Activation of NK cells by HIV-specific ADCC antibodies: role for granulocytes in expressing HIV-1 peptide epitopes

Influenza A Virus Antibodies with Antibody-Dependent Cellular Cytotoxicity Function

Influenza causes millions of cases of hospitalizations annually and remains a public health concern on a global scale. Vaccines are developed and have proven to be the most effective countermeasures against influenza infection. Their efficacy has been largely evaluated by hemagglutinin inhibition (HI) titers exhibited by vaccine-induced neutralizing antibodies, which correlate fairly well with vaccine-conferred protection. Contrarily, non-neutralizing antibodies and their therapeutic potential are less well defined, yet, recent advances in anti-influenza antibody research indicate that non-neutralizing Fc-effector activities, especially antibody-dependent cellular cytotoxicity (ADCC), also serve as a critical mechanism in antibody-mediated anti-influenza host response. Monoclonal antibodies (mAbs) with Fc-effector activities have the potential for prophylactic and therapeutic treatment of influenza infection. Inducing mAbs mediated Fc-effector functions could be a complementary or alternative approach to the existing neutralizing antibody-based prevention and therapy. This review mainly discusses recent advances in Fc-effector functions, especially ADCC and their potential role in influenza countermeasures. Considering the complexity of anti-influenza approaches, future vaccines may need a cocktail of immunogens in order to elicit antibodies with broad-spectrum protection via multiple protective mechanisms.

Monoclonal Antibody 2C6 Targets a Cross-Clade Conformational Epitope in gp41 with Highly Active Antibody-Dependent Cell Cytotoxicity

Previous studies in our laboratory characterized a panel of highly mutated HIV-specific conformational epitope-targeting antibodies (Abs) from a panel of HIV-infected long-term nonprogressors (LTNPs). Despite binding HIV envelope protein and having a high number of somatic amino acid mutations, these Abs had poor neutralizing activity. Because of the evidence of antigen-driven selection and the long CDR3 region (21 amino acids [aa]), we further characterized the epitope targeting of monoclonal Ab (MAb) 76-Q3-2C6 (2C6). We confirmed that 2C6 binds preferentially to trimeric envelope and recognizes the clades A, B, and C SOSIP trimers. 2C6 binds gp140 constructs of clades A, B, C, and D, suggesting a conserved binding site that we localized to the ectodomain of gp41. Ab competition with MAb 50-69 suggested this epitope localizes near aa 579 to 613 (referenced to HXB2 gp160). Peptide library scanning showed consistent binding in this region but to only a single peptide. Lack of overlapping peptide binding supported a nonlinear epitope structure. The significance of this site is supported by 2C6 having Ab-dependent cell cytotoxicity (ADCC) against envelope proteins from two clades. Using 2C6 and variants, alanine scanning mutagenesis identified three amino acids (aa 592, 595, and 596) in the overlapping region of the previously identified peptide. Additional amino acids at sites 524 and 579 were also identified, helping explain its conformational requirement. The fact that different amino acids were included in the epitope depending on the targeted protein supports the conclusion that 2C6 targets a native conformational epitope. When we mapped these amino acids on the trimerized structure, they spanned across oligomers, supporting the notion that the epitope targeted by 2C6 lies in a recessed pocket between two gp41 oligomers. A complete understanding of the epitope specificity of ADCC-mediating Abs is essential for developing effective immunization strategies that optimize protection by these Abs.IMPORTANCE This paper further defines the function and area of the HIV trimeric envelope protein targeted by the monoclonal antibody 2C6. 2C6 binding is influenced by amino acid mutations across two separate gp41 sections of the envelope trimer. This epitope is recognized on multiple clades (variant groups of circulating viruses) of gp41, gp140 trimers, and SOSIP trimers. For the clades tested, 2C6 has robust ADCC. As the target of 2C6 is available in the major clades of HIV and has robust ADCC activity, further definition and appreciation of targeting of antibodies similar to 2C6 during vaccine development should be considered.

