Altered immune environment in long acellular nerve allograft
Deng Pan, B.S.1; Ellen Larson, B.S.1; Susan Mackinnon, M.D.2; Matthew Wood, Ph.D.3; (1)Washington University in St. Louis, Saint Louis, MO, (2)Division of Plastic and Reconstructive Surgery, Washington University School of Medicine, Saint Louis, MO, (3)Washington University School of Medicine, Saint Louis, MO
Peripheral nerve injuries are common and often require surgical repair. Larger nerve defects require the use of autologous nerves or acellular nerve allografts (ANAs) to bridge the injury. ANAs are clinical alternatives to autologous nerve grafts, and obviates the need for additional surgery to harvest donor nerves from patients themselves. However, axonal regeneration is significantly hindered across long ANAs. While axons regenerate across 2 cm ANAs robustly, regeneration across 4 cm ANAs is severely limited. To determine how longer ANAs impede regeneration, we examined cellular populations within short (2 cm) and long (4 cm) ANAs at 2 weeks and 4 weeks post-graft. We observed that at 2 weeks, there were similar number of cells in long and short ANAs, and that macrophages (cd68+) consists most cells in ANAs of short or long length (42% vs 38%). At 4 weeks, macrophages in the short ANAs makes up a significantly greater proportion than those in long ANAs (21% vs 11%). Interestingly, we observed that in the mid-graft, angiogenesis in longer ANAs were limited compared to shorter ANAs at both 2 and 4 weeks post surgical repair. In the mid-graft, total length of blood vessels was significantly higher in the short ANAs than the long ANAs (10280 pixels vs 3020 pixels at 2 weeks, and 8241 pixels vs 4501 pixels at 4 weeks). Interestingly, T cells accumulation in the midgraft of long ANAs were also significantly reduced compared to short ANAs. To determine the impact of T cells in mediating regeneration of peripheral nerve, we utilized the RNU nude rat, which does not have mature T cells. When peripheral nerve was repaired using an isograft, there was no difference in the regenerative outcome in nude rats compared to the Lewis rats. On the other hand, when injured nerve was repaired using 4 cm ANAs, axonal regeneration was retarded as measured by axon counting (800 vs 0). Taken together, these data suggest that longer ANAs have altered immune environment with reduced T cells and macrophages, which impacted regeneration.
Fig. 1. Reduced angiogenesis in the mid-graft of long ANAs compared to short ANAs is seen both a) 2 weeks and b) 4 weeks post surgical repair. Proportion of macrophages c) and T cells d) are also reduced. Regeneration of nerve across e) isograft is not hindered by the lack of T cells, but regeneration across f) ANAs is.
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