The neutral cysteine proteinase of Entamoeba histolytica degrades IgG and prevents its binding.

Patients infected with Entamoeba histolytica generate specific IgG that does not prevent invasive amebiasis or recurrent infection. Studies investigated whether the effectiveness of the human humoral response was limited by cleavage of IgG by the extracellular neutral cysteine proteinase of E. histolytica trophozoites, one of the first amebic products to interact directly with components of host defenses. Purified proteinase cleaved polyclonal human and monoclonal murine IgG in a dose-dependent manner. Peptide sequencing of the major cleavage fragment(s), which contained the protein A binding site, suggested that cleavage occurred near the hinge region. Intact trophozoites also cleave IgG in both growth media and serum-free media. Cleaved monoclonal antibody to a 29-kDa surface antigen of E. histolytica bound to trophozoites 83.5% +/- 6.7% less than did uncleaved antibody. These results suggest that cleavage of IgG by the extracellular cysteine proteinase may limit the effectiveness of the host humoral response.