Harnessing prostaglandin E₂ signaling to ameliorate autoimmunity

Abstract

The etiology of most autoimmune diseases remains unknown; however, shared among them is a disruption of immunoregulation. Prostaglandin lipid signaling molecules possess context-dependent immunoregulatory properties, making their role in autoimmunity difficult to decipher. For example, prostaglandin E₂ (PGE₂) can function as an immunosuppressive molecule as well as a proinflammatory mediator in different circumstances, contributing to the expansion and activation of T cell subsets associated with autoimmunity. Recently, PGE₂ was shown to play important roles in the resolution and post-resolution phases of inflammation, promoting return to tissue homeostasis. We propose that PGE₂ plays both proinflammatory and pro-resolutory roles in the etiology of autoimmunity, and that harnessing this signaling pathway during the resolution phase might help prevent autoimmune attack.

Modulating the cycle of inflammation to resolve autoimmunity

The cycle of inflammation and resolution (Figure 1, Key Figure) is crucial for maintaining tissue homeostasis while allowing continuous monitoring of foreign invaders. Interruption of this normal cycle can result in loss of tissue integrity and function leading to chronic inflammation or autoimmune attack of endogenous tissues. Proper inflammation/resolution requires coordination and temporal control of a complex array of immunomodulatory factors (Figure 1). Autoimmunity serves as a particularly interesting example because improper activation of the immune system against autoantigens can lead to a local inflammatory response, which if left unresolved, may deteriorate into tissue damage. Current treatments for autoimmune diseases often treat the end point symptoms rather than the underlying origin of loss of inflammation resolution. One mediator of inflammation with important roles in all phases of inflammation in vertebrates is prostaglandin E₂ (PGE₂) (see Glossary).

Figure 1. Model for the role of PGE₂ in modulating normal progression towards inflammation.

Prostaglandin E₂ plays a complex role in modulating inflammation. In the initiating stages, PGE₂ is produced by damaged cells within a tissue/organ leading to the recruitment of neutrophils that produce proinflammatory cytokines and leukotrienes, resulting in monocyte recruitment to the site of inflammation. Macrophage efferocytosis of dying neutrophils induces macrophage-specific production of PGE₂, which is essential for facilitating a phenotypic switch that favors production of specialized pro-resolving lipid mediators such as LXA4 [61,62]. In addition, recruited macrophages secrete factors such as IL-6, IL-4, IFNγ , and IL-12 that can alter T cell polarization and activation states, as well as promote DC migration and expression of costimulatory molecules such as OX40L, CD80/CD86, or CD40. There is also evidence that PGE₂ contributes to macrophage production of anti-inflammatory factors such as IL-6, IL-1β, and IL-23, as well as blockade of further innate immune activation, allowing proper restoration of tissue homeostasis. We posit that the diverse actions of PGE₂ throughout the progression of inflammation are achieved via effects of signaling through disparate receptors, changes in the activity of synthetic enzymes (i.e. COX-1/COX-2, PGES-1/mPGES-1) and the cell of origin, as well as the general inflammatory context under which PGE₂ is acting (for example, acute injury versus the post-resolution immunosuppressive phase). This figure was created with Biorender.com

PGE₂ is a metabolite of arachidonic acid, derived from cyclooxygenase (COX) activity, and known to have a complex role in inflammation [1,2]. There is no consensus on the exact role of PGE₂ in mediating inflammation in the setting of autoimmunity. Most 20^th century studies suggested that PGE₂ could facilitate immunosuppression by favoring anti-inflammatory immune cell polarization and cytokine secretion profiles (IL-10 and TGF-β1) [3–7]. However, in the 21^st century, the focus of PGE₂ studies mostly turned to its role as an initiator and perpetuator of inflammation, promoting proinflammatory cytokine production (IL-6, IL-1β, IL-8, and IL-23), and proinflammatory cell phenotypes in both murine and human T cells and macrophages [8–11]. It is crucial that the dual role of PGE₂ in inflammation is recognized and incorporated into a wholistic therapeutic approach in a variety of inflammatory processes and autoimmune diseases.

