Lung resident memory (Trm) CD8 T cells induced by influenza A virus (IAV) are pivotal for providing heterosubtypic immunity, but are not maintained long term, causing gradual loss of protection. This contrasts sharply with long-term maintenance of Trm induced by localized infections of the skin and other tissues. Here we show that the decline in lung Trm is determined by an imbalance between apoptosis and lung recruitment and conversion to Trm of circulating memory cells. At the cellular level, circulating effector memory (Tem) rather than central memory (Tcm) cells are the precursors for conversion to lung Trm. Time-dependent changes in expression of genes critical for lymphocyte trafficking and Trm differentiation, in concert with enrichment of Tcm, diminish the capacity of circulating memory CD8 T cells to form Trm with time, explaining why IAV-induced Trm are not stably maintained. Importantly, systemic booster immunization, through increasing the number of circulating Tem cells, increases lung Trm, providing a potential new avenue for future IAV vaccines.

A paradigm shift in the understanding of the exhausted CD8+ T cell (Tex) lineage is underway. Originally thought to be a uniform population that progressively loses effector function in response to persistent antigen, single-cell analysis has now revealed that CD8+ Tex is composed of multiple interconnected subpopulations. The heterogeneity within the CD8+ Tex lineage is comprised of immune checkpoint blockade (ICB) permissive and refractory subsets termed stem-like and terminally differentiated cells, respectively. These populations occupy distinct peripheral and intratumoral niches and are characterized by transcriptional processes that govern transitions between cell states. This review presents key findings in the field to construct an updated view of the spatial, transcriptional, and functional heterogeneity of anti-tumoral CD8+ Tex. These emerging insights broadly call for (re-)focusing cancer immunotherapies to center on the driver mechanism(s) underlying the CD8+ Tex developmental continuum aimed at stabilizing functional subsets.

Activated CD8+ T cells differentiate into cytotoxic effector (TEFF) cells that eliminate target cells. How TEFF cell identity is established and maintained remains less understood. Here we show Runx3 deficiency limits clonal expansion and impairs upregulation of cytotoxic molecules in TEFF cells. Runx3-deficient CD8+ TEFF cells aberrantly upregulate genes characteristic of follicular helper T (TFH) cell lineage, including Bcl6, Tcf7 and Cxcr5. Mechanistically, the Runx3-CBFβ complex deploys H3K27me3 to Bcl6 and Tcf7 genes to suppress the TFH program. Ablating Tcf7 in Runx3-deficient CD8+ TEFF cells prevents the upregulation of TFH genes and ameliorates their defective induction of cytotoxic genes. As such, Runx3-mediated Tcf7 repression coordinately enforces acquisition of cytotoxic functions and protects the cytotoxic lineage integrity by preventing TFH-lineage deviation.

SUMMARY Lung-resident primary memory CD8+ T cell populations (Trm) induced by a single influenza infection decline within months, rendering the host susceptible to new heterosubtypic influenza infections. Here, we demonstrate that, relative to single virus exposure, repeated antigen exposure dramatically alters the dynamics of influenza-specific lung Trm populations. Lung Trm derived from repeatedly stimulated circulating memory CD8+ T cells exhibit extended durability and protective heterosubtypic immunity relative to primary lung Trm. Parabiosis studies reveal that the enhanced durability of lung Trm after multiple antigen encounters resulted from the generation of long-lasting circulating effector memory (Tem) populations, which maintained the ability to be recruited to the lung parenchyma and converted to Trm, in combination with enhanced survival of these cells in the lung. Thus, generating a long-lasting Trm precursor pool through repeated intranasal immunizations might be a promising strategy to establish long-lasting lung Trm-mediated heterosubtypic immunity against influenza.

The successful development of a mucosal vaccine depends critically on the use of a safe and effective immunostimulant and/or carrier system. This review describes the effectiveness and mode of action of an immunostimulating particle, derived from bacteria, used in mucosal subunit vaccines. The non-living particles, designated bacterium-like particles are based on the food-grade bacterium Lactococcus lactis. The focus of the overview is on the development of intranasal BLP-based vaccines to prevent diseases caused by influenza and respiratory syncytial virus, and includes a selection of Phase I clinical data for the intranasal FluGEM vaccine.