New immune cell engagers for cancer immunotherapy

There have been major advances in the immunotherapy of cancer in recent years, including the development of T cell engagers - antibodies engineered to redirect T cells to recognize and kill cancer cells - for the treatment of haematological malignancies. However, the field still faces several challenges to develop agents that are consistently effective in a majority of patients and cancer types, such as optimizing drug dose, overcoming treatment resistance and improving efficacy in solid tumours. A new generation of T cell-targeted molecules was developed to tackle these issues that are potentially more effective and safer. In addition, agents designed to engage the antitumour activities of other immune cells, including natural killer cells and myeloid cells, are showing promise and have the potential to treat a broader range of cancers.

Type I IFNs link skin-associated dysbiotic commensal bacteria to pathogenic inflammation and angiogenesis in rosacea

Rosacea is a common chronic inflammatory skin disease with a fluctuating course of excessive inflammation and apparent neovascularization. Microbial dysbiosis with a high density of Bacillus oleronius and increased activity of kallikrein 5, which cleaves cathelicidin antimicrobial peptide, are key pathogenic triggers in rosacea. However, how these events are linked to the disease remains unknown. Here, we show that type I IFNs produced by plasmacytoid DCs represent the pivotal link between dysbiosis, the aberrant immune response, and neovascularization. Compared with other commensal bacteria, B. oleronius is highly susceptible and preferentially killed by cathelicidin antimicrobial peptides, leading to enhanced generation of complexes with bacterial DNA. These bacterial DNA complexes but not DNA complexes derived from host cells are required for cathelicidin-induced activation of plasmacytoid DCs and type I IFN production. Moreover, kallikrein 5 cleaves cathelicidin into peptides with heightened DNA binding and type I IFN-inducing capacities. In turn, excessive type I IFN expression drives neoangiogenesis via IL-22 induction and upregulation of the IL-22 receptor on endothelial cells. These findings unravel a potentially novel pathomechanism that directly links hallmarks of rosacea to the killing of dysbiotic commensal bacteria with induction of a pathogenic type I IFN-driven and IL-22-mediated angiogenesis.

Antitumor immunity induced by antibody-based natural killer cell engager therapeutics armed with not-alpha IL-2 variant

Harnessing innate immunity is emerging as a promising therapeutic approach in cancer. We report here the design of tetraspecific molecules engaging natural killer (NK) cell-activating receptors NKp46 and CD16a, the β-chain of the interleukin-2 receptor (IL-2R), and a tumor-associated antigen (TAA). In vitro, these tetraspecific antibody-based natural killer cell engager therapeutics (ANKETs) induce a preferential activation and proliferation of NK cells, and the binding to the targeted TAA triggers NK cell cytotoxicity and cytokine and chemokine production. In vivo, tetraspecific ANKETs induce NK cell proliferation and their accumulation at the tumor bed, as well as the control of local and disseminated tumors. Treatment of non-human primates with CD20-directed tetraspecific ANKET leads to CD20⁺ circulating B cell depletion, with minimal systemic cytokine release and no sign of toxicity. Tetraspecific ANKETs, thus, constitute a technological platform for harnessing NK cells as next-generation cancer immunotherapies.

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Tetraspecific ANKETs constitute a technological platform to harness NK cells in cancer

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Tetraspecific ANKETs target NKp46, CD16a, IL-2Rβ, and a tumor antigen

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Tetraspecific ANKETs stimulate NK cell proliferation, activation, and antitumor functions

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In vivo, tetraspecific ANKETs promote NK cell tumor accumulation and antitumor activity

851 Harnessing innate immunity in cancer therapies: the example of natural killer cell engagers

Background

Most immunomodulatory approaches have focused on enhancing T-cell responses, with immune checkpoint inhibitors, chimeric antigen receptor T cells or bispecific antibodies. Although these therapies have led to exceptional successes, only a minority of cancer patients benefit from these treatments, highlighting the need to identify new cells and molecules that could be exploited in the next generation of immunotherapy. Given the crucial role of innate immune responses in immunity, harnessing these responses opens up new possibilities for tumor control. Antibody engineering provides us with great opportunities to induce synthetic immunity and to optimize the biological functions of innate immune cells, in particular by boosting the capacity of Natural Killer (NK) cells to kill tumor cells directly and to stimulate T-cell responses indirectly.

