CROSS-LINKING OF THE CAMPATH-1 ANTIGEN (CD52) TRIGGERS ACTIVATION OF NORMAL HUMAN T-LYMPHOCYTES

The CAMPATH-1 (CD52) antigen is a 21-28 kDa glycopeptide which is high ly expressed on lymphocytes and macrophages and is coupled to the memb rane by a glycosylphosphatidylinositol (GPI) anchoring structure. The function of this molecule is unknown, However, it is an extremely good target for complement-mediated attack and antibody-mediated cellular cytotoxicity. The humanized CAMPATH-1H antibody, which is directed aga inst CD52, is very efficient at mediating lymphocyte depletion in vivo , and is currently being used in clinical trials for lymphoid malignan cy and rheumatoid arthritis. It is therefore important to examine the functional effects of this antibody on different lymphocyte sub-popula tions. Because several other GPI-linked molecules expressed on the sur face of T lymphocytes are capable of signal transduction resulting in cell proliferation, we have investigated whether the CAMPATH-1 antigen can also mediate these effects. In the presence of phorbol esters and cross-linking anti-lg antibodies, mAbs specific for CD52 induced prol iferation and lymphokine production in highly purified resting CD4(+) and CD8(+) T lymphocytes. The rat IgG2c YTH 361.10 anti-CD52 antibody, however, was able to activate resting CD4(+) and CD8(+) T cells direc tly without cross-linking or phorbol myristate acetate in the absence of Fc-bearing cells., Anti-CD52 antibodies also augmented the anti-CD3 mediated proliferative response of CD4(+) and CD8(+) T cells when the two antibodies were co-immobilized onto the same surface or cross-lin ked in solution by the same second antibody. Both CD4(+)CD45RA and CD4 (+)CD45RO T cells were stimulated to proliferate by anti-CD52 antibodi es in the presence of appropriate co-stimulatory factors. Anti-CD52 mA bs did not, however, synergize with anti-CDS or CD28 mAb to induce CD4 (+) T cell proliferation. The activation of CD4+ T cells by anti-CD52 antibodies was inhibited by cyclosporin A, suggesting a role for the c alcineurin-dependent signal transduction pathways. Although CD52 could transduce a signal in T cells, anti-CD52 antibodies did not inhibit a ntigen-specific or polyclonal T cell responses, suggesting this molecu le does not play an essential co-stimulatory role in normal T cell act ivation.