Diverse Biological Roles of Danger Signals

As primary danger signals, PAMPs and DAMPs exhibit remarkable functional diversity, ranging from the rapid initiation of inflammatory cascades to the subtle orchestration of tissue repair. While innate immune cells are traditionally viewed as the principal decoders of these signals, it is now clear that their impact extends to adaptive lymphocytes and diverse stromal populations, which actively shape the danger response. Furthermore, emerging research is increasingly focused on the systemic effects of these molecules.

A question that often confuses researchers is how the same PAMP or DAMP can elicit divergent, and sometimes opposing, biological outcomes. To resolve this ambiguity, it is essential to recognize that the biological outcome of danger signaling is not intrinsic to the molecule itself but is fundamentally dictated by the molecular and spatiotemporal context. Factors such as molecular isoforms governed by posttranslational modifications (e.g., the redox state of HMGB1 [69]) and the synergistic cross‐talk between coincident PAMP and DAMP exposure [70] determine whether a signal promotes protective immunity or fuels maladaptive pathology.

This section highlights the universal mechanisms by which these molecules modulate host defense and stromal activities, emphasizing how these signal receivers transform ancient danger signatures into specialized biological instructions (Figure 3).

FIGURE 3: The Multifaceted roles of PAMPs and DAMPs in immunity, tissue homeostasis, and systemic integration. (A) Danger signals orchestrate immune responses by regulating leukocyte trafficking, cellular activation and plasticity, and multiple inflammatory cell death pathways. In adaptive immunity, PAMPs and DAMPs provide adjuvanticity to license DC maturation, T cell differentiation, and Treg accumulation, while also directly modulating B cell responses. (B) Beyond immune modulation, these signals reshape the nonimmune microenvironment through vascular endothelial destabilization leading to leakage, pathological bone loss via osteoclast activation, and conversely, promote tissue repair by inducing stem cell differentiation and neuro‐reparative programs. (C) At the systemic level, PAMPs and DAMPs induce trained immunity through central (bone marrow) and peripheral reprogramming, and mediate cross‐organ communication by translocating from injured tissues to distant sites via various anatomical routes, including the gut‐liver axis and placental barrier, thereby propagating systemic distress signals.

Orchestrating the Immune Landscape

Innate Immunity

PAMPs and DAMPs serve as primary drivers of leukocyte trafficking, coordinating the migration of immune cells from the vasculature to sites of infection or injury [71, 72]. These molecules function as direct chemoattractants. For instance, extracellular ATP released from damaged tissue acts as a canonical “find‐me” signal for myeloid cells [73]. Beyond acting as direct gradients, they activate tissue‐resident cells to release chemoattractive mediators. In schistosomiasis, DAMPs trigger the cross‐talk between P2Y2 and P2×7 purinergic receptors in endothelial cells, upregulating VCAM‐1 through NF‐κB signaling and thereby facilitating mononuclear cell adhesion [74]. Similarly, the bacterial metabolite ADP‐heptose engages the host sensor ALPK1, triggering a TIFA‐dependent cascade that induces robust chemokine secretion from epithelial cells [75]. Beyond local recruitment, these signals also govern systemic cell mobilization. The NLRP3‐caspase‐1 axis facilitates the egress of hematopoietic stem and progenitor cells (HSPCs) from the bone marrow by activating the complement cascade through a cocktail of DAMPs that includes HMGB1 and S100A9 [76].

Following recruitment, DAMPs and PAMPs amplify the inflammatory response by inducing cellular activation and phenotypic shifts. This process establishes a pro‐inflammatory state characterized by the release of reactive oxygen species (ROS) and cytokines like TNF‐α and IL‐8, as observed in neutrophils treated with DAMPs (e.g., methemoglobin) [77]. In myeloid cells, retroviral replication intermediates, such as intron‐containing HIV‐1 RNA, act as PAMPs that bypass traditional receptors to bind NLRP1, driving IL‐1β secretion [78]. These molecular gradients also dictate macrophage functional plasticity, utilizing metabolic and epigenetic reprogramming to steer the transition between pro‐inflammatory M1 and pro‐resolution M2 phenotypes [79]. In addition, the role of these danger signals is increasingly recognized in innate lymphoid cells (ILCs), which serve as a critical functional bridge between innate sensing and adaptive execution. NK cells, for instance, function as autonomous sensors of PAMPs. In the absence of accessory cells, specific bacterial PAMPs can trigger NK cells to rapidly secrete IFN‐γ and release preformed alpha‐defensins, cationic peptides that directly disrupt microbial membranes [41]. This direct activation is further exemplified by the fungal PAMP β‐1,3‐glucan, which engages NKp30 to initiate a Src family kinase‐dependent signaling cascade, leading to the polarized release of cytolytic granules containing perforin [80].