PGE₂ has four specific receptors in mice and humans, EP1-EP4, each with distinct ligand binding affinities and downstream signaling pathways (Box 1) [12,13]. Therefore, based on the receptors expressed, cellular responses to PGE₂ can be vastly different. Circulating or local inflammatory factors such as cytokines and chemokines can also modulate PGE₂ production and signaling [14]. As a pathway capable of influencing cell fate decisions, stress responses, and survival, modifying PGE₂ signaling has the potential not only to alter immune cell populations, but also cells of the target tissue in autoimmune attack. For example, pharmacological EP3 blockade or activation of EP4 in isolated human or murine islets ex vivo enhances survival of insulin-producing beta cells in the presence of a cytokine cocktail known to induce beta-cell death (TNFα, IL-1β, and IFNγ) [15]. In vivo manipulation of EP3 signaling in murine models of type 1 diabetes (T1D) or type 2 diabetes resulted in maintained beta-cell mass and enhanced activity of antioxidant pathways to ameliorate beta-cell stress [16,17]. Thus, targeting PGE₂ receptor activity might reduce inflammatory burden, and simultaneously protect target tissues in autoimmune diseases such as T1D, although this remains to be fully assessed. We propose that a better understanding of the factors that modulate PGE₂ activity and its dichotomous roles, and how these factors fluctuate throughout the cycle of inflammation (Figure 1) can identify windows for intervening in the PGE₂ signaling axis to delay or prevent autoimmune attack. We hypothesize that this would occur by limiting inflammation and local tissue damage, while encouraging resolution, which would be a novel and robust method for addressing both the development and termination of inflammatory responses.

BOX 1: PGE₂ SYNTHESIS AND SIGNALING.

PGE₂ is a bioactive lipid metabolite of arachidonic acid (AA). Two cyclooxygenase (COX) enzymes, COX-1 (constitutive) and COX-2 (inducible), convert AA into an unstable intermediate, prostaglandin G₂, which rapidly isomerizes to prostaglandin H₂ [67]. Prostaglandin E synthase 1 (PGES-1) converts PGH₂ into PGE₂ [67]. COX-2 is primarily expressed in fibroblasts, epithelial cells, and immune cells in the context of inflammation, and is enhanced in the presence of proinflammatory stimuli in mice and humans [68]. Both the synthetic enzymes and the receptors for PGE₂ are widely expressed, leading to a diverse array of systemic effects from PGE₂ action [69]. The effects of PGE₂ are mediated through four different G protein-coupled E prostanoid receptors, EP1, EP2, EP3 and EP4. These receptors share a similar structure, with an extracellular ligand binding domain, seven transmembrane domains, and a cytoplasmic C-terminal tail that facilitates G-protein coupling [67]. Each receptor primarily couples to a different G-protein, leading to activation of diverse downstream effectors. EP1 couples to Gαq proteins, leading to activation of phospholipase C and release of intracellular calcium as a second messenger [70]. EP2 and EP4 primarily couple to Gαs proteins, resulting in activation of adenylyl cyclase and production of cyclic adenosine monophosphate (cAMP) as a second messenger [70]. cAMP can then activate protein kinase A (PKA) or exchange protein activated by cAMP (Epac), leading to activation of diverse intracellular response pathways [70]. EP4 is more promiscuous, occasionally coupling with Gβi proteins to activate the PI3K pathway, as well as β-arrestins and EP4-associated protein (EPRAP) [70]. The EP3 receptor is primarily coupled to Gβi proteins, functioning to inhibit the activity of adenylyl cyclase, therefore prohibiting the production of cAMP [70]. In addition to activating different downstream pathways, each receptor has a different affinity for the ligand. EP3 and EP4 have the highest and remarkably similar affinities for PGE₂, followed by EP2 and finally EP1, with the lowest affinity [12]. The EP3 receptor is the only PGE₂ receptor to have multiple isoforms (generated by alternative splicing) that differ in the degree of constitutive versus ligand-induced activity. Taken together, these properties allow for PGE₂ to elicit differential responses based on the receptors expressed on the cell surface.

PGE₂ promotes immune cell activation and acquisition of proinflammatory phenotypes

PGE₂ has diverse effects on various cell populations, including altering activation and proliferation, cytokine secretion, and regulatory function [9,10,18–22]. This section explores the ways that PGE₂ can promote proinflammatory actions in T cells and macrophages (Table 1).

Table 1.

Summarized reported effects of PGE₂ on various immune cell populations.

