Immunochemical localisation of cathepsin S, cathepsin L and MHC class II-associated p41 isoform of invariant chain in human lymph node tissue

Antigen presentation by MHC class II molecules requires cysteine proteases (CP) for two convergent proteolytic processes: stepwise degradation of the invariant chain (li) and generation of immunogenic peptides. Their activity is controlled by intracellular CP inhibitors, including presumably the p41 isoform of invariant chain (p41 li), which is in vitro a potent inhibitor of cathepsin L but not of cathepsin S. In order to evaluate the inhibitory potential of p41 ii in antigen-presenting cells (APC), these three proteins were stained in lymph node tissue using specific monoclonal and polyclonal antibodies. The most abundant labelling was observed in subcapsular (corti cal) and trabecular sinuses of the lymph node. In this area the most freque nt APC were macrophages, as confirmed by the CD68 cell marker. Using confoc al fluorescence microscopy, co-localisation of p41 li with cathepsin S, but not with cathepsin L was found in these cells. Our results are consistent with the hypothesis that cathepsin S participates in degradation of the inv ariant chain, but they do not support the association between cathepsin L a nd p41 li in APC.