Extracellular matrix interacts with soluble CD95L: retention and enhancement of cytotoxicity

Fas ligand (CD95L) is synthesized both on the cell surface membrane and in a soluble form. Although CD95L contributes to immune privilege in the cornea and testis, the functions of these alternatively processed proteins are not well understood. Some reports suggest that the cytotoxicity of soluble CD95L is insignificant, whereas others show potent responses in vivo, including hepatocyte apoptosis that causes liver failure. We show here that extracellular matrix proteins interact with soluble CD95L and potentiate its pro-apoptotic activity. The cytotoxicity of supernatants from CD95L-expressing cells was increased by incubation on tissue culture plates coated with these matrix proteins; this effect was mediated by trimeric soluble CD95L. With the use of immunoprecipitation, it was found that CD95L binds directly to fibronectin. In addition, immunohistochemical analysis of the cornea revealed that soluble CD95L binds primarily to extracellular matrix. The retention of soluble CD95L on extracellular matrices is likely to play an important role in the development of peripheral tolerance in immune-privileged sites.

SM22alpha promoter targets gene expression to vascular smooth muscle cells in vitro and in vivo

BACKGROUND

Gene transfer into vascular smooth muscle cells (vsmcs) holds promise for studying the pathogenesis of arterial disorders. However, a potential limitation of vectors with heterologous promoters is organ toxicity resulting from unrestricted transgene expression. Vascular smooth muscle cell-specific gene expression could increase the safety of vectors for vascular diseases.

MATERIALS AND METHODS

To develop vectors that target gene expression to vsmcs, we constructed vectors encoding human placental alkaline phosphatase (hpAP) and chloramphenicol transferase (CAT) driven by a 441-bp region of the murine SM22alpha promoter (AdSM22alpha-hpAP).

RESULTS

Transfection of AdSM22alpha-hpAP into vascular and nonvascular cells resulted in the expression of alkaline phosphatase (AP) in primary arterial and venous smcs, but not in primary endothelial cells or National Institutes of Health (NIH) 3T3 cells. Expression of AP was observed on 32.5 +/- 1.4% of primary pig vsmcs-infected AdSM22alpha-hpAP at a multiplicity of infection (MOI) of 500; whereas, infection with AdCMV-hpAP resulted in 100 +/- 0.0% expression at a MOI of 250. In vitro, expression from the heterologous cytomegalovirus (CMV) promoter was approximately 10(3)-fold higher in vsmcs, compared with the SM22alpha promoter. Following introduction of AdSM22alpha-hpAP vectors into balloon-injured pig arteries, AP recombinant protein was detected in neointimal (2.23 +/- 1.14%) and medial (0.56 +/- 0.21%) smcs, but not in endothelial or adventitial cells. In contrast, AdCMV-hpAP vectors led to AP expression in intimal endothelial and smcs cells (39.14 +/- 10.09%) and medial smcs (2.84 +/- 1.05%). AP expression was not observed in endothelial or vsmcs following transfection with the control vector, adenoviral vector lacking E1 (AddeltaE1).

CONCLUSIONS

The SM22alpha promoter programs recombinant gene expression exclusively to vascular smcs in vitro and in vivo. Although expression levels are lower than with heterologous promoters, these vectors may provide a safe and effective tool for gene therapy of vascular diseases.

Identification of the Ebola virus glycoprotein as the main viral determinant of vascular cell cytotoxicity and injury

Here we defined the main viral determinant of Ebola virus pathogenicity; synthesis of the virion glycoprotein (GP) of Ebola virus Zaire induced cytotoxic effects in human endothelial cells in vitro and in vivo. This effect mapped to a serine-threonine-rich, mucin-like domain of this type I transmembrane glycoprotein, one of seven gene products of the virus. Gene transfer of GP into explanted human or porcine blood vessels caused massive endothelial cell loss within 48 hours that led to a substantial increase in vascular permeability. Deletion of the mucin-like region of GP abolished these effects without affecting protein expression or function. GP derived from the Reston strain of virus, which causes disease in nonhuman primates but not in man, did not disrupt the vasculature of human blood vessels. In contrast, the Zaire GP induced endothelial cell disruption and cytotoxicity in both nonhuman primate and human blood vessels, and the mucin domain was required for this effect. These findings indicate that GP, through its mucin domain, is the viral determinant of Ebola pathogenicity and likely contributes to hemorrhage during infection.

Restricted expression of an adenoviral vector encoding Fas ligand (CD95L) enhances safety for cancer gene therapy

Gene transfer of Fas ligand (CD95L) using adenoviral vectors has been shown to generate apoptotic responses and potent inflammatory reactions that can be used to induce the regression of malignancies in vivo, but these vectors also cause significant hepatotoxicity that may limit their clinical utility. Here we describe an adenoviral vector encoding CD95L with restricted gene expression that reduces its toxicity in vivo. Preclinical efficacy and gene expression studies of lineage-restricted CD95L adenoviral vectors were performed. To enhance its cytotoxicity and reduce potential systemic effects, a noncleavable CD95L was made by deleting a segment containing the cleavage site (CD95L deltaQP). Higher CD95L expression of this mutant was observed on the tumor cell surface, together with a reduction in the release of soluble CD95L. This CD95L cleavage mutant was then expressed under control of a smooth muscle-specific promoter, SM22apha, and analyzed for its ability to suppress the growth of tumors of smooth muscle origin in vivo. Growth of human leiomyosarcomas but not gliomas was inhibited after ADV gene transfer into tumor-bearing immunodeficient mice. In contrast to viral promoters, in which mortality was uniformly seen after injection of 10(12) particles, no significant hepatic injury or systemic toxicity was observed in mice, and the maximum tolerated dose was increased > or = 10- to 100-fold. These findings suggest that restricted specificity of adenoviral CD95L gene expression enhances the safety of this approach for cancer gene therapy.

Differential effects of the cyclin-dependent kinase inhibitors p27(Kip1), p21(Cip1), and p16(Ink4) on vascular smooth muscle cell proliferation

BACKGROUND

The cyclin-dependent kinase inhibitors (CKIs) have different patterns of expression in vascular diseases. The Kip/Cip CKIs, p27(Kip1) and p21(Cip1), are upregulated during arterial repair and negatively regulate the growth of vascular smooth muscle cells (VSMCs). In contrast, the Ink CKI, p16(Ink4), is not expressed in vascular lesions. We hypothesized that a variation in the inactivation of cdk2 and cdk4 during the G(1) phase of the cell cycle by p27(Kip1), p21(Cip1), and p16(Ink4) leads to different effects on VSMC growth in vitro and in vivo.

