Circulating CD33+ large mononuclear cells contain three distinct populations with phenotype of putative antigen-presenting cells including myeloid dendritic cells and CD14+ monocytes with their CD16+ subset

BACKGROUND: In peripheral blood, myeloid markers identify a heterogeneous mixture of cells in transit from the bone marrow to peripheral tissues. Similarly, HLA-class II DR expression usually identifies mononuclear cells with the potential for developing antigen-presenting activity. We gathered putative antigen presenting cells bearing myeloid markers (My-APC) to study their composition by cell surface phenotype. METHODS: To gather and dissect My-APC phenotype while excluding lymphocytes and granulocytes, we developed a strategy based on staining red cell-lysed peripheral blood and gating cells bearing myeloid markers and physical parameters of large mononuclear cells. RESULTS: Phenotypic analysis within the My-APC gate showed three distinct populations. The largest fraction was constituted by CD14+ monocytes that extended into the other two populations, each expressing gradually lower levels of CD14 surface antigen along with increasing levels of CD16 and CD2, respectively. The CD16 and CD2 expression patterns extended from CD16+CD14+ or CD2+CD14+ double- positive intermediate cells toward each single positive subset, but they were reciprocally exclusive. Interestingly, CD2+CD14- cells within the My-APC gate were equivalent to myeloid dendritic cell precursors (pre-DC) defined previously by the absence of lineage markers and expression of HLA-DR and myeloid markers. Phenotypic analysis of each population revealed differences in the expression of costimulatory molecules and CD62L. CONCLUSIONS: This novel analytical approach allowed us to distinguish circulating My-APC in three subsets and to identify relationships between monocytes and other related myeloid populations including DC.