Neutrophils (also named polymorphonuclear leukocytes or PMN) are essential components of the immune system, rapidly recruited to sites of inflammation, providing the first line of defense against invading pathogens. Since neutrophils can also cause tissue damage, their fine-tuned regulation at the inflammatory site is required for proper resolution of inflammation. Annexin A1 (AnxA1), also known as lipocortin-1, is an endogenous glucocorticoid-regulated protein, which is able to counterregulate the inflammatory events restoring homeostasis. AnxA1 and its mimetic peptides inhibit neutrophil tissue accumulation by reducing leukocyte infiltration and activating neutrophil apoptosis. AnxA1 also promotes monocyte recruitment and clearance of apoptotic leukocytes by macrophages. More recently, some evidence has suggested the ability of AnxA1 to induce macrophage reprogramming toward a resolving phenotype, resulting in reduced production of proinflammatory cytokines and increased release of immunosuppressive and proresolving molecules. The combination of these mechanisms results in an effective resolution of inflammation, pointing to AnxA1 as a promising tool for the development of new therapeutic strategies to treat inflammatory diseases.

Annexin A1 is a mediator of natural and glucocorticoid-induced resolution of inflammation, with profound effects on neutrophil apoptosis in vivo.This study aimed at assessing whether AnxA1, a downstream mediator for the anti-inflammatory effects of GCs, could affect the fate of immune cells in tissue exudates, using LPS-induced pleurisy in BALB/c mice. AnxA1 protein expression in exudates was increased during natural resolution, as seen at 48–72 h post-LPS, an effect augmented by treatment with GC and associated with marked presence of apoptotic neutrophils in the pleural exudates. The functional relevance of AnxA1 was determined using a neutralizing antibody or a nonspecific antagonist at FPR/ALXRs: either treatment inhibited both spontaneous and GC-induced resolution of inflammation. Injection of Ac2-26 (100 μg, given 4 h into the LPS response), an AnxA1-active N-terminal peptide, promoted active resolution and augmented the extent of neutrophil apoptosis. Such an effect was prevented by the pan-caspase inhibitor zVAD-fmk. Mechanistically, resolution of neutrophilic inflammation was linked to cell apoptosis with activation of Bax and caspase-3 and inhibition of survival pathways Mcl-1, ERK1/2, and NF-κB. These novel in vivo data, using a dynamic model of acute inflammation, provide evidence that AnxA1 is a mediator of natural and GC-induced resolution of inflammation with profound effects on neutrophil apoptosis.

Key Points: Plg and Pla induce macrophage reprogramming and promote resolution of acute inflammation. Plg and Pla enhance the efferocytic capacity of macrophages and override the prosurvival effect of LPS on neutrophils via annexin A1.

Glucocorticoid (GC)-induced leucine zipper (GILZ) has been shown to mediate or mimic several actions of GC. This study assessed the role of GILZ in self-resolving and GC-induced resolution of neutrophilic inflammation induced by LPS in mice. GILZ expression was increased during the resolution phase of LPS-induced pleurisy, especially in macrophages with resolving phenotypes. Pretreating LPS-injected mice with trans-activator of transcription peptide (TAT)–GILZ, a cell-permeable GILZ fusion protein, shortened resolution intervals and improved resolution indices. Therapeutic administration of TAT-GILZ induced inflammation resolution, decreased cytokine levels, and promoted caspase-dependent neutrophil apoptosis. TAT-GILZ also modulated the activation of the survival-controlling proteins ERK1/2, NF-κB and Mcl-1. GILZ deficiency was associated with an early increase of annexin A1 (AnxA1) and did not modify the course of neutrophil influx induced by LPS. Dexamethasone treatment resolved inflammation and induced GILZ expression that was dependent on AnxA1. Dexamethasone-induced resolution was not altered in GILZ^−/− mice due to compensatory expression and action of AnxA1. Our results show that therapeutic administration of GILZ efficiently induces a proapoptotic program that promotes resolution of neutrophilic inflammation induced by LPS. Alternatively, a lack of endogenous GILZ during the resolution of inflammation is compensated by AnxA1 overexpression.

Gout is a self‐limited inflammatory disease caused by deposition of monosodium urate (MSU) crystals in the joints. Resolution of inflammation is an active process leading to restoration of tissue homeostasis. Here, we studied the role of Annexin A1 (AnxA1), a glucocorticoid‐regulated protein that has anti‐inflammatory and proresolving actions, in resolution of acute gouty inflammation. Injection of MSU crystals in the knee joint of mice induced inflammation that was associated with expression of AnxA1 during the resolving phase of inflammation. Neutralization of AnxA1 with antiserum or blockade of its receptor with BOC‐1 (nonselective) or WRW₄ (selective) prevented the spontaneous resolution of gout. There was greater neutrophil infiltration after challenge with MSU crystals in AnxA1 knockout mice (AnxA1^−/−) and delayed resolution associated to decreased neutrophil apoptosis and efferocytosis. Pretreatment of mice with AnxA1‐active N‐terminal peptide (Ac_2–26) decreased neutrophil influx, IL‐1β, and CXCL1 production in periarticular joint. Posttreatment with Ac_2–26 decreased neutrophil accumulation, IL‐1β, and hypernociception, and improved the articular histopathological score. Importantly, the therapeutic effects of Ac_2–26 were associated with increased neutrophils apoptosis and shortened resolution intervals. In conclusion, AnxA1 plays a crucial role in the context of acute gouty inflammation by promoting timely resolution of inflammation.