Endothelial progenitor cells: a new player in lupus?

Patients with systemic lupus erythematosus (SLE) have a greatly increased risk of cardiovascular disease. There is growing interest in the link between vascular damage and lupus-specific inflammatory factors. Impaired endothelial repair could account for the endothelial dysfunction in this patient group. This review describes the contribution that endothelial progenitor cells could play in the pathogenesis of premature vascular damage in this disease. The methods of isolation, detection, and characterization of endothelial progenitor cells, together with their potential role in repair of the endothelium and as a therapeutic target in SLE, are discussed.

essential anti-thrombotic and barrier functions. Endothelial dysfunction represents a state of deviation from normal to a vasoconstrictive, pro coagu lant, plateletactivat ing, and anti-fi brinolytic state and is thought to have a key role in atherosclerosis and related diseases. Th ough found to be present in all stages of plaque progression, endothelial dysfunction is believed to be the key triggering factor in the initiation of athero sclerosis [6].
Endothelial progenitor cells (EPCs) represent a heterogeneous group of cells that are released from the bone marrow into the circulation and are thought to contribute to vascular homeostasis and endothelial repair. Although modulation of EPC numbers has been identifi ed with cardiovascular disease or vascular trauma (which appears to be predictive of clinical coronary events in the general population), there is still much controversy regarding their true identity and role [7]. Mechanisms by which vascular damage is repaired are not well understood, but previous reports have suggested that two groups of ECs can be detected in the peripheral circulation of individuals with vascular damage. Th e fi rst, a population of EPCs, is thought to be bone marrow-derived and involved in the repair of the endothelium. In addition, the vessel wall itself may be a source of progenitor cells involved in the repair process [8]. A second population of 'infl ammatory' or 'activated' ECs is thought to have been shed from the endothelium following an insult and this population may generate endothelial microparticles derived from damaged ECs and have a potential role in cell signaling [9]. It is therefore believed that damage to ECs can result in endothelial dysfunction and this is thought to be critical in the formation of atheroma [6]. Understanding endothelial repair is a vital step toward developing targeted therapies for this organ system and will enhance our understanding of the mechanisms of atherosclerosis in SLE. EPC quantifi cation may provide a useful marker to aid risk stratifi cation within the context of SLE, in which the Framingham risk prediction model would appear to underestimate the risk of clinical CHD. Th e purpose of this review is to highlight the controversy surrounding the current nomenclature and defi nitions used in this fi eld, so that we can strive toward reaching a consensus on the true identity and the role of this circulating cell type. We will focus on what is known

Abstract
Patients with systemic lupus erythematosus (SLE) have a greatly increased risk of cardiovascular disease. There is growing interest in the link between vascular damage and lupus-specifi c infl ammatory factors. Impaired endothelial repair could account for the endothelial dysfunction in this patient group. This review describes the contribution that endothelial progenitor cells could play in the pathogenesis of premature vascular damage in this disease. The methods of isolation, detection, and characterization of endothelial progenitor cells, together with their potential role in repair of the endothelium and as a therapeutic target in SLE, are discussed.

C haracterization of endothelial progenitor cells
EPCs were fi rst reported by Asahara and colleagues [10] in the late 1990s as CD34 + mononuclear cells with the ability to form island-like colonies in culture. Th ese bone marrow-derived cells were shown in a mouse model, under the infl uence of vascular endothelial growth factor, to have the ability to incorporate into blood vessels with experimentally induced ischemia [11]. Th ese landmark experiments have given an important insight into circulating angiogenic cells and the biology of vascular repair. Two key methods have been used to identify EPCs. Th e fi rst aims to identify cells with specifi c surface markers for EPCs by using fl ow cytometry, and the second addresses in vitro cell culture characteristics.

Flow cytometry identifi cation of endothelial progenitor cells
Flow cytometry has been shown to have good sensitivity, specifi city, and reproducibility [12], and several groups have used fl uorescently conjugated antibodies to specifi c cell surface markers for analysis and identifi cation of EPCs. Th e choice of surface markers for this cell type is the subject of much debate, and a variety of surface markers to identify EPCs have been used by diff erent groups, as reviewed by Timmermans and colleagues [13]. Th e majority of studies using fl ow cytometry have used at least one surface marker to identify a cell of immaturity (from the broad hematopoietic cell lineage) and at least one marker to identify a cell from the EC lineage. Additionally, markers to identify cells originating from a myeloid lineage have been studied.

