Complement receptor 3 (CR3): a public transducer of innate immunity signals in macrophages

Immunoregulatory function of SP-A

Surfactant protein A (SP-A), a natural immune molecule, plays an important role in lung health. SP-A recognizes and binds microbial surface glycogroups through the C-type carbohydrate recognition domain, and then binds corresponding cell surface receptors (such as C1qRp, CRT-CD91 complex, CD14, SP-R210, Toll-like receptor, SIRP-α, CR3, etc.) through collagen-like region, and subsequently mediates biological effects. SP-A regulates lung innate immunity by promoting surfactant absorption by alveolar type II epithelial cells and phagocytosis of pathogenic microorganisms by alveolar macrophages. SP-A also regulates lung adaptive immunity by inhibiting DC maturation, and T cell proliferation and differentiation. This article reviews latest relationships between SP-A and adaptive and intrinsic immunity.

Galectins Are Central Mediators of Immune Escape in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers and is highly immune tolerant. Although there is immune cell infiltration in PDAC tissues, most of the immune cells do not function properly and, therefore, the prognosis of PDAC is very poor. Galectins are carbohydrate-binding proteins that are intimately involved in the proliferation and metastasis of tumor cells and, in particular, play a crucial role in the immune evasion of tumor cells. Galectins induce abnormal functions and reduce numbers of tumor-associated macrophages (TAM), natural killer cells (NK), T cells and B cells. It further promotes fibrosis of tissues surrounding PDAC, enhances local cellular metabolism, and ultimately constructs tumor immune privileged areas to induce immune evasion behavior of tumor cells. Here, we summarize the respective mechanisms of action played by different Galectins in the process of immune escape from PDAC, focusing on the mechanism of action of Galectin-1. Galectins cause imbalance between tumor immunity and anti-tumor immunity by coordinating the function and number of immune cells, which leads to the development and progression of PDAC.

Complement Receptor 3 Mediates HIV-1 Transcytosis across an Intact Cervical Epithelial Cell Barrier: New Insight into HIV Transmission in Women

Transmission of HIV across the mucosal surface of the female reproductive tract to engage subepithelial CD4-positive T cells is not fully understood. Cervical epithelial cells express complement receptor 3 (CR3) (integrin αMβ2 or CD11b/CD18). In women, the bacterium Neisseria gonorrhoeae uses CR3 to invade the cervical epithelia to cause cervicitis. We hypothesized that HIV may also use CR3 to transcytose across the cervical epithelia. Here, we show that HIV-1 strains bound with high affinity to recombinant CR3 in biophysical assays. HIV-1 bound CR3 via the I-domain region of the CR3 alpha subunit, CD11b, and binding was dependent on HIV-1 N-linked glycans. Mannosylated glycans on the HIV surface were a high-affinity ligand for the I-domain. Man5 pentasaccharide, representative of HIV N-glycans, could compete with HIV-1 for CR3 binding. Using cellular assays, we show that HIV bound to CHO cells by a CR3-dependent mechanism. Antibodies to the CR3 I-domain or to the HIV-1 envelope glycoprotein blocked the binding of HIV-1 to primary human cervical epithelial (Pex) cells, indicating that CR3 was necessary and sufficient for HIV-1 adherence to Pex cells. Using Pex cells in a Transwell model system, we show that, following transcytosis across an intact Pex cell monolayer, HIV-1 is able to infect TZM-bl reporter cells. Targeting the HIV-CR3 interaction using antibodies, mannose-binding lectins, or CR3-binding small-molecule drugs blocked HIV transcytosis. These studies indicate that CR3/Pex may constitute an efficient pathway for HIV-1 transmission in women and also demonstrate strategies that may prevent transmission via this pathway. IMPORTANCE In women, the lower female reproductive tract is the primary site for HIV infection. How HIV traverses the epithelium to infect CD4 T cells in the submucosa is ill-defined. Cervical epithelial cells have a protein called CR3 on their surface. We show that HIV-1 binds to CR3 with high affinity and that this interaction is necessary and sufficient for HIV adherence to, and transcytosis across, polarized, human primary cervical epithelial cells. This suggests a unique role for CR3 on epithelial cells in dually facilitating HIV-1 attachment and entry. The HIV-CR3 interaction may constitute an efficient pathway for HIV delivery to subepithelial lymphocytes following virus transmission across an intact cervical epithelial barrier. Strategies with potential to prevent transmission via this pathway are presented.