Methods

In order to leverage the advantages of harnessing NK cell effector functions, we used our Antibody-based NK cell Engager Therapeutics (ANKET) molecular platform1 and designed a new generation of molecules that can engage activating receptors NKp46 and CD16, the IL-2Rβ chain and a tumor antigen in a single tetra-specific molecule (ANKET4). The variant of interleukin-2 (IL-2v) integrated in the ANKET4 molecule is unable to bind the α-subunit of its receptor to limit regulatory T cell activation and IL-2Rα-mediated toxicity.

Results

In vitro, ANKET4 provides proliferation and activation signals targeted to NK cells and induces primary human NK cell cytolytic activity and the secretion of cytokines and chemokines only after binding to the tumor target. In mouse models of both invasive and solid tumors, ANKET4 induced NK cell proliferation and accumulation at the tumor bed, and had a higher anti-tumor efficacy than approved therapeutic antibodies targeting the same tumor antigen. Mechanistically, transcriptomic analysis and in-vivo studies revealed that the geometry of the ANKET4 molecule including NKp46, CD16 and IL-2 receptor binding moieties on the same molecule was essential for its strong activity which results from a synthetic cooperativity between immunoreceptor tyrosine-based activation motif (ITAM) and cytokine signaling pathways. In non-human primates, CD20-directed ANKET4 resulted in sustained CD20+ B-cell depletion with minimal systemic cytokine release and no clinical sign of toxicity.

Conclusions

Tetra-specific ANKET4 thus constitutes a technological platform combining the induction of NK cell proliferation and effector functions with a manageable safety profile, supporting its clinical development for next-generation cancer immunotherapies.

Reference

Gauthier L, Morel A, Anceriz N, Rossi B, Blanchard-Alvarez A, Grondin G, et al. Multifunctional natural killer cell engagers targeting NKp46 trigger protective tumor immunity. Cell 2019;177(7):1701–13 e16.

Ethics Approval

Primary immune cells were purified from buffy coats from healthy donors obtained from Etablissement Francais du Sang (EFS, Marseille) with written consent from each volunteer.All mouse experiments were performed in accordance with the rules of the Innate Pharma ethics committee and were approved by the Ministère de l’Enseignement Supérieur, de la Recherche et de l’Innovation – France (APAFIS# 19272 ).All non human-primate procedures were conducted according to European guidelines for animal care and use for scientific purposes (Directive 63-2010, ”Journal Officiel des Communautés Européennes”, L276, September 22, 2010) and according to CEA institutional guidelines. The study was approved by the local ethical committee under the number A18_080 and by the French Administration (APAFIS#20525-2019050616506478 v1)

Natural killer cell engagers in cancer immunotherapy: Next generation of immuno-oncology treatments

Immuno-oncology is revolutionizing the treatment of cancers, by inducing the recognition and elimination of tumor cells by the immune system. Recent advances have focused on generating or unleashing tumor antigen-specific T-cell responses, leading to alternative treatment paradigms for many cancers. Despite these successes, the clinical benefit has been limited to a subset of patients and certain tumor types, highlighting the need for alternative strategies. One innovative approach is to broaden and amplify antitumoral immune responses by targeting innate immunity. Particularly, the aim has been to develop new antibody formats capable of stimulating the antitumor activity of innate immune cells, boosting not only their direct role in tumor elimination, but also their function in eliciting multicellular immune responses ultimately resulting in long-lasting tumor control by adaptive immunity. This review covers the development of a new class of synthetic molecules, natural killer cell engagers (NKCEs), which are built from fragments of monoclonal antibodies (mAbs) and are designed to harness the immune functions of NK cells in cancer. As currently shown in preclinical studies and clinical trials, NKCEs are promising candidates for the next generation of tumor immunotherapies.

Association of COVID-19 inflammation with activation of the C5a-C5aR1 axis

Coronavirus disease 2019 (COVID-19) is a disease caused by infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and has resulted in a pandemic1. The C5a complement factor and its receptor C5aR1 (also known as CD88) have a key role in the initiation and maintenance of several inflammatory responses by recruiting and activating neutrophils and monocytes1. Here we provide a longitudinal analysis of immune responses, including phenotypic analyses of immune cells and assessments of the soluble factors that are present in the blood and bronchoalveolar lavage fluid of patients at various stages of COVID-19 severity, including those who were paucisymptomatic or had pneumonia or acute respiratory distress syndrome. The levels of soluble C5a were increased in proportion to the severity of COVID-19 and high expression levels of C5aR1 receptors were found in blood and pulmonary myeloid cells, which supports a role for the C5a-C5aR1 axis in the pathophysiology of acute respiratory distress syndrome. Anti-C5aR1 therapeutic monoclonal antibodies prevented the C5a-mediated recruitment and activation of human myeloid cells, and inhibited acute lung injury in human C5aR1 knock-in mice. These results suggest that blockade of the C5a-C5aR1 axis could be used to limit the infiltration of myeloid cells in damaged organs and prevent the excessive lung inflammation and endothelialitis that are associated with acute respiratory distress syndrome in patients with COVID-19.

