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Cells and Matrix |
Center for Cardiovascular Repair,1 and Department of Medicine and Center for Cardiovascular Repair,2 University of Minnesota, Minneapolis, Minnesota
Correspondence to: D.A. Taylor, Center for Cardiovascular Repair, 312 Church Street SE, 7-105 Nils Hasselmo Hall, Minneapolis, Minnesota 55455 U.S.A. dataylor{at}umn.edu
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ABSTRACT |
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 TOP ABSTRACT UNMET NEED FOR EFFECTIVE...ENDOTHELIAL DAMAGE IS NOT...BONE MARROW PROGENITOR CELLS...PROGENITOR CELLS AND CELL...CARDIOVASCULAR COMPLICATIONS OF...MOVING FORWARD WITH CELL...REFERENCES |
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The recently discovered therapeutic potential of stem or progenitor
cells has initiated development of novel treatments in a number of
diseases—treatments that could not only improve patients' quality of
life, but also halt or even prevent disease progression. Hypertension;
fluctuations in glycemia, electrolytes, nutrient levels, and circulating
volume; and frequent infections and the associated inflammation all greatly
impair the endothelium in patients undergoing peritoneal dialysis. As our
understanding of the regulatory function of the endothelium advances, focus is
increasingly being placed on endothelial repair in acute and chronic renal
failure and after renal transplantation. The potential of progenitor cells to
repair damaged endothelium and to reduce inflammation in patients with renal
failure remains unexamined; however, a successful cell therapy could reduce
morbidity and mortality in kidney disease.
Important contributions have been made in identifying progenitor cell
populations in the kidney, and further investigations into the relationships
of these cells with the pathophysiology of the disease are underway. As the
kidney disease field prepares for the first human trials of progenitor cell
therapies, we deemed it important to review representative original research,
and to share our perspectives and lessons learned from clinical trials of
progenitor cell–based therapies that have commenced in patients with
cardiovascular disease.
KEY WORDS: End-stage renal disease; kidney disease; stem cells.
Kidney disease, including end-stage renal disease (ESRD), has been receiving increasing attention in recent years, given that the prevalence of the disease and the associated health care expenditures have been on the rise (1). Kidney disease is an important sequel to cardiovascular disease (CVD) (2), risk factors (obesity, metabolic syndrome, type 2 diabetes, hypertension) for which have been increasing in prevalence (3–6).
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UNMET NEED FOR EFFECTIVE THERAPIES IN KIDNEY DISEASE |
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TOPABSTRACTUNMET NEED FOR EFFECTIVE...ENDOTHELIAL DAMAGE IS NOT...BONE MARROW PROGENITOR CELLS...PROGENITOR CELLS AND CELL...CARDIOVASCULAR COMPLICATIONS OF...MOVING FORWARD WITH CELL...REFERENCES |
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Medical progress in ESRD is not showing the same leaps forward as in other disciplines. One reason for this lag is the limited number of effective drug therapies available to attenuate disease progression and to improve patient survival. Available agents include angiotensin converting-enzyme inhibitors and angiotensin II receptor blockers (9,10), statins (11), erythropoietin-like compounds (12), and to some extent, acetylsalicylic acid, especially when CVD is present (13,14); however, effective drug utilization remains a problem (14). Furthermore, the definitive options of dialysis and renal transplantation serve as major renal replacement therapies, but efficacies depend on the clinician's skills in coping with the complex process of renal failure and its consequences... and on the number of donors available. Overall, effective new interventions to prolong survival and reduce the overall costs of care represent a strong unmet need in kidney disease in general and in ESRD in particular.
