Susceptibility to merocyanine 540‐mediated photosensitization: A differentiation marker on murine hematopoietic progenitor cells

Merocyanine 540 (MC 540) is an impermeant fluorescent dye that binds preferentially to fluidlike domains of the cell membrane. Photoexcitation of membrane‐bound dye causes a breakdown of the normal permeability properties of the membrane and, eventually, cell death. We have used in vitro and in vivo clonal assays to determine the relative sensitivities of different classes of normal murine hematopoietic progenitor cells to MC 540‐mediated photo‐sensitization. Late erythroid progenitors (CFU‐E) were the most sensitive cells, followed in order of decreasing sensitivity by early erythroid progenitors (BFU‐E), megakaryocyte progenitors (CFU‐Meg), day 7‐spleen colony forming cells (day 7‐CFU‐S), granulocyte/macrophage progenitors (CFU‐GM), and day 11‐spleen colony forming cells (day 11‐CFU‐S). Bipotent progenitors of the granulocyte/macrophage lineage were more sensitive than unipotent macrophage progenitors but less sensitive than unipotent granulocyte progenitors. Progenitors giving rise to large granulocyte/macrophage colonies were more sensitive than progenitors giving rise to small colonies (“clusters”). We conclude that sensitivity to MC 540‐mediated photosensitization is developmentally regulated and that differences occur even between the most closely related classes of progenitor cells. Our findings indicate the usefulness of MC 540 as a plasma membrane probe. They also support the contention that early and late‐appearing spleen colonies are the progeny of two distinct classes of progenitor cells.