Christina Guzzo

The IL-12 Family of Cytokines in Infection, Inflammation and Autoimmune Disorders

Cytokines are critical coordinators of the immune response necessary for resolving bacterial and viral assaults on the immune system. In particular, the IL-12 family of cytokines are key players in the regulation of T cell responses. These responses are orchestrated by monocytes, macrophages, and dendritic cells which produce the members of the IL-12 family of cytokines in response to infection. IL-27 and IL-23 are two cytokines that are related to IL-12; these cytokines share homology at the subunit, receptor, and signalling levels. IL-12 is composed of p35 and p40 subunits, which, when combined together form the bioactive IL-12p70. IL-23 is composed of the IL-12p40 subunit as well as the IL-23p19 subunit, which shares homology with IL-12p35. IL-27 is composed of EBI3 and p28. These three cytokines activate similar members of the JAK/STAT signalling pathways as a result of homology in their receptor components. Production of these cytokines by activated monocytes, macrophages, and dendritic cells results in the activation and differentiation of T cells. In spite of their similarity, each of these cytokines has specific roles in the regulation of immune responses. IL-12 is required for the induction of IFN-gamma production, critical for the induction of Th1 cells. IL-27 has been shown to play a role in the induction of Th1 cells from naive T cells, whereas IL-23 has been demonstrated to play a key role in the induction of the newly described Th17 cells. Recently, a novel heterodimeric and anti-inflammatory cytokine composed of the IL-12p35 and EBI3 subunits has been identified as IL-35. The biological properties of the IL-12 family of cytokines, the signalling pathways mediated by these cytokines and their role in infection, inflammation, and autoimmune diseases will be the focus of this review.

IL-27 Enhances LPS-Induced Proinflammatory Cytokine Production via Upregulation of TLR4 Expression and Signaling in Human Monocytes

IL-27, which is produced by activated APCs, bridges innate and adaptive immunity by regulating the development of Th cells. Recent evidence supports a role for IL-27 in the activation of monocytic cells in terms of inflammatory responses. Indeed, proinflammatory and anti-inflammatory activities are attributed to IL-27, and IL-27 production itself is modulated by inflammatory agents such as LPS. IL-27 primes LPS responses in monocytes; however, the molecular mechanism behind this phenomenon is not understood. In this study, we demonstrate that IL-27 priming results in enhanced LPS-induced IL-6, TNF-α, MIP-1α, and MIP-1β expression in human primary monocytes. To elucidate the molecular mechanisms responsible for IL-27 priming, we measured levels of CD14 and TLR4 required for LPS binding. We determined that IL-27 upregulates TLR4 in a STAT3- and NF-κB–dependent manner. Immunofluorescence microscopy revealed enhanced membrane expression of TLR4 and more distinct colocalization of CD14 and TLR4 upon IL-27 priming. Furthermore, IL-27 priming enhanced LPS-induced activation of NF-κB family members. To our knowledge, this study is the first to show a role for IL-27 in regulating TLR4 expression and function. This work is significant as it reveals new mechanisms by which IL-27 can enhance proinflammatory responses that can occur during bacterial infections.

Neu1 sialidase and matrix metalloproteinase-9 cross-talk is essential for TOLL-like receptor activation and cellular signaling

The signaling pathways of mammalian Toll-like receptors (TLRs) are well characterized, but the precise mechanism(s) by which TLRs are activated upon ligand binding remains poorly defined. Recently, we reported a novel membrane sialidase-controlling mechanism that depends on ligand binding to its TLR to induce mammalian neuraminidase-1 (Neu1) activity, to influence receptor desialylation, and subsequently to induce TLR receptor activation and the production of nitric oxide and proinflammatory cytokines in dendritic and macrophage cells. The α-2,3-sialyl residue of TLR was identified as the specific target for hydrolysis by Neu1. Here, we report a membrane signaling paradigm initiated by endotoxin lipopolysaccharide (LPS) binding to TLR4 to potentiate G protein-coupled receptor (GPCR) signaling via membrane Gαi subunit proteins and matrix metalloproteinase-9 (MMP9) activation to induce Neu1. Central to this process is that a Neu1-MMP9 complex is bound to TLR4 on the cell surface of naive macrophage cells. Specific inhibition of MMP9 and GPCR Gαi-signaling proteins blocks LPS-induced Neu1 activity and NFκB activation. Silencing MMP9 mRNA using lentivirus MMP9 shRNA transduction or siRNA transfection of macrophage cells and MMP9 knock-out primary macrophage cells significantly reduced Neu1 activity and NFκB activation associated with LPS-treated cells. These findings uncover a molecular organizational signaling platform of a novel Neu1 and MMP9 cross-talk in alliance with TLR4 on the cell surface that is essential for ligand activation of TLRs and subsequent cellular signaling.

Impact of HIV infection, highly active antiretroviral therapy, and hepatitis C coinfection on serum interleukin-27.

