Utilization of CD11b knockout mice to characterize the role of complement receptor 3 (CR3, CD11b/CD18) in the growth of Mycobacterium tuberculosis in macrophages

Impact of surface receptors TLR2, CR3, and FcγRIII on Rhodococcus equi phagocytosis and intracellular survival in macrophages

The virulence-associated protein A (VapA) produced by virulent Rhodococcus equi allows it to replicate in macrophages and cause pneumonia in foals. It is unknown how VapA interacts with mammalian cell receptors, but intracellular replication of avirulent R. equi lacking vapA can be restored by supplementation with recombinant VapA (rVapA). Our objectives were to determine whether the absence of the surface receptors Toll-like receptor 2 (TLR2), complement receptor 3 (CR3), or Fc gamma receptor III (FcγRIII) impacts R. equi phagocytosis and intracellular replication in macrophages, and whether rVapA restoration of virulence in R. equi is dependent upon these receptors. Wild-type (WT) murine macrophages with TLR2, CR3, or FcγRIII blocked or knocked out (KO) were infected with virulent or avirulent R. equi, with or without rVapA supplementation. Quantitative bacterial culture and immunofluorescence imaging were performed. Phagocytosis of R. equi was not affected by blockade or KO of TLR2 or CR3. Intracellular replication of virulent R. equi was not affected by TLR2, CR3, or FcγRIII blockade or KO; however, avirulent R. equi replicated in TLR2-/- and CR3-/- macrophages but not in WT and FcγRIII-/-. rVapA supplementation did not affect avirulent R. equi phagocytosis but promoted intracellular replication in WT and all KO cells. By demonstrating that TLR2 and CR3 limit replication of avirulent but not virulent R. equi and that VapA-mediated virulence is independent of TLR2, CR3, or FcγRIII, our study provides novel insights into the role of these specific surface receptors in determining the entry and intracellular fate of R. equi.

IL-4 and helminth infection downregulate MINCLE-dependent macrophage response to mycobacteria and Th17 adjuvanticity

The myeloid C-type lectin receptor (CLR) MINCLE senses the mycobacterial cell wall component trehalose-6,6'-dimycolate (TDM). Recently, we found that IL-4 downregulates MINCLE expression in macrophages. IL-4 is a hallmark cytokine in helminth infections, which appear to increase the risk for mycobacterial infection and active tuberculosis. Here, we investigated functional consequences of IL-4 and helminth infection on MINCLE-driven macrophage activation and Th1/Th17 adjuvanticity. IL-4 inhibited MINCLE and cytokine induction after macrophage infection with Mycobacterium bovis bacille Calmette-Guerin (BCG). Infection of mice with BCG upregulated MINCLE on myeloid cells, which was inhibited by IL-4 plasmid injection and by infection with the nematode Nippostrongylus brasiliensis in monocytes. To determine the impact of helminth infection on MINCLE-dependent immune responses, we vaccinated mice with a recombinant protein together with the MINCLE ligand trehalose-6,6-dibehenate (TDB) as adjuvant. Concurrent infection with N. brasiliensis or with Schistosoma mansoni promoted T cell-derived IL-4 production and suppressed Th1/Th17 differentiation in the spleen. In contrast, helminth infection did not reduce Th1/Th17 induction by TDB in draining peripheral lymph nodes, where IL-4 levels were unaltered. Upon use of the TLR4-dependent adjuvant G3D6A, N. brasiliensis infection impaired selectively the induction of splenic antigen-specific Th1 but not of Th17 cells. Inhibition of MINCLE-dependent Th1/Th17 responses in mice infected with N. brasiliensis was dependent on IL-4/IL-13. Thus, helminth infection attenuated the Th17 response to MINCLE-dependent immunization in an organ- and adjuvant-specific manner via the Th2 cytokines IL-4/IL-13. Taken together, our results demonstrate downregulation of MINCLE expression on monocytes and macrophages by IL-4 as a possible mechanism of thwarted Th17 vaccination responses by underlying helminth infection.

Exploring the Use of Medicinal Plants and Their Bioactive Derivatives as Alveolar NLRP3 Inflammasome Regulators during Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), is a successful intracellular pathogen that is responsible for the highest mortality rate among diseases caused by bacterial infections. During early interaction with the host innate cells, M. tuberculosis cell surface antigens interact with Toll like receptor 4 (TLR4) to activate the nucleotide-binding domain, leucine-rich-repeat containing family, pyrin domain-containing 3 (NLRP3) canonical, and non-canonical inflammasome pathways. NLRP3 inflammasome activation in the alveoli has been reported to contribute to the early inflammatory response that is needed for an effective anti-TB response through production of pro-inflammatory cytokines, including those of the Interleukin 1 (IL1) family. However, overstimulation of the alveolar NLRP3 inflammasomes can induce excessive inflammation that is pathological to the host. Several studies have explored the use of medicinal plants and/or their active derivatives to inhibit excessive stimulation of the inflammasomes and its associated factors, thus reducing immunopathological response in the host. This review describes the molecular mechanism of the NLRP3 inflammasome activation in the alveoli during M. tuberculosis infection. Furthermore, the mechanisms of inflammasome inhibition using medicinal plant and their derivatives will also be explored, thus offering a novel perspective on the alternative control strategies of M. tuberculosis-induced immunopathology.

Impact of Type 2 Diabetes on the capacity of human macrophages infected with Mycobacterium tuberculosis to modulate monocyte differentiation through a bystander effect

Type 2 diabetes mellitus (T2DM) is a risk factor for the development of tuberculosis (TB) through mechanisms poorly understood. Monocytes and macrophages are key effector cells to control TB, but they are also subverted by Mycobacterium tuberculosis (Mtb). Specifically, Mtb can induce a bystander effect that skews monocyte differentiation towards macrophages with a permissive phenotype to infection. Here, we evaluated whether T2DM impacts this TB aspect. Our approach was to differentiate monocytes from healthy control (HC) subjects and T2DM patients into macrophages (MDM), and then assess their response to Mtb infection, including their secretome content and bystander effect capacity. Through flow cytometry analyses, we found a lower level of activation markers in MDM from T2DM patients in comparison to those from HC in response to mock (HLA-DR, CD86, and CD163) or Mtb challenge (CD14 and CD80). In spite of high TGF-β1 levels in mock-infected MDM from T2DM patients, cytometric bead arrays indicated there were no major differences in the secretome cytokine content in these cells relative to HC-MDM, even in response to Mtb. Mimicking a bystander effect, the secretome of Mtb-infected HC-MDM drove HC monocytes towards MDM with a permissive phenotype for Mtb intracellular growth. However, the secretome from Mtb-infected T2DM-MDM did not exacerbate the Mtb load compared to cmMTB-HC, possibly due to the high IL-1β production relative to Mtb-infected HC-MDM. Collectively, despite T2DM affecting the basal MDM activation, our approach revealed it has no major consequence on their response to Mtb or capacity to generate a bystander effect influencing monocyte differentiation.