Natural killer cells in HIV-1 infection and therapy

: Natural killer (NK) cells are important effectors of innate immunity playing a key role in the eradication and clearance of viral infections. Over the recent years, several studies have shown that HIV-1 pathologically changes NK cell homeostasis and hampers their antiviral effector functions. Moreover, high levels of chronic HIV-1 viremia markedly impair those NK cell regulatory features that normally regulate the cross talks between innate and adaptive immune responses. These pathogenic events take place early in the infection and are associated with a pathologic redistribution of NK cell subsets that includes the expansion of anergic CD56/CD16 NK cells with an aberrant repertoire of activating and inhibitory receptors. Nevertheless, the presence of specific haplotypes for NK cell receptors and the engagement of NK cell antibody-dependent cell cytotocity have been reported to control HIV-1 infection. This dichotomy can be extremely useful to both predict the clinical outcome of the infection and to develop alternative antiviral pharmacological approaches. Indeed, the administration of antiretroviral therapy in HIV-1-infected patients restores NK cell phenotype and functions to normal levels. Thus, antiretroviral therapy can help to develop NK cell-directed therapeutic strategies that include the use of broadly neutralizing antibodies and toll-like receptor agonists. The present review discusses how our current knowledge of NK cell pathophysiology in HIV-1 infection is being translated both in experimental and clinical trials aimed at controlling the infection and disease.

Development of IFN-γ secretory ELISPOT based assay for screening of ADCC responses

Antibody dependent cell mediated cytotoxicity has been established as one of the important protective immune mechanisms against HIV making it essential to evaluate it while testing immunogenicity of emerging vaccine candidates. IFN-γ secretory ELISPOT assay, widely used for evaluation of CTL response in HIV vaccine trials, was adapted for measuring ADCC responses and the results were compared with the standard ICS based assays. IFN-γ responses elicited by plasma samples of 23 HIV infected individuals against Env and Gag peptides using granulocytes as antigen presenting cells were assessed by both the methods. Supernatants of the activated cells in ELISPOT assay were also assessed for cytokine/chemokine estimation. ELISPOT assays detected significantly more ADCC responders against HIV-Env and Gag peptide pools than ICS assay. The magnitude of IFN-γ response in both the assay correlated significantly (p=0.002). NK cells were found to be the predominant cell type secreting IFN-γ in the assay. Although IFN-γ and IL-6 levels were significantly higher in supernatants of Env peptides stimulated cells, IP-10 and MCP-1α levels were found to be more against Gag peptides. Thus, IFN-γ secretory ELISPOT assay was found to be more sensitive in detecting ADCC responders than ICS assay making it a valuable tool for screening of ADCC responses in future vaccine trials. Differences in cytokine pattern of Env versus Gag stimulated cells warrants a need for investigating their role in protection against HIV infection.

Proliferation of functional human natural killer cells with anti-HIV-1 activity in NOD/SCID/Jak3(null) mice

Natural killer cells, a critical component of the innate immune system, eradicate both virus-infected cells and tumor cells through cytotoxicity and secretion of cytokines. Human NK cell research has largely been based on in vitro studies because of the lack of appropriate animal models. In this study, a selective proliferation model of functional human NK cells was established in NOD/SCID/Jak3(null) (NOJ) mice transplanted with peripheral blood mononuclear cells (PBMC) and K562 cells. The antiviral effects of NK cells were evaluated by challenging this mouse model with HIV-1. The percentage of intracellular p24(+) T cells and the amount of plasma p24 was decreased compared with NOJ mice transplanted with PBMC. Our findings indicate that NK cells have an anti-HIV-1 effect through direct cytotoxicity against HIV-1-infected cells. These mice provide an important model for evaluating human NK function against human infectious diseases such as HIV-1 and malignancies.

Antibody-dependent CD56+ T cell responses are functionally impaired in long-term HIV-1 infection

Background

Antibody-dependent cellular cytotoxicity (ADCC), which mainly mediated by natural killer (NK) cells, may play a critical role in slowing human immunodeficiency virus type-1 (HIV-1) disease progression and protecting from HIV-1 infection. Besides classic NK cells, CD56+ T cells also have some NK cell-like properties, such as the large granular lymphocyte morphology and the capacity to destroy NK-sensitive target cells. However, little is known about the potentials of antibody-dependent CD56+ T cell responses and the association between antibody-dependent CD56+ T cell responses and HIV-1 disease progression.

Results

In the present study, we showed evidences that, in addition to NK cells, CD56+ T cells could generate degranulation upon CD16 cross-linking. Ex vivo study showed that FcγRIII (CD16)-mediated CD56+ T cell responses were distinctly induced by IgG antibody-bound P815 cells. Comparatively, CD56− T cells and invariant NKT (CD3+ 6B11+) failed to induce antibody-dependent activation. Antibody-dependent CD56+ T cell responses were mainly ascribed to CD4/CD8 double negative subset and were functionally impaired in long-term HIV-1-infected former plasma donors, regardless of hepatitis C virus (HCV) coinfection status. Also, CD56+ T cell-mediated HIV-1-specific antibody-dependent responses were declined in men who have sex with men with HIV-1 infection over 3 years. Finally, we showed that matrix metalloprotease (MMP) inhibitor GM6001 could partially restored antibody-dependent CD56+ T cell responses of chronic HIV-1-infected subjects.

