Lymph node stromal cells vary in susceptibility to infection but can support the intracellular growth of Listeria monocytogenes

Lymph node stromal cells (LNSCs) are an often overlooked component of the immune system but play a crucial role in maintaining tissue homeostasis and orchestrating immune responses. Our understanding of the functions these cells serve in the context of bacterial infections remains limited. We previously showed that Listeria monocytogenes, a facultative intracellular foodborne bacterial pathogen, must replicate within an as-yet-unidentified cell type in the mesenteric lymph node (MLN) to spread systemically. Here, we show that L. monocytogenes could invade, escape from the vacuole, replicate exponentially, and induce a type I interferon response in the cytosol of 2 LNSC populations infected in vitro, fibroblastic reticular cells (FRCs) and blood endothelial cells (BECs). Infected FRCs and BECs also produced a significant chemokine and proinflammatory cytokine response after in vitro infection. Flow cytometric analysis confirmed that GFP⁺ L. monocytogenes were associated with a small percentage of MLN stromal cells in vivo following foodborne infection of mice. Using fluorescent microscopy, we showed that these cell-associated bacteria were intracellular L. monocytogenes and that the number of infected FRCs and BECs changed over the course of a 3-day infection in mice. Ex vivo culturing of these infected LNSC populations revealed viable, replicating bacteria that grew on agar plates. These results highlight the unexplored potential of FRCs and BECs to serve as suitable growth niches for L. monocytogenes during foodborne infection and to contribute to the proinflammatory environment within the MLN that promotes clearance of listeriosis.