Reactive oxygen species (ROS) are a key component of oxidative stress, and oxidative stress is a pathological condition caused by an imbalance between ROS and other oxidants and the antioxidant system in living organisms. ROS are an essential part of assessing whether the functions of a cell are running normally. In the development of AP, large numbers of immune cells are recruited to the organ. And the activation and proliferation of these types of immune cells are induced by ROS (16, 17). The relevant information is shown in Figure 1 .

ROS are mainly generated at complexes I and III, due to electron leakage, which refers to electrons from electron transport chain (ETC) partially escaping, which results in the univalent reduction of molecular oxygen (O₂) to superoxide anion (O₂ ^-·) and subsequent ROS (18). Cells possess a variety of antioxidant enzymes, such as superoxide dismutase, catalase, peroxidase, and glutathione systems, to regulate ROS levels, and overproduction of ROS exceeds the antioxidant capacity of cells, leading to oxidative stress. Oxidative stress can damage cell membranes, proteins and DNA, further exacerbating cellular damage and inflammatory responses (9, 19, 20). ROS can attack polyunsaturated fatty acids in cell membranes, leading to lipid peroxidation, which disrupts cell membrane integrity, resulting in cellular dysfunction and necrosis. As the stability of organellar membranes is compromised, for example, oxidized phospholipids (OxPL) are generated that can affect downstream biomacromolecules such as proteins, DNA, and lipids, which are integral parts of damage-associated molecular patterns (DAMPs), which we will discuss below (21).

Neutrophils release reactive oxygen species through respiratory burst, which could lead to necrosis (22). Apoptosis is genetically regulated, whereas necrosis is an uncontrolled mechanism of cell death (23). And it has been proven that the severity of AP is positively correlated with necrosis, and negatively correlated with apoptosis (24). The result of apoptosis is the clearance of cells from the body with minimal damage to the surrounding tissues, while necrosis leads to the spillage of cell contents into the surrounding tissues and their subsequent damage (25). At the same time, ROS can also promote the differentiation of T cells towards Th1 or Th17 (26).

Oxidative stress may also stimulate the activation of transcription factor and cause the excessive release of inflammatory mediators, including IL-1, IL-6, and TNF-α. Integrating our previous findings with established literature on p38MAPK signaling, we have identified that p38MAPK interacts with NADPH oxidase (NOX) to amplify reactive oxygen species generation, thereby establishing a self-perpetuating “ROS-p38MAPK” feedforward loop that exacerbates oxidative tissue damage. Based on this mechanistic insight, we hypothesize that targeted modulation of the p38MAPK pathway may attenuate inflammatory mediator expression and improve immune homeostasis, consequently mitigating oxidative stress and potentially ameliorating the clinical severity of acute pancreatitis (27–29). At different stages of AP, different concentrations of ROS produce different effects, in the acute phase, moderate amounts of ROS are beneficial for apoptosis, reduce necrosis and prevent severe pancreatic damage, but high concentrations of ROS cause pancreatic damage (30).

One experiment showed a significant increase in ROS levels in animal models of acute pancreatitis, which further promoted the activation and accumulation of inflammatory cells such as M1 macrophages and neutrophils, and also acted as a signaling molecule, leading to a significant increase in pro-inflammatory factors. The increase in ROS can disrupt the redox balance of the endoplasmic reticulum (ER), leading to ER stress and activation of the unfolded protein response (UPR), which in turn activates signaling pathways such as PKR-like ER kinase eukaryotic initiation factor 2 α (PERK-eIF2α) and activating transcription factor 6 (ATF6) and promotes the production of inflammatory factors (31).

Thus, ROS should not be considered simply as a toxic generation, but as a complex influencing factor. Certain substances that regulate ROS are listed in Table 1 .

