After three washes, plates were incubated with 100 l per well of 3,3′,5,5′-tetramethylbenzidine solution (TMB One Solution, Promega, Madison, Wisconsin) for 10 min. The expression level of hIgG in these cells was increased 40-fold over that induced directly by the ovalbumin promoter. On the other hand, hIgG was not induced by the ovalbumin promoter-driven Cre in chicken embryonic fibroblast cells. Conclusions The Cre/ em lox /em P-based system could significantly increase ovalbumin promoter-driven production of proteins of interest, specifically in oviduct cells. This expression system could be useful for producing therapeutic mAbs at high level using transgenic chickens as bioreactors. Background The market for therapeutic monoclonal antibodies (mAbs) has dramatically expanded over the past decade because of their high clinical efficacy. In the U.S., around 30 mAbs are currently approved for therapeutic use in cancers, autoimmune disorders, and infectious diseases, and the number of available mAb products is usually predicted to increase [1,2]. Although therapeutic mAbs have become a major class of drugs, their high production cost is a major obstacle. This is mainly due to the use of cultured mammalian cells in the manufacturing of mAbs, which requires a complex industrial bioreactor system. To reduce the cost of mAb production, a more convenient method to Mouse monoclonal to Rab25 replace mammalian cell culture is required. One alternative method involves generating transgenic farm animals as living bioreactors that produce high-yield therapeutic mAbs in milk or other secretory fluids, such as egg whites. The production of recombinant pharmaceutical proteins has been exhibited in transgenic Cilengitide animals including sheep, goats, cattle, rabbits, and chickens (reviewed in [3,4]). Among these animals, the use of transgenic chickens as bioreactors is usually expected to have several advantages, including a shorter timescale for setup, ease of scaling up, and small space requirements (reviewed in [5,6]). Several groups reported the production of therapeutic proteins, such as cytokines, mini-antibodies, and mAbs using transgenic chickens [7-11]. In these transgenic chickens, ubiquitous promoters were used to express the transgenic products; thus, tissue-restricted expression of exogenous proteins was not exhibited. Compared to tissue-restricted expression, ubiquitous expression of therapeutic mAbs in transgenic chickens will increase the heterogeneity of oligosaccharide structure of mAbs due to the glycosylation in various type of cells [11,12]. In addition, depending on the antigen recognition, whole-body expression of foreign mAb could be the risk of negatively affecting the development and health of the transgenic chickens. Therefore, oviduct-specific mAb expression is desirable to synthesize mAbs as a component of egg whites. Using chicken ovalbumin promoters, two groups demonstrated oviduct-specific expression of therapeutic proteins in transgenic chickens and secretion of these proteins into the egg whites [12,13] However, expression levels of exogenous proteins in the egg whites driven by ovalbumin promoters were not high ( 0.5 mg/ml, egg whites) compared to their expression in the mammalian cell culture bioreactor (1-13 mg/ml, culture media) [13,14]. Thus, a highly efficient oviduct promoter is usually demanded but such a promoter has not been developed [5]. In an attempt to increase the expression level of therapeutic mAbs in chicken oviduct cells, we developed a Cre- em lox /em P-regulated exogenous immunoglobulin G Cilengitide (IgG) expression vector. The vector consists of two tandem expression units, each made up of a strong promoter, a fluorescent gene flanked by em lox /em P or mutant em lox /em P as a stuffer fragment, and Cilengitide the gene for the heavy chain or light chain of humanized IgG (hIgG) encoding the human therapeutic mAb, trastuzumab. Trastuzumab recognizes human epidermal growth facter receptor 2 (HER2), and is clinically used to treat breast malignancy.. Cilengitide