Host non-inflammatory neutrophils mediate the engraftment of bioengineered vascular networks

Notwithstanding the remarkable progress in vascular network engineering, implanted bioengineered microvessels mostly fail to form anastomoses with the host vasculature. Here we demonstrate that implants containing assembled human vascular networks (A-grafts) fail to engraft owing to their inability to engage non-inflammatory host neutrophils upon implantation into mice. By contrast, unassembled vascular cells (U-grafts) readily engage alternatively polarized neutrophils, which in turn serve as indispensable mediators of vascular assembly and anastomosis. The depletion of host neutrophils abrogated vascularization in U-grafts, whereas an adoptive transfer of neutrophils fully restored vascularization in myeloid-depleted mice. Neutrophil engagement was regulated by secreted factors and was progressively silenced as the vasculature matured. Exogenous addition of factors from U-grafts re-engaged neutrophils and enhanced revascularization in A-grafts, a process that was recapitulated by blocking Notch signalling. Our data suggest that the pro-vascularization potential of neutrophils can be harnessed to improve the engraftment of bioengineered tissues.