Underwhelming or Misunderstood? Genetic Variability of Pattern Recognition Receptors in Immune Responses and Resistance to Mycobacterium tuberculosis

Human genetic control is thought to affect a considerable part of the outcome of infection with Mycobacterium tuberculosis (Mtb). Most of us deal with the pathogen by containment (associated with clinical "latency") or sterilization, but tragically millions each year do not. After decades of studies on host genetic susceptibility to Mtb infection, genetic variation has been discovered to play a role in tuberculous immunoreactivity and tuberculosis (TB) disease. Genes encoding pattern recognition receptors (PRRs) enable a consistent, molecularly direct interaction between humans and Mtb which suggests the potential for co-evolution. In this review, we explore the roles ascribed to PRRs during Mtb infection and ask whether such a longstanding and intimate interface between our immune system and this pathogen plays a critical role in determining the outcome of Mtb infection. The scientific evidence to date suggests that PRR variation is clearly implicated in altered immunity to Mtb but has a more subtle role in limiting the pathogen and pathogenesis. In contrast to 'effectors' like IFN-γ, IL-12, Nitric Oxide and TNF that are critical for Mtb control, 'sensors' like PRRs are less critical for the outcome of Mtb infection. This is potentially due to redundancy of the numerous PRRs in the innate arsenal, such that Mtb rarely goes unnoticed. Genetic association studies investigating PRRs during Mtb infection should therefore be designed to investigate endophenotypes of infection - such as immunological or clinical variation - rather than just TB disease, if we hope to understand the molecular interface between innate immunity and Mtb.

More than a Pore: Nonlytic Antimicrobial Functions of Complement and Bacterial Strategies for Evasion

The complement system is an evolutionarily ancient defense mechanism against foreign substances. Consisting of three proteolytic activation pathways, complement converges on a common effector cascade terminating in the formation of a lytic pore on the target surface. The classical and lectin pathways are initiated by pattern recognition molecules binding to specific ligands, while the alternative pathway is constitutively active at low levels in circulation. Complement-mediated killing is essential for defense against many Gram-negative bacterial pathogens, and genetic deficiencies in complement can render individuals highly susceptible to infection, for example, invasive meningococcal disease. In contrast, Gram-positive bacteria are inherently resistant to the direct bactericidal activity of complement due to their thick layer of cell wall peptidoglycan. However, complement also serves diverse roles in immune defense against all bacteria by flagging them for opsonization and killing by professional phagocytes, synergizing with neutrophils, modulating inflammatory responses, regulating T cell development, and cross talk with coagulation cascades. In this review, we discuss newly appreciated roles for complement beyond direct membrane lysis, incorporate nonlytic roles of complement into immunological paradigms of host-pathogen interactions, and identify bacterial strategies for complement evasion.

Innate Immune Pattern Recognition Receptors of Mycobacterium tuberculosis: Nature and Consequences for Pathogenesis of Tuberculosis

Innate immunity against Mycobacterium tuberculosis is a critical early response to prevent the establishment of the infection. Despite recent advances in understanding the host-pathogen dialogue in the early stages of tuberculosis (TB), much has yet to be learnt. The nature and consequences of this dialogue ultimately determine the path of infection: namely, either early clearance of M. tuberculosis, or establishment of M. tuberculosis infection leading to active TB disease and/or latent TB infection. On the frontline in innate immunity are pattern recognition receptors (PRRs), with soluble factors (e.g. collectins and complement) and cell surface factors (e.g. Toll-like receptors and other C-type lectin receptors (Dectin 1/2, Nod-like receptors, DC-SIGN, Mincle, mannose receptor, and MCL) that play a central role in recognising M. tuberculosis and facilitating its clearance. However, in a 'double-edged sword' scenario, these factors can also be involved in enhancement of pathogenesis as well. Furthermore, innate immunity is also a critical bridge in establishing the subsequent adaptive immune response, which is also responsible for granuloma formation that cordons off M. tuberculosis infection, establishing latency and acting as a reservoir for bacterial persistence and dissemination of future disease. This chapter discusses the current understanding of pattern recognition of M. tuberculosis by innate immunity and the role this plays in the pathogenesis and protection against TB.

Thinking Outside the Box: Innate- and B Cell-Memory Responses as Novel Protective Mechanisms Against Tuberculosis

Tuberculosis (TB) is currently the deadliest infectious disease worldwide. Failure to create a highly effective vaccine has limited the control of the TB epidemic. Historically, the vaccine field has relied on the paradigm that IFN-γ-mediated CD4+ T cell memory responses are the principal correlate of protection in TB. Nonetheless, the demonstration that other cellular subsets offer protective memory responses against Mycobacterium tuberculosis (Mtb) is emerging. Among these are memory-like features of macrophages, myeloid cell precursors, natural killer (NK) cells, and innate lymphoid cells (ILCs). Additionally, the dynamics of B cell memory responses have been recently characterized at different stages of the clinical spectrum of Mtb infection, suggesting a role for B cells in human TB. A better understanding of the immune mechanisms underlying such responses is crucial to better comprehend protective immunity in TB. Furthermore, targeting immune compartments other than CD4+ T cells in TB vaccine strategies may benefit a significant proportion of patients co-infected with Mtb and the human immunodeficiency virus (HIV). Here, we summarize the memory responses of innate immune cells and B cells against Mtb and propose them as novel correlates of protection that could be harnessed in future vaccine development programs.

Identification of an Increased Alveolar Macrophage Subpopulation in Old Mice That Displays Unique Inflammatory Characteristics and Is Permissive to Mycobacterium tuberculosis Infection

The elderly population is more susceptible to pulmonary infections, including tuberculosis. In this article, we characterize the impact of aging on the phenotype of mouse alveolar macrophages (AMs) and their response to Mycobacterium tuberculosis. Uninfected AMs were isolated from bronchoalveolar lavage of young (3 mo) and old (18 mo) C57BL/6 mice. AMs from old mice expressed higher mRNA levels of CCL2, IFN-β, IL-10, IL-12p40, TNF-α, and MIF than young mice, and old mice contained higher levels of CCL2, IL-1β, IFN-β, and MIF in their alveolar lining fluid. We identified two distinct AM subpopulations, a major CD11c⁺ CD11b^- population and a minor CD11c⁺ CD11b⁺ population; the latter was significantly increased in old mice (4-fold). Expression of CD206, TLR2, CD16/CD32, MHC class II, and CD86 was higher in CD11c⁺ CD11b⁺ AMs, and these cells expressed monocytic markers Ly6C, CX3CR1, and CD115, suggesting monocytic origin. Sorted CD11c⁺ CD11b⁺ AMs from old mice expressed higher mRNA levels of CCL2, IL-1β, and IL-6, whereas CD11c⁺ CD11b^- AMs expressed higher mRNA levels of immune-regulatory cytokines IFN-β and IL-10. CD11c⁺ CD11b⁺ AMs phagocytosed significantly more M. tuberculosis, which expressed higher RNA levels of genes required for M. tuberculosis survival. Our studies identify two distinct AM populations in old mice: a resident population and an increased CD11c⁺ CD11b⁺ AM subpopulation expressing monocytic markers, a unique inflammatory signature, and enhanced M. tuberculosis phagocytosis and survival when compared with resident CD11c⁺ CD11b^- AMs, which are more immune regulatory in nature.