METHODS AND RESULTS

The expression of p27(Kip1) and p21(Cip1) in serum-stimulated VSMCs inactivated cdk2 and cdk4, leading to G(1) growth arrest. p16(Ink4) inhibited cdk4, but not cdk2, kinase activity, producing partial inhibition of VSMC growth in vitro. In an in vivo model of vascular injury, overexpression of p27(Kip1) reduced intimal VSMC proliferation by 52% (P<0.01) and the intima/media area ratio by 51% (P<0.005) after vascular injury and gene transfer to pig arteries, when compared with control arteries. p16(Ink4) was a weak inhibitor of intimal VSMC proliferation in injured arteries (P=NS), and it did not significantly reduce intima/media area ratios (P=NS), which is consistent with its minor effects on VSMC growth in vitro.

CONCLUSIONS

p27(Kip1) and p21(Cip1) are potent inhibitors of VSMC growth compared with p16(Ink4) because of their different molecular mechanisms of cyclin-dependent kinase inhibition in the G(1) phase of the cell cycle. These findings have important implications for our understanding of the pathophysiology of vascular proliferative diseases and for the development of molecular therapies.

Nitric oxide modulates expression of cell cycle regulatory proteins: a cytostatic strategy for inhibition of human vascular smooth muscle cell proliferation

BACKGROUND

We examined the effect of NO on the proliferation and cell cycle regulation of human aortic vascular smooth muscle cells (VSMCs).

METHODS AND RESULTS

The NO donor diethylenetriamineNONOate (10(-5) to 10(-3) mol/L) inhibited proliferation in response to 10% fetal calf serum (FCS) and 100 ng/mL platelet-derived growth factor-BB in a concentration-dependent manner. This effect was not observed with disintegrated diethylenetriamineNONOate or with the parent compound, diethylenetriamine. Adenoviral transfection of endothelial NO synthase (NOS) inhibited proliferation in response to FCS, which was prevented with N(G)-nitro-L-arginine methyl ester. NOS overexpression did not inhibit proliferation in response to platelet-derived growth factor, although the transfection efficiency and protein expression were similar to those of FCS-stimulated cells. Nitrate release was selectively enhanced from FCS-treated cells, indicating that NOS was activated by FCS only. NO caused G(1) cell cycle arrest. Cytotoxicity was determined with trypan blue exclusion, and apoptosis was assessed with DNA fragmentation. Cyclin-dependent kinase 2 expression level, threonine phosphorylation, and kinase activity were inhibited. Cyclin A expression was blunted, whereas cyclin E remained unchanged. p21 expression was induced, and p27 remained unaltered. The effect on cyclin A and p21 started within 6 hours and preceded the changes in cell cycle distribution. Proliferation in response to 10% FCS was barely inhibited with 8-bromo-cGMP (10(-3) mol/L) but was blunted with both forskolin and 8-bromo-cAMP. Proliferation in response to 2% FCS was inhibited with 8-bromo-cGMP, but it did not mimic the cell cycle effects of NO.

CONCLUSIONS

NO inhibits VSMC proliferation by specifically changing the expression and activity of cell cycle regulatory proteins, which may occur independent of cGMP. Adenoviral overexpression of endothelial NOS represents a cytostatic strategy for gene therapy of vascular disease.

Atherosclerosis progression in LDL receptor-deficient and apolipoprotein E-deficient mice is independent of genetic alterations in plasminogen activator inhibitor-1

Impaired fibrinolysis has been linked to atherosclerosis in a number of experimental and clinical studies. Plasminogen activator inhibitor type 1 (PAI-1) is the primary inhibitor of plasminogen activation and has been proposed to promote atherosclerosis by facilitating fibrin deposition within developing lesions. We examined the contribution of PAI-1 to disease progression in 2 established mouse models of atherosclerosis. Mice lacking apolipoprotein E (apoE-/-) and mice lacking the low density lipoprotein receptor (LDLR-/-) were crossbred with transgenic mice overexpressing PAI-1 (resulting in PAI-1 Tg(+)/apoE-/- and PAI-1 Tg(+)/LDLR-/-, respectively) or were crossbred with mice completely deficient in PAI-1 gene expression (resulting in PAI-1-/-/apoE-/- and PAI-1-/-/LDLR-/-, respectively). All animals were placed on a western diet (21% fat and 0.15% cholesterol) at 4 weeks of age and analyzed for the extent of atherosclerosis after an additional 6, 15, or 30 weeks. Intimal and medial areas were determined by computer-assisted morphometric analysis of standardized microscopic sections from the base of the aorta. Atherosclerotic lesions were also characterized by histochemical analyses with the use of markers for smooth muscle cells, macrophages, and fibrin deposition. Typical atherosclerotic lesions were observed in all experimental animals, with greater severity at the later time points and generally more extensive lesions in apoE-/- than in comparable LDLR-/- mice. No significant differences in lesion size or histological appearance were observed among PAI-1-/-, PAI-1 Tg(+), or PAI-1 wild-type mice at any of the time points on either the apoE-/- or LDLR-/- genetic background. We conclude that genetic modification of PAI-1 expression does not significantly alter the progression of atherosclerosis in either of these well-established mouse models. These results suggest that fibrinolytic balance (as well as the potential contribution of PAI-1 to the regulation of cell migration) plays only a limited role in the pathogenesis of the simple atherosclerotic lesions observed in the mouse.

Plasminogen activator inhibitor-1 and vitronectin promote vascular thrombosis in mice

Occlusive thrombosis depends on the net balance between platelets, coagulation, and fibrinolytic factors. Epidemiologic information suggests that plasminogen activator inhibitor-1 (PAI-1), a central regulator of the fibrinolytic system, plays an important role in determining the overall risk for clinically significant vascular thrombosis. Vitronectin (VN), an abundant plasma and matrix glycoprotein, binds PAI-1 and stabilizes its active conformation. This study assessed the role of PAI-1 and VN expression in the formation of occlusive vascular thrombosis following arterial or venous injury. The common carotid arteries of 17 wild-type (WT) mice and 8 mice deficient in PAI-1 were injured photochemically while blood flow was continuously monitored. WT mice developed occlusive thrombi at 52.0 +/- 3.8 minutes (mean +/- SEM) following injury; mice deficient in PAI-1 developed occlusive thrombosis at 127 +/- 15 minutes (P <.0001). Mice deficient in VN (n = 12) developed vascular occlusion 77 +/- 11 minutes after injury, intermediate between the values observed for WT mice (P <.03) and mice deficient in PAI-1 (P <.01). PAI-1 and VN also affected the time to occlusion after injury to the jugular vein. Three WT mice developed occlusive venous thrombosis an average of 39.7 +/- 1 minutes following the onset of injury, whereas the jugular veins of 4 mice deficient in PAI-1 and 4 deficient in VN occluded 56.7 +/- 5 and 58.7 +/- 2 minutes, respectively, following injury (P <.04 and P <.01 compared to WT mice). These results suggest that endogenous fibrinolysis and its regulation by PAI-1 and VN have important roles in the development of occlusive vascular thrombosis after vascular injury. (Blood. 2000;95:577-580)

