Several years ago, when I joined Dr. Ke Cheng’s lab, the team was working on new methods to improve the efficiency of stem cell delivery for treating heart diseases. Intracoronary injection is a safe and popular way to deliver cells to the heart, but one of the biggest challenges hampering its therapeutic efficacy is the low cell retention in the heart after infusion. On average, less than 10 cells out of 100 remain in the heart 24 hours later1. The address this problem, the Cheng Lab had been focusing on the modification of stem cell surfaces for targeted delivery. One approach was to arm the stem cells with antibodies against cardiac injury biomarkers, so that the antibodies could guide the infused cells to the injury2. While this approach could moderately improve cell retention, clinical translation met several limitations: 1) antibodies from commercial sources are quite expensive; 2) the Fc region of antibodies is immunogenic, turning the injected cell into a target of the reticular endothelial system; 3) the expression of injury biomarkers in the heart are only transient, limiting the time window for efficient targeting.
While thinking about these problems, I wondered if there is something in our body that can innately find tissue injury. One day, as I was preparing my food, I accidentally cut my finger with a broken goblet. Of course, there were a few drops of blood and I applied a bandage. The next day, the incision was sealed by clotted blood because of platelets. Platelets! That was the “aha!” moment. Platelets, with their adhesion molecules, are specialized in finding injuries and wounds in our body. Could we use platelets to guide stem cells to the infarcted heart? A quick round of literature search further strengthened my hypothesis. A clinical trial ongoing in Europe at the time aimed to examine the pattern of blood cell populations in post-heart attack patients3. Interestingly, they found that platelets spontaneously form co-aggregates with CD34-positive cells (endothelial progenitor cells, one type of endogenous progenitor cells that can contribute to the healing of the heart). Analysis showed that the numbers of such co-aggregates correlate positively with the prognosis and recovery of those patients.
I quickly devised a plan: isolating platelets from the blood and co-incubating them with cardiac stem cells (CSCs), a cell type that we used in the lab for injection and therapeutic heart regeneration. In this way, I could manually make those platelet-stem cell co-aggregates and use them for in situ therapy. Alas, this plan was rejected by myself only a few hours later: as we all know, live platelets trigger the coagulation process in the blood, which is something to avoid in heart attack. Indeed, patients need to take anti-platelet medications after recovering from the procedures precisely to treat heart attack. So I had to keep the ‘good bits’ I needed from the platelets (the adhesion molecules on their membranes), but eliminate the harmful effects from activated platelets.
Luckily the bioengineers and cell biologists in the Cheng Lab gave me some hints on how to harness the needed platelet components. Instead of using live and intact platelets, I could actually generate platelet membrane nanovesicles (PNVs) from platelets. These tiny PNV bubbles are like ‘ghost’ platelets, inheriting the ‘coat’ from the platelets but lacking the intracellular machinery to stimulate clotting. Through a fusion process, we were able to decorate CSCs with PNVs to make hybrid PNV-CSCs which carried the regenerative payload from CSCs and the adhesion molecules from platelets. After verifying this modification strategy is nontoxic to CSCs and did not affect their potency, we created a rat model of heart attack to test the safety and efficacy of PNV-CSC therapy. The results indicated that PNV coating increased the retention and therapeutic benefits of CSCs. Furthermore, we translated the findings into a clinically-relevant pig model of myocardial infarction.
This project was a result of collaborative efforts of an interdisciplinary group with members from multiple units including Biomedical Engineering, Pharmacy, Cardiology, and Veterinary Medicine here at the NC State University and UNC-Chapel Hill. We think this PNV decorating approach represents a new avenue for targeted cell delivery which requires no genetic medication of the cells.
Our paper: Tang, J. et al. Targeted repair of heart injury by stem cells fused with platelet nanovesicles. Nat. Biomed. Eng. doi: 10.1038/s41551-017-0182-x.
1. Sanganalmath, S. K. & Bolli, R. Cell therapy for heart failure: a comprehensive overview of experimental and clinical studies, current challenges, and future directions. Circ. Res. 113, 810-34 (2013).
2. Cheng, K. et al. Magnetic antibody-linked nanomatchmakers for therapeutic cell targeting. Nat. Commun. 10, 4880 (2014).
3. Stellos, K. et al. Circulating platelet-progenitor cell coaggregate formation is increased in patients with acute coronary syndromes and augments recruitment of CD34+ cells in the ischaemic microcirculation. Eur. Heart J. 34, 2548-2556(2013).