Nitric oxide hinders antibody clearance from the surface of Trypanoplasma borreli and increases susceptibility to complement-mediated lysis

Trypanoplasma borreli is an extracellular blood parasite of carp belonging to the same Order (Kinetoplastida) as African trypanosomes. These mammalian parasites have developed different strategies to evade the host immune system including antigenic variation, immunosuppression and clearance of surface-bound antibodies. The latter mechanism allows trypanosomes to use their swimming movement to cause surface-bound antibodies to ‘sail’ and accumulate at the posterior end of the parasite, to be internalized via the flagellar pocket and be degraded. There is no evidence that T. borreli shows antigenic variation, but during the late phases of infection NO-mediated immunosuppression is observed. High levels of nitric oxide (NO) lead to extensive tissue nitration whereas the parasite itself is not affected. Therefore, the induction of NO has thus far been considered a parasite-driven response with immunosuppressive effects. In the present study, we show that the induction of NO, particularly during the early phase of T. borreli infections, should be re-considered an effective part of the host immune response. We show that T. borreli rapidly removes surface-bound IgM. In addition, moderate concentrations of NO, by hindering surface antibody clearance, maintain high the concentrations of membrane-bound IgM, thereby favoring antibody-dependent complement-mediated parasite lysis. We performed a comprehensive quantitative gene expression analysis of in total seven different complement factors involved in all three activation pathways, differentiating between 1 and 4 isoforms for each complement gene. Our gene expression analysis supports an important role for antibody-dependent complement-mediated lysis of T. borreli in vivo. To our knowledge, NO-dependent inhibition of antibody clearance from the surface of kinetoplastid parasites has not been investigated. Our data support a role for NO as an important player in host–parasite interactions, not only as immune suppressor (late response) but also as immune effector (early response) in infections with bloodstream parasites such as T. borreli.