Anaphylaxis is a systemic reaction with a range of clinical manifestations due to the varying involvement of several organs and systems. At the cellular and molecular levels, most existing research focuses exclusively on the immune component. However, interactions between vascular and immunological microenvironments are responsible for most manifestations of anaphylaxis, including the most severe. Therefore, precise understanding of the cellular communication between immune and resident cells within the complex pathophysiology of this pathological event is necessary. This review provides an overview of the human vascular system in anaphylaxis and seeks further understanding of the dysregulation of its underlying processes.

Anaphylaxis is considered the most serious manifestation of allergic disorders. The World Health Organization defines it as “a severe, life-threatening systemic hypersensitivity reaction characterized by being rapid in onset with potentially life-threatening airway, breathing, or circulatory problems and is usually, although not always, associated with skin and mucosal changes” (1). In the United States of America (USA) alone, annual expenditures due to anaphylaxis total 1.8 billion dollars between direct and indirect costs (2). The World Allergy Organization has found that its incidence and prevalence have increased over the last decade. In 2020, global incidence was 50–112 episodes per 100,000 people, and its lifetime prevalence was between 0.3% and 5.1%. Moreover, although the mortality rate remains low (0.05–0.51, 0.03–0.32, and 0.09–0.13 per million people/year for drug-, food-, and venom-induced lethal reactions, respectively), the rate of recurrence was 26.5%–54.0% over a follow-up time of 1.5–25 years (1, 3). However, current data underestimate the true actual rate due to factors such as the lack of a common definition of anaphylaxis or discrepancies in the methodologies used in epidemiological studies (1, 4). In addition, diagnosis of this pathologic event is based on clinical symptoms that are shared with other diseases, causing anaphylactic reactions to be underdiagnosed (5). Thus, in order to complement diagnosis, molecular markers are also evaluated. Currently, the main biomarker used in clinical practice is serum tryptase, a molecule released by the effector cells of anaphylaxis. The diagnostic reference threshold for this measurement has undergone considerable modifications to the current value of 11.4 μg/l. However, its clinical utility has several drawbacks since serum tryptase is also elevated in other conditions such as mast cell disorders (6). In addition, it is not increased in most patients who develop an anaphylactic reaction (7). This could be due to the sample collection as the peak of this protein is obtained 2 h after the reaction (8). Moreover, most studies have now demonstrated the importance of considering the basal value of serum tryptase at least 24 h after the episode (9, 10). Specifically, the increase of 20% plus 2 μg/l in the acute condition with respect to baseline revealed a substantial improvement in the diagnosis of anaphylaxis (11). Therefore, correct sampling and personalized diagnosis of each individual, as well as the need to find new biomarkers, are crucial for the correct management of this life-threatening reaction.

Several triggers can induce anaphylaxis, the most common being foods, drugs, and Hymenoptera venoms, although in some cases the cause of the reaction is unknown (idiopathic) (12–14). In particular, the etiological distribution of anaphylaxis differs with age since the most common triggers are drugs and insect stings in adults, while food is the main culprit in children (5, 15). However, regardless of the allergen, the signs and symptoms of anaphylaxis are the same and affect several systems. Cutaneous symptoms (e.g., localized urticaria, erythema, angioedema, pruritus) are the most common manifestations and are present in 80% of cases. Other systems may be affected, such as the gastrointestinal (e.g., emesis, diarrhea), nervous (e.g., confusion, drowsiness, seizure), respiratory (e.g., dyspnea, cough, wheezing, bronchoconstriction), and circulatory (e.g., palpitations, tachycardia, hypotension) (16). The cardiovascular system is highly involved during mild anaphylactic reactions and plays a key role in the most severe cases, in which anaphylactic shock could take place (17, 18). Deterioration of the circulatory system due to age, other diseases, and treatments administered such as angiotensin-converting enzyme inhibitors are some of the main risk factors associated with fatal reactions (19–21). Therefore, the cardiovascular system is essential in the development of the reaction and a key target to developing future therapeutic strategies.

