Prostaglandin I 2 and T Regulatory Cell Function: Broader Impacts

The adaptive immune system, which functions through generation of a specific response to pathogens, is composed of subsets of specialized T cells (Chaplin, 2010). However, the body's generation and maintenance of patrolling T cells that respond to exogenous antigens is not perfect, and as such autoimmune diseases and inflammation can develop. T regulatory cells (Tregs), one subset of T cells, are an essential component of the adaptive immune system as they function to resolve and suppress inflammation, including inflammation that stems from the resolution of infection and responses to other environmental factors such as diet (Kim et al., 2007; Sakaguchi et al., 2008). Tregs also promote tolerance to both self-antigens and environmental antigens, and thus act to prevent the development of autoimmune diseases (Kim et al., 2007; Sakaguchi et al., 2008). Tregs are identified by their expression of Forkhead Box p3 (Foxp3), a transcription factor critical to their suppressive capabilities. Systemic autoimmunity and inflammation, as well as conditions such as allergic disease, have been linked to Treg deficiency, which is associated with mutations in Foxp3 (Bennett et al., 2001; Brunkow et al., 2001; Wildin et al., 2001; Hori et al., 2003; Wan and Flavell, 2007; Marques et al., 2015). Tregs can be broadly characterized as thymically derived Tregs (tTregs), which are generated in the thymus and are functional upon their exit, or as peripheral or inducible Tregs (iTregs), which are generated in the presence of, and are specific to, a certain antigen (Shevach and Thornton, 2014).

Prostaglandins are lipids that exert hormone-like effects. As a group, prostaglandins may act to mediate inflammation (Ricciotti and Fitzgerald, 2011). The production of prostaglandins is initiated when arachidonic acid is released from the phospholipid membrane by phospholipase A₂ (Zeldin, 2001; Dorris and Peebles, 2012; Lau and Lui, 2021). Cyclooxygenase (COX) enzymes then generate prostaglandin H₂ (PGH₂) from arachidonic acid. Subsequently, PGH₂ is metabolized within tissues by enzymes and isomerases into the five primary prostaglandins (Zeldin, 2001; Dorris and Peebles, 2012; Lau and Lui, 2021). Prostaglandin I₂ (PGI₂) is one of these primary prostaglandins. PGI₂, in particular, is synthesized through the generation of PGH₂ by COX-2 and PGH₂'s subsequent metabolism by PGI synthase (PGIS) (Zeldin, 2001; Dorris and Peebles, 2012; Lau and Lui, 2021). PGI₂ is primarily produced by endothelial cells (Zeldin, 2001; Dorris and Peebles, 2012; Lau and Lui, 2021). When bound to its G protein-coupled receptor, termed either PTGIR or IP, PGI₂ triggers the activation of adenylyl cyclase, resulting in the production of the important second messenger cyclic AMP (cAMP) (Zeldin, 2001; Dorris and Peebles, 2012; Lau and Lui, 2021). PGI₂ has several demonstrated vasoprotective effects. PGI₂ exerts potent vasodilatory effects on the vascular endothelium (Dorris and Peebles, 2012; Lau and Lui, 2021). In addition, PGI₂ is able to prevent platelet aggregation and smooth muscle proliferation (Moncada et al., 1976; Dorris and Peebles, 2012). PGI₂ signaling also exerts powerful anti-inflammatory effects on the immune system (Takahashi et al., 2002; Nagao et al., 2003; Jaffar et al., 2007; Zhou et al., 2007a, 2007b, 2014, 2018; Toki et al., 2013). Several analogs for PGI₂ exist, these include treprostinil and iloprost, which are both Food and Drug Administration (FDA) approved and used to treat pulmonary hypertension, as well as cicaprost.

Recently, several studies across different models and diseases have investigated the interplay between PGI₂ and Tregs. An initial study that was the first to explore this area showed that iloprost increased the production of interleukin (IL)-10 from antigen-specific T cells using a model of allergic inflammation (Idzko et al., 2007). In these experiments, antigen-pulsed dendritic cells (DCs) were treated with iloprost before administration to mice that were DC-depleted. DCs are important antigen presenting cells that function to educate, activate, and polarize T cells to a specific subset depending on antigen and condition (Eisenbarth, 2019). The mice were then allergically challenged. The investigators found that the production of Th2 cytokines was reduced, whereas the production of IL-10 was increased from antigen-specific T cells (Idzko et al., 2007). These data imply that PGI₂ signaling in DCs may promote the formation of an IL-10 producing T cell subset; however, it was not investigated at this time if these cells were indeed Tregs. A subsequent study effectively investigated this question. The authors first found that iloprost promoted the formation of antigen-specific Tregs in a model of allergic inflammation (Wong et al., 2020). Furthermore, the authors reported that iloprost-treated antigen-pulsed DCs were able to promote antigen-specific Treg differentiation from naive T cells both in vitro and in vivo (Wong et al., 2020). These data demonstrate that PGI₂ signaling in DCs resulted in enhanced formation of antigen-specific Tregs.