Damage-associated molecular patterns (DAMPs) are endogenous danger molecules released by damaged or dying cells. DAMPs could activate the innate immune system by interacting with pattern recognition receptors (PRRs) (32, 33). They include ATP, the cytokine IL-1a, uric acid, the calcium-binding cytoplasmic proteins S100A8 and S100A9, and the DNA-binding nuclear protein high-mobility group box 1 (HMGB1) (34). DAMPs are recognized by PRRs, such as Toll-like receptors (TLRs) and cytoplasmic Nod-like receptors (NLRs), and also by non-PRRs, such as the receptor for advanced glycation end products (RAGE), CD44, integrins, and CD91 (15). As DAMPs are released locally and into circulation, they promote leucocytes infiltration and activation, which exacerbates pancreatic injury, systemic inflammation, and organ failure (35–37).

DAMPs normally reside inside the cell, but once the cell is damaged or dead, DAMPs were released into the extracellular space and then interact with PRRs. This leads to the recruitment of inflammatory cells and activation of adaptive immune responses. Excess DAMPs are also capable of activating signaling and sterile inflammation. DAMPs link local tissue damage and death to SIRS, leading to subsequent MOF and even death (14, 34). Interestingly, DAMPs are also involved in tissue repair (38). The detailed pathological roles of DAMPs in AP are summarized in Figure 1 .

TLR4 is localized on the plasma membrane and recognizes microbial components (54). Unlike other disease, the unique triggering factors of TLR4 are mostly endogenous, for example, histones as we have discussed above. In general, TLR4 and the subsequent pathways play a pro-inflammatory role in AP, however, there are also experiments showing that it also has a certain protective effect on AP. The research proved that the deletion of intestinal TRL4 could exacerbate AP by disrupting the intestinal flora and impairing the function of Paneth cell in mouse models, which leads to bacterial translocation (55). Therefore, although some experiments have shown that inhibiting TLR4 can improve AP (56, 57), we still need to view its effects dialectically.

Activation of TLR4 can induce the occurrence of autophagy, which is an intracellular degradation process in which damaged organelles, protein aggregates, and pathogens are enveloped by autophagosomes and subsequently degraded through fusion with lysosomes. TLR4 triggers autophagy via its downstream MyD88-dependent signaling pathway (58). In AP, the TLR4-mediated autophagy plays a dual role in exacerbation and resolution. On one hand, excessive autophagy may lead to cellular dysfunction, further exacerbating inflammation. On the other hand, moderate autophagy can clear damaged acinar cells, thereby alleviating inflammation. And one research also finds that autophagy and negatively regulate the excessive activation of TLR4 pathway in mouse models by degrading key molecules (such as MyD88 and TRAF6) within this pathway (59).

TLR4 is a pivotal receptor in the initiation and progression of AP. It primarily recognized DAMPs activating downstream inflammatory signaling pathway and promoting the release of pro-inflammatory cytokines, thereby exacerbating pancreatic injury and SIR (60). However, recent studies have revealed that in the later stages of AP, TLR4 may also contribute to inflammation resolution and tissue repair by activating cellular autophagy and suppressing excessive inflammation. This dynamic function suggests that TLR4 could serve as a potential therapeutic target of AP (58). Nevertheless, further investigation into its spatiotemporal regulatory mechanisms is required to balance pro- and anti-inflammatory effects, enabling more precise clinical intervention strategies.

There is also a close relationship between NF-κB and activation of trypsin. The production of pro-inflammatory cytokines that NF-κB pathway promote, induces dysfunction of lysosomes in pancreatic acinar cells, causing the leakage of lysosomal enzymes (such as cathepsin B), which leads to the activation of trypsinogen into trypsin (53, 60). The activation of trypsin leads to the necrosis of acinar cells, resulting in the release of pancreatic-specific DAMPs. This further amplifies inflammation through the TLR4/NF-κB pathway. These form a positive feedback loop, which is a specificity of AP, driving the progression of the disease (60). There is a study finds that in mouse models, the Bifidobacterium-derived metabolite, lactate, dampens macrophage-associated inflammatory responses both locally and systemically in AP, by suppressing NF-κB and NLRP3 inflammasome activation in a TLR4-MyD88- and NLRP3-Caspase1-dependent manner (61). However, the anti-inflammatory role of NF-κB is also being revealed. Studies have shown that the NF-κB activation protects acinar cells from inflammation-associated necroptosis, as well as NF-κB may reduce inflammation by limiting the processing and secretion of IL-1β (62), these two findings both indicate that it has a certain anti-inflammatory effect in the process of AP.