The commensal skin microbiota triggers type I IFN-dependent innate repair responses in injured skin

Skin wounds heal by coordinated induction of inflammation and tissue repair, but the initiating events are poorly defined. Here we uncover a fundamental role of commensal skin microbiota in this process and show that it is mediated by the recruitment and the activation of type I interferon (IFN)-producing plasmacytoid DC (pDC). Commensal bacteria colonizing skin wounds trigger activation of neutrophils to express the chemokine CXCL10, which recruits pDC and acts as an antimicrobial protein to kill exposed microbiota, leading to the formation of CXCL10-bacterial DNA complexes. These complexes and not complexes with host-derived DNA activate pDC to produce type I IFNs, which accelerate wound closure by triggering skin inflammation and early T cell-independent wound repair responses, mediated by macrophages and fibroblasts that produce major growth factors required for healing. These findings identify a key function of commensal microbiota in driving a central innate wound healing response of the skin.

Immuno-Oncology beyond TILs: Unleashing TILCs

Innate lymphoid cells (ILCs) are detected in multiple tumor types, but their contribution to tumor immunity remains unclear. Moral et al. show that a subset of tumor ILCs (TILCs) may participate in an organ-specific immune response and can be unleashed by PD-1 blockers to sustain tumor-specific T cell responses.

Netting Neutrophils Activate Autoreactive B Cells in Lupus

Lupus erythematosus (LE) patients develop autoantibodies that form circulating immune complexes (ICs) with extracellular self-nucleic acids. These ICs are deposited into peripheral tissues, where they trigger detrimental organ inflammation. Recent evidence suggests that ICs contain LL37-DNA complexes derived from neutrophil extracellular traps (NETs) and that LE patients develop pathogenic autoantibodies against these structures, including Abs to LL37. However, the mechanism that leads to the generation of these Abs is unknown. In this study, we show that NETs directly trigger Ab production by human memory B cells. This occurs via LL37-DNA complexes present in NETs, which have the unique ability to gain access to endosomal compartments of B cells and to trigger TLR9 activation. In LE patients, NET-derived LL37-DNA complexes trigger polyclonal B cell activation via TLR9, but also specifically expand self-reactive memory B cells producing anti-LL37 Abs in an Ag-dependent manner. These findings suggest a unique link between neutrophils and B cells in which NETs trigger a concerted activation of TLR9 and BCR leading to anti-NET autoantibody production in lupus.

TNF blockade induces a dysregulated type I interferon response without autoimmunity in paradoxical psoriasis

Although anti-tumor necrosis factor (TNF) agents are highly effective in the treatment of psoriasis, 2–5% of treated patients develop psoriasis-like skin lesions called paradoxical psoriasis. The pathogenesis of this side effect and its distinction from classical psoriasis remain unknown. Here we show that skin lesions from patients with paradoxical psoriasis are characterized by a selective overexpression of type I interferons, dermal accumulation of plasmacytoid dendritic cells (pDC), and reduced T-cell numbers, when compared to classical psoriasis. Anti-TNF treatment prolongs type I interferon production by pDCs through inhibition of their maturation. The resulting type I interferon overexpression is responsible for the skin phenotype of paradoxical psoriasis, which, unlike classical psoriasis, is independent of T cells. These findings indicate that paradoxical psoriasis represents an ongoing overactive innate inflammatory process, driven by pDC-derived type I interferon that does not lead to T-cell autoimmunity.

The pathogenesis of paradoxical psoriasis in patients receiving anti-TNF treatments for classical psoriasis is unclear. Here, the authors show that anti-TNF drugs enhance the production of type I interferon by plasmacytoid dendritic cells, causing skin lesions that, unlike classical psoriasis, lack T- cell autoimmunity.

