IL-1Ra does not play a role in the acute effects of IVIg. (A) C57BL/6 mice were injected with 50 mg IVIg and each mouse was bled at the times indicated on the x-axis. Sera were assessed for IL-1Ra by ELISA. Values are expressed in pg IL-1Ra/mL on the y-axis. Data are expressed as mean ± SEM; n = 4 mice from 2 separate experiments. (B) C57BL/6 mice and IL-1R–deficient mice were injected on days 0 to 3 with antiplatelet antibody to induce thrombocytopenia. On day 2, mice received 50 mg IVIg (arrow). Mice were bled daily for platelet enumeration by flow cytometry just prior to antiplatelet antibody injection. C57BL/6 mice (▾), IL-1R–deficient mice (▿); n = 6 mice per each group. (C) CD1 mice were injected with antiplatelet antibody and assessed as in panel B, and mice received either 50 mg IVIg on day 2 (□; arrow) or 50 μg rIL-1Ra on day 2 (▵); n = 6 mice per each group. (D) Mice were injected with antiplatelet antibody and received either 50 mg IVIg on day 2 (□; down arrow) or 1 mg rIL-1Ra on days 2 and 3 (▵; up arrows); n = 6 mice per each group. (E) CD1 mice were pretreated with 50 mg IVIg (column 2), nothing (column 3), or 1 mg rIL-1Ra (column 4) on day 0. On day 1, antiplatelet antibody was given as indicated. On day 2, all mice were bled for platelet counts; n = 6 mice per each group.