Background: The cell cytoskeleton is regulated by the ezrin-radixin-moesin (ERM) family of proteins, forming links between transmembrane proteins and the underlying actin cytoskeleton. Phosphorylation and activation of these proteins enable interactions with partners critically involved in shape regulation, such as actin filaments, transmembrane proteins, and scaffolding proteins. The MSN gene encodes moesin, which is ubiquitously expressed in lungs, spleen, kidney, endothelial cells of vessels, lymphocytes, and neutrophils. Deficiency or dysregulation of moesin, called X-linked moesin-associated immunodeficiency (X-MAID), is characterized by severe leukopenia affecting T cells, B cells, and neutrophils. To date, the clinical picture of patients with X-MAID is variable.

Aim: We describe the presentation, immune-workup, and lung histopathology findings of a young male patient with X-MAID and multi-organ involvement, whose severe pulmonary vein stenosis necessitated a double lung transplant.

Methods: A thorough review of the patient’s chart was performed.

Results: The patient presented with a history of recurrent respiratory tract infections, oral thrush, and 3 major bacterial infections requiring admission and antibiotic therapy. His immune evaluation was remarkable for low T cells, and normal numbers of B and NK cells. At age 4 years he underwent a double lung transplant due to severe pulmonary vein stenosis and pulmonary hypertension. He further developed chronic kidney injury post-transplant. Clinical trio whole exome sequencing revealed a novel hemizygous variant in the MSN gene (c.278dupT; p.L93FfsX21), predicted to cause loss-of-function in moesin. Histologic evaluation of the lung tissue before transplantation identified profound abnormalities in alveoli formation.

Conclusion: Patients with moesin deficiency may present during infancy or childhood with a severe form of the disease, including combined immunodeficiency with lymphopenia and neutropenia, while adults may have a milder clinical picture. The novel MSN mutation described here adds to the known spectrum of disease and highlights the non-redundant functions of moesin, particularly in the lung.

Statement of Novelty: We report the first lung histopathological description of an X-MAID case, in a pediatric patient with recurrent infections, cytopenia, and autoimmunity who underwent a double lung transplant.