Background & objectives: The presence of CD4+CD8+ (double positive) T cells (DPT) in the target organs of several autoimmune diseases has been reported. The aim of this study was to investigate the pathogenic role of DPT in systemic lupus erythematosus (SLE).

Methods: A total of 175 SLE cases and 125 matched healthy controls were investigated for CD3+, CD4+, CD8+ lymphocytes and DPT by flow cytometry. Serum samples from SLE patients and controls were tested for antinuclear antibody (ANA), anti-double strain deoxyribonucleic acid (anti-dsDNA), anti-U1 ribonucleoprotein (anti-U1 RNP), anti-sjogren syndrome A (anti-SSA), anti-ribosomal P protein (anti-rib-P), anti-Smith (anti-Sm), anti-Sjogren syndrome B (anti-SSB), complement 3 (C3) and complement 4 (C4).

Results: The DPT median and 5-95 per cent range of SLE cases and healthy controls were 0.50 [0.10-2.60] and 0.80 [0.20-2.74] respectively (P<0.001). SLE patients were divided into a ≥1:1000 subgroup and a <1:1000 subgroup according to the ANA titre. The DPT of the former subgroup was significantly lower than that of the latter (P=0.032). The DPT medians of positive subgroups with anti-dsDNA (P<0.001), anti-U1RNP (P=0.018), anti-SSA (P=0.021) or anti-rib-P (P=0.039) were also significantly lower than the negative subgroups. Likewise, DPT was significantly lower in SLE subgroups with low concentration of C3 or C4 than those with high concentration (P<0.006).

Interpretation & conclusions: Our findings show that the DPT cells may play a key suppressive role in the production of autoantibodies in SLE. Direct evidence that DPT regulates the pathogenesis of SLE needs to be investigated in future work.