Release of LL-37 by activated human Vgamma9Vdelta2 T cells: a microbicidal weapon against Brucella suis

The antimicrobial peptide cathelicidin drives development of experimental autoimmune encephalomyelitis in mice by affecting Th17 differentiation

Multiple sclerosis (MS) is a highly prevalent demyelinating autoimmune condition; the mechanisms regulating its severity and progression are unclear. The IL-17-producing Th17 subset of T cells has been widely implicated in MS and in the mouse model, experimental autoimmune encephalomyelitis (EAE). However, the differentiation and regulation of Th17 cells during EAE remain incompletely understood. Although evidence is mounting that the antimicrobial peptide cathelicidin profoundly affects early T cell differentiation, no studies have looked at its role in longer-term T cell responses. Now, we report that cathelicidin drives severe EAE disease. It is released from neutrophils, microglia, and endothelial cells throughout disease; its interaction with T cells potentiates Th17 differentiation in lymph nodes and Th17 to exTh17 plasticity and IFN-γ production in the spinal cord. As a consequence, mice lacking cathelicidin are protected from severe EAE. In addition, we show that cathelicidin is produced by the same cell types in the active brain lesions in human MS disease. We propose that cathelicidin exposure results in highly activated, cytokine-producing T cells, which drive autoimmunity; this is a mechanism through which neutrophils amplify inflammation in the central nervous system.

Expression of antimicrobial host defence peptides in the central nervous system during health and disease

Antimicrobial host defence peptides (HDP) are critical for the first line of defence against bacterial, viral, and fungal pathogens. Over the past decade we have become more aware that, in addition to their antimicrobial roles, they also possess the potent immunomodulatory capacity. This includes chemoattracting immune cells, activating dendritic cells and macrophages, and altering T-cell differentiation. Most examinations of their immunomodulatory roles have focused on tissues in which they are very abundant, such as the intestine and the inflamed skin. However, HDP have now been detected in the brain and the spinal cord during a number of conditions. We propose that their presence in the central nervous system (CNS) during homeostasis, infection, and neurodegenerative disease has the potential to contribute to immunosurveillance, alter host responses and skew developing immunity. Here, we review the evidence for HDP expression and function in the CNS in health and disease. We describe how a wide range of HDP are expressed in the CNS of humans, rodents, birds, and fish, suggesting a conserved role in protecting the brain from pathogens, with evidence of production by resident CNS cells. We highlight differences in methodology used and how this may have resulted in the immunomodulatory roles of HDP being overlooked. Finally, we discuss what HDP expression may mean for CNS immune responses.

Is the oral microbiome a source to enhance mucosal immunity against infectious diseases?

Mucosal tissues act as a barrier throughout the oral, nasopharyngeal, lung, and intestinal systems, offering first-line protection against potential pathogens. Conventionally, vaccines are applied parenterally to induce serotype-dependent humoral response but fail to drive adequate mucosal immune protection for viral infections such as influenza, HIV, and coronaviruses. Oral mucosa, however, provides a vast immune repertoire against specific microbial pathogens and yet is shaped by an ever-present microbiome community that has co-evolved with the host over thousands of years. Adjuvants targeting mucosal T-cells abundant in oral tissues can promote soluble-IgA (sIgA)-specific protection to confer increased vaccine efficacy. Th17 cells, for example, are at the center of cell-mediated immunity and evidence demonstrates that protection against heterologous pathogen serotypes is achieved with components from the oral microbiome. At the point of entry where pathogens are first encountered, typically the oral or nasal cavity, the mucosal surfaces are layered with bacterial cohabitants that continually shape the host immune profile. Constituents of the oral microbiome including their lipids, outer membrane vesicles, and specific proteins, have been found to modulate the Th17 response in the oral mucosa, playing important roles in vaccine and adjuvant designs. Currently, there are no approved adjuvants for the induction of Th17 protection, and it is critical that this research is included in the preparedness for the current and future pandemics. Here, we discuss the potential of oral commensals, and molecules derived thereof, to induce Th17 activity and provide safer and more predictable options in adjuvant engineering to prevent emerging infectious diseases.

Antibacterial effects of platelet-rich fibrin produced by horizontal centrifugation

Platelet-rich fibrin (PRF) has been widely used owing to its ability to stimulate tissue regeneration. To date, few studies have described the antibacterial properties of PRF. Previously, PRF prepared by horizontal centrifugation (H-PRF) was shown to contain more immune cells than leukocyte- and platelet-rich fibrin (L-PRF). This study aimed to compare the antimicrobial effects of PRFs against Staphylococcus aureus and Escherichia coli in vitro and to determine whether the antibacterial effects correlated with the number of immune cells. Blood samples were obtained from eight healthy donors to prepare L-PRF and H-PRF. The sizes and weights of L-PRF and H-PRF were first evaluated, and their antibacterial effects against S. aureus and E. coli were then tested in vitro using the inhibition ring and plate-counting test methods. Flow-cytometric analysis of the cell components of L-PRF and H-PRF was also performed. No significant differences in size or weight were observed between the L-PRF and H-PRF groups. The H-PRF group contained more leukocytes than the L-PRF group. While both PRFs had notable antimicrobial activity against S. aureus and E. coli, H-PRF demonstrated a significantly better antibacterial effect than L-PRF. Furthermore, the antimicrobial ability of the PRF solid was less efficient than that of wet PRF. In conclusion, H-PRF exhibited better antibacterial activity than L-PRF, which might have been attributed to having more immune cells.