The Mycobacterium tuberculosis capsule: a cell structure with key implications in pathogenesis

Bacterial capsules have evolved to be at the forefront of the cell envelope, making them an essential element of bacterial biology. Efforts to understand the Mycobacterium tuberculosis (Mtb) capsule began more than 60 years ago, but the relatively recent development of mycobacterial genetics combined with improved chemical and immunological tools have revealed a more refined view of capsule molecular composition. A glycogen-like α-glucan is the major constituent of the capsule, with lower amounts of arabinomannan and mannan, proteins and lipids. The major Mtb capsular components mediate interactions with phagocytes that favor bacterial survival. Vaccination approaches targeting the mycobacterial capsule have proven successful in controlling bacterial replication. Although the Mtb capsule is composed of polysaccharides of relatively low complexity, the concept of antigenic variability associated with this structure has been suggested by some studies. Understanding how Mtb shapes its envelope during its life cycle is key to developing anti-infective strategies targeting this structure at the host-pathogen interface.

Mycobacterial Phenolic Glycolipids Selectively Disable TRIF-Dependent TLR4 Signaling in Macrophages

Phenolic glycolipids (PGLs) are cell wall components of a subset of pathogenic mycobacteria, with immunomodulatory properties. Here, we show that in addition, PGLs exert antibactericidal activity by limiting the production of nitric oxide synthase (iNOS) in mycobacteria-infected macrophages. PGL-mediated downregulation of iNOS was complement receptor 3-dependent and comparably induced by bacterial and purified PGLs. Using Mycobacterium leprae PGL-1 as a model, we found that PGLs dampen the toll-like receptor (TLR)4 signaling pathway, with macrophage exposure to PGLs leading to significant reduction in TIR-domain-containing adapter-inducing interferon-β (TRIF) protein level. PGL-driven decrease in TRIF operated posttranscriptionally and independently of Src-family tyrosine kinases, lysosomal and proteasomal degradation. It resulted in the defective production of TRIF-dependent IFN-β and CXCL10 in TLR4-stimulated macrophages, in addition to iNOS. Our results unravel a mechanism by which PGLs hijack both the bactericidal and inflammatory responses of host macrophages. Moreover, they identify TRIF as a critical node in the crosstalk between CR3 and TLR4.

Complement Receptors in Myeloid Cell Adhesion and Phagocytosis

Myeloid cells make extensive use of the complement system in the context of recruitment, phagocytosis, and other effector functions. There are several types of complement receptors on myeloid cells, including G protein-coupled receptors for localizing the source of complement activation, and three sets of type I transmembrane proteins that link complement to phagocytosis: complement receptor 1, having an extracellular domain with tandem complement regulatory repeats; complement receptors 3 and 4, which are integrin family receptors comprising heterodimers of type I transmembrane subunits; and VSIG4, a member of the Ig superfamily. This review will focus on the role of the different classes of complement receptors and how their activities are integrated in the setting of immune tolerance and inflammatory responses.

Fibronectin binding protein and Ca2+ play an access key role to mediate pathogenesis in Mycobacterium tuberculosis: An overview

The anomalous distribution of adhesive proteins throughout on the cell surface of the Mycobacterium tuberculosis H₃₇ Rv and their contribution in cell surface adhesion and host-pathogen interaction remain elusive. The completion of M. tuberculosis H₃₇ Rv genome sequence analysis gives some interesting information about polymorphic GC-rich repetitive sequence (PGRS) subfamily of M. tuberculosis that encodes fibronectin binding proteins (FnBP), which have been extensively studied, but the function in the pathogenesis of most of these proteins remains unknown and unclear. This review addresses the M. tuberculosis entry mechanism in the host cell. In particular, an effort has been made to focus on several aspects, (a) association of FnBP encodes by PE_PGRS protein family of M. tuberculosis during host-pathogen interactions. (b) Effect of calcium ions in and outside of the host cell is overriding to maintenance of calcium trafficking in phagocytosis. Furthermore, FnBP may be a potential source of antigenic variation that participating in evoking immune response. M. tuberculosis entry mechanism does not have a major influence alone, involvement of calcium ions, perhaps shed light on host-pathogen interaction relationship, and could open up new avenues for development of novel drug by targeting M. tuberculosis FnBP and blockade of selective adhesions could be useful for therapeutics.

Lipoarabinomannan binding to lactosylceramide in lipid rafts is essential for the phagocytosis of mycobacteria by human neutrophils

Pathogenic mycobacteria use virulence factors, including mannose-capped lipoarabinomannan (ManLAM), to survive in host phagocytic cells, such as neutrophils. We assessed the roles of lactosylceramide (LacCer, CDw17)-enriched lipid rafts in the phagocytosis of mycobacteria by human neutrophils and in the intracellular fate of phagocytosed mycobacteria. We showed that the association of the Src family kinase (SFK) Lyn with C24 fatty acid chain-containing LacCer was essential for the phagocytosis of mycobacteria by neutrophils. Assays with LacCer-containing liposomes, LacCer-coated plastic plates, and LAM-coated beads demonstrated that the phagocytosis of mycobacteria was mediated through the binding of LacCer to LAM. Both ManLAM from pathogenic species and phosphoinositol-capped LAM (PILAM) from nonpathogenic Mycobacterium smegmatis bound equivalently to LacCer to stimulate phagocytosis. However, PILAM from an M. smegmatis α1,2-mannosyltransferase deletion mutant (ΔMSMEG_4247), lacking the α1,2-monomannose side branches of the LAM mannan core, did not bind to LacCer or induce phagocytosis. An anti-LacCer antibody immunoprecipitated the SFK Hck from the phagosomes of neutrophils that internalized nonpathogenic mycobacteria but not from those that internalized pathogenic mycobacteria. Furthermore, knockdown of Hck by short inhibitory RNA abolished the fusion of lysosomes with phagosomes containing nonpathogenic mycobacteria. Further analysis showed that ManLAM, but not PILAM, inhibited the association of Hck with LacCer-enriched lipid rafts in phagosomal membranes, effectively blocking phagolysosome formation. Together, these findings suggest that pathogenic mycobacteria use ManLAM not only for binding to LacCer-enriched lipid rafts and entering neutrophils but also for disrupting signaling through Hck-coupled, LacCer-enriched lipid rafts and preventing phagolysosome formation.

