Non-specific recognition by macrophages and mechanisms of macrophage activation

Abstract

This study deals with two related aspects of macrophage function, surface recognition and signal transmission across the plasma membrane.The introduction reviews the properties and functions of macrophages with particular reference to specific and non-specific recognition and activation as expressed by enhanced effector function. This review is set against the background of the structure and function of plasma membranes, biological recognition and mechanisms of cellular activation.The ability of macrophages to recognize a variety of foreign particles without the mediation of specific recognition molecules was investigated. In a binding assay performed at 4°C using non-opsonized bacteria, it was found that several types of Gram-positive and Gram-negative bacteria bound to normal mouse peritoneal macrophages. The binding could be inhibited by pre-incubating the macrophages at 4°C with various monosaccharides at a concentration of 10 mM. There was a very close correlation between the ability of a sugar to inhibit binding of a particular type of bacterium and the presence of that sugar in the bacterial cell wall. It was, therefore, postulated that the binding of non-opsonized bacteria by macrophages was based on the recognition of cell wall carbohydrates.The nature of the binding reaction was further studied using Corynebacterium parvum, It was found that binding at 4°C depended on the presence of both Ca⁺⁺⁻¹ and Mg⁺⁺⁻ions whilst binding at 20°C occurred to some degree when only Mg⁺⁺⁻ions were present. The binding was not mediated by cell-bound antibody as shown by experiments using mild trypsin-treatment and specific antibody, Pre-treatment of the macrophages with trypsin, pronase, ß-galactosidase and phospholipases A, C and D caused a marked reduction in binding whilst treatment with neuraminidase resulted in some increase in binding. Exposure of the macrophages to periodate also led to a decrease in binding of C. parvum, an effect largely reversed by subsequent treatment with borohydride. Recovery from the effects of enzyme treatment was rapid, but was inhibited by EDTA in the case of trypsin and ß-galactosidase. These results suggested that plasma membrane glycoproteins played an important part in the binding reaction which might involve a bridging action of divalent cations. The effect of neuraminidase was most easily explained by a reduction in cell surface negative charge.The enhancement of phosphatidylinositol turnover was investigated as a possible mechanism of signal transmission initiating the intracellular effects of an activating agent following contact with the macrophage surface. The rate of phosphatidylinositol turnover was assayed by measuring the uptake of tritiated myo-inositol into macrophage phosphatidylinositol during one hour. It was shown that two macrophage activating agents, endotoxin and C. parvum, caused an increase in phosphatidylinositol turnover after 4 hours of incubation whilst exposure to three inert particles, Staphylococcus albus, latex and colloidal carbon, had no such effect. All the particles tested were phagocytosed and it was concluded that enhanced turnover of phosphatidylinositol was an early event following exposure to activating agents that was not linked to the process of phagocytosis.Prolonged exposure to endotoxin resulted in enhanced bacteriostatic activity of the macrophages against Listeria monocytogenes. C. parvum and endotoxin also stimulated lysosomal enzyme activity of the macrophages. There was, thus, a correlation between the ability to stimulate phosphatidylinositol turnover and to induce the macrophage effector functions characteristic of the activated state.