Validity of the two-signal model for activation of CD28-deficient T lymphocytes: quantitative characterization of an alternative costimulatory function of dendritic cells

The effects of Cyclosporine A and azathioprine on human T cells activated by different costimulatory signals

Immunosuppression is an important treatment modality in transplantation and human diseases that are associated with aberrant T cell activation. There are considerable differences regarding the cellular processes targeted by the immunosuppressive drugs that are in clinical use. Drugs like azathioprine (Aza) mainly act by halting proliferation of fast dividing cells, whereas others like cyclosporine A (CsA) specifically target signaling pathways in T cells. Since the outcome of T cell responses critically depends on the quality and strength of costimulatory signals, this study has addressed the interplay between costimulation and the immunosuppressive agents CsA and Aza during the in vitro activation of human T cells. We used an experimental system that allows analyzing T cells activated in the presence of selected costimulatory ligands to study T cells stimulated via CD28, CD2, LFA-1, ICOS or 4-1BB. The mean inhibitory concentrations (IC₅₀) for Aza and CsA were determined for the proliferation of T cells receiving different costimulatory signals as well as for T cells activated in the absence of costimulation. CD28 signals but not costimulation via CD2, 4-1BB, ICOS or LFA-1 greatly increased the IC₅₀ for CsA. By contrast, the inhibitory effects of Aza were not influenced by T cell costimulatory signals. Our results might have implications for combining standard immunosuppressive drugs with CTLA-4Ig fusion proteins, which act by blocking CD28 costimulation.

Induction of T cells suppression by dendritic cells transfected with VSIG4 recombinant adenovirus

VSIG4 has been recently described as a B7 family-related protein. The immunotherapeutic potential of dendritic cells (DCs) transfected with VSIG4 recombinant adenovirus has not been characterized. In the present study, DCs were transfected with human VSIG4 (hVSIG4) recombinant adenovirus, a novel costimulatory molecule known to be a potent inhibitor of T cell activation. Transfected DCs were cocultured with allogeneic T cells and proliferation, cytokine production and T cell activation marker expression were assessed. The results showed that T cell proliferation potential, cytokine production and activation marker expression were suppressed after coculture with hVSIG4 recombinant adenovirus-transfected DCs. These findings suggest that DCs transfected with hVSIG4 recombinant adenovirus are capable of inducing allogeneic T cell suppression, which represents an ideal strategy for manipulating the immune response to transplanation or autoimmune diseases.

Dynamic control of self-specific CD8+ T cell responses via a combination of signals mediated by dendritic cells

It is acknowledged that T cell interactions with mature dendritic cells (DC) lead to immunity, whereas interactions with immature DC lead to tolerance induction. Using a transgenic murine system, we have examined how DC expressing self-peptides control naive, self-reactive CD8+ T cell responses in vitro and in vivo. We have shown, for the first time, that immature DC can also stimulate productive activation of naive self-specific CD8+ T cells, which results in extensive proliferation, the expression of a highly activated cell surface phenotype, and differentiation into autoimmune CTL. Conversely, mature DC can induce abortive activation of naive CD8+ T cells, which is characterized by low-level proliferation, the expression of a partially activated cell surface phenotype which does not result in autoimmune CTL. Critically, both CD8+ T cell responses are determined by a combination of signals mediated by the DC, and that altering any one of these signals dramatically shifts the balance between autoimmunity and self-tolerance induction. We hypothesize that DC maintain the steady state of self-tolerance among self-specific CD8+ T cells in an active and dynamic manner, licensing productive immune responses against self-tissues only when required.