Abstract 55: Romiplostim Treatment Restores GPVI Expression and Platelet Counts in Mice Lacking the ITIM-Containing Receptor G6b-B

Abstract

We recently demonstrated that the immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptor G6b-B plays a critical role in regulating platelet homeostasis. Mice lacking G6b-B exhibit a complex phenotype that includes severe macrothrombocytopenia, platelet surface immunoglobulins, reduced expression of the collagen activation receptor GPVI, and aberrant platelet function. In this study, we tested the effects of the thrombopoietin-mimetic romiplostim in ameliorating defects seen in G6b-B-deficient mice. Subcutaneous administration of romiplostim (100 μg/kg romiplostim every three days) restored platelet counts to normal levels in G6b-B-deficient mice within two weeks. Platelet surface GPVI expression was also elevated by five-fold in romiplostim-treated G6b-B-deficient mice compared with platelets from mice treated with vehicle alone, restoring platelet reactivity to the GPVI-specific agonist collagen-related peptide (CRP). In contrast, the same romiplostim regimen induced a 40% reduction in platelet surface GPVI expression in control mice, resulting in comparable levels of GPVI in romiplostim-treated G6b-B-deficient and control mice. Megakaryocyte counts were dramatically increased in the bone marrow and spleen of romiplostim-treated G6b-B-deficient and control mice, accompanied by severe myelofibrosis in both genetic backgrounds. Bone marrow-derived megakaryocytes from G6b-B-deficient mice grew normally in vitro in the presence of thrombopoietin, but exhibited reduced proplatelet formation on a fibrinogen-coated surface. Findings from this study demonstrate that romiplostim can be used to restore GPVI expression in addition to platelet counts in G6b-B-deficient mice, but has severe side-effects on bone marrow and spleen myelofibrosis. This approach can now be applied to further investigate the functional role of G6b-B in regulating platelet counts and reactivity. This work was funded by the British Heart Foundation.