Multinucleated Giant Cells Are Specialized for Complement-Mediated Phagocytosis and Large Target Destruction

Multinucleated giant cells (MGCs) form by fusion of macrophages and are presumed to contribute to the removal of debris from tissues. In a systematic in vitro analysis, we show that IL-4-induced MGCs phagocytosed large and complement-opsonized materials more effectively than their unfused M2 macrophage precursors. MGC expression of complement receptor 4 (CR4) was increased, but it functioned primarily as an adhesion integrin. In contrast, although expression of CR3 was not increased, it became functionally activated during fusion and was located on the extensive membrane ruffles created by excess plasma membrane arising from macrophage fusion. The combination of increased membrane area and activated CR3 specifically equips MGCs to engulf large complement-coated targets. Moreover, we demonstrate these features in vivo in the recently described complement-dependent therapeutic elimination of systemic amyloid deposits by MGCs. MGCs are evidently more than the sum of their macrophage parts.

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MGCs are specialized for phagocytosis of large and complement-opsonized particles

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MGCs show extensive membrane ruffles containing pre-activated complement receptor 3

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Membrane ruffles provide excess membrane for ingestion of large materials

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MGCs eliminate systemic amyloid deposits after immunotherapeutic targeting

Immunoevasion and immunosuppression of the macrophage by Mycobacterium tuberculosis

By virtue of their position at the crossroads between the innate and adaptive immune response, macrophages play an essential role in the control of bacterial infections. Paradoxically, macrophages serve as the natural habitat to Mycobacterium tuberculosis (Mtb). Mtb subverts the macrophage's mechanisms of intracellular killing and antigen presentation, leading ultimately to the development of tuberculosis (TB) disease. Here, we describe mechanisms of Mtb uptake by the macrophage and address key macrophage functions that are targeted by Mtb-specific effector molecules enabling this pathogen to circumvent host immune response. The macrophage functions described in this review include fusion between phagosomes and lysosomes, production of reactive oxygen and nitrogen species, antigen presentation and major histocompatibility complex class II expression and trafficking, as well as autophagy and apoptosis. All these are Mtb-targeted key cellular pathways, normally working in concert in the macrophage to recognize, respond, and activate 'proper' immune responses. We further analyze and discuss major molecular interactions between Mtb virulence factors and key macrophage proteins and provide implications for vaccine and drug development.

Microglia are crucial regulators of neuro-immunity during central nervous system tuberculosis

Mycobacterium tuberculosis (M. tuberculosis) infection of the central nervous system (CNS) is the most devastating manifestation of tuberculosis (TB), with both high mortality and morbidity. Although research has been fueled by the potential therapeutic target microglia offer against neurodegenerative inflammation, their part in TB infection of the CNS has not been fully evaluated nor elucidated. Yet, as both the preferential targets of M. tuberculosis and the immune-effector cells of the CNS, microglia are likely to be key determinants of disease severity and clinical outcomes. Following pathogen recognition, bacilli are internalized and capable of replicating within microglia. Cellular activation ensues, utilizing signaling molecules that may be neurotoxic. Central to initiating, orchestrating and modulating the tuberculous immune response is microglial secretion of cytokines and chemokines. However, the neurological environment is unique in that inflammatory signals, which appear to be damaging in the periphery, could be beneficial by governing neuronal survival, regeneration and differentiation. Furthermore, microglia are important in the recruitment of peripheral immune cells and central to defining the pro-inflammatory milieu of which neurotoxicity may result from many of the participating local or recruited cell types. Microglia are capable of both presenting antigen to infiltrating CD4(+) T-lymphocytes and inducing their differentiation-a possible correlate of protection against M. tuberculosis infection. Clarifying the nature of the immune effector molecules secreted by microglia, and the means by which other CNS-specific cell types govern microglial activation or modulate their responses is critical if improved diagnostic and therapeutic strategies are to be attained. Therefore, this review evaluates the diverse roles microglia play in the neuro-immunity to M. tuberculosis infection of the CNS.

ESX1-dependent fractalkine mediates chemotaxis and Mycobacterium tuberculosis infection in humans

Mycobacterium tuberculosis-induced cellular aggregation is essential for granuloma formation and may assist establishment and early spread of M. tuberculosis infection. The M. tuberculosis ESX1 mutant, which has a non-functional type VII secretion system, induced significantly less production of the host macrophage-derived chemokine fractalkine (CX3CL1). Upon infection of human macrophages ESX1-dependent fractalkine production mediated selective recruitment of CD11b+ monocytic cells and increased infection of neighbouring cells consistent with early local spread of infection. Fractalkine levels were raised in vivo at tuberculous disease sites in humans and were significantly associated with increased CD11b+ monocytic cellular recruitment and extent of granulomatous disease. These findings suggest a novel fractalkine-dependent ESX1-mediated mechanism in early tuberculous disease pathogenesis in humans. Modulation of M. tuberculosis-mediated fractalkine induction may represent a potential treatment option in the future, perhaps allowing us to switch off a key mechanism required by the pathogen to spread between cells.

Macrophages in tuberculosis: friend or foe

Tuberculosis (TB) remains one of the greatest threats to human health. The causative bacterium, Mycobacterium tuberculosis (Mtb), is acquired by the respiratory route. It is exquisitely human adapted and a prototypic intracellular pathogen of macrophages, with alveolar macrophages (AMs) being the primary conduit of infection and disease. The outcome of primary infection is most often a latently infected healthy human host, in whom the bacteria are held in check by the host immune response. Such individuals can develop active TB later in life with impairment in the immune system. In contrast, in a minority of infected individuals, the host immune response fails to control the growth of bacilli, and progressive granulomatous disease develops, facilitating spread of the bacilli via infectious aerosols coughed out into the environment and inhaled by new hosts. The molecular details of the Mtb-macrophage interaction continue to be elucidated. However, it is clear that a number of complex processes are involved at the different stages of infection that may benefit either the bacterium or the host. Macrophages demonstrate tremendous phenotypic heterogeneity and functional plasticity which, depending on the site and stage of infection, facilitate the diverse outcomes. Moreover, host responses vary depending on the specific characteristics of the infecting Mtb strain. In this chapter, we describe a contemporary view of the behavior of AMs and their interaction with various Mtb strains in generating unique immunologic lung-specific responses.

