RhoH is a negative regulator of eosinophilopoiesis.

Eosinophils are frequently elevated in pathological conditions and can cause tissue damage and disease exacerbation. The number of eosinophils in the blood is largely regulated by factors controlling their production in the bone marrow. While several exogenous factors, such as interleukin-5, have been described to promote eosinophil differentiation, comparatively little is known about eosinophil-intrinsic factors that control their de novo generation. Here, we report that the small atypical GTPase RhoH is induced during human eosinophil differentiation, highly expressed in mature blood eosinophils and further upregulated in patients suffering from a hypereosinophilic syndrome. Overexpression of RhoH increases, in a Rho-associated protein kinase-dependent manner, the expression of GATA-2, a transcription factor involved in regulating eosinophil differentiation. In RhoH-/- mice, we observed reduced GATA-2 expression as well as accelerated eosinophil differentiation both in vitro and in vivo. Conversely, RhoH overexpression in bone marrow progenitors reduces eosinophil development in mixed bone marrow chimeras. These results highlight a novel negative regulatory role for RhoH in eosinophil differentiation, most likely in consequence of altered GATA-2 levels.

DAPK2 positively regulates motility of neutrophils and eosinophils in response to intermediary chemoattractants.

The tight regulation of granulocyte chemotaxis is crucial for initiation and resolution of inflammation. Here, we show that DAPK2, a Ca(2+)/CaM-sensitive serine/threonine kinase known to modulate cell death in various cell types, is a novel regulator of migration in granulocytes. We demonstrate that human neutrophils and eosinophils express DAPK2 but unlike other leukocytes, no DAPK1 or DAPK3 protein. When DAPK activities were blocked by inhibitors, we found that neither granulocyte lifespan nor phagocytosis was affected. However, such pharmacological inactivation of DAPK activity abolished motility of granulocytes in response to intermediary but not end-target chemoattractants ex vivo. The defect in chemotaxis in DAPK2-inactive granulocytes is likely a result of reduced polarization of the cells, mediated by a lack of MLC phosphorylation, resulting in radial F-actin and pseudopod formation. As neutrophils treated with DAPKi also showed reduced recruitment to the site of inflammation in a mouse peritonitis model, DAPK2 may be a novel target for anti-inflammatory therapies.