Downregulation of immune functions following responses to pathogen infections is critical for limiting damage to the host by the immune system. T cell activity is known to be downregulated by a variety of negative regulatory mechanisms including negative checkpoint regulatory proteins, a family of CD28-related molecules. PD-1 is one such molecule that is transiently expressed on activated T cells. The ligands for PD-1 are PD-L1 and PD-L2, members of the B7 family of molecules which are upregulated on antigen presenting cells and tumor cells. Interaction of PD-1 with its ligand leads to inhibition of TCR-mediated signaling via recruitment of SHP1 and SHP2 phosphatases to the TCR synapse. In the August 2013 edition of The Journal of Immunology, Gerner et al., demonstrate that CD8 T cells initially activated in the presence of IL-12 and IFNα differentially re-express PD-1 upon antigen restimulation.
Cytokines play roles in regulation of nearly every aspect of immune responses. The cytokine milieu present during T cell activation directs differentiation into the different functional classes of CD4 T helper or CD8 T cells. This study sought to determine the differences in anti-tumor CD8 T cell effector functions mediated when T cells are activated in the presence of various cytokines. IL-12 and IFNα activate both overlapping and distinct gene programs and promote cytotoxic CD8 T cell responses. Thus, these cytokines were chosen for comparison in this study.
In this system, CD8+ OT-1 cells were activated ex-vivo in the presence of either IL-12 or IFNα, and transferred into B16-OVA tumor-bearing mice. T cells activated in the presence of IL-12 were found to mediate tumor-growth inhibition significantly better than if they had been activated in the presence of IFNα. Over time in tumor-bearing mice, transferred IFNα-matured OT-1 cells were observed to decline in number and lost the ability to produce IFNγ ex vivo upon restimulation, indicating these cells may be exhausted.
Because PD-1 is known to be a marker and mediator of T cell exhaustion, PD-1 expression was examined. Initial induction levels of PD-1 were comparable on OT-1 cells following ex vivo activation with IFNα or IL-12. Following transfer into tumor-bearing mice, PD-1 levels declined over time on both types of cells isolated from the spleen and on IL-12 matured cells isolated from the tumor. However, PD-1 expression was high on transferred IFNα-matured cells when isolated from the tumor. Similar results were seen when cells were transferred into mice that subsequently received an injection of the OVA peptide. Thus, CD8+ T cells matured in the presence of IFNα appear to re-express significantly higher levels of PD-1 upon antigen restimulation than IL-12 matured T cells.
PD-1 and PD-L1 targeting with inhibitory antibodies have emerged as promising avenues in tumor immunotherapy. In this study, anti-PD-1 antibody administration had no additional anti-tumor effect in mice that received IL-12-matured T cells, while in mice that received IFNα-matured T cells, anti-PD-1 antibodies led to inhibition of tumor-growth to a level similar to that in mice that had received IL-12-matured T cells. Thus, the relatively poor ability of IFNα-matured T cells to efficiently inhibit tumor growth appears to be largely due to PD-1 upregulation. Finally, when T cells were matured with both IL-12 and IFNα, the effect of IL-12 was dominant.
Many questions remain regarding the mechanisms mediating PD-1 re-expression in IFNα vs. IL-12 matured T cells. However, since IL-12 activity was dominant over IFNα on regulating PD-1 expression, IL-12 administration during immunotherapy regimens may enhance anti-tumor T cell responses by blocking the mechanisms by which IFNα enhances PD-1 re-expression.
Further Reading:
Cutting Edge: IL-12 and Type I IFN Differentially Program CD8 T Cells for Programmed Death 1 Re-expression Levels and Tumor Control. Gerner MY, Heltemes-Harris LM, Fife BT, Mescher MF. J Immunol. 2013 Aug 1;191(3):1011-5. doi: 10.4049/jimmunol.1300652. Epub 2013 Jun 26.
