Title:
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In Vivo Induction of Regulatory Immune Tolerance
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Low or reduced intensity conditioning (RIC) regimens for allogeneic haematopoietic
stem cell transplantation (HSCT) can be very effective at establishing donor
haematopoietic engraftment and host-versus-graft (HvG) tolerance. I have
investigated the mechanisms of HvG tolerance induction and maintenance in an
animal model. Although peripheral clonal deletion and anergy contribute to limit
unwanted immune responses, active regulation is the central mechanism of
transplantation tolerance. Several T cell subsets have been identified with the ability
to suppress immune responses to a variety of self and non-self antigens. Firstly, I
studied the role of regulatory T cells. Natural regulatory T (Treg) cells are a
subpopulation of thymus-derived CD4+ T cells which constitutively express the
interleukin-2 receptor a chain (CD25). I have observed that splenocytes from chimeric
mice inhibited donor-specific CD8+ T cell responses both in vitro and in vivo, and their
adoptive transfer facilitated donor haematopoietic engraftment. These properties were
contained within the CD4+CD25+ population, and the inhibitory effect was
antigen-dependent. The administration of anti-CD25 depleting antibodies to
conditioned recipients at time of HSCT prevented donor-recipient chimerism, but did
not affect engraftment if performed after the establishment of chimerism, thus
indicating that recipient Treg cells are required for the generation but not the
maintenance of HvG tolerance. Therefore, donor-specific Treg cells of recipient origin
are recruited when the donor antigens are present during RIC induced Treg expansion.
In the second part of my thesis, I have analysed the effect of natural killer T (NKT)
cells on HvG tolerance. NKT cells use an invariant T-cell receptor which interact with
synthetic glycolipids such as a-galactosylceramide in the context of the monomorphic
CD1 d antigen-presenting molecule. I have found that also NKT cells from the chimeric
mice showed inhibitory effects on the anti-donor T cell responses in vivo and in vitro.
In the last chapter, I have studied the use of cytotoxic pathways in HvG responses by
using FasL deficient (gld) mice and perforin deficient (prf-I-) mice. In prf-I. mice, donor
cell engraftment was higher and donor cell rejection was delayed compared with wild
type control mice and gld mice, indicating that HvG effect T cells preferentially use
perforin-granzyme pathway to reject haematopoietic donor cells. These findings have
important implications for the design of tolerogenic regimens in transplantation.
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