The potential of human neutrophil peptides in tuberculosis therapy

The problems of drug resistance and bacterial persistence in tuberculosis have prompted scientists to search for clues from the latest advances in microbiology and immunology. Recent research on human neutrophil peptides (HNPs) has highlighted their bactericidal action against Mycobacterium tuberculosis and suggested that neutrophils may play a more important defensive role in tuberculosis than previously thought. Human neutrophil peptides belong to a family of antimicrobial and cytotoxic peptides known as 'defensins'. Neutrophils use both oxidative and non-oxidative microbicidal mechanisms to provide the host with innate immunity against microbial infections. Defensins are most abundant among an array of oxygen-independent antimicrobial proteins and peptides in neutrophil granules. Defensins are effective against a wide spectrum of microbes including bacteria, viruses, fungi, spirochetes and mycobacteria. In addition to direct antimicrobial activity, HNPs can potentially influence the inflammatory or immune responses by modulating cytokine production or acting like opsonins or chemotactic factors. HNPs are active against M. tuberculosis grown in vitro or within macrophages. HNPs released by neutrophils recruited in the early lesion could attract monocytes to the site and macrophages may in vivo uptake the extracellular HNPs and kill the intracellular pathogens. As such, HNPs are potential therapeutic agents against tuberculosis. HNPs are also cytotoxic against a wide range of normal mammalian cells; however, there is evidence that defensins may not cause significant cytotoxicity at the therapeutic level. Finally, the clinical application of HNPs must be evaluated in the context of possible drug resistance, as some resistance-associated genes have been identified.