Olfa Debbeche

Antiviral NK cell responses in HIV infection: I. NK cell receptor genes as determinants of HIV resistance and progression to AIDS

NK cells play an important role in controlling viral infections. They can kill virus‐infected cells directly as well as indirectly via antibody‐dependent, cell‐mediated cytotoxicity. They need no prior sensitization and expansion for this killing. NK cells are also considered as important regulators of antiviral immune responses. They do so by secreting a multitude of soluble mediators and by directly interacting with other immune cells, e.g., dendritic cells. NK cells do not possess a single well‐defined receptor to recognize antigens on target cells. Instead, they express an array of inhibitory and activating receptors and coreceptors, which bind to their cognate ligands expressed on the surface of target cells. These ligands include classical and nonclassical MHC class I antigens, MHC‐like proteins, and a variety of other self‐ and virus‐derived molecules. They may be expressed constitutively and/or de novo on the surface of virus‐infected cells. NK cell receptors (NKRs) of the killer‐cell Ig‐like receptor (KIR) family, like their MHC class I ligands, are highly polymorphic. Several recent studies suggest that epistatic interactions between certain KIR and MHC class I genes may determine innate resistance of the host to viral infections, including HIV. In the first part of this review article, we provide an overview of the current state of knowledge of NK cell immunobiology and describe how NKR genes, alone and in combination with HLA genes, may determine genetic resistance/susceptibilty to HIV infection and the development of AIDS in humans.

Viral strategies for evading antiviral cellular immune responses of the host

The host invariably responds to infecting viruses by activating its innate immune system and mounting virus‐specific humoral and cellular immune responses. These responses are aimed at conrolling viral replication and eliminating the infecting virus from the host. However, viruses have evolved numerous strategies to counter and evade hosts antiviral responses. Providing specific examples from the published literature, we discuss in this review article various strategies that viruses have developed to evade antiviral cellular responses of the host. Unraveling these viral strategies allows a better understanding of the host‐pathogen interactions and their coevolution. This knowledge is important for identifying novel molecular targets for developing antiviral reagents. Finally, it may also help devise new knowledge‐based strategies for developing antiviral vaccines.

Antiviral NK cell responses in HIV infection: II. viral strategies for evasion and lessons for immunotherapy and vaccination

As is the case in other viral infections, humans respond to HIV infection by activating their NK cells. However, the virus uses several strategies to neutralize and evade the host’s NK cell responses. Consequently, it is not surprising that NK cell functions become compromised in HIV‐infected individuals in early stages of the infection. The compromised NK cell functions also adversely affect several aspects of the host’s antiviral adaptive immune responses. Researchers have made significant progress in understanding how HIV counters NK cell responses of the host. This knowledge has opened new avenues for immunotherapy and vaccination against this infection. In the first part of this review article, we gave an overview of our current knowledge of NK cell biology and discussed how the genes encoding NK cell receptors and their ligands determine innate genetic resistance/susceptibilty of humans against HIV infections and AIDS. In this second part, we discuss NK cell responses, viral strategies to counter these responses, and finally, their implications for anti‐HIV immunotherapy and vaccination.

Novel associations between activating killer-cell immunoglobulin-like receptor genes and childhood leukemia

Acute lymphoblastic leukemia of pre-B cells (pre-B ALL) is the most frequent form of leukemia affecting children in Western countries. Evidence is accumulating that genetic factors play an important role in conferring susceptibility/resistance to leukemia in children. In this regard, activating killer-cell immunoglobulin-like receptor (KIR) genes are of particular interest. Humans may inherit different numbers of the 6 distinct activating KIR genes. Little is known about the impact of this genetic variation on the innate susceptibility or resistance of humans to the development of B-ALL. We addressed this issue by performing a case-control study in Canadian children of white origin. Our results show that harboring activating KIR genes is associated with reduced risk for developing B-ALL in these children. Of the 6 activating KIR genes, KIR2DS2 was maximally associated with decreased risk for the disease (P = 1.14 × 10(-7)). Furthermore, our results showed that inheritance of a higher number of activating KIR genes was associated with significant reductions in risk for ALL in children. These results were also consistent across different ALL phenotypes, which included children with pre-T cell ALL. Our study provides novel insights concerning the pathogenesis of childhood leukemia in white children and has implications for the development of new immunotherapies for this cancer.