The combination of DAMPS and PRRS as an immune response to histiocyte injury such as acute pancreatitis can not only stimulate the accumulation of inflammatory cells and activate adaptive inflammatory responses, but also indirectly affect the severity of pancreatic inflammation and treatment methods through inflammatory cells.

Neutrophils are polymorphonuclear white blood cells and one of the main responders to acute inflammation (64). As the first line of the innate immune response, neutrophils are well recognized as one of the key players during acute inflammation. Data from both experimental and clinical contexts indicate that the precise localization of neutrophils to the site of inflammation plays a pivotal role in the effective elimination of infection following infection. A prominent feature of AP is the infiltration of neutrophils into the pancreas (65). They are typically the first leukocytes recruited to the site of inflammation and acquire the ability to eliminate pathogens through a variety of mechanisms (66). Numerous studies have documented that neutrophil aggregation represents a critical element contributing to the development of AP. The process of neutrophil recruitment and activation involves a series of complex cascades. In inflammatory conditions, neutrophils have been recognized as harmful cells that inappropriately damage host cells. However, neutrophils are capable of recruiting to the sterile sites of inflammation, clearing dead tissue and cells and promoting tissue repair.

The inflammatory signals released by acinar cells mediate the recruitment and activation of circulating inflammatory cells, especially neutrophils. And excessive activation of these cells triggers intense local and systemic inflammatory responses (67). The secretion of platelet activating factor (PAF) induced neutrophils to release superoxide and cause degranulation. Toxicity was then diffused by the recruitment of neutrophils (68). During the development of AP, macrophage inflammatory protein-2 (MIP-2) was formed by macrophages and damaged acinar cells (69). MIP-2 regulates neutrophil chemotaxis and tissue neutrophils (70). The classical recruitment cascade of leukocytes has been extended to include capturing, rolling, adhesion, crawling, and transmigration (64). CXCL2 is the most effective stimulator for neutrophils to recruit and infiltrate (71). Neutrophils are free-flowing cells within the bloodstream. Capturing includes two closely related steps: primary capture, implying direct neutrophil/endothelial interaction, and secondary L–selectin–mediated capturing (72). Rolling relates to sialyl-Lewis^X and the neutrophil-expressed glycosylation-dependent receptor P-selectin glycoprotein ligand 1 (PSGL-1), which, following binding to E- or P-selectin (73). During the progress of rolling and deceleration, chemokine receptors on neutrophils, such as CXCR-2 and formyl peptide receptors, with their ligands present on the endothelium successfully interact, which triggers a series of signal responses finally leading to the conformational change of integrins and shear-resisting cellular adhesion (74). Integrins, such as those that interact with the endothelial cell ligand ICAM-1, cause neutrophils to stop rolling and remain on the endothelial cell surface (75). Adhesion is followed by crawling, which shows the movement of neutrophils along the endothelium. Mac-1 is the main regulator of intravascular crawling on ICAM-1 in the microvasculature (76). At the extravasation sites for neutrophils to transendothelial, happens by a transjunctional (paracellular) or a transcellular fashion. Inflammatory signals have been proved to enhance the way (77). Upon entering the interstitium, the neutrophil migrates in the so-called “amoeboid” form. Inflammatory chemokines or chemoattractants serve as the primary inducers of neutrophil polarity and deformational movement by activating intracellular signaling pathways through GPCRs (78). CXCL12 signaling has been shown to induce chromatin compaction by promoting H4K20 dimethylation, which is essential for neutrophil migration in challenging microenvironments (79). Once neutrophils are recruited to the site of infection, the pathogens are combated. In acute pancreatitis, the chemokine and cytokine cascades that accompany inflammation are functions of neutrophils that have been recognized for many decades (67). Direct evidence has shown that activation of trypsinogen to trypsin induces neutrophil infiltration at the sites through genetic knockout models. A curious discovery reveals that trypsinogen activation is thought to induce initial neutrophil infiltration into the pancreas, and subsequently the presence of neutrophils triggers further trypsinogen production (67). Similarly, MMP-9, found in neutrophils, has been suggested as a potential prognostic marker in pancreatitis and may be involved in neutrophil trypsin activation (80). Therefore, inhibiting the activation of trypsin, and thus the activation of neutrophils and damage to pancreatic tissue, could be a positive way to treat AP.

