If you have an antibody that recognize your antigen, then you could perform an immunostaining of your APC cells and see if you can detect specific cytoplasmic signal of your antigen in a pattern of small dots (endosomes), by using confocal microscopy. However, once internalized into APC, antigens are rapidly degraded (within lysosomes). Thus I would fix the APC cells as soon as possible after their exposure to the antigen, permeabilize them and do the immunostaining. You should also use APC cells not non exposed to your antigen as a negative control.

Dear Lorenzo Bossi,

For the quantitation of the bound and internalized antigen, you could label your antigen with radioactive iodine and measure the cell associated radioactivity. To this end, simply spin your cells through a layer of a dodecane/bromododecane mix to separate the cells efficiently from the liquid, exploiting differences in density. For details on the centrifugation step, see e.g. Article In vitro phage display in a rat beta cell line: A simple app...

For subcellular localization, electron microscopy of sections embedded after staining with gold labelled antibodies should do the trick.

Demonstrating presentation by dendritic cells means having to show that a particular peptide made it into the dendritic cell and back to the surface on MHC.

This requires antigen specific T-cells to respond to the dendritic cell.

Internalization can be demonstrated by using a radioactive labeled antigen (or other label). Add to your PBMC, wait an appropriate amount of time (you will need to determine this). Then treat cells with a strong protease to strip off any antigen on the surface. Then spin the cells down, measure residual radioactivity. If you don't wish to label the antigen you can also do this with an antigen capture assay by lysing the cells and detecting antigen. Controls consist of cells at 4 degrees C during the antigen incubation. The antigen will stick, but will not be taken into the cells.