| Abstract |
Inflammatory bowel disease (IBD) is a chronic remittent or progressive inflammation
of the gastrointestinal tract that affects millions of people worldwide. This
inflammatory condition encompasses two major forms known as Crohn’s disease
(CD) and ulcerative colitis (UC). The onset of IBD typically occurs in the second and
third decades of life, with the majority of patients progressing to a relapsing and
chronic disease. Although the precise cause of IBD has not yet been fully elucidated,
recent advances in the understanding of the molecular pathogenesis of IBD have been
made owning to three related lines of investigation. First, IBD has been amenable to
the discovery of susceptibility genes. Secondly, it appears that commensal pathogens
or their products rather than conventional flora, play a pivotal role in the dysregulated
immunity typically observed in IBD cases. Thirdly, murine models that exhibit many
of the features of UC seem to be driven bacterially and have helped to unravel the
pathogenic mechanisms underlying IBD. Overall, it appears that an imbalance of the
mucosal immune system leads to the overproduction of inflammatory cytokines,
release of reactive oxygen metabolites and infiltration of immune cells into the
intestine, resulting in uncontrolled intestinal inflammation and tissue damage.
Myeloid-derived suppressor cells (MDSCs) constitute a diverse cell population made
up of immature myeloid cells (IMCs), consisting of mostly progenitor cells of
macrophages, granulocytes, dendritic cells and myeloid cells at early stages of
differentiation. In mice, MDSCs are characterized by the co-expression of Gr-1 and
CD11b markers, and are further divided into granulocytic and monocytic subset,
defined as CD11b+Ly6G+Ly6Clow and CD11b+Ly6GlowLy6C+ cells, respectively. In
humans, phenotypic characterization of MDSCs is challenging, due to lack of uniform
criteria. Nevertheless, they are most commonly characterized by the expression of the
myeloid marker CD33 and the lack of expression of the major histocompatibility
complex (MHC) class-II molecule, human leukocyte antigen (HLA-DR) and several
other lineage markers. Although most of our knowledge on the role of MDSCs in
immune responses has been based on tumor bearing mice and cancer patients,
increasing evidence has suggested their role in many pathological conditions, such as
infections, transplantation and autoimmunity. Traditionally, MDSCs are considered as
potent suppressors of immune responses through different mechanisms. However,recent literature highlighted their plasticity, denoting proinflammatory potential of
specific subsets emerging in specific microenvironment.
Despite the well-defined suppressive effects of MDSCs on T cell responses, their
function in autoimmune diseases, like IBD, is controversial. In particular, the
immunosuppressive function of MDSCs was suggested by several reports showing
that CD11b+Gr1+ MDSCs are increased during intestinal inflammation. As is the case
for murine colitis models, CD14+HLA-DRlow MDSCs with suppressive properties
were also reported to be increased in the peripheral blood (PB) of IBD patients. On
the other hand, recent studies supported a proinflammatory role of myeloid cells in
experimental colitis, demonstrated that adoptively transferred colonic Ly6Chigh cells
differentiated into inflammatory cells, contributing to intestinal inflammation.
In this PhD thesis, we investigated the immunomodulatory properties of MDSCs in
experimental inflammatory colitis and T cell-mediated immune responses in IBD
patients.
MDSCs (defined as CD14+︎HLA-︎DR-/lowCD33+CD15+ cells) numbers were
determined in PB from IBD patients. PB MDSCs function was assessed in vitro.
Experimental colitis was induced upon 2,4,6-trinitrobenzene sulfonic acid (TNBS)
treatment and MDSCs were characterized by flow cytometry. The in vivo suppressive
potential of bone marrow (BM)-derived MDSCs (BM-MDSCs) was tested by using
both depleting and adoptive transfer strategies.
MDSCs were enriched in the periphery of IBD patients during active disease. TNBS
colitis induced amplification of MDSCs, particularly of the granulocytic (Ly6G+)
subset during the effector phase of disease. Of interest, BM-MDSCs potently
suppressed CD4+ T cell responses under steady state but failed to control colitisassociated
immune responses in vivo. Mechanistically, under the colonic
inflammatory milieu MDSCs switched phenotype (decreased proportion of Gr1high
and increased numbers of Gr1low) and downregulated CCAAT/enhancer-binding
protein beta (CEBPβ) expression, a critical transcription factor for the suppressive
function of MDSCs. In accordance with the murine data, human
CD33+CD15+MDSCs from PB of IBD patients not only failed to suppress autologous
T cell responses but instead enhanced T cell proliferation in vitro.
Our findings demonstrate an aberrant function of MDSCs in experimental
inflammatory colitis and in IBD-associated immune responses in vitro. Delineation of the mechanisms that underlie the loss of MDSCs function in IBD may provide novel therapeutic targets.
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