Vascularized composite allografts consists of skin, muscle, bone, and other tissues, and is more immunogenic than solid-organ allografts. Our laboratory developed a murine model to study mechanisms of vascularized composite allotransplantation (VCA) rejection. A limb from a donor C57BL/6 (syngeneic) or BALB/c mouse (allogeneic) is placed in the cervical region of a recipient C57BL/6 mouse, and the donor's femoral artery and vein are anastomosed to the recipient's common carotid artery and jugular vein. Survival rate is >90% for syngeneic transplant recipients and 66.7% for recipients of allogeneic transplants (n = 12). The skin is monitored daily for hair loss, discoloration, and dryness.  Syngeneic grafts were monitored for >100 days and showed no evidence of rejection. In the allogeneic group, rejection was evident on day 3 post-transplant and severe rejection occurred by day 5. Histology showed leukocyte infiltration on day 3 (n=2) and total loss of tissue architecture on day 5 (n=3).
We utilized the high dimensional capacity of Mass Cytometry (Cytometry by Time of Flight; CyTOF) for unsupervised analysis of VCA immunity and designed a panel of surface and intracellular markers for the identification and characterization of leukocyte populations in the spleen and lymph nodes. We observed a higher proportion of TNFα⁺CD44^loCD62L^- CD8⁺T-cells and CD11c^hiCD11b⁺NK1.1 NK cells in the allogeneic group as compared to control .
To develop strategies to promote VCA prolongation, C57BL/6 mice were treated with donor BALB/c plasmacytoid dendritic cells (pDCs) intravenously 7 days prior to VCA. pDCs are a subset of dendritic cells that we and others have shown to prolong murine allograft survival. pDC-treated mice exhibited a delayed-rejection phenotype, showing little to no hair-loss and discoloration of the graft on day 5 post-transplant. Histology showed less leukocyte infiltration and tissue damage than the VCA untreated groups. CyTOF analysis identified an immune profile that is uniquely distinguishable from control and untreated VCA mice on the basis of principal component analysis. In addition, pDC-treated mice had a higher proportion of Gr1⁺CD11b^himyeloid-derived suppressor cells (MDSCs).  Thus, we have developed a robust VCA model to characterize the immune response and have identified a novel, promising strategy to prolong VCA survival.