The Therapeutic Effect of Immune-Modifying Microparticles in Acute Graft-Versus-Host Disease

Introduction

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an important therapy for hematologic malignancies. The success of allo-HSCT is often limited by acute graft versus host disease (AGVHD), which is a severe and often fatal complication. AGVHD is characterized by production of inflammatory mediators and recruitment and activation of immune cells accounting for damage to target organs including the liver, intestine, and skin. There are a limited number of therapies to treat aGVHD. Steroids remains the mainstay of treatment. The addition of other immunosuppressive agents have shown limited benefit and often increase the potential for life-threatening infections. Inflammatory monocytes are among the immune cells that are recruited to target organs during AGVHD. Inflammatory monocyte involvement in AGVHD has been described both in terms of their innate immune function and in their role in the activation and proliferation of T-cells by antigen presentation and cytokine secretion. Inflammatory monocyte-derived effector cells play an important role in the pathogenesis of numerous inflammatory diseases. Furthermore, clinical studies have reported that monocyte infiltration was correlated directly with AGVHD severity.

Despite the importance of inflammatory monocytes in AGVHD, there are no treatments that specifically target these cells. Immune-modifying microparticles (IMPs), derived from biodegradable poly(lactic-co-glycolic) acid (PLGA) have been shown to be taken up by inflammatory monocytes. Subsequently, these monocytes no longer traffic to sites of inflammation; rather, IMP infusion causes their sequestration in the spleen through apoptotic cell clearance.

Previous studies have shown that the administration of IMPs in murine models of myocardial infarction, colitis, peritonitis, and encephalitis markedly reduced monocyte accumulation at inflammatory foci, reduced disease symptoms, and promoted tissue repair. Therefore, we hypothesized that the administration of IMPs may reduce clinical symptoms and mortality in a murine model of AGVHD.

Methods

Murine AGVHD model: BALB/c mice were given 800 cGy total body irradiation, irradiated BALB/c mice were transplanted with 5×106 C57BL/6 bone marrow cells and 1×106 C57BL/6 spleen cells via tail vein. IMP made with PLGA (Phosphorex Inc, Hopkinton MA) was administered to the recipient mice (1.4 mg/kg body weight) by IV daily starting from day 5 to day 10 after bone marrow transplantation (BMT). PBS at the same volume was used as vehicle control.

AGVHD monitoring: Mice were individually scored three times a week for five clinical parameters (posture, activity, fur, skin and weight loss) on a scale from 0 to 2. Clinical GVHD score was assessed by summation of these parameters. Survival was monitored daily. AGVHD experiments were performed with at least five mice per group.

AGVHD histology: Tissue samples were cryo-embedded and blinded for transplant conditions. Histopathological score was assessed on H&E-stained sections.

Results

IMPs treated mice had significantly less AGVHD (average score of 2.48) than the untreated BM+Sp group (average score 3.96) starting at the time of IMP treatment (days 5-10) and remained with significantly reduced symptoms of AGVHD for the 30 day course (Figure 1). IMPs treatment also rescued BM+Sp mice from lethal AGVHD with a 30 day overall survival of 62% compared to 4% in the untreated BM+Sp group (Figure 2). Intestinal tissue from the IMPs treated mice compared to the BM+Sp mice demonstrated less evidence of AGVHD (an average score of 1.25 and 2.75, respectively). Hepatic tissue from the IMPs treated mice compared to the BM+Sp mice demonstrated less evidence of AGVHD (an average score of 1.5 and 2.42, respectively) (Figure 3). IMPs treatment significantly reduced INF-𝞬 levels in the intestinal tissues of treated mice compared to untreated BM+Sp mice.

Conclusions

Our results demonstrate that IMPs significantly reduce symptoms and mortality in a murine model of AGVHD. The reduction in circulating inflammatory monocytes in this model also reduced hepatic lymphocyte infiltration and intestinal mucosal denudation. These findings highlight the potential of IMP therapy as a specific, safe, and cost-effective tool for inhibiting inflammatory monocyte-mediated pathology in AGVHD.

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