Age-related macular degeneration (AMD), particularly its dry form, is a leading cause of irreversible blindness in older individuals with a prevalence of 0.44% globally [1]. AMD is classified into two forms: dry and wet AMD. Dry AMD, is characterized by geographic atrophy (GA) of the central retina in the late stages [1]. Dry AMD has a multifactorial etiology and is driven by genetic, environmental, and lifestyle factors ^[2][3][2,3]. The clinical manifestation of AMD encompasses the buildup of drusen, the loss of retinal pigment epithelium (RPE), and photoreceptor cells leading to a gradual loss of central vision ^[2][3][2,3]. Despite significant advances in the understanding of the disease etiology, the precise molecular and cellular mechanisms underlying the pathophysiology of dry AMD remain incompletely understood ^[3][4][5][3,4,5]. Thus, gaining deeper insights into the cellular and molecular mechanisms involved in pathogenesis of dry AMD holds significant promise in unraveling the disease’s mechanisms, identifying essential risk factors, and paving the way for development of preventive strategies. This knowledge should also facilitate the development of potential biomarkers for early detection and monitoring of the disease’s progression.

A key aspect of dry AMD pathogenesis, namely the loss of retinal pigment epithelium (RPE) cells, has been the focus of many studies. RPE is a single layer of specialized epithelial cells that support the photoreceptor cells’ integrity and function, as well as providing protective and homeostatic mechanisms against oxidative stress, and angioinflammatory and fibrotic processes. RPE cells experience significant functional changes that promote AMD progression. Notably, alterations in their phagocytic and degradative activities lead to the accumulation of lipofuscin (a metabolic byproduct of the visual cycle), the formation of drusen (extracellular deposits between the RPE and Bruch’s membrane) ^[6][7][6,7], and enhanced oxidative stress and inflammatory processes. The role of oxidative stress and inflammation in the pathogenesis of dry AMD has been widely recognized to lead to cellular damage and death. Chronic inflammation is a key contributor to the progression of AMD by promoting the release of pro-inflammatory cytokines and chemokines, as occurring during aging, further exacerbating oxidative stress and recruitment and engagement of immune cells and inflammatory processes ^[8][9][8,9].

The complement system, a component of the innate immune system, which enhances the ability to clear microbes and damaged cells, has been implicated in dry AMD pathogenesis. Genetic studies have linked variants of various complement genes, such as CFH, C3, and CFI to increased AMD risk [8]. Among the notable players is complement factor H (CFH), a crucial regulator of the alternative complement pathway. CFH operates by inhibiting the C3 convertase enzyme and promotes the decay of C3b, thereby controlling complement activation. A common polymorphism in the CFH gene (Y402H) has been strongly associated with an increased risk of developing AMD [8]. This variant presumably alters CFH function, leading to complement pathway over-activation and subsequent inflammation and tissue damage, a hallmark of AMD progression.

2. Choroidal Mast Cells and AMD

Although the precise etiology of AMD remains not fully elucidated, it is well recognized that inflammation and immune system dysregulation play crucial roles in its pathogenesis ^[10][24]. Recent studies by Lutty and colleagues have begun to explore the uncharted territory of choroidal mast cell involvement in AMD pathophysiology. Their published studies using human donor eyes began to hint to a possible association between mast cell activity and the initiation and/or progression of AMD ^[11][12][13][25,26,27]. Mast cells are a type of resident immune cell that play a crucial role in the body’s immune responses, particularly in the context of allergic reactions and other inflammatory conditions. They contain granules rich in histamine and heparin, substances that once released can provoke significant inflammatory responses ^[14][28]. Thus, mast cells that are present in the choroid, but not in the retina, may potentially offer a primary cellular target for modulation of angioinflammatory processes affecting choriocapillaris function during AMD pathogenesis.

