A new dual immunocytochemical assay for oestrogen receptor (ER) and epidermal growth factor receptor (EGFR) has been developed. It has been tested in a variety of conditions using cell culture lines and the results correlate well with those obtained from single immunocytochemical assays.
MCF-7 and A43I cells and a mixture of the two types of cell were assessed immunocytochemically for ER and EGFR. ER showed immunopositivity of 30% in MCF-7 cells, I0% in the mixture and 0% in A43I cells. EGFR demonstrated immunopositivity of 0% in MCF-7 cells, 70% in the mixture and 100% in A431 cells. Dual immunocytochemical assays using anti-ER followed by anti-EGFR monoclonal antibodies on single histological sections showed similar reactivity to the single assays. Three staining patterns were seen in the mixture: ER+/EGFR-(MCF-7 cells), ER-/EGFR-(MCF-7 cells) and ER-/EGFR+ (A431 cells). ZR-75-I and MDA-MB-231 cells and their retrovirally transfected counterparts ZR/HERc and MDA/HEGO cells were then analysed. The dual assay revealed the fourth phenotype (ER+/EGFR+)in 40% of ZR/HERc cells and in 10% of MDA/HEGO cells. This is the first description of a dual immunocytochemical assay system for ER and EGFR on single 5 ~m frozen section samples. Studies are now underway assessing breast carcinoma sections which may allow investigation of the clonality of human breast cancer.