Development of a dendritic cell-targeting lipopeptide as an immunoadjuvant that inhibits tumor growth without inducing local inflammation

Immune mediated support of metastasis: Implication for bone invasion

Bone is a common organ affected by metastasis in various advanced cancers, including lung, breast, prostate, colorectal, and melanoma. Once a patient is diagnosed with bone metastasis, the patient's quality of life and overall survival are significantly reduced owing to a wide range of morbidities and the increasing difficulty of treatment. Many studies have shown that bone metastasis is closely related to bone microenvironment, especially bone immune microenvironment. However, the effects of various immune cells in the bone microenvironment on bone metastasis remain unclear. Here, we described the changes in various immune cells during bone metastasis and discussed their related mechanisms. Osteoblasts, adipocytes, and other non-immune cells closely related to bone metastasis were also included. This review also summarized the existing treatment methods and potential therapeutic targets, and provided insights for future studies of cancer bone metastasis.

Toll-like receptor 6 inhibits colorectal cancer progression by suppressing NF-κB signaling

Background

Toll-like receptors (TLRs) are implicated in the pathogenesis and progression of inflammation-associated cancers, except their role in regulating innate immunity. Specifically, a berrant expression of TLR6 has been observed in colorectal cancers (CRC). However, the effect of abnormal TLR6 expression on CRC remians unclear. Therefore, the present study evaluated TLR6 expression in CRC, its effect on CRC proliferation, and its underlying mechanism.

Methods

The expression of TLR6 in CRC was assessed using data from TCGA, GTEx, and HPA datasets and immunohistochemical assays of tumor tissues from patients with CRC. In human CRC cell lines, TLR6 signaling was activated using the TLR6 agonist Pam2CSK4 and was blocked using antiTLR6-IgG; subsequently, cell growth, migration, invasion, cell cycle, and apoptosis were compared in CRC cells. The levels of the anti-apoptotic protein Bcl-2 and the apoptotic protein Bax were identified using western blotting. In addition, the effect of TLR6 knockdown by shRNAs in CRC cells was observed both in vitro and in vivo. Nuclear factor κB (NF-κB) level was evaluated using immunofluorescence and western bolt.

Results

TLR6 expression was significantly downregulated in CRC tissues. The activation of TLR6 by Pam2CSK4 (100 pg/mL to 10 ng/mL) inhibited the proliferation of CRC cells. Compared with blocking TLR6 signaling using antiTLR6-IgG, activating TLR6 signaling significantly inhibited CRC cell growth, migration, and invasion as well as decreased the proportion of cells in the S and G2/M phases and promoted apoptosis. Furthermore, the knockdown of TLR6 by shRNA promoted the biological activity of CRC cells both in vitro and in vivo. Moreover, the activation of TLR6 signaling by Pam2CSK4 significantly downregulated NF-κB and Bcl-2 levels but upregulated Bax levels.

Conclusion

The findings of this study demonstrate that TLR6 may play a inhibitive role in CRC tumorigenesis by suppressing the activity of NF-κB signaling.

Potent adjuvant effect elicited for tumor immunotherapy by a liposome conjugated pH-sensitive polymer and dendritic cell-targeting Toll-like-receptor ligand

The generation of DCs with augmented functions is a strategy for obtaining satisfactory clinical outcomes in tumor immunotherapy. We developed a novel synthetic adjuvant comprising a liposome conjugated with a DC-targeting Toll-like-receptor ligand and a pH-sensitive polymer for augmenting cross-presentation. In an in vitro study using mouse DCs, these liposomes were selectively incorporated into DCs, significantly enhanced DC function and activated immune responses to present an epitope of the incorporated antigen on the major histocompatibility complex class I molecules. Immunization of mice with liposomes encapsulating a tumor antigen significantly enhanced antigen-specific cytotoxicity. In tumor-bearing mice, vaccination with liposomes encapsulating a tumor antigen elicited complete tumor remission. Furthermore, vaccination significantly enhanced cytotoxicity, targeting not only the vaccinated antigen but also the other antigens of the tumor cell. These results indicate that liposomes are an ideal adjuvant to develop DCs with considerably high potential to elicit antigen-specific immune responses; they are a promising tool for cancer therapy with neoantigen vaccination.