Proinflammatory effects of PGE2
Immune Cell Population	PGE2 effect	Experimental Determination	Species	References
T cells	Increased production of proinflammatory factors such as IL-8	In vitro co-exposure with PGE2 & EP1/EP4 antagonists	Human	[8]
	Promotes Th1 phenotype in a dose-dependent manner	Inactivation of EP4 receptor specifically in mouse T cells; Agonists of each EP receptor	Mouse	[9]
	Promotes Th17 expansion	In vitro culture with PGE2 under Th17- or Th2-skewing conditions	Mouse	[20]
	Blocks Treg differentiation	In vitro differentiation with PGE2 or EP1-4 receptor antagonists	Human	[19]
Macrophages	Increased secretion of proinflammatory factors such as IL-6, IL-1β, and IL-23	Myeloid-lineage EP4 receptor inactivation in mice	Mouse	[10]
	Maintains proinflammatory character based on continued expression of proinflammatory mediators, Il1b, Il12, and Nos2	Targeted COX-2 inhibition in mice (nanotherapy or genetic inactivation)	Mouse	[26]
	Facilitated chemotaxis in a dose-dependent manner	In vitro stimulation with PGE2	Mouse	[27]
Dendritic Cells	Enhanced T cell priming & activation	In vivo irreversible EP2 and EP4 antagonists in mice	Mouse Human	[18]
	Increased expression of costimulatory molecules	In vitro maturation with PGE2	Human	[21]
Immunosuppressive effects of PGE2
Immune Cell Population	PGE2 effect	Experimental Determination	Species	References
T cells	Antiproliferative – interrupts the IL-2 pathway	In vitro exposure to PGE2	Human	[30]
	Blockade of Th1 induction	Dependent on mode of in vitro stimulation	Human	[66]
	Induces peripheral Treg	In vitro antagonism of EP2 and EP4 receptors	Human	[36]
Macrophages	Induces expression of anti-inflammatory & pro-resolving factors such as IL-10	In vitro exposure to PGE2, EP2 & EP4 agonists	Mouse	[42]
	Shifts toward anti-inflammatory M2-like phenotype based on IL-10 production and expression of markers such as CD163	In vivo antagonism of the EP4 receptor in mice	Mouse	[44]
	Induces shift to SPM production	Ptges inactivation in zebrafish	Zebrafish	[63]
Dendritic Cells	Downregulated Th1-associated chemokines such as CXCL10	In vitro exposure of EP1-4 receptor agonists	Human	[40]
	Decreased chemoattractant capabilities	In vitro maturation with PGE2	Human	[41]

T cells

PGE₂ signaling has effects on multiple different T cell phenotypes. Substantial evidence suggests that PGE₂ promotes T cell proliferation and activation. T cells produce PGE₂, and its autocrine action through EP2 and EP4 is required for priming activation, proliferation, and accumulation of CD4⁺ T helper cells in draining lymph nodes [18]. Activation of murine and human CD4⁺ T cells in vitro using anti-CD3 and anti-CD28 monoclonal antibodies, induced production of PGE₂ and increased EP2 and EP4 surface expression, as determined by flow cytometry [18]. CD4⁺ T cells isolated from EP2^−/− or EP4^−/− mice, showed blunted, but not complete loss of T cell activation by anti-CD3/anti-CD28 antibodies. Blocking of the non-inactivated receptor with specific antagonists, AH6809 (EP2) and AH23848 (EP4), resulted in complete loss of activation [18], demonstrating that both receptors play a role in this process. In vitro treatment of isogenic CD4⁺ T cells with AH6809 or AH23848 decreased the duration of T cell-dendritic cell (DC) interaction in peripheral lymph nodes in an adoptive transfer mouse model, visualized using intravital microscopy [18]. These data suggest that PGE₂ can stabilize T cell-DC interactions in the lymph node, allowing for more efficient T cell activation.

Contrary to most early reports, discussed in the next section, recent evidence suggests that PGE₂ actually enhances the Th1 phenotype. PGE₂ facilitates human T cell production of proinflammatory factors, such as the neutrophil chemoattractant IL-8, when primary peripheral T cells or Jurkat T cells are activated in vitro [8]. This can be concentration-dependent. In vitro, low doses of PGE₂ act directly through EP4 on anti-CD3/anti-CD28 antibody-activated murine CD4⁺ T cells to facilitate differentiation of naïve T cells into a Th1 phenotype; this occurs via enhanced production of Il12rb2 mRNA and protein, and synergistic amplification of IL-12 signaling, resulting in enhanced production of Th1-associated factors such as IL-2, IFNγ, and T-bet [20]. Conversely, incubation of murine CD4⁺ T cells with higher doses of PGE₂ suppressed the production of IFNγ [9]. Incubation of naïve CD4⁺ T cells from C57BL/6 mice with PGE₂ in the presence of Th1, Th2, or Th17-skewing conditions, enhanced only Th1-cell differentiation. However, when primary DCs from C57BL/6 mice were cultured in the presence of PGE₂, there was significant upregulation of the Th17-inducing cytokine, IL-23 [9]. This result implicates PGE₂ signaling in the expansion of Th17-cells, which has been supported by others [11,23–25].

