Can a repeat exposure help distinguish an allergy-causing chemical from a simple irritant in mice?

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The mouse popliteal lymph node assay (PLNA) has been proposed as an immunotoxicological test to predict which chemicals are allergenic. However, some irritants also produce a popliteal lymph node (PLN) response associated with non-specific activation.

In a controlled experiment in male ICR mice, researchers examined how PLN cellularity indices changed over time during primary and secondary responses. They used flow cytometric analysis to measure the proportions of T and B cells in the PLN. Mice received a subcutaneous injection of TNBS, an allergenic compound, or SDS, an irritant compound, into the left hind footpad, and vehicle into the right hind footpad. On day 28 after the first injection, mice received another injection at 1/10 or 1/100 of the initial dose.

After the first TNBS injection, the primary response reached its maximum PLN cellularity index of 16.5 on day 7. After the second injection, the secondary response reached its maximum index of 13.1 on day 2. A marked increase in the proportion of B cells in the PLN was also observed with TNBS.

After the first SDS injection, the primary response reached its maximum PLN cellularity index of 2.8 on day 10. However, neither a secondary response nor an increase in the proportion of B cells was observed with SDS.

The results suggest that detecting a secondary response and using flow cytometric analysis are useful for distinguishing responses to allergenic and irritant compounds in the PLNA.