Establishment and Maintenance of Conventional and Circulation-Driven Lung-Resident Memory CD8+ T Cells Following Respiratory Virus Infections

Abstract

Antigen-specific CD8⁺ tissue-resident memory T cells (T_RM cells) persist in the lung following resolution of a respiratory virus infection and provide first-line defense against reinfection. In contrast to other memory T cell populations, such as central memory T cells that circulate between lymph and blood, and effector memory T cells (T_EM cells) that circulate between blood and peripheral tissues, T_RM cells are best defined by their permanent residency in the tissues and their independence from circulatory T cell populations. Consistent with this, we recently demonstrated that CD8⁺ T_RM cells primarily reside within specific niches in the lung (Repair-Associated Memory Depots; RAMD) that normally exclude CD8⁺ T_EM cells. However, it has also been reported that circulating CD8⁺ T_EM cells continuously convert into CD8⁺ T_RM cells in the lung interstitium, helping to sustain T_RM numbers. The relative contributions of these two mechanisms of CD8⁺ T_RM cells maintenance in the lung has been the source of vigorous debate. Here we propose a model in which the majority of CD8⁺ T_RM cells are maintained within RAMD (conventional T_RM) whereas a small fraction of T_RM are derived from circulating CD8⁺ T_EM cells and maintained in the interstitium. The numbers of both types of T_RM cells wane over time due to declines in both RAMD availability and the overall number of T_EM in the circulation. This model is consistent with most published reports and has important implications for the development of vaccines designed to elicit protective T cell memory in the lung.

Keywords: CD8+ T cells; lung; memory T cell maintenance; respiratory virus infections; tissue-resident memory.

Figure 1

Analysis of lung T_RM and T_EM by parabiosis experiment. Congenically distinct mice (CD45.1⁺ and CD45.2⁺) were infected i.n. with influenza virus x31 (300 EID₅₀) and subjected to parabiotic surgery 35 days later. Day 21 after the surgery, mice were injected i.v. with 1 μg anti-CD8β 3 min prior to tissue harvest. Cells in the lung airways were recovered by BAL. Lung tissues were digested by collagenase D, and enriched by centrifugation in 40/80% Percoll gradient. Cells were stained with influenza NP_366−374/D^b tetramer and fluorescent-conjugated antibodies. Data shown are derived from a CD45.2⁺ parabiont. Plots shown in (A) indicate the gating strategy of host- and partner-derived antigen-specific CD8⁺ T cells in the spleen, lung interstitium and airways. Bar graphs show ratio of host and partner cells among i.v. antibody negative cells in individual mouse. Plots shown in (B) indicate the expression of CD69, CD103, and CD49a on host- and partner-derived NP_366−374/D^b tetramer⁺ CD8⁺ T cells in the lung airways and interstitium. Host cells are the mixture of a large proportion of T_RM (CD69⁺ CD49a⁺ CD103⁺ and CD69⁺ CD49a⁺ CD103⁻) and a minor population of T_EM (CD69⁻ CD49a⁻ CD103⁻). The former population may include a small number of circulation-driven T_RM converted from host T_EM. The data also show how circulation-driven T_RM cells are a relatively small population and are difficult to identify in individual animals.

Figure 2

A comprehensive picture of memory CD8⁺ T cell populations in the lung. (A) Memory CD8⁺ T cells in the lung interstitium comprise a major population of conventional T_RM and a smaller population of T_EM. Some of the latter also give rise to T_RM in response to TNF secreted in the conditioned lung that experience prior virus infection. Both host and partner cells in the interstitium are likely recruited to the lung airways and undergo phenotypic changes induced by environmental factors in this tissue. Although lung airway memory CD8⁺ T cells represent non-circulating population, and thus, are recognized as T_RM, continual replacement is required for their maintenance. The size of the circles indicates the relative sizes of the respective populations in the lung. (B) As T_EM cells in the circulation decrease overtime after infection, input of T_EM to the lung interstitium and airways also decrease. Full recovery from the tissue damage, and resultant decrease of the size of RAMDs leads to reduction in the number of host CD8⁺ T_RM cells in the lung interstitium and airways. Consequently, the animals lost CD8⁺ T cell-mediated protective immunity in the lung. (C) Because of the lack of local antigen, bona fide CD8⁺ T_RM cells are not generated in the lung interstitium and airways. Although some T_EM cells give rise to T_RM in the lung, the extent is less than infection-experienced lung.