Endothelial TLR4: from danger sensing to vascular remodelling

This editorial refers to ‘Endothelial TLR4 coordinates angiogenesis and inflammation through a CXCR2–neutrophil–oxidative axis’ by I. Molokotina et al., https://doi.org/10.1093/cvr/cvag211. The vascular endothelium not only functions as a passive barrier but also acts as an active sensor and regulator of inflammation.1 Endothelial cells integrate metabolic, mechanical, and inflammatory signals to regulate vascular permeability, leukocyte recruitment, and thrombosis.2 In chronic cardiovascular disease, this ability can become detrimental, particularly when endogenous danger signals produced by oxidative stress perpetuate endothelial activation.2 In a recent study published in Cardiovascular Research, Molokotina et al. showed a major leap forward in this paradigm by identifying endothelial-intrinsic Toll-like receptor 4 (TLR4) as an organizing node that couples inflammatory sensing to neutrophil recruitment, oxidative lipid production, and vascular remodelling3 (Figure 1). Combining inducible endothelial-specific TLR4 knockout mice (TLR4ECKO) together with human transcriptomic datasets and pharmacological intervention, this study suggests that endothelial TLR4 induces CXCR2 ligands, recruits neutrophils, and promotes myeloperoxidase (MPO)-associated lipid oxidation, thus creating a feed-forward inflammatory loop.3 Role of endothelial TLR4 in danger sensing and vascular remodelling. TLR4 is a well-established innate immune receptor involved in danger recognition, inflammatory activation, and angiogenesis. The study by Molokotina et al. identifies a distinct endothelial TLR4-CXCL1/2/5-CXCR2 axis that promotes recruitment of CXCR2-expressing neutrophils and enhances MPO-associated lipid oxidation, creating a feed-forward loop of vascular inflammation. These findings raise several important directions for future investigation. Figure was created with BioRender.com. TLR4 has long been implicated in innate immunity and vascular inflammation.4,5 TLR4 is expressed by multiple vascular and immune cell populations, and its expression can be induced by a plethora of ligands, including bacterial lipopolysaccharides. Previous studies mostly used global TLR4 knockout models, making it difficult to determine the specific contribution of endothelial TLR4. The use of an inducible endothelial-specific knockout in the present study addresses this issue and demonstrates that endothelial TLR4 plays a distinct role in regulating the early vascular response to tissue injury. TLR4ECKO mice exhibited impaired early venular remodelling and vascular permeability, reduced junctional loosening, and diminished neutrophil-centred inflammatory-cell recruitment in non-infectious wounds and tumours; in tumours, endothelial TLR4 deletion reduced angiogenesis, inflammatory-cell recruitment, oxidative stress, and tumour growth.3 Thus, endothelial TLR4 is not simply a pathological receptor implicated in pathogen recognition and bacterial infections. Rather, the findings suggest that its activation is part of a physiological programme that permits the vascular changes required for effective early repair. This context dependence is particularly important when considering the therapeutic implications of TLR4 inhibition. A transient inflammatory response is necessary for tissue repair, whereas persistent endothelial activation may contribute to chronic vascular dysfunction.1 The study by Molokotina et al.3 provides a mechanistic explanation for how these apparently opposing functions could arise from the same receptor. Endothelial TLR4 activation induced a prominent CXCL1/2/5 chemokine programme in mice, while human endothelial cells exposed to oxidized phospholipids showed induction of CXCL8 and related CXCR2 ligands. These transcriptional findings were supported by protein-level analyses, and pharmacological CXCR2 inhibition phenocopied endothelial TLR4 deficiency with respect to vascular remodelling and leakage.3 Thus, the study moves the field beyond the relatively long-standing concept that TLR4 promotes endothelial inflammation and instead identifies a specific inflammatory effector pathway capable of linking endothelial sensing to myeloid-cell recruitment. A second important contribution of this paper is the proposed interaction between innate immune signalling and oxidative lipid biology. TLR4ECKO wounds and tumours showed reduced MPO-positive inflammatory cells and diminished carboxyethylpyrrole accumulation, whereas combined reactive oxygen species (ROS) and MPO inhibition reduced tumour growth, vascularization, inflammatory-cell accumulation, and oxidative lipid deposition. The authors therefore propose that neutrophil-derived oxidative products generate endogenous TLR4 ligands, establishing a feed-forward loop in which endothelial TLR4 recruits neutrophils that subsequently generate additional danger signals. This framework is particularly relevant to cardiovascular disease, where oxidized phospholipids are