High mobility group protein 1 (HMGB1) is a non-histone nuclear protein with a dual function. Inside the cell, HMGB1 binds to DNA and modulates a variety of processes, including transcription. Outside the cell, HMGB1 can serve as an alarmin to mediate disease manifestations in animal models of sepsis and arthritis; in these models, blocking HMGB1 can attenuate disease.1^–3

In in vitro experiments, HMGB1 translocation and cellular release can occur during activation as well as cell death and is present in tissue in conditions, such as rheumatoid arthritis and cutaneous lupus.4 5 While original studies suggested that release occurs only with necrosis,6 more recent studies have shown that HMGB1 release also occurs in late apoptosis.7 8 As increased apoptosis and decreased clearance of apoptotic material may underlie the pathogenesis of systemic lupus erythematosus (SLE), these findings suggest that extracellular HMGB1 levels rise in this disease and promote systemic and local inflammation.

To elucidate the expression of HMGB1 in SLE, we have investigated blood levels of HMGB1, using Western blotting to analyse serum samples from a murine lupus model and patients with SLE. We obtained serum samples from MRL/MpJ-lpr/lpr and BALB/c mice purchased from the Jackson Laboratory (Bar Harbor, MA, USA). Human SLE serum samples were purchased from Immunovision (Springdale, AR, USA). For Western blotting, electrophoresis was performed using a 4–12% Bis-Tris sodium dodecyl sulphate–polyacrylamide gel electrophoresis (Invitrogen, San Diego, CA, USA). Protein was transferred on to PDVF membrane and blotted with a rabbit anti-HMGB1 polyclonal antiserum (gift of Dr Kevin Tracey, North-Shore Jewish Hospital, Long Island, NY, USA) followed by HRP-conjugated anti-rabbit IgG and Super Signal West Femto substrate (Pierce, Rockford, IL, USA). Images were captured with a CCD camera.

As data in fig 1 indicate, sera from patients with SLE as well as MRL/MpJ-lpr/lpr mice show increased levels of HMGB1 by Western blotting. To assess the extent of this increase, the density of the HMGB1 band was analysed by AlphaEasyFC version 3.1.2 and the value expressed as fold increase over control. In samples studied, for human sera, the amounts of HMGB1 increased 2.8–36-fold while, for the murine sera, the values increase 1–28-fold over controls. Furthermore, in MRL/MpJ-lpr/lpr mice, the HMGB1 levels rose with disease progression (data not shown). Together, these data indicate that HMGB1 release occurs with SLE and can produce increased levels in the sera similar to that occurs in sepsis and shock.2 9 Further analysis will be need to determine the relationship to disease activity and treatment.

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Figure 1 Detection of high mobility group protein 1 (HMGB1) in mouse and human sera. Sera from either mice (A) or human (B) were resolved on sodium dodecyl sulphate–polyacrylamide gel electrophoresis and analysed by Western blotting with an anti-HMGB1 antibody. SLE, systemic lupus erythematosus.

In the context of SLE, increased levels of other nuclear constituents in the blood (eg, DNA) have been attributed to cell death, with impaired clearance mechanisms (eg, complement deficiency) preventing normal disposal. Extracellular HMGB1 in SLE may have a similar origin and, indeed, this protein may be released along with other nuclear constituents to form immune complexes.9 While the mechanisms for HMGB1 activation require future investigation, these observations suggest that increased levels of this protein may be a marker of immune cell abnormalities in SLE and contribute to systemic inflammation and autoreactivity. As such, SLE may represent an alarmin-mediated disease.