Qa‐12—‐A NOVEL DETERMINANT OF ACTIVATED T AND B LYMPHOCYTES

SUMMARY Investigation of the antigenic phenotype of activated lymphocytes using the broadly cross‐reactive mAb 6.3.4 revealed two phenotypic alterations as compared with the resting lymphocytes. The Qa specificity Qa‐m208 disappears after lectin activation of Qa‐m208‐positive T lymphocytes. Analysis of Q7 and Q9 transfectants expressing the Qa‐2 polypeptides shows that Qa‐m208 is an epitope of the Qa‐2 antigen. Because the Qa‐2 antigens are still expressed on T lymphoblasts which have lost Qa‐m208, changes of the Qa‐2 molecules occur and result in the loss of certain epitopes. The second phenotypic change that we observed is the appearance of a novel specificity, Qa‐12. Its expression is induced by lymphocyte activation and it is expressed on lymphoblasts of both T and B cell origin. The presence of this novel non‐ubiquitous antigenic specificity is determined by the Tla region.