Belimumab After Rituximab in Resistant Primary Juvenile SS

The goal of this clinical trial is to explore the effectiveness of sequential use of rituximab and belimumab in the treatment of resistant primary juvenile Sjogren's syndrome. Does sequential use of rituximab and belimumab reduce the activity of SS in resistant patients Researchers will compare the disease activity before and after the treatment of sequential use of rituximab and belimumab to see if the therapy works to treat SS. Participants will: Recieve Rituximab each week for 2-4 times until B%\<0.5% or B#\<20×10\^6/L Recieve Belimumab 4 weeks after the last use of Rituximab, and then every 4 weeks until week 28