Although not as common as ASO-induced CLI, AICLI still represents an important subgroup of advanced PAD. Meanwhile, AICLI is characterized with higher incidence in East Asians, much lower onset age, and a higher proportion of No-option ones, which causes a tricky dilemma that conventional revascularization modalities hardly work in these patients with higher levels of expectation of limb salvage, especially in China where there is a large population of patients with AICLI [22, 23]. This is one of the reasons why we selected patients with NO-AICLI as our treatment subjects by SCT. Several previous studies have demonstrated PBDSCs transplantation presented similar outcomes with BMDSCs on limb salvage [18, 24, 25]. However, the sample size was relatively small whether in RCT or single-arm studies [26-29]. To our knowledge, the current series represents the largest number of patients with AICLI treated by PBDSCs transplantation in a single center. Meanwhile, though not designed as an RCT, the baseline characteristics of patients in PCCs and PBMNCs groups are similar in the current retrospective study, which therefore ensures the comparability of outcomes after transplantation of two types of PBDSCs. This study shows that autologous PBDSCs intramuscular transplantation was effective in improving symptoms at an early stage and the clinical efficacy persisted over the long term, without any serious adverse events. A significant improvement on clinical and hemodynamic outcomes was observed in both treatment groups without any significant difference between the groups, validating that the clinical efficacy of PCCs was not impaired due to the cell loss during PCCs isolation.
Several previous clinical studies have demonstrated the beneficial effect of SCT on limb salvage and ulcer healing in NO-AICLI compared with placebo intervention [20, 21, 30-32]. More recent data reported by Gu et al suggested that the 10-year amputation-free survival was 85.3% in patients with TAO treated by BMMNCs, similar with the long-term MAFS in this study [33]. However, such favorable limb salvage rates were not observed in ASO-induced NO-CLI by SCT. More recent studies observed that patients with NO-AICLI benefit much more from SCT than those with ASO-induced NO-CLI [19, 34]. The JUVENTAS trial, which was deemed as a well-designed double-blinded, placebo-controlled clinical trial with a relatively larger sample size, enrolled patients with ASO-induced NO-ACL as the majority of the subjects [35]. Outcomes of this study indicated autologous BMDSCs repetitive intra-vascular transplantation had no effect on the primary outcome of major amputation rates compared with the placebo group (19% vs 13%) [36]. The failure in validating the beneficial effects of BMDSCs administration in the JUVENTAS trial was partly attributed to the high proportion of enrolled aged patients with a high systemic atherosclerotic burden and prevalence of cardiovascular disease. Moreover, our recent published study has reported that age ≥50 years and arterial occlusion above the knee/elbow combined with blood fibrinogen, TcPO2, and the total transplanted CD34+ cell count were independent prognostic factors of the responders to PBDSCs -based therapeutic angiogenesis for NO-CLI, which suggested that ASO-induced NO-CLI characterized with an advanced age and a higher arterial occlusion plane was less likely to benefit from SCT [37]. Therefore, despite that the first published clinical trial on SCT for NO-CLI conducted by Tateishi-Yuyama et al enrolled patients with ASO as the major group of subjects, NO-AICLI seemed to be the better candidate in advanced PAD for SCT according to published data.
An interesting observation in the current study was that all major amputations occurred in Rutherford 5 patients. Despite equally classified into CLI and enrolled as an important subgroup in the present study, Rutherford 4 patients without tissue loss were at a much lower risk of major amputations after SCT than Rutherford 5 patients. Similar observations have been reported in recent years. Benoit et al conducted a prospective double-blinded RCT (2:1 therapy to control) of 48 patients with NO-CLI treated with BMDSCs and found that amputation rates in patients with tissue loss were significantly higher than patients with rest pain only (46.7% vs 5.6%, P= .0029) [38]. A further meta-analysis of the literature was performed and confirmed a difference in amputation rates between patients with tissue loss and rest pain [38]. These outcomes suggested that the number of enrolled Rutherford 4 patients should be limited in a clinical trial regarding SCT for NO-CLI if the primary endpoint of the trial is major amputation rate or MAFS, considering that the much lower major amputation rates in Rutherford 4 patients might dilute the event rate in the overall enrolled population making it difficult to assess the clinical efficacy. In addition, given that the much more favorable outcomes of SCT for AICLI compared with traditional modalities and the significant difference in limb salvage after SCT between patients with rest pain and tissue loss, we highly recommend to use PBDSCs transplantation as the first-choice therapy in patients with angiitis-induced Rutherford 4 class limb ischemia, avoiding that the possibility of progressing into tissue loss during the phase of traditional revascularization treatment which seldom works in AICLI.
