Evidence behind the pathophysiology of TRALI

Authors


  • 4D-S16-01

Yoke Lin Fung, Critical Care Research Group, The University of Queensland and The Prince Charles Hospital, Brisbane, Australia E-mail: ylfung@uq.edu.au

Abstract

Transfusion-related acute lung injury (TRALI) is a serious transfusion complication that may lead to significant morbidity and mortality. This has driven a significant research effort focused on understanding why and how TRALI develops. The ultimate goal must be prevention or at least mitigation of the clinical consequences of TRALI.

The underlying pathophysiology of TRALI is presently best described by two hypotheses which are not mutually exclusive. These are the antibody mediated TRALI mechanism and the two-event or priming TRALI mechanism. One of the key initial findings in TRALI research was the frequent presence of leucocyte antibodies in associated blood products, providing strong evidence for an antibody driven pathogenesis. In contrast, the two-event mechanism proposed that these transfused antibodies activated neutrophils that had first been primed by the patient’s clinical condition.

Together, data from haemovigilance programs, clinical reports and experimental findings have led several countries to introduce TRALI risk-reduction strategies. These include either limiting the transfusion of plasma from female donors or, screening female donors for the presence of leucocyte antibodies. Both approaches are justified by adoption of the immune mechanism as the prime driver of the pathogenesis of TRALI. TRALI incidence has gratifyingly been reduced by these measures. Nevertheless, TRALI cases persist and they remain a major concern because of continuing significant morbidity and mortality.

While the majority of earlier TRALI research has focused on the role of antibodies in TRALI, evidence for the role of non-antibody factors in TRALI is now growing, based on an increasing number of in vitro, ex vivo and in vivo models. This review aims to present data from such models, which are the foundation for our current understanding of the pathophysiology behind antibody mediated and non-antibody mediated TRALI.

Introduction

Transfusion-related acute lung injury (TRALI) is defined clinically as the development of hypoxaemia and non-cardiogenic pulmonary oedema either during or within 6 h of blood transfusion [1]. Despite supportive treatment, supplemental oxygen and mechanical ventilation, it has a mortality rate of 5–10% [2]. Haemovigilance programs in the USA and the UK report TRALI to be the most frequent cause of transfusion-related morbidity [3,4].

Blood transfusion is not prescribed without clinical justification. Accumulating experimental evidence about the role of cellular priming resulting from the patient’s underlying clinical condition is receiving greater acceptance as being central to TRALI pathophysiology. Priming events are thought to activate endothelial cells (ECs), causing polymorphonuclear neutrophils (PMNs) to accumulate in the pulmonary microvasculature [5–8]. Subsequent activation of primed PMNs results in an augmented respiratory burst response whereby nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is assembled and reactive oxygen species (ROS) are generated. In the absence of a microbial target, this can instead result in collateral injury to the surrounding tissues. In vitro studies show that high concentrations of priming agents or exposure to sequential priming agents can activate PMNs [9]. The ability of antibodies or non-antibody factors (NAFs) to prime the PMN respiratory burst in vitro has therefore been utilised to model their possible roles in the pathophysiology of TRALI. This paper examines current evidence behind the differing pathophysiologies of antibody mediated and non-antibody mediated TRALI.

Antibody mediated TRALI

Donor derived leucocyte antibodies (directed against human neutrophil antigens (HNA) or human leucocyte antigens (HLA) Classes I and II have been reported in approximately 80% of TRALI cases [10–12].

Antibody interactions with PMNs

It has been proposed that transfused leucocyte antibodies induce sequestration and aggregation of the recipient’s PMNs in the pulmonary microvasculature [13]. In the absence of a microbial target, the subsequent unwarranted activation of these PMNs and the concomitant activation of their respiratory burst results in damage to the pulmonary microvasculature and TRALI [13] (Fig. 1A).

Figure 1.

 Alternate pathophysiologies of TRALI. The pathophysiology of TRALI can be categorised as (i) antibody mediated (a - c) or (ii) non-antibody mediated (d). In antibody-mediated TRALI, the transfused antibody interacts with cells that express the cognate antigen (PMNs, ECs,platelets, T-lymphocytes and monocytes [a - c]). Non-antibody factors (NAFs) (e.g. lyso-PCs, neutral lipids,sCD40L and cytokines) that accumulate in blood products during storage have been implicated in non-antibody mediated TRALI (d). Common to both categories of TRALI is that the transfused stimulus is thought to result in PMN activation, assembly of PMN NADPH oxidase and subsequent release of PMN-derived ROS as well as enzymes (e). These PMN-derived ROS and enzymes damage the ECs of the pulmonary microvasculature, increasing vascular permeability and result in fluid leakage, the development of pulmonary oedema and ultimately TRALI (f).

