Antibodies are an important class of pharmaceutical products that have been successfully used in the treatment of various human diseases and conditions, such as cancer, allergic diseases, prevention of transplant rejection and host-versus-graft disease.
A major problem of antibody preparations obtained from animals is the intrinsic immunogenicity of non-human immunoglobulins in human patients. In order to reduce the immunogenicity of non-human antibodies, it has been shown that by fusing animal variable (V) region exons with human constant (C) region exons, a chimeric antibody gene can be obtained.
Humanized monoclonal antibodies have also been developed and are in clinical use. Humanized monoclonal antibodies are typically human antibodies in which some CDR residues and possibly some FR residues are substituted by residues from analogous sites in non-human animal, e.g. rodent, antibodies. Humanization can be essentially performed following the method of Winter and co-workers (Jones et al., Nature, 321: 522 (1986); Riechmann et al., Nature, 332: 323 (1988); Verhoeyen et al., Science, 239: 1534 (1988)), by substituting non-human animal, e.g. rodent, CDRs or CDR sequences for the corresponding sequences of a human monoclonal antibody.
It has been described that the homozygous deletion of the antibody heavy-chain joining region (JH) gene in chimeric and germ-line mutant mice results in complete inhibition of endogenous antibody production. Transfer of the human germ-line immunoglobulin gene array in such germ-line mutant mice will result in the production of human antibodies upon antigen challenge. See, e.g., Jakobovits et al., Proc. Natl. Acad. Sci. USA, 90: 2551 (1993); Jakobovits et al., Nature, 362: 255 (1993); Bruggemann et al., Year in Immunol., 7: 33 (1993). While this genetic engineering approach resulted in the expression of human immunoglobulin polypeptides in genetically engineered mice, the level of human immunoglobulin expression is low. This may be due to species-specific regulatory elements in the immunoglobulin loci that are necessary for efficient expression of immunoglobulins. As demonstrated in transfected cell lines, regulatory elements present in human immunoglobulin genes may not function properly in non-human animals.
Indeed, several regulatory elements in immunoglobulin genes have been described. Of particular importance are enhancers downstream (3′) of heavy chain constant regions and intronic enhancers in light chain genes. In addition, other, yet to be identified, control elements may be present in immunoglobulin genes. Studies in mice have shown that the membrane and cytoplasmic tail of the membrane form of immunoglobulin molecules play an important role in expression levels of human-mouse chimeric antibodies in the serum of mice homozygous for the human Cγ1 gene. Therefore, for the expression of heterologous immunoglobulin genes in animals it is desirable to replace sequences that contain enhancer elements and exons encoding transmembrane (M1 exon) and cytoplasmic tail (M2 exon) with sequences that are normally found in the animal in similar positions.
In addition to the issues raised by the potential immunogenicity of the non-human antibodies, the use of monoclonal antibodies in general, whether chimeric, humanized or human, is further limited by the fact that devastating diseases, such as cancer and infections with virulent pathogens, are difficult to treat by attacking one target, due to their complexity, multifactorial etiology and adaptivity. Monoclonal antibodies directed against singularly defined targets fail when those targets change, evolve and mutate. Thus, malignancies may gain resistance to standard monoclonal antibody therapies. A solution to this problem is the use of polyclonal antibodies, which have the ability to target and attack a plurality of evolving targets linked with complex diseases. Polyclonal antibodies also have the ability to neutralize bacterial and viral toxins, and direct immune responses to kill and eliminate pathogens.
Accordingly, there is a great clinical need for a new approach suitable for the large-scale production of high-titer, high-affinity, humanized poly- and monoclonal antibodies.
The introduction of human immunoglobulin genes into the genome of mice resulted in expression of a diversified human antibody repertoire in genetically engineered mice. In both mice and humans, primary antibody diversity is generated by gene rearrangement. This process results in the generation of many different recombined V(D)J segments encoding a large number of antibody molecules with different antigen binding sites. However, in other animals, like rabbits, pigs, cows and birds, primary antibody diversity is generated by substantially different mechanisms, namely templated mutations or gene conversion and non-templated mutations or hypermutation. For example, it is well established that in rabbit and chicken, VDJ rearrangement is very limited (almost 90% of immunoglobulin is generated with the 3′ proximal VH1 element) and antibody diversity is generated by gene conversion and hypermutation. In contrast, mouse and human gene conversion occurs very rarely, if at all. Therefore, it is expected that in animals that diversify their primary antibody repertoire by gene conversion and hypermutation a genetic engineering approach based on gene rearrangement will result in animals with low antibody titers and limited antibody diversity. Thus, the genetic engineering of large animals for the production of non-immunogenic antibody preparations for human therapy requires alternative genetic engineering strategies.
The production of humanized antibodies in transgenic non-human animals is described in PCT Publication No. WO 02/12437, published on Feb. 14, 2002, the disclosure of which is hereby expressly incorporated by reference in its entirety. WO 02/12437 describes genetically engineered non-human animals containing one or more humanized immunoglobulin loci which are capable of undergoing gene rearrangement and gene conversion in transgenic non-human animals, including animals in which antibody diversity is primarily generated by gene conversion to produce diversified humanized antibodies. The humanized antibodies obtained have minimal immunogenicity to humans and are appropriate for use in the therapeutic treatment of human subjects. It further describes novel nucleotide sequences from the 5′ and 3′ flanking regions of immunoglobulin heavy chain constant region segments of various non-human mammalians, such as chickens, cows, sheep, and rabbits. Recombinant vectors in which human immunoglobulin heavy chain gene segments are flanked by sequences homologous to such 5′ and 3′ sequences are shown to be useful for replacing an immunoglobulin heavy chain gene segment of a non-human animal with the corresponding human immunoglobulin heavy chain gene segment.