This study investigates the immune dysregulation in rheumatoid arthritis (RA) flares compared to drug-free remission using a combination of mass cytometry and single-cell RNA sequencing. The experimental approach involves the withdrawal of immunomodulatory drugs in RA patients to synchronize flare processes, allowing detailed characterization. Key findings include the identification of three circulating cellular subsets—CD45RO+PD1hi CD4+ and CD8+ T cells, and CD27+CD86+CD21+ B cells—that precede arthritis flare. Single-cell sequencing reveals distinct lymphocyte subsets, including cytotoxic and exhausted CD4+ memory T cells, memory CD8+CXCR5+ T cells, and IGHAI+ plasma cells, which are primed for activation in flare patients. Regulatory memory CD4+ T cells (Treg cells) increase at flare onset but exhibit dysfunctional regulatory marker expression compared to drug-free remission. Significant clonal expansion is observed in T cells but not in B cells after drug cessation, particularly in the granzyme-expressing cytotoxic subset within CD4+ memory T cells. The study suggests a model of immune dysregulation in RA flare, highlighting the role of specific lymphocyte subsets and Treg cells in maintaining drug-free remission. These findings provide insights into the underlying immunological mechanisms of RA flare and offer potential avenues for future translational research.This study investigates the immune dysregulation in rheumatoid arthritis (RA) flares compared to drug-free remission using a combination of mass cytometry and single-cell RNA sequencing. The experimental approach involves the withdrawal of immunomodulatory drugs in RA patients to synchronize flare processes, allowing detailed characterization. Key findings include the identification of three circulating cellular subsets—CD45RO+PD1hi CD4+ and CD8+ T cells, and CD27+CD86+CD21+ B cells—that precede arthritis flare. Single-cell sequencing reveals distinct lymphocyte subsets, including cytotoxic and exhausted CD4+ memory T cells, memory CD8+CXCR5+ T cells, and IGHAI+ plasma cells, which are primed for activation in flare patients. Regulatory memory CD4+ T cells (Treg cells) increase at flare onset but exhibit dysfunctional regulatory marker expression compared to drug-free remission. Significant clonal expansion is observed in T cells but not in B cells after drug cessation, particularly in the granzyme-expressing cytotoxic subset within CD4+ memory T cells. The study suggests a model of immune dysregulation in RA flare, highlighting the role of specific lymphocyte subsets and Treg cells in maintaining drug-free remission. These findings provide insights into the underlying immunological mechanisms of RA flare and offer potential avenues for future translational research.