Discussion

Major findings in this study were as follows: (1) In subjects with stable CAD, circulating CD34⁺CD133⁺ cell levels decreased with age. (2) In subjects with AMI, (a) there was no significant relationship between the number of mobilised CD34⁺CD133⁺ cells and age, and (b) peak CK level and sex (male) were negatively associated with the number of mobilised CD34⁺CD133⁺ cells. (3) As for the functions of CD34⁺CD133⁺ cell-derived EPCs, both the migratory activity and the network-forming ability with HUVECs were superior in subjects with AMI to those with stable CAD.

Circulating EPCs are known to play an important role not only in ischaemia-induced angiogenesis but also in healing the injured endothelium.1 3 4 However, the results from clinical trials involving administration of autologous bone marrow cells to patients with critical limb ischaemia or myocardial infarction are not consistent.15–18 One of the reasons for this discrepancy may be related to the fact that most patients were targeted at older subjects, whereas in animal models, most experiments were performed in young mice.19–21

Circulating CD34⁺ or CD133⁺ cells are reported to decrease with age in subjects with stable CAD.3 This explains, in part, the varying efficacy of autologous bone marrow cell transplantation into critical ischaemic limb or ischaemic myocardium. However, in the present study, the inverse relationship between age and circulating CD34⁺CD133⁺ cells was not confirmed in subjects with AMI. Our findings are consistent with the description about CD34⁺/CD117⁺ and CD34⁺/CXCR4⁺ cells in AMI by Wojakowski et al.22 These findings imply that, although circulating CD34⁺CD133⁺ cells decrease in patients with stable CAD, some patients still have the capacity to release CD34⁺CD133⁺ cells into the circulation from bone marrow in the event of AMI. Signals from ischaemic tissue may be responsible for stimulating the release of CD34⁺CD133⁺ cells. So far, VEGF is known to be one of the major angiogenic cytokines released from ischaemic tissues.3 However, VEGF did not correlate with circulating CD34⁺CD133⁺ levels in the present study. Because VEGF levels increase quickly after the onset of AMI and return to the normal range,23 the relationship between CD34⁺CD133⁺ levels and VEGF could not be observed as late as day 7. Alternatively, other angiogenic chemokines such as Angiopoietin-124 and SDF-13 may be a dominant factor affecting the mobilisation of CD34⁺CD133⁺ cells around 7 days after the onset of AMI.

In the present study, the migratory activity of EPCs and network-forming ability of EPCs were superior in subjects with AMI compared with those with stable CAD. In AMI, increased inflammatory and angiogenic cytokine/chemokine, such as VEGF13 and SDF-1,3 might improve the functions of the EPCs mobilised from bone marrow. A similar phenomenon was reported by Vöö et al, who found an enhanced chemotactic response of CD133⁺ circulating progenitor cells following AMI.25

Recently, the precise origin of these EPCs has become the subject of debate.26 Nonetheless, the importance of EPCs in maintaining vascular homeostasis remains unchanged, because they have been used clinically to induce angiogenesis,15–18 We can safely state that circulating CD34⁺CD133⁺ cells and these cell-derived EPCs are maintained in at least some AMI subjects.

The peak CK level and sex (male) were negatively associated with CD34⁺CD133⁺ cell mobilisation in AMI. Our findings are consistent with the study by Wojakowski et al indicating that the number of mobilised stem cells is negatively correlated with cardiac necrosis markers (Troponin I, CK-MB), and that the peak number of CD34 cells was shown to be significantly lower than in patients with LVEF≤40%.22 The high value of peak CK in the present study was associated with lower LVEF, which made for the mobilisation of CD34⁺CD133⁺ cells after AMI decreased.

The fact that the number of mobilised CD34⁺CD133⁺ was lower in men was also consistent with the report by Hoetzer et al, which clarified that the EPC colony-forming capacity and migratory activity were higher in women than in men.27 Although oestrogen is known to mobilise EPC from bone marrow, our female subjects were all in postmenopause. Thus, it is unlikely that oestrogen affected the mobilisation of EPCs in the present study. The circulating EPC level might be intrinsically higher in women than in men, which would explain one mechanism of slow atherosclerotic progression in women.

Limitations of our study include the fact that we measured CD34⁺CD133⁺ cell levels and EPC levels at a single time after the onset of AMI. However, we previously confirmed that EPC and CD34⁺ levels peaked during 4–7 days after the onset.13 Therefore, the number measured on day 7 after the onset could be used to indicate the circulating CD34⁺CD133⁺ cell level in subjects after AMI. Second, other types of EPCs derived from myeloid and mesenchymal were not taken into consideration. Serial blood samplings would be warranted to elucidate the behaviour of CD34⁺CD133⁺ cell and EPC levels in both CAD and AMI. Additionally, different types of EPCs should also be investigated.

In conclusion, the number of circulating CD34⁺CD133⁺ cells decreased with age in non-smoking subjects with stable CAD, but not in subjects with AMI. Furthermore, the functions of CD34⁺CD133⁺ cell-derived EPCs in subjects with AMI did not deteriorate, even in older people. Taken together, some older adults with CAD may well have the capacity to mobilise and utilise residual EPCs in the event of AMI, even if the circulating CD34⁺CD133⁺ cell level is low.