Some cellular mechanisms governing self and nonself recognition and pathogenicity in vertebrates and invertebrates relative to protistan parasites

Abstract

Knowledge about the mechanisms by which three intracellular protistan pathogens of humans, Leishmania donovani, Toxoplasma gondii, and Trypanosoma cruzi, avoid being destroyed by their host cells, and, therefore, interpreted to represent “self”, is reviewed, along with what is known about the mechanisms by which four protistan parasites of oysters ( Ostrea edulis and Crassostrea virginica), namely, Bonamia ostreae, Marteilia refringens, Hasplosporidium nelsoni, and Perkinsus marinus, become camouflaged as self. Although considerably more is known about the mechanisms by which the three pathogens of mammals avoid being killed, studies on the selected parasites of molluscs have also contributed to our understanding of how parasites avoid being destroyed by their hosts. Also, the suppression of immunocompetence in vertebrates and invertebrates is known to favor infection and virulence on the part of certain parasites. In invertebrates, such as molluscs, that possess open circulatory systems, certain environmental factors are known to reduce immunocompetence and render such hosts more susceptible to parasitism and the expression of pathogenicity by protists.