Kinetics of the inflammatory response following intramuscular injection of aluminum adjuvant

Aluminum-containing adjuvants are widely used in human and veterinary vaccines, but their mechanism of action is not well understood. Recent evidence suggests an important role for inflammation in the immune response to aluminum-adjuvanted vaccines. To better understand this process, vaccines with aluminum adjuvant were injected into naïve or previously immunized mice and the injection sites were characterized for the corresponding primary and secondary inflammatory response at different time points after immunization. Inflammatory cells appeared at the injection site between 2 h and 6 h after vaccination, dominated by neutrophils at first, followed by macrophages, and later eosinophils and MHCII+ cells. The number of cells at the injection site increased over time, except neutrophils, which decreased in number after day 2. There was extensive phagocytosis of aluminum adjuvant particles by macrophages. In secondary immunized mice, a faster and more robust recruitment of eosinophils, macrophages, and antigen presenting cells was observed at the injection site. The enhanced recruitment of inflammatory cells in previously immunized mice coincided with increased expression of relevant chemokines at the injection site. Since neutrophils accumulated first in response to aluminum-adjuvanted vaccines, their role was evaluated by depleting them prior to vaccination. Neutrophil depletion transiently reduced the recruitment of macrophages but it did not change the recruitment of eosinophils and MHCII+ cells or the quality and magnitude of the antibody response.