Psoma, A.; Muntjewerff, E. M.; Nicolasen, M. J. T.; Bottema, R.; ter Beest, M.; de Boer, R.; Bianchi, F.; Revelo, N. H.; van den Bogaart, G.

Interleukin-12 (IL-12) is a key immunostimulatory cytokine produced by dendritic cells (DCs) upon infection that plays a central role in the activation and differentiation of cytotoxic T cells. Notably, IL-12 is secreted in a highly polarized manner from late endosomes or lysosomes at the contact interface between a DC and a T cell, known as the immunological synapse. However, the signaling mechanisms initiating this spatially restricted cytokine release remain poorly defined. Here, using artificial T cell mimics, beads coated with T cell surface ligands, we show that antigen recognition induces recruitment of the DCs microtubule organizing center (MTOC) and Golgi apparatus to the immunological synapse via reverse MHC class I signaling. Our results further indicate that the signaling pathways governing MTOC polarization are highly conserved in DCs, consistent with mechanisms described in other immune cell types. As newly synthesized IL-12 traffics through the Golgi, this intracellular reorientation facilitates localized release of IL-12 directly at the site of T cell engagement, likely enabling DCs to focus their stimulatory capacity on responsive T cells within the crowded environment of secondary lymphoid organs. Furthermore, by applying spatially patterned antibody arrays targeting MHC class I, we reveal that MTOC polarization is biased toward sites exhibiting the strongest reverse MHC class I signaling. This suggests that IL-12 is preferentially released at synapses formed with T cells displaying higher T cell receptor affinity, providing a mechanism for selection of the most potent T cell.