Cells give off little membrane-bound bubbles called exosomes, which are known to contain microRNAs (miRNAs)—short stretches of RNA that can interfere with protein production. Exosomes have also been associated with cancer metastasis. According to a study published in Cell last week (October 23), tumor-linked exosomes can produce their own miRNAs from starter material, and these miRNAs can enter neighboring cells and incite them to form tumors.
“Collectively, this study unravels the possible role cancer exosomes play in inducing an oncogenic ‘field effect’ that further subjugates adjacent normal cells to participate in cancer development and progression,” Raghu Kalluri, a cancer biologist at MD Anderson Cancer Center, and his coauthors wrote in their report.
Comparing exosomes derived from normal cells to those derived from breast cancer cells, Kalluri’s
team found the proteins responsible for chopping up miRNA precursors into their mature form present only in the cancer exosomes. When normal cells were implanted into a mouse alongside cancer exosomes, those normal cells formed tumors. However, blocking an miRNA-processing protein called Dicer in the cancer exosomes obliterated tumor formation, “suggesting that miRNA biogenesis in exosomes contributes to cancer progression,” the authors wrote.
“It’s amazing—these vesicles were considered garbage cans,” Khalid Al-Nedawi, a cancer researcher at McMaster University in Hamilton, Canada, told Nature News. “This paper really brings us closer to harnessing the potential of these tiny vesicles.”
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
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
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.
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. TREG 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; TREG cells, regulatory T cells.
Sexually transmitted diseases caused by bacteria, viruses and protozoa can affect the male genital tract. Gimenes and colleagues discuss the evidence for effects of sexually transmitted pathogens on semen, sperm and male infertility.
Sexually transmitted diseases (STDs) are caused by several pathogens, including bacteria, viruses and protozoa, and can induce male infertility through multiple pathophysiological mechanisms. Additionally, horizontal transmission of STD pathogens to sexual partners or vertical transmission to fetuses and neonates is possible. Chlamydia trachomatis, Ureaplasma spp., human papillomavirus, hepatitis B and hepatitis C viruses, HIV-1 and human cytomegalovirus have all been detected in semen from symptomatic and asymptomatic men with testicular, accessory gland and urethral infections. These pathogens are associated with poor sperm quality and decreased sperm concentration and motility. However, the effects of these STD agents on semen quality are unclear, as are the effects of herpes simplex virus type 1 and type 2, Neisseria gonorrhoeae, Mycoplasma spp., Treponema pallidum and Trichomonas vaginalis, because few studies have evaluated the influence of these pathogens on male infertility. Chronic or inadequately treated infections seem to be more relevant to infertility than acute infections are, although in many cases the exact aetiological agents remain unknown.
Sexually transmitted diseases (STDs) can induce male infertility through multiple pathophysiological mechanisms
Several STD-causing agents, including bacteria, viruses and protozoa have been detected in semen from symptomatic and asymptomatic males, and can be transmitted through natural intercourse or insemination
STD pathogens can affect sperm parameters and functions, particularly when testicular, accessory gland and urethral infections localize the disease agents in proximity to semen
Several highly sensitive and specific molecular methods are now available to explore the relationship between infertility and infections of semen with STD pathogens
Chlamydia trachomatis, Ureaplasma spp., human papillomavirus, hepatitis B and C viruses, HIV-1 and human cytomegalovirus are associated with reduced sperm quality, concentration
Sexually transmitted diseases (STDs) are caused by microorganisms that colonize the female and male genital tracts, often causing only mild symptoms.1 STDs affect health and cause social and economic problems worldwide.2, 3 Despite the development of antibiotics and vaccines, and the existence of disease prevention and control programmes, these pathogens remain important causes of acute and chronic disease.1
In 2000, the WHO recognized the role of genital tract infections in human infertility.4 STDs in men cause genital injury, infections of semen, prostatitis, urethritis, epididymitis and orchitis.5 Several studies have reported that infertile men are affected by semen infection,6 mainly resulting from testicular, accessory gland and urethral infections. Semen infection also has possible involvement in pregnancy complications and, via maternal infection, in transmission of pathogenic agents to the fetus or neonate.1 STD-causing microorganisms detected in semen have been associated with poor sperm quality,7 decreases in sperm concentration and motility8, 9 and changes in other semen parameters relevant to fertility, including sperm velocity, morphology and viability, as well as seminal volume, pH, viscosity and biochemical composition.7, 8, 9
Several STD pathogens have been detected in semen from asymptomatic and symptomatic men (Figure 1 and Table 1).10, 11, 12, 13, 14 Chronic or inadequately treated infections seem to have greater association with infertility than acute infections do, although in many cases the exact aetiological agents resulting in infertility remain unknown.1 Undiagnosed, asymptomatic infections could have important implications for individual and public health.15
Figure 1: Major sexually transmitted disease pathogens detected in semen.
Inflammatory processes triggered in the male genital tract (MGT) by some STDs can lead to deterioration of spermatogenesis and obstruction of the seminal tract,16, 17 which can worsen the characteristics of semen (Figure 2 and Table 2).18 Abortive apoptosis is an important part of the control of spermatogenesis, and has been observed in spermatogonia, spermatocytes, spermatids and ejaculated spermatozoa.18, 19, 20 However, sperm apoptosis can also be associated with inflammatory conditions and oxidative stress that occur in response to infection, resulting in impaired motility and reduced capacity of spermatozoa for fertilization.18, 19, 20, 21, 22
Figure 2: Sexually transmitted disease loci in the male genital tract and their relation to infertility.
The demonstration of a microorganism in the male genital tract and seminal fluid can be a sign of an infection with pathological sequelae. The inflammatory processes triggered by infection with some pathogens can lead to deterioration of spermatogenesis and obstruction of the seminal tract.
Over the past three decades, diagnostics for STDs have depended on traditional methods, such as bacterial culture, enzyme immunoassays, fluorescent antibody staining and microscopy. In recent years, several highly sensitive and specific PCR-based diagnostic methods have become available, enabling better understanding of the relationship between infertility and infections of semen with STD pathogens.23
Figure 3: Interaction of sexually transmitted disease pathogens with spermatogenic cells and spermatozoa.
a| Schematic diagram and representative photomicrographs (with Leishman staining) of spermatogenesis, indicating pathogens that are known to attach and internalize in spermatogenic cells. b| Photomicrograph indicating sperm death by staining (red) with eosin-nigrosin, and schematic illustration of a spermatozoon showing sites of attachment and internalization of sexually transmitted disease pathogens. Internalization in the sperm head can cause DNA damage.
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