Immature hematopoietic cells
Th e most widely used marker to identify immaturity is C34 positivity. CD34 is a 110-kDa surface marker expressed on hematopoietic stem cells; however, the exact origin and signals for diff erentiation of these cells remain unclear. CD34 can be detected on approximately 0.1% of circulating mononuclear cells and is thought to act as an adhesion molecule related to endothelial and hematopoietic cells [14]. CD133 is a 120-kDa glyco protein expressed on progenitor and hematopoietic stem cells [15]. Th e exact function of CD133 is not clearly understood; however, the protein is thought to identify an immature population of EPCs. CD133 has been detected on a subpopulation of CD34 + /VEGFR2 + cells in the immature phase and has been shown to be no longer expressed on mature CD34 + /VEGFR2 + in vitro, leading to the conclusion that CD34 + /CD133 − /VEGFR2 + cells represent a more mature diff erentiated population of ECs and that CD133 is a marker of 'early' EPCs [16]. It is possible that CD34 + /VEGFR2 + and CD133 + /VEGFR2 + represent diff erent subtypes of EPCs and that CD34 + /CD133 + / VEGFR2 + can be used as a restricted EPC phenotype. One major disadvantage is that these triple-labeled cells represent a population of cells that are very small and therefore diffi cult to quantify reliably.

Endothelial lineage markers
It is widely accepted that EPCs express vascular endothelial growth factor receptor-2 (VEGFR2) -or kinase insert domain receptor (KDR) in humans -and the roles of the VEGF receptor family in angiogenesis and vasculogenesis are well recognized [17]. Other endothelial antigens include CD31 and von Willebrand factor. CD31 or platelet endothelial cell adhesion molecule-1 (PECAM-1) is known to have roles in angiogenesis, platelet function, thrombosis, and regulation of leukocyte migration through vascular walls [18]. von Willebrand factor has a role in platelet adhesion to the vascular wall injury and is a carrier protein of coagulation factor VIII [19].

Myeloid progenitor cells
Subsets of CD14 + mononuclear cells have been shown to express EC markers such as CD31 [20] and, following stimulation by angiogenic factors, express von Willebrand factor and KDR [21]. CD14 + /CD34 + cells display clonogenic properties in vitro, and importantly several recent studies have shown that CD14 + cells contribute to vasculogenesis [22,23]. Additionally, clinical correlations of CD14 + subsets suggest a reduction in the number of patients with left ventricular dys func tion [24]. However, in contrast to CD34 + /CD133 + cell numbers, CD14 + cell numbers appear not to be reduced in patients with clinical CHD or related risk factors [25]. While myeloid progenitor cells are detected in the circulation and appear to share many EP C characteristics, preliminary clinical studies would suggest a functional distinction between these subsets.

Cell-counting approaches for endothelial progenitor cell analysis
Another important factor to consider is the units in which cell populations are expressed. Some studies have favored expression of cells as a count per unit of volume of blood. EPCs are estimated to represent between 0.0001% and 0.01% of peripheral cells, making them a 'rare event' in fl ow cytometric analysis. Given the rarity of these events in peripheral blood and the variations in hemodilution between individuals (particularly in patients with cardiovascular disease), this measure can be misleading. Others have identifi ed the proportion of total cytometry events acquired. Th is approach allows the enumeration of a proportion of a consistent number of cells counted by using fl ow cytometry (for example, proportion of 500,000 labeled cells analyzed). Quantifying the total number of cells may better take into account the potential for lymphopenia in patients with SLE.
Identifi cation and quantifi cation of EPCs by using fl ow cytometry can be challenging. Despite this, associations with disease states, including coronary heart disease, have been reported [7]. Specifi c and sensitive markers and properties to discriminate and defi nitively distinguish subsets of EPCs within the hierarchy of progenitor cells from mature ECs are beginning to emerge, and it has been wisely suggested that the fi eld should adopt alternative terms for each specifi c subset. An alternative approach has been to identify viable cells with proliferative capacity in culture.

