Demonstration of heat-labile and heat-stable epitopes of Rickettsia japonica on ultrathin sections

BACKGROUND: Spotted fever group rickettsiae including Rickettsia japonica have two major polypeptides and lipopolysaccharide (LPS)-like antigen on the surface. The major part of the antibodies to these polypeptides generated by immunization with live rickettsiae are reactive with heat-labile and conformation-dependent epitopes. EXPERIMENTAL DESIGN: To demonstrate and precisely localize the heat-labile and heat-stable epitopes on the major surface polypeptides and LPS-like antigen of R. japonica, the immunocolloidal gold method was performed on ultrathin sections with polyclonal and monoclonal antibodies. The antigenicity and fine structure were preserved by using periodate-lysine-paraformaldehyde as fixative, followed by embedding with the resin Lowicryl K4M at a polymerization temperature of -35° C. RESULTS: Heat-labile and heat-stable epitopes on the two major surface polypeptides together with LPS-like antigen of R. japonica were demonstrated at the same location. These antigens were rather broadly distributed on the cell wall apart from the slime layer of the organism. The number of gold particles corresponding to the LPS-like antigen was much larger than that corresponding to the polypeptide antigen. CONCLUSIONS: Heat-labile epitopes of rickettsiae could be preserved and demonstrated by using the procedure cited here. Moreover, the precise localization of the major surface polypeptides and LPS-like antigen could be achieved in the cell wall by using ultrathin sections of infected cells instead of whole rickettsial cells. The LPS-like antigen was quantitatively predominant as judged by immunoelectron microscopy.