Evolutionary Insight into Immunothrombosis as a Healing Mechanism

Abstract

Both invertebrates and vertebrates possess a cluster of immediate and local wound-sealing, pathogen-killing, and tissue healing responses known as immunoclotting and immunothrombosis, respectively, to cope with two life-threatening emergencies, namely, bleeding and microbial invasion. Despite their convergence in function, immunoclotting and immunothrombosis are deployed by different blood cells and intravascular multidomain proteins. In vertebrates, these proteins share some domains with intrinsic chemical affinities useful in generating cooperative networks such as pathogen and damage pattern recognition molecules. Moreover, many of the proteins involved in coagulation and fibrinolysis in humans are multifunctional molecules playing roles in other processes from inflammation to healing and beyond. In our modern society, however, the interaction of activated intravascular allosteric proteins with one another and with blood cells entails vulnerabilities posing a biological paradox: intravascular proteins that locally operate as tissue repair enhancers can nevertheless generate pathogenic processes by acting systemically. In this manuscript, we contextualize and frame the coagulation system and hemostasis through an evolutionary time scale, illustrating their role as dual players in the defense against exsanguination and pathogens while significantly influencing wound healing.

Keywords: coagulation; complement system; evolution; fibrinogen; growth factors; healing; hemostasis; platelets.

Figure 1

An overview of some of the main biological and environmental events in the evolution of the vertebrate coagulation system. The Cambrian radiation was preceded by a whole-genome duplication that, together with genetic variations in the form of gene duplication, exon shuffling, and simple mutation, led to a period of great innovations, including the emergence of organisms (the earliest jawless vertebrate agnathans similar to today’s hagfish and lampreys) with a simple version of coagulation system involving prothrombin, tissue factor, fibrinogen, FVII, and X. The emergence of the contact system and the integration into the vertebrate coagulation system was gradual, and it has been associated with the water-to-land transition of vertebrates and the appearance of the first amphibians somewhere around 400–390 Mya. The origin of the megakaryocyte/platelet axis (M/Pa) remains to be determined, but it might have occurred with the emergence of the first mammals around 200–166 Mya (the red circle and black diamond represent the points in evolution where the first and second whole-genome duplication events have been proposed to occur).

Figure 2

Domain organization of some multidomain proteins that make up the intravascular innate immune cascade systems with the catalytic serine protease domain as a key enzymatic region. These proteins share some domains with intrinsic chemical affinities useful in generating cooperative networks as pathogen and damage pattern recognition molecules, among other functions. Moreover, many of the proteins involved in coagulation and fibrinolysis are unique in that a large number of functions may coexist in one molecule and domain, which communicate signals across very different pathways. In these multifunctional molecules, the noncatalytic regions are involved in morphogen signaling, cell–cell and cell–matrix adhesion and interaction, thereby playing roles in other processes from embryonic development to inflammation to healing and beyond. The numbers in the figure are for illustrative purposes and do not correspond to the order of appearance from an evolutionary point of view.

Figure 3

Immunothrombosis as a mosaic of defense mechanisms, including the activation of nociceptors, endothelial cells, fibroblasts, tissue-resident macrophages, circulating platelets and neutrophils, and plasma innate immune cascade systems, leads to curtailing and compartmentalizing the damage, thereby giving way to tissue repair. However, when the triggering emergency cannot be handled in a tissue autonomous manner (due to the patient status, a severe polytrauma, and/or the use of cutting-edge medical strategies), the numeric and topographic race between DAMPs and PAMPs, and the well-developed local checks tips the balance towards a gradual or massive entry of biotic and abiotic molecules into the bloodstream. In this new infectious or sterile inflammatory context, the blood-born cascade and the neuroendocrine systems may be systemically dysregulated, thereby causing a systemic inflammatory response syndrome (SIR). Moreover, the activation of the autonomic nervous system results in a barrier dysfunction that may lead to multiple organ dysfunction syndrome (MODS) and sepsis [33].

Figure 4

Autologous fibrin matrix (AFM) is a by-product of nature’s own healing systems that involves blood anticoagulation and mild centrifugation steps followed for fibrin formation by restoring thrombin production. The injected AFM into the damaged tissue operates as a liquid-to-gel dynamic scaffold that carries plasma- and platelet-derived growth factors, including TGF-β1, PDGF, VEGF, HGF, FXII, or IGF-1, as biological mediators for tissue repair. The gradual biodegradation of a fibrin clot mediated by the serine protease plasmin will release GFs that act as extracellular ligands by binding to transmembrane receptors on the surface of target cells, thereby activating intracellular signal transduction pathways to induce a wide range of cell specifications during inflammation and the healing process: cell survival, proliferation, migration, differentiation, transdifferentiation, maturation, and changes in protein synthesis and metabolism. These biologics are emerging as a promising therapeutic approach with antialgic, anti-inflammatory, trophic, antifibrotic, and tissue-specific angiogenesis effects. They are applied in sterile-inflammatory injuries such as osteoarthritis, tendinopathies, cartilage injuries, peripheral neuropathies, intervertebral disc degeneration, skin burns and ulcers, corneal ulcers, and dry eyes, among other conditions.