Infections by RSV are a formidable threat for certain groups of patients, especially newborns and elderly [25,41]. It is a single stranded, negative-sense enveloped RNA virus belonging to the Orthopneumovirus genus of thePneumoviridae family [42]. RSV replicates in the nasopharynx and then spreads within the epithelium of bronchi and bronchiole by cell-to-cell and producing clinical features including bronchiolitis and pneumonia [43,44,45]. RSV also productively infects non-epithelial cells; thus, it been isolated from alveolar endothelium, myocardial tissue, central nervous system, cerebrospinal fluid, endocrines and liver, during severe disease and sudden infant death [46,47,48,49]. There is no effective treatment or vaccine available for RSV; palivizumab (a humanized monoclonal antibody) is the only RSV immunoprophylaxis approved for use in specific high-risk pediatric populations [50]. The therapy is mostly supportive care combined with symptomatic treatments including modalities such as bronchodilators, epinephrine, corticosteroids, hypertonic saline, and/or supplemental oxygen [51,52].

Newborns cannot be expected to mount a sufficiently strong secretion of IFNs to respond against RSV, while the elderly do not respond well to vaccinations in general [53]. There are vaccine candidates under development, yet it remains a major challenge to immunize certain population categories [54]. It is imperative that therapies are developed to protect patients that either have not been vaccinated or cannot mount an optimal immune response to RSV. RSV infection causes respiratory symptoms that may encompass the lower respiratory tract, culminating in bronchiolitis, which in severe cases results in necrosis and the sloughing of epithelial cells into the airways, airway mucus, edema, and peribronchiolar inflammation, cumulatively resulting in airway obstruction [55,56,57]. Severe bronchiolitis is associated with the manifestation of asthma in later life [57]. Epithelial cells express cytokines IL-33, IL-25, and thymic stromal lymphopoietin (TSLP), as well as the innate immune cell-derived cytokine high mobility group box 1 (HMGB1), which activate group 2 innate lymphoid cells (ILC2). This signaling promotes the progression of T-helper type 2-mediated pulmonary disease, thus explaining the association of RSV with asthma in later life.

In vitro and in vivo experiments show that during RSV infection, epithelial cells infected with RSV express and secrete IL-1α, which activates vascular endothelial cells to express increased cell surface ICAM-1, and to a lesser extent, vascular adhesion molecule-1 (VCAM-1) and E-selectin [58]. RSV induces expression of MIP-1α in epithelial cells of the alveoli and bronchioles, as well as in adjoining capillary endothelium [59]. Adhesion experiments using polymorphonuclear leukocytes (PMN) verified an increased ICAM-1-dependent adhesion rate of PMN co-cultured with RSV-infected endothelial cells. Furthermore, the increased adhesiveness resulted in an enhanced transmigration rate of PMN [60]. ICAM-1 expression on RSV-infected endothelial cells may contribute to the enhanced accumulation of PMN into the bronchoalveolar space. The virus-induced ICAM-1 upregulation was dependent on the activity of protein kinase C, protein kinase A, phosphatidylinositol 3-kinase (PI3K), and p38 mitogen-activated protein kinase (MAPK) [60].

In lung alveoli, a gradient of CXCL8 is the most likely chemo-attractant for the neutrophils that migrate from the systemic circulation into the alveolar space [61]. Neutrophils function by releasing reactive oxygen species (ROS) and extracellular traps, undergoing degranulation and phagocytosis, and by recruiting other cell types to the site of infection such as alveolar macrophages, dendritic cells, and T-cells [62]. However, soluble endothelial cell adhesion molecules (sCAMs), such as sICAM-1, can be measured in the systemic circulation, indicating that the currently postulated neutrophil influx into the lungs should rather be regarded as a neutrophil efflux from the vasculature, involving substantial neutrophil-endothelial interactions. Endothelial cells become activated upon RSV infection, driving a ‘pro-adhesive state’ for circulating neutrophils with upregulation of endothelial ICAM-1. During RSV lower respiratory tract infections, different subsets of immature and mature neutrophils are present in the bloodstream, parallel with upregulation of integrins, lymphocyte-function associated (LFA)-1 and macrophage (Mac)-1 antigen, serving as ICAM-1 ligands [61].

RSV infection induces ROS generation, activates mitogen- and stress-activated kinases-1 (MSK1)-phospho-Ser-276 v-relreticuloendotheliosis viral oncogene homolog A (RelA) pathway required for cytokine expression [63]. Aero-allergens and respiratory viruses stimulate toll-like receptor (TLR) signaling, producing oxidative injury and inflammation [64]. Repetitive exacerbations produce complex mucosal adaptations, cell-state changes, and structural remodeling. These structural changes produce substantial morbidity, decrease lung capacity, and impair quality of life. Repetitive activation of innate signaling pathways produces a form of epigenetic ‘training’ in the cell nucleus, to induce adaptive epithelial responses [64].

