CD28 is an inducible T cell surface antigen that transduces a proliferative signal in CD3+ mature thymocytes.
The rearrangement of TCR genes during thymic ontogeny creates a repertoire of T cell specificities that is refined to ensure the deletion of autoreactive clones and the MHC restriction of T cell responses. Signals delivered via the accessory molecules CD2, CD4, and CD8 have a crucial role in this phase of T cell differentiation. Recently, CD28 has been identified as a signal transducing molecule on the surface of most mature T cells. Perturbation of the CD28 molecule stimulates a novel pathway of T cell activation regulating the production of a variety of lymphokines including IL-2. We have studied the expression and function of CD28 during thymic ontogeny, and in resting and activated PBL. A variable percentage of resting thymocytes were CD28+ (3 to 25%, n = 8), but it was found in high density only on mature CD3+(bright) CD4/CD8 cells. Both unseparated thymocytes and isolated CD3-CD28-/dull cells proliferated when stimulated with PMA plus IL-2 or PMA plus ionomycin. PMA treatment also rapidly up-regulated CD28 expression in the CD3- subset as these cells became CD3-CD28+(bright). Despite the ability of PMA to induce high density CD28 expression in CD3- cells, CD3- thymocytes did not proliferate in response to PMA plus anti-CD28 mAb, in contrast to unseparated cells. CD3+ thymocytes stimulated with immobilized anti-CD3 mAb also failed to proliferate in culture. However, the addition of either IL-2 or anti-CD28 mAb supported proliferation, suggesting that only CD3+ cells could respond to CD28 signaling. The comitogenic effect of anti-CD3 and anti-CD28 mAb was IL-2 dependent as it was abrogated by an anti-IL-2R mAb. Interestingly, the expression of CD28 on the cell surface of CD3+ cells was also inducible, as flow cytometric analysis demonstrated a 10-fold increase in cell surface CD28 by 24 to 48 h after anti-CD3 stimulation of both CD3+ thymocytes and peripheral blood T cells. This increase was accounted for by a commensurate increase in CD28 mRNA levels. Together, these results suggest that CD28 is an inducible T cell antigen in both CD3- and CD3+ cells. In addition, stimulation of the CD28 pathway can provide a second signal to support the growth of CD3+ thymocytes stimulated through the TCR/CD3 complex, and may therefore represent a mechanism for positive selection during thymic ontogeny.
Turka
L A
LA
Department of Medicine, University of Michigan, Ann Arbor 48109.
Ledbetter
J A
JA
Lee
K
K
June
C H
CH
Thompson
C B
CB
D000911:Antibodies, Monoclonal / Q000276:immunology; D000945:Antigens, Differentiation, T-Lymphocyte / Q000502:physiology*; D015152:Blotting, Northern; D018106:CD28 Antigens; D017252:CD3 Complex; D002478:Cells, Cultured; D015870:Gene Expression / Q000187:drug effects; D066298:In Vitro Techniques; D007376:Interleukin-2 / Q000502:physiology; D008213:Lymphocyte Activation; D011493:Protein Kinase C / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D011948:Receptors, Antigen, T-Cell / Q000502:physiology; D015375:Receptors, Interleukin-2 / Q000502:physiology; D015398:Signal Transduction; D013601:T-Lymphocytes / Q000502:physiology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D000911:Antibodies, Monoclonal; D000945:Antigens, Differentiation, T-Lymphocyte; D018106:CD28 Antigens; D017252:CD3 Complex; D007376:Interleukin-2; D012333:RNA, Messenger; D011948:Receptors, Antigen, T-Cell; D015375:Receptors, Interleukin-2; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate