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CAMPATH antibodies recognize CD52, a phosphatidylinositol-linked membrane protein expressed by mature lymphocytes and monocytes. Since some antigen-presenting dendritic cells (DCs) differentiate from a monocytic progenitor, we investigated the expression of CD52 on dendritic cell subsets. Four-color staining for lineage markers (CD3, 14, 16, 19, 20, 34, and 56), HLA-DR, CD52, and CD123 or CD11c demonstrated that myeloid peripheral blood (PB) DCs, defined as lineage⁻HLA-DR⁺CD11c⁺, express CD52, while expression by CD123⁺ lymphoid DCs was variable. Depletion of CD52⁺ cells from normal PB strongly inhibited their stimulatory activity in an allogeneic mixed lymphocyte reaction and also reduced the primary autologous response to the potent neoantigen keyhole limpet hemocyanin. CD52 is thus expressed by a myeloid subset of PBDCs that is strongly allostimulatory and capable of …