Murine Macrophage Requires CD11b to Recognize Talaromyces marneffei

Introduction

Talaromyces marneffei (T. marneffei) is an emerging pathogenic fungus. Macrophage-1 antigen (Mac-1, CR3, CD11b/CD18) is an important receptor on innate immune cells and can recognize pathogens. However, the importance of CR3 in phagocytosis of T. marneffei by macrophages and their responses to T. marneffei have not been clarified.

Methods

We show that interaction of mouse peritoneal macrophages (pMacs) or RAW264.7 macrophages with T. marneffei of its conidia spores and yeast cells enhances CR3 expression on macrophages. The phagocytosis rate was determined using flow cytometry, RT-PCR and Western blotting were used to detect CD11b expression, and the levels of IFN-γ, TNF-α, IL-2, IL-4, IL-6 and IL-10 in the co-culture supernatants were determined by ELISA.

Results

Incubation of mouse macrophages with T. marneffei promoted phagocytosis of T. marneffei, which was dramatically mitigated by pretreatment with anti-CD11b antibody or knockdown of CR3 expression on macrophages. Then, interferon γ, tumor necrosis factor α, IL-4, IL-10 and IL-12 production in macrophages incubation with heat-killed T. marneffei was detected. CD11b expression on mouse macrophages was upregulated by T. marneffei. Incubation of T. marneffei promoted phagocytosis of T. marneffei by macrophages and high levels of pro-inflammatory and anti-inflammatory cytokine production by macrophages, which were mitigated and abrogated by pre-treatment with anti-CD11b or knockdown of CD11b expression.

Conclusion

These data indicated that murine macrophage requires CD11b to recognize Talaromyces marneffei and their cytokine responses to heat-killed T. marneffei in vitro.

Neisseria gonorrhoeae pilin glycan contributes to CR3 activation during challenge of primary cervical epithelial cells

Expression of type IV pili by Neisseria gonorrhoeae plays a critical role in mediating adherence to human epithelial cells. Gonococcal pilin is modified with an O-linked glycan, which may be present as a di- or monosaccharide because of phase variation of select pilin glycosylation genes. It is accepted that bacterial proteins may be glycosylated; less clear is how the protein glycan may mediate virulence. Using primary, human, cervical epithelial (i.e. pex) cells, we now provide evidence to indicate that the pilin glycan mediates productive cervical infection. In this regard, pilin glycan-deficient mutant gonococci exhibited an early hyper-adhesive phenotype but were attenuated in their ability to invade pex cells. Our data further indicate that the pilin glycan was required for gonococci to bind to the I-domain region of complement receptor 3, which is naturally expressed by pex cells. Comparative, quantitative, infection assays revealed that mutant gonococci lacking the pilin glycan did not bind to the I-domain when it is in a closed, low-affinity conformation and cannot induce an active conformation to complement receptor 3 during pex cell challenge. To our knowledge, these are the first data to directly demonstrate how a protein-associated bacterial glycan may contribute to pathogenesis.

A novel immunoprecipitation strategy identifies a unique functional mimic of the glial cell line-derived neurotrophic factor family ligands in the pathogen Trypanosoma cruzi

The journey of the Chagas' disease parasite Trypanosoma cruzi in the human body usually starts in the skin after an insect bite, when trypomastigotes get through the extracellular matrix to bind specific surface receptors in the epidermis and dermis to enter cells, where they differentiate and replicate. As the infection spreads to the heart, nervous system, and other parts of the body via the circulatory system, the parasite must also cope with additional receptors in the immune system and vascular endothelium. The molecular underpinnings that govern host cell receptor recognition by T. cruzi counterreceptors remain largely unknown. Here, we describe an immunoprecipitation strategy designed to concurrently identify host receptors and complementing parasite counterreceptors. Extracellular domains of growth factor receptors fused to human immunoglobulin G (IgG) Fc were incubated with parasite lysates, immunoprecipitated on protein G-Sepharose, and eluted with Laemmli sample buffer. Possible T. cruzi counterreceptors pulled down by the receptor-Fc bait were visualized on immunoblots probed with multispecific high-affinity IgG from chronic chagasic sera and on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels stained with silver or Coomassie blue. In screening receptors important for nervous system repair, this parasite counterreceptor immunoprecipitation (PcIP) assay identified 7 to 11 polypeptides (molecular masses, 14 kDa to 55 kDa) that bound to the coreceptors of glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) GFRalpha-1, -2, and -3. Binding was specific because the T. cruzi mimic of host GFLs, named TGFL, did not react with GFL coreceptor tyrosine kinase RET and with other neurotrophic receptors. The polypeptides were located on the parasite outer membrane and bound noncovalently to each other. TGFL eluted from the GFL receptor/protein G affinity column with 0.5 M NaCl, pH 7.5, and potently promoted neurite outgrowth and cell survival in a GFL-sensitive mouse pheochromocytoma cell line. Given that GFLs are neuron survival factors crucial for development and maintenance of central and peripheral nervous systems, it may be that T. cruzi mimicry of host GFLs helps in mutually beneficial host repair of infected and damaged nervous tissue. As there are >30 growth factor receptor-Fc chimeras commercially available, this PcIP assay can be readily adapted to identify receptors/counterreceptors in other T. cruzi invasion sites and in other infections such as Lyme disease, amebiasis, and schistosomiasis.

