Transgenic approach for the study of pathogenesis induced by human viruses

An understanding of the pathogenesis of human viral diseases has been hampered by the lack of suitable animal models. However, with the advent in the last decade of transgenic technology, it is now possible to introduce one or more viral genes into the germ-line of animals. Thus, transgenic technology allows for the study of viral gene expression and function in the context of the whole animal. The focus of this review is to define the advantages and disadvantages of the transgenic approach in studies of viral pathogenesis. Studies involving a human DNA tumor virus (JCV) and a human retrovirus (HIV) will be described to illustrate these points.

Transgenic mouse model for human gastric carcinoma

To understand the pathogenesis that may be induced by human adenovirus type 12 (Ad12), we have generated transgenic mice carrying the Ad12 early region 1 under control of the mouse mammary tumor virus long terminal repeat. Eleven of 11 male founder mice, but only 2 of 12 females, died between 3 to 4 mo of age. Death was associated with presence of tumors at or near the squamocolumnar junction of the stomach. Microscopically, these multifocal tumors appeared to arise from hyperplastic epithelium and showed features consistent with adenocarcinoma or adenosquamous carcinoma. High levels of expression of both the Ad12 E1A and E1B genes were seen in the tumor-bearing stomach. Various levels of expression were also detected in other tissues, although the stomach was the only organ with detectable pathology. These observations suggest an organ-specific action of the Ad12 early region 1 gene products. This transgenic mouse model provides an experimental system for studying the development of human carcinomas at sites of transition from squamous to columnar epithelium.

Intensive induction chemotherapy for poor risk non-seminomatous germ cell tumours

An increase in initial chemotherapy intensity was evaluated in 29 patients with high risk metastatic non-seminomatous germ cell tumours (NSGCT) of the testis, defined by the presence of multiple large lung metastases, liver, bone or brain metastases, or the combination of large abdominal mass with high serum concentration of the tumour markers alpha-foetoprotein (AFP) or beta subunit of human chorionic gonadotrophin (HCG) (AFP greater than 500 ku/l or HCG greater than 1000 iu/l). Four courses of bleomycin, vincristine and cisplatin (BOP) were given at 7 day intervals, followed by three courses of etoposide, cisplatin with or without bleomycin (BEP or EP) at 21 day intervals for a total of 13 weeks of chemotherapy. Twenty-three (85%) of 27 evaluable patients have remained continuously free from disease progression at a median of 24 months (range 14-38 months) from chemotherapy and the actuarial 2 year freedom from progression rate is 86% (95% CI = 73-99%). Three patients died from non-malignant causes, two of bleomycin pneumonitis and one from complications of cystic fibrosis. Thus cause specific overall survival in the total population of treated patients is 79%. With appropriate limitation of bleomycin dosage, this approach is well tolerated and results compare favourably with less intensive induction schedules based on initial 21-28 day cycles.

Use of transgenic animals to study human retroviruses

The purpose of transgenic technology is to introduce a gene into the germline of an animal in order to investigate its proper expression in the appropriate cell types and its effect on cellular functions. This technology has been used to explore a wide variety of genetic and biological issues including stage- and tissue-specific gene expression, development, immunology, oncology, and gene therapy. The intent of this article is to discuss the value of gene transfer technology for the study of mechanisms of viral pathogenesis. We will focus mainly on our experience with two experimental systems involving human T-lymphotropic virus Type 1 (HTLV-1) and human immunodeficiency virus Type 1 (HIV-1).

The HIV tat gene induces dermal lesions resembling Kaposi's sarcoma in transgenic mice

When the human immunodeficiency virus transactivating gene under the control of the viral regulatory region is introduced into the germline of mice, skin lesions are induced that resemble Kaposi's sarcoma seen in AIDS. Our findings indicate that HIV could play a direct part in causing cancer.

