CD4 and pneumonic plague: A similar result was also observed in which lung CD4+ and CD8+ TRM cells significantly accumulated in the lungs of orally live attenuated Yersinia pestis-vaccinated mice and dramatically expanded, whereas short-term or long-term treatment of immunized mice with FTY720 did not affect lung TRM cell formation and expansion or protection against pneumonic plague [42] since the Yersinia pestis strain could translocate into the lung by itself after oral immunization.