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Heterologous expression of proteins is used widely for the biosynthesis of biologics, many of which are secreted from cells. In addition, gene therapy and messenger RNA (mRNA) vaccines frequently direct the expression of secretory proteins to nonnative host cells. Consequently, it is crucial to understand the maturation and trafficking of proteins in a range of host cells including muscle cells, a popular therapeutic target due to the ease of accessibility by intramuscular injection. Here, we analyzed the production efficiency for α1-antitrypsin (AAT) in Chinese hamster ovary cells, commonly used for biotherapeutic production, and myoblasts (embryonic progenitor cells of muscle cells) and compared it to the production in the major natural cells, liver hepatocytes. AAT is a target protein for gene therapy to address pathologies associated with insufficiencies in native AAT activity or production. AAT secretion and maturation were most efficient in hepatocytes. Myoblasts were the poorest of the cell types tested; however, secretion of active AAT was significantly augmented in myoblasts by treatment with the proteostasis regulator suberoylanilide hydroxamic acid, a histone deacetylase inhibitor. These findings were extended and validated in myotubes (mature muscle cells) where AAT was transduced using an adeno-associated viral capsid transduction method used in gene therapy clinical trials. Overall, our study sheds light on a possible mechanism to enhance the efficacy of gene therapy approaches for AAT and, moreover, may have implications for the production of proteins from mRNA vaccines, which rely on the expression of viral glycoproteins in nonnative host cells upon intramuscular injection.

Recombinant adeno-associated virus (AAV) vectors have been used to transduce murine skeletal muscle as a platform for secretion of therapeutic proteins. The utility of this approach for treating alpha-1-antitrypsin (AAT) deficiency was tested in murine myocytes in vitro and in vivo. AAV vectors expressing the human AAT gene from either the cytomegalovirus (CMV) promoter (AAV-C-AT) or the human elongation factor 1-alpha promoter (AAV-E-AT) were examined. In vitro in C2C12 murine myoblasts, the expression levels in transient transfections were similar between the two vectors. One month after transduction, however, the human elongation factor 1 promoter mediated 10-fold higher stable human AAT expression than the CMV promoter. In vivo transduction was performed by injecting doses of up to 1.4 x 10(13) particles into skeletal muscles of several mouse strains (C57BL/6, BALB/c, and SCID). In vivo, the CMV vector mediated higher levels of expression, with sustained serum levels over 800 micrograms/ml in SCID and over 400 micrograms/ml in C57BL/6 mice. These serum concentrations are 100,000-fold higher than those previously observed with AAV vectors in muscle and are at levels which would be therapeutic if achieved in humans. High level expression was delayed for several weeks but was sustained for over 15 wk. Immune responses were dependent upon the mouse strain and the vector dosage. These data suggest that recombinant AAV vector transduction of skeletal muscle could provide a means for replacing AAT or other essential serum proteins but that immune responses may be elicited under certain conditions.

Dendritic cells (DCs) instruct and activate a naive immune system to mount a response toward foreign proteins. Therefore, it has been hypothesized that an ideal vaccine strategy would be to directly introduce genes encoding antigens into DCs. To test this strategy quantitatively, we have compared the immune response elicited by a genetically transfected DC line to that induced by a fibroblast line, or standard genetic immunization. We observe that a single injection of 500-1,000 transfected DCs can produce a response comparable to that of standard genetic immunization, whereas fibroblasts, with up to 50-fold greater transfection efficiency, were less potent. We conclude that transfection of a small number of DCs is sufficient to initiate a wide variety of immune responses. These results indicate that targeting genes to DCs will be important for controlling and augmenting the immunological outcome in genetic immunization.

