Expression and modulation of cell surface determinants on human adult and neonatal monocytes

The difference between newborn and adult mononuclear cells in the antigen dose required for optimal antibody production in vitro can be ascribed to differences in the antigen-presenting capacities of the respective monocytes (Van Tol et al., 1984b). We have therefore studied the expression of cell surface determinants on human neonatal and adult monocytes by the use of monoclonal antibodies to membrane proteins including MHC antigens. No difference was observed in the expression of LeuM3 with regard to both the percentage of positive cells and the density of the respective determinant. In contrast, neonatal cells express the antigens OKM5, LFA1, OKM1 and LeuM5 at a lower density than adult cells do. The same holds for beta 2-microglobulin, but neonatal and adult monocytes express MHC class I alpha-chains at a similar density, whereas among the class II MHC antigens, HLA-DR is significantly more highly expressed on neonatal cells. This difference remains after treatment in vitro with gamma-interferon (gamma-IFN). Treatment with gamma-IFN also resulted in a less dense expression of the LeuM3 antigen. Preincubation of monocytes with LeuM3 monoclonal antibody partially abrogates subsequent upregulation of class II MHC antigens by gamma-IFN, a phenomenon observed with both neonatal and adult monocytes. These data indicate a functional involvement of LeuM3 with the cellular action of gamma-IFN. Taken together, the cell surface phenotype of neonatal monocytes is that of a highly efficient antigen presenting cell.