Immunoregulatory role of CD8alpha in the veto effect.
BACKGROUND
Allogeneic bone marrow cell (allo-BMC) infusion induces tolerance to incompatible renal allografts in rhesus macaques after depletion of peripheral T lymphocytes with cytolytic anti-T cell antibodies. The tolerogenic effect of allo-BMC, ascribed to a veto mechanism, associates with specific functional deletion of antidonor cytotoxic T lymphocyte precursor (CTLp), and is dependent on a CD8+ donor BMC subset. In previous studies, the CD8 molecule was implicated by loss of suppression after blocking interaction between CD8 on allo-BMC and major histocompatibility complex class Ialpha3 domain on CTLp. CD8 cross-linking on BMC induced secretion of active transforming growth factor-beta1 (TGF-beta1), suggesting a regulatory mechanism(s) operating via a CD8-mediated signaling pathway.


METHODS
CD8 on rhesus cells was cross-linked using IgG-conjugated beads, and TGF-beta1 mRNA and protein were quantified. CD8+ cells were tested for veto activity by mixed lymphocyte reaction (MLR)-induced cell-mediated lymphocytotoxicity (CML) assay. Activated rhesus T cells exposed to TGF-beta1 were examined for apoptosis by TdT-mediated end-labeling and annexin staining.


RESULTS
CD8 cross-linking induces accumulation of TGF-beta1 mRNA and protein. Both CD3- CD8+CD16+ and CD3+ CD8+CD16- subsets of allo-BMC up-regulate TGF-beta1 mRNA after CD8 cross-linking, and exhibit veto activity. The CD3-CD8+CD16+ subset expresses more TGF-beta1 mRNA and increased veto activity at low BMC/CTLp ratios. Exposure of activated T cells to TGF-beta1 induces apoptosis.


CONCLUSIONS
CD8+ allo-BMC are enriched for veto activity and activation via CD8 induces TGF-beta1 mRNA and protein accumulation. These results agree with the hypothesis that paracrine TGF-beta1 may be involved in peripheral deletion of alloreactive CTLp by CD8+ allo-BMC. We suggest that TGF-beta1 overexpression by donor lymphohematopoietic cells may enhance tolerance induction.
Asiedu
C
C
Department of Surgery, University of Alabama at Birmingham, 35294, USA.
Meng
Y
Y
Wang
W
W
Huang
Z
Z
Contreras
J L
JL
George
J F
JF
Thomas
J M
JM
D000818:Animals; D001854:Bone Marrow Cells / Q000166:cytology / Q000276:immunology; D016026:Bone Marrow Transplantation / Q000276:immunology*; D016827:CD8 Antigens / Q000032:analysis / Q000276:immunology*; D018414:CD8-Positive T-Lymphocytes / Q000276:immunology*; D002478:Cells, Cultured; D003432:Cross-Linking Reagents; D053938:DNA Fragmentation / Q000276:immunology*; D007108:Immune Tolerance; D007165:Immunosuppression Therapy / Q000379:methods; D008212:Lymphocyte Depletion; D017710:Lymphocyte Transfusion; D008253:Macaca mulatta; D008285:Major Histocompatibility Complex; D014176:Protein Biosynthesis; D017452:Receptors, IgG / Q000032:analysis / Q000276:immunology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016176:T-Lymphocyte Subsets / Q000276:immunology; D013601:T-Lymphocytes / Q000276:immunology; D014158:Transcription, Genetic; D016212:Transforming Growth Factor beta / Q000032:analysis / Q000096:biosynthesis / Q000235:genetics*; D014184:Transplantation, Homologous / Q000276:immunology*
D016827:CD8 Antigens; D003432:Cross-Linking Reagents; D017452:Receptors, IgG; D016212:Transforming Growth Factor beta