In vivo protection of V.opt DNA-vaccinated mice treated with PBS, anti-CD8a (dCD8), or anti-CD4 (dCD4) monoclonal antibody prior to lethal challenge. Balb/C mice were challenged with a lethal dose of 80,000 cfu Y. pestis (KIM strain) by intranasal inoculation at one week after the 4th DNA immunization and treated for 5 days with PBS (20 mice), anti-CD8a (15 mice), or anti-CD4 (10 mice) monoclonal antibody. Cumulative survival curves were plotted to show the protection for each group as indicated. The vector control group (20 mice) was included as a negative control. “*” shows the statistical difference, p<0.05 compared to the PBS-treated group.