Figure 5. Model for TRIM21‐mediated Cross‐presentation.

Immune complexes (e.g. Ab:N protein) are phagocytosed—“Uptake”—by APCs, before being moved from the endosome to the cytosol—“Import”. While multiple routes for uptake have been described, the mechanism and regulation of exogenous protein import are not well understood. Once in the cytosol, immune complexes are detected by the high‐affinity Fc receptor and E3 ubiquitin ligase TRIM21. TRIM21 catalyses ubiquitination to recruit the proteasome and cause complex degradation. Proteasome‐derived peptides can then be loaded onto MHC Class I molecules and exported to the cell surface to activate T cells.