We have identified and biochemically characterized an
antigen, 8A3, which is expressed on activated T lymphoblasts and activated platelets.
Monoclonal antibodies to 8A3 were raised against the primitive lymphoid/myeloid cell line KG1a and additionally bound to the
erythroleukemia-derived cell line HEL, whilst exhibiting little or no reactivity with a panel of other hematopoietic cell lines. The 8A3
antigen was expressed on poorly differentiated
T-cell leukemias and on
phytohemagglutinin-activated T-cells maintained in
interleukin-2 (7,000 sites/cell). This
antigen, though not detected on resting platelets, was expressed on
thrombin-activated platelets (2,000 sites/platelet).
Antibodies to 8A3 identified
polypeptides of Mr 170,000 and 150,000 in lysates of surface-iodinated KG1a cells, T lymphoblasts, and activated platelets under both reducing and nonreducing conditions. However,
peptide mapping and susceptibily to
glycosidases indicated that the 8A3
antigen was a monomeric
glycoprotein of Mr 170,000 which contained two N-linked
endoglycosidase H-sensitive
glycans, and that the Mr 150,000 structure was derived from it by proteolytic degradation. The 8A3
antigen was not detectably phosphorylated in KG1a cells in vivo, nor did
immune complexes containing it exhibit
kinase activity in vitro. Structural and serologic characteristics of the 8A3
antigen indicate that it is different from other previously described leukocyte activation
antigens including
transferrin receptors,
interleukin-2 receptors, members of the
integrin family of adhesion molecules, or "restricted" members of the
leukocyte-common antigen/CD45 cluster. Furthermore, the 8A3
antigen does not appear to be related to the other previously described activation-specific platelet molecule, GMP140/
PADGEM. This antibody may be useful in monitoring T-cell activation status in some clinical situations and in characterizing clinically relevant activation-associated platelet membrane alterations.