Condition-dependent immunoregulatory control of contact sensitivity by levamisole

The immunostimulatory activity of levamisole [1-(-),2,3,5,6-tetrahydro-6-phenylimidazo(2,1-beta)-thiazole] was investigated using oxazolone-induced contact sensitivity in C57Bl mice. Oxazolone sensitivity was induced by applying 0.1 ml of 3 or 5% oxazolone in ethanol to the shaved abdomen (day 0). Challenge with oxazolone followed on day 2 (45 h) or day 3, and was accomplished by painting a 1--5% solution of oxazolone in ethanol to the left hind paw. The response at 24 h was determined plethysmographically. Levamisole (50 mg/kg, p.o., days 0--3 or day 0 only) failed to stimulate consistently the oxazolone response in a 3-day (minimal) sensitization period regimen. Use of a subliminal (45 h) sensitization, by contrast, revealed a consistent immunostimulatory effect of levamisole (12.5--100 mg/kg, p.o., day 0 only). It is speculated that the observed difference in levamisole effectiveness is attributable to (1) the ability of levamisole to stimulate both effector and suppressor mechanisms, and (2) the apparent absence of significant suppressor influence at 2-day postsensitization, leaving only the effector mechanism to be stimulated by levamisole.