PURPOSE OF THE STUDY.
Toll-like receptors (TLR) are innate immune receptors that recognize pathogen-associated molecules and are critical for the development of proper antimicrobial responses. Children with defects in the TLR3-signaling pathway are specifically susceptible to developing herpes simplex virus (HSV) encephalitis but not disseminated infections with HSV or other viruses. Therefore, the authors investigated whether TLR3 activity in neuronal cells was necessary for controlling HSV infection in the central nervous system (CNS).
Studies were performed by using cells from patients with inherited defects in TLR3 or UNC93B, a protein necessary for TLR3-mediated signaling.
The authors used an elegant strategy of generating inducible pluripotent stem cells (iPSC) from patients. This innovative technique involves obtaining skin fibroblasts and genetically modifying them to assume an undifferentiated and self-renewing phenotype similar to embryonic stem cells. The iPSC were cultured with certain growth factors to generate neurons or glial cells, which in turn were tested for sensitivity to HSV infection.
The authors verified that neurons and glial cells derived from patient iPSC resembled primary CNS cells both molecularly and functionally. As expected, neurons and oligodendrocytes generated from patient iPSC were defective in TLR3 signaling, as these cells were unable to secrete antiviral interferon after treatment with a TLR3 agonist. When the CNS cells were infected with HSV, viral replication was significantly greater in cells generated from TLR3- or UNC93B-deficient iPSC than those from normal subjects. The enhanced viral replication in neuronal cells was associated with impaired secretion of TLR3-dependent interferon production.
TLR3 activity in neurons and glial cells is critical for controlling HSV infection in the CNS.
This intriguing study addresses multiple issues relevant to human disease pathogenesis. First, it introduces the power of using iPSC for studying the biology of human cells that are difficult to obtain or culture ex vivo. This strategy could prove useful for studying organ systems that are generally prohibitive for analyzing at the cellular level, such as the CNS or cardiovascular system. Second, it illustrates the obvious but sometimes overlooked reality that humans and rodents are not identical. Unlike humans, mice deficient in TLR3 are susceptible to systemic infections with multiple viruses, thus highlighting the importance of verifying genetic phenotypes observed in animals with patient samples. Finally, this study provides an example that antimicrobial signaling pathways in nonimmune cells such as neurons are nonetheless essential for host defense against invading pathogens.
- Copyright © 2013 by the American Academy of Pediatrics