Autoantigen TRIM21 (Ro52) assembles pro-inflammatory immune complexes following lytic cell death

Sjogrens disease (SjD) causes localised and systemic inflammation due to autoantibody production against intracellular proteins, such as TRIM21/Ro52. TRIM21 is an E3 ubiquitin ligase which binds antibody Fc domains on opsonised pathogens, which have escaped extracellular immunity and entered cytosols; TRIM21 ubiquitinates these, driving their proteasomal degradation. How and why TRIM21 becomes an autoantigen remains unclear. We show that TRIM21 is released upon lytic cell death (pyroptosis/necroptosis) but not apoptosis. Released TRIM21 binds circulating antibody Fc domains, and forms large immune complexes (ICs). These are further enhanced with TRIM21/Ro52 seropositive SjD plasma antibodies, where interactions are mediated via both Fc and F(ab)2 domains. TRIM21-ICs are taken up by macrophages, which in high interferon environments drive pro-inflammatory responses, antigen presentation, and inflammatory and metabolic transcriptional changes. Whilst many cytosolic proteins are released by dead cells, due to its high affinity for antibodies, TRIM21 can generate large ICs. This may perpetuate inflammation and antigen presentation, causing TRIM21 to be highly autoimmunogenic. One Sentence SummaryHow the intracellular protein TRIM21 becomes an autoantigen.