proMMP-9 interacts with α4β1 integrin and a high-molecular-weight CD44 isoform in B-CLL cells. Lysates from 2 × 10⁷ B-CLL cells cultured for 24 hours in RPMI/0.1% FCS were immunoprecipitated with anti–MMP-9 pAbs or with rabbit IgG (Ctl), and analyzed by gelatin zymography (A) and Western blotting (B). The results from 2 patients of the 3 analyzed with identical results are shown. (C) Lysates from 2 × 10⁷ B-CLL cells transfected with MMP-9 or control siRNA were immunoprecipitated with anti–MMP-9 pAbs and analyzed by Western blotting. The efficiency of the siRNA transfection was confirmed by gelatin zymography. (D) B-CLL cell lysates were immunoprecipitated with the indicated Abs and analyzed by gelatin zymography. Anti-HLA and anti–MMP-9 Abs were used as negative and positive controls, respectively. (E) Gelatin zymographic analysis of the cell fractions and conditioned media of the indicated B-cell types, after 24-hour cell culture.