The second and third amino acids of Pam2 lipopeptides are key for the proliferation of cytotoxic T cells

The TLR2 agonist, dipalmitoyl lipopeptide (Pam2LP), has been used as an immune adjuvant without much success. Pam2LP is recognised by TLR2/6 receptors in humans and in mice. This study examined the proliferative activity of cytotoxic T lymphocytes (CTL) using mouse Ag-presenting dendritic cells (DCs) and OT-I assay system, where a library of synthetic Pam2LP was utilised from the Staphylococcus aureus database. Ag-specific CTL expansion and IFN-γ levels largely depended on the Pam2LP peptide sequence. The first Aa is cysteine (Cys), which has an active SH residue to bridge fatty acids, and the second and third Aa are hydrophilic or non-polar. The sequence structurally adapted to the residual constitution of the reported TLR2/6 pocket. The inactive sequence contained proline or leucine/isoleucine after the first Cys. Notably, no direct activation of OT-I cells was detected without DCs by stimulation with the active Pam2LP having the Cys-Ser sequence. MyD88, but not TICAM-1 or IFN pathways, in DCs participates in DC maturation characterised by upregulation of CD40, CD80 and CD86. Hence, the active Pam2LPs appear suitable for dimeric TLR2/6 on DCs, resulting in induction of DC maturation.

Development of a vaccine based on bacteria-mimicking tumor cells coated with novel engineered toll-like receptor 2 ligands

For a successful tumor vaccine, it is necessary to develop effective immuno-adjuvants and identify specific tumor antigens. Tumor cells obtained from surgical or biopsy tissues are a good source of tumor antigens but, unlike bacteria, they do not induce strong immune responses. Here, we designed 2 novel lipopeptides that coat tumor cell surfaces and mimic bacterial components. Tumor cells coated with these lipopeptides (called bacteria-mimicking tumor cells [BMTC]) were prepared and their efficacy as a tumor vaccine examined. Natural bacterial lipopeptides act as ligands for toll-like receptor 2 (TLR2) and activate dendritic cells (DC). To increase the affinity of the developed lipopeptides for the negatively charged plasma membrane, a cationic polypeptide was connected to Pam2Cys (P2C), which is the basic structure of the TLR2 ligand. This increased the non-specific binding affinity of the peptides for the cell surface. Two such lipopeptides, P2CSK11 (containing 1 serine and 11 lysine residues) and P2CSR11 (containing 1 serine and 11 arginine residues) bound to irradiated tumor cells via the long cationic polypeptides more efficiently than the natural lipopeptide MALP2 (P2C-GNNDESNISFKEK) or a synthetic lipopeptide P2CSK4 (a short cationic polypeptide containing 1 serine and 4 lysines). BMTC coated with P2CSR11 or P2CSK11 were efficiently phagocytosed by DC and induced antigen cross-presentation in vitro. They also induced effective tumor-specific cytotoxic T cell responses and inhibited tumor growth in in vivo mouse models. P2CSR11 activated DC but induced less inflammation-inducing cytokines/interferons than other lipopeptides. Thus, P2CSR11 is a strong candidate antigen-specific immuno-adjuvant, with few adverse effects.

Toll-like Receptor Agonist Conjugation: A Chemical Perspective

Toll-like receptors (TLRs) are vital elements of the mammalian immune system that function by recognizing pathogen-associated molecular patterns (PAMPs), bridging innate and adaptive immunity. They have become a prominent therapeutic target for the treatment of infectious diseases, cancer, and allergies, with many TLR agonists currently in clinical trials or approved as immunostimulants. Numerous studies have shown that conjugation of TLR agonists to other molecules can beneficially influence their potency, toxicity, pharmacokinetics, or function. The functional properties of TLR agonist conjugates, however, are highly dependent on the ligation strategy employed. Here, we review the chemical structural requirements for effective functional TLR agonist conjugation. In addition, we provide similar analysis for those that have yet to be conjugated. Moreover, we discuss applications of covalent TLR agonist conjugation and their implications for clinical use.