γδ T cell frequencies are altered in HIV positive pregnant South African women and are associated with preterm birth

Background

Preterm birth is the leading cause of neonatal and child mortality worldwide. Maternal HIV infection and antiretroviral treatment (ART) increase the rate of preterm birth, but the underlying mechanisms remain unknown, limiting progress in prediction, prevention and treatment. While overall γδ T cell levels remain constant, acute HIV infection is associated with a depletion of the Vδ2 subset and an increase in the Vδ1 subset, which do not return to baseline with ART. γδ T cells have also been implicated in adverse pregnancy outcomes and we therefore investigated the potential association between maternal HIV infection, peripheral γδ T cell frequencies and preterm birth.

Methods

Study participants were HIV positive (n = 47) and HIV negative (n = 45) women enrolled in a prospective pregnancy cohort study at Chris Hani Baragwanath Academic Hospital in Soweto, South Africa. Women were enrolled in early pregnancy and gestational age was accurately determined by first trimester ultrasound scan. Peripheral blood samples were collected in each trimester and peripheral blood mononuclear cells isolated. Frequencies of γδ T cells, Vδ1+ and Vδ2+ γδ T cell subsets, and CCR6 chemokine receptor expression were determined by flow cytometry.

Results

Total γδ T cell levels were similar between HIV positive and HIV negative women throughout pregnancy. However, in each trimester maternal HIV infection was associated with reduced levels of the Vδ2+ subset and increased levels of the Vδ1+ subset, leading to a reversal of the Vδ1/Vδ2 ratio. Timing of ART initiation among HIV positive women did not affect levels of γδ T cells, the Vδ1+ and Vδ2+ subsets, or the Vδ1/Vδ2 ratio. Importantly, preterm birth was associated with lower total γδ T cell levels in early pregnancy and γδ T cell frequencies were lowest in HIV positive women who delivered preterm. Moreover, in the first trimester the proportion of Vδ1+ T cells that were CCR6+ was significantly reduced in HIV+ women and women who delivered preterm, resulting in the lowest proportion of CCR6+ Vδ1 T cells in HIV positive women who delivered preterm.

Conclusions

Our findings suggest that altered γδ T cell frequencies may link maternal HIV infection and preterm birth. γδ T cell frequencies in early pregnancy may serve as predictive biomarkers to identify women at risk of delivering preterm.

Key Aspects of the Immunobiology of Haploidentical Hematopoietic Cell Transplantation

Hematopoietic stem cell transplantation from a haploidentical donor is increasingly used and has become a standard donor option for patients lacking an appropriately matched sibling or unrelated donor. Historically, prohibitive immunological barriers resulting from the high degree of HLA-mismatch included graft-vs.-host disease (GVHD) and graft failure. These were overcome with increasingly sophisticated strategies to manipulate the sensitive balance between donor and recipient immune cells. Three different approaches are currently in clinical use: (a) ex vivo T-cell depletion resulting in grafts with defined immune cell content (b) extensive immunosuppression with a T-cell replete graft consisting of G-CSF primed bone marrow and PBSC (GIAC) (c) T-cell replete grafts with post-transplant cyclophosphamide (PTCy). Intriguing studies have recently elucidated the immunologic mechanisms by which PTCy prevents GVHD. Each approach uniquely affects post-transplant immune reconstitution which is critical for the control of post-transplant infections and relapse. NK-cells play a key role in haplo-HCT since they do not mediate GVHD but can successfully mediate a graft-vs.-leukemia effect. This effect is in part regulated by KIR receptors that inhibit NK cell cytotoxic function when binding to the appropriate HLA-class I ligands. In the context of an HLA-class I mismatch in haplo-HCT, lack of inhibition can contribute to NK-cell alloreactivity leading to enhanced anti-leukemic effect. Emerging work reveals immune evasion phenomena such as copy-neutral loss of heterozygosity of the incompatible HLA alleles as one of the major mechanisms of relapse. Relapse and infectious complications remain the leading causes impacting overall survival and are central to scientific advances seeking to improve haplo-HCT. Given that haploidentical donors can typically be readily approached to collect additional stem- or immune cells for the recipient, haplo-HCT represents a unique platform for cell- and immune-based therapies aimed at further reducing relapse and infections. The rapid advancements in our understanding of the immunobiology of haplo-HCT are therefore poised to lead to iterative innovations resulting in further improvement of outcomes with this compelling transplant modality.

