No abstract

Background: The posttraumatic response to traumatic brain injury (TBI) is characterized, in part, by activation of the innate immune response, including the complement system. We have recently shown that mice devoid of a functional alternative pathway of complement activation (factor B-/-mice) are protected from complement-mediated neuroinflammation and neuropathology after TBI. In the present study, we extrapolated this knowledge from studies in genetically engineered mice to a pharmacological approach using a monoclonal anti-factor B antibody. This neutralizing antibody represents a specific and potent inhibitor of the alternative complement pathway in mice.

Reduced infarct volume in TLR2-knockout mice compared with C57Bl/6 wild-type mice has recently been shown in experimental stroke and confirmed in this study. We now also show a significant decrease of CD11b-positive cell counts and decreased neuronal death in the ischemic hemispheres of TLR2-deficient mice compared with C57Bl/6wt mice 2 days after transient focal cerebral ischemia. To examine the potential benefit of intravascular TLR2 inhibition, C57Bl/6wt mice were treated intraarterially with TLR2-blocking anti-TLR2 antibody (clone T2.5) after 45 minutes of cerebral ischemia and compared with control antibody (isotype) treated wild-type mice. Whereas T2.5-treated mice had no reduction in infarct volumes at 48 hours after reperfusion, they did have decreased numbers of CD11b-positive inflammatory cells and decreased neuronal death compared with isotype-treated control mice. Comparison of the isotype antibody treatment to control (saline) treatment showed no effects on infarct volumes or neuronal survival. However, mice treated with the control isotype antibody had increased numbers of CD11b-positive inflammatory cells compared with saline-treated animals. Thus, antibody treatment itself (i.e., control isotype antibody, but potentially of any antibody) may have adverse effects and limit therapeutic benefit of anti-TLR2-antibody therapy. We conclude that TLR2 mediates leukocyte and microglial infiltration and neuronal death, which can be attenuated by TLR2 inhibition. The TLR2 inhibition in vivo improves neuronal survival and may represent a future stroke therapy.

Several reports have recently demonstrated a detrimental role of Toll-like receptors (TLR) in cerebral ischemia, while there is little information about the endogenous ligands which activate TLR-signaling. The myeloid related proteins-8 and-14 (Mrp8/S100A8; Mrp14/S100A9) have recently been characterized as endogenous TLR4-agonists, and thus may mediate TLR-activation in cerebral ischemia. Interestingly, not only TLR-mRNAs, but also Mrp8 and Mrp14 mRNA were found to be induced in mouse brain between 3 and 48 h after transient 1 h focal cerebral ischemia/reperfusion. Mrp-protein was expressed in the ischemic hemisphere, and co-labeled with CD11b-positive cells. To test the hypothesis that Mrp-signaling contributes to the postischemic brain damage, we subjected Mrp14-deficient mice, which also lack Mrp8 protein expression, to focal cerebral ischemia. Mrp14-deficient mice had significantly smaller lesion volumes when compared to wild-type littermates (130+/-16 mm(3) vs. 105+/-28 mm(3)) at 2 days after transient focal cerebral ischemia (1 h), less brain swelling, and a reduced macrophage/microglia cell count in the ischemic hemisphere. We conclude that upregulation and signaling of Mrp-8 and-14 contribute to neuroinflammation and the progression of ischemic damage.