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 one 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. Additionally, 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 demonstrates key requirements for long-lived cellular immunity to influenza virus, knowledge that could be utilized in future vaccine design.