Critical Roles for Coiled-Coil Dimers of Butyrophilin 3A1 in the Sensing of Prenyl Pyrophosphates by Human Vγ2Vδ2 T Cells

Vγ2Vδ2 T cells play important roles in human immunity to pathogens and tumors. Their TCRs respond to the sensing of isoprenoid metabolites, such as (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate and isopentenyl pyrophosphate, by butyrophilin (BTN) 3A1. BTN3A1 is an Ig superfamily protein with extracellular IgV/IgC domains and intracellular B30.2 domains that bind prenyl pyrophosphates. We have proposed that intracellular α helices form a coiled-coil dimer that functions as a spacer for the B30.2 domains. To test this, five pairs of anchor residues were mutated to glycine to destabilize the coiled-coil dimer. Despite maintaining surface expression, BTN3A1 mutagenesis either abrogated or decreased stimulation by (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate. BTN3A2 and BTN3A3 proteins and orthologs in alpacas and dolphins are also predicted to have similar coiled-coil dimers. A second short coiled-coil region dimerizes the B30.2 domains. Molecular dynamics simulations predict that mutation of a conserved tryptophan residue in this region will destabilize the dimer, explaining the loss of stimulation by BTN3A1 proteins with this mutation. The juxtamembrane regions of other BTN/BTN-like proteins with B30.2 domains are similarly predicted to assume α helices, with many predicted to form coiled-coil dimers. An exon at the end of this region and the exon encoding the dimerization region for B30.2 domains are highly conserved. We propose that coiled-coil dimers function as rod-like helical molecular spacers to position B30.2 domains, as interaction sites for other proteins, and as dimerization regions to allow sensing by B30.2 domains. In these ways, the coiled-coil domains of BTN3A1 play critical roles for its function.

γδ T Lymphocytes: An Effector Cell in Autoimmunity and Infection

γδ T cells are non-conventional lymphocytes which show several properties of innate immune cells. They present a limited TCR repertoire and circulate as cells with a pre-activated phenotype thus being able to generate rapid immune responses. γδ T cells do not recognize classical peptide antigens, their TCRs are non-MHC restricted and they can respond to pathogen-associated molecular patterns and to cytokines in absence of TCR ligands. They also recognize self-molecules induced by stress, which indicate infection and cellular transformation. All these features let γδ T cells act as a first line of defense in sterile and non-sterile inflammation. γδ T cells represent 1–10% of circulating lymphocytes in the adult human peripheral blood, they are widely localized in non-lymphoid tissues and constitute the majority of immune cells in some epithelial surfaces, where they participate in the maintenance of the epithelial barriers. γδ T cells produce a wide range of cytokines that orchestrate the course of immune responses and also exert high cytotoxic activity against infected and transformed cells. In contrast to their beneficial role during infection, γδ T cells are also implicated in the development and progression of autoimmune diseases. Interestingly, several functions of γδ T cells are susceptible to modulation by interaction with other cells. In this review, we give an overview of the γδ T cell participation in infection and autoimmunity. We also revise the underlying mechanisms that modulate γδ T cell function that might provide tools to control pathological immune responses.

Butyrophilins: an important new element of resistance

Butyrophilins belonging to the immunoglobulin superfamily are new immune system regulators because they are present on lymphocytes, dendritic cells, monocytes, macrophages, neutrophils and eosinophils, and they exert a stimulatory and (or) inhibitory effect on them. The role of butyrophilins is associated and results from their similarity to the regulatory B7 protein family involved in the modulation of immune phenomena. Butyrophilins are glycoproteins built of two extracellular immunoglobulin domains, stabilized with disulfide bonds: constant IgC, and variable IgV and a transmembrane region. Most of these proteins contain a conserved domain encoded by a single exon – B30.2, also referred to as PRYSPRY. In humans, the family of butyrophilins includes 7 butyrophilin proteins, 5 butyrophilin-like proteins and the SKINT-like factor. Butyrophilins have been also demonstrated to play a role in various infections, e.g. tuberculosis or diseases that include sarcoidosis, systemic lupus erythematosus, rheumatoid arthritis, genetic metabolic diseases, ulcerative colitis, cancer and kidney disease.

A promising sword of tomorrow: Human γδ T cell strategies reconcile allo-HSCT complications

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is potentially a curative therapeutic option for hematological malignancies. In clinical practice, transplantation associated complications greatly affected the final therapeutical outcomes. Currently, primary disease relapse, graft-versus-host disease (GVHD) and infections remain the three leading causes of a high morbidity and mortality in allo-HSCT patients. Various strategies have been investigated in the past several decades including human γδ T cell-based therapeutical regimens. In different microenvironments, human γδ T cells assume features reminiscent of classical Th1, Th2, Th17, NKT and regulatory T cells, showing diverse biological functions. The cytotoxic γδ T cells could be utilized to target relapsed malignancies, and recently regulatory γδ T cells are defined as a novel implement for GVHD management. In addition, human γδ Τ cells facilitate control of post-transplantation infections and participate in tissue regeneration and wound healing processes. These features potentiate γδ T cells a versatile therapeutical agent to target transplantation associated complications. This review focuses on insights of applicable potentials of human γδ T cells reconciling complications associated with allo-HSCT. We believe an improved understanding of pertinent γδ T cell functions would be further exploited in the design of innovative immunotherapeutic approaches in allo-HSCT, to reduce mortality and morbidity, as well as improve quality of life for patients after transplantation.

