Dear Editor,

Spleen is a secondary lymphoid organ (SLO). It is composed of white pulps, lymph node-like structures that contain T and B cells, and red pulps, which filter effete red blood cells. The architecture of spleen is supported by stromal cells that can be classified into at least four subtypes based on expression of CD31 and podoplanin (gp38): lymphatic endothelial cells (LECs, CD31+gp38+), blood endothelial cells (BECs, CD31+gp38−), fibroblastic reticular cells (FRCs, CD31−gp38+), and double-negative stromal cells (DNSCs, CD31−gp38−) (Link et al., 2007). The stromal cells form a physical framework that supports compartmentation of SLOs (Astarita et al., 2015). In addition, growing evidence indicates that stromal cells also play critical roles in immune cell homeostasis (Mueller and Germain, 2009). Stromal cells in SLOs can secrete cytokines, e.g. IL7, CCL19, and CCL21, to regulate proliferation, apoptosis, and migration of immune cells (Roozendaal and Mebius, 2011).