Abstract

Affinity purification of crude acid extracts of rabbit poly-morphonuclear leukocytes using Escherichia coli (J5) as ad-sorbent yields the bactericidal/permeability-increasing pro-tein (BPI), two 15-kD species (pl5s), and the two most potent (cationic) defensin species (neutrophil peptides [NP]-1 and-2). Tested in buffered isotonic medium, the relative anti-bacterial potency of these proteins against E. coli J5 is BPI (IC50 0.2 nM)> pl5A (10 nM)> NP- 1 (400 nM). Suble-thal doses of pl5A or NP-1 can synergize with BPI to de-crease the dose required to inhibit the growth of E. coli by up to 50-fold. BPI and pl5A display similar features of antibacterial action distinct from defensin NP-1, but NP-1 acts synergistically only with BPI and not with pl5A. All aspects of the combined action of BPI and NP-1 resemble those observed with higher concentrations of BPI alone, implying that NP-1 enhances BPI potency. Neither NP-1 nor pi5A alter the amount of BPI binding to E. coli but BPI enhances binding of pi5A to E. col, raising the possibility that synergy between these two proteins may occur at least partially at the level of binding. The potent synergistic ac-tions of these proteins can also be demonstrated against serum-resistant clinical isolates of encapsulated E. coli tested in whole blood and plasma ex vivo, suggesting that such combined action may contribute to host defense in vivo.