In previous investigations we characterized the humoral response to hnRNP-A2 in SLE patients and some clinical implications thereof [8
]. In this study we examined SLE patients and healthy controls for the presence of autoreactive T cells to hnRNP-A2 to better understand the role of this autoimmune response in the pathogenesis of SLE. The data obtained show the existence of a pronounced cellular response to hnRNP-A2 in the majority of SLE patients, which was far more vigorous than in healthy controls. In contrast, the response to the control antigen tetanus toxoid was similar in patients and controls, demonstrating the specificity of the immune response towards hnRNP-A2. Moreover, the response of SLE patients to hnRNP-A2 was of similar magnitude, or even slightly higher, than the response to tetanus toxoid, which is a recall antigen eliciting a pronounced response in the majority of individuals tested.
The occurrence of autoreactive T cells in healthy individuals is a common finding that has previously been reported for antigens associated with SLE [21
] as well as other autoimmune diseases like pemphigus foliaceus [25
] and multiple sclerosis [26
]. Thus, the presence of hnRNP-A2 auoreactive T cells in healthy controls by itself was not surprising; a striking difference, however, was the significantly higher strength of the cellular immune response observed in SLE patients, which may be considered an indication for pathogenic involvement of these autoreactive cells and/or a lack of counter-regulation in SLE patients.
Interestingly, no correlation of cellular reactivity to hnRNP-A2 with the appearance of the respective autoAbs in SLE patients could be observed. Thus, hnRNP-A2 appears to be a predominant T cell antigen, while the generation of autoAbs might represent a bystander phenomenon occurring in a subgroup of patients. However, recent data indicate that in SLE patients the humoral response to hnRNP-A2 fluctuates considerably and increases during flares. Therefore, the prevalence of these autoAbs may be considerably higher than previously assumed (unpublished observation).
Although the cellular reactivity to hnRNP-A2 appeared to be primarily a Th1 response, we observed a relatively high percentage of CD8+ TCCs in SLE patients. Of particular interest, most of these clones lacked CD28 expression and produced neither IFNγ nor IL-4, but did produce IL-10. This may indicate a special pathological role of this T cell subset because this phenotype appeared to be restricted to SLE patient derived TCCs. In previous reports, CD4+ T cells in SLE were shown to be of predominantly Th1 or Th0 subtypes [21
]. Furthermore, autoreactive CD4+ TCCs from SLE patients and healthy controls showed similar cytokine production [22
]. Whereas the relatively high proportion of hnRNP-A2 specific CD8+ TCCs might mirror the common understanding of a generally increased CD8:CD4 ratio in SLE patients [33
], there are only anecdotal reports about autoreactive CD8+ TCCs in SLE [6
] and their role is not entirely clear and may be quite complex indeed. Thus, on the one hand, the rather low IFNγ secretion of patient derived CD8+ TCCs might be due to inherent abnormalities of CD8+ (suppressor) T cell function in SLE, as shown recently by Filaci and colleagues [36
]. On the other hand, these TCCs secreted considerable amounts of IL-10 and, in contrast to the TCCs derived from healthy controls, were CD28 negative. Recently, a new T suppressor population was defined, which could be generated in vitro
from CD8+CD28- T cells [37
]. This subset exerted its regulatory and suppressive function in a cell contact independent manner via secretion of IL-10, which is in line with previous reports attributing a regulatory function to CD8+CD28- T cells [38
]. An increase of these cells in patients with SLE might thus constitute an effort of counter-regulation within the disturbed immune system of SLE patients.
Surprisingly, though, the supernatants of the hnRNP-A2 specific CD8+CD28- TCCs derived from SLE patients did not inhibit proliferation of CD4+ T cells, but instead enhanced anti-CD3/anti-CD28 induced stimulation, similar to supernatants derived from CD4+CD28+ TCCs. As IL-10 alone had an antiproliferative effect, the increased content of IL-10 in the supernatants of CD8+CD28- TCCs was obviously counteracted by other yet unknown stimulatory components of the supernatant, which remain to be identified. Of note, the lack of CD28 expression is also a hallmark of senescent T cells, which are known to be increased in various autoimmune diseases and chronic inflammatory conditions and may show a rather aggressive phenotype [41
]. Therefore, it is conceivable that the CD8+CD28- TCC may be derived from the senescent T cell pool of SLE patients; this also correlates better with our data.
Elevated IL-10 serum levels have been reported in SLE patients and correlated with clinical and serological disease activity, especially anti-DNA antibody titres [42
]. Several cell types have been implicated in the production of IL-10 [43
] and augmented IL-10 secretion has been linked to autoAb production in a model where PBMCs from patients with SLE were transferred into mice with severe combined immunodieficiency [44
]. So, altogether, the CD8+CD28- T cell subpopulation might enhance the inflammatory process in SLE patients by stimulation of B cells via IL-10. Unfortunately, because of the short life-span of the clones, additional functional assays could not be performed and remain the subject of future investigations.
Finally, the question remains why hnRNP-A2 is such a preferred target of the T cell response in SLE, while autoAbs occur less frequently and may represent an epiphenomenon of ongoing T cell autoreactivity. On the other hand, autoAb titers increase during disease flares (unpublished observation) and, importantly, autoAbs to hnRNP-A2 are, together with anti-Sm antibodies, among the earliest detectable autoAbs in MRL/lpr mice, where they were found to precede anti-dsDNA autoAbs [11
]. HnRNP-A2 is a multifunctional protein that, in the nucleus, partially colocalizes with spliceosomal complexes [13
], a preferred autoimmune target in SLE. Thus, the data obtained in the course of this study further strengthen the view of the spliceosome being one of the predominant autoimmune target structures in SLE, playing a presumably pivotal role in the pathogenesis of this disease, even though the reasons for this are still far from being fully understood.