Treg, CAR-Treg & vesicles
Regulatory T cells in keratoplasty
Ophthalmology has contributed a great deal to this biology. R. Dana's group showed that inflammation in the high-risk graft does not only activate effector lymphocytes: it also impairs Treg suppressive function, with a corresponding decrease in FOXP3 expression [6]. Several experimental strategies correct or amplify this deficit — in vivo Treg expansion with low-dose interleukin-2, delivery of suppressive myeloid cells that counter interleukin-6-induced dysfunction via interleukin-10, and subconjunctival injection of Treg. It is also known that Treg do more than regulate the alloresponse: they directly protect the corneal endothelium from death caused by effectors and cytokines, through an interleukin-10-dependent mechanism [27]. Because graft survival depends on endothelial competence, this protection has clinical significance.
Treg and CAR-Treg therapies: M. Miyara's program
In France, it is Makoto Miyara's team that is driving the therapeutic translation of these principles. Professor of immunology at Sorbonne Université and Pitié-Salpêtrière (Inserm U1135, CIMI-Paris), trained under Shimon Sakaguchi, he has authored foundational work on human Treg: the functional delineation and differentiation dynamics of FOXP3-expressing CD4+ T cells, and the role of CTLA-4 in their suppressive function, established the heterogeneity and markers of these subpopulations [28,29]. His current program focuses on immunoregulatory cell therapies in transplantation and autoimmunity: expansion of pharmaceutical-grade human Treg using a combination of interleukin-2, rapamycin and epigenetic inhibitors [30], Treg engineered with a chimeric antigen receptor (CAR-Treg), and clinical injection of expanded Treg in liver and kidney transplantation, aiming for operational tolerance — that is, graft function maintained without continuous immunosuppression [31]. This framework, that of an immunotherapy that restores tolerance rather than suppressing immunity, represents the most promising prospect for keratoplasty.
The acellular horizon: from cell therapy to vesicle therapy
Treg cell therapy, however, runs into well-identified difficulties: Treg are rare in lymphoid organs, their in vitro expansion is lengthy, and above all their phenotype is unstable, with loss of FOXP3 and possible reconversion into effector cells in an inflammatory environment. An acellular version of this biology has therefore been explored: Treg-derived extracellular vesicles (Treg-EV), exosomes and microvesicles.
Part of the Treg suppressive effect may operate through a contact-independent pathway: the release of small vesicles that transfer micro-RNAs and immunoregulatory surface proteins to their targets [32,33]. These vesicles curb effector proliferation, shift their cytokine profile (more interleukin-4 and interleukin-10, less interferon-γ, interleukin-2 and interleukin-6) and alter dendritic cell function [34]. In several transplantation models, they prolong the survival of humanized skin and renal allografts by limiting immune infiltration [32,33]. One feature makes them well suited to corneal inflammation: unlike the cells they derive from, they do not revert to an effector phenotype in an inflammatory environment, which removes the main pitfall of Treg, their instability [35].
The most recent work demonstrates the maturity of the approach. Engineered extracellular vesicles, displaying on their surface a peptide–MHC class II complex combined with interleukin-2 and TGF-β, induce antigen-specific Treg in vivo that are strongly positive for CTLA-4, PD-L1 and LAG-3, and capable of blocking lymphocyte proliferation [36]. The cornea lends itself to this principle: the ocular surface is accessible to topical or subconjunctival application, the volume to be treated is small, and an acellular, standardizable, storable therapy would escape the production, traceability and compatibility constraints of cell therapies. For high-risk grafts, Treg vesicles could constitute a treatment aimed at local restoration of tolerance.