Circulating CD2(+) monocytes are dendritic cells

Low levels of CD2 have been described on subsets of monocytes, macrophages, and dendritic cells. CD2 is expressed an about one-third of circulating mo nocytes, at levels one-half log lower than on T or NK cells, representing 2 -4% of PBMC. FAGS analysis of CD2(+) and CD2(-) monocytes revealed no signi ficant difference in the expression of adhesion molecules (CD11a/b/c), clas s II Ags (HLA-DR, -DQ, -DP), myeloid Ags (CD13, CD14, CD33), or costimulato ry molecules (CD80, CD86). Freshly isolated CD2(+) and CD2- monocytes were morphologically indistinguishable by phase contrast microscopy. However, sc anning electron microscopy revealed large prominent ruffles on CD2(+) monoc ytes in contrast to small knob-like projections on CD2(-) monocytes. After 2 days of culture, the CD2(+) monocytes largely lost CD14 expression and de veloped distinct dendrites, where as the CD2(-) monocytes retained surface CD14 and remained round or oval. Freshly isolated CD2(+) monocytes were mor e potent inducers of the allogeneic MLR and more efficiently induced prolif eration of naive T cells in the presence of HIV-1 gp120 than did CD2(-) mon ocytes. After culture in the presence of GM/CSF and IL-4, CD2(+) monocytes were up to 40-fold more potent than monocyte-derived dendritic cells or CD2 - monocytes at inducing allogeneic T cell proliferation. These findings sug gest that circulating CD2(+) and CD2(-) monocytes are dendritic cells and t he precursors of macrophages, respectively. Thus, dendritic cells are far m ore abundant in the blood than previously thought, and they and precursors of macrophages exist in the circulation as phenotypically, morphologically, and functionally distinct monocyte populations.