Subretinal allografts of ES-derived RPE cells in rhesus monkeys without immune suppression: Systemic and local immune responses associated with rejection

Abstract

Purpose: To determine viability of allogeneic rhesus monkey embryonic stem cell (ESC)-derived RPE cells transplanted to the subretinal space of the same species, and to examine the resulting local and systemic immune response.<br />

Methods: RPE cells were generated from rhesus monkey ESCs, pre-labeled with fluorescent markers, and transplanted as allografts into 9 eyes of 6 monkeys. Use of an improved transchoroidal surgical technique assured that transplanted cells remained in the subretinal space. Retinas were imaged by fundus photography and OCT prior to surgery, twice in the first week and then weekly. Peripheral blood mononuclear cells were collected weekly to monitor changes in frequency of circulating naïve and memory T- and B-cell subsets. Animals were sacrificed 1-6 weeks after surgery and retinas were examined morphologically and by immunohistochemistry.<br />

Results: Color fundus photos and OCTs revealed successful delivery of cells in all eyes. At 4 weeks post-injection, histology revealed infiltration of mononuclear cells into the choroid and subretinal space, resulting in ablation of grafted cells and disruption of host RPE and choroid in the immediate vicinity of the graft. These mononuclear cells stained positively for B-cells but negatively for T-cells. All animals showing clear histological signs of rejection also showed signs of systemic immune activation, as evidenced by 2- to 3-fold increase in T and B cell proliferation starting ~14 days after surgery, often accompanied by increased frequency of memory T and B cells, and in some cases also increased numbers of circulating lymphocytes.

Conclusions: Our data demonstrate that allogeneic cells transplanted into the subretinal space without immune suppression provoke an immune response that can compromise survival of the cells. Presence of B-cells at the graft site was unexpected given the short survival period of the grafted cells, and the longer-term nature of a typical B-cell response. It is unclear why T- and B- cell activity were upregulated systemically, while only B cells were observed in retinal tissue. These results have important implications for clinical application of cell-based therapy, underscoring the need for autologous cell sources and/or adequate T- and B-cell immune suppression to ensure graft survival.