Type I Interferon-Independent Dendritic Cell Priming and Antitumor T Cell Activation Induced by a Mycoplasma fermentans Lipopeptide

Mycoplasma fermentans-derived diacylated lipoprotein M161Ag (MALP404) is recognized by human/mouse toll-like receptor (TLR) 2/TLR6. Short proteolytic products including macrophage-activating lipopeptide 2 (MALP2) have been utilized as antitumor immune-enhancing adjuvants. We have chemically synthesized a short form of MALP2 named MALP2s (S-[2,3-bis(palmitoyloxy)propyl]-CGNNDE). MALP2 and MALP2s provoke natural killer (NK) cell activation in vitro but only poorly induce tumor regression using in vivo mouse models loading NK-sensitive tumors. Here, we identified the functional mechanism of MALP2s on dendritic cell (DC)-priming and cytotoxic T lymphocyte (CTL)-dependent tumor eradication using CTL-sensitive tumor-implant models EG7 and B16-OVA. Programmed death ligand-1 (PD-L1) blockade therapy in combination with MALP2s + ovalbumin (OVA) showed a significant additive effect on tumor growth suppression. MALP2s increased co-stimulators CD80/86 and CD40, which were totally MyD88-dependent, with no participation of toll-IL-1R homology domain-containing adaptor molecule-1 or type I interferon signaling in DC priming. MALP2s + OVA consequently augmented proliferation of OVA-specific CTLs in the spleen and at tumor sites. Chemokines and cytolytic factors were upregulated in the tumor. Strikingly, longer duration and reinvigoration of CTLs in spleen and tumors were accomplished by the addition of MALP2s + OVA to α-PD-L1 antibody (Ab) therapy compared to α-PD-L1 Ab monotherapy. Then, tumors regressed better in the MALP2s/OVA combination than in the α-PD-L1 Ab monotherapy. Hence, MALP2s/tumor-associated antigens combined with α-PD-L1 Ab is a good therapeutic strategy in some mouse models. Unfortunately, numerous patients are still resistant to PD-1/PD-L1 blockade, and good DC-priming adjuvants are desired. Cytokine toxicity by MALP2s remains to be settled, which should be improved by chemical modification in future studies.

Development of effective tumor immunotherapy using a novel dendritic cell-targeting Toll-like receptor ligand

Although dendritic cell (DC)-based immunotherapy shows little toxicity, improvements should be necessary to obtain satisfactory clinical outcome. Using interferon-gamma injection along with DCs, we previously obtained significant clinical responses against small or early stage malignant tumors in dogs. However, improvement was necessary to be effective to largely developed or metastatic tumors. To obtain effective methods applicable to those tumors, we herein used a DC-targeting Toll-like receptor ligand, h11c, and examined the therapeutic effects in murine subcutaneous and visceral tumor models and also in the clinical treatment of canine cancers. In murine experiments, most and significant inhibition of tumor growth and extended survival was observed in the group treated with the combination of h11c-activated DCs in combination with interferon-gamma and a cyclooxygenase2 inhibitor. Both monocytic and granulocytic myeloid-derived suppressor cells were significantly reduced by the combined treatment. Following the successful results in mice, the combined treatment was examined against canine cancers, which spontaneously generated like as those in human. The combined treatment elicited significant clinical responses against a nonepithelial malignant tumor and a malignant fibrous histiocytoma. The treatment was also successful against a bone-metastasis of squamous cell carcinoma. In the successful cases, the marked increase of tumor-responding T cells and decrease of myeloid-derived suppressor cells and regulatory T cells was observed in their peripheral blood. Although the combined treatment permitted the growth of lung cancer of renal carcinoma-metastasis, the marked elevated and long-term maintaining of the tumor-responding T cells was observed in the patient dog. Overall, the combined treatment gave rise to emphatic amelioration in DC-based cancer therapy.

