The inhibitory potencies of monoclonal antibodies to the macrophage adhesion molecule sialoadhesin are greatly increased following PEGylation

Antibody binding to porcine sialoadhesin reduces phagocytic capacity without affecting other macrophage effector functions

Sialoadhesin (Sn) is a macrophage-restricted endocytic receptor involved in cell-cell, cell-matrix and cell-pathogen interactions. Recently, porcine Sn (pSn) was shown to be involved in signaling and lately Sn is gaining interest as a potential target for immunotherapy. However, little is known about the effect of ligand binding to Sn on macrophage effector functions. In this study, we tested the effect of antibody binding to pSn on macrophage viability, phagocytosis of microspheres, uptake and processing of soluble antigens, reactive oxygen/nitrogen species production, MHC I and MHC II cell surface expression and cytokine production. This was done by treatment of porcine primary alveolar macrophages with the pSn-specific mAb 41D3, or an isotype-matched control mAb. No significant effect on most effector functions under study was observed, except for a significant reduction of phagocytosis. Thus, antibody binding to pSn can downregulate phagocytosis, which could have implications on homeostasis, infectious and immune diseases, and immunotherapy.

Analysis of sialoadhesin expression on mouse alveolar macrophages

Sialoadhesin (Sn) is a macrophage-restricted receptor that was first characterised on mouse resident bone marrow macrophages as a receptor that mediates the binding, without ingestion, of sheep erythrocytes. Sn is highly conserved in mammals but its expression on tissue macrophages is heterogeneous. In the mouse, high levels of erythrocytes binding are shown on macrophages from lymphoid tissues but a low erythrocytes binding activity is detectable on macrophages isolated from the broncho-alveolar space. Yet, Sn expression has been demonstrated on human, rat and pig alveolar macrophages (AM) using methods of molecular biology. Therefore, the present study aimed to investigate the expression of Sn on mouse AM in order to confirm the presence of the protein on this population of murine macrophages. Using cytometrical analyses, we showed that Sn was expressed on mouse AM surface. Following desialylation, AM largely bound erythrocytes and this binding was inhibited by 3D6, an anti-mouse Sn monoclonal antibody, in a dose-dependent manner. This indicates that Sn is expressed on mouse AM but that the sialic acid binding activity mediated by this molecule is naturally masked by endogenous sialic acid within the glycocalyx on the cell surface.

PEGylation of anti-sialoadhesin monoclonal antibodies enhances their inhibitory potencies without impairing endocytosis in mouse peritoneal macrophages

Poly(ethylene glycol) (PEG) 5 kDa and 20 kDa have been previously conjugated to two anti-sialoadhesin (Sn) monoclonal antibodies (mAbs), SER-4 and 3D6, and shown to dramatically increase their inhibitory potency in solid-phase red blood cell binding assays. In the present study, we evaluated the effect of anti-Sn SER-4 and 3D6 mAbs PEGylation on their inhibition of cell adhesion in mouse peritoneal macrophages. We also examined whether Sn-mediated PEGylation could affect plasma membrane functions of macrophages as to prevent accessibility, binding, and endocytosis of macromolecules and particles. Conjugation of PEG to plasma membrane is known to cause immune tolerance by impairing protein-protein and cell-cell interactions. PEGylation of SER-4 and 3D6 mAbs increased by 4-fold their inhibition of Sn-mediated erythrocyte binding to macrophages. PEGylated SER-4 and 3D6 mAbs did not impair macrophage membrane integrity, cell metabolism, nor pinocytosis of macromolecules and phagocytosis of latex particles. Thus, PEGylation of antibodies directed to cell surface receptors could be potentially exploited in a therapeutic setting to increase inhibitory potency of antibodies without impairing vital functions of cells.