The surface of dendritic cells (DC) has been analyzed by means of monoclonal antibodies (Ab) and lactoperoxidase (LPO)-mediated radioiodination. Antigens and other exteriorily disposed polypeptides of purified spleen DC were compared with those of tissue macrophages (Mφ), monocytes, and other bone marrow-derived elements.

Quantitative binding studies and autoradiography with (125)I-Ab established that DC expressed high levels of I-A and H-2D, 2 × 10(5) and 1 × 10(5) Ab binding sites per cell, respectively. DC from conventional, germ-free, and specific pathogen-free mice were all rich in Ia. Expression of Ia on B cells was 5-10 percent of that on DC and increased fivefold during lipopolysaccharide mitogenesis. More than 70-90 percent of purified Mφ and monocytes from specific pathogen-free mice were Ia negative, but increased levels of Ia were noted on cells from mice reared under conventional conditions. Thus large amounts of Ia on DC is a constitutive trait, whereas the expression of Ia by other cell types may be governed by the environmental and immunological status of the host.

The 2.4G2 Fc receptor Ag was not detected on DC. Peritoneal and spleen Mφ had 10(5) 2.4G2 binding sites/cell, whereas monocytes and lymphocytes were less reactive (1 × 10(4)-3 × 10(4) binding sites/cell).

Four other Mφ-related antigens were evaluated. Each had a distinctive tissue distribution and none bound exclusively to Mφ and monocytes. Neither 1.21J (Mac-1) nor F4/80 reacted with DC. Immunoprecipitation studies of externally ((125)I) and biosynthetically ([(35)S]methionine)dabeled cells confirmed the binding data.

Sensitive binding assays with (125)I-Ab confirmed previous observations that DC lack Ig and Thy-1. Lyt-1 was also not found on DC, but 5-12 percent of the cells in purified DC preparations expressed both Lyt-2 and Ia. All DC expressed the leukocyte common antigens at levels similar to other leukocytes.

The spectrum of surface polypeptides labeled by LPO-mediated iodination was different on Mφ, DC, and lymphocytes. Polypeptides migrating at molecular weights of 155,000, 85,000, and 62,000 appeared to be restricted to DC.

These observations establish that the cell surface of DC differs considerably from other leukocytes, including the blood monocyte, and suggest that the DC is part of a unique Ia-rich leukocyte differentiation pathway.

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