The reversal of established enhancement in rat cardiac allografts.

Established enhancement in rat cardiac allografts was challenged in a variety of ways in an attempt to provoke rejection. Incompatible skin grafts, injections of sensitised lymphocytes, and the administration of the macrophage-stimulating agent Corynebacterium parvum proved ineffective. However, levamisole, which stimulates both macrophages and sensitised lymphocytes, caused rejection in four of a group of six AS rats bearing enhanced (August X AS)F1 hybrid heart allografts. A combination of C. parvum and levamisole provoked rejection in all four animals in which it was used. The results suggest that an increased activity of both lymphocytes and macrophages is required to break established enhancement.