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ENDOTHELIAL DAMAGE IS NOT JUST A CARDIOVASCULAR ISSUE: IT IS AN IMPORTANT LINK IN KIDNEY DISEASE PROGRESSION |
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TOPABSTRACTUNMET NEED FOR EFFECTIVE...ENDOTHELIAL DAMAGE IS NOT...BONE MARROW PROGENITOR CELLS...PROGENITOR CELLS AND CELL...CARDIOVASCULAR COMPLICATIONS OF...MOVING FORWARD WITH CELL...REFERENCES |
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Endothelial-dependent relaxation is impaired early in atherogenesis (19). Proinflammatory cytokines impair endothelium-dependent dilatation (21). Clinical measures of endothelial dysfunction in the absence of flowlimiting lesions predict CVD events, including stroke (19,22,23). We now understand that vasodilatory, antiplatelet, and antithrombotic processes are primarily regulated at the endothelial level, and the loss of normal endothelial function may be the most important driver of the balance in favor of inflammation and thrombosis (15,19,24).
Acute renal failure (ARF) is a fulminant ischemic process that arises from toxin- or immune-mediated damage to nephrons, mainly from antibiotics, chemotherapy agents, vasodilatory shock, systemic diseases, or major surgical procedures, alone or in combination (25). Mortality rates in ARF exceed 50% (25). Several targeted pharmacologic approaches have exhibited efficacy in animals (26–28), but translation into bedside therapeutic benefit has remained problematic.
The major component of ARF (Figure 1) is systemic inflammatory response syndrome (SIRS), which is caused by tubular necrosis and leads to multiple organ failure (MOF) (29). Necrosis-based loss of tubular epithelium plays one of several central roles in ARF (30). However, it has recently been determined that, in ARF, renal endothelial cells undergo early damage (31) and become edematous, reducing microvascular perfusion by a "no reflow" mechanism (32). The high propensity of ARF patients to develop SIRS confirms that renal cells play a critical immunomodulatory role—one that primarily involves endothelial rather than epithelial cells. Similarly, MOF is a state of global massive endothelial dysfunction with a total loss of antithrombotic and anti-inflammatory functions on the microcirculatory level (33).
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The inflammatory state in ESRD is usually manifested by elevated C-reactive
protein, interleukins-1 and -8, and tumor necrosis factor 
(TNF-)
(38,39).
Of the latter molecules, IL-6 has been identified as a predictive factor
closely correlated with mortality in patients undergoing dialysis. Each
picogram of IL-6 increases the relative risk of mortality by 4.4%
(39). Aggressive endothelial
damage is responsible for the high rate of CVD and related mortality in both
CRF and ESRD (40).
Addressing endothelial damage is highly relevant to patients undergoing dialysis, because fluctuations in glucose, electrolytes, nutrients, circulatory volume, and red blood cell concentration first and foremost affect endothelium (40–42). In addition, persistent activation of renin–angiotensin–aldosterone is directly toxic to endothelium, because prolonged exposure to angiotensin II and aldosterone results in a loss of endothelial cells (19,43,44).
Even after renal transplantation, the role of endothelium does not diminish (Figure 1). Two major causes of graft failure—allograft rejection and progressive vasculopathy—show high dependence on the regulatory function of endothelium (45,46). Specifically, endothelium replacement in the graft by recipient-type cells is required for rejection, and progressive transplant vasculopathy is characterized by an accumulation of vascular smooth muscle cells, myofibroblasts, and connective tissue in the endothelium of the graft. Both processes lead to a shutdown of endothelial function (46). Therefore, to effectively intervene in various forms of kidney disease, the balance between injury and repair needs to be tipped toward promotion of repair and strengthening of the endothelium and surrounding cells.
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BONE MARROW PROGENITOR CELLS ARE IMPORTANT CONTRIBUTORS TO ENDOTHELIAL REPAIR |
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TOPABSTRACTUNMET NEED FOR EFFECTIVE...ENDOTHELIAL DAMAGE IS NOT...BONE MARROW PROGENITOR CELLS...PROGENITOR CELLS AND CELL...CARDIOVASCULAR COMPLICATIONS OF...MOVING FORWARD WITH CELL...REFERENCES |
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Currently, only a conceptual understanding of repair has been achieved. Many questions are yet unanswered, but mediation of the process by BMPCs makes sense. After all, if cells that originate from the BMPCs did not have the ability to fight inflammation and pathogens and to participate in healing, an organism would not survive for too long. When the number or function of these cells decreases because of a systemic disease (such as type 2 diabetes), aging, or toxic substances (such as those from smoking) (58–60), healthy and functioning endothelium fails (19), and adverse consequences manifest clinically as atherosclerosis, thrombosis, and their micro- and macrovascular sequelae (24).