Sensor Function for Butyrophilin 3A1 in Prenyl Pyrophosphate Stimulation of Human Vγ2Vδ2 T Cells

Vγ2Vδ2 T cells play important roles in human immunity to pathogens and in cancer immunotherapy by responding to isoprenoid metabolites, such as (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate and isopentenyl pyrophosphate. The Ig superfamily protein butyrophilin (BTN)3A1 was shown to be required for prenyl pyrophosphate stimulation. We proposed that the intracellular B30.2 domain of BTN3A1 binds prenyl pyrophosphates, resulting in a change in the extracellular BTN3A1 dimer that is detected by Vγ2Vδ2 TCRs. Such B30.2 binding was demonstrated recently. However, other investigators reported that the extracellular BTN3A1 IgV domain binds prenyl pyrophosphates, leading to the proposal that the Vγ2Vδ2 TCR recognizes the complex. To distinguish between these mechanisms, we mutagenized residues in the two binding sites and tested the mutant BTN3A1 proteins for their ability to mediate prenyl pyrophosphate stimulation of Vγ2Vδ2 T cells to proliferate and secrete TNF-α. Mutagenesis of residues in the IgV site had no effect on Vγ2Vδ2 T cell proliferation or secretion of TNF-α. In contrast, mutagenesis of residues within the basic pocket and surrounding V regions of the B30.2 domain abrogated prenyl pyrophosphate-induced proliferation. Mutations of residues making hydrogen bonds to the pyrophosphate moiety also abrogated TNF-α secretion, as did mutation of aromatic residues making contact with the alkenyl chain. Some mutations further from the B30.2 binding site also diminished stimulation, suggesting that the B30.2 domain may interact with a second protein. These findings support intracellular sensing of prenyl pyrophosphates by BTN3A1 rather than extracellular presentation.

Cancer Immunotherapy Using γδT Cells: Dealing with Diversity

The broad and potent tumor-reactivity of innate-like γδT cells makes them valuable additions to current cancer immunotherapeutic concepts based on adaptive immunity, such as monoclonal antibodies and αβT cells. However, clinical success using γδT cells to treat cancer has so far fallen short. Efforts of recent years have revealed a striking diversity in γδT cell functions and immunobiology, putting these cells forward as true “swiss army knives” of immunity. At the same time, however, this heterogeneity poses new challenges to the design of γδT cell-based therapeutic concepts and could explain their rather limited clinical efficacy in cancer patients. This review outlines the recent new insights into the different levels of γδT cell diversity, including the myriad of γδT cell-mediated immune functions, the diversity of specificities and affinities within the γδT cell repertoire, and the multitude of complex molecular requirements for γδT cell activation. A careful consideration of the diversity of antibodies and αβT cells has delivered great progress to their clinical success; addressing also the extraordinary diversity in γδT cells will therefore hold the key to more effective immunotherapeutic strategies with γδT cells as additional and valuable tools to battle cancer.

Role of non-conventional T lymphocytes in respiratory infections: the case of the pneumococcus

Non-conventional T lymphocytes constitute a special arm of the immune system and act as sentinels against pathogens at mucosal surfaces. These non-conventional T cells (including mucosal-associated invariant T [MAIT] cells, gamma delta [γδ] T cells, and natural killer T [NKT] cells) display several innate cell-like features and are rapidly activated by the recognition of conserved, stress-induced, self, and microbial ligands. Here, we review the role of non-conventional T cells during respiratory infections, with a particular focus on the encapsulated extracellular pathogen Streptococcus pneumoniae, the leading cause of bacterial pneumonia worldwide. We consider whether MAIT cells, γδ T cells, and NKT cells might offer opportunities for preventing and/or treating human pneumococcus infections.

Metabolic engineering of Salmonella vaccine bacteria to boost human Vγ2Vδ2 T cell immunity

