CD4+ T cells and immune ageing; major players and predictors for disease activity in giant cell arteritis

Age-associated alterations of the immune system (immunosenescence) have a
strong clinical impact and may contribute to susceptibility to infectious
diseases, autoimmunity and cancer in the elderly. Instruments, such as the
Groningen Frailty Index, have been devised to assess this general
age-associated susceptibility to diseases, also named frailty.
Especially the adaptive arm of the immune system is affected by ageing.
Immunosenescence is the result of 1) thymus involution leading to a steady
decline in the production of naïve T cells, 2) shrinkage of the T cell
repertoire through continuous antigen stimulation favouring the development of
functionally altered, oligolonal, senescent T cells (identified by CD28 loss
and telomeric erosion) and 3) a chronic low degree of inflammation (termed
inflamm-aging) evidenced by increased serum levels of inflammatory cytokines
and acute phase proteins. Longitudinal studies (that included healthy octo-,
nono- and centenarians) have identified a so called immune risk phenotype (IRP)
that is associated with poor immune function and increased mortality risk: a
CD4+/CD8+ T cell ratio < 1, low CD19+ B cell counts and a poor lymphocyte
proliferative response.
In general the risk of developing autoimmune disease increases with age. This
is particularly true for temporal arteritis or giant cell arteritis (GCA). GCA
manifests as a vasculitis predominantly affecting medium and large arteries.
Classically, this disease develops in older people (older than 50 years of age;
mean age 72) with an almost similar incidence in men and women. In patients
with GCA, a syndrome of systemic inflammation accompanies vascular
manifestations such as occlusive vasculopathy causing stroke, blindness and
aortic arch syndrome. Standard treatment of GCA is done with high dose
corticosteroids. Relapse of GCA is frequently seen in 50% of patients within
1-25 months.
The etiology of GCA remains obscure, but progress has been made in clarifying
the pathophysiology of the disease. Systemic inflammation in GCA is
substantiated by reports showing elevated levels of TNFα and IL-6 in patients.
Interestingly, like rheumatoid arthritis, GCA is associated with carriage of
certain HLA-DR4 alleles (HLA-DRB1*0401 and HLA-DRB1*0404), although it has
been suggested that different epitopes within these alleles are relevant in the
two diseases. In RA the HLA-DR4 alleles are associated with production of the
highly specific anti-citrullinated protein antibodies (ACPAs), but
autoantibodies have not been found in GCA so far. Nonetheless, in both diseases
a significant role for CD4+ T cells is suggested, since HLA-DR is involved in
antigen presentation to these cells. Previously, Weyand and Gorozny, proposed a
novel interpretation for the involvement of the HLA-DR4 haplotype in autoimmune
disease. They noted that HLA-DR4+ individuals (both patients and healthy
persons) experience premature ageing of their T cells as defined by shorter
telomere lengths, decreased T cell receptor diversity and loss of CD28.
Although studies have shown that aged T cells may demonstrate decreased
proliferative responses, they are known to produce considerable amounts of
IFN-γ, perforine and granzyme B. Further expansion of these aged cytotoxic T
cells may be further enhanced in chronic inflammatory disease due to persistent
immune activation, but it is also seen after CMV and EBV infection. In addition
the functionality of aged T cells is altered due to acquisition NK cell
markers. One of these molecules is CD161 that may identify T cells with tissue
migratory properties.
As expected, CD4+ T cells appear to be major players in the pathophysiology of
GCA. It is now appreciated that the naïve CD4 (helper) T cell is a
multipotential precursor which can be triggered to differentiate towards Th1,
Th2, Th17 and T regulatory phenotypes dependent on APC function and the
composition of the local cytokine milieu. Especially IFN-γ producing CD4+ T
cells are found in large numbers in affected arterial walls. Whether these
cells are Th1 cells or aged CD4+CD28- T cells is unknown. Recently IL-17
producing CD4+ T cells were also demonstrated in the inflamed arterial walls.
Studies on the functional role of T regulatory cells in GCA are lacking.
It is currently unclear how aging affects the prevalence and functionality of
different effector and regulatory T cell subsets. The altered cytokine milieu
as a consequence of inflamm-aging may 1) accelerate replicative senescence in T
cells and 2) favour a shift (away) from the T regulatory arm of the immune
system towards the pro-inflammatory Th17 effector subset associated with tissue
pathology. Both replicative senescence and the Treg to Th17 shift may be
responsible for the decline in peripheral CD4 T cell numbers (an IRP parameter)
as both senescent T cells and Th17 cells are thought to migrate to the tissues
by virtue of CD161 expression and may promote autoimmune vascular injury.

