MHC class I and II and CD80 expression by U937 cells. (A) Single-color labeling demonstrated that U937 cells have similar levels of MHC class I and II as DCs. Control U937-M3P cells were CD80⁻, and U937-CD80 cells had a high level of CD80 expression that was greater than that of DC. (B) Dual-labeling using anti-CD80 antibody and CTLA4.Ig demonstrated no CD80/CD86 expression on the control U937-M3P population and high anti-CD80 as well as CTLA4.Ig binding on the U937-CD80 population.