Prospects for genetic therapy of cardiovascular disease

Significant progress has been made in cardiovascular gene therapy. Further investigations are required to understand the basic science of vectors, mechanisms of gene delivery, vector-associated immunogenicity, and pathophysiology of vascular and myocardial diseases. In addition, catheter and stent devices will be required to deliver vectors to the vasculature and myocardium (Fig. 3). Despite these scientific challenges, molecular therapies for cardiovascular diseases are being implemented into clinical practice.

Combination gene delivery of the cell cycle inhibitor p27 with thymidine kinase enhances prodrug cytotoxicity

Cytoxicity induced by the herpesvirus thymidine kinase (TK) gene in combination with prodrugs is dependent on cell growth and leads to the elimination of genetically modified cells, thus limiting the duration of expression and efficacy of this treatment in vivo. Here, an effort was made to enhance TK/prodrug efficacy by coexpression of a cyclin-dependent kinase inhibitor (CKI), p27, to render cells resistant to TK/prodrug by inhibiting DNA synthesis. Expression of p27 by transfection substantially reduced cell cycle progression, and its activity was enhanced by mutations designed to stabilize the protein. Coexpression of p27 and TK or a p27/TK fusion protein led to greater prodrug cytotoxicity than that produced by TK alone in the Renca cell line, which is sensitive to bystander killing. Combination gene transfer of this CKI with TK therefore sustained the synthesis of TK by genetically modified cells to enhance the susceptibility of bystander cells to prodrug cytotoxicity and increased the efficacy of this gene transfer approach.

Requirements for enhanced transgene expression by untranslated sequences from the human cytomegalovirus immediate-early gene

BACKGROUND

The cytomegalovirus immediate early (CMV IE) promoter has been widely used for heterologous expression. Further enhancements of gene expression from this potent promoter may allow for the development of improved gene transfer strategies. We aimed to determine whether inclusion of the first exon (5' untranslated) and first intron of the CMV IE gene would increase heterologous transgene expression in primary target cells and to determine the sequences required for any observed increases.

MATERIALS AND METHODS

Comparisons of reporter gene expression were made following transient transfection of vascular smooth muscle cells (VSMCs) with plasmids containing the first exon and intron from the CMV IE gene or deletional mutations. Comparisons were also made using a heterologous promoter (RSV).

RESULTS

Gene expression from the CMV IE promoter was increased 5.7-fold in VSMC with the inclusion of the first exon and intron. Similar increases were seen with other target cells and from the heterologous RSV promoter. This increase was associated with an increase in steady-state mRNA. Deletion analyses demonstrated that the enhancement was dependent on the presence of the 5' portion of the first exon while deletion of large segments within the intron was associated with similar levels of expression compared with the parental plasmid.

CONCLUSIONS

Inclusion of the first exon and intron from the CMV IE gene increases expression from the CMV IE promoter. This enhancement is seen with the heterologous RSV promoter and is associated with an increase in steady-state mRNA. Deletion analyses suggest that this enhancement is associated with inclusion of sequences within the 5' portion of the first exon and inclusion of an intron.

Graft permeabilization facilitates gene therapy of transplant arteriosclerosis in a rabbit model

BACKGROUND

Smooth muscle cell (SMC) replication plays a central role in the pathogenesis of transplant arteriosclerosis. One strategy to eliminate dividing cells is to express a herpesvirus thymidine kinase (tk) gene that phosphorylates the nucleoside analogue ganciclovir into a toxic form leading to cell killing. However, medial SMCs are resistant to gene transfer unless the artery undergoes deendothelialization. We hypothesized that manipulations that increase the "porosity" of the artery can make SMCs prone to gene transfer without denudation.

METHODS AND RESULTS

In organ culture of rabbit aorta, longitudinal stretch and supraphysiological pressure applied for 3 hours during incubation with adenoviral vector facilitated gene transfer into medial SMCs without denudation. Of the SMCs, 10.2+/-3.8% expressed a reporter gene of human placental alkaline phosphatase (hpAP), whereas SMCs in control arteries did not express hpAP. To evaluate the feasibility of transgene expression in arterial grafts, we performed such permeabilization-assisted reporter gene transfer into aortas of donor Dutch Belted rabbits and transplanted them into carotid arteries of recipient New Zealand White rabbits. Unstretched transfected grafts were used as a control. SMCs expressed hpAP (7. 3+/-2.4% of cells in 2 days and 4.2+/-1.9% in 2 weeks) in stretched grafts only. In the next series of experiments, we transfected stretched grafts with ADV-tk and combined transplantation with systemic administration of ganciclovir. Stretched ADV-hpAP grafts were used as a control. In 2 weeks, the formation of intimal thickening in tk-expressing grafts was significantly reduced (P<0. 01) because of a decrease in proliferating SMCs.

CONCLUSIONS

Manipulations within target tissues can enhance the efficiency of gene transfer into SMCs. Although mechanical permeabilization is clinically problematic, in principle, targeting SMC replication may provide a genetic approach to the treatment of transplant arteriosclerosis.

Gene therapy for coronary artery disease: The University of Michigan Program

Recent studies show that the cyclin dependent kinase inhibitor KIP/CIP family members function as regulators of VSMC proliferation. The prevention and treatment of cell proliferation in arteries after percutaneous intervention, represents an attractive target for gene therapy. Targeting of cell cycle regulatory proteins might inhibit cell proliferation and migration, and induce withdrawal from the cell cycle. Furthermore, these studies suggest that genetic approaches are feasible and that local expression of a regulatory gene is sufficient to abrogate lesion formation in different animal models of vascular diseases.

