Multiple mechanisms of immune evasion by African trypanosomes.

During infection of a mammalian host, African trypanosomes are in constant contact with the host's immune system. These protozoan parasites are infamous for their ability to evade the immune responses by periodically switching their major variant surface glycoprotein (VSG), a phenomenon called antigenic variation. Antigenic variation, however, is likely to be only one of several mechanisms enabling these organisms to thrive in the face of the immune defenses. The ability to grow in high levels of interferon-gamma (IFN-gamma) and to avoid complement-mediated destruction may also facilitate the parasite's survival. In this review we summarize (i) the activation of trypanosome genes for three different VSGs during antigenic variation, (ii) the secretion of a trypanosome protein that induces host CD8 T cells to produce IFN-gamma, and (iii) the evidence for trypanosome protein similar to a surface protease of Leishmania that plays a role in resistance to complement-mediated lysis.