Receptors for complement C3 on T-lymphocytes: Relics of evolution or functional molecules?

Despite the fact that receptors for complement on T-cells have been described many years ago the function remains unclear as is the role of complement in the T-cell response. In this review we will evaluate how the accumulated wisdom concur with the current concepts of the adaptive T-cell response.

The CNS role of Toll-like receptor 4 in innate neuroimmunity and painful neuropathy

Neuropathic pain remains a prevalent and persistent clinical problem because of our incomplete understanding of its pathogenesis. This study demonstrates for the first time, to our knowledge, a critical role for CNS innate immunity by means of microglial Toll-like receptor 4 (TLR4) in the induction phase of behavioral hypersensitivity in a mouse and rat model of neuropathy. We hypothesized that after L5 nerve transection, CNS neuroimmune activation and subsequent cytokine expression are triggered by the stimulation of microglial membrane-bound TLR4. To test this hypothesis, experiments were undertaken to assess tactile and thermal hypersensitivity in genetically altered (i.e., TLR4 knockout and point-mutant) mice after L5 nerve transection. In a complementary study, TLR4 antisense oligodeoxynucleotide (ODN) was administered intrathecally to L5 spinal nerve injured rats to reduce the expression of spinal TLR4. Both the genetically altered mice and the rats treated with TLR4 antisense ODN displayed significantly attenuated behavioral hypersensitivity and decreased expression of spinal microglial markers and proinflammatory cytokines as compared with their respective control groups. This finding shows that TLR4 contributes to the initiation of CNS neuroimmune activation after L5 nerve transection. Further understanding of this early, specific, innate CNS/microglial response and how it leads to sustained glial/neuronal hypersensitivity may point to new therapies for the prevention and treatment of neuropathic pain syndromes.

Activation of complement components and reduced regulator expression in alcohol-induced liver injury in the rat

The purpose of this study was to evaluate the possible contribution of complement-mediated inflammation to the development of alcoholic liver disease. Male Wistar rats were fed ethanol by liquid diet in a model that results in continuous ethanol intoxication and induces early signs of alcoholic liver injury. After a six-week study period liver samples were analyzed for the deposition of complement components (C1, C3, and C8) and expression of cell membrane-bound regulators (Crry and CD59). Activation of the homologous complement system in vitro was tested by treating frozen liver sections with normal rat serum (NRS). Immunohistochemical analysis showed deposits of C8 in the liver sections of ethanol-treated rats. When frozen liver sections from these rats were treated with NRS, periportal deposition of both C3 and C8, but only slight C1 deposition, was observed. Immunohistochemical and Western blot analysis both revealed a reduced expression of the complement regulators Crry and CD59. These results suggest an induction of complement-activating capacity in the liver after chronic ethanol treatment. Lack of C1 deposition in the lesions suggests that complement activation occurs primarily via the alternative pathway. The reduced expression of the critical complement regulatory proteins Crry and CD59 may sensitize the liver to complement-mediated damage.

Phagosome maturation: aging gracefully

Foreign particles and apoptotic bodies are eliminated from the body by phagocytic leucocytes. The initial stage of the elimination process is the internalization of the particles into a plasma membrane-derived vacuole known as the phagosome. Such nascent phagosomes, however, lack the ability to kill pathogens or to degrade the ingested targets. These properties are acquired during the course of phagosomal maturation, a complex sequence of reactions that result in drastic remodelling of the phagosomal membrane and contents. The determinants and consequences of the fusion and fission reactions that underlie phagosomal maturation are the topic of this review.