Immunization with the adjuvant MF59 induces macrophage trafficking and apoptosis

The mechanisms associated with the immunostimulatory activity of vaccine adjuvants are still poorly understood. We have undertaken a study to determine whether antigen-presenting cell trafficking is modified by administration of the submicron emulsion adjuvant MF59. We investigated the fate of inflammatory macrophages after intramuscular injection of the antigen herpes simplex virus gD2 with fluorescence-labeled MF59. A homogeneous population of macrophages infiltrated the muscle, internalized adjuvant and expressed markers characteristic of mature macrophages over a 48-h period. Macrophage influx to the injection site was reduced by 70% in mice deficient for the chemokine receptor 2 (CCR2). Two distinct cell populations were shown to contain fluorescence-labeled MF59 in the draining lymph node at 48 h post injection. The first population had a round morphology, exhibited bright fluorescence, was located in the subcapsular sinus, and was apoptotic. The second population had a dendritic morphology, was weakly fluorescent, and was located in the T cell area where adjuvant-containing apoptotic bodies identified by TUNEL labeling were present. We propose that lymph node-resident dendritic cells can acquire antigen and MF59 after intramuscular immunization by uptake of the apoptotic macrophages.