Retrovirus-mediated gene transfer into hepatocytes in vivo results in long-term gene expression. Limitations include the need to remove two-thirds of the liver and the relatively low frequency of gene transfer. To increase gene transfer without surgical hepatectomy, mouse hepatocytes were transduced in vivo with a recombinant adenovirus that transiently expressed urokinase, resulting in high rates of asynchronous liver regeneration. During the regenerative phase, in vivo retroviral-mediated gene transfer in hepatocytes resulted in 5- to 10-fold greater transduction efficiencies than that obtained by conventional partial hepatectomy. In 3-4 weeks, the architecture and microscopic structure of the recipient livers were normal. The two-viral system of achieving permanent transgene expression from hepatocytes in vivo offers an alternative approach to current ex vivo and in vivo gene-transfer models.

Liver-directed gene therapy will be applicable to many inherited diseases. Although various protocols have been devised for in vivo delivery of retrovirus, comparison of hepatocyte transduction frequencies has been difficult due to variations in retroviral titer and a paucity of DNA data. We have previously reported an in vivo rat hepatocyte transduction technique which involves 70% hepatectomy followed 24 hr later by portal vein injection of retrovirus during hepatic in-flow occlusion. In this study, we employed this method and concentrated retroviral preparations to achieve transduction of up to 15% of hepatocytes as determined by a quantitative PCR assay. As an initial step toward identifying promoters which lead to high-level long-term expression of retroviral transduced genes, we used our in vivo delivery system to compare the Moloney murine leukemia virus long terminal repeat (LTR) promoter with the promoter for the large subunit of murine RNA polymerase II (Pol-II). Human alpha 1-antitrypsin (hAAT) was used as the reporter gene to facilitate long-term analysis of expression. Serum hAAT levels were higher for the Pol-II promoter (143 ng/ml) than for the LTR promoter (50 ng/ml). This difference was consistent with the higher transduction frequency observed for the Pol-II-hAAT vector. Although serum hAAT expression was sustained for up to 1 year in six of eight Pol-II-hAAT-transduced rats and three of five LTR-hAAT-transduced rats and was proportional to hAAT mRNA level and proviral DNA frequency, in vivo expression was significantly lower than in transduced tissue culture cells. We conclude that a high frequency of in vivo transduction can be achieved by using retroviral vectors and our rapid transduction protocol, but transduced gene expression remains a serious problem. The quantitative assays described herein will facilitate in vivo comparisons of gene regulatory elements.

Formation of the covalently stabilized complex of alpha 1-antitrypsin (alpha 1-AT) with neutrophil elastase, the archetype of serine proteinase inhibitor serpin-enzyme complexes, is associated with structural rearrangement of the alpha 1-AT molecule and hydrolysis of a reactive-site peptide bond. An approximately 4-kDa carboxyl-terminal cleavage fragment is generated. alpha 1-AT-elastase complexes are biologically active, possessing chemotactic activity and mediating increases in expression of the alpha 1-AT gene in human monocytes and macrophages. This suggested that structural rearrangement of the alpha 1-AT molecule, during formation of a complex with elastase, exposes a domain that is recognized by a specific cell surface receptor or receptors. To test this hypothesis, the known three-dimensional structure of alpha 1-AT and comparisons of the primary structures of the serpins were used to select a potentially exteriorly exposed and highly conserved region in the complexed form of alpha 1-AT as a candidate ligand (carboxyl-terminal fragment, amino acids 359-374). We show here that synthetic peptides based on the sequence of this region bind specifically and saturably to human hepatoma cells and human monocytes (Kd = 4.0 X 10(-8) M, 4.5 X 10(5) plasma membrane receptors per cell) and mediate increases in synthesis of alpha 1-AT. Binding of peptide 105Y (Ser-Ile-Pro-Pro-Glu-Val-Lys-Phe-Asn-Lys-Pro-Phe-Val-Tyr-Leu-Ile) is blocked by alpha 1-AT-elastase complexes, antithrombin III (AT III)-thrombin complexes, alpha 1-antichymotrypsin (alpha 1-ACT)-cathepsin G complexes, and, to a lesser extent, complement component C1 inhibitor-C1s complexes, but not by the corresponding native proteins. Binding of peptide 105Y is also blocked by peptides with sequence corresponding to carboxy-terminal fragments of the serpins AT III and alpha 1-ACT, but not by peptides having the sequence of the extreme amino terminus of alpha 1-AT. The results also show that peptide 105Y inhibits binding of 125I-labeled alpha 1-AT-elastase complexes. Thus, these studies demonstrate an abundant, relatively high-affinity cell surface receptor which recognizes serpin-enzyme complexes (SEC receptor). This receptor is capable of modulating the production of at least one of the serpins, alpha 1-AT. Since the ligand specificity is similar to that previously described for in vivo clearance of serpin-enzyme complexes, the SEC receptor may also be involved in the clearance of certain serpin-enzyme complexes.

