Peripheral blood T cells from normal donors, from newborns, and from patients with rheumatoid arthritis were analyzed for the presence of surface Ia antigens using monoclonal anti-Ia reagents and a fluorescence-activated cell sorter. Two out of four monoclonal anti-Ia antibodies detected Ia antigen on 2-22% (mean 7.4 ± 5.9%) of normal E rosette (+) cells. A subpopulation of E rosette (+) cells thus expresses an Ia molecule with antigenic determinants similar to the Ia on B cells. The staining is weak in comparison to the staining with anti-Ia of B cells, but could not be blocked by preincubation of the T cells with aggregated human IgG. Confirming a previous report by Yu et al. (D. T. Y. Yu, R. J. Winchester, S. M. Fu, A. Gibefsky, H. S. Ko, and H. G. Kunkel, J. Exp. Med., 151, 91, 1980) in rheumatoid arthritis more Ia-positive T cells were detected (mean 17.4 ± 12.5%) that also express a higher number of Ia molecules on their surface. On the contrary, the number of Ia(+) T cells was decreased in cord blood. Both helper T cells [OKT4(+)] and suppressor T cells [OKT8(+)] contain a subfraction that is Ia(+), as evidenced by additive staining and two-color immunofluorescence experiments. In RA, an increase in Ia(+) cytotoxic/suppressor cells is mainly responsible for the higher proportion of Ia(+) T cells. We have previously identified a subpopulation of T cells staining with both OKT8 and OKM1, a marker thought to be specific for cells of myelomonocytic lineage. We now report that this OKT8(+) OKM1(+) overlap population is almost entirely Ia(+) and therefore could represent an activated cell type. However, Ia antigen was also detected on OKT3(+) cells after removing OKM1(+) E rosette (+) cells.
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