Not exactly new news - people have known this for something like 30 years. Sheeesh.
A current paradigm states that monocytes circulate freely and patrol blood vessels but differentiate irreversibly into dendritic cells (DCs) or macrophages upon tissue entry. Here we show that bona fide undifferentiated monocytes reside in the spleen and outnumber their equivalents in circulation.
This is a study published last month, not 30 years ago.