Gene transfer into normal and atherosclerotic human blood vessels

Gene transfer to blood vessels is a promising new approach to the treatment of the vascular diseases, but the feasibility of gene transfer to adult human vessels has not been explored. We introduced an adenovirus vector encoding a marker gene human placental alkaline phosphatase into normal and atherosclerotic human vessels in organ culture. In the normal vessels, recombinant gene was expressed preferentially in the endothelial cells (approximately 100%), intimal smooth muscle cells (1.3+/-0.4%, 1.4+/-1.0%, and 3.8+/-0.8% in the internal mammary arteries, saphenous veins, and normal coronary arteries, respectively), and various adventitial cells. Advanced, complicated atherosclerotic plaques demonstrated a similar efficiency of recombinant gene expression (3.1+/-0.5% and 3.8+/-0.3% of nonendothelial intimal cells in the coronary artery and carotid artery plaques, respectively). Of these intimal cells, macrophages and smooth muscle cells expressed a transgene, identifying them as targets for gene transfer. Areas of plaque rupture and thrombus are sites of predilection for expression of recombinant genes. Collagenase and elastase treatment increased the percentage of transgenic alkaline phosphatase-positive cells 7 times (P<0.001), suggesting that the pattern of gene expression was affected by the amount of surrounding extracellular matrix. These studies demonstrate the feasibility of gene transfer to human blood vessels. However, these studies also highlight important barriers to adenoviral gene delivery to the actual normal and atherosclerotic human vessels of clinical interest.

Expression of cyclin-dependent kinase inhibitors in vascular disease

Arterial lesions in cardiovascular diseases are characterized by proliferation and migration of smooth muscle cells as well as deposition of connective tissue matrix. Factors that stimulate vascular smooth muscle cell (VSMC) proliferation are well described; however, the role of proteins that limit intimal hyperplasia is not well understood. To examine the function of Kip/Cip and INK cyclin-dependent kinase inhibitors (CKIs) in vascular diseases, the expression of p27Kip1 and p16INK was examined in VSMCs in vitro and in porcine arteries and human atherosclerosis in vivo. Western blot and fluorescence activated cell-sorting analysis demonstrated that levels of p27Kip1, but not p16INK, increased during serum deprivation of primary VSMC cultures and caused G1 arrest. p27Kip1 inhibited Cdk2 activity, suggesting that Kip CKIs promote G1 arrest in VSMCs by binding cyclin E/Cdk2. In porcine arteries, p27Kip1, but not p16INK, was constitutively expressed at low levels. Immediately after balloon injury, cell proliferation increased as p27Kip1 levels declined. Three weeks after injury, p27Kip1 was strongly expressed in intimal VSMCs when VSMC proliferation was < 2%, suggesting that p27Kip1 functions as an inhibitor of cell proliferation in injured arteries. In contrast, p16INK expression was detected only transiently early after injury. CKI expression was examined in 35 human coronary arteries, ranging from normal to advanced atherosclerosis. p27Kip1 expression was abundant in nonproliferating VSMCs and macrophages within normal (7 of 8) and atherosclerotic (25 of 27) arteries. p21Cip1 levels were undetectable in normal arteries but were elevated in atherosclerotic (19 of 27) arteries. p16INK could not be detected in normal or atherosclerotic arteries (0 of 35). Thus, the Kip/Cip and INK CKIs have different temporal patterns of expression in VSMCs in vitro and in injured arteries and atherosclerotic lesions in vivo. In contrast to p16INK, p27Kip1 likely contributes to the remodeling process in vascular diseases by the arrest of VSMCs in the G1 phase of the cell cycle.

Distinct cellular interactions of secreted and transmembrane Ebola virus glycoproteins

The mechanisms by which Ebola virus evades detection and infects cells to cause hemorrhagic fever have not been defined, though its glycoprotein, synthesized in either a secreted or transmembrane form, is likely involved. Here the secreted glycoprotein was found to interact with neutrophils through CD16b, the neutrophil-specific form of the Fc gamma receptor III, whereas the transmembrane glycoprotein was found to interact with endothelial cells but not neutrophils. A murine retroviral vector pseudotyped with the transmembrane glycoprotein preferentially infected endothelial cells. Thus, the secreted glycoprotein inhibits early neutrophil activation, which likely affects the host response to infection, whereas binding of the transmembrane glycoprotein to endothelial cells may contribute to the hemorrhagic symptoms of this disease.

Immunization for Ebola virus infection

Infection by Ebola virus causes rapidly progressive, often fatal, symptoms of fever, hemorrhage and hypotension. Previous attempts to elicit protective immunity for this disease have not met with success. We report here that protection against the lethal effects of Ebola virus can be achieved in an animal model by immunizing with plasmids encoding viral proteins. We analyzed immune responses to the viral nucleoprotein (NP) and the secreted or transmembrane forms of the glycoprotein (sGP or GP) and their ability to protect against infection in a guinea pig infection model analogous to the human disease. Protection was achieved and correlated with antibody titer and antigen-specific T-cell responses to sGP or GP. Immunity to Ebola virus can therefore be developed through genetic vaccination and may facilitate efforts to limit the spread of this disease.

Gene transfer of Fas ligand induces tumor regression in vivo

The Fas-Fas ligand (FasL) system plays an important role in the induction of lymphoid apoptosis and has been implicated in the suppression of immune responses. Herein, we report that gene transfer of FasL inhibits tumor cell growth in vivo. Although such inhibition is expected in Fas+ tumor cell lines, marked regression was unexpectedly observed after FasL gene transfer into the CT26 colon carcinoma that does not express Fas. Infection by an adenoviral vector encoding FasL rapidly eliminated tumor masses in the Fas+ Renca tumor by inducing cell death, whereas the elimination of Fas- CT26 cells was mediated by inflammatory cells. Analysis of human malignancies revealed Fas, but not FasL, expression in a majority of tumors and susceptibility to FasL in most Fas+ cell lines. These findings suggest that gene transfer of FasL generates apoptotic responses and induces potent inflammatory reactions that can be used to induce the regression of malignancies.

Inhibition of vascular smooth muscle cell proliferation and intimal hyperplasia by gene transfer of beta-interferon

BACKGROUND

Balloon injury of the arterial wall induces increased vascular smooth cell proliferation, enhanced elastic recoil, and abnormalities in thrombosis, each of which contribute to regrowth of intima and the lesion of restenosis. Several gene transfer approaches have been used to inhibit such intimal smooth muscle cell growth. In this report, adenoviral gene transfer of beta-interferon (beta-IFN) was analyzed in a porcine model of balloon injury to determine whether a secreted growth inhibitory protein might affect the regrowth of vascular smooth muscle cells in vitro and in arteries.

MATERIALS AND METHODS

An adenoviral vector encoding beta-interferon (ADV-beta-IFN) was prepared and used to infect porcine vascular smooth muscle cells in a porcine balloon injury model. Its antiproliferative effect was analyzed in vitro and in vivo.