Drug Repurposing Approach Identifies a Synergistic Drug Combination of an Antifungal Agent and an Experimental Organometallic Drug for Melanoma Treatment

By screening a drug library comprising FDA approved compounds, we discovered a potent interaction between the antifungal agent haloprogin and the experimental organometallic drug RAPTA-T, to synergistically induce cancer cell killing. The combination of these two small molecules, even at low doses, elicited an improved therapeutic response on tumor growth over either agent alone or the current treatment used in the clinic in the highly aggressive syngeneic B16F10 melanoma tumor model, where classical cytotoxic chemotherapeutic agents show little efficacy. The combination with the repurposed chemodrug haloprogin provides the basis for a new powerful treatment option for cutaneous melanoma. Importantly, because synergistic induction of tumor cell death is achieved with low individual drug doses, and cellular targets for RAPTA-T are different from those of classical chemotherapeutic drugs, a therapeutic strategy based on this approach could avoid toxicities and potentially resistance mechanisms, and could even inhibit metastatic progression.

Interleukin 23-Helper T Cell 17 Axis as a Treatment Target for Pityriasis Rubra Pilaris

Importance

Treatment of pityriasis rubra pilaris (PRP) is solely based on its resemblance to psoriasis rather than any knowledge of its pathomechanism. Insight into pathogenic mediators of inflammation is essential for targeted and valid treatment options that could replace previous serendipitous therapeutic approaches in refractory PRP.

Objective

To determine whether blockade of the interleukin 23-helper T cell 17 (IL-23-TH17) pathway with ustekinumab represents an efficacious and, based on its proinflammatory cytokine profile, targeted treatment option in PRP.

Design, Setting, and Participants

In this case report, a patient with PRP received outpatient treatment at a university hospital department of dermatology with ustekinumab according to the dosing regimen approved for psoriasis. Lesional skin biopsy samples were taken from this patient and 2 others with refractory PRP. Messenger RNA (mRNA) expression of proinflammatory innate and T-cell-derived cytokines were measured and compared with skin samples from patients with psoriasis and healthy donors. From 1 patient, lesional skin samples were taken before ustekinumab treatment and 4 and 28 weeks after treatment initiation. Follow-up was completed after 6 months.

Intervention

Subcutaneous ustekinumab, 45 mg, at weeks 0 and 4 and quarterly thereafter.

Main Outcomes and Measures

The primary outcome was to determine the changes in expression of proinflammatory innate and T-cell-derived cytokines during ustekinumab therapy. The secondary objective was to evaluate the clinical and histopathologic phenotype in relation to the mRNA expression profile of proinflammatory cytokines.

Results

In lesional PRP skin samples from a single patient, upregulated expression levels were found for most proinflammatory innate cytokines, including tumor necrosis factor (TNF), IL-6, IL-12, IL-23, and IL-1β. Among adaptive T-cell cytokines, an increase of TH1 cytokines and, in particular, TH17 cytokines IL-17A, IL-17F, and IL-22 was seen in PRP. The patient with PRP who received ustekinumab showed regression of skin lesions after 2 weeks and almost complete resolution after 1 month. Clinical and histopathologic improvement paralleled the expression levels of TH17 cytokines but not of interferon-γ and TNF, which lagged behind the amelioration.

Conclusions and Relevance

In this case report, a role of the IL-23-TH17-axis in PRP was identified, suggesting a shared pathogenic inflammatory pathway with psoriasis, despite evident clinical and histopathologic differences. In addition, this report provides a rationale for targeting the IL-23-TH17-pathway as a treatment option for refractory PRP.

Pharmacological eEF2K activation promotes cell death and inhibits cancer progression

Activation of the elongation factor 2 kinase (eEF2K) leads to the phosphorylation and inhibition of the elongation factor eEF2, reducing mRNA translation rates. Emerging evidence indicates that the regulation of factors involved in protein synthesis may be critical for controlling diverse biological processes including cancer progression. Here we show that inhibitors of the HIV aspartyl protease (HIV-PIs), nelfinavir in particular, trigger a robust activation of eEF2K leading to the phosphorylation of eEF2. Beyond its anti-viral effects, nelfinavir has antitumoral activity and promotes cell death. We show that nelfinavir-resistant cells specifically evade eEF2 inhibition. Decreased cell viability induced by nelfinavir is impaired in cells lacking eEF2K. Moreover, nelfinavir-mediated anti-tumoral activity is severely compromised in eEF2K-deficient engrafted tumors in vivo Our findings imply that exacerbated activation of eEF2K is detrimental for tumor survival and describe a mechanism explaining the anti-tumoral properties of HIV-PIs.