Exposure of naïve human T cells to PGE₂ significantly blunted the total number of regulatory T cells (Tregs) detected by flow cytometry after a seven-day in vitro stimulation protocol under Treg-inducing conditions, and decreased expression of FOXP3, a marker of mature Treg cells, suggesting that PGE₂ inhibited Treg differentiation [19]. These effects occurred even in the presence of the Treg-inducing factors TGF-β and IL-2 and were dependent on an EP2-cAMP-PKA signaling axis in T cells, suggesting direct PGE₂ activity [19]. Therefore, blocking PGE₂ might be useful in the therapeutic modulation of T cell phenotypes by reducing proinflammatory Th17 cells while increasing pro-resolving Tregs.

Macrophages

There is a large body of evidence supporting the proinflammatory effects of PGE₂ on macrophages. [10]. In one study, five-day exposure of human and murine macrophages to elevated PGE₂ following an acute wound, enhanced gene expression of proinflammatory mediators such as Il1b, Il12, and Nos2, and blunted phagocytic activity [26]. Decreasing macrophage-specific production of PGE₂ via genetic (Cox2^fl/flLyz2^Cre+) or nanocarrier-mediated pharmacological inhibition of COX-2, as well as pharmacological blockade of EP2 activity with AH6809 on macrophages, resulted in a reversal of this proinflammatory action; this was evidenced by the decreased production of proinflammatory mediators and restored phagocytic capabilities relative to controls. Moreover, these result confirm previous findings indicating that PGE₂ is an important player governing macrophage phenotypes through the progression of inflammation [26]. The effects of PGE₂ on macrophage chemotaxis are dose-dependent: low concentrations of PGE₂ facilitate migration toward chemokines, while high concentrations promote establishment of focal adhesions, reducing chemotaxis [27]. Thus, we posit that establishing a PGE₂ gradient may be key for recruiting macrophages to the site of inflammation, supporting the hypothesis that PGE₂ elicits different effects depending on stage of inflammation, site of action, and local concentrations.

PGE₂ induces regulatory and suppressive effects on immune cells

As mentioned, PGE₂ was initially appreciated primarily for its immunosuppressive effects and ability to block the initiation of inflammation. However, many of these effects, including blockade of activation, shifts toward anti-inflammatory phenotypes, and enhancement of regulatory capability, are in direct opposition to its reported proinflammatory actions described above [1,6,28–30]. This section discusses the immunosuppressive actions of PGE₂ in T cells and antigen-presenting cells (Table 1).

T cells

Interrupted IL-2 action was widely reported as a primary mechanism by which PGE₂ altered human CD3⁺ T cell activity [3,31,32]. For example, direct exposure of human lymphocytes to PGE₂ in vitro resulted in reduced IL-2 production [3]. Later studies connected reduced IL-2 and transferrin receptor expression to the anti-proliferative effects of PGE₂ on T cells, because PGE₂ and other cAMP-elevating agents, such as isoproterenol, blunted the surface expression of the transferrin and IL-2 receptors, as well as T cell production of IL-2 [30,33]. Direct exposure of human and mouse T cells to PGE₂ significantly blunted T cell proliferation in response to phytohemagglutinin, a T cell mitogen, but this was partially rescued by addition of exogenous IL-2 [30]. PGE₂ also suppressed antigen-specific T cell activation in response to specific allergens, an effect mimicked by activating EP2/EP4, but not other EP receptors [34]. This suggested that PGE₂ played an important role in T cell activation, which could be targeted by modulating its activity through EP2/EP4 receptors.

Several groups have proposed that PGE₂ enhances Treg cell differentiation and activity [35–38]. For example, PGE₂ is important in the induction of a unique subset of T cells in the periphery called adaptive regulatory T cells (T_R^adapt) [36]. When exposed to PGE₂ in the presence of anti-CD3/anti-CD28 antibodies, human CD4⁺ T cells upregulated FOXP3 and CD25 protein expression, and secreted IL-10 and TGF-β, two anti-inflammatory cytokines that are hallmarks of T_R^adapt cells [36]. PGE₂ also enhanced Treg immunosuppressive activity, blunting proliferation of effector T cells following anti-CD3/anti-CD28 antibody stimulation in vitro [39]. Recent evidence suggests that PGE₂ can facilitate the tissue-specific development of Tregs, such as in adipose tissue where addition of exogenous PGE₂ to primary adipocytes from COX-2^−/− mice fed a high fat diet led to increased Treg proliferation relative to controls [38]. Taken together, these findings highlight the concept that the effects of PGE₂ are cell type- and context-dependent.

