Microarray analysis of splenic reservoir monocytes and their blood counterparts

A current paradigm states that monocytes circulate freely and patrol blood vessels, but differentiate irreversibly into dendritic cells or macrophages upon tissue entry. Here we show that bona fide undifferentiated monocytes reside in the spleen and outnumber their equivalents in circulation. The reservoir monocytes are relatively immotile, assemble in clusters in the cords of the subcapsular red pulp, and are distinct from macrophages and dendritic cells. In response to ischemic myocardial injury, splenic monocytes increase their motility, exit the spleen en masse, accumulate in injured tissue and participate in wound healing. These observations uncover a role for the spleen as a site for storage and rapid deployment of monocytes, and identify the splenic monocyte reservoir as a resource that the body exploits to regulate inflammation. The goal of this gene expression study was to compare the gene expression of Ly-6C hi inflammatory monocytes residing in the spleen and their circulating counterparts in the blood.