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Dendritic cell activation of CD4 T cells in the lymph node draining a site of infection or vaccination is widely considered the central event in initiating adaptive immunity. The accepted dogma is that this occurs by stimulating local activation and antigen acquisition by dendritic cells, with subsequent lymph node migration, however the generalizability of this mechanism is unclear. Here we show that in some circumstances antigen can bypass the injection site inflammatory response, draining freely and rapidly to the lymph nodes where it interacts with subcapsular sinus (SCS) macrophages resulting in their death. Debris from these dying SCS macrophages is internalized by monocytes recruited from the circulation. This coordinated response leads to antigen presentation by monocytes and interactions with naïve CD4 T cells that can drive the initiation of T cell and B cell responses. These studies demonstrate an entirely novel pathway leading to initiation of adaptive immune responses in vivo.

B helper follicular T (Tfh) cells are critical for long-term humoral immunity. However, it remains unclear how these cells are recruited and contribute to secondary immune responses. Here we show that primary Tfh cells segregate into follicular mantle (FM) and germinal center (GC) subpopulations that display distinct gene expression signatures. Restriction of the primary Tfh cell subpopulation in the GC was mediated by downregulation of chemotactic receptor EBI2. Following collapse of the GC, memory Tcells persisted in the outer follicle where they scanned CD169+ subcapsular sinus macrophages. Reactivation and intrafollicular expansion of these follicular memory Tcells in the subcapsular region was followed by their extrafollicular dissemination via the lymphatic flow. These data suggest that Tfh cells integrate their antigen-experience history to focus Tcell help within the GC during primary responses but act rapidly to provide systemic Tcell help after re-exposure to the antigen.

The precise pathways of memory T-cell differentiation are incompletely understood. Here we exploit transgenic mice expressing fluorescent cell cycle indicators to longitudinally track the division dynamics of individual CD8(+) T cells. During influenza virus infection in vivo, naive T cells enter a CD62L(intermediate) state of fast proliferation, which continues for at least nine generations. At the peak of the anti-viral immune response, a subpopulation of these cells markedly reduces their cycling speed and acquires a CD62L(hi) central memory cell phenotype. Construction of T-cell family division trees in vitro reveals two patterns of proliferation dynamics. While cells initially divide rapidly with moderate stochastic variations of cycling times after each generation, a slow-cycling subpopulation displaying a CD62L(hi) memory phenotype appears after eight divisions. Phenotype and cell cycle duration are inherited by the progeny of slow cyclers. We propose that memory precursors cell-intrinsically modulate their proliferative activity to diversify differentiation pathways.

Skin-derived dendritic cells (DCs) play a crucial role in the maintenance of immune homeostasis due to their role in antigen trafficking from the skin to the draining lymph nodes (dLNs). To quantify the spatiotemporal regulation of skin-derived DCs in vivo, we generated knock-in mice expressing the photoconvertible fluorescent protein KikGR. By exposing the skin or dLN of these mice to violet light, we were able to label and track the migration and turnover of endogenous skin-derived DCs. Langerhans cells and CD103(+)DCs, including Langerin(+)CD103(+)dermal DCs (DDCs), remained in the dLN for 4-4.5 days after migration from the skin, while CD103(-)DDCs persisted for only two days. Application of a skin irritant (chemical stress) induced a transient >10-fold increase in CD103(-)DDC migration from the skin to the dLN. Tape stripping (mechanical injury) induced a long-lasting four-fold increase in CD103(-)DDC migration to the dLN and accelerated the trafficking of exogenous protein antigens by these cells. Both stresses increased the turnover of CD103(-)DDCs within the dLN, causing these cells to die within one day of arrival. Therefore, CD103(-)DDCs act as sentinels against skin invasion that respond with increased cellular migration and antigen trafficking from the skin to the dLNs.