Human natural killer (NK) cells express low-affinity Fc immunoglobulin G (IgG) receptor (FcgRIIIA/CD16). The binding of monomeric IgG (mIgG) and F(ab 0 ) 2 fragments of 3G8 anti-CD16 monoclonal antibody (mAb) to FcgRIIIA was investigated by flow cytometry. Over 90% of NK cells bound endogenous IgG, and during incubation at 37 C, the FcgRIIIA occupancy decreased slowly. Approximately 90% of NK cells bind mIgG or F(ab 0 ) 2 fragments of 3G8 anti-CD16 mAb. The calculated half-time (T 1/2 ) of in vitro mIgG dissociation from FcgRIIIA was 130 min. By cross-linking the mIgG ligand with F(ab 0 ) 2 fragments of anti-human IgG antibody, the T 1/2 decreases to 85 min. In kinetics study, it has been shown that 125 I-mIgG bound to FcgRIIIA is slowly released in the culture supernatant, maybe eluted at acid pH, or partially internalized and degraded. The binding of IgG to FcgRIIIA was increased by 53.8% on cells cultured in the presence of RU36156, a matrix metalloproteinase (MMP) inhibitor. Furthermore, an increase in phosphorylation of Lyn tyrosine kinase, after cross-linking of mIgG-FcgRIIIA complex, was observed on NK cells treated with RU36156. When the FcgRIIIA was occupied by mIgG, the capacity of NK cells to kill K562 target cells was decreased by RU36156, because the MMP inhibitor protects CD16 from proteolysis. Our data demonstrate that binding of mIgG to human NK cells is followed by ligand dissociation and/or internalization, enzymatic degradation and exocytosis. The RU36156 MMP inhibitor protects FcgRIIIA from cleavage, augments NK-cell activation and may interfere in their killing capacity.

Receptors for the Fc region of IgG (Fc + RIIIa, Fc + RIIc) and IgM (Fc ? R) were previously described on NK cells. In this work the expression of Fc receptors for IgA (Fc § R) on human NK cells and the signaling events were investigated. The Fc § R was demonstrated by flow cytometry using secretory IgA (sIgA) and anti-human IgA antibody.

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Radiolabeled protein A from Staphylococcus aureus (SpA) injected i.v. into mice and rabbits forms a soluble [(IgG)2-(SpA)1]2 complex (Mr = 684 000) which is identical in composition to that formed by SpA in vitro with an equivalent amount or an excess of IgG. A soluble rabbit IgG-SpA complex injected into a mice or rabbits dissociates completely in vivo and a new complex is formed with the IgG of the recipient animal. The half-life of SpA administered to a mouse or a rabbit is therefore the half-life of the IgG-SpA complex formed in vivo. In mice and rabbits the half-life of the complexes formed is 9 and 30 h, respectively, whereas the half-life of rabbit IgG in these animals is 106 and 153 h, respectively. Fragment B of SpA (fSpA) reacts with IgG of mouse and rabbit and forms an (IgG)1-(fSpA)1 complex. Complexes of identical composition are formed if fSpA is injected i.v. into mice and rabbits. The half-life of the complexes in mice and rabbits are much shorter than those of the corresponding free IgG in these animals (up to 15 times). This result suggests that the binding of fSpA to the CH2 and the CH3 domains of IgG alters the function of the site, which controls the catabolism of IgG and is located in the CH2 domain. By contrast, fSpA does not change the Fc receptor-binding site of IgG, indicating that the Fc receptor site and the catabolic site are unrelated to each other.

We and others have previously shown that some microorganisms, including bacteria, express on their surfaces receptors that specifically recognize extracellular matrix proteins, such as laminin, fibronectin, or both. The ability of microorganisms to adhere and to invade might depend on the existence of receptors which could, thus, be correlated with pathogenicity. In the present paper, we report the isolation of five stable cell lines that were producers of monoclonal antibodies to Staphylococcus aureus laminin receptors. One of these antibodies, which was of the immunoglobulin M isotype, blocked the binding of laminin to bacteria before and after fixation and recognized the putative 52-kilodalton laminin-binding protein in whole bacterial extracts. Also, purified receptor was isolated by immunoaffinity chromatography and shown to bind laminin. Furthermore, the same antibodies bound the 67-kilodalton putative receptor from mouse melanoma cells and gave positive immunofluorescence reactions against mammalian tumor cells. These data strongly suggest either the evolutionary conservation of at least some sequences in both procaryotic and eucaryotic laminin-binding proteins or convergent evolution and positive selection of epitopes cross-reacting with laminin. Some of these antibodies to the procaryotic protein could therefore become useful markers for the expression of laminin receptors by cancer cells.