The three human monocyte subsets: implications for health and disease

Human blood monocytes are heterogeneous and conventionally subdivided into two subsets based on CD16 expression. Recently, the official nomenclature subdivides monocytes into three subsets, the additional subset arising from the segregation of the CD16+ monocytes into two based on relative expression of CD14. Recent whole genome analysis reveal that specialized functions and phenotypes can be attributed to these newly defined monocyte subsets. In this review, we discuss these recent results, and also the description and utility of this new segregation in several disease conditions. We also discuss alternative markers for segregating the monocyte subsets, for example using Tie-2 and slan, which do not necessarily follow the official method of segregating monocyte subsets based on relative CD14 and CD16 expressions.

Chemistry-dependent adsorption of serum proteins onto polyanhydride microparticles differentially influences dendritic cell uptake and activation

The delivery of antigen-loaded microparticles to dendritic cells (DCs) may benefit from surface optimization of the microparticles themselves, thereby exploiting the material properties and introducing signals that mimic pathogens. Following in vivo administration microparticle surface characteristics are likely to be significantly modified as proteins are quickly adsorbed onto their surface. In this work we describe the chemistry-dependent serum protein adsorption patterns on polyanhydride particles and the implications for their molecular interactions with DCs. The enhanced expression of MHC II and CD40 on DCs after incubation with amphiphilic polyanhydride particles, and the increased secretion of IL-6, TNF-α, and IL-12p40 by hydrophobic polyanhydride particles exemplified the chemistry-dependent activation of DCs by sham-coated particles. The presence of proteins such as complement component 3 and IgG further enhanced the adjuvant properties of these vaccine carriers by inducing DC maturation (i.e. increased cell surface molecule expression and cytokine secretion) in a chemistry-dependent manner. Utilizing DCs derived from complement receptor 3-deficient mice (CR3(-/-) mice) identified a requirement for CR3 in the internalization of both sham- and serum-coated particles. These studies provide valuable insights into the rational design of targeted vaccine platforms aimed at inducing robust immune responses and improving vaccine efficacy.

Comparative immunological and microbiological aspects of paratuberculosis as a model mycobacterial infection

Paratuberculosis or Johne's disease of livestock, which is caused by Mycobacterium avium subsp. paratuberculosis (MAP), has increased in prevalence and expanded in geographic and host ranges over about 100 years. The slow and progressive spread of MAP reflects its substantial adaptation to its hosts, the technical limitations of diagnosis, the lack of practical therapeutic approaches, the lack of a vaccine that prevents transmission and the complexity and difficulty of the on-farm control strategies needed to prevent infection. More recently evidence has accumulated for an association of MAP with Crohn's disease in humans, adding to the pressure on animal health authorities to take precautions by controlling paratuberculosis. Mycobacterial infections invoke complex immune responses but the essential determinants of virulence and pathogenesis are far from clear. In this review we compare the features of major diseases in humans and animals that are caused by the pathogenic mycobacteria M. ulcerans, M. avium subsp. avium, M. leprae, M. tuberculosis and MAP. We seek to answer key questions: are the common mycobacterial infections of humans and animals useful "models" for each other, or are the differences between them too great to enable meaningful extrapolation? To simplify this, the immunopathogenesis of mycobacterial infections will be defined at cellular, tissue, animal and population levels and the key events at each level will be discussed. Many pathogenic processes are similar between divergent mycobacterial diseases, and at variance between virulent and avirulent isolates of mycobacteria, suggesting that the research on the pathogenesis of one mycobacterial disease will be informative for the others.

C-type lectins with a sweet spot for Mycobacterium tuberculosis

The pattern of receptors sensing pathogens onto host cells is a key factor that can determine the outcome of the infection. This is particularly true when such receptors belong to the family of pattern recognition receptors involved in immunity. Mycobacterium tuberculosis, the etiologic agent of tuberculosis interacts with a wide range of pattern-recognition receptors present on phagocytes and belonging to the Toll-like, Nod-like, scavenger and C-type lectin receptor families. A complex scenario where those receptors can establish cross-talks in recognizing pathogens or microbial determinants including mycobacterial components in different spatial and temporal context starts to emerge as a key event in the outcome of the immune response, and thus, the control of the infection. In this review, we will focus our attention on the family of calcium-dependent carbohydrate receptors, the C-type lectin receptors, that is of growing importance in the context of microbial infections. Members of this family appear to be key innate immune receptors of mycobacteria, capable of cross-talk with other pattern recognition receptors to induce or modulate the inflammatory context upon mycobacterial infection.

Adjusting to a new home: Mycobacterium tuberculosis gene expression in response to an intracellular lifestyle

Mycobacterium tuberculosis remains the most significant single species of bacteria causing disease in mankind. The ability of M. tuberculosis to survive and replicate within host macrophages is a pivotal step in its pathogenesis. Understanding the microenvironments that M. tuberculosis encounters within the macrophage and the adaptations that the bacterium undergoes to facilitate its survival will lead to insights into possible therapeutic targets for improved treatment of tuberculosis. This is urgently needed with the emergence of multi- and extensively drug resistant strains of M. tuberculosis. Significant advances have been made in understanding the macrophage response on encountering M. tuberculosis. Complementary information is also accumulating regarding the counter responses of M. tuberculosis during the various stages of its interactions with the host. As such, a picture is emerging delineating the gene expression of intracellular M. tuberculosis at different stages of the interaction with macrophages.

Complement receptor 3 deficiency influences lesion progression during Leishmania major infection in BALB/c mice

Leishmania major is an obligately intracellular protozoan parasite that causes cutaneous leishmaniasis. Like numerous intracellular pathogens, Leishmania exploits cell surface receptors as a means of entry into host cells. Complement receptor 3 (CR3; also called CD11b/CD18), a beta(2) integrin on phagocytic cells, is one such receptor. Ligation of CR3 has been shown to inhibit the production of interleukin-12, the cytokine that is pivotal in establishing the cell-mediated response necessary to combat intracellular infection. Here we investigate the role that CR3 plays in the establishment and progression of cutaneous leishmaniaisis in vivo. Dermal lesions of wild-type BALB/c mice are characteristically progressive and lead to extensive tissue necrosis coupled with elevated parasite burdens; CD11b-deficient BALB/c mice, however, demonstrate an intermediate phenotype characterized by chronic lesions and a reduced incidence of tissue damage. Infection followed by a reinfection challenge indicates that both susceptible (BALB/c) and resistant (C57BL/6) mice, regardless of CD11b status, develop resistance to L. major. In addition, CD11b does not bias the T helper cytokine response to L. major infection. Our results further indicate that CD11b is not necessary for disease resolution in resistant mice; rather, this protein appears to play a minor role in susceptibility.

