Background. Nearly half of all infiltrating leukocytes in rejecting human allografts are macrophages, yet, in comparison with T cells, much less is known about the contribution of this cell type to rejection. Our laboratory has previously described models of rejection of human skin or artery grafts in immunodeficient mouse hosts mediated by adoptively transferred allogeneic T cells. However, mature human monocyte/macrophages have consistently failed to engraft in these animals. Here, we describe the introduction of human CD68⁺ macrophages into irradiated immunodeficient mice by transplantation of enriched CD34⁺ hematopoietic stem-cells isolated from peripheral blood of G-colony-stimulating factor pretreated adults. Methods. We investigated strains of immunodeficient mice bearing human tissue grafts (skin and artery) inoculated with 1×10⁶ human CD34⁺ adult hematopoietic stem cells, peripheral blood monuclear cells autologous to the CD34 donor, or both for human cell engraftment. Results. In the absence of T cells, CD68⁺ CD14⁺ macrophages infiltrate allogeneic human skin but produce little injury or thrombosis. Both responses are enhanced when combined with adoptive transfer of T cells autologous to the hematopoietic stem cells as exemplified by the induction of the macrophage activation marker CD163. CD68⁺ macrophages also infiltrate allogeneic arterial interposition grafts, producing intimal expansion and calcification in the absence of T cells. Conclusions. These new models may be used to study the role of human macrophages in transplant rejection and other pathologies in vivo.

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