Deletion of the chemokine receptor CCR1 prolongs corneal allograft survival.

Penetrating keratoplasty has emerged as the most common form of solid tissue transplantation. Currently, more than 30,000 corneal transplantations are performed each year in the United States,1 with a 2-year success rate as high as 90% for uncomplicated first grafts performed in avascular low-risk beds.2,3 However, these results contrast sharply with the fate of corneal grafts placed in high-risk host beds in which rejection rates can exceed 70% to 90%, even with maximum local and systemic immune suppression.4,5 Eyes at high risk for corneal transplant failure include those with corneal vascularization, herpes simplex keratitis, and a history of one or more previously failed grafts.6 Although immune-mediated rejection remains the leading cause of corneal transplant failure,6,7 surprisingly, pharmacotherapy for corneal transplantation has changed little over the past several decades, with corticosteroids remaining the mainstay of therapy. However, corticosteroids are only variably effective in either the prevention or treatment of high-risk corneal graft rejection and are capable of inducing cataracts, glaucoma, and opportunistic infections.3,8 Development of safe and targeted new regimens that are effective in modulating alloimmune responses are therefore a priority in corneal transplantation. The destructive immune response to corneal grafts is effected principally by delayed-type hypersensitivity (DTH)-mediating Th1 cells,9,10 recruited into the transplant site by the local expression of chemokines.11 Chemokines are low-molecular-weight (8–11 kDa) proteins that play a critical role in immune and inflammatory responses, with nearly 60 chemokine species and over 20 chemokine receptors identified over the past 10 years.12 Although chemokines are best known for their ability to stimulate cell migration, they have additional activities that can contribute to tissue damage and inflammation, including enhancing T cell activation,13 regulating Th1/ Th2 polarization,14,15 and stimulating macrophage function and protease secretion. Work from many laboratories has demonstrated the presence of specific chemokines during the progression of allograft rejection. In the cornea, we have shown the expression of specific species, including CCL2/MCP-1, CCL3/MIP-1α, CCL4/ MIP-1β, CXCL10/IP-10, and CCL5/RANTES after corneal transplantation.11 These chemokines associate with particular receptors: CCR1 with CCL3/MIP-1α and CCL5/RANTES; CCR5 with CCL3/MIP-1α, CCL4/MIP-1β, and CCL5/RANTES; CCR2 with CCL2/MCP-1; and CXCR3 with CXCL10/IP-10. However, to date there have been no studies in which the relationship between chemokine or chemokine receptor deficiency and corneal transplant rejection has been examined. In contrast, studies in other tissues have revealed that targeted deletion of CCR1 in complete-major histocompatibility complex (MHC)–mismatched heart allografts leads to slightly increased survival compared with wild-type (WT) recipients.16,17 Similar results have been reported for rat cardiac allografts,18 and rabbit renal allografts.19 In addition, targeting CCR2 has been shown to provide transient prolongation of islet cell and heart allograft survival.20 When heart allografts are transplanted into CXCL10/ IP-10 (a potent attractant for primed Th1 cells) knockout (KO) recipients, they are rejected at the same rate as their control mice. However, when heart allografts from CXCL10/IP-10 KO donor mice are transplanted, a significantly improved survival rate was demonstrated, which was associated with the absence of NK cell infiltration into the grafts.21 Profound increases in survival rates were also reported in CXCR3 KO murine (the receptor for CXCL10/IP-10) recipients of heart allografts compared with WT control animals.22 Last, targeting CCL5/ RANTES, a late chemokine, through blocking antibodies to CCL5/RANTES after cardiac allografting,23 or by using CCR5 KO recipients for cardiac allografts,24 has also shown improvement in graft survival. In the present study, we used as corneal transplant recipients murine KO strains of different chemokine–chemokine receptors that we had correlated with corneal transplantation. We used this method to determine more precisely which specific chemokine pathways are most robustly associated with corneal allograft rejection. We demonstrate, for the first time, that selected deletion of CCR1, but not CCR5, CCR2/ CCL3(MIP-1α), CXCR3, or CXCL10/IP-10, in graft recipients, leads to significant corneal allograft survival. Further, we demonstrate that the increased survival of allografts in CCR1 KO recipients is associated with decreased expression of proinflammatory cytokines involved in allorejection, a lowered infiltration of inflammatory cells into the graft site, and suppressed DTH responses in grafted hosts.