Overall, a large number of anaphylactic mediators coming from different cellular sources are released both into the bloodstream and in local microenvironments ( Table 1 ). The nature of these molecules is very diverse: vasoactive agents, proteases, lipidic particles, cytokines, chemokines, interleukins, hormones, and neurotransmitters, among others. In addition, due to the homeostatic imbalance and acute inflammatory state characteristic of anaphylaxis, complement and contact/coagulation cascades are involved in the pathophysiology of this event leading to the release of numerous intermediate products (42, 44, 102).

Classically, the best-characterized mediators have been classified into preformed and newly synthesized. Those stored in cytoplasmic granules contain highly sulfated polysaccharides (heparin or other proteoglycans), tryptase, chymase, histamine, and PAF, being mainly released from mast cells and basophils (22, 24). However, other cellular providers, such as neutrophils or platelets, also supply key molecular effectors as PAF to the reaction (103). Molecules derived from arachidonic acid, interleukins, chemokines, and/or cytokines are also massively released (42, 104, 105). In summary, the diversity of triggers and/or signals described upstream to the cellular activation induces different degranulation strategies in mast cells (106). Probably, it also occurs with other participating cells of the anaphylactic reaction. Into the downstream signaling following cellular activation, the influx of mediators causes the recruitment of other immune cells but also the activation of resident cells that contribute, in turn, with a multitude of other anaphylactic products amplifying the molecular and cellular signaling (107, 108).

In relation, multitudes of soluble products from both the contact (bradykinin) and the complement system (C3a, C4a, C5a) are also released in the pool of mediators (109, 110). The activation of the coagulation, fibrinolytic, contact, and complement system pathways is highly involved in anaphylaxis. Peptides C3a, C4a, and C5a derived from the proteolysis of the C3, C4 and C5 components of the complement system are considered, along IgG molecules, the main elicitors of non-IgE anaphylactic reactions. These molecules, often known as anaphylatoxins, induce mast cell degranulation through its specific receptors on their surface (35). On the other hand, contact and coagulation/fibrinolytic systems are closely related to each other and are involved in the so-called non-immunologic anaphylactic reactions. Coagulation factors such as kininogen, fibrinogen, and factors V and VII are decreased in anaphylactic patients (44).

In addition, other molecular players such as extracellular vesicles (EVs), microRNAs, and metabolites are just starting to gain relevance as surrogate biomarkers in anaphylaxis but also participating in their underlying molecular pathways (13, 111–113).

Altogether, this large variety of molecular signaling pathways contributes to amplify the number and heterogeneity of processes occurring in these pathological events.

The cellular components of the vascular compartment are both target and effector resident cells in anaphylaxis. Thus, the following section expands on the morphological and functional particularities of the cardiovascular system in order to broaden its knowledge in anaphylactic reactions. The circulatory system is composed of a set of interconnected tubular organs that form a closed circuit in the human body. It is divided into the blood vascular system, which carries blood, and the lymphoid vascular system, which transports lymph (114). However, in this review, we will only focus on the former.

The functions of the blood vascular system are as follows: transportation of substances entering the body from the external environment (mainly nutrients and oxygen), transfer of molecules such as hormones, hydraulic force generation, regulation of heat, ultrafiltration in the kidney, and defense through the transport of immune cells and mediators (115). This circuit is made up of the heart and a number of different types of vessels that distribute blood to the organs. Elastic arteries emerge from the heart and branch out to the muscular arteries and arterioles. The latter give rise to the capillaries, and from these, venules and veins return blood to the heart (114, 116, 117) ( Figure 2 and Box 1 ).

