Megakaryoblastic leukemia 1 (MKL1) promotes the regulation of essential cell processes, including actin cytoskeletal dynamics by co-activating serum response factor. Recently, the first human case with MKL1 deficiency, leading to a novel primary immunodeficiency, was identified. We report a second family with two siblings with a homozygous frameshift mutation in MKL1. The index case deceased as an infant from progressive and severe pneumonia by Pseudomonas aeruginosa and poor wound healing. The younger sib was preemptively transplanted shortly after birth. The immunodeficiency was marked by a pronounced actin polymerization defect and a strongly reduced motility and chemotactic response by MKL1-deficient neutrophils. Apart from the lack of MKL1, subsequent proteomic and transcriptomic analyses of patient neutrophils revealed actin and several actin-related proteins to be downregulated, confirming a role for MKL1 as transcriptional co-regulator. Degranulation was enhanced upon suboptimal neutrophil activation, while production of reactive oxygen species was normal. Neutrophil adhesion was intact but without proper spreading. The latter could explain the observed failure in firm adherence and transendothelial migration under flow conditions. No apparent defect in phagocytosis and bacterial killing was found. Also monocyte-derived macrophages showed intact phagocytosis; lymphocyte counts and proliferative capacity were normal. Non-hematopoietic primary patient fibroblasts demonstrated defective differentiation into myofibroblasts but normal migration and filamentous actin (F-actin) content, most probably due to compensatory mechanisms of MKL2, which is not expressed in neutrophils. Our findings extend current insight into the severe immune dysfunction in MKL1 deficiency, with cytoskeletal dysfunction and defective extravasation of neutrophils as most prominent features.Copyright © 2020 American Society of Hematology.