Soluble MHC-Ig dimers, Kb-Ig, was prepared and loaded with peptides as described (17 (link)), see Supplemental Materials. Nano-aAPC were manufactured by direct conjugation of MHC-Ig dimer and anti-CD28 antibody (37.51; Biolegend) to MACS Microbeads (Miltenyi Biotec) as described previously (18 (link)).