RESULTS

Expression of recombinant porcine beta-IFN in vascular smooth muscle cells reduced cell proliferation significantly in vitro, and supernatants derived from the beta-IFN vector inhibited vascular smooth muscle cell proliferation relative to controls. When introduced into porcine arteries after balloon injury, a reduction in cell proliferation was observed 7 days after gene transfer measured by BrdC incorporation (ADV-delta E1 arteries 14.5 +/- 1.2%, ADV-beta IFN 6.8 +/- 0.8%, p < 0.05, unpaired, two-tailed t-test). The intima-to-media area ratio was also reduced (nontransfected arteries, 0.70 +/- 0.05; ADV-delta E1 infected arteries, 0.69 +/- 0.06; ADV-beta-IFN infected arteries, 0.53 +/- 0.03; p < 0.05, ANOVA with Dunnett t-test). No evidence of organ toxicity was observed, and regrowth of the endothelial cell surface was observed 3-6 weeks after balloon injury.

CONCLUSIONS

Gene transfer of an adenoviral vector encoding beta-IFN into balloon-injured arteries reduced vascular smooth muscle proliferation and intimal formation. Expression of this gene product may have potential application for the treatment of vascular proliferative diseases.

Transfection of human endothelial cells

OBJECTIVE

The introduction of recombinant genes into endothelial cells provides a method to study specific gene products and their effect on cell function. In addition, endothelial cells can be used for implantation into vessels or prosthetic vascular grafts. Because transfection efficiencies in human endothelial cells have been low, it is important to develop improved gene transfer techniques. Therefore, several transfection methods were optimized and transfection efficiencies were determined.

METHODS

Transfection by particle-mediated gene transfer (biolistics) or by cationic liposomes were optimized and compared to calcium phosphate and DEAE-dextran. Transfection efficiency was determined using either a beta-galactosidase or placental alkaline phosphatase reporter gene. The effect of promoter strength was analyzed by transfecting plasmids with either the Rous sarcoma virus (RSV) promoter or cytomegalovirus (CMV) promoter regions.

RESULTS

Optimal conditions for particle-mediated gene transfer utilized gold particles of 1.6 microns diameter, a target distance of 3 cm, helium pressures of 8.96 MPa (1300 psi) and cell confluence of 75%. Transfection with different cationic liposomes demonstrated that one compound, N-(3-aminopropyl)-N,N-dimethyl-2,3-(bis-dodecyloxy)-1-propanimi nium bromide/dioleoyl phosphatidylethanolamine (gamma AP-DLRIE/DOPE), was optimal for gene transfer when 5 micrograms of DNA and 10 to 20 micrograms of lipid was used. With both gold particles and gamma AP-DLRIE/DOPE, the alkaline phosphatase reporter was more efficient than beta-galactosidase using comparable promoters and polyadenylation sites. CMV regulatory elements were more efficient than the RSV promoter in optimizing gene expression. Optimal gene transfer efficiency was 20.28% of cells with gamma AP-DLRIE/DOPE, 3.96% with biolistics, 2.09% with calcium phosphate and 0.88% with DEAE-dextran.

CONCLUSIONS

Gene expression is detectable in a high percentage of human endothelial cells after liposome-mediated transfection when expression is controlled by a strong promoter. Particle-mediated transfection is less efficient under these conditions, but more effective than liposomes when expression is driven by a relatively weak promoter. Calcium phosphate and DEAE-dextran are less useful.

[Gene transfer of interferon beta inhibits vascular smooth muscle cell proliferation in vitro and in animal model of arterial injury]

Vascular hypertrophy may increase the blood pressure by its effect on vascular resistance. In this study, adenoviral gene transfer of IFN-beta was analysed in a porcine model of balloon injury to determine whether a secreted growth inhibitory protein might affect the regrowth of vascular smooth muscle cells (VSMC) in vitro and in arteries. An adenoviral vector encoding IFN-beta (ADV-IFN-beta) was constructed by homologous recombination between sub360 genomic DNA, an ADV 5 derivative with a deletion in the E3 region and a porcine IFN-beta expression plasmid. Its antiproliferative effect was analysed using cell proliferation assays, and used in a porcine model of balloon injury. After injury, arteries were immediately transfected with 7 x 10(9) plaques forming units of either ADV-IFN-beta or a control E1A deficient adenovirus that does not encode a recombinant protein, ADV-delta E1. The intima/media (I/M) area ratio was determined by quantitative morphometry 21 days after artery injury and gene transfer. Expression of recombinant porcine IFN-beta in VSMC reduced cell proliferation significantly in vitro, and supernatants derived from IFN-beta vector infected cells inhibited VSMC proliferation relative to controls. When introduced into porcine arteries after balloon injury, a reduction in I/M ratio of 30% was found. I/M ratio in the IFN-beta transduced arteries was 0.54 +/- 0.03 vs 0.69 +/- 0.06 in ADV-delta E1 transfected arteries and 0.702 +/- 0.05 in the non-transfected arteries. Gene transfer of an adenoviral vector encoding IFN-beta to VSMC and injured arteries reduced cell proliferation and vascular thickening. This approach is potentially applicable to vascular proliferative diseases.

[Direct gene transfer in the rat kidney in vivo]

Gene delivery to the kidney has both experimental and therapeutic potential in hypertension, although the delivery methods, distribution of transgene and subsequent inflammatory response have been poorly characterized. In adult male Sprague-Dawley rats (200 g, n = 26), the left iliac artery was catheterized and a small catheter (Microbore Tygon S-54-HL) was advanced to the origin of the left renal artery. Loops were tied transiently around the aorta and below the renal arterial bifurcation. After flushing the kidney, the renal vein was tied and 500 microL of transfection solution was instilled. After 15 min all the loops were released, the catheter was removed and the left iliac artery ligated. Both replication-defective adenovirus (ADV) constructions used were based on an Ad5 derivative with a partial E3 deletion. Virus ADV-chloramphenicol acetyl transferase (CAT) and ADV-human placental alkaline phosphatase (hpAP), 10(8), 3 x 10(8), 10(9) and 10(10) plaques forming units/mL (pfu/mL), were used respectively to compare the degree of transfection (CAT) and to localize the transgene in the kidney (hpAP), 48 h after transfection. Controls were infused with vehicle. ADV-CAT 10(10) pfu/mL induced a gene expression, respectively, 1.4 (NS), 12 (p < 0.001) and 28 (p < 0.001) fold greater than the 10(9), 3 x 10(8) and 10(8) pfu/mL formulations. HpAP staining was located in the juxta-medullary part of the cortex, predominantly in the interstitium. Genetically-modified cells were identified as endothelial cells, mainly in peritubular capillaries but also in efferent arterioles and hilar arteries. Highly efficient gene transfer achieved with ADV-hpAP 10(10) pfu/mL was associated with focal necrosis of the proximal convoluted tubules. No changes were observed with the other viral concentrations. Gene delivery, mediated by a replication-defective ADV, to one rat kidney via the renal artery, induced a dose-dependent gene expression located in endothelial cells in peritubular capillaries. Toxicity was observed only with the highest viral concentration.

