Species-Specific Chromosome Engineering Greatly Improves Fully Human Polyclonal Antibody Production Profile in Cattle.

Large-scale production of fully human IgG (hIgG) or human polyclonal antibodies (hpAbs) by transgenic animals could be useful for human therapy. However, production level of hpAbs in transgenic animals is generally very low, probably due to the fact that evolutionarily unique interspecies-incompatible genomic sequences between human and non-human host species may impede high production of fully hIgG in the non-human environment. To address this issue, we performed species-specific human artificial chromosome (HAC) engineering and tested these engineered HAC in cattle. Our previous study has demonstrated that site-specific genomic chimerization of pre-B cell receptor/B cell receptor (pre-BCR/BCR) components on HAC vectors significantly improves human IgG expression in cattle where the endogenous bovine immunoglobulin genes were knocked out. In this report, hIgG1 class switch regulatory elements were subjected to site-specific genomic chimerization on HAC vectors to further enhance hIgG expression and improve hIgG subclass distribution in cattle. These species-specific modifications in a chromosome scale resulted in much higher production levels of fully hIgG of up to 15 g/L in sera or plasma, the highest ever reported for a transgenic animal system. Transchromosomic (Tc) cattle containing engineered HAC vectors generated hpAbs with high titers against human-origin antigens following immunization. This study clearly demonstrates that species-specific sequence differences in pre-BCR/BCR components and IgG1 class switch regulatory elements between human and bovine are indeed functionally distinct across the two species, and therefore, are responsible for low production of fully hIgG in our early versions of Tc cattle. The high production levels of fully hIgG with hIgG1 subclass dominancy in a large farm animal species achieved here is an important milestone towards broad therapeutic applications of hpAbs.

Acceleration of genetic gain in cattle by reduction of generation interval.

Genomic selection (GS) approaches, in combination with reproductive technologies, are revolutionizing the design and implementation of breeding programs in livestock species, particularly in cattle. GS leverages genomic readouts to provide estimates of breeding value early in the life of animals. However, the capacity of these approaches for improving genetic gain in breeding programs is limited by generation interval, the average age of an animal when replacement progeny are born. Here, we present a cost-effective approach that combines GS with reproductive technologies to reduce generation interval by rapidly producing high genetic merit calves.

Triple immunoglobulin gene knockout transchromosomic cattle: bovine lambda cluster deletion and its effect on fully human polyclonal antibody production.

Towards the goal of producing fully human polyclonal antibodies (hpAbs or hIgGs) in transchromosomic (Tc) cattle, we previously reported that Tc cattle carrying a human artificial chromosome (HAC) comprising the entire unrearranged human immunoglobulin (Ig) heavy-chain (hIGH), kappa-chain (hIGK), and lambda-chain (hIGL) germline loci produced physiological levels of hIgGs when both of the bovine immunoglobulin mu heavy-chains, bIGHM and bIGHML1, were homozygously inactivated (bIGHM-/-, bIGHML1-/-; double knockouts or DKO). However, because endogenous bovine immunoglobulin light chain loci are still intact, the light chains are produced both from the hIGK and hIGL genomic loci on the HAC and from the endogenous bovine kappa-chain (bIGK) and lambda-chain (bIGL) genomic loci, resulting in the production of fully hIgGs (both Ig heavy-chains and light-chains are of human origin: hIgG/hIgκ or hIgG/hIgλ) and chimeric hIgGs (Ig heavy-chains are of human origin while the Ig light-chains are of bovine origin: hIgG/bIgκ or hIgG/bIgλ). To improve fully hIgG production in Tc cattle, we here report the deletion of the entire bIGL joining (J) and constant (C) gene cluster (bIGLJ1-IGLC1 to bIGLJ5-IGLC5) by employing Cre/loxP mediated site-specific chromosome recombination and the production of triple knockout (bIGHM-/-, bIGHML1-/- and bIGL-/-; TKO) Tc cattle. We further demonstrate that bIGL cluster deletion greatly improves fully hIgGs production in the sera of TKO Tc cattle, with 51.3% fully hIgGs (hIgG/hIgκ plus hIgG/hIgλ).

Physiological level production of antigen-specific human immunoglobulin in cloned transchromosomic cattle.

