A presumed antagonistic LPS identifies distinct functional organization of TLR4 in mouse microglia.

Microglia as principle innate immune cells of the central nervous system (CNS) are the first line of defense against invading pathogens. They are capable of sensing infections through diverse receptors, such as Toll-like receptor 4 (TLR4). This receptor is best known for its ability to recognize bacterial lipopolysaccharide (LPS), a causative agent of gram-negative sepsis and septic shock. A putative, naturally occurring antagonist of TLR4 derives from the photosynthetic bacterium Rhodobacter sphaeroides. However, the antagonistic potential of R. sphaeroides LPS (Rs-LPS) is no universal feature, since several studies suggested agonistic rather than antagonistic actions of this molecule depending on the investigated mammalian species. Here we show the agonistic versus antagonistic potential of Rs-LPS in primary mouse microglia. We demonstrate that Rs-LPS efficiently induces the release of cytokines and chemokines, which depends on TLR4, MyD88, and TRIF, but not CD14. Furthermore, Rs-LPS is able to regulate the phagocytic capacity of microglia as agonist, while it antagonizes Re-LPS-induced MHC I expression. Finally, to our knowledge, we are the first to provide in vivo evidence for an agonistic potential of Rs-LPS, as it efficiently triggers the recruitment of peripheral immune cells to the endotoxin-challenged CNS. Together, our results argue for a versatile and complex organization of the microglial TLR4 system, which specifically translates exogenous signals into cellular functions. Importantly, as demonstrated here for microglia, the antagonistic potential of Rs-LPS needs to be considered with caution, as reactions to Rs-LPS not only differ by cell type, but even by function within one cell type.

CD14 is a key organizer of microglial responses to CNS infection and injury.

Microglia, innate immune cells of the CNS, sense infection and damage through overlapping receptor sets. Toll-like receptor (TLR) 4 recognizes bacterial lipopolysaccharide (LPS) and multiple injury-associated factors. We show that its co-receptor CD14 serves three non-redundant functions in microglia. First, it confers an up to 100-fold higher LPS sensitivity compared to peripheral macrophages to enable efficient proinflammatory cytokine induction. Second, CD14 prevents excessive responses to massive LPS challenges via an interferon ß-mediated feedback. Third, CD14 is mandatory for microglial reactions to tissue damage-associated signals. In mice, these functions are essential for balanced CNS responses to bacterial infection, traumatic and ischemic injuries, since CD14 deficiency causes either hypo- or hyperinflammation, insufficient or exaggerated immune cell recruitment or worsened stroke outcomes. While CD14 orchestrates functions of TLR4 and related immune receptors, it is itself regulated by TLR and non-TLR systems to thereby fine-tune microglial damage-sensing capacity upon infectious and non-infectious CNS challenges.

Microglial diversity by responses and responders.

Microglia are the principal resident innate immune cells of the CNS. Their contributions to the normal development of the CNS, the maintenance and plasticity of neuronal networks and the safeguarding of proper functionality are becoming more and more evident. Microglia also survey the tissue homeostasis to respond rapidly to exogenous and endogenous threats, primarily with a protective outcome. However, excessive acute activation, chronic activity or an improper adaptation of their functional performance can foster neuropathologies. A key to the versatile response behavior of these cells is their ability to commit to reactive phenotypes, which reveal enormous complexity. Yet the respective profiles of induced genes and installed functions may build up on heterogeneous contributions of cellular subsets. Here, we discuss findings and concepts that consider the variety of microglial activities and response options as being based-at least in part-on a diversity of the engaged cells. Whether it is the production of proinflammatory cytokines, clearance of tissue debris, antigen presentation or the ability to sense neurotransmitters, microglial cells present with an unanticipated heterogeneity of their constitutive and inducible features. While the organizational principles of this heterogeneity are still largely unknown, functional implications are already perceptible.