Transient depletion of CD4 + T cells augments IL‐21‐based immunotherapy of disseminated neuroblastoma in syngeneic mice

IL‐21 is a member of the IL‐2 cytokine family, produced by CD4+ T cells. We previously showed that immunotherapy (IT) with IL‐21‐transduced neuroblastoma cells (Neuro2a/IL‐21) cured 33% of syngeneic mice bearing systemic NB. Here, we studied whether the removal of Treg cells could potentiate the therapeutic efficacy of Neuro2a/IL‐21 vaccine. The administration of anti‐CD25 mAb, which targets Treg cells, slightly potentiated the effect of vaccine IT (50% cure rate), but anti‐CD4 mAb had a more potent effect leading to 80% cure rate. Anti‐CD25 mAb, indeed, only partially depleted CD4+CD25+FoxP3+ Treg cells, whereas anti‐CD4 mAb was more effective in this respect, leading to 90% depletion of Treg cells. In mice receiving vaccine+anti‐CD4 mAb, which developed systemic immunity to NB, CD4+ T cells counts completely recovered in 90 days. Depletion of CD8+ T cells abrogated the effect of the combined IT, indicating a predominant role of these cells in driving the immune response. In addition, CD8+ T cells from cured mice coinjected with Neuro2a/parental cells (pc) in NOD‐SCID mice completely inhibited tumor growth. Spleen cells from mice receiving Neuro2a/IL‐21 vaccination showed increased expression of IFN‐alpha2, ‐beta1 and ‐gamma mRNA. Moreover, mice receiving vaccine therapy alone or vaccine+anti‐CD4 mAb showed increased IFN‐gamma serum levels and IFN‐gamma‐producing CD8+ T cells were found in spleen cells. In conclusion, anti‐CD4 mAb potentiated IL‐21‐based IT by removing Treg cells and/or their precursors and other potentially immune‐suppressive CD4+ cell subsets, thus allowing the development of an IL‐21‐driven CD8+ T cell response, which mediates NB rejection.