BMPCs have the potential to promote repair in various forms of kidney disease (Figure 1). The notion of the presence of regenerative mechanisms in the kidney is derived from clinical observations in ARF. Although mortality is more than 50% in ARF, patients who survive experience a return of kidney function to between 90% and 95% of baseline without long-term consequences (25). And because therapy of ARF is not specifically directed at tissue repair, recovery must reflect endogenous processes that take place to overcome renal ischemia. Administration of BMPCs could potentially augment the repair mechanisms while slowing destructive ischemic cascades—all geared toward increased survival of patients. However, the response to exogenous BMPCs will most likely depend on the degree of tissue ischemia, and appropriate strategies would need to be employed to minimize interference of factors such as infection that are likely to lessen the therapeutic benefit.
Therapeutic application of BMPCs in CRF and ESRD, which represent different pathophysiologic processes, has a larger goal. A major need is to reduce inflammation, which is an important component of disease progression. Our group (59) demonstrated the ability of BMPCs to reduce IL-6 levels in a mouse model of atherosclerosis, findings that have fueled hope of a similar result if these cells were to be applied in patients with advanced kidney disease or ESRD. If inflammation were to be reduced and the balance between injury and repair to be shifted toward repair, a more fully functioning endothelium could possibly be created to slow the loss of renal function.
Whether BMPCs are capable of preventing ESRD is an intriguing question. Equally important is the potential for administration of BMPCs in ESRD in the hopes of reducing CVD risk, the main contributor to mortality. Theoretically, elimination of the inflammatory milieu in the kidney could, to an extent, reduce the global proinflammatory state not only because of local paracrine action, but also, to some degree, because of systemic action. Borrowing from observations of initiation of endothelial dysfunction and occurrence of cardiac events in patients with peripheral arterial disease (61), in which the original pathophysiologic process lies far (in terms of distance) from the heart, the induction of a proinflammatory state distal to the origin results in the entire vascular tree being involved in the disease process.
For patients undergoing dialysis, BMPCs may bring much needed endothelial stabilization, given that endothelial integrity is greatly impaired because of fluctuations in glycemia, electrolyte balance, circulating volume, and nutrition status, and frequent hypertension (40–42,62). Another important contributor is susceptibility to infection (63), which on its own generates an inflammatory response... in a disease state in which no extras are needed. Application of BMPCs here may ward off inflammation, most likely over a course of administrations. Reduction of CVD risk as described above is a much needed outcome in patients undergoing dialysis, and whether BMPCs can aid current therapies in that regard is a much-needed investigation.
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PROGENITOR CELLS AND CELL THERAPY IN KIDNEY DISEASE |
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TOPABSTRACTUNMET NEED FOR EFFECTIVE...ENDOTHELIAL DAMAGE IS NOT...BONE MARROW PROGENITOR CELLS...PROGENITOR CELLS AND CELL...CARDIOVASCULAR COMPLICATIONS OF...MOVING FORWARD WITH CELL...REFERENCES |
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BMPCs in the Kidney: Bussolati et al. (47) isolated an immature population of AC133+ progenitor cells from renal cortical parenchyma, representing approximately 0.8% of the cell population. Similar cells can be routinely isolated from blood or bone marrow. In fact, the percentage of AC133+ progenitor cells found in kidney is similar to the mean percentage found in the blood of normal human volunteers in preliminary observations by our group.
