Trichinella spiralis: Activation of complement by infective larvae, adults, and newborn larvae

Abstract The ability of Trichinella spiralis to activate complement (C) has been addressed by several investigators. However, these investigators employed methods in which either detection of C fragments on the parasite surface or the adherence of leukocytes to the parasite was considered an indication of C activation. The present studies were undertaken to examine: (a) whether activation of C occurs via the classical and/or alternative pathway, (b) at which stage(s) of the parasite C activating capacity is acquired, and (c) what molecular entities of the epicuticle and/or cuticle are responsible for initiating C activation. Our studies indicate that T. spiralis activates C primarily via the alternative pathway (and weakly via the classical pathway) since incubation of parasites obtained from infected mice with either normal human serum (NHS) or Mg. EGTA-NHS, followed by incubation (1 hr, 37 °C) with antibody-sensitized sheep erythrocytes or rabbit erythrocytes, respectively, showed a time-and parasite number-dependent depletion of C. Although the three stages of T. spiralis , i.e., infective larvae, adults and newborn larvae, are capable of activating C, the newborn appears to be the most potent activator, especially when parasite number and size are taken into consideration. Further evidence of C activation is obtained from SDS-PAGE and Western blot analysis in which homogenates of parasites preincubated with NHS showed the presence of C3, C9, and Clq, whereas controls without serum were negative. Since isolated Clq was also capable of directly binding to the surface of adults and infective larvae, it is postulated that their cuticle and/or epicuticle may possess surface structures which serve as binding sites for Clq. Activation of C appears to occur in the absence of a specific antibody; it is therefore assumed that the parasite surface expressed structures that may directly bind to and activate C. Taken together, our results demonstrate that the surface of all three stages of T. spiralis possess a molecule(s) that can recognize and trigger C activation.