abundant and increasingly recognized as important mediators of vascular inflammation and vascular disorders. Another important aspect of the study is the demonstration that the biological consequences of endothelial TLR4 activation depend on its context, magnitude, timing, and duration of activation. Self-limited endothelial TLR4 signalling may support early repair by inducing CXCR2 ligands, neutrophil recruitment, and inflammatory venular remodelling. In contrast, excessive or persistent TLR4 activation may promote barrier disruption, MPO-dependent lipid oxidation, and chronic inflammatory amplification. This context dependence is consistent with prior studies showing that global TLR4 deficiency or loss-of-function delays healing in normal/non-diabetic wounds, whereas TLR4 deficiency or inhibition can improve healing in diabetic or diabetic-ischaemic wounds, where chronic hyperinflammation is dominant.6–8 It is therapeutically important because systemic inhibition of TLR4 has previously produced disappointing results in severe inflammatory disease such as sepsis,9 whereas selective interference with local vascular inflammatory circuits may offer greater specificity and safety. The study also raises an important question concerning the relationship between inflammation and angiogenesis. The suppression of tumour angiogenesis and growth following endothelial TLR4 deletion suggests that the same endothelial inflammatory programme can be co-opted by pathological tissues. This observation is relevant to cardiovascular pathology because pathological angiogenesis and inflammation are closely intertwined in several vascular diseases, including atherosclerosis. Previously, it has been reported that TLR4 is mechanosensitive and it may serve as a critical initiating step in atherogenesis10; however, whether the described endothelial TLR4–CXCR2–neutrophil–oxidative circuit operates similarly within the arterial wall remains to be established. The skin wound and tumour models provide strong evidence for the mechanism in these settings, but the atherosclerotic vessel presents a distinct microenvironment characterized by disturbed blood flow, lipid accumulation, hypoxia, and prolonged inflammatory signalling. However, several important questions remain unresolved. First, a critical next step will be to determine whether endothelial-specific TLR4 deletion in hyperlipidemic ApoE−/− or LDLR−/− mice alters different stages of atherosclerotic lesion development (from early to advanced stage), plaque composition, and plaque angiogenesis. Second, although pharmacological CXCR2 inhibition strongly supports the importance of CXCR2 signalling, genetic approaches distinguishing neutrophil CXCR2 from endothelial or other CXCR2-expressing cells would more precisely define the cellular mechanism, as CXCR2 is expressed in several cell types and its functions are context-dependent. Third, the proposed directionality of the oxidative feedback loop deserves further mechanistic resolution. Demonstrating directly that specific MPO-generated oxidized lipid species activate endothelial TLR4 in vivo, and determining which TLR4 co-receptors (such as MD2, which may directly sense oxidized lipid species) are required, would substantially strengthen the model. Finally, the therapeutic implications of these findings may extend beyond direct TLR4 inhibition. Systemic suppression of TLR4 could interfere with host defence and physiological tissue repair. The observation that CXCR2 blockade or ROS/MPO suppression can reproduce aspects of endothelial TLR4 deficiency raises the possibility that downstream components of the pathway might provide a more selective and safe therapeutic window. Such strategies could potentially attenuate chronic vascular inflammation while preserving the beneficial functions of transient endothelial TLR4 signalling. Determining whether this can be achieved will require careful attention to disease stage, tissue context, and duration of pathway activation. Taken together, the study by Molokotina et al.3 identifies endothelial TLR4 as a central node of inflammatory venular remodelling that integrates microbial and endogenous danger signals to coordinate chemokine secretion, neutrophil recruitment, oxidative stress amplification, and vascular leakage. The next challenge is to determine whether interrupting this circuit selectively within diseased arterial endothelium can dampen chronic vascular inflammation while preserving the beneficial functions of transient TLR4 signalling during tissue repair and host defence. Another challenge is to understand key factors that determine how endothelial TLR4 signalling shifts from a physiological component of vascular repair to a driver of persistent vascular inflammation. Resolving these issues may ultimately answer whether the endothelial TLR4-CXCR2-oxidative axis can be therapeutically targeted. S.X. conceptualized this paper. I.I. wrote the original draft. Both authors read and approved the final manuscript. None. None.