All blood vessels have a common structure that is mainly composed of three layers, i.e., tunica intima, tunica media, and tunica adventitia (ti, tm, and ta, respectively), but morphological and functional differences exist between them. The ti consists of a single and extensive layer of endothelial cells (ECs) that forms a physical barrier between blood and tissues, allowing the selective transport of molecules through it (132). This morphological and functional structure is named the endothelium, and it participates in a multitude of vital functions such as modulating coagulation/fibrinolysis, regulating transportation of inflammatory cells, controlling cellular metabolism, sustaining homeostasis of resident stem cells, guiding organ repair, and releasing inflammatory and angiocrine factors (133, 134). The lumen of the endothelium is covered by a set of proteoglycans and glycoproteins (glycocalyx) which also confer multifunctional and dynamic properties to this layer (135). Thus, ti plays a critical role in many physiologic and pathologic processes and as a result the endothelium is considered an organ in itself (136). At the cellular level, ECs are very heterogeneous (137). Their morphology, function, and gene and antigen composition vary between different organs and sections of the vasculature (138, 139). For instance, the capacity of EC to quickly contract in response to inflammatory and vasoactive mediators leads up to endothelial breakdown, resulting in increased vascular permeability, which occurs mainly in the microvasculature (37, 140–143). At the molecular level, two main canonical pathways regulate endothelium stability: the actin-myosin cytoskeleton and those molecular processes related to the tight and adherent junctions (144–147).

The next tunica, tm, is located in the middle area of the vessels determining their diameter and is usually composed of several layers of vascular smooth muscle cells (VSMCs) and/or elastin fibers supported mainly by a collagen matrix. VSMCs are fundamental homeostatic players in different diseases such as hypertension and atherosclerosis (148, 149). These cells contain the main molecular machinery to directly regulate the vascular tone (dilatation and constriction) mainly by changes in their intracellular Ca²⁺ (iCa²⁺) levels. Indirectly and with the main purpose of maintaining homeostasis, ECs also contribute to vascular tone modulation via synthesis and release of vasoactive substances (150, 151). Specifically, the main relaxant product released by ECs is nitric oxide (NO) (152–154).

The outer layer of the vessel is the ta, which is responsible for the integrity and resistance to physical stress of the vascular wall. This layer consists mainly of connective tissue and contains vasa vasorum. These small vessels irrigate the whole wall of the large vessel since nutrients and oxygen cannot reach all cells by diffusion (114, 116, 155–157). In addition, this layer might present nerve fibers which regulate vascular tone and which are also altered during pathological processes (158).

Decades of investigations have demonstrated the importance of specific interactions between immune and vascular cells in different diseases (159). In anaphylaxis, the participation of immune cells leading to the final release of mediators is very well defined (42). Nevertheless, their impact throughout the vascular niche is less clear. Both the rupture of the endothelial barrier and vascular tone disturbances are the essential anomalies observed during this pathologic event. In addition, phenomena occurring in the chest cavity (e.g., cardiac arrest, decreased cardiac output, hypoxia) are present in anaphylactic reactions ( Figure 3 ). Molecularly, the EC- and VSMC-signaling pathways govern these processes ( Figure 4 ). A dysfunctional endothelium is the cause of important cardiovascular diseases such as thrombosis, atherosclerosis, or hypertension. Even a damaged endothelium has been observed in patients with mastocytosis (160). Additionally, in acute inflammatory situations, as it has been observed in COVID-19 disease, ECs contribute as effector cells to the cytokine storm (161). In addition, the endothelium actively participates in the activation of the coagulation, contact, and complement in anaphylaxis (44, 162, 163). Therefore, its study is a hot topic and of great importance for the treatment of several pathologies such as anaphylaxis.

A wide variety of soluble mediators have been proposed as participants of the anaphylaxis pathophysiology (46, 71). The majority of these molecules are ligands for the largest group of membrane receptors which are those linked to heterotrimeric G proteins (GPCRs). Additionally, they have been implicated in a variety of diseases (224–226). However, GPCR downstream signaling pathways are not well characterized for every anaphylactic mediator, although their impact on vascular permeability and vasodilation is well established ( Table 2 ).

In this sense, most of the studies have mainly been carried out in experimental models (37). Specifically, anaphylactic shock depending on endothelial Gq/G11 was characterized in mice (247).

