 and 17 postinoculation, gp70 and p27 were detected in 43 to 73% of erythroid, 25 to 75% of lymphoid, and 35 to 50% of myeloid lineage cells, regardless of whether the cats were infected with FSC or F6A. Thus, anemogenic subgroup C and nonanemogenic subgroup A FeLVs have similar hemopoietic cell tropism and infection kinetics, despite their divergent effects on erythroid progenitor cell function. Acute anemia induction by subgroup C FeLV, therefore, does not reflect a unique tropism for marrow erythroid cells but rather indicates a unique cytopathic effect of the SU on erythroid progenitor cells.

Cytotoxicity in feline leukemia virus subgroup-C infected fibroblasts is mediated by adherent bone marrow mononuclear cells

The pathogenesis of retrovirus-induced erythroid aplasia in cats is unknown. In studies to define mechanisms of cytotoxicity associated with retroviral infections, bone marrow mononuclear cells (BMMC) from healthy specific pathogen-free cats were co-cultured with uninfected feline embryonic fibroblasts (FEA cells) and FEA cells infected with feline leukemia virus (FeLV) of subgroup A (FEA-A) or subgroup C (FEA-C). Moderate to marked cytotoxicity (CPE) developed in co-cultures of BMMC and FEA-C cells on Days 5 to 7 of incubation but not in co-cultures of BMMC and FEA-A or BMMC and uninfected cells (FEA-CT). Cytotoxicity was associated with adherent cells of light density (1.056) from bone marrow and peripheral blood, which were positive for alpha naphthyl butyrate esterase activity. Stimulation of adherent cells with phorbol ester or addition of recombinant human tumor necrosis factor-alpha (rhTNF-alpha) caused similar CPE in FEA-CT cells. The TNF-alpha concentrations in the culture supernatants of BMMC+FEA-C were higher than those of BMMC+FEA-A or BMMC+FEA-CT, and addition of anti-TNF antibodies to the cultures blocked the CPE. These data support the hypothesis that macrophages exposed to FeLV-C cause CPE in co-cultures of BMMC and FEA cells by a mechanism involving TNF-alpha. It is suggested that TNF-alpha may be involved in the suppression of hematopoiesis in cats which develop FeLV-C induced erythroid aplasia.

Evidence for the maintenance of hematopoiesis in a large animal by the sequential activation of stem-cell clones

To test if hematopoiesis can be maintained by the sequential activation of stem-cell clones, we performed autologous marrow transplantations with limited numbers of cells in cats heterozygous for the X chromosome-linked enzyme glucose-6-phosphate dehydrogenase (G6PD) and observed the G6PD phenotypes of erythroid and granulocyte/macrophage progenitors over time. The animals were the female offspring of Geoffroy male and domestic female cats. In repeated studies of marrow from control animals (n = 5) or experimental animals prior to transplantation (n = 3), the percent of progenitors with domestic-type G6PD did not vary. After transplantation, the peripheral blood counts, marrow morphologies, frequencies of progenitors, and progenitor cell cycle kinetics returned to normal. However, abrupt and significant fluctuations were seen in the G6PD type of progenitors from each cat during the 1-1.5 years of observation. These data cannot be explained if there were either a large or constant population of active stem cells and thus imply, in a large-animal system, that hematopoiesis was maintained through clonal succession. A stochastic model was developed to estimate the numbers of active clones and their mean lifetimes.

Viruses and bone marrow failure

Some generalizations can be drawn from a review of virus-associated bone marrow failure. The story of B19 parvovirus illustrates that viral infection may be an occult cause of marrow failure. Although the epidemiology of transient aplastic crisis suggested a viral aetiology, the implication of a single virus was surprising; the sporadic appearance of chronic bone marrow failure in immunosuppressed persons has had none of the features of a viral illness. The incrimination of parvovirus in these cases required development of specific immunological and molecular assays. Human and animal retrovirus studies have shown that small changes in the virus genome can have dramatic effects on the biology of the infectious agent and its pathogenicity in infected hosts. In Epstein-Barr virus infection, the host's immune response may play a more important role in mediating disease than virus cytotoxicity. Finally, the association of aplastic anaemia with hepatitis may be underestimated because of the inability to diagnose virus infection without obvious liver disease. The true spectrum of bone marrow disease due to virus infection is not known.

