Immune cells in experimental acute kidney injury

Nature Reviews Nephrology:

 

The immune system has a vital role in the renal response to acute kidney injury (AKI). In this Review, Hye Ryoun Jang and Hamid Rabb describe current understanding of the function of the innate and adaptive immune systems in the early and late injury phases of ischaemic and nephrotoxic AKI, and describe the influence of immune cells on recovery and long-term outcome following AKI. 

 

Abstract:

 

Acute kidney injury (AKI) prolongs hospital stay and increases mortality in various clinical settings. Ischaemia–reperfusion injury (IRI), nephrotoxic agents and infection leading to sepsis are among the major causes of AKI. Inflammatory responses substantially contribute to the overall renal damage in AKI. Both innate and adaptive immune systems are involved in the inflammatory process occurring in post-ischaemic AKI. Proinflammatory damage-associated molecular patterns, hypoxia-inducible factors, adhesion molecules, dysfunction of the renal vascular endothelium, chemokines, cytokines and Toll-like receptors are involved in the activation and recruitment of immune cells into injured kidneys. Immune cells of both the innate and adaptive immune systems, such as neutrophils, dendritic cells, macrophages and lymphocytes contribute to the pathogenesis of renal injury after IRI, and some of their subpopulations also participate in the repair process. These immune cells are also involved in the pathogenesis of nephrotoxic AKI. Experimental studies of immune cells in AKI have resulted in improved understanding of the immune mechanisms underlying AKI and will be the foundation for development of novel diagnostic and therapeutic targets. This Review describes what is currently known about the function of the immune system in the pathogenesis and repair of ischaemic and nephrotoxic AKI

 

Key points:

• Various components of the innate and adaptive immune systems are implicated in the pathogenesis and repair of acute kidney injury (AKI)

• The roles of individual immune cell types have been most thoroughly investigated in models of ischaemic AKI

• Various immune cells traffic into the post-ischaemic kidney and show changes in phenotypes and numbers depending on the time course after establishment of ischaemic AKI

• The roles of macrophages, renal dendritic cells and T regulatory cells differ according to the pathogenesis of AKI

• Although numerous studies in animal models of AKI show therapeutic potential for modulating immune cells, big hurdles must be overcome before applying these findings to patients

• Functions and interactions of specific immune cell types and humoral factors in AKI differ between human disease and animal models, and depend on the type and stage of injury

Introduction

 

Despite remarkable advances in modern medicine, acute kidney injury (AKI) still remains a challenging condition that lacks specific tools for its early diagnosis and treatment. AKI worsens the overall clinical course of affected patients by causing uraemia, acid–base or electrolyte disturbances, and volume overload. The incidence of AKI has been reported to be as high as 5% of hospitalized patients or 30% of critically ill patients.¹ The risk of chronic kidney disease and end-stage renal disease is substantially increased in patients with AKI.² Most patients with AKI are diagnosed when injury is already established and, therefore, only conservative treatment including fluid therapy and dialysis is available. To improve the clinical outcome of AKI, novel diagnostic and therapeutic strategies need to be developed. Understanding the pathophysiology of AKI is, therefore, the cornerstone of exploration of novel diagnostic and therapeutic strategies.

 

Experimental models of AKI can be divided into several categories depending on the induction method (Figure 1). In models of septic AKI, the initial immune response against foreign antigens and innate triggers causes a complex secondary inflammatory response that facilitates renal injury.³ Non-septic and septic AKI are known to have very different pathophysiological features. Septic AKI is a systemic manifestation of sepsis following exposure to foreign antigens such as bacteria or viruses; detailed discussion of septic AKI is beyond the scope of this review.

