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

J Allergy Clin Immunol. 2023 Mar;151(3):791-796.e7. doi: 10.1016/j.jaci.2022.09.033. Epub 2022 Nov 30.


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.

Keywords: TLR4/MD2; genomics; host-pathogen interactions; inborn error of immunity; inflammatory bowel disease; pediatrics.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, N.I.H., Extramural

MeSH terms

  • Cytokines / metabolism
  • Humans
  • Lipopolysaccharides* / pharmacology
  • Lymphocyte Antigen 96 / metabolism
  • Signal Transduction
  • Toll-Like Receptor 4* / genetics
  • Toll-Like Receptors / metabolism


  • Cytokines
  • Lipopolysaccharides
  • Lymphocyte Antigen 96
  • Toll-Like Receptor 4
  • Toll-Like Receptors
  • LY96 protein, human