Background: Third-party-specific cytotoxic T lymphocytes (CTL), or veto CTL, are being assessed as a cellular therapeutic for the induction of T-cell tolerance during transplantation. Conceptually, veto cell-expressed antigens (Ags) may induce B-cell immune responses, and this may have deleterious consequences. Whether veto cells induce immunity, tolerance, or are ignored by B lymphocytes has, however, not been addressed.
Methods: CTL were retrovirally transduced with a model cell surface Ag to generate veto CTL. The impact of CTL-specific Ag expression on the activation and tolerization of Ag-specific B cells was assessed in vitro and, using adoptive transfer models, in vivo.
Results: In vitro, CTL-expressed Ag induced an abortive proliferative response in specific B lymphocytes, whereby an initial proliferative burst was followed by cell death. In vivo, the administration of veto CTL also induced B-cell tolerance. Specific immunoglobulin was not detected after subsequent immunization with a veto cell-expressed Ag. Modeling of this effect with Ag-specific B-cell receptor transgenic B lymphocytes demonstrated that Ag-specific B cells were eliminated by the veto CTL; the cell division was accompanied by the exhaustion and depletion of responding cells. Veto-induced B-cell tolerance could be wholly abrogated by treatment with the toll-like receptor ligand lipopolysaccharide, implying that this tolerance resulted from the absence of adequate supplemental signals during antigenic stimulation.
Conclusions: Veto CTL are effective promoters of B-cell tolerance. Further assessment of their therapeutic potential in this regard is warranted.