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Antimicrobial peptides (AMPs) from amphibians and other eukaryotes recognize pathogenicity patterns mostly related to differences in membrane composition between the host and a variety of bacterial, fungal and protozoan pathogens. Compared to the other two groups, protozoa are fairly neglected targets in antimicrobial chemotherapy, despite their role as causative agents for scourges such as malaria, amoebiasis, Chagas' disease or leishmaniasis. Herein we review the scarce but growing body of knowledge addressing the use of amphibian AMPs on parasitic protozoa, the adaptations of the protozoan to AMP pressure and their impact on AMP efficacy and specificity, and the current and foreseeable strategies for developing AMPs into practical therapeutic alternatives against parasitic disease.

There is an increasing awareness of immune cell modulation by antimicrobial peptides. While this process often requires specific receptors for the peptides involved, several reports point out to a receptor-independent process. The cecropin A-melittin hybrid peptide CA(1-8)M(1-18) (KWKLFKKIGIGAVLKVLTTGLPALIS-amide) modifies gene expression in the macrophage line RAW 264.7 in the absence of any previous macrophage priming, suggesting a membrane permeation process. To further analyze the initial steps of this mechanism, we have studied the interaction of the peptide with these cells. Below 2 microM, CA(1-8)M(1-18) causes a concentration-dependent membrane depolarization partially reversible with time. At 2 microM, the accumulation of the SYTOX green vital dye is one half of that achieved with 0.05% Triton X-100. The binding level, as assessed by fluorescein-labeled CA(1-8)M(1-18), varies from 7.7+/-1.2 to 37.4+/-3.9 x 10(6) molecules/cell over a 0.5-4.0 microM concentration range. Electrophysiological experiments with 0.5 microM CA(1-8)M(1-18), a concentration that triggers maximal NOS2 expression and minimal toxicity, show a reversible current induction in the RAW 264.7 plasma membrane that is maintained as far as peptide is present. This activation of the macrophage involves the production of nitric oxide, a metabolite lethal for many pathogens that results from unspecific membrane permeation by antimicrobial peptides, and represents a new mode of action that may open new therapeutic possibilities for these compounds against intracellular pathogens.