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Platelets as inhibitory cells in neutrophil-mediated cytolysis
F Dallegri1, A Ballestrero, L Ottonello
1First Medical Clinic, University of Genoa Medical School, Italy.
Abstract:
Neutrophilic polymorphonuclear leukocytes (PMNs), triggered by opsonized zymosan (OPZ), lysed chicken red blood cells as measured by a chromium 51 release method. The lysis was prevented by scavengers of hypochlorous acid. When platelets were added to the cytolytic system, a dose-dependent inhibition of the lysis was observed. Moreover, platelets lowered the HOCl recovery from OPZ-triggered PMNs. A positive linear relationship was found between the extent of the lysis mediated by and the amount of HOCl recovered from PMNs. Finally, both the inhibition of the lysis and the reduction of the HOCl recovery induced by platelets were prevented by pulsing platelets with carmustine (BCNU) to block their glutathione cycle. These results suggest that platelets act by consuming via their glutathione cycle significant amounts of PMN-derived hydrogen peroxide, with a consequent impairment of the PMN HOCl production and, in turn, lytic efficiency. Consistent with such a conclusion, platelets were found to consume PMN-derived H2O2 via a BCNU-inhibitable process. Further, the platelet inhibitory activity could be abolished by the addition of an appropriate extra-flux of enzymatically generated H2O2. No evidence for a platelet-induced inhibition of OPZ-PMN interaction, PMN myeloperoxidase release, and H2O2 production was obtained. The present study provides direct evidence for a platelet-dependent mechanism capable of controlling the PMN production of highly reactive oxidants and, in turn, the PMN cytolytic activity.
Insights
Platelets control the lytic activity of neutrophils (PMNs) by consuming hydrogen peroxide (H2O2). This process reduces hypochlorous acid (HOCl) production, thereby inhibiting PMN-mediated cell lysis.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Neutrophilic polymorphonuclear leukocytes (PMNs) are crucial immune cells involved in pathogen clearance.
- PMNs generate reactive oxygen species, including hypochlorous acid (HOCl), to induce cell lysis.
- Platelets are known to modulate immune responses, but their role in regulating PMN cytotoxic activity is not fully understood.
Purpose of the Study:
- To investigate the mechanism by which platelets inhibit the cytolytic activity of PMNs.
- To determine if platelets affect PMN production of reactive oxygen species like HOCl.
- To elucidate the role of the platelet glutathione cycle in this inhibitory process.
Main Methods:
- Opsonized zymosan (OPZ)-triggered lysis of chicken red blood cells by PMNs was measured using a chromium 51 release assay.
- Hypochlorous acid (HOCl) recovery from PMNs was quantified.
- Platelets were pre-treated with carmustine (BCNU) to inhibit their glutathione cycle.
- Hydrogen peroxide (H2O2) consumption by platelets was assessed.
Main Results:
- Platelets significantly inhibited PMN-mediated red blood cell lysis in a dose-dependent manner.
- Platelets reduced HOCl recovery from stimulated PMNs.
- Inhibition of lysis and reduced HOCl recovery by platelets were reversed by BCNU treatment, indicating the involvement of the platelet glutathione cycle.
- Platelets consumed PMN-derived H2O2 via a BCNU-sensitive pathway, which was abolished by excess H2O2 addition.
Conclusions:
- Platelets inhibit PMN cytolytic activity by consuming PMN-derived hydrogen peroxide (H2O2) through their glutathione cycle.
- This H2O2 consumption impairs PMN HOCl production, leading to reduced lytic efficiency.
- Platelets possess a regulatory mechanism to control PMN production of reactive oxidants and subsequent cytotoxic effects.