Effect of interferon-gamma and human alpha 2-macroglobulin on peritoneal macrophage morphology and Ia antigen expression

Alpha-2-Macroglobulin in Inflammation, Immunity and Infections

Alpha-2-macroglobulin is an extracellular macromolecule mainly known for its role as a broad-spectrum protease inhibitor. By presenting itself as an optimal substrate for endopeptidases of all catalytic types, alpha-2-macroglobulin lures active proteases into its molecular cage and subsequently ‘flags’ their complex for elimination. In addition to its role as a regulator of extracellular proteolysis, alpha-2-macroglobulin also has other functions such as switching proteolysis towards small substrates, facilitating cell migration and the binding of cytokines, growth factors and damaged extracellular proteins. These functions appear particularly important in the context of immune-cell function. In this review manuscript, we provide an overview of all functions of alpha-2-macroglobulin and place these in the context of inflammation, immunity and infections.

Receptor-recognized alpha 2-macroglobulin-methylamine elevates intracellular calcium, inositol phosphates and cyclic AMP in murine peritoneal macrophages

Human plasma alpha 2-macroglobulin (alpha 2M) is a tetrameric proteinase inhibitor, which undergoes a conformational change upon reaction with either a proteinase or methylamine. As a result, a receptor recognition site is exposed on each subunit of the molecule enabling it to bind to its receptors on macrophages. We have used Fura-2-loaded murine peritoneal macrophages and digital video fluorescence microscopy to examine the effects of receptor binding on second messenger levels. alpha 2M-methylamine caused a rapid 2-4-fold increase in intracellular Ca2+ concentration ([Ca2+]i) within 5 s of binding to receptors. The agonists induced a focal increase in [Ca2+]i that spread out to other areas of the cell. The increase in [Ca2+]i was dependent on the alpha 2M-methylamine concentration and on the extracellular [Ca2+]. Both sinusoidal and transitory oscillations were observed, which varied from cell to cell. Neither alpha 2M nor boiled alpha 2M-methylamine, forms that are not recognized by the receptor, affected [Ca2+]i in peritoneal macrophages under identical conditions of incubation. The alpha 2M-methylamine-induced rise in [Ca2+]i was accompanied by a rapid and transient increase in macrophage inositol phosphates, including inositol tris- and tetrakis-phosphates. Native alpha 2M did not stimulate a rise in inositol phosphates. Finally, binding of alpha 2M-methylamine to macrophages increased cyclic AMP transiently. Thus receptor-recognized alpha-macroglobulins behave as agonists whose receptor binding causes stimulation of signal transduction pathways.

Induction of HLA-DR by interferon-gamma requires a trypsin-like protease

Induction of HLA-DR antigen expression by interferon-gamma (IFN-gamma) is inhibited by trypsin inhibitors and an anti-trypsin monoclonal antibody, but not by chymotrypsin inhibitors, suggesting a requirement for trypsin-like protease (TLP) activity in IFN-gamma-induced HLA-DR expression. Using p-nitroanilide and thioester substrates, TLP activity was demonstrated in cellular extracts of a hybrid epidermal cell line and judged to be essential for HLA-DR expression. TLP activity was inhibited by the trypsin inhibitors soybean trypsin inhibitor, ovomucoid trypsin inhibitor, and tosyl-lysyl-chloromethyl ketone and by an anti-trypsin monoclonal antibody, closely paralleling inhibition of HLA-DR expression by such agents. TLP activity was enhanced by exposure to trypsin-linked agarose, indicating that the protease normally exists in an inactive form, perhaps in an enzyme-inhibitor complex or as an activatable proenzyme. Finding glucocorticoids (GC) to also inhibit IFN-gamma-induced HLA-DR expression and to regulate serine protease, especially urokinase plasminogen activator (uPA), activity raised the possibility of GC regulation of TLP activity. However, TLP activity was found to be constitutively expressed, regulated by neither GC nor IFN-gamma, nor was uPA activity involved in HLA-DR regulation. Trypsin inhibitors and GC also inhibited induction of intracellular 2',5'-oligoadenylate (2-5A) synthetase by IFN-gamma. Thus, TLP activity is required for IFN-gamma induction of HLA-DR and 2-5A synthetase.

The exposure of murine macrophages to alpha 2-macroglobulin 'fast' forms results in the rapid secretion of eicosanoids

The exposure of [3H]arachidonate-radiolabelled murine peritoneal macrophages to alpha 2-macroglobulin-methylamine or alpha 2-macroglobulin-trypsin but not native alpha 2-macroglobulin (alpha 2M) results in the rapid secretion of [3H]eicosanoids. Resident peritoneal macrophages stimulated with 0.1 microM alpha 2M-methylamine exhibited an enhanced secretion within 10 min. The ability of alpha 2M 'fast' forms to stimulate secretion of [3H]eicosanoids was similar to that observed in the presence of the murine macrophage chemoattractant platelet-activating factor. As observed for total [3H]eicosanoid secretion, alpha 2M 'fast' forms also rapidly enhanced the secretion of the cAMP-elevating prostanoid, prostaglandin E2, from resident peritoneal macrophages. Stimulated secretion of prostaglandin E2 in response to 0.1 microM alpha 2M-methylamine was less rapid than that observed using 0.1 microM platelet-activating factor. Similar amounts of secreted prostaglandin E2 were present in media of macrophage cultures after 1 h exposure to the two stimuli. In the presence of 0.1 microM alpha 2M-methylamine, secreted prostaglandin E2 remained elevated, compared to the appropriate buffer control, for at least 24 h. The present results indicate that receptor recognition of alpha 2M 'fast' forms by macrophages results in the rapid stimulation of eicosanoid secretion and suggest that secretion of prostaglandin E2 and other eicosanoids may be involved in the ability of alpha 2 M 'fast' forms to regulate various macrophage functional responses.