1. Introduction
Now it is understood that asthma is a diverse illness influenced by interactions between environmental exposure and epigenetic regulation. Asthma is widely recognized to be a heterogeneous syndrome which involves airway hyperreactivity in response to multiple triggers that have distinct pathobiology [
1], which clinically manifests as cough, wheezing, and shortness of breath. The most common and therefore the better understood prototype is the disease associated with T helper type 2 (Th2) cell-mediated allergic sensitization, now known as the Type 2 (T2)-asthma [
2]. Advances in the management of T2 asthma has identified the subset of “difficult-to-control” or “severe” asthma that is poorly responsive to currently available therapies, which are most effective against T2 pattern of inflammation. [
2,
3,
4,
5] Thus, heterogeneity in disease presentation in the advent of personalized medicine has again placed the spotlight on the need for more holistic definition of asthma. In this review, we discuss recent advances in our knowledge of the pathobiology of asthma, as well as the phenotypes and endotypes of asthma and their relationship to the clinical efficacy of targeted therapies used to treat severe uncontrolled asthma.
Asthma is caused by a pathogenesis involving both the innate and adaptive immune systems and epithelial cells: mucus overproduction, airway remodeling, and bronchial blockage and hyperreactivity are the primary clinical symptoms [
6]. The different ways that asthma manifests in the various so-called "phenotypes" is explained by the intricate interactions between various immune pathways [
7]. The overexpression and activation of Th-2 cells have historically been associated with asthma as an eosinophilic mediated illness. Additionally, research has shown that there is a subtype of neutrophilic asthma that also involves T helper type 17 (Th-17) cells [
8]; the ability of type 2 innate lymphoid cells (ILC-2) and basophils to generate eosinophilic inflammation in subgroups of asthmatic patients was an additional intriguing discovery [
9]. It was important to remark this finding since it revealed how irregularly diverse the asthma really is. This notion was supported by other cytokine-targeted therapy clinical studies, which showed reduced symptoms in patients [
10,
11,
12,
13,
14,
15]. Thus, our goal completely changed to get more knowledgeable about the many categories of illness kinds to provide a safer, more accurate, and more potent course of treatment.
2. Pathogenetic Mechanism
In the past, asthmatic airway inflammation was only thought to be a component of the immune system's adaptive response. However, the identification of innate lymphoid cells (ILCs) ILC-2 and the possibility of their involvement in atopic illnesses [
16] led to a clearer comprehension of the crucial role played by innate immunity in determining the inflammatory response that distinguishes the various asthma phenotypes [
17].
ILCs are lineage and antigen negative lymphocytes, indicating they lack the markers or receptors observed on myeloid, dendritic, antigen-specific B or T cells. ILCs come in three unique varieties that have been discovered thus far: group 1 ILCs (ILC-1) producing interferon-γ (IFN-γ), ILC-2 producing cytokines traditionally linked to Th-2 cells (interleukin [IL] IL-4, IL-5, IL-13), and ILC-3 producing IL-17 and/or IL-22 [
5,
18]. ILC-2 cells are essential to the rapid inflammatory response to helminthic and viral infections [
19]. Due to the absence of antigen-specific receptors on these cells, regardless of atopic status, these cells play a crucial role in orchestrating airway inflammation in eosinophilic phenotypes of asthma [
16]. However, they respond to epithelium-derived signals mediated by "alarmins" [thymic stromal lymphopoietin - (TSLP), IL-25 and IL-33] produced by epithelial cells [
20]. The TSLP is a cytokine of the IL-2 family that is produced by a variety of cells, including lung epithelial cells [
21]. TSLP plays a vital role in mediating corticosteroid resistance associated with ILC-2 airway inflammation and in triggering the upregulation of the adaptive immune response in asthma [
22]. TSLP binds to a heterodimer surface receptor expressed on a variety of cells including T, B, natural killer (NK) cells, monocytes, basophils, eosinophils, ILC-2, dendritic cells, and epithelial cells [
23,
24,
25]. IL-33 is a member of the IL-1 family of proinflammatory cytokines and has an IL-1-like domain on one of its end chains [
26]. This is among the primary explanations why IL-33 can bind to the ST2 receptor, also known as the IL-1 like receptor, a member of the Toll-like receptor family that is expressed on numerous immune cells, including Th-2 and ILC-2 [
27]. IL-25 is the third and final target cytokine in the “alarmins” triad. This cytokine enhances the subsequent cascade of proinflammatory mediators: IL-25 secretion is determined by epithelial cell damage, which in turn is determined by protease exposure from exogenous antigens [
28]. Moreover, IL-25 induces the NF-kB signaling pathway by activating the signaling pathway. In response, the secretion of Th2 cytokines IL-4, IL-5, and IL-13 activates the Type 2 inflammatory response [
29] (
Figure 1).
Historically, asthma immunopathology was described as a linear progression that distinguished between innate and adaptive immunity. This theory led researchers to depart from the conventional cascade, but it did permit them to distinguish between the innate and adaptive immunity-mediated inflammatory response in asthma. Despite this, the pathogenesis of asthma follows a common cascade, with signaling molecules shared by both forms of immunity [
5,
30]. Regarding the roles of Th-2 and ILC-2 cells in the pathogenesis of asthma, the two immune cells share mechanistic features, the most prominent of which is the upregulation of the transcription factor GATA-3, which promotes the synthesis of type 2 cytokines and chemokine receptors [
31,
32]. Among these cytokines, IL-5 is essential for the controlled maintenance of eosinophil development in bone marrow [
33]. To influence the underlying pathogenesis of airway eosinophilia, IL-5 signals eosinophil chemotaxis so that they can survive and release subsequent cytokines. IL-5 and IL-13 induce other molecules, including eosinophil cationic protein, major basic protein, tumor necrosis factor, and eicosanoid pathway metabolites, in addition to priming bronchial hyper-reactivity. The IL-13 overexpression in the lung, in conjunction with IL-4, influences the signaling of ICAM-1 and VCAM-1 [
33,
34]. Those are released by Th-2 cells and causes perivascular and peribronchial infiltrates to penetrate the lung interstitium, as well as bronchial smooth muscle hyperreactivity, chemokine induction, and epithelial injury.
