Characterization of factor H-related cell membrane molecules expressed by human B lymphocytes and neutrophil granulocytes

The human factor H protein family comprises six plasma glycoproteins. Earli er we described a membranal factor H-related (mFHR) molecule that is expres sed by human B lymphoblastoid cell lines and exerts cofactor activity. In o ur present study we screened human blood cells for the presence of mFHR pro teins and further characterized these molecules. By cytofluorimetry it is s hown that the factor H-specific rabbit antiserum reacts strongly with B cel ls and neutrophil granulocytes, but not with T cells and monocytes. On B ly mphocytes mFHR is shown to be down-regulated upon activation of the cells v ia sig. In experiments studying which short consensus repeat (SCR) domains are part of the cell membrane proteins we found that antibodies raised agai nst SCRs 1-4, 19-20 and FHR-3 bound to neutrophils but not to B cells. Whil e mFHRs derived both from B cells and granulocytes are shown to bind hepari n, their size and structure are different as revealed by Western blotting. A further characteristic of the granulocyte-derived mFHR is its sensitivity to the PI-specific PLC gamma enzyme. These data demonstrate the existence of new members of the FHR protein family, as two distinct, membranal forms are identified. Based on the differences, the B cell derived molecule is te rmed mFHR-1 and the neutrophil derived protein mFHR-2. (C) 2001 Elsevier Sc ience B.V. All rights reserved.