Monocyte recruitment, antigen degradation and localization in cutaneous leishmaniasis

The relationship between the destruction of Leishmania, the recruitment of monocytes and macrophage activity in the lesions of cutaneous leishmaniasis (CL) was studied in 53 biopsies representing the phases of evolution of the infection. Lysozyme, amastigotes and their degradation products were located by their specific antibodies. A rising level of monocyte influx was found to correlate with the degradation and solubilization of antigen, a falling level with final clearance. Differences in the results supported the previous concept of macrophage activation and macrophage lysis as alternative mechanisms for the elimination of Leishmania. Macrophage activation appeared to coincide with re-phagocytosis of externalized antigenic products of different type and origin. Macrophage lysis was a fully effective mechanism only when the antigen was contained within a focalized granuloma before mass lysis. Failing this, degradation and clearance of antigen were incomplete, and residues were sequestered on the periphery of the lesion where they bound to collagen and epidermis with consequential tissue damage. Antigen was demonstrated on the surface of lightly parasitized macrophages but not heavily infected ones. Other cells bound antigen without ingesting it, a process which might allow antigen presentation though it would also favour survival of parasites within the cell.