DCs are recognized as the most powerful antigen-presenting cells in the immune system (99). DCs include multiple subtypes, such as conventional DCs (cDCs), plasmacytoid DCs (pDCs) and monocyte-derived DCs (moDCs). cDCs originated from bone marrow pluripotent hematopoietic stem cells while moDCs are differentiated from monocytes at the site of inflammation (100). pDCs are mainly involved in anti-viral infections, and there are currently few studies on pDCs and acute pancreatitis, and it may be involved in the immune response in AP by producing IFN-I (101). In contrast, moDCs have poor migration capabilities but are capable of producing inflammatory cytokines and activating T cells to mediate inflammatory responses in inflamed tissues (100).

Upon exposure to inflammatory stimuli, such as DAMPs, as discussed previously, immature DCs were activated and then triggered both adaptive and innate immune responses (102). Activated DCs promoted the recruitment of myeloid cells in the early AP (103). DCs were considered to promote the vitality of the pancreas during AP rather than damaging it. In type B coxsackievirus-induced acute pancreatitis, inflamed pancreatic acinar cells secreted CCL17, which was identified and combined with CCR4 on DCs. And the combination induced DCs to infiltrate the spot of inflammation. Then, it triggered further Th1 immune response (104), and protected the pancreas by reducing tissue injury. Another study showed that in the progression of AP in mouse models, acinar cells could express DC-SIGN, which could trigger the differentiation of naive CD4⁺ T cells into CD4⁺/IFN-γ⁺ Th1 and CD4⁺/IL-17A⁺ Th17 cells (the functions of Th17 and other T cells types will be discussed below) (105). Also, dendritic cells regulate the polarization direction of macrophages by secreting cytokines and chemokines. For example, IL-12 secreted by dendritic cells can promote macrophage polarization toward the M1 type and enhance their pro-inflammatory functions, whereas IL-10 secreted can induce macrophages to polarize to the M2 type and exert anti-inflammatory and tissue repair functions (106). This will be described in more detail in the following section regarding macrophage. During AP, DAMPs also activate DCs via PRRs, including TLR4 and the NLRP3 inflammasome. These damage signals trigger the NF-κB and MAPK pathways within DCs, promoting the transcription, synthesis and release of IL-33. IL-33 then binds to the ST2 receptor on mast cells, leading to their degranulation (107, 108). These will be explained in more detail in Figure 3 .

Lymphocytes represent a specific type of white blood cell, produced by lymphoid organs with immune recognition functions. They are capable of participating in the body’s immune response. Lymphocytes can be subdivided into three main categories: natural killer (NK) cells, B lymphocytes (B cells), and T lymphocytes (T cells). This classification is based on the presence and characteristics of surface molecules, as well as the functions they perform. Humoral immunity and cellular immunity are mediated by B lymphocytes and T lymphocytes, respectively.

It has been demonstrated that, in addition to innate immune cells, T lymphocytes are present in the normal pancreas. During the course of acute pancreatitis, a continuous influx of T lymphocytes from the blood vessels results in a notable increase in the number of T lymphocytes within the pancreas (109).