Toll-like receptor 2 ligand and interferon-γ suppress anti-tumor T cell responses by enhancing the immunosuppressive activity of monocytic myeloid-derived suppressor cells

CD11b⁺Gr1⁺ myeloid-derived suppressor cells (MDSCs) suppress activation/proliferation of cytotoxic T cells, thereby hindering cancer immunotherapy. MDSCs are increased after adjuvant therapy with toll-like receptor (TLR) 2 ligands, such as Pam2CSK4, in tumor-bearing mice. However, it remains unknown if the activation of TLR2 in MDSCs affects their function and the therapeutic efficacy of TLR2 ligand. Here, we show that TLR2 signaling in CD11b⁺Ly6G⁻Ly6C^high monocytic MDSCs (M-MDSCs), but not CD11b⁺Ly6G⁺Ly6C^low granulocytic MDSCs (G-MDSCs), enhances their immunosuppressive activity, thereby limiting anti-tumor T cell responses induced by TLR2-activated dendritic cells (DCs). iNOS induction was critical for Pam2CSK4-enhanced T cell suppression by M-MDSCs. iNOS was expressed in M-MDSC-derived macrophages, but not undifferentiated M-MDSCs, in cocultures with CD8⁺ T cells, CD11c⁺ DCs, antigen peptide and Pam2CSK4. Pam2CSK4 increased the differentiation frequency of M-MDSCs to macrophages, and iNOS expression required interferon-γ (IFN-γ) production by CD8⁺ T cells that had been transiently stimulated by M-MDSC-derived macrophages in an antigen/TLR2-dependent manner. Although Pam2CSK4 triggered DC maturation and tumor regression via induction of tumor antigen-specific cytotoxic T lymphocyte (CTL) responses in tumor-bearing mice, Pam2CSK4 plus antigen increased the frequency of iNOS⁺ macrophages in the tumor. Treatment with iNOS inhibitor enhanced the therapeutic efficacy of Pam2CSK4. Hence, the results suggest that TLR2 ligand and T cell-derived IFN-γ enhance M-MDSC-mediated immunosuppression, which may negatively regulate anti-tumor CTL response.

Clients and Oncogenic Roles of Molecular Chaperone gp96/grp94

As an endoplasmic reticulum heat shock protein (HSP) 90 paralogue, glycoprotein (gp) 96 possesses immunological properties by chaperoning antigenic peptides for activation of T cells. Genetic studies in the last decade have unveiled that gp96 is also an essential master chaperone for multiple receptors and secreting proteins including Toll-like receptors (TLRs), integrins, the Wnt coreceptor, Low Density Lipoprotein Receptor-Related Protein 6 (LRP6), the latent TGFβ docking receptor, Glycoprotein A Repetitions Predominant (GARP), Glycoprotein (GP) Ib and insulin-like growth factors (IGF). Clinically, elevated expression of gp96 in a variety of cancers correlates with the advanced stage and poor survival of cancer patients. Recent preclinical studies have also uncovered that gp96 expression is closely linked to cancer progression in multiple myeloma, hepatocellular carcinoma, breast cancer and inflammation-associated colon cancer. Thus, gp96 is an attractive therapeutic target for cancer treatment. The chaperone function of gp96 depends on its ATPase domain, which is structurally distinct from other HSP90 members, and thus favors the design of highly selective gp96-targeted inhibitors against cancer. We herein discuss the strategically important oncogenic clients of gp96 and their underlying biology. The roles of cell-intrinsic gp96 in T cell biology are also discussed, in part because it offers another opportunity of cancer therapy by manipulating levels of gp96 in T cells to enhance host immune defense.

Polymer-Based DNA Delivery Systems for Cancer Immunotherapy

The use of gene delivery systems for the expression of antigenic proteins is an established means for activating a patient’s own immune system against the cancer they carry. Since tumor cells are poor antigen-presenting cells, cross-presentation of tumor antigens by dendritic cells (DCs) is essential for the generation of tumor-specific cytotoxic T-lymphocyte responses. A number of polymer-based nanomedicines have been developed to deliver genes into DCs, primarily by incorporating tumor-specific, antigen-encoding plasmid DNA with polycationic molecules to facilitate DNA loading and intracellular trafficking. Direct in vivo targeting of plasmid DNA to DC surface receptors can induce high transfection efficiency and long-term gene expression, essential for antigen loading onto major histocompatibility complex molecules and stimulation of T-cell responses. This chapter summarizes the physicochemical properties and biological information on polymer-based non-viral vectors used for targeting DCs, and discusses the main challenges for successful in vivo gene transfer into DCs.

