Splenic fibroblastic reticular cells regulate dendritic cell maturation and T-DC interactions required for antiviral immunity
Stromal cells construct the architecture of the spleen and provide crucial signals that influence the homeostasis and migration of immune cells in the steady state. Fibroblastic Reticular Cells (FRCs) respond to infection via the regulation of many key structural and functional components. Yet, how spleen FRCs contribute to the induction and maintenance of antiviral T cell responses is still poorly defined. We found that ablation of splenic FRCs impacted the generation of virus-specific CD8 T cell during lymphocytic choriomeningitis virus (LCMV) infection in mice. Imaging revealed that during T cell priming in the presence of FRCs, CD8 T cells clustered in the T cell zone with type 1 conventional dendritic cells (cDC1) before moving to the marginal zone to interact with virus-infected cells. In the absence of FRCs, T cells instead clustered with virus-infected cells and cDC1s in the marginal zone, undergoing suboptimal priming due to reduced TCR signalling and unstable DC-T cell interactions. We found that FRCs regulated the early inflammatory wave required for optimal DC activation in vivo. The absence of FRCs thus resulted in diminished DC activation and impaired capacity to prime T cell responses. We further revealed that the presence of an intact FRC network was crucial for the generation of functional effector T cells. Importantly, these early cellular events orchestrated by FRCs also regulated the generation of protective memory T cells upon resolution of infection. Altogether, our study reveals important roles for FRCs for induction of antiviral T cell responses through support for DC maturation, effector CTL generation and the formation of protective memory T cells. Overall design: To investigate how FRC regulate early immune responses to LCMV infection, we transferred transgenic tdTomato+ P14 CD8 T cells into control and CCL19-DTR mice, followed by DT treatment to deplete white pulp CCL19+ FRCs and subesquantly infected mice with LCMV Armonsg. Spleens were harvested 1.5 days post-infection, digested and different cell populations were sorted by flow cytometry. We sorted 6 fractions consisting of tdTomato+ P14 cells, CD157+PDPN+ stromal cells, CD157-PDPN- stromal cells, CD11c+ myeloid cells, CD11c- myeloid cells, and total splenocytes. Sorted cells were then mixed in equal ratios and subjected to 10X single cell  sequencing.