Transplantation is currently the only form of treatment for many blinding diseases
caused by opacity of the cornea. Corneal graft rejection is the single most important
reason for corneal graft failure and survival rates have not improved over the decades.
Alternative investigative interventions have been studied in experimental models of
corneal transplantation for many years. IDO catabolises tryptophan and as a
consequence of this and the production of tryptophan catabolites (kynurenines)
downregulates T cell responses. Overexpression of IDO by virus-mediated cDNA
transfer to donor cornea ex vivo prior to transplantation prolongs graft survival.
As the overall objective of this project was to develop IDO or kynurenines as
potential therapeutic targets for preventing corneal allograft rejection, the first part of
this thesis examined potential methods of upregulation of IDO expression in the
cornea, concentrating on the effect of UVB exposure on IDO expression.
Upregulation of IDO at the mRNA level but not the protein or functional level was
demonstrated in response to UVB exposure.
The second part of this thesis examined whether kynurenines could mimic the effect
of IDO in prolonging corneal graft survival. It was demonstrated that the effect of
IDO overexpression in delaying graft rejection could be replicated by both the
systemic and local administration of kynurenines. Furthermore the potential
mechanism of action of this observed effect was investigated at the in vitro and in
vivo levels with kynurenines demonstrating selective T cell cytotoxicity in vitro as
well as local and systemic depletion of lymphocyte counts after both local and
systemic administration of kynurenines respectively.
Given the potential of using kynurenines as a therapeutic strategy in patients, the third
part of this thesis examined the effect of a commercially available tryptophan
metabolite- Tranilast- on allograft survival. Significant prolongation in graft survival
was observed and the potential mechanisms of action of this drug were analysed.
Tranilast inhibited T cell proliferation through cell cycle arrest as a result of
upregulation of p21 and p15 (cell cycle inhibitors) and downregulation of Cyclin E. An associated reduction in IL-2 production was also observed allowing us to
characterise a novel mechanism for Tranilast induced CD4+ T cell anergy.
Further to emerging evidence indicating a role for the aryl-hydrocarbon receptor
(AhR) in CD4 T cell differentiation and a tryptophan photo-product as a ligand of the
AhR, the fourth part of this thesis examined the potential of kynurenines (3-
hydroxykynurenine-3HK) as well as Tranilast as agonists of the AhR. 3HK and
Tranilast were found to activate the AhR and the downstream signalling pathway.
Collectively, the data represented in this thesis helps to explain the effects of IDO and
kynurenines in prolonging corneal allograft survival and helps to shed light on the
direct immunoregulatory mechanism of IDO and kynurenines. The identification of a
commercially available drug effective in prolonging corneal allograft survival may
prove a useful adjunct or alternative to topical corticosteroids that can be transferred
from the laboratory to clinical practice.