Human retroviruses: their role in neoplasia and immunodeficiency

Human T-cell Lymphotropic Viruses: Review and Prospects for Antiviral Therapy

The human T-cell lymphotropic viruses types I and II (HTLV-I, II) pose challenges to researchers and clinicians who seek to unveil mechanisms of viral transformation and pathogenesis. HTLV-I infection in humans is associated with a wide array of primary and secondary diseases ranging from mild immunosuppression to adult T-cell leukaemia/lymphoma and HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a neurological degenerative syndrome. As retroviruses, HTLV-I and II share similar replicative cycles with human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome. However, in contrast to HIV-I which destroys CD4+ T cells, HTLV-I and II can preferentially transform a CD4+ T-cell subset to an unrestricted growth state.

HTLV-I and II, along with simian T-lymphotropic virus (STLV) and bovine leukaemia virus (BLV), form a phylogenetic group which is distinct from ungulate, non-human primate and human lentiviruses such as visna, simian immunodeficiency virus (SIV), and human immunodeficiency viruses types 1 and 2. The proviral genome of HTLV-I is flanked at the 5′ and 3′ ends by long terminal repeats (LTR) and is further subdivided into structural gag and env genes, a pro gene encoding an aspartyl protease, a pol gene which encodes reverse transcriptase and endonuclease, and the regulatory gene elements tax and rex. Regions within the LTR contain recognition sites for cellular proteins and the tax gene product that collectively promote viral expression. Tax-mediated activation of cellular genes involved in growth and differentiation is suspected to play a dominant role in the leukaemogenic process associated with HTLV-I infection. Differential rex-regulated splicing of viral message gives rise to transcripts encoding the polyprotein precursor gag-pro-pol (unspliced), envelope (single spliced), or tax/rex (doubly spliced). The 100nm HTLV virion contains an electron-dense core surrounding a divalent-single stranded DNA genome. This core is in turn enclosed by concentric shells of matrix protein and an outer lipid bilayer, the latter acquired as the virus buds from the surface of the infected cell. Envelope glycoproteins associated with the outside of this lipid bilayer can interact with viral receptors on cells and mediate virus entry. Antiviral strategies have been directed at inhibiting viral entry into cells (sulphated and non-sulphated polysaccharides, vaccines), blocking of viral replication (AZT, suramin), intracellular immunization (transdominant repression of rex), and elimination of virus infected cells (IL-2 receptor-directed toxins). Serological screening of the blood supply and curtailing breast feeding of children by HTLV-I + mothers have likely had a major impact in preventing HTLV-I infection.

Isolation of a bovine plasma fibronectin-containing complex which inhibits the expression of bovine leukemia virus p24

Bovine leukemia virus (BLV) is a deltaretrovirus that infects cattle worldwide. In agriculturally intensive regions, approximately 30% of dairy cows are BLV infected. Like the human T-cell leukemia virus (HTLV), there is a lengthy period of viral quiescence after initial infection with BLV. Unlike HTLV, BLV resides predominantly in B cells. Lymphoma is observed in less than 10% of BLV-infected adult cattle. Although viremia is undetectable in vivo, BLV-infected peripheral blood mononuclear cells readily become productive when cultured in vitro. Productivity is markedly diminished when cultures are supplemented with bovine plasma. This inhibitory activity of bovine plasma has been attributed to the "plasma blocking factor" (PBF). Here, we describe the purification of a PBF whose activity was resistant to heating to 65 degrees C for 10 min and was attributable to a fibronectin-containing complex of approximately 320 kDa under nonreducing conditions. By use of two-dimensional polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization-time of flight (mass spectrometry), a protein with a size of 220 kDa and a pI of 5.4 was identified as a member of the fibronectin group of molecules. Both the purified protein and the commercially available bovine fibronectin inhibited BLV production in naturally infected peripheral blood mononuclear cells, although the fibronectin was less biologically active.

Lead-induced endoplasmic reticulum (ER) stress responses in the nervous system

Lead (Pb) poisoning continues to be a significant health risk because of its pervasiveness in the environment, its known neurotoxic effects in children, and potential endogenous exposure from Pb deposited in bone. New information about mechanisms by which Pb enters cells and its organelle targets within cells are briefly reviewed. Toxic effects of Pb on the endoplasmic reticulum (ER) are considered in detail, based on recent evidence that Pb induces the expression of the gene for 78-kD glucose-regulated protein (GRP78) and other ER stress genes. GRP78 is a molecular chaperone that binds transiently to proteins traversing through the ER and facilitates their folding, assembly, and transport. Models are presented for the induction of ER stress by Pb in astrocytes, the major cell type of the central nervous system, in which Pb accumulates. A key feature of the models is disruption of GRP78 function by direct Pb binding. Possible pathways by which Pb-bound GRP78 stimulates the unfolded protein response (UPR) in the ER are discussed, specifically transduction by IRE1/ATF6 and/or IRE1/JNK. The effect of Pb binding to GRP78 in the ER is expected to be a key component for understanding mechanisms of Pb-induced ER stress gene expression.

