TLR9 and endogenous adjuvants of the whole blood-stage malaria vaccine

Modulation of the Innate Immune Response by Targeting Toll-like Receptors: A Perspective on Their Agonists and Antagonists

Toll-like receptors (TLRs) are a class of proteins that recognize pathogen-associated molecular patterns (PAMPs) and damaged-associated molecular patterns (DAMPs), and they are involved in the regulation of innate immune system. These transmembrane receptors, localized at the cellular or endosomal membrane, trigger inflammatory processes through either myeloid differentiation primary response 88 (MyD88) or TIR-domain-containing adapter-inducing interferon-β (TRIF) signaling pathways. In the last decades, extensive research has been performed on TLR modulators and their therapeutic implication under several pathological conditions, spanning from infections to cancer, from metabolic disorders to neurodegeneration and autoimmune diseases. This Perspective will highlight the recent discoveries in this field, emphasizing the role of TLRs in different diseases and the therapeutic effect of their natural and synthetic modulators, and it will discuss insights for the future exploitation of TLR modulators in human health.

Immune effector mechanisms in malaria: An update focusing on human immunity

The past decade has witnessed dramatic decreases in malaria-associated mortality and morbidity around the world. This progress has largely been due to intensified malaria control measures, implementation of rapid diagnostics and establishing a network to anticipate and mitigate antimalarial drug resistance. However, the ultimate tool for malaria prevention is the development and implementation of an effective vaccine. To date, malaria vaccine efforts have focused on determining which of the thousands of antigens expressed by Plasmodium falciparum are instrumental targets of protective immunity. The antigenic variation and antigenic polymorphisms arising in parasite genes under immune selection present a daunting challenge for target antigen selection and prioritization, and is a given caveat when interpreting immune recall responses or results from monovalent vaccine trials. Other immune evasion strategies executed by the parasite highlight the myriad of ways in which it can become a recurrent infection. This review provides an update on immune effector mechanisms in malaria and focuses on our improved ability to interrogate the complexity of human immune system, accelerated by recent methodological advances. Appreciating how the human immune landscape influences the effectiveness and longevity of antimalarial immunity will help explain which conditions are necessary for immune effector mechanisms to prevail.

TACI Contributes to Plasmodium yoelii Host Resistance by Controlling T Follicular Helper Cell Response and Germinal Center Formation

The delay in parasite-specific B cell development leaves people in malaria endemic areas vulnerable to repeated Plasmodium infections. Here, we investigated the role of transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), a molecule involved in the generation of antigen-specific antibody secreting cells, in host response to non-lethal Plasmodium yoelii infection. We found that TACI deficiency not only resulted in higher peak parasitemia levels in P. yoelii challenged mice, but also led to a delay in parasite clearance and anti-P. yoelii Merozoite Surface Protein 1(C-terminal 19-kDa fragment [rMSP-1₁₉]) protein and anti-rMSP-1₁₉ and anti-P. yoelii IgG antibody development. There was also a delay in the generation of splenic high affinity antibody secreting cells that recognize rMSP-1₁₉ protein as compared to wild-type mice. Interestingly, coinciding with the delay in parasite clearance there was a delay in the resolution of T follicular helper (T_FH) cell and germinal center (GC) B cell responses in TACI -/- mice. The persistence of T_FH and GC B cells is likely a result of enhanced interaction between T_FH and GC B cells because inducible costimulator ligand (ICOSL) expression was significantly higher on TACI -/- GC B cells than wild-type cells. The difference in the kinetics of GC reaction appeared to also impact the emergence of plasma cells (PC) because there was a delay in the generation of TACI -/- mice PC. Nevertheless, following the recovery from P. yoelii infection, TACI -/- and wild-type mice were both protected from a rechallenge infection. Establishment of protective B cell response was responsible for the resolution of parasitemia because B cells purified from recovered TACI -/- or wild-type mice were equally protective when introduced to naïve wild-type mice prior to P. yoelii challenge. Thus, despite the increased susceptibility of TACI -/- mice to P. yoelii infection and a delay in the development of protective antibody levels, TACI -/- mice are able to clear the infection and resist rechallenge infection.