Acute myeloid leukaemia-derived Langerhans-like cells enhance Th1 polarization upon TLR2 engagement

Langerhans cells (LCs) represent a highly specialized subset of epidermal dendritic cells (DCs), yet not fully understood in their function of balancing skin immunity. Here, we investigated in vitro generated Langerhans-like cells obtained from the human acute myeloid leukaemia cell line MUTZ-3 (MUTZ-LCs) to study TLR- and cytokine-dependent activation of epidermal DCs. MUTZ-LCs revealed high TLR2 expression and responded robustly to TLR2 engagement, confirmed by increased CD83, CD86, PD-L1 and IDO expression, upregulated IL-6, IL-12p40 and IL-23p19 mRNA levels IL-8 release. TLR2 activation reduced CCR6 and elevated CCR7 mRNA expression and induced migration of MUTZ-LCs towards CCL21. Similar results were obtained by stimulation with pro-inflammatory cytokines TNF-α and IL-1β whereas ligands of TLR3 and TLR4 failed to induce a fully mature phenotype. Despite limited cytokine gene expression and production for TLR2-activated MUTZ-LCs, co-culture with naive CD4(+) T cells led to significantly increased IFN-γ and IL-22 levels indicating Th1 differentiation independent of IL-12. TLR2-mediated effects were blocked by the putative TLR2/1 antagonist CU-CPT22, however, no selectivity for either TLR2/1 or TLR2/6 was observed. Computer-aided docking studies confirmed non-selective binding of the TLR2 antagonist. Taken together, our results indicate a critical role for TLR2 signalling in MUTZ-LCs considering the leukemic origin of the generated Langerhans-like cells.

Pros and Cons of Antigen-Presenting Cell Targeted Tumor Vaccines

In therapeutic antitumor vaccination, dendritic cells play the leading role since they decide if, how, when, and where a potent antitumor immune response will take place. Since the disentanglement of the complexity and merit of different antigen-presenting cell subtypes, antitumor immunotherapeutic research started to investigate the potential benefit of targeting these subtypes in situ. This review will discuss which antigen-presenting cell subtypes are at play and how they have been targeted and finally question the true meaning of targeting antitumor-based vaccines.

The TLR-2/TLR-6 agonist macrophage-activating lipopeptide-2 augments human NK cell cytotoxicity when PGE2 production by monocytes is inhibited by a COX-2 blocker

Macrophage-activating lipopeptide-2 (MALP-2) is a potent inducer of proinflammatory cytokine secretion by macrophages, monocytes, and dendritic cells. MALP-2 was reported to be involved in natural killer (NK) cell activation and ensuing tumor rejection. However, the mechanism of MALP-2-mediated NK cell activation remained unclear. Therefore, we studied the effects of MALP-2 on cultured human NK cells. We found that MALP-2 had no direct effect on NK cells. Instead, MALP-2 acted on monocytes and triggered the release of different molecules such as interleukin (IL)-1β, IL-6, IL-10, IL-12, IL-15, interferon gamma-induced protein (IP-10), and prostaglandin (PG)-E2. Our data show that monocyte-derived IP-10 could significantly induce NK cell cytotoxicity as long as the immunosuppression by PGE2 is specifically inhibited by cyclooxygenase (COX)-2 blockade. In summary, our results show that MALP-2-mediated stimulation of monocytes results in the production of several mediators which, depending on the prevailing conditions, affect the activity of NK cells in various ways. Hence, MALP-2 administration with concurrent blocking of COX-2 can be considered as a promising approach in MALP-2-based adjuvant tumor therapies.