Peripheral tolerance to tumor antigens (Ags) is a major hurdle for antitumor immunity. Draining lymph nodes are considered the privileged sites for Ag presentation to T cells and for the onset of peripheral tolerance. Here, we show that the spleen is fundamentally important for tumor-induced tolerance. Splenectomy restores lymphocyte function and induces tumor regression when coupled with immunotherapy. Splenic CD11b+Gr-1intLy6Chi cells, mostly comprising proliferating CCR2+-inflammatory monocytes with features of myeloid progenitors, expand in the marginal zone of the spleen. Here, they alter the normal tissue cytoarchitecture and closely associate with memory CD8+ T cells, cross-presenting tumor Ags and causing their tolerization. Because of its high proliferative potential, this myeloid cell subset is also susceptible to low-dose chemotherapy, which can be exploited as an adjuvant to passive immunotherapy. CCL2 serum levels in cancer patients are directly related to the accumulation of immature myeloid cells and are predictive for overall survival in patients who develop a multipeptide response to cancer vaccines.
► The spleen is a crucial organ for tumor-induced tolerance ► Chemotherapy enhances the efficacy of passive immunotherapy by eliminating splenic CD11b+Gr-1int cells ► CD11b+Gr-1int splenocytes comprise CCR2+ cycling myeloid precursors that cross-tolerize Ag-specific CD8+ T lymphocytes ► CCL2 serum levels are predictive of clinical outcome in patients who respond to cancer vaccination
