Monoclonal antibodies (mAb) are antibodies that are identical because they were produced by one type of immune cell, all clones of a single parent cell. Given any substance, it is possible to create monoclonal antibodies that specifically bind to that substance; they can then serve to detect or purify that substance. This has become an important tool in biochemistry, molecular biology and medicine.
If a foreign substance (an antigen) is injected into a vertebrate such as a mouse or a human, some of the immune system's B-cells will turn into plasma cells and start to produce antibodies that bind to that antigen. Each B-cell produces only one kind of antibody, but different B-cells will produce structurally different antibodies that bind to different parts of the antigen. This mixture of antibodies is known as polyclonal antibodies.
To produce monoclonal antibodies, one removes B-cells from the spleen of an animal that has been challenged with the antigen. These B-cells are then fused with myeloma tumor cells that can grow indefinitely in culture (myeloma is a B-cell cancer). This fusion is done by making the cell membranes more permeable. The fused hybrid cells (called hybridomas) will multiply rapidly and indefinitely (since they are cancer cells after all) and will produce large amounts of antibodies. The hybridomas are sufficiently diluted and grown, thus obtaining a number of different colonies, each producing only one type of antibody. The antibodies from the different colonies are then tested for their ability to bind to the antigen (for example with a test such as ELISA), and the most effective one is picked out.
Monoclonal antibodies can be produced in cell culture or in animals. When the hybridoma cells are injected in mice (in the peritoneal cavity, the gut), they produce tumors containing an antibody-rich fluid called ascites fluid.
Once monoclonal antibodies for a given substance have been produced, they can be used to detect for the presence and quantity of this substance, for instance in a Western blot test (to detect a substance in a solution) or an immunofluorescence test (to detect a substance in a whole cell). Monoclonal antibodies can also be used to purify a substance with techniques called immunoprecipitation and affinity chromatography.
In medicinal treatments, the small variation (if any) in recognizing the antigen helps to reduce side effects. However, there are drawbacks to using monoclonal antibodies as opposed to polyclonals. Each B-lymphocyte produces antibodies that are specific not to an antigen, but to an epitope of that antigen. An epitope is a small piece of the antigen to which the antibody binds. Polyclonal antibodies bind to many epitopes of a given antigen, while monoclonals bind to a single epitope. In the processing of antibodies, certain binding capabilities are degraded. If the monoclonal antibody is susceptible to such degradation, it is useless. Polyclonals will still be useful even if certain epitope-binding species are degraded.
Another problem in medical applications is that the standard procedure of producing monoclonal antibodies yields mouse antibodies, and these are rejected by the human immune system. Various approaches to overcome this problem have been tried. In one approach, one takes the DNA that encodes the binding portion of monoclonal mouse antibodies and merges it with human antibody producing DNA, in order to make bacteria produce antibodies that are half mouse and half human. Another approach involves genetically engineered mice that produce more human-like antibodies.
Monoclonal antibodies for cancer treatment
One possible treatment for cancer involves monoclonal antibodies that bind only to cancer cells specific antigen and induce an immunological response on the target cancer cell. Such mAb could also be modificated for delivery of a toxin, radioisotope, cytokine or other active conjugate; it is also possible to design bispecific antibodies that can bind both to target antigen and conjugate or effector cell. The picture below shows all these possibilities:
Monoclonal antibodies for cancer. ADEPT, antibody directed enzyme prodrug therapy; ADCC, antibody dependent cell-mediated cytotoxicity; CDC, complement dependent cytotoxicity; MAb, monoclonal antibody; scFv, single-chain Fv fragment. Modified from Carter P: Improving the efficacy of antibody-based cancer therapies. Nat Rev Cancer 2001;1:118-129
The idea of a "magic bullet" was first proposed by Paul Ehrlich who in the beginning of the century figured that if a compound could be made that selectively targeted a disease causing organism, then a toxin for that organism could be delivered along with the agent of selectivity.
In the 1970s, the B-cell cancer myeloma was known, and it was understood that these cancerous B-cells all produce a single type of antibody. This was used to study the structure of antibodies, but it was not possible to produce identical antibodies specific to a given antigen.
The process of producing monoclonal antibodies described above was invented by Georges K÷hler and CÚsar Milstein in 1975; they shared the Nobel Prize in Physiology or Medicine in 1984 for the discovery.
The key idea was to use a line of myeloma cells that had lost their ability to secrete antibodies, and come up with a technique to fuse these cells with healthy antibody producing B-cells.