Structural-functional relationship of pathogen-associated molecular patterns: lessons from BCG cell wall skeleton and mycoplasma lipoprotein M161Ag

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.

Update on Chlamydia trachomatis Vaccinology

Attempts to produce a vaccine to protect against Chlamydia trachomatis-induced trachoma were initiated more than 100 years ago and continued for several decades. Using whole organisms, protective responses were obtained. However, upon exposure to C. trachomatis, disease exacerbation developed in some immunized individuals, precluding the implementation of the vaccine. Evidence of the role of C. trachomatis as a sexually transmitted pathogen started to emerge in the 1960s, and it soon became evident that it can cause acute infections and long-term sequelae in women, men, and newborns. The main focus of this minireview is to summarize recent findings and discuss formulations, including antigens, adjuvants, routes, and delivery systems for immunization, primarily explored in the female mouse model, with the goal of implementing a vaccine against C. trachomatis genital infections.

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

Materials used for the past 30 years as immunoadjuvants induce suboptimal antitumor immune responses and often cause undesirable local inflammation. Some bacterial lipopeptides that act as Toll-like receptor (TLR) 2 ligands activate immune cells as immunoadjuvants and induce antitumor effects. Here, we developed a new dendritic cell (DC)-targeting lipopeptide, h11c (P2C-ATPEDNGRSFS), which uses the CD11c-binding sequence of intracellular adhesion molecule-1 to selectively and efficiently activate DCs but not other immune cells. Although the h11c lipopeptide activated DCs similarly to an artificial lipopeptide, P2C-SKKKK (P2CSK4), via TLR2 in vitro, h11c induced more effective tumor inhibition than P2CSK4 at low doses in vivo with tumor antigens. Even without tumor antigens, h11c lipopeptide significantly inhibited tumor growth and induced tumor-specific cytotoxic T cells. P2CSK4 was retained subcutaneously at the vaccination site and induced severe local inflammation in in vivo experiments. In contrast, h11c was not retained at the vaccination site and was transported into the tumor within 24 hr. The recruitment of DCs into the tumor was induced by h11c more effectively, while P2CSK4 induced the accumulation of neutrophils leading to severe inflammation at the vaccination site. Because CD11b+ cells, but not CD11c+ cells, produced neutrophil chemotactic factors such as macrophage inflammatory protein (MIP)-2 in response to stimulation with TLR2 ligands, the DC-targeting lipopeptide h11c induced less MIP-2 production by splenocytes than P2CSK4. In this study, we succeeded in developing a novel immunoadjuvant, h11c, which effectively induces antitumor activity without adverse effects such as local inflammation via the selective activation of DCs.

Adjuvant engineering for cancer immunotherapy: Development of a synthetic TLR2 ligand with increased cell adhesion

The development of effective immunoadjuvants for tumor immunotherapy is of fundamental importance. The use of Mycobacterium bovis bacillus Calmette-Guérin cell wall skeleton (BCG-CWS) in tumor immunotherapy has been examined in various clinical applications. Because BCG-CWS is a macromolecule that cannot be chemically synthesized, the development of an alternative synthetic molecule is necessary to ensure a constant supply of adjuvant. In the present study, a new adjuvant was designed based on the structure of macrophage-activating lipopeptide (MALP)-2, which is a Toll-like receptor (TLR)-2 ligand similar to BCG-CWS. Macrophage-activating lipopeptide-2, [S-(2,3-bispalmitoyloxypropyl)Cys (P2C) - GNNDESNISFKEK], originally identified in a Mycoplasma species, is a lipopeptide that can be chemically synthesized. A MALP-2 peptide was substituted with a functional motif, RGDS, creating a novel molecule named P2C-RGDS. RGDS was selected because its sequence constitutes an integrin-binding motif and various integrins are expressed in immune cells including dendritic cells (DCs). Thus, this motif adds functionality to the ligand. P2C-RGDS activated DCs and splenocytes more efficiently than MALP-2 over short incubation times in vitro, and the RGDS motif contributed to their activation. Furthermore, P2C-RGDS showed higher activity than MALP-2 in inducing migration of DCs to draining lymph node, and in inhibiting tumor growth in vivo. This process of designing and developing synthetic adjuvants has been named "adjuvant engineering," and the evaluation and improvement of P2C-RGDS constitutes a first step in the development of stronger synthetic adjuvants in the future.

