Two types of Ia-positive T cells. Synthesis and exchange of Ia antigens.

Two distinct types of Ia-positive T cells have been described. One type represents a blastoid T cell responding from stimulation by mitogens, antigens, and in allogeneic and autologous mixed lymphocyte culture reactions. This is a large cell that is strongly positive for Ia antigens as measured by a variety of different antisera. The other general type is a smaller cell with a lower expression of Ia antigens that is found at low levels in normal peripheral blood and is markedly elevated in various pathological states. It also rises rapidly after inoculation with tetanus toxoid and PPD in sensitized individuals. This cell does not incorporate thymidine and is enriched in the Tgamma fraction; it can be markedly concentrated from normal lymphocytes, and current evidence indicates that it is a T cell. The marked elevation of this cell in the blood of patients with rheumatoid arthritis is of special interest. Considerable evidence indicates that, at least in certain instances, the Ia antigens are synthetized by the cells that carry them. Incorporation of labeled amino acid experiments and the in vitro translation results presented above indicate this. However, the ready exchange of Ia antigens between cells in the experiments described indicates that uptake from other cells may be a significant source.