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.

We sought to provide phenotypic and mechanistic insights into human MD2 deficiency.

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.

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.

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.