Very late activation antigen-5 (VLA-5) expression decreases in monocytes leaving injured cardiac tissue

We have recently shown that monocytes accumulate in ischemie myocardium within one hour of reperfusion after occlusion of a circumflex coronary artery in dogs. By the third hour, some transform into phagocytic macrophages; others leave the tissue via the draining lymph. We postulated that the expression of adhesion molecules might determine which monocytes entering injured cardiac tissue remain in the heart and which leave via the draining lymph. Less than 25% of monocytes appearing in the lymph at three hours were positive for VLA-5, a receptor for fibronectin. whereas 95% of blood monocytes and 88% of monocytes in cardiac lymph collected before occlusion were positive for VLA-5. This apparent ioss of VLA-5 expression on iymph monocytes as compared to blood monocytes occured only in animals with infarcts. The change was selective since cell expression of VLA-4, another fibronectin receptor that binds to a different region of the molecule, was not altered. Decreased expression of VLA- 5, measured by fluorescence staining, was also seen when human monocytes in blood were incubated with the 120 kD proteolytic fragment of fibronectin that contains the arginine-glycine-aspartic acid (RGD) sequence binding to VLA-5. This change occurred at 37C, but not at 4oC, and was progressive over several hours, indicating that a simple blocking of antibody binding by the 120 kD fragment was not responsible. Western blot analysis showed that canine cardiac lymph contained the 120 kD fragment, but not intact fibronectin or other fragments. This indicates thai, the 120 kD fragment is available to influence VLA 5 expression by monocytes in canine cardiac ex tracellular fluids, and may influence the ability of these cells to remain in the heart by decreasing their adherence to nbronectin in tissue matrix. .