Immune sensing of nucleic acids in inflammatory skin diseases

Endosomal and cytosolic nucleic acid receptors are important immune sensors required for the detection of infecting or replicating viruses. The intracellular location of these receptors allows viral recognition and, at the same time, avoids unnecessary immune activation to self-nucleic acids that are continuously released by dying host cells. Recent evidence, however, indicates that endogenous factors such as anti-microbial peptides have the ability to break this protective mechanism. Here, we discuss these factors and illustrate how they drive inflammatory responses by promoting immune recognition of self-nucleic acids in skin wounds and inflammatory skin diseases such as psoriasis and lupus.

Sex bias in susceptibility to MCMV infection: implication of TLR9

Toll-like receptor (TLR)-dependent pathways control the activation of various immune cells and the production of cytokines and chemokines that are important in innate immune control of viruses, including mouse cytomegalovirus (MCMV). Here we report that upon MCMV infection wild-type and TLR7^−/− male mice were more resistant than their female counterparts, while TLR9^−/− male and female mice showed similar susceptibility. Interestingly, 36 h upon MCMV infection TLR9 mRNA expression was higher in male than in female mouse spleens. MCMV infection led to stronger reduction of marginal zone (MZ) B cells, and higher infiltration of plasmacytoid dendritic cells and neutrophils in wild-type male than female mice, while no such sex differences were observed in TLR9^−/− mice. In accordance, the serum levels of KC and MIP-2, major neutrophil chemoattractants, were higher in wild-type, but not in TLR9^−/−, male versus female mice. Wild-type MCMV-infected female mice showed more severe liver inflammation, necrosis and steatosis compared to infected male mice. Our data demonstrate sex differences in susceptibility to MCMV infection, accompanied by a lower activation of the innate immune system in female mice, and can be attributed, at least in a certain degree, to the lower expression of TLR9 in female than male mice.

TLR8 deficiency leads to autoimmunity in mice

TLRs play an essential role in the induction of immune responses by detecting conserved molecular products of microorganisms. However, the function of TLR8 is largely unknown. In the current study, we investigated the role of TLR8 signaling in immunity in mice. We found that Tlr8(-/-) DCs overexpressed TLR7, were hyperresponsive to various TLR7 ligands, and showed stronger and faster NF-κB activation upon stimulation with the TLR7 ligand R848. Tlr8(-/-) mice showed splenomegaly, defective development of marginal zone (MZ) and B1 B cells, and increased serum levels of IgM and IgG2a. Furthermore, Tlr8(-/-) mice exhibited increased serum levels of autoantibodies against small nuclear ribonucleoproteins, ribonucleoprotein, and dsDNA and developed glomerulonephritis, whereas neither Tlr7(-/-) nor Tlr8(-/-)Tlr7(-/-) mice showed any of the phenotypes observed in Tlr8(-/-) mice. These data provide evidence for a pivotal role for mouse TLR8 in the regulation of mouse TLR7 expression and prevention of spontaneous autoimmunity.

Contribution of TLR7 and TLR9 signaling to the susceptibility of MyD88-deficient mice to myocarditis

Toll-like receptors (TLRs) are evolutionary conserved molecules that recognize various microbial components and host-derived agonists from damaged cells and play a central role in innate and adaptive immunity. It has been reported that MyD88, the adaptor molecule downstream of all TLRs, except TLR3, is essential for initiation of experimental autoimmune myocarditis (EAM). To determine the role of the intracellular TLRs in EAM, TLR3(-/-), TLR7(-/-), and TLR9(-/-) mice were immunized with cardiac alpha-myosin heavy chain peptide (MyHC-alpha) in Complete Freund's Adjuvant (CFA) and their EAM scores and associated immunological responses were compared to wild-type (WT) and MyD88(-/-) mice. MyD88(-/-) mice were completely resistant to EAM and had a profound defect in all the parameters we tested. Myocardial cellular infiltration and in vitro proliferation of MyHC-alpha-restimulated splenocytes were markedly reduced in TLR7(-/-) mice, while TLR3(-/-) and TLR9(-/-) mice showed similar inflammatory cell infiltration in the heart-like WT mice. Thus, the resistance of MyD88(-/-) mice to EAM can be attributed to a certain degree to TLR7 signaling. Moreover, upon murine cytomegalovirus-induced myocarditis, we found that the severity of myocardial inflammation was higher in TLR9(-/-) and MyD88(-/-) mice compared with WT, TLR3(-/-), or TLR7(-/-) mice and paralleled the ability of the mice to fight the viral infection.

