TLR4 Accessory Molecule RP105 (CD180) Regulates Monocyte-Driven Arteriogenesis in a Murine Hind Limb Ischemia Model

Abstract

AimsWe investigated the role of the TLR4-accessory molecule RP105 (CD180) in post-ischemic neovascularization, i.e. arteriogenesis and angiogenesis. TLR4-mediated activation of pro-inflammatory Ly6Chi monocytes is crucial for effective neovascularization. Immunohistochemical analyses revealed that RP105+ monocytes are present in the perivascular space of remodeling collateral arterioles. As RP105 inhibits TLR4 signaling, we hypothesized that RP105 deficiency would lead to an unrestrained TLR4-mediated inflammatory response and hence to enhanced blood flow recovery after ischemia.Methods and ResultsRP105−/− and wild type (WT) mice were subjected to hind limb ischemia and blood flow recovery was followed by Laser Doppler Perfusion Imaging. Surprisingly, we found that blood flow recovery was severely impaired in RP105−/− mice. Immunohistochemistry showed that arteriogenesis was reduced in these mice compared to the WT. However, both in vivo and ex vivo analyses showed that circulatory pro-arteriogenic Ly6Chi monocytes were more readily activated in RP105−/− mice. FACS analyses showed that Ly6Chi monocytes became activated and migrated to the affected muscle tissues in WT mice following induction of hind limb ischemia. Although Ly6Chi monocytes were readily activated in RP105−/− mice, migration into the ischemic tissues was hampered and instead, Ly6Chi monocytes accumulated in their storage compartments, bone marrow and spleen, in RP105−/− mice.ConclusionsRP105 deficiency results in an unrestrained inflammatory response and monocyte over-activation, most likely due to the lack of TLR4 regulation. Inappropriate, premature systemic activation of pro-inflammatory Ly6Chi monocytes results in reduced infiltration of Ly6Chi monocytes in ischemic tissues and in impaired blood flow recovery.

TLR4 Deters Perfusion Recovery and Upregulates Toll-like Receptor 2 (TLR2) in Ischemic Skeletal Muscle and Endothelial Cells

“…Our current studies add to these findings, showing that the suppression of inflammation was associated with a more rapid angiogenesis and perfusion recovery. Similar findings were reported in mice lacking CD180, a molecule that attenuates TLR4-induced inflammation, where profound inflammation and compromised arteriogenesis were observed (40,41). In another report, TLR4KO mice undergoing myocardial ischemia/reperfusion developed smaller infarct areas and less myocardial-activated caspase-3 staining compared with wild-type mice (42).…”

Ager Deletion Enhances Ischemic Muscle Inflammation, Angiogenesis, and Blood Flow Recovery in Diabetic Mice

“…In the case of the toll receptor family, for example, it was shown that extensive cross-talk between Tlr4 and Tlr2 is required in mediating adaptive responses to hind limb ischemia, when tested in non-diabetic mice. 59, 60 The present work adds RAGE to the cadre of genes that exhibit tissue microenvironment-dependent roles in inflammation and tissue regeneration. In this context, a burgeoning body of evidence links RAGE to opposing outcomes in murine models of infection challenge.…”

RP105 Protects Against Apoptosis in Ischemia/Reperfusion-Induced Myocardial Damage in Rats by Suppressing TLR4-Mediated Signaling Pathways

“…It is a key negative regulator of TLR4 signaling pathways, and may do so as RP105/myeloid differentiation protein-1(MD-1) or TLR4/MD-2 complexes [13,14]. The RP105/MD-1 [13,14].…”

“…The radioprotective 105 kDa protein (RP105), a single-pass type I membrane protein belonging to the toll-like receptor family, is a specific inhibitor of the TLR4-triggered response [13][14][15][16]. The expression of RP105 is strongly correlated with the inflammatory response of MIRI [12].…”