Natural resistance against lymphoma grafts conveyed by H-2Dd transgene to C57BL mice

The H-2Dd transgenic strain D8 on C57BL background was more resistant to subcutaneous challenge of RBL-5 lymphoma cells than B6 controls. The direct role of the H-2Dd antigen was investigated by the use of (D8 x B6)F1 crosses and (D8 B6) x B6 backcrosses. The latter showed cosegregation with regard to Dd antigen expression and lymphoma resistance, both of which were inherited in a pattern consistent with control by a single dominant gene. The rejection potential in (D8 x B6)F1 mice appeared as strong as that seen in crosses between B6 and MHC congenic mice (on B10 background) carrying H-2Dd. The lymphoma resistance could be abrogated by treatment with anti-asialo GM1 antiserum or anti-NK 1.1 mAb, indicating a role for NK cells.

Expression of heat-shock and glucose-regulated genes: differential effects of glucose starvation and hypertonicity

The hsp and grp gene families encode structurally-related proteins which are overexpressed upon physiological stress and which presumably play critical roles in protecting cells against different environmental insults. While the heat-shock proteins induced by hyperthermia have been localized to the nucleus, the glucose-regulated proteins activated by glucose deprivation have been identified in the endoplasmic reticulum. To further our understanding of the regulation of these genes, we have analyzed the expression of individual members of the two gene families under different environmental states. We have observed that, while related members of the same gene family may be coordinately induced under one physiologic state, they may also be differentially activated under a different environmental condition. Furthermore, the induction may be gradual but persistent or abrupt but transient. We have also found that related members of the two gene families may similarly be coordinately or differentially induced in response to different environmental abuses. Our observations are consistent with the belief that the regulation of the expression of stress proteins is complex.

Multiple endocrine neoplasia induced by the promiscuous expression of a viral oncogene

There is increasing evidence for the importance of events that govern and influence the interaction between the transformed cell and its host being ultimately responsible for the establishment of the cancer phenotype. To derive an animal model that will allow us to define some of these phenomena at the molecular level, we have chosen to induce the expression of a viral oncogene in all tissue types, with the hope of identifying sites that are more susceptible to malignant transformation. When the gene for simian virus 40 large tumor antigen (T antigen) was placed under the control of a major histocompatibility complex class I gene enhancer, the resulting transgenic mice not only developed choroid plexus papillomas, as seen with wild-type simian virus 40, but also lymphoid hyperplasia and multiple endocrine neoplasias. The development of lymphoid hyperplasia was preceded by an elevated level of expression of T antigen in these tissues at an early age. Surprisingly, the striking thymic hyperplasia has not been observed to progress toward malignancy. The multiple endocrine neoplasias developed later in life and involved the pancreas, pituitary, thyroid, adrenals, and testes. While not preceded by an elevated level of expression of T antigen, once endocrine tumors appeared they quickly progressed toward malignant growth. Although other tissues also exhibited a basal level of expression of the viral oncogene similar to that detected in endocrine tissues, they rarely developed tumors. This transgenic mouse model seems particularly suitable for a molecular understanding of events responsible for certain tissue types being so much more susceptible to neoplastic conversion, with others being so refractory.

Rejection of B16 melanoma induced by expression of a transfected major histocompatibility complex class I gene

Transfection of a functional major histocompatibility complex class I gene into certain tumor cells, induced by oncogenic viruses or chemical carcinogens, can effectively abrogate their tumorigenic activity. Since experimentally induced tumors possess strong tumor-specific transplantation antigens, expression of cell surface class I antigens may present the tumor cells to appropriate immune effector cells. Most spontaneously arising tumors do not possess tumor-specific transplantation antigens, and their tumorigenicity may not be affected by the expression of a transfected class I gene. We demonstrate that the poorly immunogenic B16-BL6 melanoma can be rendered nontumorigenic in syngeneic mice by the expression of the class I H-2K antigen but not the class II I-A antigen. Furthermore, the poorly tumorigenic, class I-expressing B16-BL6-transfected cells can effectively immunize syngeneic C57BL/6 mice against the highly tumorigenic, class I-deficient B16-BL6 parental cells. Our success in experimentally manipulating the tumorigenicity of a spontaneously derived neoplasm offers hope for a potential modality for the effective treatment of human cancer.