Therapeutic human polyclonal antibodies (hpAbs) derived from pooled plasma from human donors are Food and Drug Administration approved biologics used in the treatment of a variety of human diseases. Powered by the natural diversity of immune response, hpAbs are effective in treating diseases caused by complex or quickly-evolving antigens such as viruses. We previously showed that transchromosomic (Tc) cattle carrying a human artificial chromosome (HAC) comprising the entire unrearranged human immunoglobulin heavy-chain (hIGH) and kappa-chain (hIGK) germline loci (named as κHAC) are capable of producing functional hpAbs when both of the bovine immunoglobulin mu heavy-chains, bIGHM and bIGHML1, are homozygously inactivated (double knockouts or DKO). However, B lymphocyte development in these Tc cattle is compromised, and the overall production of hpAbs is low. Here, we report the construction of an improved HAC, designated as cKSL-HACΔ, by incorporating all of the human immunoglobulin germline loci into the HAC. Furthermore, for avoiding the possible human-bovine interspecies incompatibility between the human immunoglobulin mu chain protein (hIgM) and bovine transmembrane α and ß immunoglobulins (bIgα and bIgß) in the pre-B cell receptor (pre-BCR) complex, we partially replaced (bovinized) the hIgM constant domain with the counterpart of bovine IgM (bIgM) that is involved in the interaction between bIgM and bIgα/Igß; human IgM bovinization would also improve the functionality of hIgM in supporting B cell activation and proliferation. We also report the successful production of DKO Tc cattle carrying the cKSL-HACΔ (cKSL-HACΔ/DKO), the dramatic improvement of B cell development in these cattle and the high level production of hpAbs (as measured for the human IgG isotype) in the plasma. We further demonstrate that, upon immunization by tumor immunogens, high titer tumor immunogen-specific human IgG (hIgG) can be produced from such Tc cattle.

Normal development following chromatin transfer correlates with donor cell initial epigenetic state.

If the full potential of chromatin transfer (CT) technology is to be realized for both animal production and biomedical applications it is imperative that the efficiency of the reprogramming process be improved, and the potential for deleterious development be eliminated. Generation of the first cloned animals from adult somatic cells demonstrated that development is substantially an epigenetic process (Wilmut I, Schnieke AE, McWhir J, Kind AJ, Campbell KH, 1997. Viable offspring derived from fetal and adult mammalian cells. Nature. 385(6619): 810-813.). In this study, we provide preliminary evidence that the epigenetic state of the donor cell, may be valuable in assessing potential cloning success. We have measured key indicators of cellular epigenetic state in both serially derived cell populations of the same genetic origin, but differing in epigenomic status, and in a distinct cohort of donor cell populations with diverse genetic origins and epigenomic status. Specifically, the relative abundance of particular histone modifications in donor populations prior to manipulation has been correlated with the measurable variance in reprogramming efficiencies observed following CT, as defined by the number of resulting live births and healthy progeny, and the concomitant incidence of deleterious growth measures (notably the appearance of large offspring syndrome (LOS)). Thus, we suggest that the likely outcome and relative success of cloning may be predictable based on the expression of discriminating histone marks present in the donor cell population before CT. This approach may provide the basis of a prognostic signature for the future evaluation and risk assessment of putative donor cells prior to CT, and thus increase future cloning success and alleviate the incidence of abnormal development.

Transcriptional reprogramming of gene expression in bovine somatic cell chromatin transfer embryos.

BACKGROUND: Successful reprogramming of a somatic genome to produce a healthy clone by somatic cells nuclear transfer (SCNT) is a rare event and the mechanisms involved in this process are poorly defined. When serial or successive rounds of cloning are performed, blastocyst and full term development rates decline even further with the increasing rounds of cloning. Identifying the "cumulative errors" could reveal the epigenetic reprogramming blocks in animal cloning. RESULTS: Bovine clones from up to four generations of successive cloning were produced by chromatin transfer (CT). Using Affymetrix bovine microarrays we determined that the transcriptomes of blastocysts derived from the first and the fourth rounds of cloning (CT1 and CT4 respectively) have undergone an extensive reprogramming and were more similar to blastocysts derived from in vitro fertilization (IVF) than to the donor cells used for the first and the fourth rounds of chromatin transfer (DC1 and DC4 respectively). However a set of transcripts in the cloned embryos showed a misregulated pattern when compared to IVF embryos. Among the genes consistently upregulated in both CT groups compared to the IVF embryos were genes involved in regulation of cytoskeleton and cell shape. Among the genes consistently upregulated in IVF embryos compared to both CT groups were genes involved in chromatin remodelling and stress coping. CONCLUSION: The present study provides a data set that could contribute in our understanding of epigenetic errors in somatic cell chromatin transfer. Identifying "cumulative errors" after serial cloning could reveal some of the epigenetic reprogramming blocks shedding light on the reprogramming process, important for both basic and applied research.

Antigen-specific human polyclonal antibodies from hyperimmunized cattle.