The immune recognition by environmental allergens is initiated by antigen presenting cells, such as dendritic cells (DCs) (but also B lymphocytes and other cell types); these induce the maturation of T-nave cells into Th-2 cells, which can produce cytokines such as IL-4, IL-5, and IL-13. [
6,
27]. Th-2 cells produce IL-4, & IL-13 which stimulates B cell differentiation into plasma cells and switches antibody production to the IgE isotype [
31]. Cross-linking between allergen specific IgE molecules, the allergen, and effector cells such as mast cells and basophils determines the release of mediators such as histamine, tryptase, leukotrienes, and prostaglandins, which can trigger asthma symptoms [
27]. In addition, IL-4 contributes to the polarization of T-naive cells towards Th2 cells, thereby enhancing the entire Th-2-mediated inflammatory response [
30,
35]. Other relevant activities of IL-4 in the pathogenesis of asthma include the induction of VCAM-1 expression, which leads the migration of eosinophils, basophils, monocytes, and T cells to the site of allergic inflammation, and the induction of mucin gene expression, resulting in higher airway mucus production [
36,
37]; intriguingly, nonallergic asthmatics with eosinophilic airway inflammation had elevated IL-5 levels (as do allergic patients), suggesting that IL-5 plays a crucial role in determining eosinophilic airway inflammation even in the absence of an allergic stimulus[
38]. More recently, a second mechanism of mast cell upregulation has been described; it involves the synthesis of IL-9 by both Th-2 and ILC-2 cells [
5,
39].
IL-17 (IL-17A and IL-7F) is the central cytokine in another adaptive immune system mechanism implicated in the pathogenesis of asthma, particularly neutrophilic phenotypes [
40]. Th-17 cells, other T cells, NK cells, ILC-3, and mast cells produce this cytokine. With an abundance of IL-17A and IL-17F, the neutrophil influx is partially regulated by the stimulation of airway epithelial cell and stromal cell cytokine production, which influences neutrophil chemotaxis to the scene. IL-17 is the predominant signaling molecule that induces the activation of its receptor in the pathogenesis of neutrophilic asthma, and it is expressed on smooth muscle cells, leading to hypertrophy of smooth muscle cells [
41]. In addition to being expressed on epithelial fibroblasts, macrophages, and endothelial cells, the IL-17 receptor is also expressed on epithelial fibroblasts, which helps to explain the potential function of IL-17 in airway remodeling [
42]. Researchers believe that neutrophil-released cytokines, such as IL-6, TGF-beta, IL-1beta, TNF-alpha, IL-21, and IL-23, can activate key transcription factors such as STAT3 and RORC2 to promote the differentiation of naive CD4+ T cells into Th-17cells [
43].
Over sixty genetic loci have been linked to asthma, and some of these loci have been linked to severe asthma. Genome-wide association studies (GWAS) in children and adults identified 5 loci associated with severe exacerbations and implicated multiple genes involved in immune responses, including IL33, IL1RL1, and CDHR3[
44]. GWAS identified 24 loci associated with moderate-to-severe asthma [
44,
45]. Several studies have investigated gene expression in asthma that is persistent and severe. These investigations have been conducted on a variety of cell types, including airway epithelial cells, whole blood, sputum, and bronchioalveolar lavage, and it is essential to understand that gene expression is tissue-specific [
46,
47,
48,
49]. Moreover, some studies [
50,
51] have compared differences in gene expression response to specific interventions. Multiple patterns of gene expression have, not surprisingly, been identified, and these signatures suggest that multiple mechanisms contribute to persistent and severe asthma. Numerous of these mechanisms are linked to altered immune responses. Although sputum proteomics is a relatively new field of study in asthma, few studies have examined proteomics and epigenetics in severe asthma [
52,
53,
54,
55].
6. Conclusions
Asthma is a complicated, multi-factorial illness that manifests differently in distinct patient subgroups and is brought on by the varied expression of inflammatory pathways involving both innate and adaptive immune systems. The simplistic concept of asthma as a singular disease characterized by chronic airway inflammation, bronchial hyperreactivity, airway obstruction, and airway remodeling is therefore no longer applicable. In this context, a more personalized approach to asthmatic patients, employing so-called precision medicine to better characterize patients into phenotypes and endotypes and to select the most appropriate drug for everyone (a "tailored treatment" approach) is now required, especially for patients with severe asthma.
The identification of phenotypes requires careful evaluation of any single clinical aspect (e.g., the presence of atopy, comorbidities, clinical presentations), of lung function patterns (e.g., the degree of bronchial reversibility, the presence of fixed airway obstruction, the degree of airway hyperreactivity), and of sputum and systemic inflammatory involvement (e.g., eosinophilic, neutrophilic, mixed). The precision medicine approach to asthma is an entirely new paradigm, and it will provide not only the opportunity to treat patients more effectively and appropriately, but also new insights into those immunological aspects of asthma that require further study.