Cell culture techniques to identify endothelial progenitor cells
While fl ow cytometry is an excellent technique for the enumeration of these low-frequency EPCs in peripheral blood, cell culture, by using specifi c surface markers, has the advantage of allowing the expansion of cell numbers and enabling qualitative characteristics to be gathered. In the landmark publication cited above, Asahara and colleagues [10] described EPCs as in vitro cells with the ability to adhere to fi bronectin and form colonies or colony-forming units (CFUs) as early as after 3 days in culture. Cells in culture were shown to have typical EC characteristics such as uptake of Dil-acLDL (Dil-labeled acetylated low-density lipoprotein) and expression of surface markers, including CD31, KDR, CD34, and Tie-2 [10]. Th is initial study used CD34 + -and KDR + -enriched cells in culture; however, since bone marrow EPCs fail to expand and diff erentiate in vitro, several investigators have subsequently used unfraction ated mononuclear cells and included a pre-plating stage to increase specifi city and exclude other cell types. Between 4 and 9 days after culture, this modifi ed protocol identifi es CFUs termed 'early outgrowth' colonies or CFU-Hill [26] ( Figure 1). Another commonly used proto col identifi es 'late outgrowth' colonies derived from adherent mononuclear cells following 7 to 21 days in culture [27].
Early-and late-outgrowth cells share a number of features, including uptake of Dil-acLDL and expression of CD31, but diff er with regard to surface expression of CD133. Th ey are likely to represent EPCs at diff ering stages of maturation. It should be noted that EPCs identifi ed by using cell culture are unlikely to represent the same cells identifi ed by using fl ow cytometry; however, with the aid of proteomic studies, knowledge of the precise nature of these diff ering EPC subtypes is becoming more refi ned [28]. Th e ultimate aim of understanding EPC biology rests with elucidating their role in vivo and whether they might be prognostic or diagnostic factors. Several groups have successfully used CFU number as a biomarker in several diff erent disease states, including CHD-related diseases [29] and infl ammatory rheumatic diseases [30], so fur ther ing our understanding of the in vivo role of these subsets of circulating progenitor cells holds considerable promise for their clinical application.

Working toward a consensus for the identity of the endothelial progenitor cell
Th e observations reported by Asahara and colleagues [10] undeniably resulted in a paradigm shift within the fi eld of vasculogenesis by highlighting the role of EPCs in vascular repair. Prior to experiments conducted by Asahara and colleagues in 1997, it was largely believed that 'vasculogenesis' or the de novo formation of vessels from uncommitted precursor cells occurred only in the developing fetus. However, more than a decade after the initial landmark experiments, there remains a lack of standardization regarding the identifi cation and defi nition of EPCs. Th e use of diff ering surface markers and cell culture techniques between studies makes comparison of results diffi cult. Many groups have used CD34, CD133, and VEGFR2 surface markers in various combinations and successfully identifi ed inverse correlations to disease states; however, recent reports have challenged the origin of cells with these markers and provide evidence that some of these cell populations, initially described as EPCs, are in fact myeloid-derived hematopoetic precursors [31]. Some investigators have advocated the incorporation of a common leukocyte marker CD45 within fl ow cytometric analysis and suggested that EPCs can be identifi ed only within the CD45 − fraction of cells [32].
Cells enumerated by using fl ow cytometry are insuffi cient in number to allow the biology of the EPCs to be examined. Culture methods provide a means by which a large number of cells can be isolated but may not be an accurate refl ection of EPCs in the in vivo situation. In addition, recent studies have demonstrated that other immune cell types may have the ability to mimic the morphological characteristics of EPCs in culture [33,34]. A degree of plasticity at the progenitor cell level in response to diff ering biological signals may explain some of these fi ndings. It is possible that cells identifi ed as EPCs are not directly incorporated into the regenerating endothelium but exert indirect but equally important angiogenic paracrine eff ects that result in endothelial repair [35]. Clearly, there is a need for studies to examine the biology of EPCs as well as elucidation of homing and diff erentiation signals to enable a better understanding of this unique cell type and the role of EPCs in health and disease. More methodological consistency and evidence of robust reproducibility and reliability of techniques are needed in future studies with clinical outcomes.