 

Immune mechanisms were not expected to have an important role in models of aseptic AKI, but numerous studies conducted over the past two decades have revealed that inflammatory processes mediated by the immune system are crucial in mediating renal injury.³ Immune mechanisms involved in the pathogenesis of renal injury have been studied most extensively in models of ischaemic AKI employing cold or warm ischaemia. Both types of ischaemia occur during organ transplantation; cold ischaemia starts when the organ is cooled with cold perfusion solution after procurement, and lasts until the temperature of the organ reaches the physiologic temperature. Thereafter, warm ischaemia begins, and ends when perfusion is restored after completion of surgical anastomosis. Thus, two distinct periods of warm ischaemia occur in the transplantation setting—during organ retrieval and implantation.⁴ Interestingly, the nephrotoxicity induced by cisplatin, a chemotherapeutic agent, has many pathophysiological features that overlap with those of ischaemia–reperfusion injury (IRI).

 

Both innate and adaptive immune systems are directly involved in the pathogenesis of ischaemic AKI. Various cellular and humoral immune system components contribute to AKI, some of which are also thought to be involved in the repair process following IRI.^{5, 6} The healthy kidney produces hormones that influence the immune system, such as vitamin D (calcitriol) and erythropoietin,⁷ and the renal tubular epithelium expresses Toll-like receptors (TLRs), which critically contribute to activation of the complement system and recruitment of immune cells in response to inflammatory stimuli.^{8, 9} Several types of resident immune cells, such as dendritic cells, macrophages, mast cells and lymphocytes are homeostatically maintained in the normal kidney, although these cells constitute a small population.^{10, 11, 12, 13} Under normal conditions, the renal mononuclear phagocytes mainly comprise macrophages located in the renal medulla and capsule and renal dendritic cells found in the tubulointerstitium.^{10, 11, 14} In mice, renal dendritic cells show a specific CD11c⁺CD11b⁺EMR1(F4/80)⁺CX₃CR1 (CX₃C-chemokine receptor)⁺CD8⁻CD205⁻ phenotype, and have a similar transcriptome as dendritic cells residing in other nonlymphoid tissues.^{15, 16} Dendritic cells are recruited to the kidney by a CX₃CR1–CX₃CL1 (CX₃C-chemokine ligand 1, also known as fractalkine) chemokine pair,¹⁷ and have an important role in local injury or infection. Dendritic cells not only function as a potent source of other factors, such as neutrophil-recruiting chemokines and cytokines,^{12, 18} but also present antigens to T cells. Intrarenal macrophages exert homeostatic functions by phagocytosis of antigens in the kidney and undergo phenotypic changes that enable them to participate in both inflammatory and anti-inflammatory processes.¹⁴ Both dendritic cells and macrophages contribute substantially to homeostasis and regulation of immune responses (as resident renal mononuclear phagocytes) in the normal kidney. Mast cells also reside in the tubulointerstitium and mediate pathogenic processes in crescentic and other forms of glomerulonephritis. However, the exact roles of dendritic cells, macrophages and mast cells in the normal kidney are yet to be elucidated.^{19, 20, 21} Lymphocytes, including both T cells and B cells, have been found in normal mouse kidneys even after extensive exsanguination and perfusion.²² Intrarenal resident T cells show distinctly different phenotypes from T cells in spleen and blood; those from normal mouse kidneys contain an increased percentage of CD3⁺CD4⁻CD8⁻ double-negative T cells. Intrarenal T cells also show a high proportion of activated, effector and memory phenotypes, whereas a small percentage of regulatory T cells and natural killer (NK) T cells exist in perfused and exsanguinated mouse kidney.²²

 

In this Review, we describe how immune cells participate in the pathogenesis of AKI, focusing on ischaemic and nephrotoxic AKI. Immune system function in septic AKI is only outlined in this article, because the pathophysiology of septic AKI includes both immune responses to various foreign antigens and secondary systemic inflammatory responses, which are distinctly different to the immune responses that occur in aseptic AKI.

 

Figure 1: Experimental models of AKI.

