Abstract:Polymorphonuclear neutrophils (PMNs) play pivotal roles as phagocytic cells in immune defence against bacteria and parasites, exerting their effects by production of reactive oxygen species, several cytokines, chemokines and by phagocytotic reaction. In our investigation of properties of activated PMNs, we discovered that one of the two kinds of mouse anti‐human CD19 monoclonal antibodies (mAbs) clone SJ25‐C1, weakly binds to freshly prepared PMNs. Moreover, the treatment of freshly prepared PMNs with anti‐CD19 mAb (clone SJ25‐C1) at 37°C for 6 h induces the production and the secretion of tumour necrosis factor α (TNF α) by PMNs in vitro which was detectable in culture supernatants by bioassay using mouse cell line L929 cells. The concentration of TNF α secreted into the culture supernatant of PMNs cultured in the presence of anti‐CD19 mAb (clone SJ25‐C1) was higher than those of PMNs treated at 37°C for 6 h with various PMN activators, such as anti‐CD24 mAb, granulocytes‐macrophage colony stimulation factor (GM‐CSF) or interferon γ (IFN γ). In contrast, another clone of anti‐CD19 mAb, HD37, did not bind to freshly prepared PMNs and failed to produce TNF α. To confirm that anti‐CD19 mAb (clone SJ25‐C1)‐treated PMNs definitely produce TNF α, we measured the levels of intracellular expression of TNF α in PMNs permeabilized by saponin. These cells were treated with fluorescence‐conjugated mouse anti‐human TNF α mAb for detection of intracellular TNF α expression. Consequently, large amounts of intracellular TNF α were detected in PMNs treated with anti‐CD19 mAb (clone SJ25‐C1) but not in those treated with anti‐CD19 mAb (clone HD37). Copyright © 2004 John Wiley & Sons, Ltd.