Immunization with the outer membrane proteins OmpK17 and OmpK36 elicits protection against Klebsiella pneumoniae in the murine infection model

Klebsiella pneumoniae is a Gram-negative bacterium that is increasingly reported as a serious nosocomial and community-acquired pathogen. In the current study, two K. pneumoniae antigens, OmpK17 and OmpK36, as well as their fusion protein cognate F36/17 were investigated as potential vaccine candidates in a murine infection model. Three immunoadjuvants, namely the Gram-positive Enhancer Matrix (GEM) adjuvant, synthetic hemozoin (Hz) adjuvant and incomplete Freund's adjuvant (IFA) were evaluated. Genes of OmpK17 and OmpK36 antigens as well as their fusion protein were cloned in Escherichia coli for recombinant expression. Mice were immunized thrice with the individual recombinant purified antigens adjuvanted with one of the three adjuvants. Two weeks after the last booster, animals were challenged with a lethal dose of K. pneumoniae and immune protection parameters were assessed. Animals immunized with GEM- or Hz-adjuvanted K. pneumoniae antigens did not show significant protection upon bacterial challenge. Animals immunized with subcutaneous IFA-adjuvanted antigens showed the best results with survival percentages of 50, 60 and 50% for groups immunized with OmpK17, OmpK36 and F36/17, respectively. Serum IgG1, rather than IgG2a, antibodies were the most prevalent following vaccination indicating bias towards T helper type 2 (Th2) immune response. Opsonophagocytic assays demonstrated significant percentage killing in case of animals immunized with IFA-adjuvanted antigens. Overall, OmpK17 and OmpK36 are promising vaccine antigens which are worthy of further optimization of the immunization conditions, particularly the used immunoadjuvants, in order to achieve full protection against K. pneumoniae.

TLR3 and TLR9 agonists improve postexposure vaccination efficacy of live smallpox vaccines

Eradication of smallpox and discontinuation of the vaccination campaign resulted in an increase in the percentage of unvaccinated individuals, highlighting the need for postexposure efficient countermeasures in case of accidental or deliberate viral release. Intranasal infection of mice with ectromelia virus (ECTV), a model for human smallpox, is curable by vaccination with a high vaccine dose given up to 3 days postexposure. To further extend this protective window and to reduce morbidity, mice were vaccinated postexposure with Vaccinia-Lister, the conventional smallpox vaccine or Modified Vaccinia Ankara, a highly attenuated vaccine in conjunction with TLR3 or TLR9 agonists. We show that co-administration of the TLR3 agonist poly(I:C) even 5 days postexposure conferred protection, avoiding the need to increase the vaccination dose. Efficacious treatments prevented death, ameliorated disease symptoms, reduced viral load and maintained tissue integrity of target organs. Protection was associated with significant elevation of serum IFNα and anti-vaccinia IgM antibodies, modulation of IFNγ response, and balanced activation of NK and T cells. TLR9 agonists (CpG ODNs) were less protective than the TLR3 agonist poly(I:C). We show that activation of type 1 IFN by poly(I:C) and protection is achievable even without co-vaccination, requiring sufficient amount of the viral antigens of the infective agent or the vaccine. This study demonstrated the therapeutic potential of postexposure immune modulation by TLR activation, allowing to alleviate the disease symptoms and to further extend the protective window of postexposure vaccination.

The genetics of innate immunity sensors and human disease

Since their discovery, innate immunity microbial sensors have been increasingly studied and shown to play a critical role in innate responses to microbes in several experimental in vitro, ex vivo, and animal models. However, their role in the human response to infection in natural conditions has just started to be deciphered, by means of clinical studies of primary immunodeficiencies and epidemiological genetic studies. Here, we summarize the major findings concerning the genetic diversity of the various families of microbial sensors in humans, and of other molecules involved in the signaling pathways they trigger. Specifically, we review the genetic associations, revealed by both clinical and epidemiological genetics studies, of microbial sensors from five different families: Toll-like receptors, C-type lectin receptors, NOD-like receptors, RIG-I-like receptors, and cytosolic DNA sensors. In particular, we consider the relationships between variation at the genes encoding these molecules and susceptibility to and the severity of infectious diseases and other clinical conditions associated with immune dysfunction, including autoimmunity, inflammation, allergy, and cancer. Despite the fact that the genetic links between innate immunity sensors and human disorders remain still limited, human genetics studies are increasingly improving our understanding of the genuine functions of microbial sensors and downstream signaling molecules in the natural setting.