Mycobacterium tuberculosis Cpn60.2 and DnaK are located on the bacterial surface, where Cpn60.2 facilitates efficient bacterial association with macrophages

Mycobacterium tuberculosis, the causative agent of tuberculosis, initially contacts host cells with elements of its outer cell wall, or capsule. We have shown that capsular material from the surface of M. tuberculosis competitively inhibits the nonopsonic binding of whole M. tuberculosis bacilli to macrophages in a dose-dependent manner that is not acting through a global inhibition of macrophage binding. We have further demonstrated that isolated M. tuberculosis capsular proteins mediate a major part of this inhibition. Two-dimensional polyacrylamide gel electrophoresis analysis of the capsular proteins showed the presence of a wide variety of protein species, including proportionately high levels of the Cpn60.2 (Hsp65, GroEL2) and DnaK (Hsp70) molecular chaperones. Both of these proteins were subsequently detected on the bacterial surface. To determine whether these molecular chaperones play a role in bacterial binding, recombinant Cpn60.2 and DnaK were tested for their ability to inhibit the association of M. tuberculosis bacilli with macrophages. We found that recombinant Cpn60.2 can inhibit approximately 57% of bacterial association with macrophages, while DnaK was not inhibitory at comparable concentrations. Additionally, when polyclonal F(ab')(2) fragments of anti-Cpn60.2 and anti-DnaK were used to mask the surface presentation of these molecular chaperones, a binding reduction of approximately 34% was seen for anti-Cpn60.2 F(ab')(2), while anti-DnaK F(ab')(2) did not significantly reduce bacterial association with macrophages. Thus, our findings suggest that while M. tuberculosis displays both surface-associated Cpn60.2 and DnaK, only Cpn60.2 demonstrates adhesin functionality with regard to macrophage interaction.

Surfactant protein A modulates cell surface expression of CR3 on alveolar macrophages and enhances CR3-mediated phagocytosis

Pulmonary surfactant protein A (SP-A), a member of the collectin family, plays an important role in innate immune defense of the lung. In this study, we examined the role of SP-A in modulating complement receptor-mediated phagocytosis. Complement receptors (CR), CR3 (CD11b), and CR4 (CD11c) were expressed at reduced levels on the surface of alveolar macrophages from Sp-a(-/-) compared with Sp-a(+/+) mice. Administration of intratracheal SP-A to Sp-a(-/-) mice induced the translocation of CR3 from alveolar macrophage intracellular pools to the cell surface. Intratracheal challenge with Haemophilus influenza enhanced CR3 expression on the surface of alveolar macrophages from Sp-a(-/-) and Sp-a(+/+) mice, but relative expression remained lower in the Sp-a(-/-) mice at all time points post-inoculation. The effects of SP-A on macrophage and neutrophil CR3 redistribution between intracellular and cell surface pools were restricted to cells isolated from the lung. SP-A augmented CR3-mediated phagocytosis in a manner that was attenuated by N-glycanase or collagenase treatment of SP-A, implicating the N-linked sugar and collagen-like domains in that function. The binding of CR3 to SP-A was calcium dependent and mediated by the I-domain of CR3 and to a lesser extent by the CR3 lectin domain. Mapping of the domains of SP-A that were required for optimal binding to CR3 revealed that the N-linked sugars were more critical than the collagen-like domain or the extent of oligomeric assembly. We conclude that SP-A modulates the cell surface expression of CR3 on alveolar macrophages, binds to CR3, and enhances CR3-mediated phagocytosis.

Histoplasma capsulatum cyclophilin A mediates attachment to dendritic cell VLA-5

Histoplasma capsulatum (Hc) is a pathogenic fungus that replicates in macrophages (Mphi). In dendritic cells (DC), Hc is killed and fungal Ags are processed and presented to T cells. DC recognize Hc yeasts via the VLA-5 receptor, whereas Mphi recognize yeasts via CD18. To identify ligand(s) on Hc recognized by DC, VLA-5 was used to probe a Far Western blot of a yeast freeze/thaw extract (F/TE) that inhibited Hc binding to DC. VLA-5 recognized a 20-kDa protein, identified as cyclophilin A (CypA), and CypA was present on the surface of Hc yeasts. rCypA inhibited the attachment of Hc to DC, but not to Mphi. Silencing of Hc CypA by RNA interference reduced yeast binding to DC by 65-85%, but had no effect on binding to Mphi. However, F/TE from CypA-silenced yeasts still inhibited binding of wild-type Hc to DC, and F/TE from wild-type yeasts depleted of CypA also inhibited yeast binding to DC. rCypA did not further inhibit the binding of CypA-silenced yeasts to DC. Polystyrene beads coated with rCypA or fibronectin bound to DC and Mphi and to Chinese hamster ovary cells transfected with VLA-5. Binding of rCypA-coated beads, but not fibronectin-coated beads, was inhibited by rCypA. These data demonstrate that CypA serves as a ligand for DC VLA-5, that binding of CypA to VLA-5 is at a site different from FN, and that there is at least one other ligand on the surface of Hc yeasts that mediates binding of Hc to DC.

Stage-specific sampling by pattern recognition receptors during Candida albicans phagocytosis

Candida albicans is a medically important pathogen, and recognition by innate immune cells is critical for its clearance. Although a number of pattern recognition receptors have been shown to be involved in recognition and phagocytosis of this fungus, the relative role of these receptors has not been formally examined. In this paper, we have investigated the contribution of the mannose receptor, Dectin-1, and complement receptor 3; and we have demonstrated that Dectin-1 is the main non-opsonic receptor involved in fungal uptake. However, both Dectin-1 and complement receptor 3 were found to accumulate at the site of uptake, while mannose receptor accumulated on C. albicans phagosomes at later stages. These results suggest a potential role for MR in phagosome sampling; and, accordingly, MR deficiency led to a reduction in TNF-α and MCP-1 production in response to C. albicans uptake. Our data suggest that pattern recognition receptors sample the fungal phagosome in a sequential fashion.

Infection by Candida albicans has increased as a result of immunosuppression associated with AIDS and organ transplantation. We assessed the role of three pattern recognition receptors, namely Dectin-1 (a beta glucan receptor), the type 3 complement receptor (CR3), and the mannose receptor, in mediating uptake of this fungus. These receptors are known to recognize structures on the C. albicans cell wall, but their exact contribution to binding and uptake is still unclear. We show that only Dectin-1 plays a major role in binding and uptake of C. albicans. Furthermore, we are the first to find that these receptors sample the internalized particle in a sequential manner; intracellular mannose receptor is recruited later and is involved in secretion of immune modulators. These findings provide a better understanding of the innate immune mechanisms involved in protection against this medically important fungal pathogen.

