The pathway of cross‐presentation is influenced by the particle size of phagocytosed antigen

Cross-presentation is the presentation by MHC class I of antigenic peptides from exogenous proteins that have been internalised and processed by professional antigen presenting cells, eg. dendritic cells (DC). We have investigated the influence of particle size and antigen load on cross-presentation following antigen delivery on microspheres (MS). Cross-presentation from small particles (0.8 ?m) is sensitive to proteasome inhibition and the blockade of ER-resident MHC class I complex export, whereas cross-presentation from larger particles (aggregated clumps of 0.8 ?m MS) is resistant to these antagonists. This observation may have been overlooked previously, due to the heterogeneity of particle size and MS uptake in unsorted DC populations. Whilst larger particles carry more antigen, we show that antigen load does not influence the cross-presentation pathway utilised. Whereas early endosome autoantigen 1 (EEA1) could be observed in all phagosomes, we observed endoplasmic reticulum SNARE of 24 kDa (ERS24) and cathepsin S in association with 3.0 ?m and aggregated 0.8 ?m MS, but not individual 0.8 ?m MS. A potential mechanism underlying our observations may be the activation of ?-catenin by disruption of E-cadherin-mediated adhesion. Activated ?-catenin was detected in the cytoplasm of cells after phagocytosis of MS (highest levels for the largest particles). We propose that particle size can direct the use of different pathways for the cross-presentation of an identical antigen. Furthermore, these pathways have differing yields of MHC class I-peptide complexes, which is an important variable in designing vaccination strategies for maximal antigen expression and CD8(+) T cell priming.