The amplification cascade intensifies when DAMPs and PAMPs trigger regulated cell death pathways, thereby eliminating infected or damaged cells [81]. Apoptosis, traditionally considered immunologically silent, can become inflammatory under certain contexts. For example, PAMPs accelerate neuron apoptosis in the presence of amyloid pathology, thereby amplifying neuron inflammation [82]. Necroptosis represents a more overtly inflammatory death pathway, exemplified by viral Z‐form RNA activating the ZBP1/RIPK3/MLKL axis to eliminate infected cells [83]. Furthermore, PAMP and DAMP signaling can initiate other specialized inflammatory cell death modalities: NETosis induced by Candida albicans traps and neutralizes extracellular pathogens [84, 85], whereas pyroptosis facilitates defense responses against intracellular infection [86]. PANoptosis represents the most complex integration of death pathways, in which simultaneous inflammasome activation by multiple ligands (e.g., PAMP/heme or heme/cytokine combinations) triggers multiple forms of programmed inflammatory cell death [87]. In this context, NLRP3, AIM2, NLRC4, and Pyrin assemble into a large multiprotein complex alongside ASC, caspase‐1, caspase‐8, and RIPK3 that drives PANoptosis, ensuring robust host defense against pathogens like HSV‐1 that might otherwise evade individual death pathways [88].

Adaptive Immunity

Beyond their well‐established roles in innate activation, PAMPs and DAMPs exert a profound influence on adaptive immunity by directly modulating lymphocyte activity and restructuring the microenvironment that governs antigen‐specific responses.

The transition from a transient innate response to sustained adaptive surveillance requires more than just antigen recognition. It necessitates “adjuvanticity,” a quality primarily provided by the PAMP/DAMP axis. While antigenicity provides specificity, danger signals provide the requisite “second signals” for optimal immunogenicity. These signals drive DC maturation and enhance antigen cross‐presentation, thereby licensing the initiation of CD8+ T cell‐mediated antitumor or antipathogen immunity [89]. Furthermore, DAMPs such as HMGB1 can be released by DCs into the immunological synapse, where they bind to RAGE on T cells to stabilize the DC‐T cell interface and lower the threshold for TCR activation [90].

Once primed, T cell differentiation is further fine‐tuned by the local DAMP landscape. In chronic infections, the DAMP IL‐33 counterbalances type I interferon effects to prevent premature exhaustion of Tcf‐1+ progenitor cells, thereby ensuring sustained differentiation of functional CD8+ T cells [91]. Conversely, during the oral wound‐healing process, the IL‐33/ST2 axis exhibits a pro‐reparative role; ST2 is preferentially expressed on regulatory T cells (Tregs), where IL‐33 signaling promotes Treg accumulation to quench excessive inflammation and accelerate tissue closure [92]. Beyond individual cell modulation, persistent IL‐33 signaling acts as a potent driver of tertiary lymphoid structures (TLSs) in chronic inflammatory contexts such as colitis, facilitating localized immune aggregates that bridge the gap between acute damage and long‐term surveillance [93].

Within the tumor microenvironment, the impact of DAMPs is characterized by a striking duality. On one hand, IL‐33 can orchestrate potent anti‐tumor CD4+ and CD8+ T cell responses through annexin A1 signaling, and its presence can enhance the efficacy of chemotherapies like 5‐fluorouracil by priming the T cell compartment [94]. On the other hand, certain danger signals foster immune evasion. For instance, the release of the DAMP IL‐1α during tumor necroptosis can paradoxically impair myeloid cell function, subsequently suppressing CD8+ T cell recruitment and fostering an immunosuppressive milieu that facilitates malignancy [95].

Finally, DAMPs directly engage the humoral arm of adaptive immunity. Following myocardial infarction, heart‐derived DAMPs such as HMGB1 and HSP60 activate B cells via Toll‐like receptor signaling, promoting their rapid differentiation into antibody‐producing plasma cells [43]. This direct linkage ensures that the humoral system is mobilized not only against foreign pathogens but also in response to the molecular signatures of structural tissue failure, facilitating a comprehensive systemic defense [43].