To date, the first-line and most effective anaphylaxis treatment is intramuscular administration of adrenaline/epinephrine (245). It is indicated after recognition of symptoms due to its ability to directly remedy alterations in the cardiovascular system caused by the reaction. Its administration prevents cardiovascular collapse and enhances blood flow during anaphylactic shock (12, 248, 249). Mechanistically, this drug resolves anaphylaxis via its different adrenergic receptors ( Figure 5 ). Epinephrine exerts its vasoconstrictor action through α-adrenergic receptors on VSMCs. Its activation causes stimulation of PLC-β, which cleaves PIP2 into DAG and IP3. This increased intracellular IP3 binds to its receptors, leading to higher iCa²⁺ levels and subsequent myosin phosphorylation (250, 251). The result of this pathway is a peripheral vasoconstriction that reverses peripheral vasodilation alleviating hypotension and reducing edema (252, 253). On the other hand, epinephrine, through β-adrenergic receptors and subsequent Gα(s) activation, results in enhanced AC activity, which promotes cAMP formation and the stability of the ECs barrier (254). In addition, through β1 receptors it increases heart rate and its force of contraction, while β2 receptor-mediated action restores bronchoconstriction and reduces the release of inflammatory mediators by the main immune effector cells, mast cells, and basophils (251, 252, 255). In bronchial smooth muscle cells (BSMCs), epinephrine restores its constriction through the β2-adrenergic receptor. cAMP production activates PKA, which phosphorylates and inactivates the MLCK. These facts stop the downstream signal for contraction and thus relax BSMCs (251). However, this drug administration is not always effective and patients may still die.

Epinephrine would be ineffective in cardiopathy patients receiving β-blockers, and glucagon is occasionally administered. The action of this drug is not mediated by adrenergic receptors and allows reversal of refractory hypotension and bronchospasm (1, 249). Moreover, there are other complementary therapies which may be useful but should never replace the first-line treatment. These include supplemental oxygen, β2-agonists to reverse bronchospasm, intravenous crystalloid solutions to restore circulatory volume, antihistamines to control cutaneous symptoms, and glucocorticosteroids to prevent biphasic reactions and reduce inflammation (256).

Currently, various investigations on potential alternative drugs for the treatment of anaphylaxis are being conducted, although they are not implemented in clinical routine yet. Methylene blue is an inhibitor of NO pathways and has been reported to be a safe medication for refractory hypotension underlying anaphylactic shock (257). On the other hand, sugammadex, a g-cyclodextrin, has been shown to encapsulate and inactivate neuromuscular blocking agents, which may be triggers of this pathological event. However, its usefulness remains controversial since a recent study of perioperative anaphylaxis treated with this drug concluded that it did not modify the reaction (258). In addition, promising results have been obtained from mouse models where PAF antagonists have been shown to mitigate the severity and duration of anaphylaxis (259).

Otherwise, desensitization interventions have been proposed for the long-term treatment and prevention of anaphylaxis (260). Specifically, biological agents are emerging as a potential adjuvant for this process (36, 261). Among them, omalizumab, a monoclonal antibody against IgE, has been shown to be a successful treatment in reducing the number and severity of anaphylactic reactions (262). In addition, several therapies are appearing for the prevention of food-induced anaphylaxis. These are aimed to modulate specific immune pathways in different ways, such as antibodies against the main cytokines involved in the response (anti-IL-33, anti-IL-5, etc.) or immunotherapies to polarize the Th2 response characteristic of allergic reactions to Th1 and T regulatory (Treg) ones through toll-like receptor (TLR) agonists (TLR4 and TLR9), nanoparticles, probiotics, or IFNγ, among others (263–265).

Understanding the extent of human vessels in anaphylaxis is the challenge we have taken on in this study. Although the vascular system functions as a whole, specific features are identified in the different types of vessels. Their specialization is based on the morphology, functionality, and molecular signaling pathways of the vascular cells composing those regions (138). Therefore, expanding the knowledge of human vessels in anaphylaxis is crucial to implementing tools based on their underlying molecular mechanisms. It would help to achieve better quality of acute and long-term clinical management of anaphylactic patients.