Antigen-specific human polyclonal antibodies (hpAbs), produced by hyperimmunization, could be useful for treating many human diseases. However, yields from available transgenic mice and transchromosomic (Tc) cattle carrying human immunoglobulin loci are too low for therapeutic applications. We report a Tc bovine system that produces large yields of hpAbs. Tc cattle were generated by transferring a human artificial chromosome vector carrying the entire unrearranged, human immunoglobulin heavy (hIGH) and kappa-light (hIGK) chain loci to bovine fibroblasts in which two endogenous bovine IgH chain loci were inactivated. Plasma from the oldest animal contained >2 g/l of hIgG, paired with either human kappa-light chain (up to approximately 650 microg/ml, fully human) or with bovine kappa- or lambda-light chain (chimeric), with a normal hIgG subclass distribution. Hyperimmunization with anthrax protective antigen triggered a hIgG-mediated humoral immune response comprising a high proportion of antigen-specific hIgG. Purified, fully human and chimeric hIgGs were highly active in an in vitro toxin neutralization assay and protective in an in vivo mouse challenge assay.

Production of cattle lacking prion protein.

Prion diseases are caused by propagation of misfolded forms of the normal cellular prion protein PrP(C), such as PrP(BSE) in bovine spongiform encephalopathy (BSE) in cattle and PrP(CJD) in Creutzfeldt-Jakob disease (CJD) in humans. Disruption of PrP(C) expression in mice, a species that does not naturally contract prion diseases, results in no apparent developmental abnormalities. However, the impact of ablating PrP(C) function in natural host species of prion diseases is unknown. Here we report the generation and characterization of PrP(C)-deficient cattle produced by a sequential gene-targeting system. At over 20 months of age, the cattle are clinically, physiologically, histopathologically, immunologically and reproductively normal. Brain tissue homogenates are resistant to prion propagation in vitro as assessed by protein misfolding cyclic amplification. PrP(C)-deficient cattle may be a useful model for prion research and could provide industrial bovine products free of prion proteins.

Sequential targeting of the genes encoding immunoglobulin-mu and prion protein in cattle.

Gene targeting is accomplished using embryonic stem cells in the mouse but has been successful, only using primary somatic cells followed by embryonic cloning, in other species. Gene targeting in somatic cells versus embryonic stem cells is a challenge; consequently, there are few reported successes and none include the targeting of transcriptionally silent genes or double targeting to produce homozygotes. Here, we report a sequential gene targeting system for primary fibroblast cells that we used to knock out both alleles of a silent gene, the bovine gene encoding immunoglobulin-mu (IGHM), and produce both heterozygous and homozygous knockout calves. We also carried out sequential knockout targeting of both alleles of a gene that is active in fibroblasts, encoding the bovine prion protein (PRNP), in the same genetic line to produce doubly homozygous knockout fetuses. The sequential gene targeting system we used alleviates the need for germline transmission for complex genetic modifications and should be broadly applicable to gene functional analysis and to biomedical and agricultural applications.

Cloned calves from chromatin remodeled in vitro.

We have developed a novel system for remodeling mammalian somatic nuclei in vitro prior to cloning by nuclear transplantation. The system involves permeabilization of the donor cell and chromatin condensation in a mitotic cell extract to promote removal of nuclear factors solubilized during chromosome condensation. The condensed chromosomes are transferred into enucleated oocytes prior to activation. Unlike nuclei of nuclear transplant embryos, nuclei of chromatin transplant embryos exhibit a pattern of markers closely resembling that of normal embryos. Healthy calves were produced by chromatin transfer. Compared with nuclear transfer, chromatin transfer shows a trend toward greater survival of cloned calves up to at least 1 mo after birth. This is the first successful demonstration of a method for directly manipulating the somatic donor chromatin prior to transplantation. This procedure should be useful for investigating mechanisms of nuclear reprogramming and for making improvements in the efficiency of mammalian cloning.

Cloned transchromosomic calves producing human immunoglobulin.

Human polyclonal antibodies (hPABs) are useful therapeutics, but because they are available only from human donors, their supply and application is limited. To address this need, we prepared a human artificial chromosome (HAC) vector containing the entire unrearranged sequences of the human immunoglobulin (hIg) heavy-chain (H) and lambda (lambda) light-chain loci. The HAC vector was introduced into bovine primary fetal fibroblasts using a microcell-mediated chromosome transfer (MMCT) approach. Primary selection was carried out, and the cells were used to produce cloned bovine fetuses. Secondary selection was done on the regenerated fetal cell lines, which were then used to produce four healthy transchromosomic (Tc) calves. The HAC was retained at a high rate (78-100% of cells) in calves and the hIg loci underwent rearrangement and expressed diversified transcripts. Human immunoglobulin proteins were detected in the blood of newborn calves. The production of Tc calves is an important step in the development of a system for producing therapeutic hPABs.