TRYPANOSOMA-CRUZI TRANS-SIALIDASE - ENHANCEMENT OF VIRULENCE IN A MURINE MODEL OF CHAGAS-DISEASE

Trypanosoma cruzi, the etiological agent of Chagas' disease, expresses a trans-sialidase at highest levels in infective trypomastigotes, whe re it attaches to the plasma membrane by a glycophosphoinositol linkag e. Bound enzyme sheds into the extracellular milieu in a soluble form. Experiments performed in vitro suggest that the trans-sialidase parti cipates in several parameters of T. cruzi-host interactions, like cell adhesion and complement resistance. However, the role that membrane-b ound 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 th at had been sensitized for various times with the purified protein. A single dose of either endogenous or recombinant trans-sialidase inject ed into the connective tissues of BALB/c mice greatly enhanced parasit emia and mortality. Maximum enhancement was achieved with 1-2-h primin g. Injection of the enzyme after the parasites had been established in the inoculation site had little, if any, consequence in modifying vir ulence. The enhancement did not seem to be through a direct effect of the enzyme on trypomastigote-host cell interactions because it occurre d when the sites of trans-sialidase sensitization and parasite inocula tion 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-sia lidase because it did not occur in mice primed with Newcastle virus si alidase, which has the same substrate specificity as the T. cruzi enzy me, or with the sialidase from the bacterium Vibrio cholerae, whose su bstrate specificity is broader than the trypanosome sialidase. Further more, no enhancement of virulence occurred after sensitization with an other adhesion protein (penetrin) purified from T. cruzi trypomastigot es 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 tin y doses, similar to 1-2 mu g/kg, raising the possibility that neutrali zation of the enzyme with specific probes could impair the development of Chagas' disease. In fact, a monoclonal antibody specific for the t andem repeat in the trans-sialidase COOH terminus enhanced infection o f BALB/c mice, in agreement with earlier experiments in vitro, whereas antibodies against an amino acid sequence in the Cys region had the o pposite effect.