Enzyme treatment of Trypanosoma danilewskyi (Laveran & Mesnil) increases its susceptibility to lysis by the alternative complement pathway of goldfish, Carassius auratus (L.)

Abstract This study examined whether in vitro ‐cultured Trypanosoma danilewskyi were susceptible to lysis in the presence or absence of anti‐parasite antibodies and complement. Cultured trypanosomes were resistant to lysis by either immune or non‐immune goldfish serum. However, trypanosomes treated with the proteolytic enzyme trypsin, which destroys surface proteins of the parasites, became susceptible to lysis when exposed to either immune or non‐immune goldfish serum. The lysis by goldfish serum was dependent on the presence of heat‐labile factors and occurred at 4 and 20 °C. The lysis was also dependent on the presence of Mg 2+ ions but not Ca 2+ ions. Furthermore, treatment of the parasites with different sialidases did not enhance their susceptibility to lysis by goldfish serum. Trypsinized parasites regained resistance to lysis after at least 6‐h cultivation in the absence of trypsin and the restoration of full resistance was observed after 24‐h cultivation. The resistance to lysis was abrogated when the protein synthesis inhibitor, puromycin, was added to the cultures. These results suggest that trypsinized trypanosomes were susceptible to lysis by goldfish complement (alternative pathway) and that protective surface proteins of the parasite were required for the resistance of normal trypanosomes to lysis.