TAP1 protein levels in primary tonsillar B lymphocytes are reduced to about one third after incubation with IL-10 or IL-10 containing supernatants (supernatant from B95.8 cells, VFS, supernatant-derived Cos7 cells transiently transfected with expression plasmids of hIL-10 and vIL-10, or 0.03 μg/mL purified rIL-10) in comparison with cell culture medium (RPMI). After 48 hours of incubation, TAP1 protein is reduced but still detectable, presumably due to its extended protein half life. An in vitro-generated LCL (Be 4.6, lane 7) produces significantly lower levels of TAP1 as compared with primary B cells. Equal amounts of total cellular protein were loaded in each lane. TAP1 protein was visualized after Western blotting with a monoclonal antibody directed against TAP1.