Studies on mechanisms involved in macrophage recognition of apoptotic neutrophils

• Main Author: McCutcheon, Judith Clare
• Published: University of Edinburgh 1995
• Subjects: 616.079
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.666150

The macrophage integrin αvβ3 and CD36 have been proposed to act in concert to bind thrombospondin which forms a molecular bridge between the surface of the macrophage and the apoptotic neutrophil. This thesis has investigated the regulatory mechanisms underlying this macrophage phagocytic system. Phagocytosis of apoptotic neutrophils by monocyte-derived macrophages was found to be modulated by short term pre-treatment of macrophages with activators of protein kinase C and protein kinase A. In contrast, FcR-mediated phagocytosis was unaffected by protein kinase activation, suggesting specific modulation of this macrophage phagocytic system. Examination of the molecules thought to be involved in this phagocytic pathway revealed that thrombospondin production by macrophages was undetectable after 30 minutes and no change in thrombospondin binding or surface expression of αvβ3 and CD36 occurred following protein kinase activation. However, altered distribution of the β3 integrin subunit did occur with a more localised distribution observed following activation of protein kinase C. Surprisingly, a number of monoclonal antibodies against αvβ3 did not inhibit phagocytosis and αvβ3 was not found to colocalise with the macrophage actin cytoskeleton or with ingested apoptotic neutrophils. These results question the involvement αvβ3 of in the phagocytosis of apoptotic neutrophils. Protein kinase A activation was seen to cause disruption of the macrophage actin cytoskeleton. Other implicated inhibitors were subsequently found to disrupt the macrophage actin framework. Since cytoskeletal integrity is required for cell adhesion processes, the adhesion state of the macrophage during phagocytosis of apoptotic neutrophils was investigated. Adherence of macrophages to extracellular matrix proteins was found to increase levels of phagocytosis compared to control conditions. By contrast, very little phagocytosis was observed when the assay was performed in suspension.