Antibodies to HNA-3a and HNA-2a have been implicated in multiple TRALI events and insights into their mechanism of action have been revealed through a series of in vitro and ex vivo experiments. One of the earliest ex vivo studies demonstrated that the combination of HNA-3a positive PMNs with complement and anti-HNA-3a, causes TRALI [14]. In vitro studies have subsequently demonstrated that both the leucoagglutinating HNA-3a antibody [15] and the non-agglutinating monoclonal antibody to HNA-2a (CD177) [16] are able to prime for N-formylmethionyl-leucyl-phenylalanine (fMLP)-activated respiratory burst and cause endothelial damage in the presence of PMNs [16,17]. In 2006, ex vivo studies showed that the ability of CD177 alone, to induce TRALI, required PMNs having high HNA-2a density [16]. However, TRALI also occurred if PMNs with low HNA-2a density were first primed with fMLP, demonstrating the potentiating effect of priming [16]. Interestingly, the principal difference between these two ex vivo animal models was that TRALI induced by anti-HNA-3a was complement-dependent [14], whereas TRALI induced by anti-HNA-2a was not [16]. The fact that anti-HNA-3a is a leucoagglutinin and targets a transmembrane glycoprotein associated antigen [15,18–20], in contrast with antibody to CD177 which is not a leucoagglutinin [16] and targets a glycosyl-phosphatidylinositol (GPI) linked antigen [15], suggests that characteristics of both the antibody and target antigen can determine the clinical outcome.

In a recent in vivo model, rats primed with LPS and infused with monoclonal antibodies (MoAb) (OX18 or OX27) directed against major histocompatibility complex Class I (MHC-I) developed acute lung injury (ALI) [8]. This study provided important insight into the central role of PMNs in TRALI as it (i) demonstrated that MHC-I antibodies primed the PMN respiratory burst, (ii) provided histological evidence that these MHC-I antibodies were localised on PMN surfaces and were absent from endothelium or airway epithelial cells of the alveoli and (iii) confirmed that PMNs expressing cognate antigen were essential.

Monoclonal antibody 34-1-2s interactions with endothelial cells, platelets and lymphocytes

The anti-mouse MHC-I MoAb (34-1-2s) has been used in multiple in vivo murine TRALI models [7,21–26]. This MoAb appears to bind to PMNs but does not cause activation of the PMN respiratory burst [22]. A series of sophisticated experiments confirmed the central role of PMNs in this MoAb TRALI murine model and further proposed that PMN Fc gamma receptors (FcR) were essential for ALI to occur [21,22]. FcR–/– mice only developed ALI after adoptive transfer of FcR+ PMNs [22]. Looney et al. hypothesised that 34-1-2s first bound to pulmonary ECs, then PMNs became tethered to these via their FcRs, with resultant PMN activation leading to ALI [22]. Subsequent studies have shown that the 34-1-2s mediated murine TRALI was also dependent on the presence of platelets, as platelet-depletion or aspirin treatment prevented ALI [7]. Most recently, another group reported that T-lymphocytes played a significant role in modulating 34-1-2s mediated TRALI reactions [26].

Whilst the information gathered from this series of murine TRALI models has provided informative and interesting insight into the possible involvement of MHC-I antibodies in ALI (Fig. 1B), some limitations of these models need to be appreciated. Firstly, the leucocyte antibodies implicated in human TRALI are polyclonal and have HNA and/or HLA specificity, whilst 34-1-2s is an IgG2a anti-MHC-I MoAb. It is also interesting that during the search for an antibody to be used in this model, 34-1-2s was the only MoAb hybridoma of over 20 tested, which was able to induce TRALI [21,24], suggesting that specific serological characteristics of this MoAb were essential to precipitating the ALI insult.