Experimental strategies for modification of histocompatibility antigens in tumor cells

Cancer may be thought of as an immunological disorder that arises because certain 'transformed' cells, endowed with the propensity to divide, have learned to evade detection by the immune system. The prospect of intervention by 'immunotherapy' depends very much on our ability to either [1] render cancer cells more recognizable to the immune system, or [2] potentiate the immune system towards a more effective recognition of cancer cells. There is now direct evidence that suppression of the major histocompatibility complex class I antigens, a family of cell-surface glycoproteins required for the presentation of cancer cells to the immune system, is directly responsible for the ability of tumor cells to escape immune surveillance. It has been shown that cancer cells can be made immunogenic either by the expression of an exogenous class I gene introduced by DNA-mediated gene transfer, or by the derepression of endogenous class I genes with interferon; these cells are efficiently rejected by the immune system. Even more interesting is the finding that the immune system can be potentiated to reject tumors by immunization with homologous tumor cells that have been manipulated to express normal levels of class I antigens. Since increasing numbers of human tumors have been found to have greatly reduced levels of class I antigens, these findings suggest a direct route to immunotherapy that involves debulking of the tumor mass, raising the level of class I antigens in a small number of explanted tumor cells, and re-immunizing the host.

Immunotherapy of a murine tumor with interleukin 2. Increased sensitivity after MHC class I gene transfection

We have shown that two weakly immunogenic MCA sarcomas developed in our laboratory that are sensitive to high-dose IL-2 immunotherapy express class I MHC in vivo and in vitro. Two nonimmunogenic MCA sarcomas are relatively insensitive to IL-2 therapy and express minimal or no class I MHC molecules in vivo and in vitro. To study the role of MHC in the therapy of tumors with IL-2, a class I-deficient murine melanoma, B16BL6, was transfected with the Kb class I gene. Expression of class I MHC rendered B16BL6 advanced pulmonary macrometastases sensitive to IL-2 immunotherapy. 3-d micrometastases of CL8-2, a class I transfected clone of B16BL6, were significantly more sensitive to IL-2 therapy than a control nontransfected line. Expression of Iak, a class II MHC molecule, had no effect on IL-2 therapy of transfectant pulmonary micrometastases in F1 mice. By using lymphocyte subset depletion with mAbs directed against Lyt-2, therapy of class I transfectant macrometastases with high-dose IL-2 was shown to involve an Lyt-2 cell. In contrast, regression of micrometastases treated with low-dose IL-2 involved Lyt-2+ cells, but regression mediated by high doses of IL-2 did not. We hypothesize that both LAK and Lyt-2+ T cells effect IL-2-mediated elimination of micrometastases, but only Lyt-2+ T cells are involved in macrometastatic regression. Low doses of IL-2 stimulate Lyt-2+ cells to eliminate class I-expressing micrometastases, but high doses of IL-2 can recruit LAK cells to mediate regression of micrometastases independent of class I expression. Only high-dose IL-2, mediating its effect predominantly via Lyt-2+ cells, is capable of impacting on MHC class I-expressing macrometastases. Macrometastases devoid of class I MHC antigens appear to be resistant to IL-2 therapy.

A transgenic class I antigen is recognized as self and functions as a restriction element

The function of a transgenic Dd class I molecule in the induction of immunologic tolerance to major histocompatibility complex antigens and in directing major histocompatibility complex restriction in C57BL/6 mice were investigated. All of the transgenic Dd mouse strains were found to be tolerant for the Dd antigen. Spleen cells from transgenic mice were immunocompetent but consistently failed to generate an anti-Dd cytotoxic T lymphocyte response in vitro, and skin grafts between transgenic Dd mice were not rejected. These data suggests that the Dd antigen was recognized as a self molecule. In addition, the transgenic Dd mice generated antigen-specific Dd-restricted cytotoxic T lymphocyte, indicating that the Dd antigen also functioned as a restriction element for antigen recognition. These observations demonstrate the usefulness of the transgenic mouse system for studying class I antigen expression and function.