In vivo gene delivery to the pulmonary circulation in rats: transgene distribution and vascular inflammatory response

Although gene delivery to the pulmonary circulation has both experimental and therapeutic potential, the delivery methods, distribution of transgene, and subsequent inflammatory response have been poorly characterized to date. To address these issues, we utilized a 0.76-mm OD (outside diameter) end hole catheter inserted into the internal jugular vein of adult Sprague-Dawley rats, directing the tip into a pulmonary capillary wedge position. We then compared infusion of polycationic lipid:DNA complexes to replication-defective adenovirus with respect to magnitude and distribution of transgene expression using either chloramphenicol acetyltransferase (CAT) or human placental alkaline phosphatase (hpAP) reporter genes. Both lipid:DNA and adenovirus resulted in detectable transgene expression, though maximum lung CAT activity using lipid (gamma AP-DLRIE/DOPE) was approximately 2% of maximum activity using adenovirus (Ad-CAT). Further characterization of expression after transfection with 10(8) pfu (plaque forming units) of Ad-CAT demonstrated persistence of transgene for at least 14 days (lung CAT activity 27% of maximum). Alkaline phosphatase staining demonstrated that both large and small pulmonary arteries as well as the alveolar wall expressed transgene. Although little inflammatory response was detected in conduit arteries, a predominantly mononuclear cell infiltrate surrounded small pulmonary arteries as well as the alveolar spaces in transfected areas of lung. We conclude that percutaneous catheter-mediated gene delivery to the pulmonary circulation in rats using non-viral and viral vectors is feasible. Although an inflammatory response to first generation replication-defective adenovirus was detected, it appeared to be largely restricted to the distal pulmonary circulation and airspace. This technique should prove useful for investigations requiring overexpression of novel genes in the pulmonary artery wall, and could ultimately be used to develop gene-based therapies for pulmonary vascular diseases.

Expression of a recombinant preproendothelin-1 gene in arteries stimulates vascular contractility

Endothelin (ET)-1 is a potent vasoconstrictor peptide that is elevated in cardiovascular diseases. However, the biological function of ET-1 gene expression within arteries in vivo has not been determined. The effects of ET-1 gene expression were investigated using gene-transfer methods on porcine vascular cells in vitro and porcine arteries in vivo. Transfection of vascular cells with a vector encoding for human preproendothelin-1 cDNA (pVR-ppET) resulted in significant increase in active ET-1 levels in culture supernatant compared with nontransfected cells (P < 0.05). This supernatant contracted rat aortic strips at concentrations 10-fold lower than synthetic ET-1 protein, which was inhibited by the ET-A receptor antagonist BQ-123. Transfection of pVR-ppET into pig iliofemoral arteries resulted in an increase in contractile responses to angiotensin I compared with control vessels (P < 0.05), in contrast to serotonin, phenylephrine, synthetic ET-1, and angiotensin II. A mitogenic effect of recombinant ET-1 on intimal cell growth was not observed. These findings demonstrate that expression of a recombinant ET-1 gene in vivo augments vascular contractility due to an increased sensitivity to angiotensin I, suggesting a role for ET-1 in the pathogenesis of cardiovascular diseases.

Combination gene transfer to potentiate tumor regression

Recent efforts to treat malignancy using gene transfer have met with varying degrees of success. In this paper, we report the results of studies using two recombinant adenoviral vectors to examine the efficacy of combination gene transfer to cause tumor regression in vivo. One of these vectors encodes the murine MHC class I gene, H-2Kb (ADV-Kb), which induces an immune response that stimulates tumor regression. The second vector encodes the human p21 cyclin dependent kinase inhibitor (ADV-p21). This gene product arrests cell cycle progression and prevents proliferation of tumor cells. Both vectors were tested in a murine model in vivo for antitumor effect. As previously shown, a significant reduction of tumor size was observed with each vector. Combination treatment, in which both vectors were administered, resulted in a trend toward a reduced tumor growth greater than with either vector alone. In order to characterize the mechanism of tumor regression, cytolytic T lymphocyte (CTL) assays against the allogeneic molecule, H-2Kb, were performed. Mice treated with ADV-Kb showed specific CTL activity against the H-2Kb molecule, demonstrating that the immune response against the H-2Kb gene product involved in tumor regression was potentiated by expression of the p21 gene which affects cell cycle progression.

Gene and other biological therapies for vascular diseases

Gene transfer and antisense therapy offer novel approaches to the study and treatment of vascular diseases. The localized nature of vascular diseases like restenosis has made the application of genetic material an attractive therapeutic option. Viral and nonviral vectors have been developed to facilitate the entry of foreign DNA or RNA into cells. Vector improvement and production, demonstration of vector safety and demonstration of therapeutic efficacy are among the main present challenges. Various strategies have already been shown to be successful in preventing restenosis in animal models and include: the transfer of the herpes simplex virus thymidine kinase associated with ganciclovir: transfection of the cell cycle regulatory genes encoding for the active form of retinoblastoma gene product (Rb) or the cyclin-dependent kinase inhibitor p21, and antisense therapy. Therapeutic angiogenesis using gene transfer is a new strategy for the treatment of severe limb ischemia. Transfection of DNA encoding for the vascular endothelial growth factor has resulted in increasing collateral flow in animal models of peripheral ischemia. This approach is currently being investigated in a clinical trial in patients with distal ischemia. Other potential targets for genetic treatment in cardiovascular diseases include thrombosis, extracellular matrix synthesis and lipid metabolism.

Immune response in human melanoma after transfer of an allogeneic class I major histocompatibility complex gene with DNA-liposome complexes

Analysis of the antitumor immune response after gene transfer of a foreign major histocompatibility complex class I protein, HLA-B7, was performed. Ten HLA-B7-negative patients with stage IV melanoma were treated in an effort to stimulate local tumor immunity. Plasmid DNA was detected within treated tumor nodules, and RNA encoding recombinant HLA-B7 or HLA-B7 protein was demonstrated in 9 of 10 patients. T cell migration into treated lesions was observed and tumor-infiltrating lymphocyte reactivity was enhanced in six of seven and two of two patients analyzed, respectively. In contrast, the frequency of cytotoxic T lymphocyte against autologous tumor in circulating peripheral blood lymphocytes was not altered significantly, suggesting that peripheral blood lymphocyte reactivity is not indicative of local tumor responsiveness. Local inhibition of tumor growth was detected after gene transfer in two patients, one of whom showed a partial remission. This patient subsequently received treatment with tumor-infiltrating lymphocytes derived from gene-modified tumor, with a complete regression of residual disease. Thus, gene transfer with DNA-liposome complexes encoding an allogeneic major histocompatibility complex protein stimulated local antitumor immune responses that facilitated the generation of effector cells for immunotherapy of cancer.

