Allogeneic grafts and the trouble with “non-self” recognition
Allogeneic hematopoietic cell transplantation is an effective therapy for many blood cancers and bone marrow disorders, where functional hematopoietic cells from a matched donor are engrafted into a recipient. The graft-versus-leukemia or tumor effect can have profoundly positive clinical outcomes. In contrast, a major limitation to the success of allogeneic transplantation is the development of Graft versus Host Disease (GvHD). This disease occurs when engrafted T cells recognize the recipient tissues as “non-self” and mount an immune response against them (Ferrara et al., 2009).
The development of GvHD is not uncommon, with an estimated 60% of allogeneic transplant recipients suffering from the disease (Harris et al., 2012). Incidence of GvHD can be minimized by ensuring that donor and recipient cells are closely “matched”, where compatible human leukocyte antigen (HLA) type is critical for graft success (Howard et al., 2015). Given that there simply are not enough familial donors to meet the need for transplants, it follows that the percentage of unrelated donors will rise, increasing the potential incidence of GvHD (Ferrara et al., 2009). Investigating new therapeutic strategies to prevent and treat GvHD will likely prove increasingly crucial.
While the primary effector of GvHD is known to rest in the donor helper T cell compartment, endothelial dysfunction is also implicated in its development (Mir et al., 2017). In particular, decreased endothelial thrombomodulin denotes endothelial impairment, with concomitant degradation of the cytoprotective effects of protease-activated protein C (aPC) mediated by thrombomodulin (Andrulis et al., 2012). Multiple lines of evidence implicate aPC and thrombomodulin in preventing GvHD. Indeed, treatment of mice with recombinant thrombomodulin reduces symptoms of GvHD (Ikezoe et al., 2015). Also, aPC expands the regulatory T cell compartment in a mouse model of type 1 diabetes (Xue et al., 2012) and the presence of regulatory T cells following transplantation can reduce alloreactivity of a graft (Ball et al., 2008). These findings highlight the potential benefits of manipulating the thrombomodulin-aPC axis in the treatment and prevention of GvHD.
A new paradigm in GvHD prevention
In a study recently published in Nature Communications, Ranjan and colleagues investigated the role of aPC in the development of GvHD, and identified that pre-transplant treatment with aPC could serve as an effective strategy to minimize GvHD. To determine if endogenous aPC was capable of protecting against GvHD, these investigators studied a transgenic mouse that expresses hyperactivated aPC. When engrafted with bone marrow and T cells from a non-matched donor, these mice exhibited reduced signs of GvHD (Ranjan et al., 2017). Moreover, the authors demonstrated that pre-treatment of pan-T cells with aPC limited T cell reactivity in vitro, expanded the population of regulatory T cells, and inhibited onset of GvHD in vivo (Ranjan et al., 2017).
Ranjan et al. extended their in vitro findings and in vivo studies using immunocompetent mice to a humanized model of GvHD. NOD.Cg-Prkdcscid Il2rgtm1Wjl H2-Ab1tm1Gru Tg(HLA-DRB1)31Dmz/SzJ (Stock# 017637) are highly immunodeficient on account of the NOD inbred background, the Pkdcscid, and Il2rgtm1Wjl mutations. This mouse also lacks mouse MHC class II and expresses a transgene for human HLA-DR4, making it a useful model for GvHD when engrafted with human cells that are DR4-negative. Ranjan and colleagues engrafted these mice with human T cells (CD4+, DR4-) with or without pre-incubation of these cells with aPC. Survival, clinical presentation, and pathological analysis indicated that pre-incubation of CD4+, DR4- T cells with aPC ameliorated GvHD in this humanized model. Of note, aPC pre-treatment protected against GvHD while preserving the clinically desirable graft versus leukemia effect, as evidenced by increased survival in mice engrafted with a leukemia cell line along with aPC pre-treated allogeneic T cells (Ranjan et al., 2017).
Summary
The collection of experiments conducted by Ranjan and colleagues highlights an exciting therapeutic strategy for the prevention of GvHD while maintaining the graft versus leukemia capabilities of allogeneic hematopoietic transplants. Targeting the thrombomodulin-aPC axis may lessen the risk of complications associated with allogeneic hematopoietic cell transplantation therapies.
At JAX, the preclinical GvHD services utilize human peripheral blood mononuclear cells (PBMCs) engrafted with high efficiency into NSG™ mice, resulting in a robust and reproducible study platform. Each study is supported by PhD-level study directors from the initial design to the final report, and includes midpoint and terminal blood samples, survival curves, comparison to clinically approved agents (belatacept), and optional FACS blood analysis, tissue collection, additional treatment arms, and alternative benchmarking agent (etanercept). JAX has extensive experience conducting large-scale GvHD studies for commercial clients.
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