Response by Luo et al to Letter to the Editor by Davidson, et al

In Response

We thank Takov et al for their interest in our study¹. In their letter to the editor, Takov et al raise three concerns regarding our study.

First, Takov et al pointed out the necessity of including the vehicle control and the importance of administration technique in the in vivo mouse myocardial infarction experiments. Previous study has indicated that vehicle control and sham-operated control show similar effect on post MI healing². We also completely agree with Takov et al that administration technique is very important in cell transplantation. In our study, we intramyocardially inject 1×10⁵ synMSC particles or MSCs in 50 μl PBS into 4 sites in the border zone of infarction with a 30-gauge needle. This technique has been well established in various previous studies in our labs^3-5.

Secondly, Takov et al questioned about our claim regarding the superiority of synMSCs over MSCs in immune tolerance. They asked about the unrecognition of synMSCs by macrophages since the MSC membrane coating on synMSCs might expose the inner (cytosolic) side located phosphatidylserine to the outer side. This is a great question and we admit that our process did not rule out the presence of a “eat me” signal on synMSCs. However, to allow immune reaction to propagate, cell death is needed. In the case of synMSC, since there is nothing to “eat”, such reaction could be amplified. The rationale to inject freeze/thawed synMSCs or MSCs is to compare their cryopreservation stability. Cryopreservation is needed for many cell therapy practice. However as no cryopreservation method is perfect, cell death and/or apoptosis is inevitable⁶. Also, temperature fluctuations occur during the storage/shipping process⁷. In such cases, the dead or dying cells will be produced and once administered they will be tagged by macrophages. Since synMSCs are not living cells at all, there will be no cell death signal to be generated for inflammatory cells to tag. Finally, Takov et al mentioned that recruited macrophages by MSCs conditioned medium observed at day 7 and day 14 post-myocardial infarction may be beneficial in wound healing and angiogenesis⁸. We agree with that. Indeed, because synMSCs contain the MSCs conditioned medium, the recruited macrophages at late phase may be a potential mechanism of cardioprotective effects.

Thirdly, Takov et al are also interested to know how the exosomes contained within the conditioned medium contribute to the regenerative potential of synMSCs. Since synMSCs contain concentrated conditioned media from MSCs and exosomes are a part of MSC secretome, we expect to have MSC exosomes in synMSCs, along with other soluble proteins. One limitation of study is that we did not delineate the relative roles of exosomes and soluble proteins in synMSCs to promote heart repair.

After all, we appreciated the comments from Takov et al, and we believe with some further optimization and development, synthetic stem cells represent a promising strategy and alternative to stem cell therapies.