Overproduction of HMGB1, which occurs in various tumor cells, is associated with the hallmarks of cancer (e.g., tumor angiogenesis, growth, inflammation, invasion and metastasis) (27
). HMGB1 plays multiple roles in either anti- or pro-tumor effects (23
). Recombinant human HMGB1 induces a distinct form of cell death in cancer cells that may provide therapeutic benefits for cancer patients (39
). HMGB1 released from dying tumor cells after chemo-, viro- or radiation-therapy, as a ‘danger’ molecule, stimulates DC maturation and tumor antigen presentation via TLR2/4 or activates innate immunity, leading to an antitumor immune response (29
). However, in the physiological condition (tumor progression), the role of HMGB1 in tumor immune suppression is largely unknown even though it has been reported that HMGB1 produced by tumor cells exhibits the inhibitory effect on DC in both mice and humans (32
The observations in this work suggest that tumor cell-derived HMGB1 may suppress a naturally acquired immune effector cell (CD8)- or cytokine (IFN-γ)-dependent antitumor response, probably by enhancing tumor-associated Treg to produce IL-10, which is necessary for immune suppression in vitro
and in vivo
. Treg-derived IL-10 may dampen DC, CD4 or CD8 T cell function to diminish the priming of tumor-specific CD8 T cells. DC have been suggested to be the most relevant targets of Treg in vivo
and decommissioning of DC probably is a realistic mechanism underlying Treg-mediated immune suppression (40
). The mechanisms underlying Treg-derived IL-10-mediated suppression of the priming of antitumor CD8 T cells via a DC effect will be precisely dissected such as using the tetramer analysis of antitumor CD8 T cell frequencies in the future work.
IL-10-producing Treg have been shown to be highly suppressive (14
). The in vitro
data indicate that tumor cell-derived HMGB1 may act, as an extracellular signal, on tumor-associated Treg to promote IL-10 production for an enhanced suppressive functionality. Whether HMGB1-KD tumor immunity in vivo
involves interference with IL-10 producing-Treg needs to be investigated in the future studies. In a burn injury model, massive HMGB1 released from burn injury has been suggested to activate Treg via RAGE to produce (detectable) IL-10 in vitro
). It is possible that tumor cell-derived HMGB1 may interact with RAGE (or other putative HMGB1 receptors) on Treg to activate p38 MAP kinase, extracellular signal-regulated kinase1/2 and Jun N-terminal kinase, leading to the activation of transcriptional factors (activator protein 1 and NFκB) for IL-10 production in Treg (19
). The exact intrinsic molecular pathway triggered by tumor cell-derived HMGB1 and whether anti-HMGB1 treatment alters the downstream signaling pathway need to be elucidated in the future studies.
HMGB1 KD in tumor cells resulted in a CD8 T cell- or IFN-γ-dependent tumor rejection, clearly suggesting that HMGB1 KD-mediated antitumor activity in vivo
is due to naturally acquired antitumor immunity but not modulation on tumor cells for death suggested in the in vitro
prostate tumor cell study (42
). HMGB1 KD may render tumor cell susceptibility to cytotoxic T lymphocyte-mediated killing via altering tumor phenotype. Since Treg depletion provoked antitumor immunity and tumor elevated and activated Treg which suppressed antitumor immunity (34
), attenuation of the ability of tumor cells to in vivo
expand and activate Treg by HMGB1 KD may allow for CD8 T cells to operate in an unopposed manner leading to enhanced CD8 T cell responses. How HMGB1 KD reduce tumor cell capacity to in vivo
induce Treg is still mysterious. It is possible that HMGB1 KD may modulate tumor cells to produce immune stimulatory molecules and/or to reduce secrete immune suppressive factors.
The conflicting data shown in the literature and here on HMGB1 in tumor immune responses may be explained by the different sources of HMGB1 under different tumor cell conditions. HMGB1 released from dying tumor cells post chemo-, viro- or radiation-therapy may complex with soluble moieties in tumors (e.g., nucleic acids, microbial products, and cytokines) to exert its inflammatory properties (29
). HMGB1 released (secreted) from tumor cells in the physiological condition (tumor progression) may not be able to do so or may be specifically modified posttranslationally (e.g., oxidation) to exhibit its ability to promote tumor invasion, metastasis or immune tolerance (23
). Indeed, the state of oxidation of HMGB1 is critical in determining immune response vs. non-responsiveness (43
RAGE-HMGB1 blockade by administration of soluble RAGE, anti-RAGE and/or anti-HMGB1 Ab inhibits tumor growth and metastases (44
). Since HMGB1 KD in tumor cells uncovered naturally acquired CD8 T cell-dependent antitumor immunity, we are actively investigating whether anti-HMGB1 treatment [e.g., intratumorally blocking HMGB1 signaling using: i) anti-HMGB1 mAb (neutralizing HMGB1 signaling), ii) siRNA HMGB1 (HMGB1 KD) or iii) glycyrrhizin (a specific inhibitor of extracellular HMGB1) (inhibiting HMGB1)] can be used therapeutically to rescue an antitumor CD8 T cell response to established tumors.
In sum, the data suggest a new function for tumor cell-derived HMGB1 in suppressing naturally acquired CD8 T cell-dependent antitumor immunity probably via promoting tumor-associated IL-10-producing Treg.