Milk samples were thawed at 23°C and exosomes were isolated from 1 mL human milk by using differential ultracentrifugation as previously described with minor modifications [11 (link)]. Experimental details have been deposited in EV-Track (https://evtrack.org) and can be accessed through EV-Track ID: EV210145. Protocols used are consistent with recommendations by the International Society for Extracellular Vesicles and the NIH exRNA Consortium [13 , 15 (link)]. Exosomes were re-suspended in 200 μL PBS and counts and sizes were assessed using a NanoSight NS300 instrument. Exosome proteins were extracted using ice-cold radioimmunoprecipitation assay buffer (Sigma-Aldrich) with protease inhibitor cocktail (Sigma-Aldrich). Antibodies against CD-9 (GeneTex, Irvine, CA), CD63 (Santa Cruz, Dallas, TX) and Alix (Santa Cruz) were used as positive controls; Antibodies against α-tubulin (Santa Cruz) and integrin-β (Abcam, Cambridge, MA) were used to assess contamination with microvesicles. We probed human caseins in EV preparations from human milk with anti-human β-casein (Novus Biologicals, Littleton, CO). Anti-α-lactalbumin (Abcam) and anti-butyrophilin (R&Dsystems, Minneapolis, MN) were used to probe whey proteins and lipid globules, respectively.