A transgenic class I antigen is recognized as self and functions as a restriction element

The function of a transgenic Dd class I molecule in the induction of immunologic tolerance to major histocompatibility complex antigens and in directing major histocompatibility complex restriction in C57BL/6 mice were investigated. All of the transgenic Dd mouse strains were found to be tolerant for the Dd antigen. Spleen cells from transgenic mice were immunocompetent but consistently failed to generate an anti-Dd cytotoxic T lymphocyte response in vitro, and skin grafts between transgenic Dd mice were not rejected. These data suggests that the Dd antigen was recognized as a self molecule. In addition, the transgenic Dd mice generated antigen-specific Dd-restricted cytotoxic T lymphocyte, indicating that the Dd antigen also functioned as a restriction element for antigen recognition. These observations demonstrate the usefulness of the transgenic mouse system for studying class I antigen expression and function.

Functional expression of a heterologous major histocompatibility complex class I gene in transgenic mice

The regulated expression of major histocompatibility complex class I antigens is essential for assuring proper cellular immune responses. To study H-2 class I gene regulation, we have transferred a foreign class I gene to inbred mice and have previously shown that the heterologous class I gene was expressed in a tissue-dependent manner. In this report, we demonstrate that these mice expressed the transgenic class I molecule on the cell surface without any alteration in the level of endogenous H-2 class I antigens. Skin grafts from transgenic mice were rapidly rejected by mice of the background strain, indicating that the transgenic antigen was expressed in an immunologically functional form. As with endogenous H-2 class I genes, the class I transgene was inducible by interferon treatment and suppressible by human adenovirus 12 transformation. Linkage analysis indicated that the transgene was not closely linked to endogenous class I loci, suggesting that trans-regulation of class I genes can occur for class I genes located outside the major histocompatibility complex.

Reconstitution of a functional interleukin 2 receptor in a nonlymphoid cell

The maintenance of T lymphocytes which are important effectors of immune responses requires the T cell growth factor, interleukin 2 (IL-2). The binding of IL-2 to specific cell-surface receptors (IL-2R) has previously been shown to be essential to the growth and proliferation of activated lymphocytes. A human IL-2R cDNA sequence, placed under the control of the SV40 transcriptional promoter and enhancer, has been transfected into murine L cells. Single cell analysis by autoradiography was used to show that fibroblastic L cells, stably expressing human IL-2R, respond to stimulation with IL-2 by DNA synthesis and proliferation. This response is specifically blocked by the addition of an anti-IL-2R monoclonal antibody, anti-Tac, as previously reported. Neither nonspecific antisera nor 7G7/B6, an anti-IL-2R monoclonal antibody which does not interfere with IL-2 binding to its receptor, had any effect on this response. The induction of DNA synthesis by IL-2 is both rapid and dose-dependent. The ability of IL-2 to stimulate these transfected L cells to proliferate demonstrates that a lymphoid environment is not required for the functional interaction between IL-2 and its receptor, and provides a unique model system for the investigation of the molecular basis for the cellular events mediated by IL-2.

Reconstitution of a functional interleukin 2 receptor in a nonlymphoid cell

The maintenance of T lymphocytes which are important effectors of immune responses requires the T cell growth factor, interleukin 2 (IL-2). The binding of IL-2 to specific cell-surface receptors (IL-2R) has previously been shown to be essential to the growth and proliferation of activated lymphocytes. A human IL-2R cDNA sequence, placed under the control of the SV40 transcriptional promoter and enhancer, has been transfected into murine L cells. Single cell analysis by autoradiography was used to show that fibroblastic L cells, stably expressing human IL-2R, respond to stimulation with IL-2 by DNA synthesis and proliferation. This response is specifically blocked by the addition of an anti-IL-2R monoclonal antibody, anti-Tac, as previously reported. Neither nonspecific antisera nor 7G7/B6, an anti-IL-2R monoclonal antibody which does not interfere with IL-2 binding to its receptor, had any effect on this response. The induction of DNA synthesis by IL-2 is both rapid and dose-dependent. The ability of IL-2 to stimulate these transfected L cells to proliferate demonstrates that a lymphoid environment is not required for the functional interaction between IL-2 and its receptor, and provides a unique model system for the investigation of the molecular basis for the cellular events mediated by IL-2.

