Studies on the mechanisms of immune evasion in Trypanosoma carassii infections of the goldfish (Carassius auratus L.)

Parasites possess variety of mechanisms to modulate or evade host defence systems to maintain chronic infection and ensure their transmission. The protozoan parasite Trypanosoma carassii is infective to a number of freshwater fish species and can cause significant mortality in aquaculture. T. carassii shares similarities with both T. brucei, the causative agent of sleeping sickness and T. cruzi, which causes Chagas’ disease. It is not known how T. carassii escapes host immune responses.;My doctoral research focused on the interactions between T. carassii and its fish host with emphasis on the strategies used by the parasite to evade fish immune host defense.. and establish long lasting infections. I identified at the molecular level the key antigens of T. carassii present in excretory/secretory (ES) products and surface protein fractions of the parasite. As a first step toward understanding the importance of select antigens of T. carassii, I produced the recombinant proteins and characterized their roles in immune evasion.;Like heat shock protein 70 of other pathogens, I found that T. carassii hsp70 was immunogenic and was present in both ES products and surface protein fraction of the parasite. Recombinant parasite hsp70 significantly increased expression of pro-inflammatory genes and enhanced inflammatory response of goldfish macrophages. In contrast, another parasite surface molecule, glycoprotein 63 (Gp63) down-regulated both pathogen and cytokine-induced inflammatory responses of goldfish monocytes and macrophages. Parasite gp63 was associated with macrophages and appeared to interfere with signalling mechanisms. Since complement-mediated lysis is one of the main host defence responses against trypanosomes, I cloned and characterized parasite surface molecule called calreticulin. Recombinant T. carassii calreticulin bound to first component of complement, C1q, of not only goldfish but also humans. Further, recombinant T. carassii calreticulin inhibited C1q-dependent hemolysis.;T. carassii infection of goldfish induced increased expression of pro- and anti-inflammatory cytokines. Increased cytokine mRNA levels were observed during the acute phase of infection, and then they returned to normal levels or were down-regulated during the elimination phase of the infection. These findings demonstrate that parasite surface molecules and those found in ES fraction have the capacity to manipulate host inflammatory and antimicrobial responses, thereby ensuring persistence of T. carassii in its host.