Infection of adult thymus with murine retrovirus induces virus-specific central tolerance that prevents functional memory CD8+ T cell differentiation.

In chronic viral infections, persistent antigen presentation causes progressive exhaustion of virus-specific CD8+ T cells. It has become clear, however, that virus-specific naïve CD8+ T cells newly generated from the thymus can be primed with persisting antigens. In the setting of low antigen density and resolved inflammation, newly primed CD8+ T cells are preferentially recruited into the functional memory pool. Thus, continual recruitment of naïve CD8+ T cells from the thymus is important for preserving the population of functional memory CD8+ T cells in chronically infected animals. Friend virus (FV) is the pathogenic murine retrovirus that establishes chronic infection in adult mice, which is bolstered by the profound exhaustion of virus-specific CD8+ T cells induced during the early phase of infection. Here we show an additional evasion strategy in which FV disseminates efficiently into the thymus, ultimately leading to clonal deletion of thymocytes that are reactive to FV antigens. Owing to the resultant lack of virus-specific recent thymic emigrants, along with the above exhaustion of antigen-experienced peripheral CD8+ T cells, mice chronically infected with FV fail to establish a functional virus-specific CD8+ T cell pool, and are highly susceptible to challenge with tumor cells expressing FV-encoded antigen. However, FV-specific naïve CD8+ T cells generated in uninfected mice can be primed and differentiate into functional memory CD8+ T cells upon their transfer into chronically infected animals. These findings indicate that virus-induced central tolerance that develops during the chronic phase of infection accelerates the accumulation of dysfunctional memory CD8+ T cells.

Interstitial-resident memory CD8+ T cells sustain frontline epithelial memory in the lung.

Populations of CD8+ lung-resident memory T (TRM) cells persist in the interstitium and epithelium (airways) following recovery from respiratory virus infections. While it is clear that CD8+ TRM cells in the airways are dynamically maintained via the continuous recruitment of new cells, there is a vigorous debate about whether tissue-circulating effector memory T (TEM) cells are the source of these newly recruited cells. Here we definitively demonstrate that CD8+ TRM cells in the lung airways are not derived from TEM cells in the circulation, but are seeded continuously by TRM cells from the lung interstitium. This process is driven by CXCR6 that is expressed uniquely on TRM cells but not TEM cells. We further demonstrate that the lung interstitium CD8+ TRM cell population is also maintained independently of TEM cells via a homeostatic proliferation mechanism. Taken together, these data show that lung memory CD8+ TRM cells in the lung interstitium and airways are compartmentally separated from TEM cells and clarify the mechanisms underlying their maintenance.

CXCR6 regulates localization of tissue-resident memory CD8 T cells to the airways.

Resident memory T cells (TRM cells) are an important first-line defense against respiratory pathogens, but the unique contributions of lung TRM cell populations to protective immunity and the factors that govern their localization to different compartments of the lung are not well understood. Here, we show that airway and interstitial TRM cells have distinct effector functions and that CXCR6 controls the partitioning of TRM cells within the lung by recruiting CD8 TRM cells to the airways. The absence of CXCR6 significantly decreases airway CD8 TRM cells due to altered trafficking of CXCR6-/- cells within the lung, and not decreased survival in the airways. CXCL16, the ligand for CXCR6, is localized primarily at the respiratory epithelium, and mice lacking CXCL16 also had decreased CD8 TRM cells in the airways. Finally, blocking CXCL16 inhibited the steady-state maintenance of airway TRM cells. Thus, the CXCR6/CXCL16 signaling axis controls the localization of TRM cells to different compartments of the lung and maintains airway TRM cells.

Specific niches for lung-resident memory CD8+ T cells at the site of tissue regeneration enable CD69-independent maintenance.

CD8+ tissue-resident memory T cells (TRM cells) reside permanently in nonlymphoid tissues and provide a first line of protection against invading pathogens. However, the precise localization of CD8+ TRM cells in the lung, which physiologically consists of a markedly scant interstitium compared with other mucosa, remains unclear. In this study, we show that lung CD8+ TRM cells localize predominantly in specific niches created at the site of regeneration after tissue injury, whereas peripheral tissue-circulating CD8+ effector memory T cells (TEM cells) are widely but sparsely distributed in unaffected areas. Although CD69 inhibited sphingosine 1-phosphate receptor 1-mediated egress of CD8+ T cells immediately after their recruitment into lung tissues, such inhibition was not required for the retention of cells in the TRM niches. Furthermore, despite rigid segregation of TEM cells from the TRM niche, prime-pull strategy with cognate antigen enabled the conversion from TEM cells to TRM cells by creating de novo TRM niches. Such damage site-specific localization of CD8+ TRM cells may be important for efficient protection against secondary infections by respiratory pathogens.