Human Vγ2Vδ2 T cells monitor isoprenoid metabolism by recognizing foreign (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), a metabolite in the 2-C-methyl-D-erythritol-4-phosphate pathway used by most eubacteria and apicomplexan parasites, and self isopentenyl pyrophosphate, a metabolite in the mevalonate pathway used by humans. Whereas microbial infections elicit prolonged expansion of memory Vγ2Vδ2 T cells, immunization with prenyl pyrophosphates or aminobisphosphonates elicit short-term Vγ2Vδ2 expansion with rapid anergy and deletion upon subsequent immunizations. We hypothesized that a live, attenuated bacterial vaccine that overproduces HMBPP would elicit long-lasting Vγ2Vδ2 T cell immunity by mimicking a natural infection. Therefore, we metabolically engineered the avirulent aroA(-) Salmonella enterica serovar Typhimurium SL7207 strain by deleting the gene for LytB (the downstream enzyme from HMBPP) and functionally complementing for this loss with genes encoding mevalonate pathway enzymes. LytB(-) Salmonella SL7207 had high HMBPP levels, infected human cells as efficiently as did the wild-type bacteria, and stimulated large ex vivo expansions of Vγ2Vδ2 T cells from human donors. Importantly, vaccination of a rhesus monkey with live lytB(-) Salmonella SL7207 stimulated a prolonged expansion of Vγ2Vδ2 T cells without significant side effects or anergy induction. These studies provide proof-of-principle that metabolic engineering can be used to derive live bacterial vaccines that boost Vγ2Vδ2 T cell immunity. Similar engineering of metabolic pathways to produce lipid Ags or B vitamin metabolite Ags could be used to derive live bacterial vaccine for other unconventional T cells that recognize nonpeptide Ags.

A comprehensive summary of LL-37, the factotum human cathelicidin peptide

Cathelicidins are a group of antimicrobial peptides. Since their discovery, it has become clear that they are an exceptional class of peptides, with some members having pleiotropic effects. Not only do they possess an antibacterial, antifungal and antiviral function, they also show a chemotactic and immunostimulatory/-modulatory effect. Moreover, they are capable of inducing wound healing, angiogenesis and modulating apoptosis. Recent insights even indicate for a role of these peptides in cancer. This review provides a comprehensive summary of the most recent and relevant insights concerning the human cathelicidin LL-37.

Type I interferon induction during influenza virus infection increases susceptibility to secondary Streptococcus pneumoniae infection by negative regulation of γδ T cells

The majority of deaths following influenza virus infection result from secondary bacterial superinfection, most commonly caused by Streptococcus pneumoniae. Several models have been proposed to explain how primary respiratory viral infections exacerbate secondary bacterial disease, but the mechanistic explanations have been contradictory. In this study, mice were infected with S. pneumoniae at different days after primary influenza A (X31) virus infection. Our findings show that the induction of type I interferons (IFNs) during a primary nonlethal influenza virus infection is sufficient to promote a deadly S. pneumoniae secondary infection. Moreover, mice deficient in type I interferon receptor (IFNAR knockout [KO] mice) effectively cleared the secondary bacterial infection from their lungs, increased the recruitment of neutrophils, and demonstrated an enhanced innate expression of interleukin-17 (IL-17) relative to wild-type (WT) mice. Lung γδ T cells were responsible for almost all IL-17 production, and their function is compromised during secondary S. pneumoniae infection of WT but not IFNAR KO mice. Adoptive transfer of γδ T cells from IFNAR KO mice reduced the susceptibility to secondary S. pneumoniae infection in the lung of WT mice. Altogether, our study highlights the importance of type I interferon as a key master regulator that is exploited by opportunistic pathogens such as S. pneumoniae. Our findings may be utilized to design effective preventive and therapeutic strategies that may be beneficial for coinfected patients during influenza epidemics.

Full restoration of Brucella-infected dendritic cell functionality through Vγ9Vδ2 T helper type 1 crosstalk

Vγ9Vδ2 T cells play an important role in the immune response to infectious agents but the mechanisms contributing to this immune process remain to be better characterized. Following their activation, Vγ9Vδ2 T cells develop cytotoxic activity against infected cells, secrete large amounts of cytokines and influence the function of other effectors of immunity, notably cells playing a key role in the initiation of the adaptive immune response such as dendritic cells. Brucella infection dramatically impairs dendritic cell maturation and their capacity to present antigens to T cells. Herein, we investigated whether V T cells have the ability to restore the full functional capacities of Brucella-infected dendritic cells. Using an in vitro multicellular infection model, we showed that: 1/Brucella-infected dendritic cells activate Vγ9Vδ2 T cells through contact-dependent mechanisms, 2/activated Vγ9Vδ2 T cells induce full differentiation into IL-12 producing cells of Brucella-infected dendritic cells with functional antigen presentation activity. Furthermore, phosphoantigen-activated Vγ9Vδ2 T cells also play a role in triggering the maturation process of dendritic cells already infected for 24 h. This suggests that activated Vγ9Vδ2 T cells could be used to modulate the outcome of infectious diseases by promoting an adjuvant effect in dendritic cell-based cellular therapies.

Peripheral human γδ T cells control growth of both avirulent and highly virulent strains of Francisella tularensis in vitro

In this paper we evaluate the role of human γδ T cells in control of Francisella tularensis infection. Using an in vitro model of infection, a reduction in bacterial numbers was detected in the presence of human γδ T cells for both attenuated LVS and virulent SCHU S4 strains of F. tularensis. Antibody neutralisation of IFN-γ caused an increase in survival of F. tularensis LVS suggesting that γδ T cell-mediated control of F. tularensis infection is partially mediated by IFN-γ.

