Acquired CD8 T Cell-Dependent Antitumor Regulatory T Cells and Uncovers Naturally Attenuates Their Ability To Induce Knockdown of HMGB1 in Tumor Cells

Although high mobility group box 1 (HMGB1) in tumor cells is involved in many aspects of tumor progression, its role in tumor immune suppression remains elusive. Host cell-derived IL-10 suppressed a naturally acquired CD8 T cell-dependent antitumor response. The suppressive activity of tumor-associated Foxp3 + CD4 + CD25 + regulatory T cells (Treg) was IL-10 dependent. Neutralizing HMGB1 impaired tumor cell-promoted IL-10 production by Treg. Short hairpin RNA-mediated knockdown of HMGB1 (HMGB1 KD) in tumor cells did not affect tumor cell growth but uncovered naturally acquired long-lasting tumor-specific IFN-g– or TNF-a–producing CD8 T cell responses and attenuated their ability to induce Treg, leading to naturally acquired CD8 T cell-or IFN-g–dependent tumor rejection. The data suggest that tumor cell-derived HMGB1 may suppress naturally acquired CD8 T cell-dependent antitumor immunity via enhancing Treg to produce IL-10, which is necessary for Treg-mediated immune suppression. D uring progression, tumor-associated regulatory T cells (Treg) inhibit naturally acquired antitumor immune responses (1). Tumor cell-derived factors (e.g., TGF-b, indoleamine 2, 3-dioxygenase, stem cell factor, CCL22, and yet-to-be-identified factors) contribute directly to Treg or CD25 2 CD4 + T cells and/or indirectly to dendritic cells (DC), plasma-cytoid DC, myeloid-derived suppressor cells, or B cells for Treg expansion, conversion, activation, and/or recruitment (2–9). How tumor cell-derived factors suppress naturally acquired antitumor immunity via Treg is poorly understood from a mechanistic point of view. IL-10 inhibits antitumor activity in both mice and humans (10– 12). CD4 + T regulatory type 1 cells (Tr1) do not express high levels of CD25 or Foxp3 but produce significant IL-10 to mediate immune suppression in vitro and in vivo (13). Treg also produce IL-10, and IL-10–producing Treg have been shown to be highly suppressive (14). In both mouse and human tumor studies, although Treg-derived IL-10 mediates immune suppression in vitro (15, 16), whether it happens in vivo remains unclear (17). Importantly , the signals derived by tumor cells acting on Treg to promote IL-10 production are largely unknown (14). High mobility group box 1 (HMGB1), a small protein of 215 aa residues with extensive various posttranslation modifications, is a highly conserved protein in the nucleus, cytoplasm, or extra-cellular environment (e.g., released from cells via necrosis and autophagy, secreted from inflammatory or cancer cells) with multiple distinguished functions (e.g., binding/bending DNA to facilitate transcription factor assembly on site-specific DNA targets , promoting autophage, inducing cell death, acting as a sig-naling molecule to alert innate immunity) …