A new cationic liposome DNA complex enhances the efficiency of arterial gene transfer in vivo

An important goal of gene therapy for cardiovascular diseases and cancer is the development of effective vectors for catheter-based gene delivery. Although adenoviral vectors have proven effective for this purpose in animal models, the ability to achieve comparable gene transfer with nonviral vectors would provide potentially desirable safety and toxicity features for clinical studies. In this report, we describe the use of a new cationic DNA-liposome complex using an improved expression vector and lipid, N-(3-aminopropyl)-N, N-dimethyl-2,3-bis(dodecyloxy)-1-propaniminium bromide/dioleyl phosphatidylethanolamine (GAP-DL-RIE/DOPE) to optimize catheter-mediated gene transfer in porcine arteries. The efficiency of this vector was compared to DNA alone, DNA with a previously described cationic liposome complex, (+/-)-N-(2-hydroxyethyl)-N, N-dimethyl-2,3-bis(tetradecyloxy)-1-propanaminium bromide (DMRIE/DOPE), and a replication-defective adenoviral vector in a porcine artery gene transfer model. When used in optimal ratios, GAP-DL-RIE/DOPE liposomes provided a 15-fold higher level of gene expression in arteries compared to DNA alone or DMRIE/DOPE. Gene expression was observed in intimal and medial cells. However, when compared to adenoviral vectors (10(10) pfu/ml), gene expression following GAP-DLRIE/DOPE transfection was approximately 20-fold lower. Following intravenous injection of GAP-DLRIE/DOPE in mice, biochemical, hematological, and histopathological abnormalities were not observed. Significant improvements in the efficacy of arterial gene expression can be achieved by optimization of transfection condition with DNA-liposome complexes in vivo that may prove useful for arterial gene delivery in cardiovascular diseases and cancer.

Gene transfer approaches to the regulation of vascular cell proliferation

Considerable progress has been made in identifying potential targets for treating vascular proliferative diseases. In this review, we discuss gene transfer approaches to regulating smooth muscle cell proliferation after vascular injury using the cell cycle specific proteins, p21, delta Rb and HSV-tk. Results from these studies suggest that replicating smooth muscle cells and macrophages are inhibited in vivo in several animal models of restenosis, including hyperlipidaemic vessels. Identification of appropriate vascular diseases and improvements in gene delivery and vectors will require careful optimization in order to develop effective molecular therapies for human vascular diseases.

Direct transfer of a foreign MHC gene into human melanoma alters T cell receptor V beta usage by tumor-infiltrating lymphocytes

The direct introduction of foreign genes into tumors shows promise as a therapeutic modality to enhance tumor immunogenicity. Hence, melanoma nodules were directly injected with a vector encoding an allogeneic MHC class I molecule, HLA-B7. Tumor-infiltrating lymphocytes (TIL) were isolated from cutaneous melanoma biopsies before and after HLA-B7 gene transfer. TIL were expanded in interleukin-2 (IL-2) by standard techniques for approximately 4 weeks, then analyzed for T cell receptor V beta usage by quantitative reverse transcriptase polymerase chain reaction (RT-PCR). Prior to gene transfer. TIL V beta usage was found to be highly restricted, the only one to four V beta families being expressed and one or two of these families representing more than 90% of the repertoire. As anticipated, TIL V beta usage varied among patients expressing different HLA types. However, V beta 13 was over-represented in that six of eight patients utilized V beta 13 as a dominant family, regardless of HLA type. Following HLA-B7 gene transfer, TIL V beta usage was markedly altered: (1) V beta families that dominated following gene transfer differed from the V beta families utilized by TIL prior to treatment, and (2) introduction of the HLA-B7 gene resulted in a more diverse repertoire with an increase in the number of V beta families represented. In two patients, TIL were evaluated before treatment and from multiple, distinct melanoma nodules following gene transfer. In these two patients, a comparison was made between TIL V beta profiles obtained after treatment from nodules that had been injected with the HLA-B7 gene or left untreated. Interestingly, the V beta repertoires of TIL from uninjected nodules following gene transfer were similar to that of TIL from injected nodules, rather than pretreatment TIL. These data demonstrate a direct immunological effect of foreign MHC gene transfer into human melanoma, and suggest that local expression of an allogeneic MHC molecule generates systemic alterations in the antitumor immune response.

Role of the p21 cyclin-dependent kinase inhibitor in limiting intimal cell proliferation in response to arterial injury

Arterial injury induces a series of proliferative, vasoactive, and inflammatory responses that lead to vascular proliferative diseases, including atherosclerosis and restenosis. Although several factors have been defined which stimulate this process in vivo, the role of specific cellular gene products in limiting this response is not well understood. The p21 cyclin-dependent kinase inhibitor affects cell cycle progression, senescence, and differentiation in transformed cells, but its expression in injured blood vessels has not been investigated. In this study, we report that p21 protein is induced in porcine arteries following balloon catheter injury and suggest that p21 is likely to play a role in limiting arterial cell proliferation in vivo. Vascular endothelial and smooth muscle cell growth was arrested through the ability of p21 to inhibit progression through the G1 phase of the cell cycle. Following injury to porcine arteries, p21 gene product was detected in the neointima and correlated inversely with the location and kinetics of intimal cell proliferation. Direct gene transfer of p21 using an adenoviral vector into balloon injured porcine arteries inhibited the development of intimal hyperplasia. Taken together, these findings suggest that p21, and possibly related cyclin-dependent kinase inhibitors, may normally regulate cellular proliferation following arterial injury, and strategies to increase its expression may prove therapeutically beneficial in vascular diseases.