As described above, patients with tissue loss correlates a relatively poorer prognosis than patients with rest pain after SCT. Despite this, in the current study, the major amputation rates of 5.5% in patients with tissue loss were still rather encouraging given that these patients were at a huge risk of major amputations with traditional therapies. SCT is characterized with a lower and slow-acting improvement of limb perfusion. Therefore, cell therapy was considered challenging to reverse more acute, diffuse, and critical ischemia. For this reason, Rutherford 6 patients with major tissue loss were excluded from the current study. Almost all clinical trials on SCT for CLI categorized the extent of tissue loss by the Rutherford classification. In studies containing Rutherford 6 patients, Madaric et al reported an association between Rutherford 6 limb ischemia and a negative therapeutic outcome of SCT, which was in consisitent with the results of the PROVASA trial where Rutherford 6 patients at baseline did not benefit from SCT [39, 40]. However, as reported by Mills et al, the Rutherford classification of lower extremity ischemia lacks sufficient detail such like depth of the wound and presence and severity of infection with respect to wound categorization, failing to achieve more precise stratification among patients with tissue loss to aid in selection of the best therapy [41]. The WIFI classification has been regarded as a more objective and accurate classification of the ischemia-induced lower extremity wound based on the degree of ischemia, wound extent, gangrene, and infection, creating a more reasonable gradient of limb perfusion required for wound healing [41]. There is a great potential for application of the WIFI classification in future clinical trials on SCT, as more accurate and detailed stratification of patients with tissue loss would yield a better platform for performing more meaningful comparisons, thereby determining the optimal target population that could benefit from SCT among patients with tissue loss.
The mechanism of improvement of limb ischemia after SCT remains to be investigated. Unlike with mechanical revascularization of relatively larger arteries in conventional open and endovascular interventions, the mechanism of SCT is more complicated and mainly based on angiogenesis by differentiation of transplanted stem cells into endothelial lineage cells composing the structure of micro-circulation [3, 42-44]. Paracrine action and anti-inflammation effects have also recently been demonstrated as important roles in SCT [45-47]. Hemodynamic parameters like ABI and TcPO2 were commonly used in evaluating lower limb perfusion and regarded as effective noninvasive modalities correlating well with clinical severity parameters after revascularization interventions [48]. Considering that the functional mechanism of cell therapy is totally different from revascularization modalities, application of conventional hemodynamic parameters, especially ABI which reflects the blood flow in relatively large size of artery, in predicting the clinical trend after SCT remains to be studied [26, 37, 38]. Fujita et al observed that the time course of the improvement on clinical parameters were not parallel with functional parameters after SCT. It was also reported that the change of ABI after SCT was rather limited despite the great improvement of the clinical status [49]. Similarly, though the mean ABI and TcPO2 values in the current study significantly increased compared with the baseline, whereas the degree of change is still modest compared with the dramatic improvement of ischemic symptoms. These results suggested that clinical severity parameters including ulcer healing and Rutherford class might be more valid endpoints for assessing the treatment efficacy than hemodynamic parameters in trials investigating SCT for CLI at least for now. Hence, with SCT more commonly used in limb ischemia, apart from continuing exploring the mechanism of perfusion restoration after SCT, it is also of great significance to find a safe, fast, and easily repeatable hemodynamic testing modality more appropriate for cell therapy to accurately monitor the change of lower-extremity perfusion, thereby allowing vascular specialists to refine our current understanding of the disease process while assessing wound healing potential, optimizing the clinical decision making, and improving outcomes after SCT.
The advantages of the present study include its well-designed inclusion criteria for SCT, a larger sample size, comparability of both groups of patients in terms of their baseline characteristics, prospective follow-up, and comprehensive assessments. The main limitation of this study is that the analysis was retrospectively in nature although the data were collected prospectively. In addition, while the mean follow-up period of enrolled patients was relatively longer, the proportion of patients completing a 5-year follow-up was small in the PBMNCs group.