Immunoglobulin subclass responses of wild brown rats to Sarcocystis singaporensis

Immunoglobulin subclass responses of wild brown rats (Rattus norvegicus) fr om southeastern Asia to the endemic cyst-forming coccidian Sarcocystis sing aporensis were characterised. The antibody response of brown rats to wild-t ype parasites thigh reproductive capacity) showed a Th1 profile during acut e infection, namely elevated concentrations of parasite-specific IgG2b and IgG2c and absence of IgG1. Chronic infection (bradyzoite development) resul ted in a mixed Th1/Th2 pattern whereby significant concentrations of IgG1 a ppeared. A primary infection with 1000 sporocysts eight days before challen ge induced protection, accompanied by significant concentrations of IgA and IgG2, particularly IgG2a. Western blot analysis of rat sera, using sporozo ite and bradyzoite-extracts as antigen, revealed that IgG1, IgG2a, and IgG2 b predominantly recognised molecules between 70-80 kDa in one or the other stage. Some of the antibodies were possibly directed against a 79 kDa heat shock protein of sporozoites. An apparent unresponsiveness to molecules in the low molecular weight range, particularly of bradyzoite antigens, was ob served. This was abrogated by infection of rats with an avirulent strain of S. singaporensis (low reproductive capacity) indicating that a parasite th at was less adapted to its host provoked a stronger immune response. These results suggest the existence of an immune evasion strategy used by Sarcocy stis and show that wild rodents may be suitable as immunological research o bjects, reflecting the natural situation. (C) 2001 Australian Society for P arasitology Inc. Published by Elsevier Science Ltd. All rights reserved.