Tolerance without toxicity? α1-antitrypsin as a novel alternative to immunosuppression

Keywords::

• AAT
• allotransplantation
• graft-versus-host disease
• tolerance

Historical perspective

Although the protease inhibitor activity of human plasma was discovered by Fermi and Pernossi in 1894 [1,2], it was not until 1955 that Schultze et al. first isolated the serum trypsin inhibitor responsible for this activity and named it α1-antitrypsin (AAT) [1,3]. Laurell and Eriksson subsequently described the absence of bands correlating to AAT deficiency in two patients [4]. Over the next 50 years, an overwhelming preponderance of published literature has focused on what goes wrong when AAT levels are low as a function of faulty production due to a variety of genetic mutations – that is, faulty protease inhibitor activity – and the beneficial impact of replacement therapy. However, in recent years, a new paradigm has emerged implicating AAT as a potential key player in immunoregulation across multiple settings, including transplant tolerance, graft-versus-host disease (GvHD) and autoimmunity. Several expertly written and outstanding reviews have recently been published [1,5], and this editorial will likely serve to raise mechanistic questions and implications of these recent observations.

AAT & allotransplantation

AAT is a 52-kDa glycoprotein, which is encoded by the gene SERPINA1. It is the most abundant endogenous serine protease inhibitor in circulation and plays a critical role in the pathophysiology of chronic obstructive pulmonary disease and certain liver diseases [6]. The classic hypothesis proposes that abnormally low levels of AAT result in unopposed neutrophil elastase activity, destruction of the elastin matrix and lung inflammation [5]. Emerging experimental data show that AAT serves as an endogenous inhibitor of proinflammatory cytokine production and can also modulate in vivo inflammatory/immune-mediated diseases, including induction of tolerance to allografts and mitigation of GvHD. Evidence suggests that AAT’s immunomodulatory function goes beyond its ability to inhibit neutrophil elastase [7–9]. At our institution, which has a particular interest in the immunobiology of GvHD, we have demonstrated – in several clinically relevant in vivo experimental allogeneic bone marrow transplantation models – that brief administration of human AAT reduced proinflammatory cytokine production via direct modulation of dendritic cell responses, enhanced IL-10 (anti-inflammatory cytokine) secretion, expanded donor Tregs and mitigated experimental GvHD [8]. Of particular relevance is the finding that donor–effector T-cell function was not affected by AAT, thereby suggesting that the graft-versus-tumor effect may be preserved [8]. Marcondes et al. similarly demonstrated that AAT abrogated mortality from GvHD, in part through reduction of IL-32 [9]. These complement earlier observations by Lewis et al. [10,11], and Koulmanda et al. [12] showed that treatment with AAT can shift the cytokine environment from proinflammatory to anti-inflammatory and enhance tolerance induction in the context of allograft transplantation. Lewis et al. demonstrated that monotherapy with AAT improved the acceptance rates of allogeneic islet β-cell grafts and normalized glucose levels in mice in a dose-dependent fashion [10]. Of particular relevance was the demonstration that the benefit of AAT monotherapy for 2 weeks persisted for several weeks and retransplantation of islet cell allografts resulted in the acceptance of same-strain grafts and the rejection of third-strain grafts, thus confirming the induction of specific immune tolerance by AAT in this model [11]. Koulmanda et al. demonstrated allograft tolerance with an increase in Treg and a reduction in the proinflammatory component [12]. Analogous to the protective effect mediated by AAT on β-islet cells from cytokine-mediated attack in diabetes, it remains to be determined whether the GvHD target organs are directly protected by AAT. Nonetheless, these observations raise important questions about the mechanisms that might be critical for the immunomodulatory effects of AAT.

Does the ‘deficiency paradigm’ help us in understanding the immunosuppressive effect of AAT?

When viewed as a simple loss of protease function, AAT deficiency does little to elucidate the potentially immunosuppressive effects of AAT. However, when one considers the exaggerated inflammatory phenotype seen in patients with AAT as a form of ‘gain-in-function’ secondary to endoplasmic reticulum (ER) stress signaling due to overaccumulation of AAT polymers, the broad implications for immunomodulation by AAT become very intriguing. For example, the traditional view of liver injury/disease in the setting of AAT deficiency is that accumulation of AAT polymers in the ER of hepatocytes causes cellular damage, although the mechanisms remain unclear [5]. However, the concept of ER stress as a result of ‘polymer overload’ in AAT deficiency may provide an important common thread that links ER function and dysfunction to the activation of protein kinases implicated in immunity and inflammation, including the signaling pathway for NF-κB [13,14]. This link between ER stress as a result of the accumulation of ordered polymerized protein within the ER to the activation of NF-κB might serve as the basis for an anti-flammatory effect of supraphysiologic levels of ‘normal’ AAT and is supported by the inhibition of Toll-like-receptor-induced inflammation and NF-κB translocation in dendritic cells by AAT in a murine model [15]. In addition, the inhibitory effect of AAT on monocyte/macrophage function as manifested by a decrease in TNF-α secretion and an increase in IL-10 secretion has been associated with elevation in cAMP and activation of cAMP-dependent PKA [16]. An intriguing line of evidence demonstrated that AAT depletes lipid raft cholesterol from the cell membranes and thus might mitigate immune cell responses [17]. Recent observations also suggest that post-translational modifications of AAT (oxidation/glycosylation) have a substantial impact on its functions [1]. Therefore, it is possible that despite the measured levels of total AAT, the amount of appropriately modified AAT might be relatively deficient in the context of inflammation. Alternatively, although the measured systemic level of AAT is mainly derived from the liver [6], AAT is also produced by monocytes and macrophages, which can contribute to tissue levels of AAT. Thus, in the context of inflammatory diseases, the relevant tissues might still be relatively deficient in AAT despite the high serum levels [18,19].

AAT as a novel agent for improving outcomes after transplantation

Unfortunately, there is little controversy that there has not been a significant advance in the management of the life-threatening complication of GvHD despite the introduction of a multitude of immunosuppressive agents that were either not effective in controlling GvHD or were associated with unacceptable increases in infection and relapse [20]; novel effective approaches are clearly needed.

How do we reconcile the notion of infusing supraphysiologic AAT levels to decrease inflammation and GvHD in allogeneic bone marrow transplantation patients with no known abnormalities in the levels or function of AAT? In addition to the reasons outlined earlier for the potential of relative deficiency in these patients, one must be cautious in drawing conclusions in this regard, as no well-controlled systematic data are available regarding the relative levels of AAT in patients undergoing immunomodulation such as allogeneic stem cell or even pancreatic islet cell transplants. A key attribute that may distinguish AAT from other immunomodulatory interventions is the lack of increased risk of infection despite its many years of use. When one considers the remarkable safety record over tens of thousands of AAT infusions in the last several decades, particularly in regard to infectious complications, we suggest that the time is right for the translation of the in vivo experimental observations summarized earlier to be taken to the bedside. Proof-of-principle clinical trials in GvHD and potentially other inflammatory diseases will need to be performed to test the promise of this novel approach.

Financial & competing interests disclosure

P Reddy is supported by NIH grants AI075284 and CA143379. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.