We have examined the capacity of monocytes from patients with leprosy to undergo activation and the capacity of mononuclear cells from these patients to incorporate [3H]thymidine and produce monocyte-activating cytokines. Monocytes from patients with either lepromatous or tuberculoid leprosy were activated by concanavalin A (Con A)-induced mononuclear cell supernatants generated from the leukocytes of a normal person. Monocytes activated by these supernatants strongly inhibited L. pneumophila multiplication, and the degree of inhibition was comparable in both groups of patients. Mononuclear cells from patients with either form of leprosy responded comparably to Con A with vigorous [3H]thymidine incorporation. Mononuclear cells from patients with tuberculoid leprosy also vigorously incorporated [3H]thymidine in response to M. leprae antigens. In contrast, mononuclear cells from patients with lepromatous leprosy did not exhibit significant [3H]thymidine incorporation in response to M. leprae antigens. The capacity of mononuclear cells to generate monocyte-activating cytokines generally paralleled their capacity to incorporate [3H]thymidine in response to Con A and M. leprae. Mononuclear cells from patients with either form of leprosy responded to Con A with the production of cytokines (supernatants) able to activate normal monocytes, expressed by inhibition of L. pneumophila multiplication. However Con A-induced supernatants from patients with lepromatous leprosy were less potent than Con A-induced supernatants from patients with tuberculoid leprosy. Mononuclear cells from patients with tuberculoid leprosy responded to M. leprae antigens with the production of potent monocyte-activating supernatants. In contrast, mononuclear cells from patients with lepromatous leprosy did not produce monocyte-activating cytokines in response to M. leprae antigens. These studies support the hypothesis that the immunological defect in lepromatous leprosy results from a failure to activate mononuclear phagocytes rather than from an intrinsic inability of these cells to be activated. We suggest that the failure to activate mononuclear phagocytes stems from defective production of monocyte-activating cytokines in response to M. leprae antigens.

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