A newly described cytokine, interleukin-27 (IL-27), that activates naive CD4 T cells, has recently been shown to be an anti-HIV cytokine. However, the effect of HIV infection on IL-27 expression has not been characterized. We found that clinical characteristics, including HIV viral load, hepatitis C virus coinfection, and CD4 T cell counts, were associated with changes in serum IL-27. Overall, our results suggest circulating HIV may suppress IL-27, a critical concept in treatment development with this cytokine.

Quaternary contact in the initial interaction of CD4 with the HIV-1 envelope trimer

Binding of the gp120 envelope (Env) glycoprotein to the CD4 receptor is the first step in the HIV-1 infectious cycle. Although the CD4-binding site has been extensively characterized, the initial receptor interaction has been difficult to study because of major CD4-induced structural rearrangements. Here we used cryogenic electron microscopy (cryo-EM) to visualize the initial contact of CD4 with the HIV-1 Env trimer at 6.8-Å resolution. A single CD4 molecule is embraced by a quaternary HIV-1–Env surface formed by coalescence of the previously defined CD4-contact region with a second CD4-binding site (CD4-BS2) in the inner domain of a neighboring gp120 protomer. Disruption of CD4-BS2 destabilized CD4-trimer interaction and abrogated HIV-1 infectivity by preventing the acquisition of coreceptor-binding competence. A corresponding reduction in HIV-1 infectivity occurred after the mutation of CD4 residues that interact with CD4-BS2. Our results document the critical role of quaternary interactions in the initial HIV-Env-receptor contact, with implications for treatment and vaccine design.

Tyrosine sulfation in the second variable loop (V2) of HIV-1 gp120 stabilizes V2–V3 interaction and modulates neutralization sensitivity

Significance Despite intensive efforts, the structure of the native HIV-1 envelope trimer—the sole relevant target for vaccine design—has remained elusive. Our work identifies a key structural constraint that stabilizes the native envelope conformation and modulates its sensitivity to neutralization. We show that this constraint is established by previously unrecognized sulfated tyrosines within the second variable loop (V2) of the envelope glycoprotein subunit gp120, which mediate intramolecular interaction with the base of the third variable loop, V3. Strikingly, the V2 sulfotyrosines functionally mimic those present in the N terminus of the CCR5 coreceptor, which bind to the same V3 region. Our results shed light on the mechanisms adopted by HIV-1 to elude immunologic control and open new perspectives for vaccine design. Elicitation of broadly neutralizing antibodies is essential for the development of a protective vaccine against HIV-1. However, the native HIV-1 envelope adopts a protected conformation that conceals highly conserved sites of vulnerability from antibody recognition. Although high-definition structures of the monomeric core of the envelope glycoprotein subunit gp120 and, more recently, of a stabilized soluble gp140 trimer have been solved, fundamental aspects related to the conformation and function of the native envelope remain unresolved. Here, we show that the conserved central region of the second variable loop (V2) of gp120 contains sulfated tyrosines (Tys173 and Tys177) that in the CD4-unbound prefusion state mediate intramolecular interaction between V2 and the conserved base of the third variable loop (V3), functionally mimicking sulfated tyrosines in CCR5 and anti–coreceptor-binding-site antibodies such as 412d. Recombinant gp120 expressed in continuous cell lines displays low constitutive levels of V2 tyrosine sulfation, which can be enhanced markedly by overexpression of the tyrosyl sulfotransferase TPST2. In contrast, virion-associated gp120 produced by primary CD4+ T cells is inherently highly sulfated. Consistent with a functional role of the V2 sulfotyrosines, enhancement of tyrosine sulfation decreased binding and neutralization of HIV-1 BaL by monomeric soluble CD4, 412d, and anti-V3 antibodies and increased recognition by the trimer-preferring antibodies PG9, PG16, CH01, and PGT145. Conversely, inhibition of tyrosine sulfation increased sensitivity to soluble CD4, 412d, and anti-V3 antibodies and diminished recognition by trimer-preferring antibodies. These results identify the sulfotyrosine-mediated V2–V3 interaction as a critical constraint that stabilizes the native HIV-1 envelope trimer and modulates its sensitivity to neutralization.

The CD8-Derived Chemokine XCL1/Lymphotactin Is a Conformation-Dependent, Broad-Spectrum Inhibitor of HIV-1

CD8+ T cells play a key role in the in vivo control of HIV-1 replication via their cytolytic activity as well as their ability to secrete non-lytic soluble suppressive factors. Although the chemokines that naturally bind CCR5 (CCL3/MIP-1α, CCL4/MIP- 1β, CCL5/RANTES) are major components of the CD8-derived anti-HIV activity, evidence indicates the existence of additional, still undefined, CD8-derived HIV-suppressive factors. Here, we report the characterization of a novel anti-HIV chemokine, XCL1/lymphotactin, a member of the C-chemokine family that is produced primarily by activated CD8+ T cells and behaves as a metamorphic protein, interconverting between two structurally distinct conformations (classic and alternative). We found that XCL1 inhibits a broad spectrum of HIV-1 isolates, irrespective of their coreceptor-usage phenotype. Experiments with stabilized variants of XCL1 demonstrated that HIV-1 inhibition requires access to the alternative, all-β conformation, which interacts with proteoglycans but does not bind/activate the specific XCR1 receptor, while the classic XCL1 conformation is inactive. HIV-1 inhibition by XCL1 was shown to occur at an early stage of infection, via blockade of viral attachment and entry into host cells. Analogous to the recently described anti-HIV effect of the CXC chemokine CXCL4/PF4, XCL1-mediated inhibition is associated with direct interaction of the chemokine with the HIV-1 envelope. These results may open new perspectives for understanding the mechanisms of HIV-1 control and reveal new molecular targets for the design of effective therapeutic and preventive strategies against HIV-1.