Antigen-Presenting Cells

As the primary producers of PGE₂, myeloid cells such as macrophages and DCs are an essential part of the puzzle for understanding PGE₂ activity in inflammation. DCs matured in vitro with PGE₂ demonstrate enhanced expression of chemokines associated with Th2-like T cell responses (e.g., CCL17 and CCL22) [40,41]. In contrast, Th1-like chemokines (e.g., CXCL10) are downregulated, suggesting that PGE₂-driven T cell fate may be due at least in part to DC activity, though this remains to be interrogated experimentally [40]. Compared to human monocytes matured into DCs in vitro in the absence of PGE₂, DCs matured in the presence of PGE₂ exhibit blunted expression of CCL19, a chemokine that attracts and activates naïve CD4⁺ T cells [[41].

Macrophages and their monocyte precursors also have context-dependent responses to PGE₂. Exposure of monocytes or macrophages to PGE₂ in vitro results in diminished expression of proinflammatory mediators such as IL-1β [4,7]. Simultaneous exposure of bone marrow-derived macrophages (BMDM) from C57BL/6 mice to PGE₂ and lipopolysaccharide (LPS) in vitro results in an anti-inflammatory phenotype characterized by increased production of IL-10 mRNA and protein, and expression of well-known markers of regulatory macrophages (e.g., Tnfsf14 (aka LIGHT), Sphkl, Arg1) [42][42]. Additional evidence of an anti-inflammatory role for PGE₂ includes studies showing that PGE₂ further enhances the expression of pro-resolving or anti-inflammatory (M2-like) macrophage markers in the presence of IL-4 [43]. Furthermore, in vivo treatment of db/db mice (a murine model of type 2 diabetes) with an EP4 agonist, ONO-AE1-329, decreased the pancreatic ratio of M1-like:M2-like macrophages relative to the vehicle-treated group; similarly, in vitro cultures of proinflammatory M1-like peritoneal macrophages with ONO-AE1-329 shifted the polarization state of the macrophages to resemble an M2-like state [44]. These findings highlight the potential of PGE₂ as an emerging pro-resolving therapeutic pathway in chronic inflammation.

Involvement of PGE₂ in Autoimmunity

PGE₂ signaling is implicated in several autoimmune conditions, including T1D, rheumatoid arthritis (RA), multiple sclerosis (MS), inflammatory bowel disease (IBD), and systemic lupus erythematosus (SLE) [45–49]. Evidence suggests that this pathway may play a specific role in either initiation of inflammation or failed resolution, each of which might be therapeutically targeted [50,51]. Autoimmune diseases such as RA, SLE, MS, IBD, T1D, and psoriasis, are associated with an elevated ratio of Th17 cells to Tregs in target tissues, consistent with the concept that PGE₂ can facilitate the expansion of Th17 cells [52]. Therefore, we argue that the PGE₂ axis may constitute a target to reduce the expansion of Th17 cells while promoting the differentiation of Tregs. Promoting Treg differentiation is currently being evaluated in clinical trials for the treatment of new onset T1D in which expanded autologous Tregs are being administered in an attempt to preserve beta-cell mass and prolong function [ClinicalTrials.gov, NCT02691247ⁱ and NCT02772679ⁱⁱ] [53,54].

Cumulative evidence to date suggests that in the context of autoimmunity, PGE₂ is mainly proinflammatory, exacerbating autoimmune disease [11,47,48,51,55]. For example, in RA, PGE₂ activity on antigen-presenting cells such as macrophages and DCs via EP2 leads to upregulation of IL-23, encouraging pathogenic Th17 cell expansion [55]. Another group investigated the role of PGE₂/EP4 in a murine model of MS, experimental autoimmune encephalomyelitis (EAE). In this study, bone marrow-derived cells from COX-2^−/− or EP4^−/− mice were transplanted into irradiated wild type hosts in which EAE was then induced, generating chimeric mice harboring peripheral immune cells with inactivation of either COX-2 or EP4 [47]. Animals receiving EP4^−/− or COX-2^−/− bone marrow-derived cells exhibited delayed EAE disease onset, reduced plasma concentrations of proinflammatory cytokines IL-6 and IL-17, and decreased gene expression of matrix metalloprotease 9. These changes were associated with diminished monocyte and macrophage infiltration into the lumbar spinal cord [47]. In nonobese diabetic (NOD) mice, genetic ablation of EP4 specifically on macrophages resulted in decreased gene expression of proinflammatory cytokines including Il6, Il1b, Ccr7, and Il23 relative to controls, although progression of T1D did not change [10].

