Comparison of a radioisotopic incorporation method and the mouse ear swelling test (MEST) for contact sensitivity to weak sensitizers.

A radioisotopic incorporation assay utilizing [125I]iododeoxyuridine was compared to the standard mouse ear swelling test (MEST) for the strong sensitizers dinitrofluorobenzene and oxazolone, and for the three weak sensitizers ethylenediamine (EDA), glutaraldehyde, and nickel sulfate. Mice were sensitized epicutaneously on the abdomen for 4 consecutive days prior to challenging the left ear with the test agent and the right ear with the vehicle. A comparison of the mean difference between the test and the control ears showed that measuring reactivity 48 hr postchallenge on Day 7 is the most sensitive time period in the radioisotopic incorporation method. Both the isotopic and MEST assays gave positive results with the potent sensitizers, although the response detected by isotopic labeling of emigrating cells was up to 1000-fold greater than that determined by ear swelling measurements. No response was detected to the moderate to weak sensitizer EDA in either assay. Reactivity to glutaraldehyde was not detected by the radioisotopic assay but was minimally responsive and significant by the MEST. The opposite was true for nickel sulfate where minimal but significant reactivity was seen in the isotopic assay but not in the MEST. Although the radioisotopic assay had the advantages of being more quantitative and of having improved sensitivity, it was of no greater value than the MEST for detecting weak sensitizers. It was concluded that the mouse was not a suitable model for routinely detecting reactivity to weak sensitizers regardless of which of the two assays were used.