(A) Flow cytometric analysis shows the downregulation of surface MHC class I molecules on peripheral blood B cells after incubation for 2 days with B95.8 supernatant, VFS, or hIL-10 (200 pg/mL) or vIL-10 (200 pg/mL) derived from transiently transfected Cos7 cells. Results are given as mean fluorescence intensity values. The mean value of a reference antibody was less than 5 as a negative control (not shown). The mean fluorescence value of the control cells incubated with cell culture medium was 313 (RPMI 313). (Thin line) MHC class I expression on control cells incubated with cell culture medium, containing 1% of supernatant from untransfected Cos7 cells. (Thick line) MHC class I expression on the same cells treated with either B95.8, VFS, vIL-10, or hIL-10, which showed mean fluorescence intensity values of 139, 147, 178, and 196, respectively. (B) The same effect was observed with cells incubated in cell culture medium containing purified recombinant human IL-10 (0.03 μg/mL rIL-10) in comparison with cell culture medium only. Mean fluorescence values were 1,373 and 598, respectively.