TLR3 and Rig-like receptor on myeloid dendritic cells and Rig-like receptor on human NK cells are both mandatory for production of IFN-gamma in response to double-stranded RNA

Cross-talk between NK cells and dendritic cells (DCs) is critical for the potent therapeutic response to dsRNA, but the receptors involved remained controversial. We show in this paper that two dsRNAs, polyadenylic-polyuridylic acid and polyinosinic-polycytidylic acid [poly(I:C)], similarly engaged human TLR3, whereas only poly(I:C) triggered human RIG-I and MDA5. Both dsRNA enhanced NK cell activation within PBMCs but only poly(I:C) induced IFN-gamma. Although myeloid DCs (mDCs) were required for NK cell activation, induction of cytolytic potential and IFN-gamma production did not require contact with mDCs but was dependent on type I IFN and IL-12, respectively. Poly(I:C) but not polyadenylic-polyuridylic acid synergized with mDC-derived IL-12 for IFN-gamma production by acting directly on NK cells. Finally, the requirement of both TLR3 and Rig-like receptor (RLR) on mDCs and RLRs but not TLR3 on NK cells for IFN-gamma production was demonstrated using TLR3- and Cardif-deficient mice and human RIG-I-specific activator. Thus, we report the requirement of cotriggering TLR3 and RLR on mDCs and RLRs on NK cells for a pathogen product to induce potent innate cell activation.

Brucella control of dendritic cell maturation is dependent on the TIR-containing protein Btp1

Brucella is an intracellular pathogen able to persist for long periods of time within the host and establish a chronic disease. We show that soon after Brucella inoculation in intestinal loops, dendritic cells from ileal Peyer's patches become infected and constitute a cell target for this pathogen. In vitro, we found that Brucella replicates within dendritic cells and hinders their functional activation. In addition, we identified a new Brucella protein Btp1, which down-modulates maturation of infected dendritic cells by interfering with the TLR2 signaling pathway. These results show that intracellular Brucella is able to control dendritic cell function, which may have important consequences in the development of chronic brucellosis.

A key determinant for intracellular pathogenic bacteria to induce infectious diseases is their ability to avoid recognition by the host immune system. Although most microorganisms internalized by host cells are efficiently cleared, Brucella behave as a Trojan horse causing a zoonosis called brucellosis that affects both humans and animals. Here we show that pathogenic Brucella are able to target host cell defense mechanisms by controlling the function of the sentinels of the immune system, the dendritic cells. In particular, the Brucella TIR-containing protein (Btp1) targets the Toll-like receptor 2 activation pathway, which is a major host response system involved in bacterial recognition. Btp1 is involved in the inhibition of dendritic cell maturation. The direct consequence is a control of inflammatory cytokine secretion and antigen presentation to T lymphocytes. These bacterial proteins are not specific for Brucella and have been identified in other pathogens and may be part of a general virulence mechanism used by several intracellular pathogens to induce disease.

Involvement of Toll-like receptor 5 in the recognition of flagellated bacteria

Toll-like receptors (TLRs) are key components of the immune system that detect microbial infection and trigger antimicrobial host defense responses. TLR5 is a sensor for monomeric flagellin, which is a component of bacterial flagella known to be a virulence factor. In this study we generated TLR5-deficient mice and investigated the role of TLR5 signaling in the detection of flagellin and antibacterial immune responses to Salmonella typhimurium and Pseudomonas aeruginosa. We found that TLR5 is essential for the recognition of bacterial flagellin both in vivo and ex vivo. TLR5 contribution to antibacterial host response to i.p. infection with S. typhimurium or intranasal administration of P. aeruginosa may be masked by TLR4 or other sensing mechanisms. By using radiation bone marrow chimera, we showed that upon i.p. injection of flagellin immune responses are mediated by lymphoid cells, whereas resident cells are required for the initiation of response upon intranasal flagellin administration. These results suggest that flagellin recognition in different organs is mediated by distinct TLR5-expressing cells and provide insights into the cooperation of the TLR5 and TLR4 signaling pathways used by the innate immune system in the recognition of bacterial pathogens.