Modeling TLR8 Gain of Function Disease

About This Grant

Project Summary The goal of this project is to understand the mechanisms and potential therapeutic targets of a novel inborn error of immunity (IEI) caused by variants in toll like receptor 8 (TLR8). We recently identified genetic variants in TLR8 leading to gain-of-function (GOF) of the encoded protein as the cause of a new IEI presenting with profound neutropenia with recurrent infections, lymphoproliferation, T and B cell abnormalities, and bone marrow failure. Toll-like receptor 8 (TLR8) is an endosomal TLR encoded on the X chromosome that recognizes single-stranded RNA (ssRNA) and is expressed in neutrophils and other myeloid cells, including myeloid progenitors. Relatively little is known about human TLR8, largely because the murine equivalent differs in structure and ability to sense ssRNA. There is no available targeted therapy for patients with TLR8 GOF; however, patients have benefited from hematopoietic stem cell therapy, confirming that disease is driven by the immune system. There is a gap in our understanding of how GOF in TLR8 protein function leads to the clinical features of this newly recognized disease. As TLR8 expression is myeloid-specific and, in most cases of TLR8 GOF disease, expressed in a mosaic fashion (with approximately 10-20% of cells harboring TLR8 variants), we hypothesize that myeloid cells expressing mutant TLR8 produce inflammatory cytokines leading to T cell activation, and this inflammatory state and dysfunctional T cells contribute to the disease in patients including lymphoproliferation and bone marrow failure. Given the differences between mouse and human TLR8, we recently generated transgenic mice that conditionally express either WT or GOF human TLR8. Herein, we will use these novel models to address several gaps in our knowledge of TLR8 biology and TLR8 GOF disease, as well as to test potential therapies. In Aim 1, we will identify mechanisms of TLR8 GOF disease, using our transgenic mouse models and human cell xenografts to determine how TLR8 GOF myeloid cells influence hematopoiesis and T cell development and identify putative disease-driving cytokines. In addition, we will formally test the role of T cells in the disease pathogenesis by crossing TLR8 GOF mice to T-cell deficient mice and determining the effects on hematopoiesis, cytokine production and disease progression. In Aim 2, we will identify therapeutic targets of TLR8 GOF disease using inhibitors of TLR8 signaling and blocking candidate cytokines in our transgenic mice. Together, these studies will lead to an increased understanding of the pathology of TLR8 GOF and will identify therapeutic targets. The long-term goal of this application and our work is to develop a mechanistic understanding of how TLR8 GOF alters the immune response to enable improved therapies for patients.