Aldh1a2+ fibroblastic reticular cells regulate lymphocyte recruitment in omental milky spots (single cell RNA-Seq)

Milky spots, lymphoid clusters present in visceral adipose tissue omentum, play central roles in the regulation of abdominal immunity. Milky spots exhibit hybrid nature between secondary lymph organs and ectopic lymphoid tissues, yet the mechanism of their development and maturation is poorly understood. Here, we identified a subset of fibroblastic reticular cells (FRCs) that are uniquely present in omental milky spots. These FRCs were characterized by the expression of retinoic acid converting enzyme, Aldh1a2, and endothelial cell marker, Tie2, in addition to canonical FRC-associated genes. Diphtheria toxin-mediated ablation of Aldh1a2+ FRCs resulted in the alteration in milky spot structure with a significant reduction in size and cellularity. Mechanistically, Aldh1a2+ FRCs regulated the display of chemokine CXCL12 on high endothelial venules (HEVs), which recruit blood-borne lymphocytes from circulation. We further found that Aldh1a2+ FRCs are required for the maintenance of peritoneal lymphocyte composition. These results illustrate the homeostatic roles of FRCs in the formation of non-classical lymphoid tissues. single cell RNA-Seq of omental leukocytes of WT and Postn-DTR mice at 5 days post DT injection