Moesin
T cell aggregation
X-linked primary immunodeficiency associated with hemizygous mutations in the moesin (MSN) gene.
<h4>Background</h4>We investigated 7 male patients (from 5 different families) presenting with profound lymphopenia, hypogammaglobulinemia, fluctuating monocytopenia and neutropenia, a poor immune response to vaccine antigens, and increased susceptibility to bacterial and varicella zoster virus infections.<h4>Objective</h4>We sought to characterize the genetic defect involved in a new form of X-linked immunodeficiency.<h4>Methods</h4>We performed genetic analyses and an exhaustive phenotypic and functional characterization of the lymphocyte compartment.<h4>Results</h4>We observed hemizygous mutations in the moesin (MSN) gene (located on the X chromosome and coding for MSN) in all 7 patients. Six of the latter had the same missense mutation, which led to an amino acid substitution (R171W) in the MSN four-point-one, ezrin, radixin, moesin domain. The seventh patient had a nonsense mutation leading to a premature stop codon mutation (R533X). The naive T-cell counts were particularly low for age, and most CD8<sup>+</sup> T cells expressed the senescence marker CD57. This phenotype was associated with impaired T-cell proliferation, which was rescued by expression of wild-type MSN. MSN-deficient T cells also displayed poor chemokine receptor expression, increased adhesion molecule expression, and altered migration and adhesion capacities.<h4>Conclusion</h4>Our observations establish a causal link between an ezrin-radixin-moesin protein mutation and a primary immunodeficiency that could be referred to as X-linked moesin-associated immunodeficiency.