Regulation of cellular proliferation and intimal formation following balloon injury in atherosclerotic rabbit arteries

Injury to atherosclerotic arteries induces the expression of growth regulatory genes that stimulate cellular proliferation and intimal formation. Intimal expansion has been reduced in vivo in nonatherosclerotic balloon-injured arteries by transfer of genes that inhibit cell proliferation. It is not known, however, whether vascular cell proliferation can be inhibited after injury in more extensively diseased atherosclerotic arteries. Accordingly, the purpose of this study was to investigate whether expression of recombinant genes in atherosclerotic arteries after balloon injury could inhibit intimal cell proliferation. To test this hypothesis, we examined the response to balloon injury in atherosclerotic rabbit arteries after gene transfer of herpesvirus thymidine kinase gene (tk) and administration of ganciclovir. Smooth muscle cells from hyperlipidemic rabbit arteries infected with adenoviral vectors encoding tk were sensitive to ganciclovir, and bystander killing was observed in vitro. In atherosclerotic arteries, a human placental alkaline phosphatase reporter gene was expressed in intimal and medial smooth muscle cells and macrophages, identifying these cells as targets for gene transfer. Expression of tk in balloon-injured hyperlipidemic rabbit arteries followed by ganciclovir treatment resulted in a 64% reduction in intimal cell proliferation 7 d after gene transfer (P = 0.004), and a 35-49% reduction in internal area 21 d after gene transfer, compared with five different control groups (P < 0.05). Replication of smooth muscle cells and macrophages was inhibited by tk expression and ganciclovir treatment. These findings indicate that transfer of a gene that inhibits cellular proliferation limits the intimal area in balloon-injured atherosclerotic arteries. Molecular approaches to the inhibition of cell proliferation in atherosclerotic arteries constitute a possible treatment for vascular proliferative diseases.

Genetic therapy

Vascular gene transfer is the introduction of foreign DNA into host cells within the vessel wall. Expression of recombinant genes within vascular cells has been demonstrated in normal, injured and atherosclerotic arteries. Transfer of genes with biological activity has provided insights into the role of specific genes in normal and pathological states. The development of gene transfer as a form of vascular therapy will require safe and effective vectors and intravascular delivery systems. Viral and non-viral vectors and several delivery catheters are being evaluated to determine their clinical applicability. The current applications of vascular gene transfer to cardiovascular diseases are the prevention of restenosis following arterial injury and induction of angiogenesis in occlusive vascular disease.

Expression and function of a recombinant PDGF B gene in porcine arteries

Platelet-derived growth factor (PDGF) B is a mitogen and chemoattractant for smooth muscle cells in vitro, and expression of a recombinant PDGF B gene in porcine arteries stimulates intimal thickening. To define the mechanisms by which PDGF B gene expression induces intimal thickening in vivo, we examined its effects on smooth muscle cell proliferation and migration, extracellular matrix synthesis, and inflammatory cell infiltration in intimal lesions of pig arteries after direct gene transfer of a recombinant PDGF B gene. PDGF B gene expression was associated with rapid formation of an intima, including 3- to 10-fold increases in intimal thickness and intima-to-media area ratio 4 to 21 days after gene transfer compared with control transfected arteries. Intimal smooth muscle cell proliferation was detected at 2 days, peaked at 7 days (P < .01), and declined by 14 days, although the total number of intimal nuclei progressively increased to 21 days (P < .01). Calculations of expected-to-observed ratios of intimal cells, based on BrdC proliferation indexes, demonstrated that the increases in intimal cell number on days 2 through 7 could not be accounted for by proliferation alone, suggesting that recombinant PDGF BB acts to stimulate cell proliferation and migration of smooth muscle cells into the intima. Extracellular matrix deposition and procollagen synthesis were observed after 7 days (P < .01) and were associated with a decline in cell density in the intima, suggesting that extracellular matrix synthesis may contribute to progressive intimal thickening in response to PDGF B gene expression. There was minimal accumulation of inflammatory cells, including macrophages and CD3(+) lymphocytes, in transfected arteries. These data suggest that PDGF B gene expression promotes intimal expansion by both proliferation and migration of smooth muscle cells followed by synthesis of extracellular matrix and therefore acts through several mechanisms to play a role in the pathogenesis of intimal lesions in vivo.

Gene therapy for vascular diseases

Arterial injury induces the synthesis of gene products that stimulate smooth muscle cell (SMC) migration and proliferation, leading to intimal hyperplasia. This process contributes to the pathogenesis of many cardiovascular diseases, including vascular proliferative diseases and atherosclerosis. Molecular approaches to the inhibition of SMC proliferation could potentially limit intimal expansion following vascular injury. During the past five years, there has been considerable interest in developing methods for the introduction and expression of recombinant genes in the vasculature. Gene transfer offers novel approaches to the study and treatment of vascular diseases. To date, gene transfer to the vasculature has been employed largely for two purposes: (1) examination of the expression and function of recombinant genes in vivo; and (2) development of potential new therapies for vascular diseases.

Direct gene transfer for treatment of human cancer

Genetic instability within malignant cells gives rise to mutant proteins which can be recognized by the immune system. Recognition of tumor-associated antigens by T lymphocytes could thus contribute to the elimination of neoplastic cells. We have developed a molecular genetic intervention for the treatment of malignancies based upon the knowledge that foreign major histocompatibility complex (MHC) proteins expressed on the cell surface are efficient at stimulating an immune response. Expression of this foreign MHC gene within tumors induced a cytotoxic T cell response to the introduced gene. More importantly, the immune system recognized tumor-specific antigens on unmodified tumor cells as foreign. Growth of the tumors diminished, and in many cases, there was complete regression. This research provides evidence that direct gene transfer in vivo can induce cell-mediated immunity against specific gene products, and offers the potential for effective immunotherapy for the treatment of cancer and infectious diseases in man. Our laboratory conducted a phase I clinical trial to determine the safety and efficacy of this treatment in humans. These studies suggest that direct gene transfer provides a safe and feasible approach for the treatment of human cancer. More recent developments using combination gene therapy and the initiation of a second human trial with improvements on this technology have been implemented. Finally, we have begun to define mechanisms of resistance to immune recognition by established malignancies.

The p21 cyclin-dependent kinase inhibitor suppresses tumorigenicity in vivo

The p21 gene encodes a cyclin-dependent kinase inhibitor that affects cell-cycle progression, but the potential of this gene product to serve as a tumour suppressor in vivo has not been established. In this report, we show that the growth of malignant cells in vitro and in vivo is inhibited by expression of p21. Expression of p21 resulted in an accumulation of cells in G0/G1, altered morphology, and cell differentiation, but apoptosis was not induced. Introduction of p21 with adenoviral vectors into malignant cells completely suppressed their growth in vivo and also reduced the growth of established pre-existing tumours. Gene transfer of p21 may provide a molecular genetic approach to arresting cancer cell growth by committing malignant cells irreversibly to a pathway of terminal differentiation.