Renal AC133+ cells expressed Pax-2, which is a renal epithelial progenitor–cell marker, and co-expressed CD73, CD44, and CD29, which are markers of bone marrow–derived mesenchymal stem cells (MSCs). In vitro, these cells differentiated into epithelium and endothelium, but not into blood cells or adipocytes. In addition, the AC133+ cells formed vessel structures in Matrigel plates (Becton–Dickinson, Mountain View, CA, U.S.A.), suggesting their capacity for neovascularization. When injected into SCID (severe combined immunodeficiency disease) mice with a glycerol-induced tubular injury, the cells integrated into the tubules. These findings support the idea that cells found in blood and bone marrow (but also perhaps resident in native tissue) can participate in renal repair.
Resident Stem Cells in the Kidney: The discovery of resident cardiac stem cells has changed the view of the biology of the heart (64). What was once viewed as a terminally differentiated non-regenerating organ is now viewed as a more dynamic one. For example, myocyte mitosis is now known to occur in the fetal, neonatal, adult, and hypertrophied heart, and a pool of undifferentiated cells present in the myocardium (albeit at low numbers) appear to be activated by injury (64). Similar milestones have yet to be achieved in the kidney, although progress is being made (65,66).
Multipotent resident renal stem cells have not yet been found in the kidney. However, the field is rapidly moving forward, and so it will not be surprising if such cells are identified in the near future. Oliver et al. (67) made some promising steps in that direction when they uncovered evidence in favor of the existence of resident stem-cell pools in the renal papilla. Iwatani and colleagues (48) have suggested that renal stem cells may reside in the bone marrow and take up residence in the kidney when needed, because bone marrow stem cells were shown to be able to form mesangial cells, tubular cells, and podocytes. Studies examining lineages in kidney differentiation and identifying the markers of renal progenitor cells have implicated the importance of CD24 and cadherin-11, plus 21 associated genes... findings that, of course, open more questions (65).
Acute Renal Failure: Acute renal failure is an active target for cell-based repair. Patschan's group (68,69) advanced the understanding of the mobilization and homing of endothelial progenitor cells (EPCs) after acute renal injury (unilateral renal artery clamping) in mice. Renal ischemia mobilized EPCs to the spleen within 6 hours. Ischemic preconditioning prevented that process and directed the cells to the medullopapillary region by day 7. Transplantation of these cells into wild-type mice with ARF improved renal function.
Increased renal microcirculation and function were also demonstrated by Brodsky et al. (31), who infused human umbilical vein endothelial cells into athymic nude rats after renal artery clamping. Alternatively, Arriero and co-workers (70) transplanted skeletal muscle stem cells into an ischemic kidney, achieving differentiation into endothelial lineage, engraftment, and a modest anti-ischemic effect.
The Rhlow Lin–Sca-1+ c-Kit+ cells from Rosa-26 mice
exhibited a capacity for tubular epithelial repair when transplanted into
female mice with ARF (71).
Mobilization of BMPCs with cyclophosphamide and granulocyte
colony–stimulating factor (G-CSF) significantly increased circulating
progenitors in mice with ARF
(72), but no protective effect
was observed. Instead, renal injury actually worsened and mortality increased,
suggesting that mobilization alone may not be a good therapeutic target. On
the other hand, in a cisplatin-induced ARF model, G-CSF mobilized Lin–
CD34+ c-Kit+ cells with positive results on tubular necrosis
(73) and apoptosis
(74). In the same model, MSCs
given 1 day after cisplatin protected against renal functional impairment at
days 4 and 5 by repopulating the damaged epithelial lining of the tubules
(75). The benefit seen with
MSCs might be attributable to upregulation of IL-10, basic fibroblast growth
factor, and Bcl-2, and downregulation of IL-1β, TNF-, and
interferon 
(76).
However, whether such alterations in cytokine levels remain beneficial is
unclear. These data are promising, but they clearly point to a strong need for
diligent immunologic surveillance through serial cytokine measurements in
patients who will eventually receive cell therapy for ARF.
CRF and ESRD: Choi et al. reported a 45% reduction in circulating CD34+ cells in CRF as compared with levels in healthy controls (77). Differentiation and migratory capacity were approximately 75% lower in CRF patients.
