Detection of Ia alloantigens on phytohemagglutinin-stimulated T blast cells.

The expression of Ia antigens on leukoagglutinin-induced blast cells was studied using a sensitive protein A-rosetting technique and quantitative absorption with conventional and monoclonal Ia antibodies. By direct testing, 40 to 60% of CBA (Iak) blast cells were shown to be Ia+ (Iak+, Ia.17+, and Ias-), expressing both glycoprotein and glycolipid Ia.17 determinants. The strong reactivity of the Thy-1.2 and Lyt-1.1 antibodies with leukoagglutinin blast cells (85%+) suggested that most of the Ia+ cells were T cells. Absorption studies confirmed the expression of Ia antigens on blast cells and excluded the possibility that the reaction of the Iak antiserum with blast cells represented antiviral reactivity. The Ia+-rosetting and Ia--nonrosetting blast cells were separated on a Percoll density gradient and typed with monoclonal anti-T cell antibodies. Both Ia+ and Ia- populations contained at least two T cell subpopulations: one Lyt-1.1+, 2.1+ and the other Lyt-1.1+, 2.1-. These studies indicate that during activation, certain T cell subpopulations express Ia antigens not found on resting T cells and emphasize the importance of such findings in determining the role of Ia antigens in immune responses.