Inhibition of platelet aggregation by myeloid leukaemic cells demonstrated in vitro.

The effect of myeloid leukaemic cells, cells of the promyelocytic cell line HL-60 and normal polymorphonuclear granulocytes (PMN), enclosed in dialysis tubes, on the aggregation of allogeneic normal platelets after induction with arachidonic acid (AA) and with adenosine diphosphate (ADP) was investigated in vitro. Inhibition of aggregation was seen after preincubation of the platelets with leukaemic blood or bone marrow cells from 7/14 patients belonging to various FAB groups and with HL-60 cells, but not with normal PMN (14/14 cases). A dose-dependent inhibition was seen after lysis of some leukaemic cells with anti-human rabbit antiserum and active complement. Lysis of normal PMN inhibited platelet aggregation slightly and inconstantly and only at higher cell concentrations. Platelet inhibitory activity was not related to a higher rate of metabolism of the leukaemic cells which inhibited platelet aggregation since they did not differ from the cells not inhibiting aggregation with respect to heat production. Neither was a non-specifically increased cell membrane permeability the cause of the release of platelet inhibitory factor(s) since the release of 51Cr-labelled leukaemic cells was not related to the inhibition of platelet aggregation.