Development of a transendothelial shuttle by macrophage modification.

One of the limiting factors in tissue regeneration, particularly in the context of chronic disease such as myodystrophy, motor neuron disease, sarcopenia, and cardiovascular disease, is limited availability of stem cells. We propose employing autologous macrophages to deliver stem cells, thereby facilitating tissue regeneration, by a novel and relatively non-invasive therapeutic intervention. Circulatory monocytic cells of M1 phenotype have capacity for transendothelial migration to infiltrate damaged tissue, making them ideal delivery vehicles. However, in order to deliver viable stem cells, these macrophages must undergo phagosome maturation arrest. Our aim was to induce phagosome maturation arrest in prepolarised M1 macrophages, whilst maintaining capacity for phagocytic engulfment (including phagosome formation) and transendothelial migration. Primary human M1 macrophages were treated with a wortmannin-concanamycin A-chloroquine cocktail to induce arrest. Modified cells were allowed to ingest 4.5 μm protein-coated fluorescent latex beads (simulated stem cells), before migratory capacity in response to MCP-1 was assessed over a 2-hr period in a Transwell co-culture system. Data indicate that phagosome acidification (as indicated by pHrodo®) was prevented in treated cells, effectively limiting digestion of ingested "cargo" (1.23 ± 0.26% vs. 7.52 ± 0.98% in controls; p < .0001). Neither phagocytic engulfment capacity (68.67 ± 3.51% vs. 61.19 ± 4.68%) nor migratory capacity (70.14 ± 12.6 vs. 72.86 ± 16.0 migrated cells per well) was compromised. We conclude that macrophages were successfully modified into transendothelial delivery vehicles, without compromising required functionality. This delivery system can be exploited to develop a novel method for focussed stem cell and/or drug delivery.