2.1. Choroidal Mast Cells and Angioinflammatory Processes

Mast cells, as crucial effector cells of the innate immune system, have emerged as significant contributors to various inflammatory and autoimmune diseases ^[15][16][30,31]. Choroidal mast cells are strategically located in close proximity to the choroidal vasculature, enabling their active involvement in modulation of angioinflammatory processes. Mast cells are known for their role in allergic reactions, but their function extends beyond this classic role. They are considered as extrahepatic producers of complement proteins and express various complement receptors, including those for C3a and C5a ^[17][32]. The complement system, which enhances the ability of antibodies and phagocytic cells to eliminate microbes and damaged cells, engages with mast cells in an intricate manner. The C3a and C5a not only trigger mast cells expressing C3aR and C5aR, but also act as chemoattractants for mast cells originating from various tissue locations ^[16][31]. Mast cells also participate in regulating tissue healing and fibrosis, processes that are crucial for repairing injured tissues. However, they can lead to pathological outcomes if not properly controlled ^[18][33]. Type 2 immunity, characterized by the production of interleukin (IL)-4, IL-5, IL-9, and IL-13, is frequently observed in tissues during allergic inflammation or infection with helminth parasites. Many pivotal cell types linked with type 2 immune responses, including mast cells, contribute to tissue repair after injury ^[18][33]. Mast cells in cancers are found to exhibit a continuous phenotypic expansion specific to the tumor microenvironment ^[19][34]. Thus, mast cells can adjust their features in reaction to specific conditions within their microenvironment, potentially influencing disease progression and shaping the response to immunotherapy. In this way, mast cells perform a multifaceted part within the immune system, contributing to inflammatory responses, tissue repair, and the immune response. Understanding the precise mechanisms by which choroidal mast cells contribute to these processes during AMD will have significant clinical importance.

2.2. Mast Cells in Retinal Inflammation and Oxidative Stress

Although the retina lacks resident mast cells, systemic activity of mast cells is shown to play a significant role in retinal inflammation and oxidative stress, and pathogenesis of various ocular diseases including retinopathy of prematurity ^[20][35]. As indicated, mast cells are a component of the innate immune system and are known for their role in allergic reactions and anaphylaxis. They are also recognized as key players in the regulation of inflammation and immune responses under various pathological conditions. In the context of retinal inflammation, mast cells are involved in the release of pro-inflammatory mediators such as histamine, proteases, cytokines, and chemokines. They produce reactive oxygen species (ROS), which enhance oxidative stress. Mast cells are activated by a multitude of signals, including those from the complement system, cytokines, and toll-like receptors (TLRs). Activated mast cells release a variety of mediators, which contribute to inflammation and oxidative stress. Clare et al. ^[21][36] reported that mast cell activity contributes to the pathogenesis of retinal diseases by promoting inflammation and oxidative stress. They found that mast cells can be activated by oxidative stress, leading to the release of pro-inflammatory mediators. Wang et al. provided a comprehensive overview of the role of S100A8/A9, a heterodimer expressed in neutrophils and monocytes, in inflammation. S100A8/A9 is released during inflammation and plays a critical role in modulating inflammatory responses by stimulating leukocyte recruitment and inducing cytokine secretion. The authors also suggested that S100A8/A9 could be a potential therapeutic target for inflammation-associated diseases, including retinal inflammation ^[22][37]. Rübsam et al. discussed the role of inflammation in diabetic retinopathy, a common complication of diabetes that can lead to blindness. The authors highlighted the involvement of various inflammatory mediators, including those produced by mast cells, in the pathophysiology of diabetic retinopathy ^[23][38]. One of the key signaling pathways involved in mast cell activation is the high-affinity IgE receptor (FcεRI) signaling pathway. Cross-linking of FcεRI by antigen-bound IgE leads to the activation of Src family kinases, which in turn activate downstream signaling molecules such as Syk, phosphoinositide 3-kinase (PI3K), and mitogen-activated protein kinases (MAPKs). These signaling events lead to the release of preformed inflammatory mediators from mast cell granules, as well as the synthesis of new mediators ^[24][39]. In retinopathy of prematurity and diabetic retinopathy, mast cell activation contributes to retinal vascular leakage and neovascularization ^[20][35], and systemic inhibition of mast cell activation is protective. In AMD, choroidal resident mast cells could promote inflammation and neovascularization through the release of various cytokines and angioinflammatory factors. They can contribute to oxidative stress through the release of ROS and other pro-oxidant mediators ^[25][40]. In addition, oxidative stress can further activate mast cells, creating a vicious cycle of inflammation and oxidative damage. Although current studies highlight the crucial role of mast cells in tissue inflammation and oxidative stress, more research is needed to fully appreciate the true impact of mast cells on these processes, and to develop effective therapeutic strategies targeting mast cells.