Interactions of the human pathogenic Brucella species with their hosts

Brucellosis is a zoonotic infection caused primarily by the bacterial pathogens Brucella melitensis and B. abortus. It is acquired by consumption of unpasteurized dairy products or by contact with infected animals. Globally, it is one of the most widespread zoonoses, with 500,000 new cases reported each year. In endemic areas, Brucella infections represent a serious public health problem that results in significant morbidity and economic losses. An important feature of the disease is persistent bacterial colonization of the reticuloendothelial system. In this review we discuss recent insights into mechanisms of intracellular survival and immune evasion that contribute to systemic persistence by the pathogenic Brucella species.

Functional expression of NOD2 in freshly isolated human peripheral blood γδ T cells

γδ T cells play an important role in anti-infective immunity. The major subset of human γδ T cells selectively recognizes phosphorylated bacterial metabolites of the isoprenoid biosynthesis pathway, so-called phosphoantigens. The activation of γδ T cells is modulated by functionally expressed innate immune receptors, notably Toll-like receptor 2 and 3. It was also reported that in vitro expanded γδ T cells respond to muramyl dipeptide (MDP), the minimal peptidoglycan motif activating the nucleotide-binding oligomerization domain containing 2 (NOD2) receptor, although it is unknown whether ex vivo isolated human γδ T cells express functional NOD2. Here, we report that freshly isolated, highly purified peripheral blood γδ T cells express NOD2 mRNA and detectable amounts of NOD2 protein. The biologically active MDP L-D isomer but not the inactive D-D isomer augmented the interferon-γ (IFN-γ) secretion in phosphoantigen-stimulated peripheral blood mononuclear cells. Moreover, a moderate but reproducible and statistically significant increase in IFN-γ secretion was also observed when highly purified peripheral blood γδ T cells were activated by T cell receptor cross-linking in the presence of MDP. Taken together, our results indicate that in addition to the T cell receptor and Toll-like receptors, circulating human γδ T cells express NOD2 as a third class of pattern recognition receptor for sensing bacterial products.

Murine and bovine γδ T cells enhance innate immunity against Brucella abortus infections

γδ T cells have been postulated to act as a first line of defense against infectious agents, particularly intracellular pathogens, representing an important link between the innate and adaptive immune responses. Human γδ T cells expand in the blood of brucellosis patients and are active against Brucella in vitro. However, the role of γδ T cells in vivo during experimental brucellosis has not been studied. Here we report TCRδ^−/− mice are more susceptible to B. abortus infection than C57BL/6 mice at one week post-infection as measured by splenic colonization and splenomegaly. An increase in TCRγδ cells was observed in the spleens of B. abortus-infected C57BL/6 mice, which peaked at two weeks post-infection and occurred concomitantly with diminished brucellae. γδ T cells were the major source of IL-17 following infection and also produced IFN-γ. Depletion of γδ T cells from C57BL/6, IL-17Rα^−/−, and GMCSF^−/− mice enhanced susceptibility to B. abortus infection although this susceptibility was unaltered in the mutant mice; however, when γδ T cells were depleted from IFN-γ^−/− mice, enhanced susceptibility was observed. Neutralization of γδ T cells in the absence of TNF-α did not further impair immunity. In the absence of TNF-α or γδ T cells, B. abortus-infected mice showed enhanced IFN-γ, suggesting that they augmented production to compensate for the loss of γδ T cells and/or TNF-α. While the protective role of γδ T cells was TNF-α-dependent, γδ T cells were not the major source of TNF-α and activation of γδ T cells following B. abortus infection was TNF-α-independent. Additionally, bovine TCRγδ cells were found to respond rapidly to B. abortus infection upon co-culture with autologous macrophages and could impair the intramacrophage replication of B. abortus via IFN-γ. Collectively, these results demonstrate γδ T cells are important for early protection to B. abortus infections.

An evolutionary strategy for a stealthy intracellular Brucella pathogen

Brucella is an intracellular bacterial pathogen that causes abortion and infertility in mammals and leads to a debilitating febrile illness that can progress into a long lasting disease with severe complications in humans. Its virulence depends on survival and replication properties in host cells. In this review, we describe the stealthy strategy used by Brucella to escape recognition of the innate immunity and the means by which this bacterium evades intracellular destruction. We also discuss the development of adaptive immunity and its modulation during brucellosis that in course leads to chronic infections. Brucella has developed specific strategies to influence antigen presentation mediated by cells. There is increasing evidence that Brucella also modulates signaling events during host adaptive immune responses.

