Interleukin-la (IL-la) Induces Thrombocytosis in Mice: Possible Implication

We administered recombinant human interleukin-la (ILla). the common mediator of inflammation process, to C,,B1/6 male mice (0.5 pg, every 12 hours over five times) intraperitoneally and consequently induced a remarkable thrombocytosis. Day 1 was designated as the following day of the last injection in the morning. A significant thrombocytosis was observed on days 1 through 5 with a peak on day 2 (162 f 9 x 104/mm3) compared with the control mice injected with heated IL-la (101 k 11 x 104/mm3). A striking increase in mean size of marrow megakaryocytes was noted on days 1 and 2. The incorporation of selenomethionine into circulating platelets as a measure of platelet production was about 2.3 times higher in IL-18treated mice than in control mice. To determine which factor(s) is responsible for elicited thrombocytosis, the in vitro studies and bioassays for several hematopoietic factors were performed. IL-lB by itself did not stimulate megakaryocytopoiesis in vitro, suggesting that the thrombocytosis is attributed to other factor(s) via IL-la stimulation. Serum colony-stimulating factor (CSF) activity after a single IL-la (0.5 pg) injection, monitored by colony assay