Endogenous synthesis of Ia antigens by a cloned T cell line.

The four Ia polypeptides in mice, A alpha, A beta, E beta, and E alpha, are expressed predominantly on B lymphocytes and certain macrophage populations. Whether T lymphocytes express Ia antigens has been controversial. Although Ia antigens have been demonstrated on small populations of mitogen-activated and alloreactive lymphocytes, the T cell origin of these Ia antigens has been doubted. It has been suggested that Ia antigens on T cells are passively acquired from non-T lymphocytes. The present study analyzes the endogenous synthesis and expression of Ia antigens by an alloreactive-derived T cell clone. This clone, PLT-24.2.Cl, was derived initially from an alloreactive cell population, B6 anti-B10.K. The cell line was cloned by limiting dilution and maintained by periodic boosting with the stimulator cells. After 4 months of sequential boosting, it was possible to maintain the cloned cells in the absence of irradiated B10.K filler spleen cells. It has now been in culture for over a year in the absence of both filler cells and exogenous growth factors (e.g., IL-2). The expression of Ia antigens on the cell surface of the cloned T cell line was detected by using monoclonal antibodies and cytofluorometry. The cloned cells were positive for Thy.1, Lyt.1, and Lyt.2, and negative for surface immunoglobulin. The cells expressed all of the Ia antigens normally found on an I-Ab derived cell, i.e., Ia.8 and Ia.15, and lacked Ia.7. The cells lacked any of the Ia antigens expressed on the original stimulator cells, B10.K. These results show that PLT-24.2.Cl expresses syngeneic Ia antigens. The endogenous synthesis of Ia antigens by these cell lines was confirmed by biosynthetic labeling with radiolabeled amino acid precursors and indirect immunoprecipitation on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. The Ia antigens synthesized by the T cell clone are structurally identical to those synthesized by the syngeneic B lymphocytes, as shown by tryptic peptide mapping using high-pressure liquid chromatography. These studies prove that the Ia antigens predominantly expressed on B lymphocytes and some macrophage-derived cells can also be synthesized and expressed by T lymphocytes. Since Ia antigens are involved in numerous immune phenomena and cell-cell interactions, this information would have to be taken into account in proposing mechanisms by which Ia-restricted helper and suppressor cells are generated.