Search for correlates of resistance to virulent challenge in mice immunized with Coxiella burnetii.

Mice immunized with live phase I or phase II Coxiella burnetii, with killed phase I or phase II organisms or with trichloroacetic acid (TCAE) or phenol (PE) extracts were resistant to intraperitoneal infection with phase I C. burnetii irrespective of whether or not they displayed phase I antibody response at the time of virulent challenge. Increased phagocytosis of purified phase I organisms by blood leukocytes or peritoneal exudate cells (PEC) was noticed only in mice with phase I agglutinating antibodies in their sera or peritoneal washings. Passive transfer of resistance was made possible only by sera containing phase I agglutinating antibodies. Adoptive transfer of immunity by spleen cells, but not by PEC, was achieved providing that these cells were taken from mice immunized with live phase I C. burnetii.