There are also select studies showing that PGE₂ may be anti-inflammatory or play a dual role in autoimmunity. Dose-dependent treatment of CD8⁺ T cells isolated from NOD mice with EP receptor agonists, sulprostone, misoprostol, and 19^ROH-PGE₂, revealed that EP2 and EP4 receptors contributed to PGE₂-regulation of T cell receptor-triggered IFNγ secretion [56]. Additionally, in vitro incubation of primary human hematopoietic stem and progenitor cells with PGE₂ imbued them with a regulatory phenotype, as evidenced by the increased expression of the immunosuppressive molecule, PD-L1 [57]. Recent clinical uses of anti-PD-L1 for the treatment of some cancers has led to an increase in autoimmunity in some patients, highlighting the important role of this molecule in suppressing autoimmunity [58,59]. Finally, in the context of EAE, genetic and pharmacologic manipulations revealed that EP2 and EP4 facilitated the opposing roles of PGE₂ indeed, the prostaglandin promoted expansion of Th1 and Th17 cells through EP2 (a proinflammatory effect) but decreased initial immune cell infiltration and demyelination in the central nervous system [60]. Whether the beneficial effects of EP4 are mediated by suppressing a breach in the blood brain barrier requires robust investigation. Nevertheless, these EP2 and EP4 opposing effects highlight the crucial nature of receptor expression on the ultimate downstream action of PGE₂. Therefore, we propose that differential receptor targeting might unlock the potential to drive PGE₂ activity towards pro-resolving effects that could be beneficial in the treatment of chronic inflammatory diseases (see Clinician’s Corner).

CLINICIAN’S CORNER:

• A healthy inflammatory response must be paired with an equally important ‘resolution’ phase. While the early phase of inflammation draws immune cells to the site of injury to neutralize the source of injury, the resolution phase is necessary for the removal of dead cells, tissue repair, and a return to homeostasis. Failure of resolution leads to chronic inflammation and is known to be causal in the development and progression of several chronic inflammatory diseases, including atherosclerosis, rheumatoid arthritis, and obesity. [62]

• Targeting pro-inflammatory pathways in clinical trials (e.g. neutralizing antibodies to IL-1β) has proven to be a flawed strategy as this leads to immunosuppression, infections, and even death [71]. However, therapies that aim to enhance resolution do not impair host defense and are therefore attractive emerging therapeutic targets. There are several pro-resolving therapeutics currently in development or early-phase clinical trials for cardiovascular disease [72], cancer (TP-317), and autoimmune diseases such as Sjogren’s syndrome (NCT04684654)ⁱⁱⁱ and Crohn’s disease (TP-317).

• In addition to the specialized pro-resolving mediators (SPMs), the PGE₂ pathway may be a novel pro-resolving therapeutic target. PGE₂ is currently only approved for clinical use in obstetric indications [73]; however, the recent finding that PGE₂ can play a pro-resolving role suggests that targeting PGE₂ receptors to enhance macrophage efferocytosis activity might be beneficial for the treatment of certain chronic diseases. In support of this concept, there are ongoing clinical trials using anti-CD47 monoclonal antibodies to promote efferocytosis and enhance the macrophage pro-reparative phenotype in cancer [74,75] and atherosclerotic cardiovascular disease [72].

• Pharmacological blockade of COX-2, which catalyzes the first step in the production of PGE₂, has led to increased incidences of heart attack and stroke, resulting in the withdrawal of several drugs from the market. Their use led to blunted production of PGE₂, highlighting the potentially cardioprotective role of PGE₂. [76,77]. Thus, targeting PGE₂ receptors rather than ligand production might be therapeutically beneficial, although this remains speculative. The ability of PGE₂ to interact with multiple receptors (EP1-4), each with unique downstream signaling functions (see Box 1), provides an opportunity to tailor therapeutic strategies to favor the activation of pro-resolving pathways.

• While more research is needed to fully understand the spectrum of PGE₂ and EP receptor function, clinicians should be aware of this emerging paradigm by which targeting pro-resolving pathways might help treat chronic inflammatory diseases.