Influenza viruses have a single negative-stranded segmented RNA genome; deadliest in history is H1N1, an Alphainfluenzavirus of the family Orthomyxoviridae [65,66]. SARS-CoV-2 in contrast, the causative agent of the COVID-19 pandemic, belongs to the positive-strand RNA viruses of the genus Beta coronavirus [67]. Both viruses constitute very significant health burdens worldwide, due to the lack of effective treatments, and are still under research for the generation of vaccines that will offer lasting protection against emerging variants [68,69]. In influenza virus infection, pulmonary endothelial cells play a central role in regulating both innate immune cell recruitment as well as innate cytokine and chemokine production [4]. In victims of the 2009 pandemic influenza A/H1N1 infection, tissues of bronchial mucosa, lung, myocardium, gastrocnemius, and liver that were investigated by light microscopy and transmission electron microscopy, viral particles were found in all samples, frequently located in endothelium, epithelium, and muscle cells [70]. Cultured ECs respond to infection and iron incubation with increased production of IL-6. Iron, the generation of intracellular hydroxyl radical and NF-κB activity are essential in cellular activation, suggesting that ROS generated in the Haber–Weiss reaction are essential in invoking an immunological response to infection by ECs [71].

In patients who died from SARS-CoV-2 or influenza (H1N1)-associated respiratory failure, the histologic pattern in the peripheral lung was diffuse alveolar damage with perivascular T-cell infiltration. The lungs from patients with COVID-19 also showed distinctive vascular features, consisting of severe endothelial injury associated with the presence of intracellular virus and disrupted cell membranes. Histologic analysis of pulmonary vessels in patients with COVID-19 showed widespread thrombosis with microangiopathy. Alveolar capillary micro-thrombi were 9 times as prevalent in patients with COVID-19 as in patients with influenza (p < 0.001). In lungs from patients with COVID-19, the amount of new vessel growth—predominantly through a mechanism of intussusceptive angiogenesis—was 2.7 times as high as that in the lungs from patients with influenza (p < 0.001) [72].

Although respiratory viruses initially infect the airway epithelium, it is a compromise in vascular integrity that causes alveolar damage [4]. Indeed, the compromise in vascular integrity distinguishes influenza H1N1 from SARS-CoV-2 infections, and this divergence in effects can be attributed to difference in the patterns of expression and secretion of inflammatory mediators. The main difference between influenza and SARS-CoV-2 infections is the ability of SARS-CoV-2 to elicit dysfunction of the blood vessels. This difference can be attributed to a divergent expression of signaling molecules that cause the pathology that involves blood vessels. A comparison between immunological factors produced during the influenza and SARS-CoV-2 infection suggests that although both infections raise levels of T-helper type I mediators, SARS-CoV-2 also distinctly increases T-helper type II (Th2) mediators (IL-4, IL-5, IL-10, IL-13), as well as the allergy mediator [73]. In contrast, H1N1 severe cases show high expression of surfactant protein D at the alveolar epithelium [73]. H1N1 infections have shown more efficient activation of reparative macrophages of the M2 subtype [74]. This might suggest a more efficient repair capacity for the H1N1-infected lung.

One hypothesis is that severe SARS-CoV-2-driven pneumonia causes respiratory failure via pulmonary microthrombi and endothelial dysfunction [75]. A considerable body of evidence suggests that SARS-CoV-2, unlike other related viruses, infects and replicates within ECs, which may explain a significant portion of the observed clinical pathology [76,77]. On the contrary, certain data that show an inability of SARS-CoV-2 to directly infect and lyse endothelial cells without angiotensin-converting enzyme-2 (ACE2) expression explain the lack of vascular hemorrhage in COVID-19 patients and indicate that the endothelium is not a primary target of SARS-CoV-2 infection [78]. Although the detection of SARS-CoV-2 has not been singularly linked to bronchiolitis, with the exception of necrotizing bronchiolitis [79], it has been proposed that co-infections of SARS-CoV2 with other viruses, most notably RSV, are associated with a severe course of bronchiolitis in patients [80]. Furthermore, SARS-CoV-2 can directly infect engineered human blood-vessel organoids in vitro. EC involvement was demonstrated across vascular beds of different organs in a series of patients with COVID-19 and SARS-CoV-2 can directly infect engineered human blood-vessel organoids in vitro [81].

The binding site of the SARS-CoV-2 viral spike protein on the surface of cells is the receptor “angiotensin converting enzyme 2 (ACE2)”, which functions to protect against hypertension, cardiovascular and lung diseases, and diabetes mellitus [82]. In an experimental setting, loss of ACE2 function in the mouse lung during endotoxin inhalation led to release of inflammatory chemokines such as C-X-C motif chemokine 5 (CXCL5), macrophage inflammatory protein-2 (MIP2), C-X-C motif chemokine 1 (KC), and pluripotent cytokine TNF-α from airway epithelia, increased neutrophil infiltration, and exaggerated lung inflammation and injury [83]. By immunohistochemistry, flow cytometry and RNA sequencing, the lung could show expression of ACE2, mainly in alveolar macrophages, and subsets of type II alveolar epithelial cells [84,85,86,87,88].

SARS-CoV-2 infection can result in diverse, multiorgan pathology, the most significant being in the lungs (diffuse alveolar damage in its different phases, micro-thrombi, bronchopneumonia, necrotizing bronchiolitis, viral pneumonia), heart (lymphocytic myocarditis), kidney (acute tubular injury), central nervous system (micro-thrombi, ischemic necrosis, acute hemorrhagic infarction, congestion, and vascular edema), lymph nodes (hemophagocytosis and histiocytosis), bone marrow, and vasculature (deep vein thrombosis) [79].