Identification and characterization of splenic adherent cells forming densely-packed colonies.

It is thought that the spleen contains stem cells that differentiate into somatic cells other than immune cells. We investigated the presence of these hypothetical splenic cells with stem cell characteristics and identified adherent cells forming densely-packed colonies (Splenic Adherent Colony-forming Cell; SACC) in the spleen. Splenic Adherent Colony-forming Cell was positive for alkaline phosphatase staining and stage-specific embryonic antigen (SSEA)-1 antigen. However, the self-renewal properties of SACCs were limited because they stopped cell proliferation once colonies visible to the naked eye were formed. Gene expression analyses by semi-quantitative RT-PCR revealed the significant expression of c-Myc and Klf4, whereas faint or no expression was evident for Nanog, Oct3/4, and Sox2. Global expression analyses by DNA microarray and subsequent gene ontology analyses revealed that the expression levels of genes related to the immune system were significantly lower in SACCs than in control splenic cells. In contrast, genes unrelated to the immune system, such as those involved in cell adhesion and axon guidance, were relatively highly expressed in SACCs compared with control splenic cells. Taken together, we identified a novel cell type residing in the spleen that is different from the hypothetical splenic stem cell, but which bears some, but not all, characteristics that represent an undifferentiated state.