PGE₂ facilitates the efficient resolution of inflammation

Perhaps the most crucial example of context-dependent PGE₂ action is the resolution of inflammation. Following a normal inflammatory response, neutrophils undergo apoptosis and are cleared via efferocytosis, which activates an anti-inflammatory program and initiates the switch towards resolution (Figure 1) [61,62]. The steps that initiate resolution are not completely defined, but PGE₂ has been implicated as an important player [61]. In a zebrafish model of acute injury and inflammation, PGE₂ action on macrophages, induced a shift in arachidonate metabolism by inducing the activity of 15-lipoxygenase, thereby decreasing the production of proinflammatory leukotrienes and favoring production of lipoxin-A4 (LXA4), a specialized pro-resolving mediator (SPM), and part of the “lipid mediator class switch” that facilitates neutrophil removal from the site of inflammation [63] Confirming the PGE₂requirement for efficient resolution of inflammation, inhibition of microsomal-PGE₂ synthase-1 (mPGES-1) using a specific antagonist (mPGES-1 inhibitor compound III) resulted in incomplete neutrophil removal at the site of inflammation despite increased numbers of monocytes and macrophages [64].

In addition to promoting the production of SPMs, PGE₂ is implicated in maintaining a post-resolution phase of impaired innate immune activation, preventing reactivation of the immune system and the subsequent entrance into a cycle of chronic unresolved inflammation [61,65]. Resolution of zymosan-induced peritonitis in C57BL/6 mice is associated with enhanced expression of COX-1 and mPGES-1 in macrophages, leading to increased PGE₂ production relative to the earliest, acute stages of inflammation [65]. Additionally, monocyte and macrophage expression of Ptger2 (EP2) and Ptger4 (EP4) is increased post-inflammation, suggesting an autocrine mechanism for the PGE₂-mediated pro-reparative phenotype [65]. Perhaps most relevant, PGE₂ decreased the production of dsDNA autoantibodies in a dose-dependent manner in the mouse model of chronic zymosan-induced peritonitis; this suggested that PGE₂ in the post-resolution phase might prevent the development of autoimmune manifestations such as SLE, a possibility that certainly warrants further investigation [65]. Thus, we argue that a period of immunosuppression following inflammation resolution might be essential to preventing ongoing chronic inflammation or autoimmune activation.

Concluding remarks

A common thread underlying autoimmune diseases is the persistent and unresolved tissue damage stemming from failure to resolve an initial inflammatory event. Elucidating the normal cycle of inflammation, the key factors regulating this process, and the cellular targets for these factors can direct improved therapies for either preventing or ameliorating autoimmunity. As G protein-coupled receptors, PGE₂ receptors may represent druggable targets to modulate the cycle of inflammation and promote resolution to restore tissue homeostasis (see Clinician Corner). We recognize that the cellular source, timing, and specific receptors expressed are key elements that help determine the outcome/response to PGE₂ signaling. Much of the available data have been generated using in vitro assays and mouse models and the translation of this to human autoimmunity needs to be verified in many cases. Additionally, the development of highly-specific receptor agonists or antagonists that can home to desired target cells/tissues might overcome concerns about possible systemic effects on broadly-expressed PGE₂ receptors. Ultimately, a better understanding of this complex network in the setting of autoimmunity is needed to realize the therapeutic potential of the PGE₂ pathway (see Outstanding Questions).

Outstanding Questions.

• Would most or all autoimmune diseases benefit from a more finely-tuned approach to modulating different phases of the inflammation cycle and harness the beneficial effects of inflammatory signals such as PGE₂, while negating the deleterious effects?

• How does changing the cytokine milieu affect PGE₂ production and signaling throughout the inflammatory cycle?

• What are the relative contributions to homeostasis of tissue-derived, versus immune cell-derived PGE₂ in inflammation, tissue damage, and resolution?

• How does the array of PGE₂ receptors on immune cells and tissues change during the inflammatory process?

• Can existing animal models of autoimmune disease be better leveraged to accurately define the failure of resolution and how it leads to chronic tissue damage? Do we need new models that more accurately reflect the reality of the situation in certain patients?

• Can approaches be developed to target receptor-specific modulators to cells/tissues of interest to prevent the possible systemic effects of such receptors on other tissues where they are expressed?

Highlights.

• PGE₂ plays different roles through the inflammation and resolution cycle, acting in both proinflammatory and anti-inflammatory capacities.

• Dysregulated PGE₂ production and signaling has been implicated in the progression and chronic nature of certain autoimmune diseases.

• PGE₂ is produced by and can directly affect both immune cell populations and cells within the organ/tissue.

• Recent data indicate that PGE₂ plays a pivotal role is the resolution of inflammation.

• We hypothesize that enhancing PGE₂ signaling during the resolution phase of inflammation promotes tissue homeostasis, halts the inflammation cycle, and ameliorates certain autoimmune diseases.

Significance statement:

A better understanding of PGE₂ signaling is needed to realize the therapeutic potential of this pathway in modulating the inflammation cycle, promoting resolution, and restoring tissue homeostasis in settings of autoimmunity.

