Infection with the Endonuclear Symbiotic Bacterium Holospora obtusa Reversibly Alters Surface Antigen Expression of the Host Paramecium caudatum.

It is known that the ciliate Paramecium cell surface including cilia is completely covered by high-molecular-mass GPI-anchored proteins named surface antigens (SAgs). However, their functions are not well understood. It was found that ciliate Paramecium caudatum reversibly changes its SAgs depending on the absence or presence of the endonuclear symbiotic bacterium Holospora obtusa in the macronucleus. Immunofluorescence microscopy with a monoclonal antibody produced SAg of the H. obtusa-free P. caudatum strain RB-1-labeled cell surface of the H. obtusa-free P. caudatum RB-1 cell but not the H. obtusa-bearing RB-1 cell. When this antibody was added to the living P. caudatum RB-1 cells, only H. obtusa-free cells were immobilized. An immunoblot with SAgs extracted from Paramecium via cold salt/ethanol treatment showed approximately 266-kDa SAgs in the extract from H. obtusa-free cells and 188 and 149-kDa SAgs in the extract from H. obtusa-bearing cells. H. obtusa-free RB-1 cells produced from H. obtusa-bearing cells via treatment with penicillin-G-potassium re-expressed 266-kDa SAg, while the 188 and 149-kDa SAgs disappeared. This phenotypic change in the SAgs was not induced by degrees of starvation or temperature shifts. These results definitively show that Paramecium SAgs have functions related to bacterial infection.