A role for glycogen synthase kinase-3 in antagonizing mycobacterial immune evasion by negatively regulating IL-10 induction

Mtb dysregulates monocyte/macrophage functions to produce a large amount of the immunosuppressive cytokine IL-10. An important function of IL-10 in promoting Mtb survival is the suppression of antigen presentation of monocytes/macrophages to T cells. This dampens the host immune responses and provides an opportunity for immune evasion. GSK3 has been shown to control the balance between pro- and anti-inflammatory cytokine productions. Here, we investigated whether GSK3 regulates IL-10 expression and mediates a protective role upon live mycobacterial challenge using BCG as a model. Our results showed that BCG increased Akt phosphorylation and inhibited GSK3 activity, resulting in increased IL-10 production. We confirmed further that suppression of GSK3 activities by a specific chemical inhibitor strongly enhanced BCG-induced IL-10 production. We also showed that IL-10 secreted by BCG-infected human PBMo was a major suppressor of subsequent IFN-gamma production by PBMC and HLA-DR expression on PBMo in response to BCG. Neutralization of PBMo-secreted IL-10 by anti-IL-10 antibodies restored the IFN-gamma production and HLA-DR surface expression. Taken together, GSK3 negatively regulates mycobacteria-induced IL-10 production in human PBMo. The kinase may play a role in restoring IFN-gamma secretions and subsequent antigen presentation in response to mycobacterial infection. In conclusion, our results suggest a significant role for GSK3 in guarding against mycobacterial evasion of immunity via IL-10 induction in the host.

Mycobacteria and allergies

Exposure to mycobacteria was inevitable throughout mammalian evolution. Most mycobacteria are saprophytic environmental organisms that are enormously abundant in soil and untreated water and evoke immune responses in the residents of developing countries. A few species are pathogens. For example Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), infects approximately 1/3 of the world's population. Many individuals also receive vaccination with the Bacille Calmette Guérin (BCG), which is an attenuated form of the organism causing bovine TB. In order to understand the possible role that mycobacteria might have in the increases in allergic disorders over the last decades, it is necessary to dissect out these different mycobacterial influences. Above all it is essential, when analysing tuberculin test results, to distinguish between individuals who have latent TB and those who do not. Only then can probable effects of diverse types of exposure emerge. There is no doubt that in animal models mycobacteria can both prevent and treat allergic responses either by boosting Th1 or by driving allergen-specific regulatory T cells (RegT). Clinical trials in man remain inconclusive.

New packaging method of mycobacterial cell wall using octaarginine-modified liposomes: enhanced uptake by and immunostimulatory activity of dendritic cells

Despite the potential of mycobacterial cell wall (CW) components to serve as immunotherapeutic agents, this application is hampered by the molecules' unfavorable physicochemical properties, such as its high molecular weight, poor solubility and negatively charged nature. Here we describe a new mycobacterial CW delivery system that uses an efficient and simple packaging method. This is achieved by incorporating mycobacterial CW into liposomes and attaching arginine octamers (R8) to the liposome surface. R8-modified liposomes improve the uptake of mycobacterial CW by dendritic cells (DC) and enhance its immunostimulatory activity. High R8 surface density promoted high levels of mycobacterial CW uptake by DC compared to low density R8-modified liposomes. Maturation markers (CD80, CD86, MHC Class II molecules) showed significantly enhanced expression on DC pulsed with high density R8-modified liposomes containing mycobacterial CW. Moreover, R8-modified liposomes with mycobacterial CW incorporated induced production of IL-12 p40 by DC, at levels similar to those produced by lipopolysaccharide-pulsed DC. We assert that R8-modified liposomes with mycobacterial CW incorporated should have tremendous potential as immune-potentiating agents.

Enhancement of immunologic tumor regression by intratumoral administration of dendritic cells in combination with cryoablative tumor pretreatment and Bacillus Calmette-Guerin cell wall skeleton stimulation

PURPOSE

We developed an effective immunotherapy, which could induce antitumor immune responses against shared and unique tumor antigens expressed in autologous tumors.

EXPERIMENTAL DESIGN

Intratumoral administration of dendritic cells is one of the individualized immunotherapies; however, the antitumor activity is relatively weak. In this study, we attempted to enhance the antitumor efficacy of the i.t. dendritic cell administration by combining dendritic cells stimulated with Bacillus Calmette-Guerin cell wall skeleton (BCG-CWS) additionally with cryoablative pretreatment of tumors and analyzed the therapeutic mechanisms.

