Melanocortin receptor 1 (MCR1) function is critical for murine corneal allograft survival

Abstract

Purpose : After corneal transplantation the rate of corneal endothelial cell (CEnC) loss far exceeds that seen with normal aging. The underlying mechanisms are poorly understood. Alpha-melanocyte stimulating hormone (α-MSH) is a neuropeptide that is found in the aqueous humor with both neurocytoprotective and immunomodulatory effects. We found high expression of melanocortin receptor 1 (MCR1) on CEnCs. The goal of this study was to determine the effect of α-MSH/MCR1 signaling on allograft survival using MCR1 knock-out mice.

Methods : Corneal transplantation was performed using donor corneas derived from MCR1 KO (KO group) or wild type (WT group) C57BL/6 mice. These corneas were transplanted into BALB/c recipient mice. After transplantation, subconjunctival injection of either α-MSH (10^-4 M) or PBS was given 2x/week for eight weeks. Corneal opacity and neovascularization (NV) were assessed clinically and corneal thickness was measured using OCT. Additionally, leukocytes were assessed using flow cytometry in the draining lymph nodes (dLNs) of recipient mice on days 7 and 14.

Results : None of the grafts receiving donor corneas derived from MCR1 KO mice survived by week 5, regardless of subconjunctival α-MSH treatment. In contrast, 83% of α-MSH treated (n=10/12) and 43% of PBS treated (n=5/12) grafts receiving donor corneas derived from WT mice survived by week 8 after transplantation (p<0.0001). A significantly lower opacity score (1.1±0.2, p<0.0001), NV score (2.0±0.0, p=0.007) and corneal thickness (190±31µm, p=0.0001) were observed in the grafts receiving WT donor and treated with α-MSH, compared to grafts receiving WT donor and treated with PBS (2.6±0.2; 3.4±0.4; 308±26µm) and grafts receiving KO donor (α-MSH treated: 2.6±0.2; 3.5±0.2; 287±20µm; PBS treated: 2.7±0.2; 3.6±0.2; 287±18µm) (day 7). α-MSH treatment (regardless of WT or KO donors) resulted in a decrease in the frequencies of CD45⁺B220^-CD11b⁺ (p=0.008) and CD3⁺CD4⁺IFN-Y⁺ T cells (p=0.01) and an increase in the MFI of CD4⁺FoxP3⁺ (p=0.001) cells, compared to the PBS treated groups.

Conclusions : In addition to its well-known immunomodulatory effect, α-MSH has a pivotal cytoprotective effect on CEnCs and loss of α-MSH receptor (MCR1) function in grafts significantly decreases long-term graft survival.

This is a 2020 ARVO Annual Meeting abstract.