title: HMGB1 mediates endogenous TLR2 activation and brain tumor regression.
abstract: BACKGROUND
  Glioblastoma multiforme (GBM) is the most aggressive primary brain tumor that carries a 5-y survival rate of 5%. Attempts at eliciting a clinically relevant anti-GBM immune response in brain tumor patients have met with limited success, which is due to brain immune privilege, tumor immune evasion, and a paucity of dendritic cells (DCs) within the central nervous system. Herein we uncovered a novel pathway for the activation of an effective anti-GBM immune response mediated by high-mobility-group box 1 (HMGB1), an alarmin protein released from dying tumor cells, which acts as an endogenous ligand for Toll-like receptor 2 (TLR2) signaling on bone marrow-derived GBM-infiltrating DCs.
  
  
  RESULTS
  Using a combined immunotherapy/conditional cytotoxic approach that utilizes adenoviral vectors (Ad) expressing Fms-like tyrosine kinase 3 ligand (Flt3L) and thymidine kinase (TK) delivered into the tumor mass, we demonstrated that CD4(+) and CD8(+) T cells were required for tumor regression and immunological memory. Increased numbers of bone marrow-derived, tumor-infiltrating myeloid DCs (mDCs) were observed in response to the therapy. Infiltration of mDCs into the GBM, clonal expansion of antitumor T cells, and induction of an effective anti-GBM immune response were TLR2 dependent. We then proceeded to identify the endogenous ligand responsible for TLR2 signaling on tumor-infiltrating mDCs. We demonstrated that HMGB1 was released from dying tumor cells, in response to Ad-TK (+ gancyclovir [GCV]) treatment. Increased levels of HMGB1 were also detected in the serum of tumor-bearing Ad-Flt3L/Ad-TK (+GCV)-treated mice. Specific activation of TLR2 signaling was induced by supernatants from Ad-TK (+GCV)-treated GBM cells; this activation was blocked by glycyrrhizin (a specific HMGB1 inhibitor) or with antibodies to HMGB1. HMGB1 was also released from melanoma, small cell lung carcinoma, and glioma cells treated with radiation or temozolomide. Administration of either glycyrrhizin or anti-HMGB1 immunoglobulins to tumor-bearing Ad-Flt3L and Ad-TK treated mice, abolished therapeutic efficacy, highlighting the critical role played by HMGB1-mediated TLR2 signaling to elicit tumor regression. Therapeutic efficacy of Ad-Flt3L and Ad-TK (+GCV) treatment was demonstrated in a second glioma model and in an intracranial melanoma model with concomitant increases in the levels of circulating HMGB1.
  
  
  CONCLUSIONS
  Our data provide evidence for the molecular and cellular mechanisms that support the rationale for the clinical implementation of antibrain cancer immunotherapies in combination with tumor killing approaches in order to elicit effective antitumor immune responses, and thus, will impact clinical neuro-oncology practice.
authors.affiliation: Department of Medicine, Board of Governors' Gene Therapeutics Research Institute, Cedars Sinai Medical Center, Los Angeles, California, United States of America
authors.forename: James F
authors.initials: JF
authors.lastname: Curtin
authors.forename: Naiyou
authors.initials: N
authors.lastname: Liu
authors.forename: Marianela
authors.initials: M
authors.lastname: Candolfi
authors.forename: Weidong
authors.initials: W
authors.lastname: Xiong
authors.forename: Hikmat
authors.initials: H
authors.lastname: Assi
authors.forename: Kader
authors.initials: K
authors.lastname: Yagiz
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Edwards
authors.forename: Kathrin S
authors.initials: KS
authors.lastname: Michelsen
authors.forename: Kurt M
authors.initials: KM
authors.lastname: Kroeger
authors.forename: Chunyan
authors.initials: C
authors.lastname: Liu
authors.forename: A K M Ghulam
authors.initials: AK
authors.lastname: Muhammad
authors.forename: Mary C
authors.initials: MC
authors.lastname: Clark
authors.forename: Moshe
authors.initials: M
authors.lastname: Arditi
authors.forename: Begonya
authors.initials: B
authors.lastname: Comin-Anduix
authors.forename: Antoni
authors.initials: A
authors.lastname: Ribas
authors.forename: Pedro R
authors.initials: PR
authors.lastname: Lowenstein
authors.forename: Maria G
authors.initials: MG
authors.lastname: Castro
mesh_terms: D000256:Adenoviridae / Q000235:genetics / Q000378:metabolism; D000818:Animals; D001932:Brain Neoplasms / Q000276:immunology / Q000378:metabolism* / Q000628:therapy; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D005260:Female; D005434:Flow Cytometry; D005822:Genetic Vectors; D005909:Glioblastoma / Q000276:immunology / Q000378:metabolism; D024243:HMGB1 Protein / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008822:Mice, Transgenic; D051195:Toll-Like Receptor 2 / Q000378:metabolism*
chemical_list: D024243:HMGB1 Protein; D051195:Toll-Like Receptor 2