Blood proteins from transgenic animal bioreactors

Abstract

T HE TRANSMISSION of viruses by blood transfusion and replacement therapy with human plasma proteins has caused serious concern in recent years, eliciting several changes in the preparation of blood products. The selection of donors, rigorous screening of collected blood, and virucidal procedures introduced in the purification of plasma proteins have nearly eliminated the problems of human immunodeficiency virus and hepatitis B virus transmission, and plasma-derived proteins are generally considered to be safe. ~ Past experiences of plasma fractionators with product safety have made them cautious, as shown by the voluntary withdrawal of intravenous immunoglobulin (IGIV) products in 1994 after reports of hepatitis C transmission in Europe, of albumin and factor VIII (FVIII) concentrates containing a unit of blood from a Creutzfeldt-Jakob disease (CJD)-affected donor, and several intramuscular ilnmunoglobulin products in early 1995. The discussion on the contamination of plasma products by infectious agents that could transmit neurodegenerative diseases like CJD or Gerstmann-Str~iussler-Scheinker disease, on transfusion-associated bacterial sepsis and parasitic Chagas disease is ongoing. The production of recombinant human plasma proteins is considered to be an alternative, which will increase the available amount and safety of product. More important, the lower cost of production will permit its use in prophylaxis or for oral administration. As a result, new treatment regimens, such as the topical application of a~ antitrypsin (a1AT) in psoriasis may become feasible. Indeed, the increasing acceptance of recombinant FVIII (rFVIII) by both clinicians and patients has provided added impetus to the search for new sources of plasma proteins. The widespread use of transgenic animals in basic research, 2 development of human disease models, 3 and genetic improvement of livestock 4 have led to considerable interest in employing genetically engineered animals for the production of therapeutic human proteins, 5,6 as an alternative or a complement to eukaryotic cell culture systems. Human plasma proteins being considered for expression are those commonly used in therapy, such as factor IX (FIX), 7,8 FVIII, 9 protein C (PC), 1~ antithrombin III (ATIII) (H. Meade, personal communication, February 1995), as well as serum albumin (HSA) ~2, ~I-AT, 13-14 hemoglobin (Hb),~s and fibrinogen (Velander et al: Unpublished observations, February 1994). In the transgenic approach, synthesis of a recombinant protein is targeted to a selected cell type or organ, enabling the product to be harvested from body fluids like milk, blood, saliva, or urine. Since 1987, numerous proteins with therapeutic potential have been successfully collected by noninvasive methods from transgenic mice or farm animals like rabbits, sheep, pigs, and goats, 16A7 leading to the commercial development of well-established technologies. Questions with respect to transgene inheritance and stability, appropriate posttranslational modifications on heterologous proteins, industrial production procedures, and regulatory affairs have now emerged, as there are limited data published on the long-term effects of foreign protein expression on transgenic animal bioreactor (TAB). In this review, the authors discuss the benefits and limitations of transgenic production of human blood proteins, and express their views on its possible impact on transfusion medicine.