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Abstract
Major histocompatibility complex (MHC) class II molecules acquire antigenic peptides
after degradation of the invariant chain (Ii), an MHC class II-associated protein
that otherwise blocks peptide binding. Antigen-presenting cells of mice that lack
the protease cathepsin S fail to process Ii beyond a 10 kDa fragment, resulting in
delayed peptide loading and accumulation of cell surface MHC class II/10 kDa Ii complexes.
Although cathepsin S-deficient mice have normal numbers of B and T cells and normal
IgE responses, they show markedly impaired antibody class switching to IgG2a and IgG3.
These results indicate cathepsin S is a major Ii-processing enzyme in splenocytes
and dendritic cells. Its role in humoral immunity critically depends on how antigens
access the immune system.