Assembly of the immunoproteasome in cDC2 allows for cross-presentation of antigens under stimulatory conditions in vivo

Abstract

Dendritic cells are major initiators of immune responses, but are also responsible for the maintenance of peripheral T cell tolerance. A critical decision for induction of immune activation or immune tolerance is dependent on the activation status of DCs, which could be induced by the presence of diverse pathogens. For the recognition of pathogen associated molecular patterns (PAMPs) DCs express pattern recognition receptors (PRR), such as Toll-like receptors (TLR), or C-type lectin receptors (CLR). Previously, we have demonstrated, that steady-state DC subpopulations are specialized regarding processing and presentation of captured antigens. Therefore, DCs tightly control and direct activation of different T cell responses. Here, we investigated how TLR-mediated activation of murine cDC1 and cDC2 DCs influences the expression of molecules involved in the MHC class I antigen processing machinery. We provide evidence that activation of cDC2 DCs induces upregulation of Calreticulin, Calnexin, Erap1, ERp57, TAP1, TAP2, and Tapasin under certain immunostimulatory conditions in vivo. Importantly, delivery of targeted antigens to cDC2 DCs in combination with specific TLR-ligands allowed for cross-presentation and activation of CD8+ T cells in wildtype but also BATF3-/-(missing cDC1 DCs) mice in vivo. Moreover, the complete knock out of the three immunoproteasome subunits (β1i, β2i, β5i) hindered the cross-presentation of antigens by cDC2, but not cDC1 DCs. Our findings suggest that DCs harbor a strong flexibility in counteracting infections and tumor development if they are appropriately stimulated.