Immune Surveillance by Effector and Memory CD8+ T Cells

During priming, CD8+ T cells integrate a plethora of signals that affect their differentiation into subsets of CD8+ T cells with distinct migratory properties and functions. Given that CD8+ T cells exert their protective function via cell-cell contacts, the migratory patterns and spatial distribution of CD8+ T cell subsets induced by primary challenge are of critical importance to the host. Dendritic cells (DCs), as the primary initiators of these responses, play a pivotal role in shaping the size and differentiation status of CD8+ T cells that emerge. However, inadequate markers for CD8+ T cell subsets have hindered study of their lineage relationships, as well as their migratory behaviors. Here, we use a novel marker for identification of CD8+ T cell subsets to interrogate whether subsets of DCs skew CD8+ T cell fate decisions, the differentiation pattern of CD8+ T cell subsets, and the migratory behavior of these CD8+ T cell subsets. Within secondary lymphoid organs (SLOs), CD8+ T cells encounter subsets of DCs that may differentially impact subsequent T cell fate decisions. While distinct DC subsets were found to influence CD8+ T cell priming and subsequent differentiation in vitro, these differences were masked when priming occurred in vivo. This prompted us to delve deeper into how CD8+ T cell subsets are defined in vivo. Classically, memory CD8+ T cells are divided into two subsets: central memory T cells (TCM) and effector memory T cells (TEM). Based on the variable expression of the chemokine receptor, CX3CR1, we define TCM as CX3CR1- and TEM as CX3CR1high. Additionally, a previously undefined subset of T cells was identified that express intermediate levels of CX3CR1. Flow cytometric analysis of the subsets migrating through murine peripheral tissues in the memory phase established CX3CR1int cells as the dominant subset, thus these cells have been termed peripheral memory T cells (TPM). Lineage tracing of these three subsets established a uni-directional relationship where increasing levels of CX3CR1 marked further terminal differentiation: TCM (CX3CR1-) to TPM (CX3CR1int) to TEM (CX3CR1hi). The finding that TPM, and not TEM, migrated through peripheral tissues was intriguing because it contradicted previous studies suggesting that TEM had this migratory pattern. To resolve this contradiction, we visualized the migration of TEM precursors, CX3CR1hi effector T cells (TEff), by intravital multi-photon microscopy (IV-MPM) in real-time. Surprisingly, CX3CR1hi TEff adhered to and patrolled along the dermal endothelium of mice. Specifically, migration was enriched along arteriolar endothelium and tended to be against blood flow. Patrolling occurred for both CX3CR1hi TEff and TEM and was limited to CX3CR1hi CD8+ T cells and CX3CR1hi monocytes, but was not dependent on functional CX3CR1. Moreover, addition of cognate antigen (Ag) resulted in rapid stopping of Ag-specific T cells, suggesting that patrolling T cells scan arteriolar endothelium for cognate Ag. Together, these results challenge the paradigm that TEM function by migration through peripheral tissues and establish a new migratory behavior by TEM and their effector precursors that promotes intravascular scanning of arteriolar endothelium.Medical Science