CREB-binding protein (CBP) and p300 are histone acetyltransferases that also function as transcriptional coactivators. Inhibition of CBP/p300 in human Tregs that were differentiated ex vivo resulted in reduced expression of Foxp3 and inhibited other factors that mediate Treg suppression (Ghosh et al., 2016). Interestingly, a separate study reported that an important acetylation target of CBP/p300 within Tregs was PGIS, resulting in enhanced production of PGI₂ by Tregs, creating a positive feedback loop through which endogenous PGI₂ then promotes Treg differentiation (Castillo et al., 2019). This study overarchingly determined that PGI₂ is important for Treg differentiation through examination of the mechanisms by which CBP/p300 promotes Treg function and integrity.

A study recently completed by our group directly investigated the impact of PGI₂ on Treg function, stability, and differentiation using models of allergic disease. Utilizing mice deficient in IP (IP KO), we found that PGI₂ signaling promoted the function of both tTreg and iTreg in the setting of allergic airway inflammation (Norlander et al., 2021). Moreover, PGI₂ signaling enhanced Foxp3 expression in tTreg and the polarization of naive T cells to iTreg (Norlander et al., 2021). Furthermore, PGI₂ augmented Treg stability and prevented the pathogenic reprogramming of Tregs in allergic inflammation (Norlander et al., 2021). Finally, these studies were extended to human cells. PGI₂ promoted the differentiation of naive human T cells to Treg and that this was associated with repression of β-catenin signaling (Norlander et al., 2021). Our study provides further evidence demonstrating that PGI₂ signaling enhances Treg differentiation. In addition, our study demonstrates that PGI₂ signaling is also important for Treg stability.

Not all studies have uniformly determined that PGI₂ promotes Treg differentiation. One study showed that culturing naive T cells with iloprost had the opposite effect. In fact, their data demonstrated that, in vitro, PGI₂ signaling promotes Th17 cell differentiation, a proinflammatory T cell subset that produces IL-17A, thereby suppressing Treg differentiation (Liu et al., 2013; Golubovskaya and Wu, 2016). They also determined that this effect was mediated through cAMP and STAT3 (Liu et al., 2013). Thus, PGI₂ may not promote Treg differentiation under all conditions or in all disease states.

Another important question is whether Tregs impact the production of PGI₂ from other cell types. This question was examined in a study utilizing models of pulmonary hypertension. Data show that compared with male rats that lacked Tregs, female rats that lacked Tregs developed more severe pulmonary hypertension when treated with either hypoxia or a vascular endothelial growth factor receptor 2 (VEGFR2) inhibitor to induce disease (Tamosiuniene et al., 2018). The enhanced severity of pulmonary hypertension in Treg-deficient female rats was demonstrated to be due to decreased levels of PGI₂ (Tamosiuniene et al., 2018). Reconstituting rats with Tregs increased plasma levels of PGI₂ and prevented the development of pulmonary hypertension (Tamosiuniene et al., 2018). Furthermore, Tregs promoted the production of PGI₂ from cardiac endothelial cells when the two cell types were cultured in vitro (Tamosiuniene et al., 2018). Thus, these data suggest that PGI₂ is not only important for the formation and development of Tregs, but also reciprocally that PGI₂ bioavailability is promoted by Tregs in certain disease models.

In conclusion, overarchingly it appears that PGI₂ signaling promotes the formation and function of Tregs, be that directly or indirectly. Furthermore, PGI₂ signaling may promote the production of PGI₂ from Tregs and other cell types (Fig. 1). Thus, PGI₂ may be the first known pharmacological agent that promotes Treg differentiation and function in vitro and in vivo. As there are several analogs of PGI₂ that are FDA approved and in use to treat patients with pulmonary hypertension, it is plausible that these analogs could quickly be repurposed to treat inflammatory disorders, such as allergy and asthma, or autoimmune diseases by which they would promote Treg function to mitigate disease symptoms.

OPEN IN VIEWER

FIG. 1.

Schematic of the interplay of PGI₂ and Tregs. PGI₂ signaling in DCs promotes the differentiation of naive T cells to Treg. PGI₂ signaling within the Treg enhances Treg stability, promotes the differentiation of naive T cells to Treg, and these functional Tregs suppress allergic inflammation and pulmonary hypertension. Tregs promote the production of PGI₂ from endothelial cells. Created with BioRender.com. DC, dendritic cell; PGI₂, prostaglandin I₂; Tregs, T regulatory cells.