Abstract 303: Arteriogenesis Requires Leukocytic Co-Signaling of TLR2 and TLR4
Background: Collateral artery growth (arteriogenesis) is promoted by the accumulation of leukocytes in the perivascular tissue. Ligand binding to Toll-Like Receptors (TLR), a family of 13 receptors which forms an important part of the innate immune system, leads to activation of inflammatory cells such as leukocytes. We therefore hypothesized that TLRs, in particular TLR expression on leukocytes, plays a prominent role in adaptive arteriogenesis.
Methods: Localization of TLR expression in growing collateral arteries was assessed by immunohistochemistry. The role of TLR signaling during arteriogenesis was assessed in TLR2−/− mice on a C57BL/6J background, TLR4−/− mice on a BALB/c background and their respective controls after femoral artery occlusion. Furthermore, 6 weeks old mice underwent lethal irradiation and subsequent bone marrow (bm) transplantation before undergoing femoral artery occlusion 6 weeks later. Wild-type mice (WT) received either WT bm or TLR −/− bm, whereas TLR −/− mice received WT bm. On day 7, microsphere perfusion measurements were performed to assess collateral function.
Results: Immunohistochemistry revealed that TLRs are mainly expressed by infiltrating leukocytes during arteriogenesis. In TLR2 and TLR4 deficient mice, collateral perfusion was significantly reduced compared to controls (perfusion occluded vs. unoccluded hind limb: TLR 2 −/−: 37.7 ± 1.8 %; C57BL6J: 54.9 ± 6.0%; TLR 4−/−: 15.8 ± 5.1%; Balb-C: 25.1± 5.1 %; p<0.01) After transplantation of WT bm into WT mice, collateral function was decreased to a minor, non significant degree compared to WT mice (C57BL/6J into C57BL/6J: 49.7 ± 2.6%; BALB/c bm into BALB/c: 23.1 ± 1.0%. After transplantation of TLR −/− bm into WT mice, arteriogenesis was significantly reduced (TLR2 −/− bm into C57BL/6J: 38.2 ± 3.5%; TLR4 −/− bm into BALB/c: 16.7 ± 1.5%), whereas WT bm transplantation into TLR −/− mice normalized the arteriogenic response (C57BL/6J bm into TLR2 −/−: 46.5 ± 5.5%; BALB/C bm into TLR4 −/−: 22.6 ± 3.3%).
Conclusion: Arteriogenesis is impaired under conditions of TLR2 or TLR4 deficiency. Among the many different cell types that are able to express Toll-Like receptors, TLR signaling of circulating leukocytes seems to be of major importance for adaptive arteriogenesis.