Conclusions

Our results suggested that CD56+ T cells could mediate ADCC responses and the responses were impaired in chronic HIV-1 infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0313-6) contains supplementary material, which is available to authorized users.

Antibody-Dependent Cellular Cytotoxicity Activity of Effector Cells from HIV-Infected Elite and Viral Controllers

Carriage of alleles encoding certain inhibitory natural killer (NK) cell receptor/HLA ligand KIR3DL1/HLA-B combinations is associated with protection from HIV infection and slow time to AIDS, implicating NK cells in HIV control. NK cells also mediate antibody-dependent cellular cytotoxicity (ADCC). ADCC has been identified as a correlate of protection in secondary analyses of the modestly protective RV144 Thai HIV vaccine trial. In ADCC, HIV envelope (Env)-specific antibodies (Abs) bridge HIV-infected or gp120-coated target cells and NK cells expressing CD16 receptors for Ab Fc domains. CD16 engagement activates NK cells to secrete cytokines/chemokines, degranulate, deliver granzyme B (GrB) to target cells, and cytolysis. A subset of HIV+ subjects, known as slow progressors (SPs), maintains low-level viremia without treatment. HIV+ SPs versus progressors have higher titers and/or a greater breadth of ADCC-competent Abs. Investigations of the functional capacity of NK effector cells following CD16 engagement in HIV+ subjects are lacking. We used the ADCC-GranToxiLux (ADCC-GTL) assay to assess the frequency of GrB+ (%GrB+) cells generated by effector cells from 37 HIV+ SPs and 15 progressors to gp120-coated CEM.NKr.CCR5 target cells in the presence of anti-Env Abs. Subject groups were stratified according to whether or not they carried educating KIR3DL1/HLA-B combinations able to confer NK cells with functional potential. No differences were observed in %GrB+ target cells generated by effector cells from carriers of educating versus noneducating KIR3DL1/HLA-B pairs. The absence of an effect of NK cell education on this readout may be due to loss of the ability of educated NK cells from SPs to respond to Ab-dependent stimulation and/or the lower frequency of KIR3DL1⁺ than KIR3DL1^- NK cells that coexpress CD16. That KIR/HLA genotypes have minimal impact on interindividual differences in ADCC potency has relevance for therapeutic interventions that target ADCC for HIV control.

Comparing high-throughput methods to measure NK cell-mediated antibody dependent cellular cytotoxicity during HIV-infection

HIV-specific binding antibody responses, including those mediating antibody-dependent cellular cytotoxicity (ADCC), provided the best functional correlate of lower risk of infection in the RV144 HIV-1 vaccine clinical trial. The aim of this study was to compare two high-throughput flow cytometry based methods to measure HIV-specific ADCC responses, the GranToxilux and PanToxilux assays. Plasma from nine HIV-1 seropositive individuals was screened for binding antibody titres against HIV-1 subtype C gp120 by ELISA and western blot. Plasma from six HIV-negative individuals was included as controls. Both ADCC assays used subtype C gp120-coated CEM.NKRCCR5 cells as targets. The PanToxilux assay (which measured both granzyme B and caspase activity) measured higher levels of direct natural killer (NK) cell killing of K562 tumour cells than the GranToxilux assay (granzyme B alone; p<0.05). In ADCC assays in which NK cell killing was directed against gp120-coated CEM.NKRCCR5 cells in an antibody-dependent manner, plasma from HIV-positive individuals yielded significantly higher levels of ADCC activity than the HIV-negative controls. In contrast to direct killing, the GranToxilux assay measured similar levels of ADCC killing as the PanToxilux assay but had significantly lower background cytotoxicity against target cells coated with HIV negative serum. In conclusion, the PanToxilux assay was more sensitive for detecting direct NK cell killing of K562 cells than the GranToxilux assay, although the GranToxilux assay performed better at detecting HIV-specific ADCC activity, because of lower background cytotoxicity from HIV-negative serum. This is the first study to compare GranToxilux and PanToxilux ability to detect ADCC during HIV infection.

