Macrophages require plasminogen to transverse a fibrin matrix in vitro. RAW 264.7 cells (A) and BMDMs (B) were plated into uncoated transwell membranes (1 and 2) or into transwell membranes onto which fibrin had been polymerized (3 and 4) and induced to migrate with C5a in the absence (1 and 3) or presence (2 and 4) of plasminogen. A significant increase in the cellular migration was noted with plasminogen alone (1 vs 2), consistent with previous reports. Cells were essentially unable to migrate in the presence of fibrin without plasminogen (3). However, addition of plasminogen to the cells permitted migration in response to C5a (4). (C-J) Representative images of transwell membranes after overnight incubation with or without fibrin matrix; low power imaging (10×) with higher power magnification (as noted by square outline). Panels D, F, H, and J show the higher power imaging of the transwell membrane. In the absence of fibrin, cells readily migrated without (C-D) or with (E-F) plasminogen. In the presence of fibrin, essentially no cellular migration was noted in the absence of plasminogen (G-H). However, addition of plasminogen restored migration (I-J). Scale bars in panels C, E, G, and I indicate 1 mm. Scale bars in D, F, H, and J indicate 100 µm.