Long-term maintenance of lung resident memory T cells is mediated by persistent antigen

Tissue-resident memory T cells (T_RM) in the lungs are pivotal for protection against repeated infection with respiratory viruses. However, the gradual loss of these cells over time and the associated decline in clinical protection represent a serious limit in the development of efficient T cell based vaccines against respiratory pathogens. Here, using an adenovirus expressing influenza nucleoprotein (AdNP), we show that CD8 T_RM in the lungs can be maintained for at least 1 year post vaccination. Our results reveal that lung T_RM continued to proliferate in situ 8 months after AdNP vaccination. Importantly, this required airway vaccination and antigen persistence in the lung, as non-respiratory routes of vaccination failed to support long-term lung T_RM maintenance. In addition, parabiosis experiments show that in AdNP vaccinated mice, the lung T_RM pool is also sustained by continual replenishment from circulating memory CD8 T cells that differentiate into lung T_RM, a phenomenon not observed in influenza-infected parabiont partners. Concluding, these results demonstrate key requirements for long-lived cellular immunity to influenza virus, knowledge that could be utilized in future vaccine design.