INTRODUCTION: Macrophages are components of the innate immune response and quickly infiltrate tissues during inflammation. Their role in antibody-mediated rejection (AMR) remains unclear. We have reported murine CCR5−/−/CD8−/− recipients produce high titers of antibody and reject MHC-mismatched heart allografts. The current studies were conducted to investigate the role of graft-derived monocyte/macrophage chemoattractant Monocyte Chemoattractant Protein-1 (MCP1) in AMR and how macrophages may participate in antibody-mediated allograft injury.

METHODS: Complete MHC-mismatched A/J or MCP1−/− A/J hearts were transplanted into CCR5−/−/CD8−/− C57BL/6 recipients. Some of the recipients were treated with 250 Μg of anti-CD40L mAb (MR1) on days 0 and 1. Allografts were harvested on day 7 or 14 post-transplant and mRNA expression levels were determined by qRT-PCR and graft infiltrating cells were assessed by immunohistochemistry. Serum antibody titers were assessed by flow cytometry. Donor-reactive CD4 T cells producing IFN-Γ were enumerated by ELISPOT.

RESULTS: CCR5−/−/CD8−/− C57BL/6 recipients rejected A/J allografts with high titers of donor-reactive antibody and diffuse C4d deposition. The survival of MCP1−/− A/J allografts was slightly but significantly longer than that of A/J wild type allografts (MST: A/J wild type, day 7.5 vs. MCP1−/− A/J, day 9.0). The infiltration of macrophages and neutrophils into the allograft was decreased in MCP1−/− A/J compared to A/J wild type allografts. MCP1−/− A/J allografts induced significantly lower antibody titers than A/J wild type allografts at day 7. The mRNA expression of MCP1, IL-1Β, CCL5, Perforin and FasL was decreased in MCP1−/− A/J grafts compared to AJ wild type grafts at day 7. The number of donor-specific CD4 T cells producing IFN-Γ in the recipient spleen was decreased in MCP1−/− A/J grafts compared to AJ wild type grafts. Strikingly, the survival of MCP1−/− A/J allografts was much longer than wild-type A/J allografts in recipients treated with peritransplant anti-CD40L mAb (MST: day 17.5 vs. 46) with lower titers of antibody induced to the MCP-1-deficient allografts.

CONCLUSIONS: Our results indicate that allograft-derived MCP-1 plays an important role in the development of donor-reactive antibody and in directing macrophage and donor-reactive CD4 T cell allograft infiltration during AMR.

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