Improved quantification of cell survival on stromal monolayers by flow cytometric analyses

The ex vivo survival of leukemic cells maintained on bone marrow stroma is an important tool for the investigation of cell survival and leukemogenesis. Currently, ex vivo survival of leukemic cell survival is measured by coculture on stromal cell monolayers. In these assays, we postulated that two important sources of error might be introduced through either variations in flow volume or in donor stromal cells.

A previously reported coculture assay that maintains leukemic cells on bone marrow stromal cells was employed.

We identified two means of optimizing the coculture assay. First, biologically inert beads having well-characterized fluorescent properties were added to each sample to mathematically adjust for flow-based variations in volume acquisition. The inclusion of fluorescent beads to the basic stromal cell assay showed a significantly lower coefficient of variation as compared to samples analyzed without beads or manually counted using a hemacytometer. Second, in order to minimize variability in bone marrow hematopoietic function between donors, an adherent stromal cell line known to support hematopoiesis (HS-5) was used. When normal human donor stromal cells were used, variability in the survival of leukemic cells was observed on stromal cells derived from different donors. In contrast, statistically significant variability in survival of leukemic cells was not seen on HS-5 monolayers. Finally, we demonstrate that patient-derived leukemic samples may be examined for cell survival using these modifications.

The novel use of fluorescent beads and a hematopoietic-supportive stromal cell line together makes the quantification of stroma-supported cell survival more reproducible, accurate, and amenable to patient-derived samples. These improvements in flow cytometry-based cell quantification are an important step in establishing a role for stromal cell assays in the study of leukemia biology and therapy.