Glossary

Adaptive regulatory T cells (T_R^adapt)

regulatory CD4⁺ T cells induced in the periphery from CD25⁻ T cells following prolonged or repeated antigenic stimulation

COX-1

cyclooxygenase isoform; key enzyme in the synthesis of prostaglandins (PGs) from arachidonic acid; constitutively expressed in most tissues and considered responsible for homeostatic PG production

COX-2

second cyclooxygenase isoform; expression is inducible following proinflammatory stimuli

Costimulatory molecules

secondary signal from antigen presenting cells required for T cell activation, in addition to T cell receptor engagement

dsDNA autoantibodies

autoantibodies against double-stranded DNA; commonly detected in specific rheumatic/autoimmune diseases

Effector T cells

T cells executing immune functions; they proliferate in response to an activating stimulus and can be inhibited by T_regs

Efferocytosis

process by which macrophages endocytose apoptotic cells, inducing a phenotypic switch

Experimental autoimmune encephalomyelitis (EAE)

common murine model of multiple sclerosis or cell-mediated organ specific autoimmunity; immune cells are sensitized to myelin antigens via a strong adjuvant

Focal adhesions

macromolecular assemblies which transmit mechanical force and cellular signals between cells and the surrounding extracellular matrix

Inflammatory bowel disease (IBD)

condition of long-standing, often autoimmune, inflammation in the digestive tract, causing significant damage to the intestinal wall

Lipid mediator class switch

switch in macrophage lipid mediator production over the course of inflammation; from favoring leukotrienes and prostaglandins to favoring specialized pro-resolving mediators

Leukotrienes

another family of proinflammatory bioactive lipid derived from arachidonic acid and 5-lipoxygenase

Microsomal prostaglandin E synthase (mPGES)

membrane-bound protein found in the microsomal fraction of cellular lysates; catalyzes the conversion of PGG₂ into PGE₂

Multiple Sclerosis (MS)

progressive neurodegenerative disease characterized by autoimmune-mediated destruction of oligodendrocytes and myelin sheath in the central nervous system; results in autonomic and sensorimotor defects, visual disturbances, ataxia, fatigue, difficulty in thinking, and emotional dysregulation

Nonobese diabetic (NOD) mouse

common murine model for spontaneous autoimmune diabetes

PGE₂

lipid immunomodulator molecule derived from arachidonic acid via cyclooxygenase and prostaglandin E synthase enzyme activity

Proinflammatory (M1-like) Macrophages

“classically-activated” macrophages; produce factors to recruit and activate other immune cells

Pro-resolving or anti-inflammatory (M2-like) macrophages

produce resolving factors to activate processes such as apoptosis and efferocytosis; anti-inflammatory macrophages produce cytokines which directly inhibit proinflammatory activity

Rheumatoid Arthritis (RA)

autoimmune condition characterized by inflammatory changes in the synovial fluid, causing pain and swelling of the joints, destruction of cartilage and bone; associated with the production of autoantibodies

Regulatory T cells (T_regs)

CD4⁺ Foxp3⁺ T cells; essential for regulating peripheral tolerance, limiting immune activation via secretion of IL-10 and TGFβ

Specialized pro-resolving mediators (SPMs)

produced after initial acute inflammation, limit neutrophil infiltration, counter-regulate cytokine/chemokine production, induce neutrophil apoptosis and macrophage efferocytosis, reprogram macrophages, promote immune cell egress from sites of inflammation, instruct suppressive cells, and induce tissue repair

Systemic lupus erythematosus (SLE)

autoimmune disorder directed against connective tissue, resulting in diverse clinical manifestations (ie. synovitis and joint pain, atypical cardiac symptoms, and lupus nephritis)

Th1-like CD4⁺ T cells

helper cells producing IFNγ, TNFα, and IL-2 leading to cell-mediated responses; frequently implicated in autoimmune diseases

Th17-like T cells

CD4⁺ T helper cells producing IL-17, IL-21, and IL-22 to provide immunity to extracellular pathogens; subset most prominently associated with autoimmunity

Th2-like T cells

CD4⁺ T helper cells producing IL-4, IL-5, and IL-13, generally mediating humoral responses; primarily associated with allergic diseases

Type one diabetes (T1D)

autoimmune condition characterized by the progressive destruction of insulin-producing beta cells in the pancreas, preventing proper maintenance of glucose homeostasis

Zymosan-induced peritonitis

model of systemic inflammatory response syndrome; local inflammation of the peritoneum is induced by injection of the yeast-derived particle zymosan