sdAb19 blocks Nef-PAK2 association and restores actin remodeling after TCR engagement and cofilin deregulation. (A) Jurkat cells expressing WT or F195A Nef-GFP and increasing amounts of sdAb19 were subjected to anti-GFP immunoprecipitation and subsequent in vitro kinase assay. Nef-associated PAK2 activity is shown by the phosphorylated 62-kDa band (p-PAK2, IVKA). (B) Representative micrographs of the cells used in panel A. Cells were plated onto anti-CD3 or poly-L-lysine (PLL)–coated cover glasses (top and bottom panels, respectively), fixed and stained either with phalloidin to reveal F-actin (top) or for p-cofilin (bottom). (C) Frequency of the cells shown in panel B that are able to form F-actin–rich circumferential rings and with high p-cofilin levels. Values are the means of 3 independent experiments, and error bars represent 1 SD from the mean; ≥ 100 cells were analyzed per transfection.