PH Nibbering¹, E Broug-Holub², AC Bezemer¹, R Jansen¹, JGJ van de Winkel³, MF Geertsma¹.

1 Dept. of Infectious Diseases, University Hospital, Leiden, The Netherlands.
2 Dept. of Cell Biology and Immunology, Free University, Amsterdam, The Netherlands.
3 Dept. of Experimental Immunology, University Hospital, Utrecht, The Netherlands.

Received 12/7/95; Accepted 12/29/95; On-line 1/1/96

Phagocytes, i.e. granulocytes, monocytes and macrophages, play an important role in the resistance against infections by virtue of their ability to phagocytize and subsequently kill microorganisms. These cells express several receptors involved in the antimicrobial functions, including receptors for the Fc part of immunoglobulin G (Fcgamma receptors, FcgammaR), complement components and specific glycosylated molecules (1).

Three major classes of FcgammaR on human leukocytes are currently recognized; FcgammaRI (CD64), FcgammaRII (CD32) and FcgammaRIII (CD16). These receptors can be distinguished on basis of differences in their primary sequences, molecular size, ligand affinity and specificity, cellular distribution, and reactivity with monoclonal antibodies (mAb) against FcgammaR (2-3). FcgammaRI, a 72 kDa protein that binds monomeric IgG with high affinity, is well-expressed by cells of the mononuclear phagocyte lineage and (at low levels) neutrophils. The low affinity 40 kDa FcgammaRII and the 50-80 kDa FcgammaRIII glycoprotein bind only complexed IgG. FcgammaRII, which is the most widely distributed IgG receptor class, is expressed by all phagocytes, B lymphocytes, and several other cell-types including platelets, specialized endothelial and epithelial cells and Langerhans' cells (4). Human FcgammaRII proteins can be divided into two groups, FcgammaRIIa which is predominantly found on phagocytes, and FcgammaRIIb which is preferentially expressed on B cells (5). FcgammaRIII is found on neutrophils, NK cells, monocytes and macrophages (5). In humans, at least two genes code for FcgammaRI, three genes code for FcgammaRII and two genes code for FcgammaRIII (5).

FcgammaR occupancy can initiate a variety of biological functions of phagocytes, e.g. phagocytosis and intracellular killing of microorganisms, production of reactive oxygen intermediates (ROI), antibody dependent cell-mediated cytotoxicity, production and release of inflammatory mediators, and enhancement of antigen presentation (6-7). Since professional phagocytes express more than one class of FcgammaR on their membrane and the specificity of this receptor for ligands is relative rather than absolute (5), it is difficult to determine the relative contribution of the various classes of FcgammaR to initiation of biological functions. In previous studies, anti-FcgammaR mAb were used to investigate the relative contribution of the various classes of this receptor in the intracellular killing of bacteria by human monocytes and the signal transduction pathways involved (8-10). It was demonstrated that anti-FcgammaRI or FcgammaRII mAb as well as F(ab')₂ fragments of these mAb efficiently stimulated the intracellular killing of Staphylococcus aureus by human monocytes (8). The possibility that stimulation of the killing process after FcgammaR cross-linking by anti-FcgammaR mAb and bridging antibody differs from that after addition of serum and IgG could not be completely excluded in these studies. This possible drawback can be circumvented by determining the effects of physiological stimuli on the antimicrobial functions of FcgammaR-negative cells transfected with cDNA coding for human FcgammaR. It has been reported that such cells are able to phagocytize opsonized particles (11-15). The aim of the present study was to investigate whether mouse 3T6 mouse fibroblasts transfected with cDNA encoding for human FcgammaRIIa (FcgammaRIIa-expressing cells) are capable of phagocytosis and intracellular killing of serum-opsonized Staphylococcus aureus.