DLS shows presence of bovine serum albumin (BSA) (A) or IgG (B) alone at concentrations matching biological fluids does not cause NP aggregation, but does increase NP diameter with increasing BSA and IgG concentration and is stable over time. n = 6 from two independent experiments each containing triplicate measurements (A). n = 9 from three independent experiments containing triplicate measurements. **p < 0.05, ****p < 0.0001 using two-way ANOVA to determine if protein concentration affects NP diameter. Error bars represent the standard error of the mean. C, D) Representative TEM micorographs show intracellular membrane-bound vesicles, which are empty in untreated mice (C) and contain stable NPs in treated mice (D, arrow). Dashed line indicates cell membrane separating intracellular ductal compartment and the ductal lumen.