Mast cells are multifunctional immune effector cells derived from the bone marrow whose survival is critically dependent on cytokine signaling. These cells are involved in a variety of physiological and pathological processes, including but not limited to allergic responses, inflammatory pathways, immune homeostasis and tissue regeneration (118). In pancreatic tissue, mast cells are abundant and predominantly localized around vascular structures, lymphatic vessels and nerve fiber terminals within the pancreatic stroma. In addition, mast cells have a heightened sensitivity to even subtle environmental perturbations (119). They contain a substantial reservoir of mediators, including cytokines, histamine, proteolytic enzymes and platelet-activating factor, which can be rapidly released upon cellular activation, leading to degranulation. Upon activation by triggers such as IL-33 secreted by dendritic cells, mast cells undergo calcium influx, leading to the fusion of cytoplasmic granules with the cell membrane. This results in the rapid release of pre-formed mediators like histamine, proteases, and heparin, which increase vascular permeability and promote inflammation. Simultaneously, mast cells synthesize and release cytokines (e.g., TNF-α, IL-6), chemokines, and lipid mediators (e.g., prostaglandins, leukotrienes), amplifying the inflammatory response and recruiting other immune cells to the site of injury (120). This process significantly influences lymphocyte adhesion and chemotaxis, thereby initiating a spectrum of biological responses, including vascular dynamics within the organism. The specific distribution of mast cells and their rapid response to stimuli indicate that mast cells can form the body’s first line of defense against danger. The degranulation process is shown in Figure 3 .

Mast cell activation is an early event in the pathogenesis of acute pancreatitis (121). It is known that in the early stages of acute pancreatitis, in addition to acinar cell destruction, injury factors stimulate the activation of mast cells, which activate and release a series of pro-inflammatory cytokines. Cytokines set off a chain reaction through a complex cytokine network, exacerbating pancreatic damage and inflammatory transmission. IL-1 cytokine system is activated in the early stages of acute pancreatitis, promoting destruction of pancreatic tissue and exacerbating the condition. As a member of the IL-1 cytokine superfamily (122), IL-33 can not only induce inflammation by using cell death as an alarm in certain situations, but also inhibit inflammatory signaling by regulating nuclear gene transcription (123, 124).

In experimental studies of acute pancreatitis and endothelial barrier dysfunction, Marwan Dib et al. (125) found that mast cells are involved in exudative pancreatic vascular disease, which occurs with increased permeability of pancreatic tissue capillaries. Plasma and inflammatory cells infiltrate into the pancreatic interstitium, activating various biochemical pathways and causing pancreatic damage and dysfunction. This damaging disease is linked to the activation and release of inflammatory factors such as histamine and leukotrienes by mast cells.

Macrophages are a type of innate immune cell that reside in the abdominal cavity (in close proximity to the pancreas) and in the tissues surrounding the pancreas. They can co-mediate and amplify the inflammatory cascade reaction in the AP process with neutrophils, lymphocytes and other immune cells, thereby influencing the severity of AP (53, 126–128). Prior to the development of AP within the body, a multitude of pathological pathogenic factors (such as biliary diseases, alcohol consumption, smoking, hyperlipidemia, and genetics) first result in a sustained elevation of Ca²⁺ within the cytoplasm of pancreatic acinar cells, premature expression of trypsinogen, activation of the NF-κB inflammatory signaling pathway, and a reduction in Ca²⁺ levels (129, 130). During the early phase of AP, stimulated pancreatic acinar cells secrete monocyte chemoattractant protein-1 (MCP-1/CCL2), which mediates the recruitment and migration of CCR2-expressing inflammatory monocytes (131). The CCL2 chemotactic gradient within tissues directs the polarized migration of monocytes toward pancreatic lesions, where infiltrating monocytes differentiate into M1-polarized macrophages. Damaged pancreatic acinar cells, early-infiltrated neutrophils, and M1 macrophages collectively amplify the secretion of pro-inflammatory chemokines and cytokines, thereby initiating a feedforward inflammatory cascade that exacerbates acinar cell necrosis (132). This leads to extensive destruction of pancreatic tissue and, in some cases, spread of inflammation to specific distant organs, which can result in multi-organ failure or intractable systemic inflammation. It is therefore possible to discuss the chemokines and inflammatory mediators that play an important role in the inflammatory cascade of AP in order to identify specific targeted drugs for AP.

Basophils and eosinophils, two key granulocytes in the immune response, have been shown to play a role in various inflammatory responses that contribute to maintaining the body’s health (141). However, their involvement in the pathogenesis and progression of acute pancreatic inflammation has been less studied compared to other immune cells.