Chimeric Antigen Receptor (CAR) T cell therapy has proven to be an effective strategy against hematological malignancies but persistence and activity against solid tumors must be further improved. One emerging strategy for enhancing efficacy is based on directing CAR T cells to antigen presenting cells (APCs). Activation of CAR T cells at the immunological synapse (IS) formed between APC and T cell is thought to promote strong, persistent antigen-specific T cell-mediated immune responses but requires integration of CAR ligands into the APC/T-cell interface. Here, we demonstrate that CAR ligand functionalized, lipid-coated, biodegradable polymer nanoparticles (NPs) that contain the ganglioside GM3 (GM3-NPs) bind to CD169 (Siglec-1)-expressing APCs and localize to the cell contact site between APCs and CAR T cells upon initiation of cell conjugates. The CD169 APC/CAR T-cell interface is characterized by a strong optical colocalization of GM3-NPs and CARs, enrichment of F-actin, and recruitment of ZAP-70, indicative of integration of GM3-NPs into a functional IS. Ligands associated with GM3-NPs localized to the APC/T-cell contact site remain accessible to CARs and result in robust T-cell activation. Overall, this work identifies GM3-NPs as a potential antigen delivery platform for active targeting of CD169 expressing APCs and enhancement of CAR T-cell activation at the NP-containing IS.