Clinical impact of endothelial progenitor cells and cardiovascular disease
Within the human clinical setting, a recent study has demonstrated positive outcomes in seven out of nine patients who have critical limb ischemia and who were able to avoid amputation following the intramuscular injection of autologous CD133 + cells into the aff ected limb [36]. Similar results have been observed by other groups that used autologous peripheral stem cells or unselected bone marrow mononuclear cells [37,38]. Importantly, these studies have demonstrated the safe use of cell therapy methods. Within the context of CHD, several clinical trials in the last 5 years have assessed the eff ectiveness of cell therapy as an adjunct to bestavailable care after an acute myocardial infarction. A controlled trial demonstrated short-term improvement of left ventricular function following intracoronary infusions of autologous bone marrow progenitor cells and sustained clinical benefi t with reduced rates of death, clinical coronary event, and heart failure at 2 years of follow-up [39]. One small controlled study undertook intracoronary infusions of enriched CD133 + cells and demonstrated a small short-term (4 months) benefi t of left ventricular function but noted an increase in coronary events (11 compared with four in the control group) [40].
Cell therapy is a growing and intriguing fi eld, and there is evidence in several animal models that stem cell transfusion and systemic transfusion of expanded ex vivo EPCs can induce functionally intact endothelial regeneration [41]. Future studies need to consider source, puri fication methods, timing, and dosing of cell infusions. Th e sample sizes to date have been too small to assess the infl uence of baseline infarct size or severity of left ventricular dysfunction. Although the benefi ts observed to date have been modest, cell therapy in the context of acute myocardial infarction appears to be safe and shows much promise. Th e long-term effi cacy and safety require further clarifi cation. Whether stem cells do indeed contribute to vasculogenesis directly or result in cardiomyo cyte replenishment or mediate an eff ect indirectly via trophic eff ects is not clear from clinical cell therapy studies in CHD but may be elucidated by taking a step back to laboratory experiments.

Factors infl uencing endothelial progenitor cell generation and turnover
Endogenous factors that infl uence the number and function of EPCs are of much interest. Exposure to diff erent lifestyle factors and several pharmacological agents has been shown to infl uence EPC levels. Both smoking cessation and increased physical activity have been shown to be benefi cial to EPC number [42,43]. Studies have demonstrated an increase in EPC number following administration of HMG-CoA (3-hydroxy-3-methyl glutarylcoenzyme A) reductase inhibitors independently of choles terol reduction and this might be another mechanism by which statin medications exert a cardioprotective eff ect [44]. Angiotensin-converting enzyme inhibiting agents and angiotensin 2 receptor blockers have also been shown to have a positive eff ect on EPC number and function in the clinical setting [45]. Other pharmacological strategies that have been shown to enhance EPC number include chemokines and cytokines such as erythropoietin and VEGF [46,47].