CD43 controls the intracellular growth of Mycobacterium tuberculosis through the induction of TNF-alpha-mediated apoptosis

Establishment of Tuberculosis infection begins with the successful entry and survival of the pathogen within macrophages. We previously showed that macrophage CD43 is required for optimal uptake and growth inhibition of Mycobacterium tuberculosis both in vitro and in vivo. Here, we explore the mechanisms by which CD43 restricts mycobacterial growth in murine macrophages. We found that although M. tuberculosis grows more readily in resting CD43-/- macrophages, priming of cells with IFN-gamma returns the bacterial growth rate to that seen in CD43+/+ cells. To discern the mechanisms by which M. tuberculosis exhibits enhanced growth within resting CD43-/- macrophages, we assessed the induction of inflammatory mediators in response to infection. We found that absence of CD43 resulted in reduced production of TNF-alpha, IL-12 and IL-6 by M. tuberculosis-infected macrophages. We also found that infected resting, but not activated CD43-/- macrophages, showed decreased apoptosis and increased necrosis. Exogenous addition of the pro-inflammatory cytokine TNF-alpha restored control of M. tuberculosis growth and induction of apoptosis to CD43+/+ levels. We propose that CD43 is involved in the inflammatory response to M. tuberculosis and, through the induction of pro-inflammatory mediators, can regulate apoptosis to control intracellular growth of the bacterium.

Mycobacterium paratuberculosisand the bovine immune system

Mycobacterium aviumsubspeciesparatuberculosis(M. paratuberculosis) is the causative agent of Johne’s disease, a deadly intestinal ailment of ruminants. Johne’s disease is of tremendous economic importance to the worldwide dairy industry, causing major losses due to reduced production and early culling of animals. A highly controversial but developing link between exposure toM. paratuberculosisand human Crohn’s disease in some individuals has led to the suggestion thatM. paratuberculosisis also a potential food safety concern. As with many other mycobacteria,M. paratuberculosisis exquisitely adapted to survival in the host, despite aggressive immune reactions to these organisms. One hallmark of mycobacteria, includingM. paratuberculosis, is their propensity to infect macrophages. Inside the macrophage,M. paratuberculosisinterferes with the maturation of the phagosome by an unknown mechanism, thereby evading the host’s normal first line of defense against bacterial pathogens. The host immune system begins a series of attacks againstM. paratuberculosis-infected macrophages, including the rapid deployment of activated γδ T cells, CD4+T cells and cytolytic CD8+T cells. These cells interact with the persistently infected macrophage and with each other through a complex network of cytokines and receptors. Despite these aggressive efforts to clear the infection,M. paratuberculosispersists and the constant struggle of the immune system leads to pronounced damage to the intestinal epithelial cells. Enhancing our ability to control this important and tenacious pathogen will require a deeper understanding of howM. paratuberculosisinterferes with macrophage action, the cell types involved in the immune response, the cytokines these cells use to communicate, and the host genetic factors that control the response to infection.

CD43 is required for optimal growth inhibition of Mycobacterium tuberculosis in macrophages and in mice

We explored the role of macrophage (Mphi) CD43, a transmembrane glycoprotein, in the pathogenesis of Mycobacterium tuberculosis. Using gene-deleted mice (CD43-/-), we assessed the association of the bacterium with distinct populations of Mphi and found that CD43-/- Mphi bound less M. tuberculosis than CD43+/+ Mphi. Increased infective doses did not abrogate this difference. However, reduced association due to the absence of CD43 could be overcome by serum components. Mphi from heterozygote mice, which express 50% of wild-type CD43, bound more bacteria than CD43-/- but less than CD43+/+, proving that the gene dose of CD43 correlates with binding of M. tuberculosis. Furthermore, the reduced ability of CD43-/- Mphi to bind bacteria was restricted to mycobacterial species. We also found that the survival and replication of M. tuberculosis within Mphi was enhanced significantly in the absence of CD43, making this the first demonstration that the mechanism of mycobacterial entry influences its subsequent growth. Most importantly, we show here that the absence of CD43 in mice aerogenically infected with M. tuberculosis results in an increased bacterial load during both the acute and chronic stages of infection and more rapid development of granulomas, with greater lung involvement and distinctive cellularity.

Absence of complement receptor 3 results in reduced binding and ingestion of Mycobacterium tuberculosis but has no significant effect on the induction of reactive oxygen and nitrogen intermediates or on the survival of the bacteria in resident and interferon-gamma activated macrophages

The interaction of host macrophage (Mphi) and Mycobacterium tuberculosis (Mtb) is mediated by cell surface receptors and is important in establishing intracellular infection. Mphis can kill invading organisms via reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI). Using a Complement Receptor 3 (CR3) knockout mouse model we have examined whether the presence of CR3 affects the binding and uptake of viable Mtb by Mphis, the survival of the ingested bacteria and the induction of ROI and RNI during this interaction. We show that, although CR3 plays a role in the uptake of viable Mtb, the receptor plays no role in the subsequent survival of the bacteria. The finding holds true for resident Mphis and for interferon-gamma (IFN-gamma) activated Mphis, both in the absence and presence of serum opsonins. Activation of Mphi populations with IFN-gamma significantly inhibits the growth of Mtb in host Mphis and enhances the production of ROI and RNI. However, the presence of CR3 was not critical in any of these mechanisms. Furthermore, we demonstrate that the control of intracellular growth of Mtb in IFN-gamma activated Mphis is not mediated by a direct effect of RNI.

The glycan-rich outer layer of the cell wall of Mycobacterium tuberculosis acts as an antiphagocytic capsule limiting the association of the bacterium with macrophages

Mycobacterium tuberculosis, the causative agent of tuberculosis, is a facultative intracellular pathogen that infects macrophages and other host cells. We show that sonication of M. tuberculosis results in the removal of material from the surface capsule-like layer of the bacteria, resulting in an enhanced propensity of the bacteria to bind to macrophages. This effect is observed with disparate murine and human macrophage populations though, interestingly, not with freshly explanted alveolar macrophages. Enhanced binding to macrophages following sonication is significantly greater within members of the M. tuberculosis family (pathogens) than within the Mycobacterium avium complex (opportunistic pathogens) or for Mycobacterium smegmatis (saprophyte). Sonication does not affect the viability or the surface hydrophobicity of M. tuberculosis but does result in changes in surface charge and in the binding of mannose-specific lectins to the bacterial surface. The increased binding of sonicated M. tuberculosis was not mediated through complement receptor 3. These results provide evidence that the surface capsule on members of the M. tuberculosis family may be an important virulence factor involved in the survival of M. tuberculosis in the mammalian host. They also question the view that M. tuberculosis is readily ingested by any macrophage it encounters and support the contention that M. tuberculosis, like many other microbial pathogens, has an antiphagocytic capsule that limits and controls the interaction of the bacterium with macrophages.

