We have previously shown that macrophage infection with Mycobacterium tuberculosis and M. bovis bacillus Calmette-Guérin (BCG) partially inhibits MHC class II surface expression in response to IFN-gamma. The present study examined the nature of class II molecules that do in fact reach the surface of infected cells. Immunostaining with specific Abs that discriminate between mature and immature class II populations showed a predominance of invariant chain (Ii)-associated class II molecules at the surface of BCG-infected cells suggesting that mycobacteria specifically block the surface export of peptide-loaded class II molecules. This phenotype was due to inhibition of IFN-gamma-induced cathepsin S (Cat S) expression in infected cells and the subsequent intracellular accumulation of alphabeta class II dimers associated with the Cat S substrate Ii p10 fragment. In contrast, infection with BCG was shown to induce secretion of IL-10, and addition of blocking anti-IL-10 Abs to cell cultures restored both expression of active Cat S and export of mature class II molecules to the surface of infected cells. Consistent with these findings, expression of mature class II molecules was also restored in cells infected with BCG and transfected with active recombinant Cat S. Thus, M. bovis BCG exploits IL-10 induction to inhibit Cat S-dependent processing of Ii in human macrophages. This effect results in inhibition of peptide loading of class II molecules and in reduced presentation of mycobacterial peptides to CD4(+) T cells. This ability may represent an effective mycobacterial strategy for eluding immune surveillance and persisting in the host.