Defining the Ability of Cyclophosphamide Preconditioning to Enhance the Antigen-specific CD8⁺ T-cell Response to Peptide Vaccination: Creation of a Beneficial Host Microenvironment Involving Type I IFNs and Myeloid Cells

Abstract

Although cyclophosphamide (CTX) has been clearly shown to enhance active specific and adoptive immunotherapies, the mechanism(s) underlying these beneficial effects have not been clearly defined. To define the impact of CTX preconditioning on the antigen-specific CD8 T-cell response to peptide vaccination, we used an adoptive transfer model based on the OT-1 T-cell receptor transgenic mouse. CTX preconditioning dramatically enhanced the antigen-specific CD8 T-cell response to peptide vaccination. Specifically, CTX significantly enhanced the expansion and function of responding CD8 T cells as demonstrated by flow cytometry and cytokine production. In parallel experiments, we attempted to define the mechanism(s) underlying these beneficial effects of CTX therapy. CTX therapy increased the relative number and activation status of myeloid dendritic cells, and was associated with the induction of significant levels of the inflammatory cytokines interferon-α, monocyte chemoattractant protein-1, and IL-6. Adoptive transfer experiments into type I IFNR^−/− and CR3^−/− mice confirmed that the beneficial effects of CTX were at least partially dependent on type I interferons and myeloid cells. Adoptive transfer of up to 150×10⁶ naive spleen cells at the time of antigen-specific CD8 T-cell transfer did not abrogate the effects of CTX therapy, suggesting that the creation of a niche in the immune system may not be required. CTX decreased the absolute, but not relative number of CD4⁺CD25⁺ T_reg cells, consistent with the possibility that regulatory T cells may be targeted by CTX therapy. Of note, combination therapy with CTX and a synthetic TLR3 agonist further enhanced the antigen-specific CD8⁺ T-cell response. Taken together, our data suggest that CTX modulates specific components of the innate immune system resulting in a beneficial host microenvironment. Specific targeting of these components may enhance the effectiveness of CTX preconditioning for adoptive immunotherapy.