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Differential Role of CCR2 in Islet and Heart Allograft Rejection: Tissue Specificity of Chemokine/Chemokine Receptor Function In Vivo1

Chemokines have a pivotal role in the mobilization and activation of specific leukocyte subsets in acute allograft rejection. However, the role of specific chemokines and chemokine receptors in islet allograft rejection has not been fully elucidated. We now show that islet allograft rejection is associated with a steady increase in intragraft expression of the chemokines CCL8 (monocyte chemoattractant protein-2), CCL9 (monocyte chemoattractant protein-5), CCL5 (RANTES), CXCL-10 (IFN-γ-inducible protein-10), and CXCL9 (monokine induced by IFN-γ) and their corresponding chemokine receptors CCR2, CCR5, CCR1, and CXCR3. Because CCR2 was found to be highly induced, we tested the specific role of CCR2 in islet allograft rejection by transplanting fully MHC mismatched islets from BALB/c mice into C57BL/6 wild-type (WT) and CCR2-deficient mice (CCR2−/−). A significant prolongation of islet allograft survival was noted in CCR2−/− recipients, with median survival time of 24 and 12 days for CCR2−/− and WT recipients, respectively (p < 0.0001). This was associated with reduction in the generation of CD8+, but not CD4+ effector alloreactive T cells (CD62LlowCD44high) in CCR2−/− compared with WT recipients. In addition, CCR2−/− recipients had a reduced Th1 and increased Th2 alloresponse in the periphery (by ELISPOT analysis) as well as in the grafts (by RT-PCR). However, these changes were only transient in CCR2−/− recipients that ultimately rejected their grafts. Furthermore, in contrast to the islet transplants, CCR2 deficiency offered only marginal prolongation of heart allograft survival. This study demonstrates the important role for CCR2 in early islet allograft rejection and highlights the tissue specificity of the chemokine/chemokine receptor system in vivo in regulating allograft rejection.