Suppressed natural killer cell activity during episodes of erythema nodosum leprosum in lepromatous leprosy.

Natural killer (NK) cell activity was investigated in peripheral blood mononuclear cells of patients with leprosy. The NK activity of patients with borderline or lepromatous leprosy did not differ significantly from that of normal subjects. However, in a group of patients with lepromatous leprosy undergoing an episode of erythema nodosum leprosum (ENL), NK activity was significantly depressed. In four patients with ENL, NK activity was virtually abolished. Depressed NK activity could not be attributed to the effects of corticosteroid therapy, nor did a serum factor appear to be responsible. Evidence was obtained that depressed NK activity in patients with ENL was not due to dysfunction of the NK cells themselves; they functioned normally when separated from the individual's total mononuclear cell population. Additional cell depletion studies suggested that the patients' monocytes were responsible for the observed depression of NK activity.