Background/objectives B-cells are key players in immune responses and autoimmunity through antibody production, antigen presentation and cytokine production. Fine tuning of B-cell activation depends on convergent signals from the B-cell receptor (BCR) and costimulatory membrane receptors. We studied the expression and function of the recently characterised costimulator HVEM and its receptors, and investigated its role in B-cell activation and differentiation in healthy individuals and patients with systemic lupus erythematosus (SLE).
Materials/methods Peripheral blood mononuclear cells were isolated from healthy volunteers and patients with active SLE. Expression of HVEM and its ligands, BTLA and LIGHT, were assessed by flow cytometry at basal conditions and following activation. Immunomagentically-sorted CD19+total, CD19+CD27-naïve, or CD19+CD27+memory B-cells were stimulated with anti-IgM±IL-21 in presence or not of plate-bound HVEM. Fc chimeric protein to evaluate effects on cell proliferation (CFSE assay), apoptosis (Annexin/7AAD assay), activation (CD80/86, CD40), cytokine production (IL-6) and differentiation (CD38, Ig secretion).
Results Active SLE patients had significantly higher proportion of circulating HVEM-expressing CD4+T-cells than healthy individuals (95.4 ± 1.3% vs. 85.4 ± 3.9%, p = 0.013). Both patients and controls demonstrated high-level expression of BTLA on freshly isolated peripheral blood B-cells, whereas LIGHT had minimal basal expression and was induced following BCR activation. HVEM. Fc crosslinking on B-cells activated with anti-IgM±IL-21 led to significantly reduced cell proliferation (by 72 ± 3%) and membrane CD80/CD86 expression (by 29 ± 4%). Notably, in 5–8 days cell cultures, HVEM. Fc induced the differentiation of both CD27- and CD27+ B-cells, as evidenced by the increased expression of CD27/CD38 (CD27+: 27.9% vs. 18.4%; CD38+: 25.0% vs. 21.5%) and CD38/CD138, respectively (CD38+ +: 6.0% vs. 3.8%). This finding was accompanied by strong anti-apoptotic effect of HVEM. Fc treatment. Further experiments are underway to elucidate the molecular mechanisms of HVEM. Fc-mediated effects on B-cells and the potential therapeutic use of HVEM/BTLA manipulation in SLE.
Conclusions HVEM exerts significant B-cell co-stimulatory effects through regulation of proliferation, apoptosis and differentiation. Increased HVEM expression on SLE CD4+T-cells might contribute to enhanced memory and plasma cell differentiation observed in the disease.