Modulation of the Nonimmune Microenvironment

Beyond their immunomodulatory roles, DAMPs and PAMPs function as potent bioactive cues that reshape the nonimmune landscape, directly influencing vascular integrity, tissue remodeling, and structural homeostasis across multiple organ systems.

Vascular leakage has emerged as a critical pathological feature during circulatory failure, triggered by the inflammatory response following recognition of both PAMPs and DAMPs [96]. This hyperpermeability exacerbates circulatory collapse through hypovolemia and contributes to secondary microcirculatory disorders and organ dysfunction via interstitial edema formation. At the mechanistic level, PAMPs and DAMPs activate multiple pathways in microvascular endothelial cells, shifting them from a quiescent, barrier‐stabilized phenotype to an activated, hyperpermeable state. This transition involves destabilization of adherens and tight junctions, particularly through Src and RhoA kinase‐mediated phosphorylation and endocytosis of VE‐cadherin, alongside glycocalyx degradation, oxidative stress, and endothelial cell death [96]. Viral products further compromise endothelial integrity; for instance, treatment with NS1 alone disrupts endothelial cell monolayer integrity in an in vitro model of vascular leak, demonstrating how pathogen‐derived factors directly undermine barrier function [97]. Beyond acute permeability changes, DAMPs contribute to chronic vascular remodeling. Calprotectin (S100A8/A9) has been identified as a contributor to vascular calcification in chronic kidney disease, representing a potential therapeutic target for this complication [98].

The skeletal microenvironment is particularly sensitive to PAMP and DAMP signaling, which can drive pathological bone loss through multiple mechanisms. While traditional views focused on immune cell‐mediated bone loss, recent evidence reveals that matrix‐embedded osteocytes act as primary “sentinels” that sense bacterial PAMPs through a MYD88‐regulated signaling pathway [44]. Upon PAMP recognition, osteocytes produce significantly higher titers of the pro‐osteoclastogenic cytokine RANKL compared with osteoblasts [44]. Mechanistically, the activation of the ERK‐CREB/STAT3 signaling axis increases the binding of these transcription factors to Rankl enhancers. Simultaneously, DAMP/PAMP signaling suppresses the K48‐ubiquitination of CREB and STAT3, preventing their degradation and ensuring sustained high expression of Rankl [44]. Furthermore, virulence lipids derived from Porphyromonas gingivalis (a key periodontitis pathogen) can accelerate osteoclastogenesis independently of canonical HMGB1 signaling, highlighting the diversity of PAMP‐driven bone destruction [99]. This “danger signal‐bone” axis is also evident in sterile contexts. For example, osteocyte necrosis triggered by bone fracture results in excessive DAMP release. These DAMPs bind to Mincle on macrophages to induce osteoclast differentiation [100].

In contrast to their destructive roles, PAMPs and DAMPs are indispensable for initiating wound healing and tissue reparative programs. A striking example is the ALPK1‐TIFA signaling axis in the intestinal epithelium [45]. The PAMP ADP‐heptose triggers NF‐kB signaling, which initially induces TNF‐dependent apoptosis in damaged intestinal stem cells. However, this acute injury signal subsequently activates a TGF‐β and YAP‐based resurgence stem cell program. This process drives the dedifferentiation of Paneth cells into multipotent stem cells, effectively rebuilding the epithelial barrier. In models of radiation‐induced injury or DSS‐induced colitis, the absence of this PAMP‐sensing axis leads to severely impaired regenerative capacity. Similar reparative logic applies to the central nervous system, where specific DAMPs liberated poststroke coordinate the transition from acute neuroinflammation to long‐term neuronal repair and functional recovery [101].

Systemic and Cross‐Organ Effect

The influence of PAMPs and DAMPs is not confined to the site of initial insult; rather, these molecules serve as systemic messengers that reprogram the host immune rheostat and mediate complex interorgan communication. This integration occurs through the induction of innate immune memory and the translocation of danger signals across anatomical barriers, leading to long‐term functional shifts or distal organ dysfunction.