Antithymocyte globulin treatment of retrovirus-induced feline erythroid aplasia: in vivo and in vitro studies

The Kawakami-Theilen strain of feline leukemia virus (FeLV-KT) was used experimentally to produce erythroid aplasia in cats. The in vivo effects of goat anti-feline-thymocyte globulin (ATG) on hematopoiesis were investigated in FeLV-negative normal and FeLV-positive anemic cats. Treatment was initiated in anemic cats between 4 and 6 weeks postinoculation (PI) when erythroid progenitors were reduced to 10% of normal levels. During the first 2 weeks of treatment, ATG significantly increased the numbers of erythroid precursors in bone marrow from 15 to 35% in anemic cats and from 28 to 43% in normal cats. ATG stimulated a twofold increase of CFU-E and a threefold increase of CFU-GM in normal cats between 2 and 4 weeks after initiation of treatment but had no effect on CFU-E or CFU-GM in anemic cats. The in vivo effects of ATG were transient despite weekly treatment. Cats treated with normal globulin were not significantly different from untreated anemic control cats. In vitro treatment of low density bone marrow mononuclear cells with ATG plus complement increased CFU-E and BFU-E of bone marrow from cats prior to inoculation but not from viremic cats. These results indicate that, although ATG stimulates erythropoiesis and granulopoiesis in normal cats, it does not reverse retrovirus-induced erythroid aplasia.

Effects of increasing cyclic AMP or calcium on feline erythroid progenitors in vitro: normal cells are stimulated while cells from retrovirus-infected cats are suppressed

Pharmacologic modulators of cyclic 3',5'-adenosine monophosphate (cAMP) and calcium were added to cultures of bone marrow cells from normal cats and cats with retrovirus-induced erythroid aplasia (EA). Treatment with the following reagents increased the number of erythroid progenitors (CFU-e and BFU-e) in cells from normal cats: isoproterenol, dibutyryl cAMP, forskolin, RO-20-1724 and A23187. However, treatment of cells collected from viremic cats not only failed to enhance CFU-e and BFU-e but inhibited their growth. These studies suggest that EA is related to a non-reversible block of primitive erythroid progenitors or to direct inhibition of BFU-e and CFU-e growth.

Strong sequence conservation among horizontally transmissible, minimally pathogenic feline leukemia viruses

We report the first complete nucleotide sequence (8,440 base pairs) of a biologically active feline leukemia virus (FeLV), designated FeLV-61E (or F6A), and the molecular cloning, biological activity, and env-long terminal repeat (LTR) sequence of another FeLV isolate, FeLV-3281 (or F3A). F6A corresponds to the non-disease-specific common-form component of the immunodeficiency disease-inducing strain of FeLV, FeLV-FAIDS, and was isolated from tissue DNA of a cat following experimental transmission of naturally occurring feline acquired immunodeficiency syndrome. F3A clones were derived from a subgroup-A-virus-producing feline tumor cell line. Both are unusual relative to other molecularly cloned FeLVs studied to date in their ability to induce viremia in weanling (8-week-old) cats and in their failure to induce acute disease. The F6A provirus is organized into 5'-LTR-gag-pol-env-LTR-3' regions; the gag and pol open reading frames are separated by an amber codon, and env is in a different reading frame. The deduced extracellular glycoproteins of F6A, F3A, and the Glasgow-1 subgroup A isolate of FeLV (M. Stewart, M. Warnock, A. Wheeler, N. Wilkie, J. Mullins, D. Onions, and J. Neil, J. Virol. 58:825-834, 1986) are 98% homologous, despite having been isolated from naturally infected cats 6 to 13 years apart and from widely different geographic locations. As a group, their envelope gene sequences differ markedly from those of the disease-associated subgroup B and acutely pathogenic subgroup C viruses. Thus, F6A and F3A correspond to members of a highly conserved family and represent prototypes of the horizontally transmitted, minimally pathogenic FeLV present in all naturally occurring infections.

