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Ischemia–reperfusion injury (IRI) is a common postoperative complication that is a major cause of both short- and long-term mortality in lung recipients. Characterized by poor lung function and the appearance of radiographic infiltrates, currently there is no effective therapy for lung transplant-mediated IRI. Over the past two decades, data from experimental models of lung transplantation show that most pulmonary damage is controlled by a macrophage–neutrophil axis. First, responding mainly to the accumulation of damage-associated molecular patterns (DAMPs), macrophages release reactive oxygen intermediates and inflammatory mediators that stimulate the expression of adhesion molecules and promote the dissolution of endothelial cell tight junctions to initiate inflammatory cell infiltration. Then neutrophils, the most common infiltrating cells in injured lung grafts, release the bulk of proteolytic enzymes that further catalyze the breakdown of homeostatic barriers leading to the loss of efficient gas exchange.

Although there is little question that macrophages and neutrophils are the predominant effectors of lung parenchyma injury, it is also becoming clear that other immune cells regulate their destructive activity. Invariant natural killer T cells (iNKTs) are known to interact with both macrophages and neutrophils. However, as compared to other lymphocyte populations, less is known about iNKTs, which have been typically characterized for their ability to recognize lipid antigens in the context of nonclassical MHC I-like C1D molecules through a highly restricted T cell receptor (TCR) repertoire. In humans, iNKTs are exceedingly rare cells, making up approximately 2% of circulating T lymphocytes. Nevertheless, they have enormous immunoregulatory potential and can release large amounts of immune mediators within hours of activation. A distinct iNKT population that expresses IL-17 and is critical to promote lung neutrophil accumulation in response to microbial infection has been identified in mice. Recently, IL-17+ iNKTs have also been shown to exacerbate lung IRI [1] but what has remained largely mysterious is how these cells become initially activated.

In this issue, Sharma et al. may have solved the conundrum [2]. Using a mouse hilar clamp lung IRI model, they demonstrate an intriguing link between the release of the DAMP High Mobility Group Box 1 protein (HMGB1) and IL-17 expression by iNKTs. HMGB1, a chromatin-associated protein, is a well-established mediator of solid organ injury and is secreted by inflamed macrophages [3]. Several receptors are triggered by HMGB1, including Toll-like receptor (TLR)2, TLR4 and the receptor for advanced glycation endproducts (RAGE). iNKTs can express all of these receptors, but RAGE deficiency restricted to iNKTs was sufficient to decimate IL-17 expression, attenuate neutrophil infiltration and preserve lung function. Additionally, hypoxic stress applied to an alveolar macrophage line drove HMGB1 secretion that in turn induced RAGE-dependent IL-17 production by iNKTs, suggesting that macrophages send signals to iNKTs to promote lung graft IRI.

These results are surprising, as RAGE is expressed on many cell types and can bind other DAMPs, including advanced glycation end products and some S100 protein family members. Moreover, these data may explain why IL-17 is so rapidly generated in response to lung reperfusion and further define HMGB1's role in inducing neutrophil-mediated responses. Thus in this context, iNKTs may serve to amplify HMGB1 signals since IL-17 not only drives chemokine production that stimulates neutrophil migration, but also induces the expression of G-CSF, a cytokine that exacerbates lung graft IRI by stimulating the release and production of granulocytes from the bone marrow [4].

The recent observation of elevated soluble RAGE levels in human lung recipient plasma with primary graft dysfunction [5], a form of graft injury mediated primarily by IRI, point a to a clinically relevant pathophysiological role for RAGE ligands. However, it remains to be determined if HMGB1-RAGE engagement on iNKTs plays a significant role in exacerbating lung transplant-mediated IRI. The data in this report were generated from non-transplant models, which cannot account for the effects of cold preservation, alloimmunity or immunosuppression—any of which may obviate the inflammatory contributions of iNKTs. Also, unlike in mice, there is little evidence for the existence of a Th17-like iNKT subset in humans. Nonetheless, this study does raise some interesting questions. For example, is antigen presentation to iNKTs required to promote lung graft IRI and if so, what is the identity of these antigen-presenting cells? Although iNKT activation can occur in the absence of TCR engagement, liver and kidney IRI has been shown to be ameliorated by CD1 blockade. Additionally, in the absence of infection, TLR agonist-stimulated antigen presenting cells can be potent activators of iNKTs through presentation of lipid self-antigens. As HMGB1 is a TLR agonist known to augment antigen presentation, this raises the possibility that iNKTs make direct contact with macrophages via CD1 to become sufficiently activated to promote lung injury. The recent advances in intravital 2-photon pulmonary imaging [6] and the use of CD1 neutralizing antibodies could be used to answer these and other questions to better understand the role of iNKTs in lung IRI.

Acknowledgments

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  2. Acknowledgments
  3. Disclosure
  4. References

AEG is supported by grants from The National Heart, Lung and Blood Institute (1R01HL094601, 1R01HL113436) and the Barnes Jewish Hospital Foundation.

Disclosure

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  2. Acknowledgments
  3. Disclosure
  4. References

The authors of this manuscript have no conflicts of interest to disclose as described by the American Journal of Transplantation.

References

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  2. Acknowledgments
  3. Disclosure
  4. References
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    Kreisel D, Sugimoto S, Tietjens J, et al. Bcl3 prevents acute inflammatory lung injury in mice by restraining emergency granulopoiesis. J Clin Invest 2011; 121: 265276.
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