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.

Introduction: NF-κB proteins are transcription factors that modulate various functions of the immune system. NF-κB2 (or p100/p52) has particularly important roles in B cell development and function. Primary immunodeficiency due to mutations in the NFKB2 gene, encoding NF-κB2, range from combined immunodeficiency with susceptibility to viral or opportunistic infections to primarily antibody deficiency.Methods: A comprehensive chart review of our patient was performed.Results: Our patient, currently a 19-year-old male, presented with multiple autoimmune cytopenia resistant to treatment and generalized granulomatous lymphadenopathy. Whole exome sequencing identified a novel pathogenic variant in NFKB2 (c.1700C>T; p.A567V) that is the cause of our patient’s presentation.Conclusion: We present a novel pathogenic variant in NFKB2 with an unusual presentation.Statement of novelty: Here, we report a novel mutation in NFKB2 and the clinical presentation of the affected patient, which helps in further understanding the NF-κB2 pathway and its associated disease.