Activation of rabbit TLR9 by different CpG-ODN optimized for mouse and human TLR9

Synthetic CpG-oligodeoxynucleotides (CpG-ODN) are potent adjuvants that accelerate and boost antigen-specific immune responses. Toll-like receptor 9 (TLR9) is the cellular receptor for these CpG-ODN. Previous studies have shown species-specific activation of mouse TLR9 (mTLR9) and human TLR9 (hTLR9) by their optimized CpG-ODN. The interaction between rabbit TLR9 (rabTLR9) and CpG-ODN, however, has not been previously investigated. Here, we cloned and characterized rabTLR9 and comparatively investigated the activation of the rabbit, mouse, and human TLR9 by CpG-ODN. The complete open reading frame of rabTLR9 encodes 1028 amino acid residues, which share 70.6% and 75.5% of the identities of mTLR9 and hTLR9, respectively. Rabbit TLR9 is preferentially expressed in immune cells rich tissues, and is localized in intracellular vesicles. While mTLR9 and hTLR9 displayed species-specific recognition of their optimized CpG-ODN, rabbit TLR9 was activated by these CpG-ODN without any preference. This result suggests that rabTLR9 has a broader ligand-recognition profile than mouse and human TLR9.

Functionally Distinct Subpopulations of CpG-Activated Memory B Cells

During the human B cell (Bc) recall response, rapid cell division results in multiple Bc subpopulations. The TLR-9 agonist CpG oligodeoxynucleotide, combined with cytokines, causes Bc activation and division in vitro and increased CD27 surface expression in a sub-population of Bc. We hypothesized that the proliferating CD27^lo subpopulation, which has a lower frequency of antibody-secreting cells (ASC) than CD27^hi plasmablasts, provides alternative functions such as cytokine secretion, costimulation, or antigen presentation. We performed genome-wide transcriptional analysis of CpG activated Bc sorted into undivided, proliferating CD27^lo and proliferating CD27^hi subpopulations. Our data supported an alternative hypothesis, that CD27^lo cells are a transient pre-plasmablast population, expressing genes associated with Bc receptor editing. Undivided cells had an active transcriptional program of non-ASC B cell functions, including cytokine secretion and costimulation, suggesting a link between innate and adaptive Bc responses. Transcriptome analysis suggested a gene regulatory network for CD27^lo and CD27^hi Bc differentiation.

Antitumor effect of malaria parasite infection in a murine Lewis lung cancer model through induction of innate and adaptive immunity

Background

Lung cancer is the most common malignancy in humans and its high fatality means that no effective treatment is available. Developing new therapeutic strategies for lung cancer is urgently needed. Malaria has been reported to stimulate host immune responses, which are believed to be efficacious for combating some clinical cancers. This study is aimed to provide evidence that malaria parasite infection is therapeutic for lung cancer.

Methodology/Principal Findings

Antitumor effect of malaria infection was examined in both subcutaneously and intravenously implanted murine Lewis lung cancer (LLC) model. The results showed that malaria infection inhibited LLC growth and metastasis and prolonged the survival of tumor-bearing mice. Histological analysis of tumors from mice infected with malaria revealed that angiogenesis was inhibited, which correlated with increased terminal deoxynucleotidyl transferase-mediated (TUNEL) staining and decreased Ki-67 expression in tumors. Through natural killer (NK) cell cytotoxicity activity, cytokine assays, enzyme-linked immunospot assay, lymphocyte proliferation, and flow cytometry, we demonstrated that malaria infection provided anti-tumor effects by inducing both a potent anti-tumor innate immune response, including the secretion of IFN-γ and TNF-α and the activation of NK cells as well as adaptive anti-tumor immunity with increasing tumor-specific T-cell proliferation and cytolytic activity of CD8⁺ T cells. Notably, tumor-bearing mice infected with the parasite developed long-lasting and effective tumor-specific immunity. Consequently, we found that malaria parasite infection could enhance the immune response of lung cancer DNA vaccine pcDNA3.1-hMUC1 and the combination produced a synergistic antitumor effect.

Conclusions/Significance

Malaria infection significantly suppresses LLC growth via induction of innate and adaptive antitumor responses in a mouse model. These data suggest that the malaria parasite may provide a novel strategy or therapeutic vaccine vector for anti-lung cancer immune-based therapy.