Preferential Th1 cytokine profile of phosphoantigen-stimulated human Vγ9Vδ2 T cells

Human Vγ9Vδ2 T cells recognise pyrophosphate-based antigens (phosphoantigens) and have multiple functions in innate and adaptive immunity, including a unique ability to activate other cells of the immune system. We used flow cytometry and ELISA to define the early cytokine profiles of Vγ9Vδ2 T cells stimulated in vitro with isopentenyl pyrophosphate (IPP) and (E)-4-hydroxy-3-methyl-but-2 enyl pyrophosphate (HMB-PP) in the absence and presence of IL-2 and IL-15. We show that fresh Vγ9Vδ2 T cells produce interferon-γ (IFN-γ) and tumour necrosis factor-α (TNF-α) within 4 hours of stimulation with phosphoantigen, but neither IL-10, IL-13, nor IL-17 was detectable up to 72 hours under these conditions. Cytokine production was not influenced by expression or lack, thereof, of CD4 or CD8. Addition of IL-2 or IL-15 caused expansion of IFN-γ-producing Vγ9Vδ2 T cells, but did not enhance IFN-γ secretion after 24–72 hours. Thus, phosphoantigen-stimulated Vγ9Vδ2 T cells have potential as Th1-biasing adjuvants for immunotherapy.

Cell-mediated immunity in human brucellosis

Brucella can parasitize within human antigen-presenting cells modifying phagocytosis, phagolysosome fusion, antigen presentation, cytokine secretion, and apoptosis. Subversion of innate immune mechanisms by Brucella leads to defective Th1 immune responses and T-cell anergy in chronic brucellosis patients. This review summarizes the cellular immune responses in brucellosis, based on data derived exclusively from human cells or cell lines.

Gammadelta T cell effector functions: a blend of innate programming and acquired plasticity

Gammadelta T cells have several innate cell-like features that allow their early activation following recognition of conserved stress-induced ligands. Here we review recent observations revealing the ability of gammadelta T cells to rapidly produce cytokines that regulate pathogen clearance, inflammation and tissue homeostasis in response to tissue stress. These studies provide insights into how they acquire these properties, through both developmental programming in the thymus and functional polarization in the periphery. Innate features of gammadelta T cells underlie their non-redundant role in several physiopathological contexts and are therefore being exploited in the design of new immunotherapeutic approaches.

Vgamma2Vdelta2 T Cell Receptor recognition of prenyl pyrophosphates is dependent on all CDRs

gammadelta T cells differ from alphabeta T cells in the Ags they recognize and their functions in immunity. Although most alphabeta TCRs recognize peptides presented by MHC class I or II, human gammadelta T cells expressing Vgamma2Vdelta2 TCRs recognize nonpeptide prenyl pyrophosphates. To define the molecular basis for this recognition, the effect of mutations in the TCR CDR was assessed. Mutations in all CDR loops altered recognition and cover a large footprint. Unlike murine gammadelta TCR recognition of the MHC class Ib T22 protein, there was no CDR3delta motif required for recognition because only one residue is required. Instead, the length and sequence of CDR3gamma was key. Although a prenyl pyrophosphate-binding site was defined by Lys109 in Jgamma1.2 and Arg51 in CDR2delta, the area outlined by critical mutations is much larger. These results show that prenyl pyrophosphate recognition is primarily by germline-encoded regions of the gammadelta TCR, allowing a high proportion of Vgamma2Vdelta2 TCRs to respond. This underscores its parallels to innate immune receptors. Our results also provide strong evidence for the existence of an Ag-presenting molecule for prenyl pyrophosphates.

Human brucellosis is characterized by an intense Th1 profile associated with a defective monocyte function

In animal models, a defective Th1 response appears to be critical in the pathogenesis of brucellosis, but the Th1 response in human brucellosis patients remains partially undefined. Peripheral blood from 24 brucellosis patients was studied before and 45 days after antibiotherapy. Twenty-four sex- and age-matched healthy donors were analyzed in parallel. Significantly increased levels of interleukin 1beta (IL-1beta), IL-2, IL-4, IL-6, IL-12p40, gamma interferon (IFN-gamma), and tumor necrosis factor alpha (TNF-alpha), but not of IL-10, in serum and/or significantly increased percentages of samples with detectable levels of these cytokines, measured by enzyme-linked immunosorbent assays (ELISA), were found for untreated brucellosis patients, but these levels were reduced and/or normalized after treatment. Flow cytometry studies showed that the intracytoplasmic expression of IFN-gamma, IL-2, and TNF-alpha, but not that of IL-4, by phorbol myristate-activated CD4(+) CD3(+) and CD8(+) CD3(+) T lymphocytes was significantly increased in untreated brucellosis patients and was also partially normalized after antibiotherapy. The percentage of phagocytic cells, the mean phagocytic activity per cell, and the phagocytic indices for monocytes at baseline were defective and had only partially reverted at follow-up. T lymphocytes from untreated brucellosis patients are activated in vivo and show Th1 cytokine production polarization, with strikingly high serum IFN-gamma levels. In spite of this Th1 environment, we found deficient effector phagocytic activity in peripheral blood monocytes.

