I

Figure 1 Comparison of conventional anti-sera production in vitro. By cloning the B cell-derived hybridoma lines, the antibody (Ab) products of each individual clone can be separated and propagated indefinitely. In this way individual epitopes (Ep) of an antigen (Ag) can be analyzed because of the monospecificity of monoclonal antibodies.

The method most commonly in use was developed by Littlefield and relies on the initial isolation of mutant plasmacytomas (Figure 2) lacking the enzymes responsible for the reincorporation of purines and pyrimidines into DNA and RNA (Table 1). Cells that have mutations resulting in the functional absence of these enzymes will be the only cells to survive when these toxic base analogs are incorporated into the growth media. Thymidine kinase-negative (TK ) or hypoxanthine guanine phos-phoribosyl transferase-negative (HGPRT-) plasmacytoma cells can be selected, cloned and isolated in this way.

Once such mutant lines have been isolated it is

Table 1 Toxic base analogs and associated enzyme deficiency

Salvage pathways

Hypoxanthine

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