The tat gene of human T-lymphotropic virus type 1 induces mesenchymal tumors in transgenic mice

Human T-lymphotropic virus type 1 (HTLV-1) is a suspected causative agent of adult T-cell leukemia. One of the viral genes encodes a protein (tat) that not only results in transactivation of viral gene expression but may also regulate the expression of certain cellular genes that are important for cell growth. Transgenic mice that expressed the authentic tat protein under the control of the HTLV-1 long terminal repeat were generated, and cell types that are permissive for the viral promoter and the effects of the tat gene on these cells were studied. Three of eight founder mice with high levels of expression of the transgene in muscle were bred and then analyzed. All developed soft tissue tumors at multiple sites between 13 to 17 weeks of age. This phenotype was transmitted to nine of nine offspring that inherited the tat gene and were available for analysis. The remaining five founders expressed the transgene in the thymus, as well as in muscle. This second group of mice all exhibited extensive thymic depletion and growth retardation; in all of these mice, death occurred between 3 to 6 weeks of age before tumors became macroscopically visible. The tat gene under the control of the HTLV-1 regulatory region showed tissue-specific expression and the tat protein efficiently induced mesenchymal tumors. The data establish tat as an oncogenic protein and HTLV-1 as a transforming virus.

A transgenic mouse model for human neurofibromatosis

Human T-lymphotropic virus type 1 (HTLV-1) has been associated with the neurologic disorder tropical spastic paraparesis and possibly with multiple sclerosis. The tat gene of HTLV-1 under control of its own long terminal repeat is capable of inducing tumors in transgenic mice. The morphologic and biologic properties of these tumors indicate their close resemblance to human neurofibromatosis (von Recklinghausen's disease), the most common single gene disorder to affect the nervous system. The high spontaneous incidence of this disease, together with the diverse clinical and pathologic features associated with it, suggests that environmental factors may account for some of the observed cases. Multiple tumors developed simultaneously in the transgenic tat mice at approximately 3 months of age, and the phenotype was successfully passed through three generations. The tumors arise from the nerve sheaths of peripheral nerves and are composed of perineural cells and fibroblasts. Tumor cells from these mice adapt easily to propagation in culture and continue to express the tat protein in significant amounts. When transplanted into nude mice, these cultured cells efficiently induce tumors. Evidence of HTLV-1 infection in patients with neural and other soft tissue tumors is needed in order to establish a link between infection by this human retrovirus and von Recklinghausen's disease and other nonlymphoid tumors.

The released interleukin 2 receptor binds interleukin 2 efficiently

The released interleukin 2 receptor (IL 2R) molecule was characterized in order to clarify its biochemical structure and to determine its functional capacity. Enzymatic digestions demonstrated that the released IL 2R, like the cell surface IL 2R, is a complex glycoprotein, modified by the addition of both N- and O-linked carbohydrates and sialic acid residues. It has a peptide backbone that is approximately 10 Kd smaller than that of its membrane-associated counterpart. Affinity chromatography demonstrated that released IL 2R from either an HTLV-I-positive T cell line (HUT-102) or PHA-activated normal peripheral lymphocytes binds efficiently to purified recombinant IL 2. Furthermore, the interaction between the growth factor and the released receptor does not appear to require any accessory molecules. These observations suggest a potentially significant role for the released IL 2R in the regulation of IL 2-dependent lymphocyte functions.

