Role of hypochlorous acid and chloramines in the extracellular cytolysis by neutrophil polymorphonuclear leukocytes

The lysis of human red blood cells (HRBC) by neutrophil polymorphonuclear leukocytes (PMN), triggered with opsonized zymosan (OPZ) particles, was inhibited by azide, catalase, Cl^- -free medium and amino acids indicating the involvement of myeloperoxidase (MPO), hydrogen peroxide (H₂O₂), Cl^- ions and hypochlorous acid (HOCl) respectively. Thus, the cytolytic process depends on the following reaction: H₂O₂ + Cl^-(MPO)→(H⁺)HOCl+H₂O. Because the oxidizing agent HOCl is also the precursor of the chloramines, a group of oxidants formed by the reaction between HOCl and PMN-derived ammonia (NH₄⁺) or amines (R-NH₂), the observed HRBC lysis can be theoretically due to HOCl and/or chloramines. Nevertheless, we found that PMN-mediated cytotoxicity occurs as an unidirectional process, being HRBC targets lysed and PMN unaffected. This finding indicates that the cytotoxin must be relatively more efficient against HRBC as compared with PMN. In fact, reagent HOCl (used at concentrations comparable to those generated by PMN) but not chloramines displayed such a type of property. Taken together, the data suggest that HRBC are killed by PMN-derived HOCl without the requirement for chloramines: this implies that NH₄⁺ and R-NH₂, released by PMN, act as down-modulators of the cytotoxic process, serving as HOCl trapping agents.