To induce effective adaptive immune responses to antigens (i.e., virus, bacteria) that enter into a body for the first time, dendritic cells need to capture antigens, undergo maturation and present antigens to T cells. Therefore, to induce maturation of dendritic cells, effective vaccines usually contain adjuvants that activate dendritic cells. However, here we found that transient depletion of regulatory T cells induces maturation of antigen-presenting dendritic cells and succeeds in generating adaptive immune responses against SARS-CoV-2 antigens without adjuvants. Our finding is important in basic immunology and may help the current vaccine strategy to improve the responses. This research is mainly conducted by Professor Sayuri Yamazaki and Assistant Professor Ryuta Uraki#, Department of Immunology, Nagoya City University Graduate School of Medical Sciences under collabraition with Professor Yoshihiro Kawaoka, National Center for Global Health and Medicine Research Institute, Institute of Medical Science, University of Tokyo, and University of Wisconsin-Madison, USA. #Current Senior Reseacher in National Center for Global Health and Medicine Research Institute and Institute of Medical Science, University of Tokyo.