Belimumab in SLE Synovial Inflammation and Lymph Nodes

The primary goal of this study is to investigate the effects of belimumab on the composition of lymph nodes and the inflamed synovial tissue as well as (subsets of) immune cells in the peripheral blood using advanced flow cytometry methods. In addition, immunological alterations in lymphoid tissue and inflamed synovial tissue of SLE patients will be identified and correlated with disease stage/phenotype, prognosis, and belimumab treatment response. In this way novel biomarkers may be identified and validated that can be used for personalized medicine in SLE. The specific types of immunological alterations that will be studied include: * Genetic, epigenetic and transcriptional alterations of immune cells and stromal cells, including single cell RNA sequencing (scRNAseq) * TCR and BCR repertoire * Phenotype and function of (auto-reactive) T and B cells * Cytokine production by immune cells and stromal cells * Signalling events in immune cells and stromal cells * Antigen presentation by immune cells and stromal cells All of these parameters will be compared between lymph nodes, blood and synovium (if applicable) in SLE patients before and after belimumab treatment. Research Hypothesis Patients with SLE contain activated immune cells in their lymph nodes and inflamed synovial tissue. Furthermore, activation of stromal cells such as fibroblasts and endothelial cells may also be increased compared to (historical) healthy controls. In addition, dominant T cell and B cell clones may be present. Belimumab treatment will result in reduced inflammation in the synovial tissue and perhaps even normalization of lymph node architecture.