Expression of lactoferrin on human granulocytes: analysis with polyclonal and monoclonal antibodies

Lactoferrin (LF), an iron-binding protein present in specific granules of neutrophils, is expressed on membrane after granulocyte activation. It may represent a target for anti-neutrophil cytoplasmic antibodies (ANCA) in patients affected by some immunomediated diseases. We recently produced two MoAbs, AGM 2.29 and AGM 10.14, that recognize two spatially distant epitopes of human LF. In this study we perform a cytometric analysis in order to evaluate the expression of LF on the surface of granulocytes obtained from freshly drawn blood or after purification, in both the presence and absence of stimuli. Our results demonstrate that LF is not constitutively expressed on membrane of circulating neutrophils. After priming with phorbol myristate acetate (PMA) or tumour necrosis factor-alpha (TNF-alpha), an increased mean fluorescence intensity (MFI) was obtained on neutrophils stained with polyclonal anti-LF antibodies and with AGM 2.29. The kinetics of LF expression during activation demonstrated a progressive increase in MFI within 45 min. No increase in MFI was documented when primed granulocytes were stained with MoAb AGM 10.14, thus indicating that the epitope recognized by AGM 10.14 is not exposed at the cell surface. Following membrane permeabilization, performed in order to analyse the binding of anti-LF MoAbs to cytoplasmic LF, a marked increase in MFI was obtained by staining granulocytes with both anti-LF MoAbs. Indirect immunofluorescence (IIF) analysis confirmed that AGM 2.29 and AGM 10.14 reacted with human granulocytes, showing a cytoplasmic pattern on formalin-acetone-fixed neutrophils and a perinuclear one on ethanol-fixed cells.