INTERFERON-GAMMA MODULATES CYTOSOLIC-FREE CALCIUM IN HUMAN NEUTROPHILIC GRANULOCYTES

To investigate the role of cytosolic free calcium ([Ca2+](i) in interf eron-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 cytomet ric measurements of single, non-adherent, Fluo-3 loaded cells, and spe ctrofluorometry of Indo-1 loaded PMN in suspension. IFN-gamma increase d 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/inosito l phosphate system. The [Ca2+](i) increase was abolished in Ca2+ -free test buffer. Furthermore, the baseline [Ca2+](i) level was found to b e slightly increased in IFN-gamma primed PMN as analysed with flow cyt ometry. On the other hand, these [Ca2+](i) responses were not detectab le with the other methods used. We suggest that IFN-gamma increases th e plasma membrane permeability for calcium in PMN, and substantiate th is by demonstrating compliance with a capacitative model for intracell ular calcium regulation. Mathematical modeling also suggested that IFN -gamma primed human PMN may sequester 13% more Ca2+ than unprimed cell s in fMLP-insensitive intracellular stores. Thus, the Ca2+ responses t o IFN-gamma are modest and not easily detectable with some of the meth ods currently in use. They nevertheless explain why fMLP elicits brisk er responses from PMN after IFN-gamma priming.