Positive responses to imipramine in the popliteal lymph node assay are due to primary irritation

The popliteal lymph node (PLN) assay has long been proposed as a tool to detect immunotoxicants with the potential to induce systemic autoimmunity. A major problem hampering the further validation of this assay is the need to rule out irritants that cause false-positive PLN responses. The anti-depressant, imipramine, has not been reported to induce systemic autoimmune reactions in treated patients, but has been repeatedly found positive in the PLN assay, suggesting that this is a false-positive response. To test this hypothesis, the effects of imipramine were compared to those of 50% ethanol in C57B1/6 mice. Footpad edema was evidenced in the few days after injection of both ethanol and imipramine. T-cell depletion using monoclonal antibodies against either CD4+ or CD8+ T-lymphocytes prior to the PLN assay did not influence the responses to either ethanol or imipramine. Cytokine (TNFalpha, IL-1alpha, IL-1beta, IL-2R, IL-6, IL-12 and IFN-gamma) fingerprinting of the PLNs after injection of ethanol and imipramine evidenced the same pattern of responses. These results indicate a closely similar pattern of responses following the footpad injection of either imipramine or ethanol. The conclusion can be drawn that imipramine induces positive responses in the PLN assay via primary (nonspecific) irritation.