Overexpression of Bcl-3 enhances the retraction of fibrin clots. (A) Western analysis of Bcl-3 protein in lysates of CHO cells that express integrin α_IIbβ₃ (cell line PLA1) or wild-type CHO cells that are deficient in α_IIbβ₃ (cell line LK444) after transfection with cDNA for Bcl-3 or with empty vector. (B) Mock-transfected integrin α_IIbβ₃–expressing CHO cells (control) or integrin α_IIbβ₃–expressing CHO cells that overexpressed Bcl-3 (see panel A) were examined for the ability to retract fibrin clots. The cells were incubated with exogenous-labeled fibrinogen and stimulated with thrombin for 2 hours as in Figure 3. Mock-transfected cells formed a loose, gelatinous clot (the black arrow points to the edge of the cell-fibrin complex) with a rim of clear media at its edges. Bcl-3–overexpressing cells induced a compact retracted clot (black arrow) adherent to the stir bar at the bottom of the tube (white arrows) with clear media above, although the clots were not as tight as those formed by human control platelets activated by thrombin (Figure 3, tubes B,D). Panels in this figure are representative of 3 separate experiments. For a color figure of panel B that provides additional detail of the fibrin clots, see Figure S1 (available on the Blood website; see the Supplemental Figure link at the top of the online article).