For years, the classic cellular and molecular mechanism described for anaphylaxis has been the cross-linking of FcϵRI-bound allergen–IgE complexes activating mast cells/basophils and the consequent release of mediators (5). More recent studies shed light about the relevance of other immune effector cells in anaphylaxis such as monocytes, macrophages, neutrophils, eosinophils, and platelets (266). Unfortunately, most of this evidence is based on murine models but knowledge about its involvement in human anaphylaxis still remains scarce (12). Due to the features of these innate immune cells, the main research focus in molecular anaphylaxis is on the immune system. However, anaphylaxis is a systemic reaction in which several organs and systems are involved. Therefore, simultaneously to the inflammatory response, the vasculature actively participates in such pathophysiological process. Epinephrine is the treatment of choice to palliate anaphylactic symptoms accordingly to guidelines (245). It is the “safeguard” molecule which triggers both endogenous and exogenous mechanisms through its adrenergic receptors (1). Therefore, the downstream signaling pathways of these receptors involve a dual behavior (constriction and relaxation processes) in vascular and bronchial smooth muscle cells. In addition, both cell types, either located within the vessel wall or conforming to organs, would relate to the variety of clinical consequences and the severity grade of the reactions.

In this context, the wide endothelium surface would also be contributing differentially to the pathophysiology of the anaphylaxis. The endothelium arises as an important organ-cell like in anaphylaxis playing a role not only in the control of fluids and the vascular tone but also as an activation surface for the coagulation, contact, and complement systems (44, 162, 163). Moreover, ECs release relevant anaphylactic mediators such as NO, although it is likely that other molecules (presumably cytokines or interleukins) could also be released contributing to the pool of mediators in the reaction. However, the exact contribution of these cells as provider of mediators to the anaphylactic cellular microenvironments existing between immune and resident niches is unknown.

Altogether, we can summarize that the permeable and/or vasomotor capacity of the different anaphylactic mediators (NO, tryptase, histamine, bradykinin, or a multitude of other scarce or unknown ones) would determine the magnitude of the anaphylactic events. Furthermore, the role of their receptors (GPCRs) in human anaphylaxis is not yet fully understood, but they result to be essential in preventing or inducing vascular effects. Therefore, GPCRs are the most promising therapeutic targets of study nowadays and the degree of responsiveness of ECs and VSMCs is an important factor to determine in anaphylaxis.

Due to the complexity to investigate in human subjects the cellular and molecular mechanisms of anaphylaxis, the improvement and reproducibility of animal—or in vitro human—models is necessary. This fact is one of the major limitations to study this pathological event. A big piece of the literature around it derives from animal models. For that reason, conclusions must be taken cautelous when translated at human reactions. Strategies based on functional in vitro studies by using human cell cultures combined with sera would provide clues about cellular and molecular happenings occurring in specific anaphylactic microenvironments. Therefore, abundant investigations are aimed at finding vascular targets revealing future alternative strategies to treat or prevent anaphylaxis. Efforts must be focused on alternative therapies called to specifically modulate the whole or parts of the vascular system that benefit the clinical management of these life-threatening events in the near future.

Conceptualization and—original draft preparation, VE. Writing—review and editing, EN-B, SF-B, AY-M, and VE. Funding acquisition, VE. All authors contributed to the article and approved the submitted version.

This research was supported by grants from the Instituto de Salud Carlos III (PI18/00348 and PI21/00158) and FEDER Thematic Networks and Cooperative Research Centers RETICS ARADyAL RD16/0006/0013. This work was also supported by the SEAIC (19_A08) and Alfonso X el Sabio University Foundations. EN-B was granted by funding from the Community of Madrid included in the project FOOD-AL (CM_P2018/BAAA-4574).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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