BOTH IGM AND IGG ANTI-VSG ANTIBODIES INITIATE A CYCLE OF AGGREGATION-DISAGGREGATION OF BLOOD-STREAM FORMS OF TRYPANOSOMA-BRUCEI WITHOUT DAMAGE TO THE PARASITE

Bloodstream forms of Trypanosoma brucei, when aggregated in the presen ce of either acute immune plasma, acute immune serum, purified IgM ant i-VSG antibodies or purified IgG anti-VSG antibodies, subsequently dis aggregated with a t(1/2) for disaggregation of 15 min at 37 degrees C as long as the trypanosomes were metabolically active at the beginning of the experiment and maintained during the experiment in a suitable supporting medium. The t(1/2) for disaggregation was found to be direc tly dependent upon temperature and inversely proportional to the antib ody concentration. The trypanosomes were always motile and metabolical ly active during aggregation and after disaggregation and were fully i nfective for a mammalian host following disaggregation as well as able to grow and divide normally during axenic culture. The disaggregation was strictly energy dependent and was inhibited when intracellular AT P levels were reduced by salicylhydroxamic acid or following addition of oligomycin while respiring glucose. In addition the process of disa ggregation was dependent upon normal endosomal activity as evidenced b y its sensitivity to a wide variety of inhibitors of various endosomal functions. Disaggregation was not due to separation of immunoglobulin chains by either disulphide reduction or disulphide exchange reaction s and gross proteolytic cleavage of the immunoglobulins attached to th e surface of the parasite was not detected. In addition, gross cleavag e or release of the VSG from the surface of the cell did not occur dur ing disaggregation but proteolytic cleavage of a small proportion of e ither the VSG or the immunoglobulins could not be eliminated from cons ideration. Finally the mechanism of disaggregation was found to be a r egulated process, independent of Ca2+ movements but dependent upon the activity of protein kinase C or related kinases and inhibited by the activity of protein kinase A as evidenced by the effects of a panel of inhibitors and cAMP analogues on the process of disaggregation. The m echanism of disaggregation displayed by trypanosomes aggregated by ant i-VSG antibody is proposed to form part of the parasite's defence agai nst the host immune system and functions to aid survival of trypanosom es in the presence of antibody in the host prior to the occurrence of a VSG switching event. (C) 1998 Francqui Foundation. Published by Else vier Science B.V. All rights reserved.