Antibodies recognizing CD24 LAP epitope on human T cells enhance CD28 and IL-2 T cell proliferation

Membrane expression of the CD24 molecule on activated T lymphocytes is not eluci- dated fully. We previously described the intracel- lular and cell-surface expression of the CD24 sialic acid-dependent epitope(s) on phytohemagglutinin- activated peripheral blood mononuclear cells. However, the CD24 core protein was not detected previously on human T cells. This study reinvesti- gated the expression and role of CD24 in T cell subsets. We analyzed binding of anti-CD24 mono- clonal antibodies (mAbs) to sialic and leucine-ala- nine-proline (LAP) epitopes in resting and acti- vated, normal T lymphocytes. CD24 LAP and CD24 sialic epitopes were detected on activated CD4- and CD8-positive cells. Although expression of CD24 sialic epitopes remained stably expressed in interleukin (IL)-2-dependent cultures, T cell ex- pression of the LAP epitope was transient. Anti- LAP antibodies strongly enhanced the response of T cells to a combination of anti-CD3/CD28 mAbs and enhanced proliferative response induced by recombinant IL-2. We found similarities in the tis- sue distribution and function of the human CD24 LAP molecule and the murine, heat-stable antigen, which suggests that CD24 might function as a sig- naling molecule on human T cells. J. Leukoc. Biol. 69: 215-223; 2001.