Trypanosoma cruzi, the etiological agent of
Chagas' disease, expresses a
trans-sialidase at highest levels in infective trypomastigotes, where it attaches to the plasma membrane by a glycophosphoinositol linkage. Bound
enzyme sheds into the extracellular milieu in a soluble form. Experiments performed in vitro suggest that the
trans-sialidase participates in several parameters of T. cruzi-host interactions, like cell adhesion and
complement resistance. However, the role that membrane-bound and soluble
trans-sialidase plays in the
infection of mammals is not understood. To begin to study the role the
enzyme may play in vivo, T. cruzi trypomastigotes were inoculated subcutaneously into mice that had been sensitized for various times with the purified
protein. A single dose of either endogenous or recombinant
trans-sialidase injected into the connective tissues of BALB/c mice greatly enhanced
parasitemia and mortality. Maximum enhancement was achieved with 1-2-h priming. Injection of the
enzyme after the parasites had been established in the inoculation site had little, if any, consequence in modifying virulence. The enhancement did not seem to be through a direct effect of the
enzyme on trypomastigote-host cell interactions because it occurred when the sites of
trans-sialidase sensitization and parasite inoculation were physically separate. Rather, virulence enhancement seemed to depend on inflammatory cells, since priming with
trans-sialidase had no significant effect in
severe combined immunodeficiency mice, which lack functional T and B lymphocytes. However, antibody response to T. cruzi in the
trans-sialidase-primed BALB/c mice was the same as in the control animals. Virulence enhancement was specific for the
trans-sialidase because it did not occur in mice primed with Newcastle virus
sialidase, which has the same substrate specificity as the T. cruzi
enzyme, or with the
sialidase from the bacterium Vibrio cholerae, whose substrate specificity is broader than the trypanosome
sialidase. Furthermore, no enhancement of virulence occurred after sensitization with another adhesion
protein (penetrin) purified from T. cruzi trypomastigotes and engineered bacteria, nor with bacterial
lipopolysaccharide. The virulence-promoting activity of soluble
trans-sialidase in the mouse model may be physiologically relevant because it was achieved with tiny doses, approximately 1-2 microgram/kg, raising the possibility that neutralization of the
enzyme with specific probes could impair the development of
Chagas' disease. In fact, a
monoclonal antibody specific for the tandem repeat in the
trans-sialidase COOH terminus enhanced
infection of BALB/c mice, in agreement with earlier experiments in vitro, whereas
antibodies against an amino acid sequence in the Cys region had the opposite effect.