In addition, an interesting feature of GCA is that it frequently associates (in
40-50% of cases) with polymyalghia rheumatica (PMR), although PMR may also
present as an isolated disease. The annual incidence of PMR in persons > 50
years varies from 13 per 100 000 in Southern Europa to almost 70 per 100 000 in
Scandinavia. Patients with PMR typically present with muscle stiffness/pain and
a high ESR. However, the pathological substrate of PMR appear to be synovitis,
instead of inflammation of muscles. Even in the absence of typical GCA
symptoms, PMR patients may actually have GCA in 9-21% of cases, as
population-based studies have shown. It*s feasible to think that the
pathogeneses of GCA and PMR are somehow related, yet distinct at the same time.
Interestingly, evidence is present suggesting that T cells may be the defining
factor. It has been shown that resident dendritic cells in large/medium sized
arteries of GCA patients are activated, resulting in attraction and activation
of T cells in the artery wall. However, when the authors looked at the
large/medium sized arteries of PMR patients, they surprisingly found activated
dendritic cells in the artery walls as well. Yet, these activated dendrtic
cells were not accompagnied by the presence of activated/differentiated T
cells. So the vessel wall of PMR patients may actually be *ready* to receive T
cells, but the T cells won*t infiltrate the blood vessel, and therefore most
PMR patients don*t have vasculitis. Using an animal model, the same authors
further confirmed this idea by showing that T cells of GCA could indeed readily
infiltrate the artery samples of PMR patients, but not that of controls
persons. So T cells seem to determine whether a patient has PMR only, or GCA as
well. But how T cells of PMR patients differ from T cells in GCA patients
remains unknown.

GCA and PMR patients in the long run go into remission. We would like to
know what the characteristics are of peripheral CD4 + T-cells, B-cells and
monocytes, in patients in remission. It is also unclear which genes are
involved in the pathogenesis of GCA and PMR. From the genetic profiles we hope
to get more insight in the pathways important in the development of GCA and/
or PMR.

Primary Objective: to study the differences in CD4+ T cell subsets
(Th1/Th2/Th17/Tregulatory/Tsenescent) between GCA patients and healthy controls
at diagnosis and during follow up and in remission.

Secondary Objective(s):
- To characterize the cell subsets that migrate into the arterial wall in GCA
- To identify a biomarker (CD4+ T cell diversity, the IRP, cytokines)
predicting relapse in GCA
- To study the relation between the IRP and frailty (as assessed by the GFI)
- To identify a biomarker (CD4+ T cell diversity, the IRP, cytokines)
distinguishing PMR from GCA or the combination of both diseases.
- To characterize the T cell subsets that migrate into the joints of PMR
patients
- To characterize the genes involved in the pathogenesis of
GCA and PMR

Observational study with assessment of CD4+ T cell subsets and cytokines in the
blood, and in PMR patients also synovial fluid and tissue.

The burden for patients is the filling in of questionaires and the extra blood
withdrawal during regular visits. Healthy and infectious controls will have to
pay a visit to the outpatient clinic, fill in questionnaires and give blood.
The risks associated with the questionnaires and venapunction in this study we
consider as 'nihil'. Although a common procedure in rheumatology,
arthrocentesis (joint puncture) may lead to infection in rare cases (<0,1%).

Patients and controls will gain no direct benefit from the study. This study
is directed to unravel pathogenetic mechanisms involved in GCA and PMR. We hope
that it will lead to new targets for therapy (for example CD161). But the most
important benefit foreseen will be the identification of biomarkers (IRP/CD4+ T
cell diversity/CD161 expression) that predict which patients are at risk for
relapse after withdrawal of corticosteroid therapy and should continue therapy
to prevent morbidity like blindness, and which patients are likely to remain
disease free and can be safely withdrawn from therapy limiting high cumulative
doses of corticosteroids. Furthermore we hope to find markers that will allow a
better discrimination between PMR and GCA patients, and especially biomarkers
that predict which PMR patients develop GCA. In addition we will study T cell
subsets in the joints of PMR patients, which may guide future specific
therapeutic studies in PMR.
Finally we hope to link aging of the immune system with frailty in elderly. In
times of rising age expectancies and demographic shifts towards older
populations, we believe that more insights in the role of the immune system in
the ageing process, will be useful to develop healthy ageing strategies in the
future.