Comparison of antigenic reactivity between intracellular and extracellular rickettsiae of spotted fever group.

Antigens of intracellular rickettsiae in Vero cells persistently infected with Japanese YH strain of the spotted fever group (Rickettsia japonica sp. nov.) reacted in the immunofluorescence test with sera from patients to the same extent by IgG antibody titration as did antigens of extracellular rickettsiae prepared from culture fluid of lytically infected BHK cells. On the other hand, IgM antibody titers measured with intracellular rickettsiae in Vero cells were significantly higher in some cases than those obtained with extracellular rickettsiae. Antigens of intracellular rickettsiae in BHK cells infected with the Japanese strain reacted with patient's sera to the same extent as those in Vero cells.