The mechanisms of the impact of CRF on EPCs are multifactorial, but several key mediators, such as parathyroid hormone, cytokines (for example, IL-6), and uremic toxins (78–80) are likely to be involved through interactions with erythropoietin (81) or as mediators of ongoing endothelial dysfunction. Administration of recombinant human erythropoietin (RHuEPO) increased EPCs by 312% compared with baseline and improved tube formation, showing EPC mobilization (82). The expression of vascular endothelial growth factor did not change after administration of rHuEPO, indicating that RHuEPO activates the Akt protein kinase pathway, but not the growth factors (82).
Dialysis: As in patients with CRF, EPC counts in patients on dialysis were lower and the functional capacity of the cells was less than that seen in normal volunteers (83). Long-term hemodialysis increased the number of EPCs by approximately 26%; however, cell function remained markedly downregulated (84). That increase in EPC count most likely means that dialytic removal of toxins triggers a reparatory response; however, because the function of the EPCs does not improve, endogenous repair will likely be inefficient.
Nocturnal hemodialysis positively affected both the number and function of EPCs in 10 patients studied by Chan and colleagues (85). The mechanism of this specific effect is not fully understood, although nocturnal hemodialysis is known to lower blood pressure better than the conventional hemodialysis does. Performing the procedure nocturnally may therefore allow for a better-functioning endothelium via a more effective repair mechanism, because synthesis of nitric oxide occurs primarily during the night hours (19).
Peritoneal dialysis has become an attractive treatment modality for ESRD because of low rates of peritonitis, similar or better survival than that seen in hemodialysis, lower cost, and adequate clearances (86,87). Peritoneal dialysis is an effective treatment for patients awaiting transplantation and for children (87). As larger numbers of patients are maintained on peritoneal dialysis for 10 years or more, treatment of endothelial dysfunction and prevention of long-term changes of the peritoneal membrane through reendothelialization with BMPCs may one day become routine clinical practice... after the many related questions are answered.
Genetic Disease: Recently, transplantation of wild-type bone marrow into irradiated mice with mutations in glomerular basement membrane type IV collagen improved architecture and histology findings, and significantly reduced proteinuria (88). These demonstrated effects of the bone marrow graft offer a potential for therapeutic benefit in patients with Alport syndrome.
Experimental Glomerulonephritis: Uchimura et al. (89) recently demonstrated incorporation and functional activity of culture-modified BMPCs into the glomerular endothelial lining in a rat model of glomerulonephritis. Injury was ameliorated as a result.
After Renal Transplantation: Renal transplantation restored circulating EPCs to levels comparable to those seen in healthy controls (90). However, EPC counts correlated with uremia and graft function during follow-up post transplantation.
Steiner and colleagues (91) demonstrated that median EPC counts post kidney transplantation are inversely associated with body mass index, mean arterial pressure, and history of cardiovascular disease—the factors that reflect endothelial health. Interestingly, patients that received azathioprine or ARBs had lower EPC counts.
Soler et al. (92) presented contradictory data showing that, following renal transplantation, EPC counts were not restored to the levels seen in normal individuals. However, in multivariate analysis, variables that reflect endothelial health [lower body mass index, lipids (as seen in the results from Steiner and co-authors)] predicted higher EPC counts. This discrepancy might have been caused by less healthy (from a cardiovascular standpoint) patients in the Soler study, or use of statins in the Steiner group, or a multitude of other reasons.
Mechanisms of EPC regulation post transplantation are complex, given that the injury–repair balance is an integral part of acute allograft rejection and chronic post-transplant vasculopathy (45,46). Understanding of the pathophysiologic processes may allow for applications of BMPCs to tip the balance toward repair and to preserve the functions of the allograft, thereby reducing the need for re-transplantation (Figure 1).
