Abstract

Cathepsin L is a lysosomal cysteine protease involved in intracellular protein degradation. Recently, several new cysteine proteases have been identified. Human cathepsin V, a thymus and testisspecific human cysteine protease, 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 cathepsin 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, (M105 and H102) were specific for cathepsin L, while three (N135, B145 and D24) crossreacted with cathepsin V. None of the mAbs crossreacted with cathepsins B, H and S. We have developed a sandwich enzymelinked immunosorbent assay (ELISA) for quantifying cathepsin L. This sandwich ELISA uses a combination of two monoclonal antibodies which recognize different, nonoverlapping epitopes on the cathepsin L molecule. The lower detection limit of the sandwich ELISA was 5 ng of cathepsin L per ml.