Potential Role of Interleukin-18 in the Immunopathogenesis of AIDS: Involvement in Fratricidal Killing of NK Cells

ABSTRACT We had shown earlier that the concentrations of circulating interleukin-18 (IL-18) are increased significantly in human immunodeficiency virus (HIV)-infected persons compared to HIV-seronegative healthy subjects. In the present study, we investigated the consequences of these elevated levels of IL-18 on natural killer (NK) cells and the immunopathogenesis of AIDS. We show here an inverse correlation between IL-18 concentrations and absolute numbers of various subsets of NK cells in infected persons. Recombinant human IL-18 caused increased death of a human NK cell line, as well as of primary human NK cells in vitro. The IL-18-mediated cell death was dependent upon Fas-FasL interactions and tumor necrosis factor alpha. IL-18 induced the expression of FasL on NK cells, increased the transcription from the human FasL promoter, reduced the expression of Bcl-XL in NK cells, and increased their sensitivity to FasL-mediated cell death. These results suggest that increased IL-18 concentrations present in the circulation of HIV-infected persons contribute to the immunopathogenesis of AIDS by altering NK cell homeostasis.

Effects of the I559P gp41 Change on the Conformation and Function of the Human Immunodeficiency Virus (HIV-1) Membrane Envelope Glycoprotein Trimer

The mature human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer is produced by proteolytic cleavage of a precursor and consists of three gp120 exterior and three gp41 transmembrane subunits. The metastable Env complex is induced to undergo conformational changes required for virus entry by the binding of gp120 to the receptors, CD4 and CCR5/CXCR4. An isoleucine-to-proline change (I559P) in the gp41 ectodomain has been used to stabilize soluble forms of HIV-1 Env trimers for structural characterization and for use as immunogens. In the native membrane-anchored HIV-1BG505 Env, the I559P change modestly decreased proteolytic maturation, increased the non-covalent association of gp120 with the Env trimer, and resulted in an Env conformation distinctly different from that of the wild-type HIV-1BG505 Env. Compared with the wild-type Env, the I559P Env was recognized inefficiently by polyclonal sera from HIV-1-infected individuals, by several gp41-directed antibodies, by some antibodies against the CD4-binding site of gp120, and by antibodies that preferentially recognize the CD4-bound Env. Some of the gp120-associated antigenic differences between the wild-type HIV-1BG505 Env and the I559P mutant were compensated by the SOS disulfide bond between gp120 and gp41, which has been used to stabilize cleaved soluble Env trimers. Nonetheless, regardless of the presence of the SOS changes, Envs with proline 559 were recognized less efficiently than Envs with isoleucine 559 by the VRC01 neutralizing antibody, which binds the CD4-binding site of gp120, and the PGT151 neutralizing antibody, which binds a hybrid gp120-gp41 epitope. The I559P change completely eliminated the ability of the HIV-1BG505 Env to mediate cell-cell fusion and virus entry, and abolished the capacity of the SOS Env to support virus infection in the presence of a reducing agent. These results suggest that differences exist between the quaternary structures of functional Env spikes and I559P Envs.

Herpes Simplex Virus Type 1-induced FasL Expression in Human Monocytic Cells and Its Implications for Cell Death, Viral Replication, and Immune Evasion

Herpes simplex virus type 1 (HSV-1) is a ubiquitously occurring pathogen that infects humans early in childhood. The virus persists as a latent infection in dorsal root ganglia, especially of the trigeminal nerve, and frequently becomes reactivated in humans under conditions of stress. Monocytic cells constitute an important component of the innate and adaptive immune responses. We show here for the first time that HSV-1 stimulates human FasL promoter and induces de novo expression of FasL on the surface of human monocytic cells, including monocytes and macrophages. This virus-induced FasL expression causes death of monocytic cells growing in suspension, but not in monolayers (e.g., macrophages). The addition of a broad-spectrum caspase inhibitor, as well as anti-FasL antibodies, reduced cell death but increased viral replication in the virus-infected cell cultures. We also show here for the first time that the virus-induced de novo expression of FasL on the cell surface acts as an immune evasion mechanism by causing the death of interacting human CD4+ T cells, CD8+ T cells, and natural killer (NK) cells. Our study provides novel insights on FasL expression and cell death in HSV-infected human monocytic cells and their impact on interacting immune cells.

Potential role of IL-18 in the immunopathogenesis of AIDS, HIV-associated lipodystrophy and related clinical conditions.

IL-18 is a pleiotropic and multifunctional proinflammatory cytokine that is often produced in response to a viral infection. The biological activities of the cytokine are tightly controlled by its natural antagonist, IL-18 binding protein (IL-18BP), as well as by activation of caspase-1, which cleaves the precursor form of IL-18 into its biologically mature form. The cytokine plays an important role in both innate and adaptive antiviral immune responses. Depending upon the context, it can promote TH1, TH2 and TH17 responses. Increased serum concentrations of IL-18 and concomitantly decreased concentrations of its natural antagonist have been described in HIV-infected persons as compared to HIV-seronegative healthy subjects. We discuss in this review article how increased biological activities of IL-18 contribute towards immunopathogenesis of AIDS, HIV-associated lipodystrophy syndrome and related metabolic disturbances. While the advent of potent anti-HIV drugs has significantly enhanced life span of HIV-infected patients, it has also increased the number of these patients suffering from metabolic disorders. The cytokine may prove to be a useful target for therapeutic intervention in these patients.