Models of AKI can be broadly categorized according to whether foreign antigens are involved (aseptic or septic AKI). Each category can be subdivided according to the method used to induce AKI. Ischaemic AKI is induced by ischaemia–reperfusion injury and by the type of ischaemia (warm or cold). Nephrotoxic AKI is induced by nephrotoxic agents, such as cisplatin. Abbreviation: AKI, acute kidney injury.

 

Figure 2: Major effector cells of both innate and adaptive immune systems contribute to the establishment of renal injury in ischaemic AKI.

An immune response is initiated in post-ischaemic kidneys by resident immune cells and is potentiated by a rapid influx of immune cells through the disrupted endothelium. TLRs, adhesion molecules and DAMPs released from dying cells facilitate the recruitment and activation of various immune cells including neutrophils, macrophages, dendritic cells, NK cells, T cells and B cells during the early injury phase. Activation of the complement system and increased production of proinflammatory cytokines and chemokines are important promoters of leucocyte infiltration into the post-ischaemic kidney. Major effector cells of the innate immune system, such as macrophages, dendritic cells and NK cells are involved in the pathogenesis of renal injury after IRI. T cells, the major effector cells of the adaptive immune system, also substantially contribute to the development of renal injury from the early to late injury phase. Plasma cells seem to participate in the tubular damage process during the late injury phase. Abbreviations: AKI, acute kidney injury; DAMPs, damage-associated molecular patterns; HIF, hypoxia-inducible factor; IRI, ischaemia–reperfusion injury; NK, natural killer; TLR, Toll-like receptor; T_REG cell, regulatory T cell. Modified with permission from Elsevier © Jang, H. R. & Rabb, H. The innate immune response in ischemic acute kidney injury. Clin. Immunol. 130, 41–50 (2009).

Figure 3: Immune modulation during the repair phase of ischaemic AKI is a key factor in determining the outcome of AKI.

T_REG cells, B cells and macrophages have substantial roles in determining whether repair results in tubular regeneration or atrophy and interstitial fibrosis. T_REG cells and M2 macrophages have important roles in tubular regeneration, whereas B cells enhance tubular atrophy and suppress tubular regeneration. Humoral factors, such as proinflammatory or anti-inflammatory cytokines and chemokines, also change the intrarenal microenvironment and affect phenotype switching of macrophages. The exact mechanisms by which these immune processes regulate tubular atrophy or regeneration are not yet known. Abbreviations: AKI, acute kidney injury; IL-10, interleukin-10; TGF-β, transforming growth factor β; T_REG cells, regulatory T cells. Modified with permission from Elsevier © Jang, H. R. & Rabb, H. The innate immune response in ischemic acute kidney injury. Clin. Immunol. 130, 41–50 (2009).

 

Figure 4: Important immune cells in each phase of renal IRI.

 Neutrophils and NK T cells infiltrate the post-ischaemic kidney in the early injury phase and contribute to initiation of the inflammatory cascade. NK cells also contribute to renal tissue injury in the early injury phase. Renal dendritic cells increase in number and are activated to mediate inflammation from the early to late injury phase. Macrophages have diverse roles throughout the pathogenesis of renal IRI. In the injury phase, M1 macrophages contribute to inflammation and tissue injury, whereas M2 macrophages exert anti-inflammatory functions in post-ischaemic kidneys and facilitate renal tubular regeneration during the recovery phase. T cells also show dynamic changes in number and phenotype depending on the phase of renal IRI. CD4⁺ T cells have a substantial role in inducing renal tissue damage in the early injury phase. T_REG cells increase in the late injury phase and facilitate tubular regeneration in the recovery phase. B cells are activated and differentiate in the injury phase, and limit tubular regeneration in the recovery phase. Abbreviations: DAMPs, damage-associated molecular patterns; IRI, ischaemia–reperfusion injury; NK, natural killer; TLR, Toll-like receptor; T_REG cells, regulatory T cells.

Hye Ryoun Jang^1, & Hamid Rabb^2, 

 

Published online 21 October 2014