CD154 costimulated ovine primary B cells, a cell culture system that supports productive infection by bovine leukemia virus

Bovine leukemia virus (BLV) is closely associated with the development of B-cell leukemia and lymphoma in cattle. BLV infection has also been studied extensively in an in vivo ovine model that provides a unique system for studying B-cell leukemogenesis. There is no evidence that BLV can directly infect ovine B cells in vitro, and there are no direct data regarding the oncogenic potential of the viral Tax transactivator in B cells. Therefore, we developed ovine B-cell culture systems to study the interaction between BLV and its natural target, the B cell. In this study, we used murine CD154 (CD40 ligand) and gamma-chain-common cytokines to support the growth of B cells isolated from ovine lymphoid tissues. Integrated provirus, extrachromosomal forms, and viral transcripts were detected in BLV-exposed populations of immature, rapidly dividing surface immunoglobulin M-positive B cells from sheep ileal Peyer's patches and also in activated mature B cells isolated from blood. Conclusive evidence of direct B-cell infection by BLV was obtained through the use of cloned B cells derived from sheep jejunal Peyer's patches. Finally, inoculation of sheep with BLV-infected cultures proved that infectious virus was shed from in vitro-infected B cells. Collectively, these data confirm that a variety of ovine B-cell populations can support productive infection by BLV. The development of ovine B-cell cultures permissive for BLV infection provides a controlled system for investigating B-cell leukemogenic processes and the pathogenesis of BLV infection.

In vivo rescue of a silent tax-deficient bovine leukemia virus from a tumor-derived ovine B-cell line by recombination with a retrovirally transduced wild-type tax gene

The lack of bovine leukemia virus (BLV) expression is a consistent finding in freshly isolated ovine tumor cells and in the B-cell lines derived from these tumors. In order to gain further insight into the mechanisms of BLV silencing in these tumors, we have used the YR2 B-cell line, which was derived from the leukemic cells of a BLV-infected sheep. This cell line contains a single, monoclonally integrated, silent provirus, which cannot be reactivated either by stimulation in vitro or by in vivo injection of the tumor cells or cloned proviral DNA in sheep. Sequence analysis of the tax gene from the YR2 cell line identified two G-to-A transitions (G7924 to A7924 and G8149 to A8149) that result in E-to-K amino acid changes at positions 228 and 303 in the Tax protein. Following retroviral vector-mediated transfer of a wild-type tax gene into YR2 cells, we showed that BLV mRNA, viral proteins, and virions were produced, demonstrating that the cellular factors required for virus expression were present in the original YR2 cell line. Injection of this transduced YR2 cell line in sheep led to the rescue of replication-competent BLV proviruses. The integrated competent proviruses exhibited unique chimeric tax genes, which arose from homologous recombination between the transduced wild-type tax and the YR2-derived tax sequences. Furthermore, in one of these functional recombinant proviruses, only the A8149-to-G8149 reversion was present, providing clear evidence that the defect underlying the silent phenotype in YR2 cells results from a single C-terminal E303-to-K303 amino acid substitution in the BLV Tax protein. Our observations suggest that a single strategically located mutation in tax provides a mechanism for BLV inactivation in B-cell tumors.

Acquired immune deficiency syndrome caused by hyperimmunity

Normal immunocytes including T and B cells are equilibrated by a reciprocal attacking mechanism called a network. Continuous disequilibrium of this network results in general immunodeficiency with oligo- and polyclonal hyperimmunity, for example, T-cell activation due to spontaneous reticuloendotheliosis, paraneoplastic autoimmune syndromes, and human immunodeficiency virus infection. In these disorders, reciprocal self-reactivity, including autologous graft-versus-host reaction, plays a role in the immunodeficiency. A priori self-targeting immunity is a key mechanism to explain autoimmunity in the acquired immunodeficiency syndrome. In the treatment of the immunodeficiency due to hyperimmunity, I propose immunological suppression by agents and reconstitution of the network by bone-marrow transplantation.

Human T-cell lymphotrophic virus type-I (HTLV-I) retrovirus and human disease

Human T-cell lymphotrophic virus type-I (HTLV-I) was the first pathogenic retrovirus identified in humans. HTLV-I is now linked to a number of clinical diseases, most notably adult T-cell leukemia/lymphoma and the syndrome known as HTLV-associated myelopathy or tropical spastic paraparesis (HAM/TSP). For the emergency physician practicing among patients from high-risk groups, HTLV-I infection and its associated diseases are presenting an increasing challenge. This report describes its transmission, seroprevalence, associated diseases, and methods to control the spread of this retrovirus.