Cardiovascular disease, endothelial progenitor cells, and systemic lupus erythematosus
Over the past decade, quantitative analysis of EPCs by using both fl ow cytometry or cell culture experiments or both has shown that EPC numbers vary in relation to cardiovascular risk factors and atherosclerotic disease in the general population [7,48], and more recently these fi ndings have been extended to patients with SLE (Table 1). A variety of markers have been used to identify and characterize EPCs within SLE. Westerweel and colleagues [49] studied 15 women who had SLE and a mean age of 37 years (± 3 years). Th e women, who had inactive disease for at least a year prior to the study and were taking low doses of predniso lone (<10 mg), were compared with 15 age-matched healthy females who were 37 (± 3) years old. Th e authors demonstrated a reduction of the overall CD34 + fraction of cells as well as a reduction of CD34 + KDR + EPCs in patients with SLE compared with healthy controls. EPC number correlated positively with ankle brachial pressure index (R 2 = 0.532, P = 0.042) and inversely with cholesterol level (R 2 = −0.587, P = 0.021). Th ere was no association with disease-related factors, including disease activity, therapy, or serological fi ndings. Early-outgrowth colony formation/CFU was not diff erent between the two groups, and the infl uence of CHD risk factors was not assessed [49]. Denny and colleagues [50] defi ned EPCs by using CD34/CD133 cell markers in lineage-negative cells in SLE compared with controls. Th e overall cohort consisted of 135 patients with SLE and 60 controls with mean ages of 38 (± 3) and 41 (± 1) years, respectively. It should be noted that 55% of the controls and 95% of patients with SLE were females. Th e authors demonstrated that a reduction of CD34 + / CD133 + EPCs in SLE compared with controls remained statistically signifi cant when the analysis was limited to females only. In addition, the authors documented a correlation of EPCs with the SLE Disease Activity Index (SLEDAI) (R 2 = 0.42, P <0.01). EPCs were shown to have a reduced capacity to diff erentiate into mature ECs after 7 to 21 days in culture [50]. Moonen and colleagues [51] assessed CD34 + /CD133 + EPCs in 44 patients who had quiescent SLE and who were 40 ± 12 years old and in 35 age-matched female controls who were 41 ± 12 years old. Th e authors demonstrated a reduction of CD34 + /CD133 + EPCs and CFUs in SLE compared with controls. No correlation was noted between EPC or CFU number with regard to disease-related factors. CHD-related factors were not analyzed [51]. Whereas these studies all noted a reduction of EPCs in SLE, they varied with regard to clinical correlates. Th e populations studied were of a similar age. In a recent study, Baker and colleagues [52] investigated the correlation of EPCs with subclinical atherosclerosis. In this small study, a reduction of CD34 + / CD133 + /VEGF + cells was demonstrated between patients with SLE and age-matched healthy controls; however, the numbers of CD34 + /CD133 + cells were similar between the groups. Neither coronary artery calcifi cation scores nor carotid intima media thickness was correlated to EPC number [52]. Lastly, Deng and colleagues [53] defi ned EPCs as cells positive for CD34, Dil-acLDL, and UEA and found no statistically signifi cant diff erence between patients with active SLE and age-and gendermatched controls; however, there was impairment of various functional properties of cells in SLE, including proliferation, adhesion, and migration capacity, and increased expression of pro-infl ammatory cytokines [53].
Other groups have also shown that EPC number, as measured by fl ow cytometry, is similar in SLE and controls and therefore focused on CFU formation. Grisar and colleagues [54] studied 31 females who had SLE and who were 35 (± 2) years old and compared them with 14 healthy female controls who were 39 (± 5) years old. Th e authors found no diff erence in CD34 + /CD133 + /KDR + cells between SLE and controls. EPCs did appear to correlate with previous renal disease but not with other clinical parameters. Th e authors also reported on CFU number in 13 patients with SLE and 12 controls (ages were not reported in this subgroup) and found that CFU number was similar between the two groups. However, the authors reported a reduction in migratory and adhesive properties of cells in patients with SLE [54]. Ablin and colleagues [55] reported no signifi cant diff erence in CFU number of 28 patients who had SLE and who were 38 (± 15) years old and 50 controls who were 43 (± 18) years old. Patients with SLE included males, and patients with a history of high-dose steroid, cyclophosphamide, or mycophenolate mofetil use were excluded, suggesting that the group studied may have been of a 'milder' phenotype. Th e authors did note an impaired adhesion of cells to fi bronectin in patients with SLE, and this was interpreted as a marker of impaired function. CFU number or adhesion scores did not vary with disease activity. Similarly, Lee and colleagues [56] enumerated CFUs from 70 patients with SLE and 31 controls who had median (interquartile range) ages of 35 (27 to 45) and 36 (26.5 to 44) years, respectively. Ninetytwo percent of patients with SLE and 84% of the controls were female. Th e authors demonstrated that the signifi cantly reduced number of CFUs in SLE compared with controls was independent of leukopoenia. Th e authors also quantifi ed CD34 + /KDR + cells by using fl ow cytometry and demon strated a reduction in patients with SLE compared with controls. Neither CD34 + /KDR + cells nor CFUs were found to have any signifi cant correlation to disease-related factors or CHD risk factors. However, CFUs did have an inverse correlation with C-reactive protein [56]. Ebner and colleagues [57] measured EPCs in 19 women who had SLE and who were 36 (± 6) years old and in 19 female controls who were 37 (± 8) years old. Th e authors demonstrated reduced CD34 + /KDR + cells in SLE but an increased level of CD133 + /KDR + cells in comparison with controls. Clinical correlates were not analyzed. Th e authors suggested that low levels of endothelial damage may result in increased mobilization of immature CD133 + /KDR + cells.
Overall, studies of EPCs measured by using fl ow cytometry have consistently demonstrated a reduction in CD34 + /KDR + cells in comparison with controls [49,56,57]. Two studies have also demonstrated a reduction in CD34/CD133 + cells [50,51]. Grisar and colleagues [54] measured the smaller population of CD34 + /CD133 + / KDR + cells and found no diff erence between SLE and controls. It may be that CD133 expression is rapidly lost in this population of cells. Similarly, our group found no diff erence in the number of CD34 + /CD133 + cells between 56 patients with SLE and 48 healthy matched controls (0.03% (± 0.02%) versus 0.02% (± 0.02%), P = not significant) but did demonstrate a signifi cant reduction of CFUs in patients with SLE (7 (± 6) versus 12 (± 12), P = 0.05). Neither CFUs nor CD34 + /CD133 + cells were related to SLE or CHD risk factors [58]. Possible explanations for the discrepancy of the fi ndings between diff erent studies may be related to small sample sizes, diff erences within the study population, or method of EPC isolation and identifi cation. A crucial consideration is the diff erence in the methodology used by diff erent groups to quantify cells. Some groups have included absolute counts per volume of blood [49], whereas others have measured percentage of the mononuclear cell population [54]. Most EPC quantifi cation methods using fl ow cytometry identify cells within the lymphocyte gate or involve a lymphocyte separation step. However, lymphopenia among patients with SLE may possibly result in an over-or underestimation of EPCs in this condition. In contrast, it is also possible that cytopenias may result in bone marrow stimulation and the increased production of progenitor cells.
Th e results of culture studies have been more consistent, and whereas some studies have had small sample sizes, the larger of these studies [56] demonstrated a reduction in CFUs in SLE compared with controls. Th e smallest of the studies [51] included patients with a mild phenotype and did not fi nd any diff erence in CFU number in patients with SLE, suggesting that CFU number may be related to overall infl ammatory burden over time. Th e discordance between fi ndings using CFUs and EPCs may provide complementary rather than contradictory results. It is likely that CFUs measure a distinct myeloid-derived cell (now termed earlyoutgrowth EPC) compared with the population of CD34 + or CD133 + cells measured by using fl ow cytometry, and this may explain the discrepancy observed when these two measures were used. CFU measures are also likely, to some extent, to be a refl ection of EPC functional ability to migrate to a cluster of cells and adhere to fi bronectin. It may well be that, while progenitors are generated in adequate numbers by patients with SLE, their ability to mature and function diff ers from healthy individuals, resulting in fewer eff ective cells in patients with SLE and thus contributing to increased endothelial dysfunction. Th is hypothesis is supported by the observation that the migratory and proliferative capacities of EPCs are both reduced in SLE [54,57].
Studies in SLE to date have been too small to detect clinical correlates to EPCs or CFUs. All studies have been cross-sectional and primarily included patients with quiescent disease, although some groups have selected patients with active disease for study; however, the infl uence of infl ammatory burden over time or vasculitic manifestations remains unclear. Th e eff ect of anti-metabolite therapies such as azathioprine and mycophenolate remains unclear, and it is possible that these drugs may be toxic to CD34 + cells. Furthermore, Grisar and colleagues [54] documented EPC dysfunction in patients receiving anti-malarial therapy, although the small sample size makes robust conclusions diffi cult.

