Fibronectin is elevated in the cerebrospinal fluid of patients with bacterial meningitis and enhances Toll-like receptor induced inflammation in primary mouse microglial cell cultures

Objectives: Toll-like receptors (TLR) play a key role in the recognition of products from virtually all classes of pathogenic organisms. Fibronectin, an extracellular matrix (ECM) protein, is discussed to be a potent stimulator of the innate immune system through Toll-like-receptor 4. In central nervous system (CNS) infections several ECM proteins and bacterial compounds are elevated in the cerebrospinal fluid (CSF). For this reason, we hypothetized that endogenous and pathogen-derived molecules may jointly stimulate the innate immune system and increase the neuronal damage in bacterial meningitis.

Methods: Levels of fibronectin were measured in the CSF of patients suffering from bacterial meningitis, multiple sclerosis and with an elevated CSF protein due to other causes and compared to healthy controls. In primary cultures of mouse microglial cells the interaction of endogenous and exogenous stimulators of the innate immunity was examined after application of defined Toll-like receptor (TLR) agonists [lipopolysaccharide (LPS) (TLR4), tripalmytoyl-cysteinyl-seryl-(lysyl)3-lysine (Pam3Cys) (TLR2) and single-stranded unmethylated cytosine-guanosine (CpG) oligodesoxynucleotide (TLR9)] alone and in combination with fibronectin. Supernatants of stimulated glial cultures and unstimulated controls were analysed for nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha).

Results: Fibronectin was elevated in the CSF of patients suffering from bacterial meningitis and with an elevated CSF protein due to other causes, but not in patients with multiple sclerosis and in healthy control patients. Co-administration of fibronectin in a concentration which occures in bacterial meningitis [10µg/ml] with CpG, LPS or Pam3Cys to primary cultured microglial cells led to an additive release of NO and TNF-alpha.

Conclusion: The inflammatory reaction to the TLR agonists CpG, LPS and Pam3Cys in primary cultured microglial cells is enhanced by fibronectin in concentrations occuring in CSF during CNS infections. Exogenous-endogenous Co-activation leads to stronger microglial stimulation and may be in part responsible for neuronal damage occuring in these diseases.