Production and characterization of monoclonal antibodies that localize human thymidylate synthase in the cytoplasm of human cells and tissue.

Thymidylate synthase (TS; EC 2.1.1.45) is an important cellular enzyme that converts dUMP to dTMP, which is essential for DNA biosynthesis. In addition, TS is an important cellular target for the fluoropyrimidine cytotoxic drugs that are widely used in the treatment of solid tumors. We have generated five monoclonal antibodies against human TS using a recombinant human TS enzyme. These antibodies react specifically with human TS and display negligible cross-reactivity with other cellular proteins found in human cells. Binding affinity studies demonstrate that all antibodies form a tight interaction with recombinant human TS enzyme (Kd range = 0.3-11.0 nM). All antibodies display reactivity on enzyme-linked immunosorbent assay and immunoprecipitation. On Western blot analysis each detects a protein of approximately 36 kDa molecular mass under denaturing conditions. In addition to their reactivity on immunoprecipitation and Western analysis, two of the antibodies, TS 106 and TS 109, are reactive on immunohistochemical staining of human colon carcinoma cell lines and tissue, producing a granular cytoplasmic staining pattern. Specificity for TS is demonstrated by the lack of staining with preimmune IgG and the disappearance of the signal when the antibodies are preabsorbed with recombinant human TS enzyme. Quantitation of TS by Western blot analysis and biochemical FdUMP binding assay in 5-fluorouracil-resistant colon carcinoma cell lines (NCI H630R10, NCI H630R1) and a sensitive colon carcinoma cell line (NCI H630) revealed a 36- and 6-fold increase in TS in the resistant cell line as measured by the biochemical assay compared to a 39- and 10.6-fold increase as measured by densitometric analysis of the Western blot. These comparative studies of immunohistochemical, Western, and biochemical analyses reveal that the immunological detection of TS in human colon cell lines is a sensitive and quantitative assay. Thus the ability of these antibodies to detect TS in human cancer cells and tissue may allow measurement of TS in human tissues by quantitative immunohistochemistry in studies of drug resistance and for determination of proliferative rates.