An autoaggressive process against bystander tissues in HTLV-I-infected individuals: a possible pathomechanism of HAM/TSP

Human T lymphotropic virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a well-defined clinico-pathological entity in which the virus infection and the host immune responses are involved in the pathomechanism. It is generally agreed that the virus infection precedes the development of HAM/TSP and the infection is persistent during the course of disease. However, what plays the key role for the development of HAM/TSP remains to be elucidated. In this article, we emphasise the importance of the unique nature of HTLV-I-infected cells, which may have a potential ability to produce viral antigens outside of the blood flow, and we also review a variety of evidences supporting the following proposal. In our hypothesis, the supply of infected T cells from blood flow to central nervous system (CNS) is primary for the development of CNS lesions. Both anatomically determined hemodynamic conditions and adhesion molecule-mediated interactions between circulating infected T cells and endothelial cells may contribute to the localization of the main lesions. Following an induction of the HTLV-I antigens on the surface of infected T cells in CNS compartment, expansion of the responses of immunocompetent T cells against the viral proteins may result in CNS tissue damage which may be mediated by released cytokines. This is the first attempt to implicate a bystander damage mechanism in a human disease as an essential pathomechanism.

Benign monoclonal T cell proliferation in HTLV-I infection

A spectrum of carrier states in human T lymphotropic virus type-I (HTLV-I) infection is proposed. The suspected process of clonal selection of HTLV-I infected cells results in the spectrum of nonfatal states from polyclonal to monoclonal virus integration. This idea is based on the discovery that monoclonal proviral HTLV-I DNA was detected in the fresh peripheral blood lymphocytes from some patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a nonfatal chronic neurological disorder.

Comparison of the levels of inositol metabolites in transformed haemopoietic cells and their normal counterparts

We have compared the levels of inositol metabolites in three pairs of normal and transformed cells which have been matched with respect to their cell lineage, differentiation and proliferation status: (i) normal human myeloid blast cells and the human promyelocytic leukaemic cell line, HL60; (ii) human umbilical-cord T-helper cells and C8166 cells, a HTLV-1-transformed T-helper cell line; and (iii) an interleukin 3-dependent long-term culture of murine pro-B-cells (BAF3) and BAF3 cells transformed by transfection with the bcr-abl oncogene. Complex patterns of inositol metabolites were present in each of the cell populations. Although there were a number of differences in the levels of certain inositol metabolites between individual cell populations in the paired groups, we did not observe any consistent difference in the levels of inositol metabolites between the proliferating normal and transformed cells. In particular, our data do not support the reported correlation between elevated glycerophosphoinositol (GroPIns) levels and transformation of cells by membrane and cytoplasmic oncogenes which has been reported by other workers. All the cells contained high concentrations of Ins(1,3,4,5,6)P5 (between 12 and 55 microM) and InsP6 (between 37 and 105 microM). The HTLV1-transformed T-helper cells had particularly high levels of total inositol phosphates (predominantly GroPIns, an unidentified inositol bisphosphate and InsP6). The observations are discussed with reference to cell transformation and to the differentiation status of the paired populations.

Identification of alternatively spliced mRNAs encoding potential new regulatory proteins in cattle infected with bovine leukemia virus

The polymerase chain reaction was used to detect and characterize low-abundance bovine leukemia virus (BLV) mRNAs. In infected cattle we could detect spliced mRNA with a splice pattern consistent with a Tax/Rex mRNA, as well as at least four alternatively spliced RNAs. Two of the alternatively spliced mRNAs encoded hitherto unrecognized BLV proteins, designated RIII and GIV. The Tax/Rex and alternatively spliced mRNAs could be detected at their highest levels in BLV-infected cell cultures; the next highest levels were found in samples from calves experimentally infected at 6 weeks postinoculation. Alternatively spliced mRNAs were also expressed, albeit at lower levels, in naturally infected animals; they were detected by a nested polymerase chain reaction. Interestingly, the GIV mRNA was specifically detected in naturally infected cows with persistent lymphocytosis and in two of five calves at 6 months after experimental infection with BLV. Furthermore, the calf with the strongest signal for GIV had the highest lymphocyte counts. These data may suggest a correlation between expression of the GIV product and development of persistent lymphocytosis. Some of the donor and acceptor sites in the alternatively spliced mRNAs were highly unusual. The biological mechanisms and significance of such a choice of unexpected splice sites are currently unknown.

Acute adult T-cell leukemia/lymphoma presenting with florid cutaneous disease

Adult T-cell leukemia/lymphoma (ATLL) is characterized by cutaneous disease, hypercalcemia, associated HTLV-I infection, and a fulminant course refractory to therapy. A patient with acute ATLL is described, and the natural history of ATLL is reviewed.