Macrophage--Mycobacterium tuberculosis interactions: role of complement receptor 3

Tuberculosis is the leading infectious disease in the world. Mycobacterium tuberculosis, the causal agent of this disease, invades macrophages and can replicate inside them. Because invasion of macrophages is a critical step for establishing a mycobacterial infection, there is much interest in understanding the mechanisms for M. tuberculosis entry into macrophages. Complement receptor 3 (CR3) is a heterodimeric surface receptor with multiple binding sites, which can mediate complement-opsonized as well as nonopsonic entrance of M. tuberculosis into macrophages. Here, we describe and discuss the role of CR3 in macrophage[bond]M. tuberculosis interactions. The actual information suggests that CR3 mediates a substantial amount of M. tuberculosis binding to macrophages, but CR3 is not related to the mechanisms that allow mycobacteria to survive and replicate intracellularly. Understanding the mechanisms of macrophage[bond]M. tuberculosis interaction will help developing more effective methods to prevent and treat tuberculosis in the future.

The fibrinolytic system in dissemination and matrix protein deposition during a mycobacterium infection

The fibrinolytic system is known to play an important role in the inflammatory response to bacterial infections. In the present study, relationships between protein components of the fibrinolytic system and infectivity by Mycobacterium avium were analyzed. Infections were initiated through noninvasive intratracheal administration of M. avium 724 in mice individually deficient for plasminogen, tissue-type plasminogen activator, urokinase-type plasminogen activator, and urokinase-type plasminogen activator receptor, along with wild-type control mice. There were no differences in lung colony counts among all mouse genotypes throughout a 10-week infection. However, in tissue-type plasminogen activator and plasminogen-deficient mice an earlier dissemination of M. avium to other organs was observed. Nevertheless, the M. avium growth rates in the liver, spleen, and lung did not differ between the various mouse populations throughout a 10-week infection. Histochemical and immunohistochemical analyses at 5 and 10 weeks after infection demonstrated that plasminogen-deficient mice, compared to wild-type mice, had enhanced fibrin and fibronectin deposition, as well as increased neutrophil infiltration within liver granulomas. These results suggest that plasmin(ogen) plays a role in the turnover of extracellular matrix proteins within granulomas and has a limited effect in the early dissemination of M. avium from lungs. Thus, plasmin(ogen) functions in limiting progressive fibrosis in the granuloma during a chronic mycobacterial infection.

Dectin-1 is a major beta-glucan receptor on macrophages

Zymosan is a beta-glucan- and mannan-rich particle that is widely used as a cellular activator for examining the numerous responses effected by phagocytes. The macrophage mannose receptor (MR) and complement receptor 3 (CR3) have historically been considered the major macrophage lectins involved in the nonopsonic recognition of these yeast-derived particles. Using specific carbohydrate inhibitors, we show that a beta-glucan receptor, but not the MR, is a predominant receptor involved in this process. Furthermore, nonopsonic zymosan binding was unaffected by genetic CD11b deficiency or a blocking monoclonal antibody (mAb) against CR3, demonstrating that CR3 was not the beta-glucan receptor mediating this activity. To address the role of the recently described beta-glucan receptor, Dectin-1, we generated a novel anti-Dectin-1 mAb, 2A11. Using this mAb, we show here that Dectin-1 was almost exclusively responsible for the beta-glucan-dependent, nonopsonic recognition of zymosan by primary macro-phages. These findings define Dectin-1 as the leukocyte beta-glucan receptor, first described over 50 years ago, and resolves the long-standing controversy regarding the identity of this important molecule. Furthermore, these results identify Dectin-1 as a new target for examining the immunomodulatory properties of beta-glucans for therapeutic drug design.

Role of complement in Mycobacterium avium pathogenesis: in vivo and in vitro analyses of the host response to infection in the absence of complement component C3

We investigated the importance of the host complement system in the pathogenesis of disease mediated by the intramacrophage pathogen Mycobacterium avium. Mycobacteria opsonized with complement are efficiently ingested by macrophages through various complement receptors. Furthermore, unlike other bacteria, mycobacteria can activate both the alternative and classical complement pathways in the absence of specific antibodies. Therefore, to examine the role of complement in the mycobacterial infection process in vivo, mice deficient in complement component C3 were infected with M. avium. Surprisingly, C3-deficient mice infected intravenously with M. avium displayed no difference in bacterial burden or granulomatous response compared to wild-type control mice. C3-sufficient mice and C3-deficient mice were equally susceptible to infection by M. avium regardless of the genotype at the bcg locus, a locus known to confer susceptibility to infection with intracellular pathogens. In vitro studies using mouse bone marrow-derived macrophages resulted in significant M. avium invasion of macrophages in the absence of C3; however, the kinetics of infection were delayed compared to complement-mediated invasion. The data indicate that complement does not play an essential role in mediating M. avium infections in the mouse and suggest either that other invasion mechanisms can compensate for the absence of complement-mediated entry or that complement is not a major mycobacterial opsonin in vivo.

Mycobacterial protein HbhA binds human complement component C3

Mycobacterium tuberculosis and Mycobacterium avium are facultative intracellular pathogens that are able to survive and replicate in mononuclear phagocytes. Human complement component C3 has previously been shown to mediate attachment and phagocytosis of these bacteria by mononuclear phagocytes. In this study, a C3 ligand affinity blot protocol was used to identify a 30-kDa C3-binding protein in M. tuberculosis and Mycobacterium smegmatis and a 31-kDa C3-binding protein in M. avium. The C3-binding proteins in M. tuberculosis and M. avium localized to the cell membrane fraction and partitioned to the detergent fraction during Triton X-114 phase partitioning. The C3-binding protein from M. tuberculosis was partially purified using a cation exchange column and was shown to bind concanavalin A. The N terminus and an internal fragment of the partially purified C3-binding protein were subjected to amino acid sequence analysis. The resulting amino acid sequences matched the M. tuberculosis heparin-binding hemagglutinin (HbhA) protein. Recombinant full-length HbhA and the C terminus of HbhA fused to maltose-binding protein, but not recombinant HbhA lacking the C-terminal region, bound human C3. Recombinant full-length HbhA coated on polystyrene beads, was found to enhance the adherence and/or phagocytosis of the coated beads to J774.A1 cells in both the presence and absence of human serum. The presence of complement-sufficient serum increased the adherence of the HbhA-coated beads to the J774.A1 cells in a C3-dependent manner. If HbhA within the bacterial cell membrane functions similarly to isolated HbhA, this protein may enhance the adherence and phagocytosis of M. tuberculosis and M. avium to mononuclear phagocytes through the binding of C3 and interaction with C3 receptors on mononuclear phagocytes.