Antigen-presenting cell lines internalize peptide antigens via fluid-phase endocytosis

In this report, we present experimental evidence that antigen-presenting cell lines take up peptide antigens in a manner consistent with fluid-phase endocytosis. Using the fluid phase endocytic marker inulin and a mathematical model for fluid phase uptake, we have found a basal uptake rate constant of approximately 0.9-2 microns 3/cell minutes in A20, TA3, and J774 cells. An influenza virus peptide, PB2(303-313), the octapeptide, angiotensin II, an ovalbumin peptide, OVA(323-339), and a guinea pig myelin basic protein peptide, MBP(72-86), have uptake rate constants comparable to inulin, i.e., between 1 and 4 microns3/cell minutes in A20 cells. However, another influenza virus peptide, PB2(146-159), has an uptake rate constant approximately sixfold higher than that found for inulin in A20 cells. We have also determined that the peptide antigens we tested are retained in A20 cells similarly to inulin, with half-times calculated to be from 2 to 13 min as compared to 2 min for inulin. Notably, these results were obtained over short incubation times (up to 20 min) and under conditions that restrict peptide proteolysis and also protein synthesis. We conclude from these studies that peptide antigens enter antigen-presenting cells via fluid-phase endocytosis.