Renal Tubule Assist Device: A bioartificial tubule has been developed (93,94). It uses epithelial progenitor cells cultured on biomatrix-coated hollow-fiber membranes that are water- and solute-permeable, allowing for metabolic and endocrine function with the assist of a peristaltic pump. Technical aspects are described elsewhere (93,94). The feasibility of this renal assist device has been shown in uremic dogs (95). The renal assist device reabsorbed 40%–50% of ultrafiltrate volume, occasioned no ammonia excretion, saw the presence of glutathione processing, and achieved normal levels of 1,25-(OH)2-D3. In phase I and II trials in patients with ARF and MOF on continuous venous hemofiltration, the renal assist device demonstrated safety, durability, and functionality (96).
Best for Last—The Promise of a Bioartificial Kidney: Efforts are currently underway in our laboratory to use decellularization–recellularization to create an autologous bioartificial heart, kidney, and other organs. That method may hold significant advantages over conventional biopolymer patches, because it uses the native scaffold. An autologous artificial organ may one day be a part of renal replacement therapy—the question being not "if," but "when."
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CARDIOVASCULAR COMPLICATIONS OF RENAL DISEASE: CELL THERAPY OFFERS HOPE |
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TOPABSTRACTUNMET NEED FOR EFFECTIVE...ENDOTHELIAL DAMAGE IS NOT...BONE MARROW PROGENITOR CELLS...PROGENITOR CELLS AND CELL...CARDIOVASCULAR COMPLICATIONS OF...MOVING FORWARD WITH CELL...REFERENCES |
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Applications of blood mononuclear cells (BMNCs), skeletal myoblasts (SKMBs), and MSCs in CVD have matured into clinical trials. Although not all trials have produced homogenous results, there are substantial reasons for optimism. We have proceeded, at high speed, from improvement of left ventricular function with BMNCs in acute myocardial infarction to a reduction in death and re-infarction, with revascularization at 1 year post infarction (98).
Administration of human SKMBs in patients with heart failure has not proceeded as smoothly as has treatment of acute myocardial infarction with BMNCs, mostly because of ventricular arrhythmias in initial trials (resulting in overt skepticism). In a recent trial, concomitant administration of amiodarone improved this issue (99), but transplantation of SKMBs in the presence of an internal cardioverter–defibrillator is being tested as an alternative strategy. The Myogenesis Heart Efficiency and Regeneration Trial will provide insights into the outcomes of treatment of heart failure with SKMBs.
Evidence suggests that, after injection into the myocardium, MSCs differentiate into cardiomyocyte-like cells (100). Transplanted MSCs engraft at high numbers in an infarcted heart, leading to increased neovascularization and improved regional contractility and overall left ventricular diastolic function (101). Studies of MSCs in acute myocardial infarction and hear failure are ongoing at this time; however, given that these cells are very likely to be "immunoprivileged," good results from clinical trials may lead to rapid development of commercial celltherapy products.
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MOVING FORWARD WITH CELL THERAPY |
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TOPABSTRACTUNMET NEED FOR EFFECTIVE...ENDOTHELIAL DAMAGE IS NOT...BONE MARROW PROGENITOR CELLS...PROGENITOR CELLS AND CELL...CARDIOVASCULAR COMPLICATIONS OF...MOVING FORWARD WITH CELL...REFERENCES |
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Cell therapy is an exciting twenty-first century approach to treating kidney disease. Its potential is great; the hope is significant. As investigators, we have an opportunity to break new boundaries and to conquer new frontiers. However, those achievements will require that we share our ideas, share our data, share our understanding of if and how certain approaches may work, and most of all, share our mistakes so that new modalities emerge and patients benefit.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTUNMET NEED FOR EFFECTIVE...ENDOTHELIAL DAMAGE IS NOT...BONE MARROW PROGENITOR CELLS...PROGENITOR CELLS AND CELL...CARDIOVASCULAR COMPLICATIONS OF...MOVING FORWARD WITH CELL...REFERENCES |
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and IL-1β in patients on maintenance hemodialysis. Cell Mol
Immunol 2006; 3:151
-4.[Medline] by peripheral blood CD3+ cells in intermittent allergic
rhinitis. Inflamm Res 2005;54
: 91-5.[Medline]
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