Cell-density dependence of host-defense peptide activity and selectivity in the presence of host cells

Host-defense peptides (HDPs) are promising compounds against multidrug-resistant microbes. In vitro, their bactericidal and toxic concentrations are significantly different, but this might be due to the use of separate assays, with different cell densities. For experiments with a single cell type, the cell-density dependence of the active concentration of the DNS-PMAP23 HDP could be predicted based on the water/cell-membrane partition equilibrium and exhibited a lower bound at low cell counts. On the basis of these data, in the simultaneous presence of both bacteria and an excess of human cells, one would expect no significant toxicity, but also inhibition of the bactericidal activity due to peptide sequestration by host cells. However, this inhibition did not take place in assays with mixed cell populations, showing that for the HDP esculentin-1a(1-21)NH2, a range of bactericidal, nontoxic concentrations exists and confirming the effective selectivity of HDPs. Mixed-cell assays might be necessary to effectively asses HDP selectivity.