Human dendritic cells express a 95 kDa activation/differentiation antigen defined by CMRF-56

Despite the unique functions of dendritic cells (DC), only two cell surface antigens (CMRF-44 and CD83) with relatively restricted expression on human DC have been described to date. We describe a third mAb, CMRF-56, which re cognizes another DC early activation/differentiation antigen with limited e xpression on other haemopoietic cell populations. Circulating blood leukocy tes did not express the CMRF-56 antigen and following either in vitro cultu re or activation of PBMC populations. CMRF-56 antigen expression was detect ed only on DC and a subpopulation of CD19(+) lymphocytes. Circulating blood DC were CMRF-56(-) but induced expression within 6 h of in vitro culture. This, together with the finding that tonsil and synovial fluid DC upregulat e the antigen following short-term in vitro culture, confirmed that CMRF-56 recognizes an early activation antigen on DC. Isolated Langerhan's cells, dermal DC, migratory dermal DC and monocyte derived DC (GM-CSF/IL-4/TNF alp ha) also espress the CMRF-56 antigen antigen modulation studies demonstrate d that the amount of cell surface bound CMRF 56 and CMRF-44 (but not CD83) mAb was dramatically reduced by short-term incubation at 37 degrees C. This effect was not due to internalization and the reduction in CMRF-56 binding was a reversible temperature-dependent process. In contrast, the decrease in CMRF-44 binding was irreversible, suggesting that following ligation the CMRF-44 antigen undergoes an irreversible conformational change or sheddin g at 37 degrees C. Western blotting confirmed that CMRF-56 recognizes a pre viously undescribed 95 kDa activation antigen whose cellular distribution a nd expression kinetics overlaps with, but is clearly distinguishable from, that that of the CD83 and CMRF-44 antigens. CMRF-56 therefore provides a us eful additional marker for studies on human DC.