Trained Immunity

Trained immunity represents a form of innate immune memory in which prior exposure to microbial products or endogenous danger signals induces long‐term functional reprogramming of innate immune cells and their progenitors through sustained metabolic and epigenetic modifications [102]. Evolutionarily, trained immunity has emerged as a beneficial host defense mechanism that provides nonspecific protection against subsequent heterologous infections. However, its dysregulation can precipitate maladaptive outcomes, fueling the pathogenesis of autoimmune disorders and chronic inflammatory diseases [103].

The role of PAMPs in inducing trained immunity has been extensively characterized across both central and peripheral compartments. Centrally, the administration of β‐glucan, a prototypical trained‐immunity‐inducing agonist, drives the expansion of myeloid lineage progenitors in the bone marrow. This process is orchestrated by elevated signaling of cytokines such as IL‐1β and GM‐CSF, alongside metabolic adaptations in glucose utilization and cholesterol biosynthesis [104]. In the periphery, LPS‐induced innate memory in airway‐resident macrophages confers robust protection against acute challenges, such as pneumococcal infection, without altering the stability of the resident macrophage pool [105]. Remarkably, the paradigm of trained immunity has recently expanded to include nonimmune cells. Viral PAMPs, such as poly(I:C), can induce an “epigenetic signature” in airway epithelial cells, leading to exaggerated IL‐6 release, which is critically associated with exacerbation in experimental and clinical asthma [106].

While microbial motifs were the first identified trainers, accumulating evidence confirms that endogenous DAMPs play an equally pivotal role [107]. For instance, heme has been identified as a potent inducer of long‐term trained immunity in myeloid cells. Mechanistically, heme exposure leads to enrichment of H3K27ac (histone H3 lysine 27 acetylation) at the promoters of pro‐inflammatory genes such as TNF and IL‐8 [108]. This epigenetic remodeling is accompanied by the calibrated activation of the Syk and JNK pathways, thereby lowering the threshold for subsequent inflammatory responses.

Collectively, these findings position trained immunity as a central mechanism through which PAMPs and DAMPs exert prolonged effects on host defense and inflammatory pathology. The capacity to induce innate immune memory, whether protective or pathogenic, represents a fundamental dimension of PAMP/DAMP biology with profound implications for cancer immunotherapy and the management of chronic inflammatory diseases.

Cross‐Organ Effect

The pathogenesis of inflammatory disorders frequently involves the coordinated dysfunction of multiple organ systems, in which primary damage to local tissue can precipitate concomitant pathologies at distant anatomical sites. A critical question in systemic biology is how this interorgan communication is sustained. Emerging evidence positions PAMPs and DAMPs as central molecular vehicles that facilitate this “organ crosstalk” by traversing physiological barriers and disseminating inflammatory signals through the systemic circulation [9].

PAMPs are no longer viewed as merely localized infectious markers but as systemic modulators that can redefine the health of remote organs. Under conditions of intestinal dysbiosis, gut‐derived PAMPs translocate across the compromised epithelial barrier into the portal circulation and eventually reach the liver. Here, the intracellular sensor NOD2 recognizes muramyl dipeptide (MDP), a PAMP motif present in both Gram‐positive and Gram‐negative bacteria. This sensing mechanism translates gut‐derived signals into pro‐tumorigenic pathways in the liver, significantly contributing to hepatocarcinogenesis [109]. Perhaps more striking is the ability of PAMPs to bypass the placental barrier. Bacterial cell wall peptidoglycan (CW), a universal ligand for TLR2, has been shown to traverse the murine placenta and enter the developing fetal brain. This systemic translocation activates the CW‐TLR2 signaling axis within the fetal neuroenvironment, potentially disrupting neurodevelopment and predisposing the offspring to postnatal cognitive and behavioral disorders [110].

In addition to PAMPs, endogenous DAMPs released from necrotic or ischemic tissues also serve as potent systemic messengers of distress, often driving lethal systemic inflammation and multiple organ dysfunction syndrome (MODS). Following severe trauma or tissue injury, cell‐free mtDNA is liberated from the damaged parenchyma into the systemic circulation. Once released in the blood, mtDNA activates circulating neutrophils by inducing rapid Ca2+ signaling and p38 MAPK phosphorylation. This molecular activation drives neutrophil chemotaxis and degranulation at remote sites, such as the lungs and liver, leading to neutrophil‐mediated organ injury that mimics the inflammatory profile of sepsis [111]. Similarly, in skeletal muscle I/R injury, the DAMP HMGB1 translocates from the nucleus to the cytoplasm and is subsequently released into the systemic circulation upon reperfusion. This circulating HMGB1 serves as a long‐range signal that communicates the local ischemic insult to the entire host, potentially triggering secondary inflammatory cascades in distal organs [112].

