Experimental cutaneous leishmaniasis. V. Protective immunity in subclinical and self-healing infection in the mouse

This study shows how infection of CBA mice with L. tropica can be manipulated so as to mimic the principal features of both subclinical and self-healing cutaneous leishmaniasis in man. CBA mice were infected with graded inocula of L. tropica promastigotes. The pattern of primary infection was found to be dependent on dose of infecting organisms: mice given low dose inocula (10(2), 10(3)) developed subclinical infections; those given high dose inocula (10(4), 10(5), 10(6)) developed overt, clinical lesions. Size and duration of lesions, and antibody production were directly proportional to dose; delayed hypersensitivity responses were inversely proportional to dose. Protective immunity to challenge infection was induced by both subclinical and clinical infection; and was manifest both during and after the healing stages of primary lesions. Protective immunity was also induced by artificial immunization with sonicated promastigotes in adjuvants but was only manifest if the challenge dose was not too large. The course of challenge infections differed depending on the method of immunization, i.e. whether by infection or artificial immunization. Lymphoid cells from immune CBA mice conferred protection on recipient syngeneic CBA mice against challenge infection; serum from immune mice did not, but suspension of immune peritoneal cells in immune serum enhanced their protective capacity. The experimental induction of protective immunity by low-dose infection, without a clinical allergic response at the site of inoculation, is of importance in designing an immunoprophylactic approach to human leishmaniasis.