Autoantibodies directed against nucleosomal antigens such as dsDNA and histones are a hallmark of systemic lupus erythematosus (SLE). Histone H1 constitutes the major antigen in LE cell formation and antibodies to H1 have been shown to be more specific for SLE than other anti-histone antibodies [1,2]. In order to address the role of histone H1-specific T cells in SLE we investigated the ex vivo cellular reactivity to histone antigen in SLE patients and controls, and characterised H1-specific T-cell clones.
Peripheral blood mononuclear cells of 39 SLE patients and 20 healthy controls (HC) were exposed to purified histones (H1, H2A, H2B, H3, H4), and proliferation as well as cytokine production was measured. In addition, H1-specific T-cell clones were drawn by limiting-dilution cloning of T-cell lines. T-cell phenotyping was done by FACS analysis and the cytokines IL-4, interferon gamma (IFN-γ) and tumour necrosis factor alpha (TNF-α) were measured in the supernatants by ELISA.
After stimulation with histone H1, SLE patients showed a significantly elevated proliferative response as measured by stimulation indices (SI) when compared with HC (SI mean ± SD, 2.2 ± 1.4 versus 1.5 ± 0.4, P < 0.03) and a positive response (i.e. SI > 2) in 16 patients compared with only two responders among HC (Fig. 1). The proliferative response to H2A was also elevated in SLE patients (SI, 2.9 ± 2.5 versus 1.7 ± 0.8 in HC, P < 0.03), whereas the response to H2B, H3 and H4 did not differ between patients and HC. Cellular responses to H1 correlated with the presence of anti-histone antibodies but not with clinical features or SLE disease activity. H1-specific T-cell clones obtained from one SLE patient and two HC showed a Th1-like phenotype producing large amounts of IFN-γ and TNF-α but no IL-4.
Figure 1. Elevated proliferative responses to histone H1 in patients with systemic lupus erythematosus (SLE). Proliferation of peripheral blood mononuclear cells from 39 SLE patients and 20 healthy controls (HC) was analysed by measuring [3H]TdR incorporation, and a stimulation index (SI) > 2 was considered a positive response.
The increased Th1 autoreactivity to histones H1 and H2A in SLE patients suggests that these T cells play an important role in the pathogenesis of SLE not only by driving autoantibody responses but also by virtue of production of the proinflammatory cytokines IFN-γ and TNF-α.