HLA Class II antibody interaction with monocytes

Antibodies to HLA Class II have been associated with many TRALI cases [27,28]. However, their role in the pathogenesis of TRALI has been uncertain as PMNs do not constitutively express HLA Class II antigens [29]. Studies show that HLA Class II antibodies interact with antigen-matched monocytes in vitro to (i) release mediators including leukotriene B4 (LTB4), cytokines (e.g. growth related oncogene alpha (GROα), interleukin-8 (IL-8) and tumour necrosis factor alpha (TNF-α) [29–32], (ii) increase adhesion molecule expression (including vascular cell adhesion molecule-1 and leucocyte-function-associated molecule-1) [30] and (iii) cause damage to endothelial cells via monocyte-derived mediators [29,31,32]. This in vitro data has been corroborated by the results of a recent ex vivo rat lung model wherein HLA Class II antibodies provoked monocyte activation generating monocyte-derived mediators which activated PMNs, resulting in increased endothelial permeability [29] (Fig. 1C). Together, these observations build a plausible case for an HLA Class II antibody TRALI mechanism that would benefit from in vivo modelling.

Non-antibody mediated TRALI

It is important to note that in about 20% of TRALI cases, no leucocyte antibodies are detected [10–12], suggesting a non-antibody mediated mechanism for TRALI. Induction of TRALI by the transfusion of stored packed red blood cells (PRBC) or stored platelets into LPS primed rats [8,33–36] or LPS primed sheep [6,37] incriminates NAFs in stored blood products in TRALI pathogenesis. That PMN depletion prevented TRALI in the rat model, indicated that PMNs were central in a proposed non-antibody mediated mechanism of TRALI [8].

Non-antibody factors (NAFs)

Potential NAFs that accumulate during the routine storage of blood products include lyso-phosphatidylcholines (lyso-PCs) [9,36,38–41], neutral lipids [34,38–41], sCD40L [42–45], cytokines [37,45,46] and microparticles (MPs) [47,48]. Some NAFs have the ability to prime for PMN respiratory burst [9,34,36,38–40,43]. Furthermore, lyso-PCs and sCD40L have been shown to induce TRALI in in vitro models [9,43], and lyso-PCs induce TRALI in ex vivo models [40,41] (Fig. 1D). In vivo rat models have further verified that purified lyso-PCs or neutral lipids induce TRALI [8,34]. Importantly, the introduction of leucoreduction appears to have the additional benefit of removing the rogue lyso-PCs and sCD40L in stored PRBC [34,43].

The first large animal (sheep), two event, in vivo model of TRALI was recently reported [6,37]. The larger size of the sheep relative to rodent models provided the advantage of using routine hospital instrumentation to track in detail the respiratory and haemodynamic changes associated with the development of TRALI. Interestingly, while stored PRBC or stored platelets both induced TRALI in 80% of sheep, the haemodynamic changes observed with the former were more severe [6,37]. Analysis of the cytokine profile of the stored PRBC and platelets revealed differences between them, which may account for the dissimilar haemodynamic outcomes [6,37]. Whether any of these cytokines individually or in concert directly contributed to the observed TRALI is yet to be investigated.

RBC Duffy expression

As inflammatory conditions are not uncommon in transfusion recipients, the RBC’s ability to scavenge inflammatory chemokines through the RBC Duffy antigen has been investigated [49]. Stored RBCs were found to have reduced Duffy expression, resulting in reduced chemokine scavenging function [50]. This study found that transfusion of Duffy negative RBC into Duffy wild type endotoxaemic mice caused lung injury [50]. It also found that a reduction in RBC chemokine scavenging capability (a consequence of the reduced Duffy expression) in transfused stored RBCs could augment existing lung inflammation [50]. Thus it may be speculated that in situations of massive transfusion, the compromised chemokine scavenging function of stored PRBC could increase susceptibility to TRALI.

Conclusions

Case reports, haemovigilance reports, in vitro, ex vivo and in vivo models have together provided a valuable source of information that informs our current understanding of the pathophysiology of TRALI. PMNs appear to be a key effector cell in both antibody mediated TRALI and non-antibody mediated TRALI. But now, there is accumulating evidence that other cell types (ECs, monocytes, lymphocytes and platelets), may also actively contribute to the pathophysiology of TRALI. At this time, the involvement of these other cells appears to be inextricably linked to the serological characteristics of the specific causative antibody. As clinical cases of TRALI always involve polyclonal antibodies, careful consideration should be given to the interpretation and translation of data from models using MoAbs.

Despite TRALI-risk-reduction strategies such as using male predominant plasma rich products or screening female donors for HNA or HLA antibodies, TRALI still remains the most frequent cause of transfusion-related mortality [3,4]. There has also been little change to the number of TRALI fatalities associated with cellular blood products [3,4]. Thus, as the evidence behind the association of cellular products with NAFs and their role in non-antibody mediated TRALI grows, it may now be time to focus research on defining its mechanism and to begin considering strategies to minimise this form of TRALI.

Disclosures

No potential conflict of interests to declare.