Collectively, these systemic and cross‐organ integration mechanisms underscore that PAMPs and DAMPs are not merely local byproducts of injury, but are active participants in the global regulation of host physiology and the systemic progression of disease.

Interplay Between PAMPs and DAMPs

The host immune system operates within a complex molecular environment where exogenous microbial products and endogenous damage signals coexist. The interplay between PAMPs and DAMPs encompasses a spectrum of interactions ranging from synergistic amplification to mutual antagonism, collectively shaping inflammatory outcomes through distinct mechanistic pathways. Understanding this dynamic crosstalk is essential for deciphering the pathophysiology of conditions like sepsis and for designing rational therapeutic interventions that account for the integrated danger context rather than targeting individual signals in isolation.

Synergistic Amplification of Inflammation

Research has revealed that the concurrent presence of PAMPs and DAMPs frequently triggers inflammatory responses that exceed the sum of individual stimuli, with the nature and magnitude of synergy depending critically on the specific molecular context. In some cases, DAMPs alone are insufficient to activate inflammation but become potent amplifiers in the presence of microbial products. For example, while low‐dose extracellular RNA alone does not promote the pro‐inflammatory activation of astrocytes, it induces robust cytokine expression when combined with the synthetic PAMP molecule Pam2CSK4 [70]. Alternatively, DAMPs with intrinsic stimulatory capacity can have their effects markedly amplified by concomitant PAMP exposure, as seen when methemoglobin‐induced production of ROS and pro‐inflammatory cytokines in neutrophils is further intensified by the addition of the LTA complex [77].

This cooperative behavior operates through at least two distinct mechanistic pathways. One major mechanism involves transcriptional reprogramming, in which DAMP‐PAMP co‐stimulation activates transcription factors that upregulate pattern recognition receptor expression, creating a feed‐forward amplification loop. A primary example is the DAMP heme, released during hemolysis, which synergizes with LPS via TLR2/4‐dependent IRF1 activation; this process leads to the transcriptional upregulation of NLRP12, which qualitatively alters gene expression to prime cells for heightened inflammasome activity [113]. Beyond transcriptional reprogramming, DAMPs and PAMPs also collaborate through threshold modulation, wherein endogenous DAMPs lower the detection threshold for microbial products. Extracellular HSP60 binds LPS and facilitates microbe recognition by reducing the threshold for PAMP detection and enhancing TLR signaling, effectively functioning as a molecular bridge that sensitizes the innate immune system to concomitant microbial challenges [114].

Inhibitory and Regulatory Interactions

Paradoxically, DAMP‐PAMP interactions are not invariably pro‐inflammatory; under specific contexts, endogenous damage signals exert inhibitory effects that restrain microbial product‐driven inflammation, revealing built‐in regulatory circuits that prevent excessive immune activation and preserve tissue integrity. Receptor desensitization through internalization is a major inhibitory mechanism in which DAMPs induce the endocytosis of PAMP receptors, rendering cells hyporesponsive to subsequent microbial stimulation. In a mouse model simulating long‐bone fracture, LPS promotes macrophage necroptosis through TLR4 signaling, yet this PAMP‐driven cell death is ameliorated by HMGB1 released from damaged tissue [115]. Mechanistically, HMGB1 engages RAGE, upregulating caveolin‐1 expression through RAGE‐MyD88‐dependent Cdc42 activation and subsequent recruitment of the transcription factor Sp1, leading to caveolae‐mediated TLR4 internalization and desensitization, thereby reducing macrophage necroptosis. Another distinct regulatory pathway involves independent priming defense, wherein DAMPs can provide an independent priming defense that restricts pathogen replication before the canonical PAMP‐mediated interferon response is fully mounted. Oxidized lipids exemplify this protective dimension [116]. Exposure of cells to these DAMPs prior to vesicular stomatitis virus infection triggers a rapid, interferon‐independent antiviral state by preventing viral entry and restricting the percentage of productively infected cells, thereby serving as an early homeostatic brake that limits viral load during the nascent stages of PAMP‐mediated viral inferno responses.