2.3. Mast Cells in the Choroid

AMD is a progressive retinal disease that leads to the loss of central vision. It is characterized by the accumulation of drusen (extracellular deposits) between RPE and Bruch’s membrane, leading to RPE and photoreceptor cell death ^[26][41]. Mast cells are known to be involved in inflammatory responses and are found in various tissues throughout the body, including the choroid. They are well recognized as part of the innate immune system and for their role in allergic reactions. Mast cells contain granules packed with inflammatory mediators like histamine and various proteases, which are released upon activation ^[27][42]. This release, known as degranulation, rapidly provides a variety of inflammatory mediators into the surrounding tissue. These mediators can cause vasodilation, recruit other immune cells to the site of inflammation, and stimulate the production of cytokines and chemokines, further enhancing inflammation. In AMD, chronic inflammation and oxidative stress contribute to the degeneration of the RPE and the underlying choroidal vasculature. The resident choroidal mast cells, through their release of inflammatory mediators, could contribute to this chronic inflammation and oxidative stress associated with pathogenesis of AMD ^[13][27]. Furthermore, mast cells are involved in the activation of the complement system. Although dysregulation of the complement system is implicated in the pathogenesis of AMD ^[28][43], the underlying mechanisms remain unknown. Choroidal mast cells can release proteases that activate the complement system, potentially contributing to complement dysregulation in AMD ^[16][31]. Ogura et al. reported that mast cells are abundantly present in the choroid and their continuous stimulation and degranulation, particularly through mast-cell-derived tryptase, could be central to the progression of GA in a preclinical model. These studies demonstrated that the degeneration of RPE cells and retinal and choroidal thinning, which are characteristics of GA, were driven by chronic stimulation and activation of choroidal mast cells. They showed interventions targeting mast cell degranulation or inhibiting tryptase activity could prevent the development and progression of the disease phenotypes ^[12][26]. Thus, it is reasonable to propose that choroidal mast cells, through their role in inflammation, oxidative stress, and complement activation, are key contributors to the pathogenesis of AMD.

2.4. Choroidal Mast Cell Activation in AMD

The implications of choroidal mast cell activation in AMD are profound and multifaceted, with recent research beginning to elucidate the complex interplay between choroidal mast cells, RPE cells, and pathogenesis of dry AMD ^[11][25]. Activated choroidal mast cells release various mediators such as histamine, cytokines, and proteases ^[14][29][28,44]. In the context of dry AMD, choroidal mast cell activation is linked to the degeneration of the RPE and photoreceptors in preclinical models. The RPE cells are essential for visual function, and their degeneration is a hallmark of dry AMD. Recent in vitro studies show that mast cells induce RPE cell death by release of granules containing tryptase, a potent protease. The signaling pathways involved in this process are complex and involve multiple steps. Upon activation, mast cells release tryptase, which cleaves and activates protease-activated receptor-2 (PAR-2) on the surface of RPE cells. PAR-2 activation triggers a signaling cascade involving the phosphorylation of the MAPKs including ERKs1/2 and JNK, leading to the activation of the transcription factor AP-1. AP-1 induces the expression of the death receptor FasL, which binds to its receptor Fas on the same cell, triggering apoptosis ^[11][30][25,45]. Importantly, this process seems to be exacerbated by oxidative stress, a condition known to contribute to the pathogenesis of dry AMD. Oxidative stress can enhance the release of tryptase from mast cells, further promoting RPE cell death ^[31][46]. In addition to their direct effects on RPE cells, choroidal mast cells could also contribute to dry AMD through their interactions with other immune cells. For instance, mast cells could recruit and activate macrophages, which release additional pro-inflammatory and cytotoxic factors, potentially exacerbating RPE damage ^[29][44]. Thus, choroidal mast cell activation could play a significant role in the pathogenesis of dry AMD, primarily through the induction of RPE cell death. This process involves complex signaling pathways and is influenced by factors such as oxidative stress. However, whether this occurs in vivo and the role changes in TSP1 levels play remain unknown.