Humanized Antibodies: Key Technology for Effective Immunotherapies

Harnessing the power of the immune system for therapeutic use in human disease is not a new idea, but recent advances in biotechnology have brought new precision to the way physicians and researchers approach therapy development.  Monoclonal antibodies (mAbs) have offered real progress toward fighting many autoimmune diseases and several forms of cancer, turning immunotherapy into a multibillion dollar segment of the biopharmaceutical industry.  An estimated 37 million people are afflicted with cancer or an autoimmune disease in the United States alone, making advances in these therapies impactful for improving survival rate and quality of life for millions of patients world-wide.  As more antigens are linked to cancer, promising mAb therapies are emerging which target and block certain cancer-specific antigens.  These antigens are often functional parts of the cancer cells, or aid in the function of cells and expedite cancer growth.  MAbs are also developed to target cancer cells in the body by attaching to them, thus marking them to be eliminated by the body’s immune system.  Conjugated mAbs use specific antibodies as a homing device to deliver a deadly dose of cancer-killing agents or radioactive substances to cancerous cells in the body.  Autoimmune disorders often manifest with a concentrated attack on a specific organ system caused by immune reactivity to particular self antigens.  Identifying these antigens as the targets of mAb therapies could offer significant progress in treating diseases including multiple sclerosis, psoriasis, rheumatoid arthritis, Crohn’s disease and ulcerative colitis.

Antibody therapy as we know it today began in 1975, when scientists Cesar Milstein and Georges J. F. Kohler pioneered technology to produce monoclonal antibodies by creating the first hybridoma.  To produce hybridoma cells, scientists inject mice with an antigen linked with the particular immune response they are interested in triggering.  Mice are then screened for production of the desired antibodies and if a sufficient level is detected, B cells (the type of cells that produce antibodies) are harvested from the spleen to be used in the hybridoma.  Spleen cells on their own have a very limited lifespan, so they must be fused with immortal myeloma cells to increase their longevity and ability to reproduce.  This resulting hybrid cell can multiply indefinitely and is capable of producing antibodies at a volume large enough to be used for therapeutic or diagnostic applications. These initial antibodies were murine, meaning both cell lines were derived from mice.  However, differences between mouse and human immune systems caused clinical failure of many murine antibody therapies due to immunogenicity.  This undesired response to immunotherapy happens when the antibody being introduced is seen as a foreign protein by the body’s immune system and prompts a sever immune response in the patient.  Unlike vaccines, activating the immune system in this way can render mAbs ineffective or trigger an allergic reaction in the body such as anaphylaxis, or cause the rapid release of proinflammatory cytokines, known as cytokine release syndrome.

To decrease the chance of immunogenicity, chimeric antibodies were developed which fused murine antibody variable (antigen binding) regions with human antibody constant (effector) regions.  Lower immunogenicity allows chimeric antibodies to be used in biotherapeutics, assay development, and diagnostics.  As antibody engineering technology improved, the first humanized antibodies were created hoping to fully address the issue of immunogenic response in patient populations.  However, immunogenicity still proves to be an obstacle in immunotherapies, prompting the FDA to publish a guidance document for the industry on immunogenicity assessment for therapeutic protein products.  For biopharmaceutical companies seeking to launch new immunotherapies, the production and validation of humanized antibodies is a critical component in drug research.  There are several methods of humanization employed in antibody engineering:

• CDR grafting – Combines antibody variables called complementarity-determining regions (CDRs) which determine where antibodies bind to a particular antigen, with human constants.  Antibody specificity and antigen affinity are retained by utilizing residues associated with antigen binding. This results in an antibody that is mostly human, with only CDRs from nonhuman origin.
• Phage display – A process of using simple organisms, such as bacteriophages, to display antibodies or antibody fragments which are genetically fused to the phage coat protein.  The bacteriophage are genetically engineered through repeated cycles of antigen-guided selection, used to create a human phage display library, and then screened for binding affinity to a specific antigen.
• Transgenic animals – Mice are genetically engineered with introduced human antibody heavy and light chain gene sequences, along with targeted modification of endogenous mouse antibody genes in order to suppress their expression.  What results is a transgenic mouse which can produce fully human antibody repertoires.

Antibody engineering techniques vary depending on the target antigen and application, however robust characterization is an essential part of successful antibody production.  Assays to determine appropriate end-use effectiveness include screening for a cross-reaction with other protein species, checking for affinity requirements, application-specific viability such as immunohistochemistry, and inclusion of control studies at each stage.  Due to the complexity of antibody engineering and rigor required in mAb production, working with knowledgeable collaborators is key in the success of humanization service projects.  

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