RESULTS

These two modifications (cryoablation of tumors and BCG-CWS stimulation of dendritic cells) significantly increases the antitumor effect on both the treated tumor and the untreated tumor, which was distant at the opposite side, in a bilateral s.c. murine CT26 colon cancer model. Further analysis of the augmented antitumor effects revealed that the cryoablative pretreatment enhances the uptake of tumor antigens by the introduced dendritic cells, resulting in the induction of tumor-specific CD8(+) T cells responsible for the in vivo tumor regression of both treated and remote untreated tumors. This novel combination i.t. dendritic cell immunotherapy was effective against well-established large tumors. The antitumor efficacy was further enhanced by depletion of CD4(+)CD25(+)FoxP3(+) regulatory T cells.

CONCLUSIONS

This novel dendritic cell immunotherapy with i.t. administration of BCG-CWS-treated dendritic cells following tumor cryoablation could be used for the therapy of cancer patients with multiple metastases.

Gene-inducing program of human dendritic cells in response to BCG cell-wall skeleton (CWS), which reflects adjuvancy required for tumor immunotherapy

Adjuvants induce the expression of a number of genes in dendritic cells (DCs), which facilitate effective antigen-presentation and cytokine/chemokine liberation. It has been accepted that the toll-like receptor (TLR) family governs the adjuvant activity in DCs. An adjuvant with a long history is mycobacteria in an oil-in-water emulsion, namely Freund's complete adjuvant. Since the active center for the adjuvancy in mycobacteria is the cell-wall skeleton (CWS), we used the bacillus Calmette-Guerin cell-wall skeleton (BCG-CWS) to test DC maturation by GeneChip analysis. We identified the genes supporting an efficient DC response and output. Approximately 2000 genes were up-regulated by BCG-CWS stimulation. BCG-CWS-, peptidoglycan (PGN)- and lipopolysaccharide (LPS)-stimulation generally up-regulated some gene clusters including genes for inflammatory cytokines (TNF, IL1alpha, IL1beta, IL6, IL12 p40, IL23 p19, etc.), chemokines (CCL20, IL8, etc.), cell adhesion molecules (ICAM-1, etc.), apoptosis-related proteins (GADD45B, BCL2A1, etc.), metabolic enzymes (PTGS2, SOD2, etc.) and miscellaneous proteins (EHD1, TNFAIP6, etc.). LPS-stimulation, but not BCG-CWS- or PGN-stimulation, up-regulated the interferon-inducible antiviral proteins, including IFIT1, IFIT2, IFIT4, CXCL10, ISG15, OASL, IFITM1 and MX1. We also found that the BCG-CWS- or PGN-stimulation up-regulated CXCL5, MMP1, etc. We discussed their properties in association with TLRs and recently discovered TLR adapters.

Differential induction of gene promoter constructs by constitutively active human TLRs

Antigen presenting cells can sense microorganisms through activation of members of the Toll like receptor family (TLRs), which initiate signals leading to transcription of many inflammation-associated genes. TLRs and IL-1R, through their TIR domains, activate NFkappaB and mitogen-activated protein kinase pathways and upregulate a set of specific target genes. Recent evidence points to several differences in signaling pathways activated by individual TLRs. To evaluate the basic signaling potential of individual TIR signaling domains, we generated constitutively active versions of all known human TLRs by fusing mouse CD4 extracellular portion with the TLR transmembrane and TIR domains. A panel of promoters from genes known to be activated by TLRs as well as artificial promoter constructs with transcription factor binding sites were selected to measure their response in the presence of constitutively active CD4TLR fusion molecules. These studies show for the first time that a unique panel of promoters appears to be highly induced by CD4TLR1, 6 (TLRs that usually function through heterodimerisation with TLR2), and CD4TLR10. We also observed that CD4TLR4 is the most potent gene activator compared to all other ten human TLRs. Preliminary analyses of several promoter deletions showed that TLRs use different sequence elements to activate these reporters. In addition, since different ligands for a single TLR (e.g., TLR9) can induce different pathways, the CD4TLR fusions seem to activate all the pathways and therefore can be used to assess the overall signaling capacity of a given TLR. Finally, analysis of promoter constructs induced by the only orphan TLR, TLR10, allowed the identification of the ENA78 promoter as a tool for screening its ligands.

Innate immune responses to mycobacteria and the downregulation of atopic responses

PURPOSE OF REVIEW

Exposure to certain environmental microorganisms can promote the induction of T regulatory cells via the innate immune system. This review explores the possibility that reduced exposure to such organisms is leading to increased immunoregulatory disorders in a subset of individuals in whom this regulatory T-cell-inducing pathway is less efficient. We concentrate on mycobacteria and on asthma, because these are well documented.

