ATCS Inc has extensive experience to offer the antibody humanization service for therapeutic and diagnostic development. We have successfully performed dozens of mouse humanization projects during the past decade. We also provide humanization service to antibodies derived from other species, such as non-human primates (NHP), rabbits, dogs, chickens, llama and etc.
Humanization is important for reducing the immunogenicity of monoclonal antibodies derived from xenogeneic sources (commonly rodents) and for improving their activation of the human immune system. Since the development of the hybridoma technology, a large number of rodent monoclonal antibodies with specificity for antigens of therapeutic interest have been generated and characterized. Rodent antibodies are highly immunogenic in humans, which limits their clinical applications, especially when repeated administration is required. Importantly, they are rapidly removed from circulation and can cause systemic inflammatory effects as well. As a means of circumventing these problems, we have developed three antibody humanization strategies that can preserve the specificity and affinity of the antibody toward the antigen while significantly or completely eliminating the immunogenicity of the antibody in humans. The first approach is CDR grafting and the second is chain shuffling. These two methods are all based on phage display of humanized scFv variants and selection of high-affinity humanized binders through bio-panning. The third method, humanized IgG library screening, is somehow unique. We will make a library of humanized whole IgG to be displayed on the surface of mammalian cells and then high-affinity binders will be sorted by FACS.
CDR Grafting & SDR Grafting
We have established a CDR (complementarity-determining region) grafting platform, which is featured with randomization of a small set of framework residues using phage display technology and computer modeling. In this platform, six CDR loops comprising the antigen-binding site are grafted into corresponding human framework regions. Unfortunately, simple grafting of the rodent CDRs into human frameworks does not always reconstitute the binding affinity and specificity of the original antibody because framework residues are involved in antigen binding, either indirectly, by supporting the conformation of the CDR loops, or directly, by contacting the antigen. For this reason, we have developed a computer modeling method to randomize certain framework residues in addition to CDR grafting. The grafted CDRs together with the randomized residues are cloned into a phage display library and the humanized antibodies with the best affinity are selected by screening of the library. This approach allows the epitope specificity of the original antibody to be retained. Of note, humanization by this approach is not 100% since the CDR regions are still of rodent origin.
To reduce the immunogenicity of CDR-grafted humanized antibodies, the murine content in the CDR-grafted humanized antibodies is minimized through SDR grafting. Within each CDR, there are more variable positions that are directly involved in the interaction with antigen, i.e., specificity-determining residues (SDRs), whereas there are more conserved residues that maintain the conformations of CDRs loops. SDRs may be identified from the 3D structure of the antigen-antibody complex and/or the mutational analysis of the CDRs. An SDR-grafted humanized antibody is constructed by grafting the SDRs and the residues maintaining the conformations of the CDRs onto the human template, and its immunogenic potential is evaluated by measuring the reactivity to the sera from patients who had been immunized with the parental antibody.
We have also optimized a chain shuffling strategy that is an entirely selective humanization strategy based on the construction and screening of two chimeric phage display libraries. In this approach, the light chain of the rodent antibody is first replaced by light chains in one of our well-tested human antibody libraries; the resulting hybrid antibody library is then screened by panning against the particular antigen. After that, the heavy chain of the selected hybrid antibodies is replaced by heavy chains of the human antibody library. Subsequent screening of this secondary chimeric library will produce fully humanized antibodies. Since phage display library screening mimics in vivo antibody selection and evolution procedure, chain shuffling can result in humanized antibodies whose affinities are higher than that of the original antibody. Also, this sequential chain shuffling procedure can generate several versions of humanized antibodies with different sequences. The production of multiple humanized antibodies retaining the same epitope specificity is important in therapeutic regimens that call for long-term treatment with antibodies in which anti-idiotypic responses might be avoided by the administration of alternative antibodies.
As elaborated above, the "chain shuffling" method based on chimeric phage display library construction and screening allow full, i.e. 100% humanization of a mouse antibody. We would like to propose using our HuScL-2TM Phage Display Naive Human scFv Library with a complexity of 1.42×109 transformants as the backbone of the chimeric libraries and the donor of human VL and VH chains.
Humanized IgG Library Screening
In this method, a mammalian cell surface display library is made to display full-size humanized IgG variants that are further selected using FACS. First, we will select an acceptor human VH and a receptor VL from a subgroup of human antibodies based on consensus sequences. Next, CDR grafting is conducted as described above. In order to increase (or keep) the affinity of the humanized antibodies, a mammalian cell surface display IgG library is created to display all possible variants of the humanized IgG. Since the size limit for the mammalian cell surface displays IgG library, decisions must be made as to which amino acids to diversify and to what extent so that there are fewer nonsense antibody mutants that waste the capacity of the library. Back mutations in framework regions are designed based on computer modeling or antigen/antibody structure information. Also, we will distinguish between residues with solvent-accessible side chains from those with buried side chains. We have confirmed that randomization of residues with buried side chains is a waste of library sequence space. Also, we do not mutate glycines and tryptophans since changes in these residues usually abolish binding. Sometimes, we study the AA sequence of the parent antibody to find out conserved framework positions (in comparison with germ-line and antibody subfamily sequences). We may then introduce mutations to the positions in the framework regions that are not conserved. Supposedly, these regions will be antigen-specific and changes in these regions may increase affinity but not immunogenicity. In order to create the largest possible library, trimer codon technology is employed to randomize those defined framework residues on both humanized heavy-chain and humanized light-chain.
Finally, the humanized IgG library is created, in which the cDNA of the heavy chain variants (with positions to be randomized using trimer codons) are cloned into a mammalian cell surface display vector, while the cDNA of the light chain variants are inserted into a separate mammalian expression vector to be expressed as free (not membrane-anchored proteins). The cDNA of the heavy-chain library and the light-chain library is then mixed and transfected into 293 cells for surface display of functional whole-size humanized IgG variants. Top binders are then sorted by FACS. After stringent selections, a fraction of ELISA-positive mutant binders will have a higher affinity than the parent antibody. The affinity of the positive clones will be ranked using the Surface Plasmon Resonance instrument Biacore. Top 3-5 clones will then be expressed and purified to measure the affinity.
This method allows the selection of humanized antibodies in a full-size IgG format that retains or increases the original affinity of the mouse antibody. Also, in comparison with the above elaborated bacterial phage display based methods, this mammalian cell surface display approach allows selection of high affinity humanized antibodies in a dimeric IgG format; selected IgG are immediately good for further downstream applications—avoiding time-consuming conversion of scFv to IgG and codon optimization of converted IgG (for expression in mammalian cells), which sometime deselect some humanized scFvs. Also, some good antibodies have sequences that prevent protein synthesis and phage display in bacterial cells; these problems can be well overcome in this exclusively mammalian cells-based system.
Features of Our Humanization Services
There are two features in our antibody humanization services that single out us from our peers. First of all, antibody affinity maturation is an integrated step in our humanization procedure, thus there is no need to improve the affinity after humanization. Secondly, in addition to the common computational and biochemical methods, we have developed a proprietary in vivo approach to evaluate the immunogenicity of the humanized antibodies in primates. The immunogenicity measured in primates is the closest one that may mimic the true immunogenicity of the humanized antibodies in humans.
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