Human MD2 deficiency-an inborn error of immunity with pleiotropic features.

BACKGROUND: Toll-like receptors (TLRs) are important pattern recognition receptors that sense microbes and control host defense. Myeloid differentiation protein 2 (MD2) is the indispensable coreceptor for TLR4, facilitating the binding to the gram-negative bacterial cell wall component LPS and activation of downstream signaling. OBJECTIVE: We sought to provide phenotypic and mechanistic insights into human MD2 deficiency. METHODS: To elucidate the genetic cause in a patient with very early onset inflammatory bowel disease, we performed whole-exome sequencing and studied the functional consequences of the identified mutation in LY96 (encoding for MD2) in genetically engineered induced pluripotent stem cell-derived macrophages with knockout of MD2 or knockin of the patient-specific mutation, including TLR4-mediated signaling, cytokine production, and bacterial handling. RESULTS: Whole-exome sequencing identified a homozygous in-frame deletion in the LY96 gene (c.347_349delCAA; p.Thr116del) in a patient with very early onset inflammatory bowel disease and a sibling presenting with pneumonia and otitis media. Induced pluripotent stem cell-derived macrophages with knockout of MD2 or expression of the Thr116del mutation showed impaired activation of nuclear factor kappa B and mitogen-activated protein kinase signaling as well as TLR4 endocytosis on challenge with LPS or bacteria. In addition, MD2-deficient macrophages showed decreased cytokine expression (eg, IL-6, TNF, and IL-10) in response to LPS or gram-negative but not gram-positive bacteria. CONCLUSIONS: Human MD2 deficiency causes defective TLR4 signaling in response to LPS or gram-negative bacteria. The clinical manifestations and expressivity might be variable due to unknown secondary risk factors. Because TLR4 represents a therapeutic target for multiple inflammatory conditions, our study may provide insights into potential side effects of pharmacological TLR4 targeting.