MONOCLONAL ANTIBODIES FOR CLINICAL USE
There are many clinical uses for antibodies. They are used in diagnostic procedures (including the ELISA-see later discussion), for pregnancy testing, and to detect the presence of proteins characteristic of particular disease-causing agents. In the future, they may be used to specifically kill cancer cells or destroy viruses. Such uses need relatively large amounts of a pure antibody that specifically recognizes a single antigen. Even if an experimental animal is inoculated with a purified single antigen, its blood serum will contain a mixture of antibodies to that antigen. Remember that a single antigen has multiple epitopes, and thus antibodies will vary in both specificity and affinity. Nowadays, such a mixture is referred to as polyclonal antibody because it results from antibody production by many different clones of B cells, which all recognized the same antigen. Such a mixture is of little use either for a specific, accurate assay or for other techniques in biotechnology.
Somehow, a single line of B cells all making one particular antibody must be isolated and grown in culture. Such a pure antibody made by a single line of cells is known as a monoclonal antibody. Unfortunately, B cells live for only a few days and survive poorly outside the body. The solution to this problem is to use cancer cells. Myelomas are naturally occurring cancers derived from B cells and which therefore express immunoglobulin genes. Like many tumor cells, myeloma cells will continue to grow and divide in culture forever if given proper nutrients. To make monoclonal antibodies, the relatively delicate B cell, which is making the required antibody, is fused to a myeloma cell (Fig. 6.8). (To avoid confusion, a myeloma that has lost the ability to make its own antibody is used.) The resulting hybrid is called a hybridoma. In principle it can live forever in culture and will make the desired antibody.
In practice, an animal, such as a mouse, is injected with the antigen against which antibodies are needed. When antibody production has reached its peak, a sample of antibody-secreting B cells is removed from the animal. These are fused to immortal myeloma cells to give a mixture of many different hybridoma cells. The tedious part comes next. Many individual hybridoma cell lines must be screened to find one that recognizes the target antigen. Once found, the hybridoma is grown in culture to give large amounts of the monoclonal antibody.