[Local reactions after diphtheria-tetanus-acellular pertussis vaccines in mice; changes in histopathology at the injection site].

Diphtheria-tetanus-acellular pertussis vaccine (DTaP) developed in Japan is now widely used worldwide. DTaP is safer than the diphtheria-tetanus-whole-cell pertussis vaccine (DTwP) and has fewer severe side effects, but local reactions such as redness, swelling, and induration are still reported. The pathophysiological mechanism of these reactions is controversial. To clarify the cause of local reactions, we conducted studies using the mouse model. After administering either one or two abdominal subcutaneous DTaP inoculations, we observed changes in histopathology at the injection site at 24h, 48h, and 7 days. The control group, inoculated with physiologic saline, showed no significant changes either pathologically or with the naked eye. All mice after DTaP vaccination showed indurations at the injection site. Pathologically, we watched leukocyte invasion into or around the site, especially neutrophils and eosinophils. After the first vaccination, the extent of the invasion was strong 24h and 7 days later. At 24h following the second vaccination, a dramatic leukocyte invasion seen persisted at 7days. At 7 days after the first vaccination, peripheral fibrosis had begun, and when a second vaccination was administered, it began even earlier at the second site. These histopathological changes show that local reactions are caused by both inflammatory and allergic responses. Because this mouse study resulted in the same pattern of reactions observed in humans, this method will be useful for studies focusing on local reactions.