Anti-cathepsin L monoclonal antibodies that distinguish cathepsin L from cathepsin V

Cathepsin L is a lysosomal cysteine protease involved in intracellular prot ein degradation. Recently, several new cysteine proteases have been identif ied. Human cathepsin V, a thymus- and testis-specific human cysteine protea se, shares 78% sequence identity with human cathepsin L. Due to the strong sequence similarity, highly selective reagents are needed to elucidate the physiological functions of the two enzymes. Monoclonal antibodies (mAbs) have been prepared against recombinant human c athepsin L, Antibodies produced by five clones reacted with procathepsin L and mature cathepsin L. They also reacted with cathepsin L in complex with a peptide fragment, which is identical to the alternatively spliced segment of the p41 form of MHC Class II associated invariant chain. Two mAbs, (M10 5 and H102) were specific for cathepsin L, while three (N135, B145 and D24) crossreacted with cathepsin V. None of the mAbs cross-reacted with catheps ins B, H and S. We have developed a sandwich enzyme-linked immunosorbent as say (ELISA) for quantifying cathepsin L, This sandwich ELISA uses a combina tion of two monoclonal antibodies which recognize different, non-overlappin g epitopes on the cathepsin L molecule. The lower detection limit of the sa ndwich ELISA was 5 ng of cathepsin L per ml.