Feline leukemia/sarcoma viruses and immunodeficiency

This chapter discusses the structure feline leukemia virus (FeLV) and pathogenesis of lymphomas and leukemias BY FeLV. FeLV is quite similar to the better-studied murine leukemia viruses in structure and genetic map. The virus particles bud from cytoplasmic membranes into either extracellular spaces or into vacuoles. FeLV has long been considered a typical noncytopathogenic, longlatency leukemia virus based on its behavior in fibroblasts in vitro . Recent evidence suggests that its in vivo behavior in critical target hemolymphatic tissues is as likely to be cytopathic as transforming. The type of FeLV-related disease that occurs and the disease-free interval probably are influenced by viral envelope proteins and glycoproteins and the consequences of proviral integration. FeLV subgroup specificity apparently determines when and what type of disease will occur. The ecotropic FeLV-A is the most frequent subgroup found in pet cats and is transmitted contagiously. Immunosuppression is the most frequent and the most devastating manifestation of FeLV viremia in clinical and experimental studies. It seems that multiple cell types and multiple processes are involved in the development of feline retrovirus-induced immunosuppression. Although no solid evidence is available for the malfunctioning of cat T helper cells because of the paucity of T-cell specific markers, the circumstantial evidence provided thus far indicates an impaired T helper function in FeLV-infected cats similar to that observed in humans infected with HIV. Studies on the pathogenesis of FeLV-induced immunosuppression might provide a valuable mode for a better understanding and means of control of human AIDS.

Retrovirus-induced feline pure red cell aplasia. Hematopoietic progenitors are infected with feline leukemia virus and erythroid burst-forming cells are uniquely sensitive to heterologous complement

Feline leukemia virus subgroup C/Sarma (FeLV-C) induces pure red cell aplasia (PRCA) in cats. Just before the onset of anemia, erythroid colony-forming cells (CFU-E) become undetectable in marrow culture, yet normal frequencies of erythroid burst-forming cells (BFU-E)- and granulocyte-macrophage colony-forming cells (CFU-GM) persist. To determine if erythroid progenitors were uniquely infected with retrovirus, marrow mononuclear cells from cats viremic with FeLV-C were labeled with monoclonal antibodies to gp70 and then analyzed with a fluorescence-activated cell sorter. Both erythroid and granulocyte-macrophage progenitors were among cells sorting positively, suggesting that infection of BFU-E alone did not result in PRCA. The results were confirmed by complement (C') lysis studies using baby rabbit or guinea pig sera as sources of C'. These studies also suggested that BFU-E from cats with PRCA were unusually sensitive to C' alone, without the addition of antibody. In further studies, we demonstrated that C' activation was via the classical pathway and that C' sensitivity was unique to BFU-E and not a property of CFU-E, CFU-GM, or progenitors that were capable of giving rise to BFU-E in suspension culture. As BFU-E from cats viremic with FeLV-A/Glasgow-1 or the Rickard strain of feline leukemia virus were not sensitive to C', this finding may relate to the pathogenesis of feline PRCA. We hypothesize that, in cats viremic with FeLV-C, the abnormal C' sensitivity of BFU-E leads to the absence of CFU-E and anemia.

Retrovirus-induced feline pure red cell aplasia: the kinetics of erythroid marrow failure

Cats viremic with feline leukemia virus subgroup C (FeLV-C) develop pure red cell aplasia (PRCA) characterized by the loss of detectable late erythroid progenitors (CFU-E) in marrow culture. Normal numbers of early erythroid progenitors (BFU-E) and granulocyte-macrophage progenitors (CFU-GM) remain, suggesting that the maturation of BFU-E to CFU-E is impaired in vivo. We have examined the cell cycle kinetics of BFU-E and their response to hematopoietic growth factor(s) to better characterize erythropoiesis as anemia develops. Within 3 weeks of FeLV-C infection, yet 6-42 weeks before anemia, the traction of BFU-E in DNA synthesis as determined by tritiated thymidine suicide increased to 43 +/- 4% (normal 23 +/- 2%) while there was no change in the cell cycle kinetics of CFU-GM. In additional studies, we evaluated the response of marrow to the hematopoietic growth factor(s) present in medium conditioned by FeLV-infected feline embryonic fibroblasts (FEA/FeLV CM). With cells from normal cats or cats viremic with FeLV-C but not anemic, a 4-fold increase in erythroid bursts was seen in cultures with 5% FEA/FeLV CM when compared to cultures without CM. However, just prior to the onset of anemia, when the numbers of detectable CFU-E decreased, BFU-E no longer responded to FEA/FeLV CM in vitro. BFU-E from anemic cats also required 10% cat or human serum for optimal in vitro growth. These altered kinetics and in vitro growth characteristics may relate to the in vivo block of BFU-E differentiation and PRCA. Finally, when marrow from cats with PRCA was placed in suspension culture for 2 to 4 days in the presence of cat serum and CM, the numbers of BFU-E increased 2- to 4-fold although no CFU-E were generated. By 4 to 7 days, CFU-E were detected, suggesting that conditions contributing to the block of erythroid maturation did not persist. The suspension culture technique provides an approach to study further the defect in erythroid differentiation characteristic of feline PRCA.

