<p>Antibodies.</p>

The autoimmune disease systemic lupus erythematosus (SLE) is associated with genetic variants in the X-linked gene CXORF21, which encodes the protein TASL. TASL acts as an adaptor in the IRF5 pathway and is necessary for the phosphorylation of IRF5 in response to TLR7 or TLR9 stimulation. Here, we investigate the role of TASL in the humoral immune response, and in the development of lupus in the B6.MRLlpr murine model of SLE. We find that while TASL is dispensable for their development, it is required for the full activation of B cells via TLR9 stimulation, and consequent interferon signaling and inflammatory cytokine expression. Additionally, TASL is crucial for the emergence of age-associated B cells (ABCs), a B cell population derived from the extrafollicular response that increases with age and is expanded in autoimmune disease, and the production of IgG2c antibodies. We also find that deletion of TASL prevents the onset of autoimmunity in the genetically-determined B6.MRLlpr model of lupus.