Immunomagnetic bead isolation of megakaryocytes from guinea-pig bone marrow: effect of recombinant interleukin-6 on size, ploidy and cytoplasmic fragmentation.

Guinea-pig bone marrow megakaryocytes were isolated using an antibody to platelet glycoprotein Ib and a second antibody conjugated to magnetic beads. The procedure yielded an average of 644,800 megakaryocytes from two guinea-pigs with an average viability of 83%. All of the platelet glycoprotein Ib positive cells also expressed the platelet glycoprotein IIb-IIIa complex. The size and ploidy of megakaryocytes isolated by this technique were analysed in the presence of 10 ng/ml of interleukin-6 (IL-6). Without IL-6 megakaryocyte size increased significantly after 24 h, but an even larger increase in size occurred in the presence of IL-6. The modal ploidy class was 16N with an average of 19% 2N, 2.6% 4N, 16.4% 8N, 50.8% 16N and 11.1% 32N cells as determined by flow cytometry. Measurements made by microspectrophotometry were in close agreement. After 24 h incubation there was a significant rise in the percentage of 2N and 32N cells. The ploidy distribution after 24 h with IL-6 was the same as the control. Megakaryocytes cultured in the absence of serum on collagen gels did not form pseudopods and fragment, as occurs with serum (Leven et al, 1987). Addition of IL-6 to the serum-free cultures caused megakaryocytes to form extensive proplatelet extensions. We conclude that large numbers of pure guinea-pig bone marrow megakaryocytes can be isolated by immunomagnetic bead selection, including low ploidy immature megakaryocytes. Spontaneous maturation occurred as evidenced by the increase in megakaryocyte size and ploidy. IL-6 altered megakaryocyte size and morphology but not ploidy, indicating that these different characteristics of megakaryocytes may be regulated separately.