Molecular analysis and pathogenesis of the feline aplastic anemia retrovirus, feline leukemia virus C-Sarma

We describe the molecular cloning of an anemogenic feline leukemia virus (FeLV), FeLV-C-Sarma, from the productively infected human rhabdomyosarcoma cell line RD(FeLV-C-S). Molecularly cloned FeLV-C-S proviral DNA yielded infectious virus (mcFeLV-C-S) after transfection of mammalian cells, and virus interference studies using transfection-derived virus demonstrated that our clone encodes FeLV belonging to the C subgroup. mcFeLV-C-S did not induce viremia in eight 8-week-old outbred specific-pathogen-free (SPF) cats. It did, however, induce viremia and a rapid, fatal aplastic anemia due to profound suppression of erythroid stem cell growth in 9 of 10 inoculated newborn, SPF cats within 3 to 8 weeks (21 to 58 days) postinoculation. Thus, the genome of mcFeLV-C-S encodes the determinants responsible for the genetically dominant induction of irreversible erythroid aplasia in outbred cats. A potential clue to the pathogenic determinants of this virus comes from previous work indicating that all FeLV isolates belonging to the C subgroup, an envelop-gene-determined property, and only those belonging to the C subgroup, are potent, consistent inducers of aplastic anemia in cats. To approach the molecular mechanism underlying the induction of this disease, we first determined the nucleotide sequence of the envelope genes and 3' long terminal repeat of FeLV-C-S and compared it with that of FeLV-B-Gardner-Arnstein (mcFeLV-B-GA), a subgroup-B feline leukemia virus that consistently induces a different disease, myelodysplastic anemia, in neonatal SPF cats. Our analysis revealed that the p15E genes and long terminal repeats of the two FeLV strains are highly homologous, whereas there are major differences in the gp70 proteins, including five regions of significant amino acid differences and apparent sequence substitution. Some of these changes are also reflected in predicted glycosylation sites; the gp70 protein of FeLV-B-GA has 11 potential glycosylation sites, only 8 of which are present in FeLV-C-S.

Multilineage, non-species specific hematopoietic growth factor(s) elaborated by a feline fibroblast cell line: enhancement by virus infection

In studies designed to determine the role of feline leukemia virus (FeLV) in the pathogenesis of marrow failure in the cat, we tested medium conditioned by uninfected and FeLV-infected feline embryonic fibroblasts (FEA) for its effect on hematopoietic colony growth in culture. As opposed to an inhibitory effect, we found that the conditioned medium (CM) from FEA or FEA/FeLV increased the in vitro growth of multiple hematopoietic progenitor cell types including erythroid burst-forming cells (BFU-E), granulocyte/macrophage colony-forming cells, megakaryocytic colony-forming cells, and mixed-cell colony-forming cells. Furthermore, CM enhanced the growth of progenitors in cultures of mouse or human marrow cells, as well as cat marrow cells. Stimulation of feline BFU-E was most marked with an increment in growth of 400% over control. The human burst promoting activity (BPA) of the CM was equivalent or better than other CM available in our laboratory. The evidence suggest that the growth-promoting activity is a constitutive product(s) released by FEA which was enhanced eightfold with virus infection. Studies with non-adherent and T-lymphocyte-depleted human marrow cells and human peripheral blood cells suggest that the growth factor(s) acts directly on progenitor cells and not through readily identified accessory cells. These findings are consistent with the concept that mesenchymal cells such as fibroblasts have the capacity to release hematopoietic growth factor(s) capable of acting on primitive hematopoietic progenitors. The results provide an example of how injury of such cells, through virus infection, may enhance growth factor(s) release and influence the hematopoietic microenvironment.