Our data show significantly increased serum levels of MIF in patients with pSS, especially in those with increased γ-globulins. Hypergammaglobulinemia has been linked with the extent of histopathological salivary gland abnormalities and has been proposed as an activation marker in pSS [18
]. An increased production of γ-globulins results from polyclonal B-cell hyperactivity [20
]. MIF can provide signals for B cells to proliferate [21
]. It has been shown that neutralization of MIF significantly inhibits antibody production in vivo
]. Increased production of MIF might therefore contribute to hypergammaglobulinemia and possibly reflects disease activity of pSS.
MIF has been associated with various autoimmune diseases [7
]. The induction and regulation of MIF in autoimmune diseases is not well characterized [23
]. It has been shown that low concentrations of glucocorticoids induce MIF production from macrophages; this could be part of a counter-regulatory system that functions to control immune responses [24
]. Previous results showing increased levels of MIF in patients with systemic lupus erythematosus could partly be explained by corticosteroid use [10
]. However, because our patients with pSS did not receive any glucocorticoids, the increased MIF levels could not be explained by this argument.
MIF has been shown to be increased in acute inflammation and a correlation with CRP concentrations has been described [8
]. As the acute-phase reactant CRP was not elevated in our cohort, the increased MIF levels in patients with pSS cannot be explained by differences in the extent of acute-phase response. Hence, specific mechanisms of MIF induction in pSS remain to be elucidated.
There was an increased prevalence of HLA-DR3 in our cohort of patients with pSS, confirming previous findings [26
]. Antibodies against Ro and La have been shown to be associated with HLA-DR3, possibly as a result of HLA haplotype-dependent differences in presentation of autoantigens and subsequent stimulation of the immune response [26
]. We detected no associations of MIF levels with the HLA-DR genotype, suggesting that HLA-DR polymorphism does not have a major role in the generation of MIF.
We found a negative correlation of MIF with IL-10-secreting PBMC. In addition, we observed a tendency towards an increased number of IL-10-secreting PBMC in our cohort, as reported previously [27
]. It has been shown in vitro
that IL-10 inhibits MIF synthesis [28
]. Moreover, neutralization of MIF leads to an increase of IL-10 production in an animal model [29
]. IL-10 has been described as a potent macrophage deactivator that inhibits cytokine production by activated macrophages [30
]. Our data might indicate downregulation of MIF by IL-10 in vivo
and suggest that IL-10 and MIF are part of a negative regulatory circuit. It might be assumed that IL-10 counteracts MIF-induced inflammatory processes such as activation of macrophages, as reported previously [28
We found no association of MIF with other cytokine-secreting PBMC in our patients. It has been shown that MIF can upregulate proinflammatory cytokines including TNF-α in vitro
]. However, other authors have not identified any TNF-inducing effect of MIF on PBMC [32
]. Recently it has been shown that MIF alone is not sufficient to induce cytokine expression: co-stimulators such as lipopolysaccaride are necessary to induce the secretion of TNF-α and IL-1 [33
]. This suggests that MIF may act to modulate and amplify the response to lipopolysaccharide in sepsis. In pSS, MIF apparently has no inducing effect on TNF-α or other proinflammatory cytokines analysed, presumably because of a lack of such co-stimulatory factors.
The percentage of CD4/CD71+
T cells was significantly increased in patients with pSS compared with healthy controls, as reported previously [34
]. We did not find an association of MIF with various activation markers on T helper cells, B cells or macrophages, although MIF has been identified as an activator of B and T cells as well as macrophages [4
]. Recently it has been suggested that MIF is a critical effector of organ injury in systemic lupus erythematosus in the absence of major changes in T-cell and B-cell markers or alterations in autoantibody production [35
]. Most probably this observation holds also true for pSS.
MIF has no homology with any other proinflammatory cytokine, and the mechanisms by which MIF exerts its biological effects are not yet fully understood [33
]. It is possible that MIF mediates organ injury directly, because it has been shown that MIF induces the production of matrix metalloproteinase-9 [36
], which has been implicated in the pathogenesis of pSS [37
]. MIF can stimulate the inducible nitric oxide synthase and increase the production of nitric oxide, which can directly mediate cell injury [38
]. It has been suggested that nitric oxide contributes to inflammatory damage and acinar cell atrophy in Sjögren's syndrome [39
Our three patients with B-cell lymphoma had increased MIF levels compared with healthy controls, but there were no significant differences from patients with pSS without B-cell lymphoma.
Patients with pSS are at increased risk of developing B-cell non-Hodgkin's lymphoma [1
]. It has been suggested that MIF provides a link between inflammation and tumorigenesis [21
MIF expression is increased in sporadic human colorectal adenomas [41
]. MIF has been shown to decrease the tumor suppressor activity of p53 and to upregulate Bcl-2 expression [42
], which has been suggested to be important in B-cell monoclonal proliferation and malignant transformation in pSS [43
]. A deficiency of p53 tumor suppressor activity is associated with the development of low-grade mucosa-associated lymphoid tissue lymphoma [44
]. It has been shown in a lymphoma mouse model that loss of MIF markedly delays the onset of B-cell lymphoma development in vivo