Interferon-alpha, endothelial progenitor cells, and systemic lupus erythematosus
An interesting link between interferon-alpha (IFN-α) and EPCs is emerging within the SLE literature and may provide a mechanistic explanation for some observations. IFN-α is a member of the type 1 IFN cytokine family. Th ese proteins have a role in immune defense, and increased levels are observed in the context of viral infections. Th ey also have a role in physiological growth and diff erentiation [59]. IFN-α has been demonstrated to inhibit angiogenesis and vascular tumor growth [60]. Increased levels of IFN-α can be detected during a fl are in SLE and have a central role in SLE pathogenesis [61]. Direct quantifi cation assays of IFN are unreliable, and therefore downstream eff ects of IFN are used as surrogate markers. Lee and colleagues [56] demonstrated a reduction of CFU formation in those SLE patients with high expression of the IFN-I-stimulated gene MX1 (n = 19) and an inverse correlation of MXI with CFUs. Importantly, high IFN-I levels were associated with impaired endothelial function as measured by using peripheral arterial plathysmography [56]. Similarly, Denny and colleagues [50] demonstrated that supernatant from SLE ECs caused increased induction of IFN-inducible genes in comparison with healthy controls. Furthermore, the addition of recombinant IFN-α had a toxic eff ect on CD133 + cells in culture, resulting in CD133/CD34 + apoptosis. Th e authors used an anti-IFN-α monocloncal antibody to block IFN pathways in SLE EPCs in culture and demonstrated a normalization of function [50]. In addition, studies from the Kaplan laboratory [62] enabled the proposal of an interesting concept by using data from NZB/W lupus-prone mice: the authors showed that type I IFNs may play an important role in premature vascular damage and, potentially, atherosclerosis development in SLE. Based on the fi ndings from a study by Lian and colleagues [63], they suggest that the bone marrow niche may be exposed to elevated levels of IFN more than other peripheral compartments and that this, in turn, could eff ect EPC properties with regard to their mobilization and homing to damaged tissue [63]. Th ese early observations require further investigation as they may provide a pathway to restoration of EC function in SLE.

