Superior growth of glycophosphatidy linositol-anchored protein-deficient progenitor cells in vitro is due to the higher apoptotic rate of progenitors with normal phenotype in vivo

Objective Recently, phenotypically normal CD34 cells from the marrow of patients with paroxysmal nocturnal hemoglobinuria (PNH) were reported to show impaired growth and elevated Fas receptor expression as compared to glycophosphatidylinositol-anchored protein (GPI-AP)-deficient CD34 cells and CD34 cells from normal individuals. These results are consistent with the theory that PNH cells have an intrinsic growth advantage, but their superior expansion in vitro could also be the outcome of selective extrinsic pressure in vivo. Material and Methods Growth characteristics, competitive features, and susceptibility to apoptosis of sorted normal or GPI-AP–deficient CD34+ cells derived from PNH patients were assessed in suspension and methylcellulose cultures. Results When we directly compared the growth of patientsʹ CD34 cells, separated based on expression of GPI-AP CD55 and CD59, in most of the patients studied, mutant CD34 cells showed higher progeny production and outgrew phenotypically normal CD34 cells derived from PNH patients in mixing experiments. However, their proliferation rate did not exceed that of control CD34 cells. To determine whether deficient growth of phenotypically normal CD34 cells in PNH was secondary to a pre-existing in vivo insult, we determined the fraction of apoptotic cells within fresh normal and PNH CD34 cells. Normal CD34 cells from PNH patients showed a high proportion of apoptotic cells and higher Fas expression, while GPI-AP–deficient and control CD34 cells showed similar, low rates of apoptosis. After correction for pre-existing apoptosis, the proliferation potential of normal and PNH CD34 cells was similar. Conclusions These results strongly suggest that clonal expansion of GPI-AP–deficient progenitor cells from PNH patients is due to their selection in the hostile marrow environment of the patient.