Role of the plasma membrane in signal transduction in human polymorphonuclear leukocytes

Neutrophil priming in host defense: role of oxidants as priming agents

Neutrophils play an essential role in the body's innate immune response to infection. To protect the host, these phagocytic cells possess an impressive array of microbicidal weapons that can be brought to bear on an invading pathogen, including a variety of toxic oxygen radical species and proteolytic enzymes. Although the neutrophil response is designed to restrict the damage to the smallest possible region where the pathogen is located, some of the damaging agents inevitably leak into the surrounding areas where they have the capacity to inflict tissue damage at sites of inflammation. Thus, it is essential that the host defense response of these cells is finely tuned to result in the appropriate level of response to any given situation. One of the regulatory mechanisms implicated in controlling neutrophil responses is priming. Through the action of priming agents, the level of activation and subsequent responses of the cell can be regulated so that a continuum of activation states is achieved. In this review, we describe key features of the priming response in host defense and disease pathogenesis and focus on the unique role of reactive oxygen species as priming agents.

Superoxide release is involved in membrane potential changes in mouse peritoneal macrophages

Participation of reactive oxygen species (ROS) in the changes in macrophage membrane potential resulted from effects of different agonists has been studied. Treatment of macrophages with chemotactic peptide fMLP or platelet-activating factor (PAF) caused a brief depolarization followed by a long-lasting hyperpolarization. Lipopolysaccharide and interferon-gamma only depolarized the plasma membrane. Chemiluminescence measurements indicated that only fMLP and PAF activated macrophages to release ROS. The hyperpolarization response of the cell was significantly decreased in the presence of superoxide dismutase (but not catalase). Moreover, the O2.- -generating system, xanthine plus xanthine oxidase, caused a marked hyperpolarization. In all the cases, the hyperpolarization induced by fMLP, PAF and O2.- -generating system was found to depend on the concentration of intracellular Ca2+ and extracellular K+. Furthermore, in the presence of quinidine, a blocker of Ca2+-dependent K+ conductance fMLP and PAF caused only prolonged depolarization while the effect of O2.- was reduced to a minimum. These data suggest that the macrophage hyperpolarization response to fMLP and PAF involves superoxide-mediated Ca2+-dependent alteration of the relative membrane permeability to K+.

Adaptive and maladaptive mechanisms of cellular priming

OBJECTIVE

The mechanisms of cellular priming resulting in both adaptive and maladaptive responses to subsequent injury and strategies for manipulating this priming to constructive therapeutic advantage are explored.

BACKGROUND DATA

A cell is prepared or educated by an initial insult (priming stimulus). Investigations in both laboratory animals and humans indicate that cells, organs, and perhaps even whole patients respond differently to a proximal second insult ("second hit") by virtue of this prior environmental history. The opportunity to achieve the primed state appears to be conserved across almost all cell types. The initial stimulus transmits a message to the cellular machinery that influences the cell's response to a subsequent challenge. This response may result in an exaggerated inflammatory response in the case of the neutrophil (an often maladaptive process) or an improved tolerance to injury by the myocyte (adaptive response). Our global hypothesis is that cellular priming is a conserved, receptor-dependent process that invokes common intracellular targets across multiple cell types. We further postulate that these targets create a language based on the transient phosphorylation and dephosphorylation of intracellular enzymes that is therapeutically accessible.

CONCLUSIONS

Priming is a conserved, receptor-dependent process transduced by means of intracellular targets across multiple cell types. The potential therapeutic strategies outlined involve the receptor-mediated manipulation of cellular events. These events are transmitted through an intracellular language that instructs the cell regarding its behavior in response to subsequent stimulation. Understanding these intracellular events represents a realistic goal of priming and preconditioning biology and will likely lead to clinical control of the primed state.

Flow cytometry reveals different lag times in rapid cytoplasmic calcium elevations in human neutrophils in response to N-formyl peptide

Flow cytometric analyses were performed to study intracellular single-cell calcium transients ([Ca2+]i) in suspended human neutrophils during the initial phase of N-formyl peptide stimulation. Thereby, two neutrophil populations became apparent. Early maximally Ca(2+)-responding (high fluorescence) neutrophils and not-yet Ca(2+)-responding (low fluorescence) neutrophils, but no neutrophils with intermediate levels of [Ca2+]i were detected. Within 7 s the number of low fluorescence neutrophils decreased and the number of high fluorescence neutrophils increased maximally. This suggests that [Ca2+]i transients occurred abruptly in individual neutrophils within a time interval below 1 s. At lower N-formyl peptide concentrations the lag times of individual neutrophils and the interval time of maximal activation of the [Ca2+]i-responding neutrophil population increased, however the percentage of [Ca2+]i-responding cells decreased. Surprisingly, no influence of the N-formyl peptide concentration on the [Ca2+]i-induced fluorescence signal of the individual cell was observed: it was always in an almost maximal range or not responding. In parallel, binding studies performed with fluorescein-labeled N-formyl peptide revealed that the heterogeneity of [Ca2+]i-responding cells cannot be explained by different receptor occupancy. In summary, this study demonstrates that [Ca2+]i transients induced by N-formyl peptides in suspended individual human neutrophils occur very rapidly in an almost "all-or-none manner" and that the mean increasing fluorescence signal of a calcium indicator within a whole neutrophil population results from varying lag times of the individual cells, rather than from the mean simultaneous progress of many cells.