Human gammadelta T cells produce the protease inhibitor and antimicrobial peptide elafin

Human gammadelta T cells rapidly secrete pro-inflammatory cytokines in response to T cell receptor-dependent recognition of pyrophosphates produced by many bacteria and parasites. In further support of an important role of gammadelta T cells in the immune defence against infection, human gammadelta T cells have been shown to produce the antimicrobial peptide LL37/cathelicidin. In this study, we have investigated whether gammadelta T cells can produce additional antimicrobial peptides. To this end, we have screened human gammadelta T cell clones by RT-PCR for mRNA expression of a broad range of antimicrobial peptides. While alpha-defensins were absent and beta-defensins (HBD1) present only in rare gammadelta T cell clones, elafin mRNA was induced by supernatant of Pseudomonas aeruginosa grown under static conditions. Elafin is a protease inhibitor that also displays antimicrobial activity. Constitutive intracellular expression of elafin was demonstrated by flow cytometry and Western blot analysis. Furthermore, trappin-2 (pre-elafin) could be immunoprecipitated in cell lysates but also in the supernatant of gammadelta T cells stimulated by Ps. aeruginosa supernatant. Taken together, our studies reveal a novel effector function of gammadelta T cells which might be important for local immune defence.

Staphylococcus epidermidis isolated from newborn infants express pilus-like structures and are inhibited by the cathelicidin-derived antimicrobial peptide LL37

Coagulase-negative staphylococci and its subtype Staphylococcus epidermidis are major indigenous Gram-positive inhabitants of the human skin. Colonization occurs in direct connection with birth and terrestrial adaptation. This study focuses on factors that may influence skin colonization of the newborn infant that relates to the immune status of both the bacteria and the host. Skin is an effective barrier against bacteria, and this function is partly mediated by the presence of antimicrobial peptides including human cathelicidin peptide LL37. Gram-positive bacteria have been described to have adhesive pili on their surface that mediates specific attachment to the host. Here, we identify, by negative staining transmission electron microscopy (EM), two different types of pilus-like structures commonly expressed on S. epidermidis isolated from newborn infants. We also show that the cathelicidin antimicrobial peptide LL37, constitutively expressed in the skin barrier of the newborn, significantly inhibited growth of S. epidermidis indicating its importance for the ecological stability of the skin microbiota. Further studies are required to elucidate molecular mechanisms of host-microbe interactions, both for the maintenance of a mutually beneficial homeostatic relationship and for the protection of self when it results in overt disease.

Acute infection by conjunctival route with Brucella melitensis induces IgG+ cells and IFN-gamma producing cells in peripheral and mucosal lymph nodes in sheep

The early distribution of Brucella melitensis and the immune response induced in lymphoid tissues and lymph nodes (LN) draining the upper respiratory tract were analysed in sheep. An experimental acute infection was performed by inoculating the sheep with the virulent H38 strain of B. melitensis by the conjunctival route. The infection was rapidly controlled at the site of inoculation but resulted in a local and systemic dissemination of brucellae mainly in the pharyngeal tonsil, local and peripheral LN and the spleen. The control of the infection was associated with the induction of a specific immune response characterized by an increase in IgG+ cells, the production of IFN-gamma and IL-10 by cells from draining parotid, retropharyngeal and submaxillary LN, but also from more distant peripheral prescapular and mesenteric LN. IFN-gamma was produced by CD4+, CD8+ and CD4(-)CD8(-)gammadelta(-) cells and probably contributed to the control of both local and systemic infection.

Vaccines against intracellular bacterial pathogens

There is a long history of remarkable success in developing vaccines against bacteria that are extracellular pathogens. In general, the development of vaccines against intracellular bacterial pathogens has proven to be more challenging. Typically, such vaccines need to induce a range of immune responses, including antibody, CD4(+) and CD8(+) T cell responses. These responses can be induced by live attenuated vaccines, but eliciting these responses with non-living vaccines has proven to be difficult. The difficulties appear to be related partly to the problems associated with the identification of protective antigens and partly with the difficulties associated with inducing CD8(+) T cell responses.

Identification and isolation of Brucella suis virulence genes involved in resistance to the human innate immune system

Brucella strains are facultative intracellular pathogens that induce chronic diseases in humans and animals. This observation implies that Brucella subverts innate and specific immune responses of the host to develop its full virulence. Deciphering the genes involved in the subversion of the immune system is of primary importance for understanding the virulence of the bacteria, for understanding the pathogenic consequences of infection, and for designing an efficient vaccine. We have developed an in vitro system involving human macrophages infected by Brucella suis and activated syngeneic gamma9delta2 T lymphocytes. Under these conditions, multiplication of B. suis inside macrophages is only slightly reduced. To identify the genes responsible for this reduced sensitivity, we screened a library of 2,000 clones of transposon-mutated B. suis. For rapid and quantitative analysis of the multiplication of the bacteria, we describe a simple method based on Alamar blue reduction, which is compatible with screening a large library. By comparing multiplication inside macrophages alone and multiplication inside macrophages with activated gamma9delta2 T cells, we identified four genes of B. suis that were necessary to resist to the action of the gamma9delta2 T cells. The putative functions of these genes are discussed in order to propose possible explanations for understanding their exact role in the subversion of innate immunity.