Implications and future directions
Initial fi ndings from studies suggest the potential use of EPCs as a novel biomarker in risk stratifi cation of CHD or some insight into future organ-specifi c therapeutic targets in SLE. However, many questions remain un answered. Larger studies are required to verify these preliminary results and provide robust conclusions, in particular, to ascertain the infl uence of drugs such as statins. Longitudinal studies that focus on assessment before and after commencement of anti-metabolite therapies are needed. Given the low prevalence of SLE and diversity of clinical manifestations and therefore exposures, it is envisaged that multi-center collaborations should be encouraged.
Several groups have demonstrated reduced EPCs as measured by culture methods in SLE, but the precise characterization of EPCs requires further clarifi cation of the mechanisms underpinning abnor malities in EC function (for example, homing signals to sites of damaged endothelium require further investiga tion). To date, studies have compared patients with SLE with healthy controls. However, a more relevant compari son group may be a cohort with another chronic infl am matory condition to elucidate the relative contri bu tion to atherosclerosis in SLE.
Studies have suggested that type I IFN may contribute to EPC dysfunction in SLE and may suggest a potential role for their rescue by type I IFN-blocking agents (either IFN-antagonizing antibodies or receptor blockers) currently under development. Implications of promoting vasculogenesis/blocking type I IFN, however, need to be considered in the wider clinical context. Th ere is potential for exacerbation of viral diseases or retinopathies. Also, the eff ect on tumor formation in this vulnerable patient group or the eff ect on manifestations such as infl ammatory arthritis, in which it is thought that pathological angiogenesis occurs, is unknown.
Laboratory observations to date would suggest that there is an excess of EPC dysfunction in SLE, and clinical studies in the 'general' population would suggest that this can be of relevance to clinical coronary events. However, within SLE, prospective studies using clinical outcomes are required (a) to determine the consequence of low EPC number, (b) to establish whether the ex vivo situation is of relevance and refl ects in vivo eff ects, (c) to determine whether low EPC number does contribute to an excess of CHD, and (d) to assess whether therapeutic targeting of EPCs will enhance vascular repair and improve long-term outcomes in SLE.