IL-27 increases BST-2 expression in human monocytes and T cells independently of type I IFN

IL-27 modulates inflammatory responses by influencing cytokine secretion and CD4 T cell differentiation. Recently, IL-27 was demonstrated to inhibit HIV replication by inducing type I interferon (IFN) expression and subsequent IFN-dependent expression of apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like (APOBEC)-3 family members, a group of antiviral cytidine deaminases. To characterize other anti-viral genes modulated by IL-27, we examined another IFN-responsive gene: tetherin/bone marrow stromal cell antigen 2 (BST-2). Our study shows that IL-27 can directly induce BST-2 expression, independently of an intermediary type I IFN response. Quantitative RT-PCR analysis demonstrated IL-27-induced BST-2 mRNA expression as early as 2h after exposure of cells to IL-27. In the presence of the type I IFN-neutralizing protein, B18R, IL-27-induced BST-2 expression was maintained, demonstrating that IFN is not an intermediary in IL-27-induced BST-2. Taken together, our findings identify a novel function of IL-27 as a direct stimulator of BST-2 expression.

The TLR2 agonists lipoteichoic acid and Pam3CSK4 induce greater pro-inflammatory responses than inactivated Mycobacterium butyricum.

The innate immune system can recognize pathogen-associated molecular patterns (PAMP) through toll-like receptors (TLRs). TLR stimulation by TLR-ligands (TLR-L) induces several genes that can regulate the immune response. In this study, we compared the ability of diverse TLR2-L to activate professional antigen presenting cells (pAPCs). We found that in comparison to whole non-replicating microorganism Mycobacterium butyricum, the smaller components; lipoteichoic acid and Pam3CSK4 significantly enhanced the expression of several pro-inflammatory mediators. These included IL-6, TNF-α and nitric oxide both at the mRNA and the protein levels. Moreover, the higher response was associated with a differential activation of nuclear transcription factor kappa-B (NF-κB) by the diverse TLR2-L. However, all three ligands enhanced antigen cross-presentation and T cell induction after virus infection to the same extent. In conclusion, the data highlight the potential for small components of TLR agonists to induce superior inflammatory immune responses than whole microbial preparation in the field of vaccine studies.

Virion incorporation of integrin α4β7 facilitates HIV-1 infection and intestinal homing

Anti–integrin α4β7 therapy might directly interfere with the ability of HIV to home to intestinal reserviors. Taking HIV to the gut Antiretroviral therapy (ART) effectively limits HIV replication, but HIV+ individuals are medicated for life because ART withdrawal results in rebound of persistent virus. Developing therapies that keep viral loads low in the long term and prevent reinfection remains an important goal—one emerging approach is an antibody against integrin α4β7. Integrin α4β7 is a receptor that facilitates homing of CD4+ T cells to the gut, a key site for HIV persistence. ART-suppressed macaques that received antibodies against integrin α4β7 controlled the virus even after ART withdrawal. Here, Guzzo et al. demonstrate that integrin α4β7 is incorporated into the HIV envelope, suggesting that antibody treatment may directly interfere with the ability of HIV to target intestinal tissues. Their results change our perception of the role of integrin α4β7, a promising therapeutic target in HIV pathogenesis. The intestinal mucosa is a key anatomical site for HIV-1 replication and CD4+ T cell depletion. Accordingly, in vivo treatment with an antibody to the gut-homing integrin α4β7 was shown to reduce viral transmission, delay disease progression, and induce persistent virus control in macaques challenged with simian immunodeficiency virus (SIV). We show that integrin α4β7 is efficiently incorporated into the envelope of HIV-1 virions. Incorporated α4β7 is functionally active as it binds mucosal addressin cell adhesion molecule–1 (MAdCAM-1), promoting HIV-1 capture by and infection of MAdCAM-expressing cells, which in turn mediate trans-infection of bystander cells. Functional α4β7 is present in circulating virions from HIV-infected patients and SIV-infected macaques, with peak levels during the early stages of infection. In vivo homing experiments documented selective and specific uptake of α4β7+ HIV-1 virions by high endothelial venules in the intestinal mucosa. These results extend the paradigm of tissue homing to a retrovirus and are relevant for the pathogenesis, treatment, and prevention of HIV-1 infection.