The released interleukin 2 receptor binds interleukin 2 efficiently

The released interleukin 2 receptor (IL 2R) molecule was characterized in order to clarify its biochemical structure and to determine its functional capacity. Enzymatic digestions demonstrated that the released IL 2R, like the cell surface IL 2R, is a complex glycoprotein, modified by the addition of both N- and O-linked carbohydrates and sialic acid residues. It has a peptide backbone that is approximately 10 Kd smaller than that of its membrane-associated counterpart. Affinity chromatography demonstrated that released IL 2R from either an HTLV-I-positive T cell line (HUT-102) or PHA-activated normal peripheral lymphocytes binds efficiently to purified recombinant IL 2. Furthermore, the interaction between the growth factor and the released receptor does not appear to require any accessory molecules. These observations suggest a potentially significant role for the released IL 2R in the regulation of IL 2-dependent lymphocyte functions.

Expression of major histocompatibility complex class I antigens as a strategy for the potentiation of immune recognition of tumor cells

Like many primary tumors, human adenovirus type 12 (Ad12)-transformed mouse cells express greatly reduced levels of the major histocompatibility complex (MHC) class I antigens and are highly tumorigenic in immunocompetent hosts. Expression of a transfected class I gene by these cells can abrogate their tumorigenicity. Both the K and the L class I genes can suppress the malignant phenotype. Previous studies showed that interferon can induce class I gene expression in certain Ad12-transformed cells and can suppress their tumorigenic phenotype. We now demonstrate that preimmunization of mice with a nontumorigenic dose of interferon-treated Ad12-transformed tumor cells can afford protection against a subsequent challenge by a tumorigenic dose of untreated Ad12-transformed tumor cells. Similar immunity can also be induced by using cells transfected with the K gene, and the observed protection appears specific to Ad12-transformed cells. Significant protection can be achieved even if immunization is provided subsequent to the tumor challenge. Since increasing numbers of human tumors have been found to have reduced levels of MHC class I antigens, the prospect of therapy by immunization with the parental tumor cells that have been manipulated to induce class I gene expression offers an attractive experimental model.

Early regions of JC virus and BK virus induce distinct and tissue-specific tumors in transgenic mice

JC virus and BK virus are ubiquitous human viruses that share sequence and structural homology with simian virus 40. To characterize tissue-specific expression of these viruses and to establish model systems for the study of human viral-induced disease, transgenic mice containing early regions of each of the viruses were produced. The viral sequences induced tumors in a distinct and tissue-specific manner that was similar to their tissue tropism in humans. Ten JC virus-containing founder mice were produced, of which 5 survived to maturity. Four of them developed adrenal neuroblastomas, which metastasized to several other tissues. JC virus tumor-antigen RNA was detected at high levels in the tumor tissues and at low levels in the normal tissues of these mice. One of the three BK virus-containing mice was abnormally shaped and died at 2 weeks of age. The other two BK virus-containing mice developed primary hepatocellular carcinomas and renal tumors and died at 8-10 months of age. BK virus tumor-antigen RNA was expressed in tumor tissues of both mice. Since each of the viruses retained the general tissue tropism that it exhibits in humans, these data suggest that transgenic mice harboring human viruses will be useful as animal models for viral-induced diseases.

Developmental and tissue-specific regulation of the Q10 class I gene by DNA methylation

The H-2 class I genes encode cell-surface glycoproteins that play a critical role in the immune presentation of aberrant cells. The Q10 class I gene, however, encodes a secreted glycoprotein that is highly homologous to the membrane-bound molecules. While the H-2 genes are activated in all tissue types, the expression of the Q10 gene is restricted to only the liver. Analysis of DNA from different tissues revealed a unique methylation profile for the Q10 gene in liver. Developmental activation of this gene in newborn mice is also reflected by a coordinated temporal change in DNA methylation. By comparing the methylation profiles between congenic mice, which differed in their levels of expression of the Q10 gene, it is observed that methylation at the 3'-flanking region correlates with expression. Methylations were at both CG and CC sequences. Since treatment of newborns with 5-azacytidine, which led to inhibition of methylation, resulted in the suppression of Q10, we conclude that hypermethylation in the 3'-flanking region is responsible, at least in part if not in full, for the activation of the Q10 gene in the liver.