Interferon-gamma modulates cytosolic free calcium in human neutrophilic granulocytes

To investigate the role of cytosolic free calcium ([Ca2+]i in interferon-gamma (IFN-gamma) pre-activation (priming) of human neutrophilic granulocytes (PMN) we used three different fluorescence methods, i.e. digital imaging of single, adherent, Fura-2 loaded cells, flow cytometric measurements of single, non-adherent, Fluo-3 loaded cells, and spectrofluorometry of Indo-1 loaded PMN in suspension. IFN-gamma increased the [Ca2+]i level in single, adherent PMN during the second phase of the fMLP response. The bacterial peptide fMLP (N-formyl-L-methionyl-L-leucyl-L-phenylalanine) is a known stimulant of the calcium/inositol phosphate system. The [Ca2+]i increase was abolished in Ca(2+)-free test buffer. Furthermore, the baseline [Ca2+]i level was found to be slightly increased in IFN-gamma primed PMN as analysed with flow cytometry. On the other hand, these [Ca2+]i responses were not detectable with the other methods used. We suggest that IFN-gamma increases the plasma membrane permeability for calcium in PMN, and substantiate this by demonstrating compliance with a capacitative model for intracellular calcium regulation. Mathematical modeling also suggested that IFN-gamma primed human PMN may sequester 13% more Ca2+ than unprimed cells in fMLP-insensitive intracellular stores. Thus, the Ca2+ responses to IFN-gamma are modest and not easily detectable with some of the methods currently in use. They nevertheless explain why fMLP elicits brisker responses from PMN after IFN-gamma priming.