A quantitative study of methanol/sorbitol co-feeding process of a Pichia pastoris Mut⁺/pAOX1-lacZ strain.

BACKGROUND: One of the main challenges for heterologous protein production by the methylotrophic yeast Pichia pastoris at large-scale is related to its high oxygen demand. A promising solution is a co-feeding strategy based on a methanol/sorbitol mixture during the induction phase. Nonetheless, a deep understanding of the cellular physiology and the regulation of the AOX1 promoter, used to govern heterologous protein production, during this co-feeding strategy is still scarce. RESULTS: Transient continuous cultures with a dilution rate of 0.023 h(-1) at 25°C were performed to quantitatively assess the benefits of a methanol/sorbitol co-feeding process with a Mut+ strain in which the pAOX1-lacZ construct served as a reporter gene. Cell growth and metabolism, including O2 consumption together with CO2 and heat production were analyzed with regard to a linear change of methanol fraction in the mixed feeding media. In addition, the regulation of the promoter AOX1 was investigated by means of ß-galactosidase measurements. Our results demonstrated that the cell-specific oxygen consumption (qO2) could be reduced by decreasing the methanol fraction in the feeding media. More interestingly, maximal ß-galactosidase cell-specific activity (>7500 Miller unit) and thus, optimal pAOX1 induction, was achieved and maintained in the range of 0.45 ~ 0.75 C-mol/C-mol of methanol fraction. In addition, the qO2 was reduced by 30% at most in those conditions. Based on a simplified metabolic network, metabolic flux analysis (MFA) was performed to quantify intracellular metabolic flux distributions during the transient continuous cultures, which further shed light on the advantages of methanol/sorbitol co-feeding process. Finally, our observations were further validated in fed-batch cultures. CONCLUSION: This study brings quantitative insight into the co-feeding process, which provides valuable data for the control of methanol/sorbitol co-feeding, aiming at enhancing biomass and heterologous protein productivities under given oxygen supply. According to our results, ß-galactosidase productivity could be improved about 40% using the optimally mixed feed.

The presentation of neuroendocrine self-peptides in the thymus: an essential event for individual life and vertebrate survival.

Confirming Burnet's early hypothesis, elimination of self-reactive T cells in the thymus was demonstrated in the late 1980s, and an important question immediately arose about the nature of the self-peptides expressed in the thymus. Many genes encoding neuroendocrine-related and tissue-restricted antigens (TRAs) are transcribed in thymic epithelial cells (TECs). They are then processed for presentation by proteins of the major histocompatibility complex (MHC) expressed by TECs and thymic dendritic cells. MHC presentation of self-peptides in the thymus programs self-tolerance by two complementary mechanisms: (1) negative selection of self-reactive "forbidden" T cell clones starting already in fetal life, and (2) generation of self-specific thymic regulatory T lymphocytes (tTreg cells), mainly after birth. Many studies, including the discovery of the transcription factors autoimmune regulator (AIRE) and fasciculation and elongation protein zeta family zinc finger (FEZF2), have shown that a defect in thymus central self-tolerance is the earliest event promoting autoimmunity. AIRE and FEZF2 control the level of transcription of many neuroendocrine self-peptides and TRAs in the thymic epithelium. Furthermore, AIRE and FEZF2 mutations are associated with the development of autoimmunity in peripheral organs. The discovery of the intrathymic presentation of self-peptides has revolutionized our knowledge of immunology and is opening novel avenues for prevention/treatment of autoimmunity.

Generation of high-affinity human antibodies by combining donor-derived and synthetic complementarity-determining-region diversity.

Combinatorial libraries of rearranged hypervariable V(H) and V(L) sequences from nonimmunized human donors contain antigen specificities, including anti-self reactivities, created by random pairing of V(H)s and V(L)s. Somatic hypermutation of immunoglobulin genes, however, is critical in the generation of high-affinity antibodies in vivo and occurs only after immunization. Thus, in combinatorial phage display libraries from nonimmunized donors, high-affinity antibodies are rarely found. Lengthy in vitro affinity maturation is often needed to improve antibodies from such libraries. We report the construction of human Fab libraries having a unique combination of immunoglobulin sequences captured from human donors and synthetic diversity in key antigen contact sites in heavy-chain complementarity-determining regions 1 and 2. The success of this strategy is demonstrated by identifying many monovalent Fabs against multiple therapeutic targets that show higher affinities than approved therapeutic antibodies. This very often circumvents the need for affinity maturation, accelerating discovery of antibody drug candidates.

Rapid generation of functional human IgG antibodies derived from Fab-on-phage display libraries.

We introduce a procedure for the rapid generation of fully human antibodies derived from "Fab-on-phage" display libraries. The technology is based on the compatibility of display vectors and IgG expression constructs, and allows reformatting of individual Fab clones to IgG, as well as reformatting of antibody repertoires. Examples of batch reformatting of an uncharacterized Fab repertoire and of a pool of Fabs, previously analyzed at the phage level, are presented. The average transient expression levels of the IgG constructs in HEK293T cells are above 10 microg/ml, allowing the use of conditioned media in functional assays without antibody purification. Furthermore, we describe a high-throughput purification method yielding IgG amounts sufficient for initial antibody characterization. Our technology allows the generation and production of antigen-specific complete human antibodies as fast or even faster than raising monoclonal antibodies by conventional hybridoma techniques.

Use of Staby(®) technology for development and production of DNA vaccines free of antibiotic resistance gene.

The appearance of new viruses and the cost of developing certain vaccines require that new vaccination strategies now have to be developed. DNA vaccination seems to be a particularly promising method. For this application, plasmid DNA is injected into the subject (man or animal). This plasmid DNA encodes an antigen that will be expressed by the cells of the subject. In addition to the antigen, the plasmid also encodes a resistance to an antibiotic, which is used during the construction and production steps of the plasmid. However, regulatory agencies (FDA, USDA and EMA) recommend to avoid the use of antibiotics resistance genes. Delphi Genetics developed the Staby(®) technology to replace the antibiotic-resistance gene by a selection system that relies on two bacterial genes. These genes are small in size (approximately 200 to 300 bases each) and consequently encode two small proteins. They are naturally present in the genomes of bacteria and on plasmids. The technology is already used successfully for production of recombinant proteins to achieve higher yields and without the need of antibiotics. In the field of DNA vaccines, we have now the first data validating the innocuousness of this Staby(®) technology for eukaryotic cells and the feasibility of an industrial production of an antibiotic-free DNA vaccine. Moreover, as a proof of concept, mice have been successfully vaccinated with our antibiotic-free DNA vaccine against a deadly disease, pseudorabies (induced by Suid herpesvirus-1).