A highly conserved gp120 inner domain residue modulates Env conformation and trimer stability

ABSTRACT Previous studies have shown that highly conserved residues in the inner domain of gp120 are required for HIV-1 envelope glycoprotein (Env) transitions to the CD4-bound conformation (A. Finzi, S. H. Xiang, B. Pacheco, L. Wang, J. Haight, et al., Mol Cell 37:656–667, 2010, http://dx.doi.org/10.1016/j.molcel.2010.02.012; A. Desormeaux, M. Coutu, H. Medjahed, B. Pacheco, A. Herschhorn, et al., J Virol 87:2549–2562, 2013, http://dx.doi.org/10.1128/JVI.03104-12). Moreover, W69, a highly conserved residue located at the interface between layer 1 and layer 2 of the inner domain, was recently shown to be important for efficient Env recognition by CD4-induced (CD4i) antibodies capable of potent antibody-dependent cellular cytotoxicity (W. D. Tolbert, N. Gohain, M. Veillette, J. P. Chapleau, C. Orlandi, et al., 2016, Structure 24:697–709, http://dx.doi.org/10.1016/j.str.2016.03.005; S. Ding, M. Veillette, M. Coutu, J. Prevost, L. Scharf, et al., 2016, J Virol 90:2127–2134, http://dx.doi.org/10.1128/JVI.02779-15). We evaluated the contribution of the hydrophobicity of W69 to conformational changes of Env by replacing it with a series of residues with aliphatic or aromatic side chains of decreasing chain length. We have found that the hydrophobicity of residue 69 is important for Env processing, CD4 binding, and its transition to the CD4-bound conformation. The most deleterious effect was observed when W69 was replaced with alanine or glycine residues. However, the functions lost due to W69 mutations could be progressively restored with amino acids of increasing aliphatic chain length and fully recovered with residues bearing an aromatic ring. Interestingly, poor CD4 binding of W69A could be fully restored by introducing a compensatory mutation within layer 2 (S115W). Structural studies of HIV-1 gp120 coree W69A/S115W mutant bound to the CD4 peptide mimetic M48U1 and Fab of anti-cluster A antibody N60-i3 revealed no perturbations to the overall structure of the double mutant compared to the wild-type protein but identified higher mobility within the interface between layer 1 and layer 2, the bridging sheet region, and the CD4 binding site. IMPORTANCE HIV-1 Env transitions to the CD4-bound conformation are required for viral entry. Previous studies identified a highly conserved residue of the inner domain, W69, as being involved in these conformational transitions (A. Finzi, S. H. Xiang, B. Pacheco, L. Wang, J. Haight, et al., Mol Cell 37:656–667, 2010, http://dx.doi.org/10.1016/j.molcel.2010.02.012). Here, we show that W69, located at the interface between gp120 and gp41 in the PGT151-bound trimer, plays a critical role in the interprotomer signaling induced by CD4 binding. This new information might be useful in immunogen design.

Residues in the gp41 Ectodomain Regulate HIV-1 Envelope Glycoprotein Conformational Transitions Induced by gp120-Directed Inhibitors

ABSTRACT Interactions between the gp120 and gp41 subunits of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer maintain the metastable unliganded form of the viral spike. Binding of gp120 to the receptor, CD4, changes the Env conformation to promote gp120 interaction with the second receptor, CCR5 or CXCR4. CD4 binding also induces the transformation of Env into the prehairpin intermediate, in which the gp41 heptad repeat 1 (HR1) coiled coil is assembled at the trimer axis. In nature, HIV-1 Envs must balance the requirements to maintain the noncovalent association of gp120 with gp41 and to evade the host antibody response with the need to respond to CD4 binding. Here we show that the gp41 HR1 region contributes to gp120 association with the unliganded Env trimer. Changes in particular amino acid residues in the gp41 HR1 region decreased the efficiency with which Env moved from the unliganded state. Thus, these gp41 changes decreased the sensitivity of HIV-1 to cold inactivation and ligands that require Env conformational changes to bind efficiently. Conversely, these gp41 changes increased HIV-1 sensitivity to small-molecule entry inhibitors that block Env conformational changes induced by CD4. Changes in particular gp41 HR1 amino acid residues can apparently affect the relative stability of the unliganded state and CD4-induced conformations. Thus, the gp41 HR1 region contributes to the association with gp120 and regulates Env transitions from the unliganded state to downstream conformations. IMPORTANCE The development of an efficient vaccine able to prevent HIV infection is a worldwide priority. Knowledge of the envelope glycoprotein structure and the conformational changes that occur after receptor engagement will help researchers to develop an immunogen able to elicit antibodies that block HIV-1 transmission. Here we identify residues in the HIV-1 transmembrane envelope glycoprotein that stabilize the unliganded state by modulating the transitions from the unliganded state to the CD4-bound state.