Deleterious changes to the T-cell compartment following immunotherapy

The combination of anti-CD40 and interleukin-2 is a potent immunotherapy regimen that results in synergistic antitumor responses. This has been demonstrated in multiple murine tumor models of metastatic disease with various tumor types. The primary antitumor responses elicited by this combination are capable of inducing tumor regression and prolonged survival. However, the generation of secondary T-cell responses after irradiated tumor vaccine is abrogated after anti-CD40 and IL-2. This abrogation also occurs after other immunotherapeutic approaches that prompt the production of large amounts of interferon-gamma (IFN-g). These observations correlated with a significant skewing of the T-cell compartment. First, we observed a selective decreased of conventional CD4+ T cells following immunotherapy. Second, we observed a >5-fold expansion of memory phenotype cells, which were incapable of generating responses to new antigens. The data presented here suggest that despite initial tumor regression, potent systemic immunotherapy may impair responses to new immunological challenges. Selective CD4+ T-cell death after immunotherapy results in an alteration in the ratio of CD4+ T cells to CD8+ T cells and impairs the generation of a secondary immune response. Our data suggest that this phenomenon after immunotherapy is the result of the selective up-regulation of programmed death-1 (PD-1) and its IFN-g-responsive ligand, B7-H1. We show that the expression of PD-1 is restricted to the surface of Foxp3 neg CD4+ T cells and that CD8+ T cells and CD4+ Foxp 3+ regulatory T cells remain PD-1 low after immunotherapy. Furthermore, the expression of PD-1 correlates with CD4+ T-cell death after immunotherapy. In the absence of IFN-g either by the use of mice lacking IFN-g (IFN-g-/-) or the receptor for IFN-g (IFN-gR-/-), B7-H1 remains low after immunotherapy. Subsequently, CD4+ T cells expand in response to immunotherapy in the absence of IFN-g-responsive B7-H1. We observed a significant expansion of memory phenotype T cells after cytokine-based immunotherapy, which correlated with impairment of proliferative responses to new antigens. Memory T cells are more sensitive to cytokine stimulation than naive T cells. Therefore, we used a young thymectomized mouse model to determine if pre-existing memory T cells were preferentially expanded by immunotherapy. The thymectomized mouse model allowed us to evaluate long-term T-cell responses to immunotherapy in the absence of de novo T-