At 2 or 4 days or 1, 2, or 4 weeks after the first inoculation, rats (n ¼ 5 per time-point) were anesthetized by intraperitoneal injection of sodium pentobarbital (50 mg/kg, Abbott Laboratories, North Chicago, IL) and then perfused via the ascending aorta for 2 min with 1% paraformaldehyde in phosphate-buffered 0.9% NaCl (PBS, pH 7.4) at 120-140 mmHg.