Sjögren's disease and systemic lupus erythematosus overlap syndrome as distinct entity at crossroads of two autoimmune disorders: clinical characterisation from two Italian reference centres for both the diseases.

Objectives: To characterise the overlap syndrome between Sjögren's disease (SjD) and systemic lupus erythematosus (SLE).

Methods: Consecutive patients clinically defined as affected by SjD and SLE overlap syndrome (SjD-SLE), belonging to two Italian rheumatology centres were classified following the application of both the SjD and SLE classification criteria. Clinical, functional, ultrasound and histological data were compared with patients suffering from only SjD or SLE.

Results: Compared to SjD controls, SjD-SLE patients were younger at onset (p<0.0001). Schirmer's test and parotid swelling were comparable between the two groups, while unstimulated sialometry was more impaired in the SjD controls (p=0.0001). SjD-SLE cases showed increased joint (p=0.009), mucocutaneous (p<0.0001), renal (p=0.001) involvement, and serositis (p<0.0001). Ultrasound changes in the major salivary glands were prevalent in SjD controls, while the histological findings of the minor salivary glands were similar. Furthermore, SjD-SLE cases presented a higher prevalence of anti-SSA (p<0.0001) and lower presence of rheumatoid factor (p=0.008) and serum cryoglobulins (p=0.035). Compared to SLE controls, SjD-SLE were older (p=0.044). The frequency of extra-glandular manifestations of SjD-SLE was similar compared to SLE, including renal involvement. SjD-SLE patients showed higher prevalence of anti-SSA and anti-SSB (p<0.0001), C4 reduction (p=0.011), and leukopenia (p=0.025).

Conclusions: Our data further highlight the limitations of the application of the current classification criteria in overlap syndrome, since they are primarily based on clinical manifestations and common autoantibodies. Molecular signatures may explain clinical similarities and differences among systemic autoimmune diseases, and they may be particularly helpful in overlap syndromes.