We explored the possible role of transforming growth factor beta 1 (TGF-beta), a cytokine that appears to be an important modulator of inflammation and tissue repair, in regulation of human plasma protein synthesis during the acute-phase response. In Hep 3B cells, TGF-beta led to increased secretion of the positive acute-phase proteins alpha 1-protease inhibitor and alpha 1-antichymotrypsin and decreased secretion of the negative acute-phase protein albumin. In Hep G2 cells, after incubation with TGF-beta, the same changes in secretion of alpha 1-protease inhibitor, alpha 1-antichymotrypsin, and albumin were observed, as well as decreased secretion of both the negative acute-phase protein alpha-fetoprotein and the positive acute-phase protein fibrinogen. In addition, TGF-beta modulated the effects of interleukin 6; these cytokines, in combination, were additive in inducing synthesis and secretion of alpha 1-protease inhibitor and alpha 1-antichymotrypsin and in decreasing secretion of albumin and alpha-fetoprotein. TGF-beta inhibited the induction of fibrinogen caused by interleukin 6. The effects on alpha 1-protease inhibitor were confirmed by metabolic labeling in Hep 3B cells and by demonstrating increased accumulation of specific mRNA in Hep G2 cells, and the effects on fibrinogen were confirmed in Hep 3B cells by studies of mRNA for the alpha chain of fibrinogen. TGF-beta had no effect on haptoglobin or alpha 1-acid glycoprotein secretion, either directly or in the presence of interleukin 6, which is capable of inducing these proteins. These studies demonstrate that TGF-beta can affect hepatic synthesis and secretion of a subset of acute-phase proteins, both directly and by modulating the effect of interleukin 6. The affected group of plasma proteins is distinct from those affected by other recognized acute-phase protein-inducing cytokines. These findings support the view that combinations of cytokines mediate the response of the hepatocyte to inflammatory stimuli.

Alpha 1-Antitrypsin (alpha 1AT) deficiency is a hereditary disorder characterized by reduced serum levels of alpha 1AT, resulting in destruction of the lower respiratory tract by neutrophil elastase. As an approach to augment alpha 1AT levels in this disorder with physiologically normal human alpha 1AT, we have integrated a full-length normal human alpha 1AT cDNA into the genome of mouse fibroblasts. To accomplish this, the retroviral vector N2 was modified by inserting the simian virus 40 early promoter followed by the alpha 1AT cDNA. Southern analysis demonstrated that the intact cDNA was present in the genome of selected clones of the transfected murine fibroblasts psi 2 and infected NIH 3T3. The clones produced three mRNA transcripts (5.8, 4.8, and 2.4 kilobases) containing human alpha 1AT sequences, secreted an alpha 1AT molecule recognized by an anti-human alpha 1AT antibody, with the same molecular mass (52 kDa) as normal human alpha 1AT and that complexed with and inhibited human neutrophil elastase. The psi 2 produced alpha 1AT was glycosylated, and when infused intravenously into mice, it had a serum half-life similar to normal alpha 1AT purified from human plasma and markedly longer than that of nonglycosylated human alpha 1AT cDNA-directed yeast-produced alpha 1AT. These studies demonstrate the feasibility of using a retroviral vector to insert the normal human alpha 1AT cDNA into non-alpha 1AT-producing cells, resulting in the synthesis and secretion of physiologically "normal" human alpha 1AT.