RECENT FINDINGS

The blood cells of the children of farmers, who are partly protected from allergies, express increased levels of messenger RNA encoding CD14 and TLR2, and polymorphisms of CD14 are linked to allergic manifestations in some studies. Polymorphisms of TLR2 (which recognizes mycobacterial components in concert with CD14) are involved in the pattern of response to mycobacteria, and in the type of leprosy that develops. Similarly, polymorphisms of Nramp1, which affect the response to mycobacteria, are linked with the diseases of immunodysregulation that are increasing in parallel with allergic disorders. Moreover, congenic mice bearing different variants of Nramp1 differ in their allergic responses. These parallels are suggestive, in view of the observation that a saprophytic environmental mycobacterium is a potent inducer of regulatory T cells, and has shown significant effects in several phase I/II studies in man.

SUMMARY

The components of the innate immune system that are involved in responses to mycobacteria overlap with those implicated in allergic disorders. Polymorphisms might define the subset of individuals who develop immunoregulatory disorders. Understanding the role of the innate immune system will facilitate the design of clinical trials using microbial products.

Adjuvant-Mediated Tumor Regression and Tumor-Specific Cytotoxic Response Are Impaired in MyD88-Deficient Mice

The Mycobacterium bovis bacillus Calmette-Guérin cell-wall skeleton (BCG-CWS) activates Toll-like receptor (TLR) 2 and TLR4, but unlike the typical TLR4 agonist bacterial lipopolysaccharide barely induces type 1 IFN. BCG-CWS has been used for adjuvant immunotherapy for patients with cancer. We investigated the adjuvant potential of BCG-CWS for induction of CTLs subsequent to TLR-mediated dendritic cell (DC) maturation, using a syngeneic mouse tumor model (B16 melanoma in C57BL/6). We evaluated the retardation of tumor growth and cytotoxic response in wild-type and MyD88-/- mice immunized with tumor debris and/or BCG-CWS. Delays in tumor growth and cytotoxic response were induced by immunization with a mixture of BCG-CWS emulsion and the tumor. BCG-CWS was capable of activating DCs ex vivo by the criteria of CD80/CD86 up-regulation and cytokine (interleukin-12, tumor necrosis factor-alpha) induction. Efficient tumor suppression and ex vivo cytokine induction did not occur in MyD88-deficient mice and cells, suggesting that the MyD88 adapter is crucial for induction of tumor cytotoxicity. Because TLR4 is involved in both MyD88-dependent and -independent pathways and the latter affects DC maturation, our findings indicate that both pathways cooperate to induce CTL-based tumor immunity.

Simultaneous blocking of human Toll-like receptors 2 and 4 suppresses myeloid dendritic cell activation induced by Mycobacterium bovis bacillus Calmette-Guérin peptidoglycan

The Mycobacterium bovis bacillus Calmette-Guérin (BCG) cell wall skeleton (CWS) consists of mycolic acids, arabinogalactan, and peptidoglycan (PGN) and activates Toll-like receptor 2 (TLR2) and TLR4. Here we investigated the ability of the essential portion of highly purified BCG CWS to support the TLR agonist function by using the following criteria: myeloid dendritic cell (DC) maturation, i.e., tumor necrosis factor alpha (TNF-alpha) production and CD83/CD86 up-regulation. The purified PGN region was sufficient to activate TLR2 and TLR4 in mouse DCs and macrophages; in TLR2 and TLR4 double-knockout cells the BCG PGN-mediated TNF-alpha production ability was completely impaired. Likewise, stimulation with BCG CWS of HEK293 cells expressing either human TLR2 or TLR4, MD-2, and CD14 resulted in NF-kappa B activation as determined by a reporter assay. Notably, specific blockers of extracellular human TLR2 (an original cocktail of monoclonal antibodies TLR2.45 and TH2.1) and TLR4 (E5531) inhibited BCG CWS-mediated NF-kappa B activation by 80%. Using this human TLR blocking system, we tested whether human myeloid DC maturation was TLR2 and TLR4 dependent. BCG PGN-mediated DC maturation was blocked by 70% by suppression of both TLR2 and TLR4 and by 30 to 40% by suppression of either of these TLRs. Similar but less profound suppression of BCG CWS-mediated DC maturation was observed. Hence, the presence of BCG PGN is a minimal requirement for activation of both TLR2 and TLR4 in human DCs, unlike the presence of PGNs of gram-positive bacteria, which activate only TLR2. Unexpectedly, however, BCG PGN, unlike BCG CWS, barely activated NF-kappa B in HEK293 cells coexpressing TLR2 plus TLR1, TLR2 plus TLR4, TLR2 plus TLR6, or TLR2 plus TLR10, suggesting that PGN receptors other than TLR2 and TLR4 present on human DCs but not on HEK293 cells are involved in TLR signaling for DC activation.