Translational physiology: origin and phenotype of lung side population cells.

Side population (SP) cells, a rare cell type identified by their ability to efflux the vital dye Hoechst 33342, are highly enriched for stem cell activity. Bone marrow (BM) SP cells uniformly express the pan-hematopoietic marker CD45, whereas tissue SP cells are heterogeneous in CD45 expression. In previous studies, we found that CD45 is expressed on 75% of lung SP cells. By performing whole BM transplantations, we determined that CD45-positive and CD45-negative lung SP cells are marrow derived. Transplantation of 200 highly purified BM SP cells indicated that both lung SP cell subtypes are derived from this marrow cell type. Morphologically, CD45-positive lung and BM SP cells possess similar features. They are small, round, and contain scant cytoplasm. CD45-negative lung SP cells are larger and contain abundant granular cytoplasm. Gene expression patterns for hematopoietic transcription factors GATA-1, GATA-2, and PU.1 further differentiated SP marrow and lung subtypes. By immunostaining for alpha-smooth muscle actin and cytokeratin, we found significant differences in the relative expression patterns of these markers in lung and marrow SP cell subtypes. In summary, these findings demonstrate that lung SP cells are derived from the BM and that CD45-positive and -negative subtypes can be distinguished by morphological differences and gene expression patterns.