The role of HIV-specific antibody-dependent cellular cytotoxicity in HIV prevention and the influence of the HIV-1 Vpu protein

There is growing interest in the role of anti-HIV antibody-dependent cellular cytotoxicity (ADCC) antibodies in the prevention and control of HIV infection. Passive transfer studies in macaques support a role for the Fc region of antibodies in assisting in the prevention of simian-human immunodeficiency virus (SHIV) infection. The Thai RV144 HIV-1 vaccine trial induced anti-HIV ADCC antibodies that may have played a role in the partial protection observed. Several observational studies support a role for ADCC antibodies in slowing HIV disease progression. However, HIV evolves to escape ADCC antibodies and chronic HIV infections causes dysfunction of effector cells such as natural killer (NK) cells that mediate the ADCC functions. Further, four recent studies show that the HIV-1 Vpu protein, by promoting release of virions, reduces the capacity of ADCC antibodies to recognize HIV-infected cells. The review dissects some of the recent research on HIV-specific ADCC antibodies and discusses mechanisms to further harness ADCC antibodies in the prevention and control of HIV infection.

Slaying the Trojan horse: natural killer cells exhibit robust anti-HIV-1 antibody-dependent activation and cytolysis against allogeneic T cells

Many attempts to design prophylactic human immunodeficiency virus type 1 (HIV-1) vaccines have focused on the induction of neutralizing antibodies (Abs) that block infection by free virions. Despite the focus on viral particles, virus-infected cells, which can be found within mucosal secretions, are more infectious than free virus both in vitro and in vivo. Furthermore, assessment of human transmission couples suggests infected seminal lymphocytes might be responsible for a proportion of HIV-1 transmissions. Although vaccines that induce neutralizing Abs are sought, only some broadly neutralizing Abs efficiently block cell-to-cell transmission of HIV-1. As HIV-1 vaccines need to elicit immune responses capable of controlling both free and cell-associated virus, we evaluated the potential of natural killer (NK) cells to respond in an Ab-dependent manner to allogeneic T cells bearing HIV-1 antigens. This study presents data measuring Ab-dependent anti-HIV-1 NK cell responses to primary and transformed allogeneic T-cell targets. We found that NK cells are robustly activated in an anti-HIV-1 Ab-dependent manner against allogeneic targets and that tested target cells are subject to Ab-dependent cytolysis. Furthermore, the educated KIR3DL1(+) NK cell subset from HLA-Bw4(+) individuals exhibits an activation advantage over the KIR3DL1(-) subset that contains both NK cells educated through other receptor/ligand combinations and uneducated NK cells. These results are intriguing and important for understanding the regulation of Ab-dependent NK cell responses and are potentially valuable for designing Ab-dependent therapies and/or vaccines.

IMPORTANCE

NK cell-mediated anti-HIV-1 antibody-dependent functions have been associated with protection from infection and disease progression; however, their role in protecting from infection with allogeneic cells infected with HIV-1 is unknown. We found that HIV-1-specific ADCC antibodies bound to allogeneic cells infected with HIV-1 or coated with HIV-1 gp120 were capable of activating NK cells and/or trigging cytolysis of the allogeneic target cells. This suggests ADCC may be able to assist in preventing infection with cell-associated HIV-1. In order to fully utilize NK cell-mediated Ab-dependent effector functions, it might also be important that educated NK cells, which hold the highest activation potential, can become activated against targets bearing HIV-1 antigens and expressing the ligands for self-inhibitory receptors. Here, we show that with Ab-dependent stimulation, NK cells expressing inhibitory receptors can mediate robust activation against targets expressing the ligands for those receptors.

Flow cytometry-based assay to study HIV-1 gp120 specific antibody-dependent cellular cytotoxicity responses

Increased attention on the role of Fc-mediated effector functions against HIV-1 has led to renewed interest into the role that antibody-dependent cellular cytotoxicity (ADCC) could play in controlling viral transmission and/or the rate of disease progression. While (51)Chromium release assays have traditionally been used to study ADCC responses against HIV-1, a number of alternative flow-cytometry-based assays were recently developed. In this study, an alternative flow-cytometry-based assay was established to allow non-radioactive measurement of ADCC-mediated elimination of HIV-1 gp120 envelope glycoprotein (Env)-coated target cells. This assay relies on staining target and effector cells with different dyes, which allows precise gating and permits the calculation of the number of surviving target cells by normalization to flow-cytometry particles. By using small concentrations of recombinant gp120 Env, suitable targets cells that recapitulate the ADCC response mediated against HIV-1-infected cells were generated. Finally, this method was applied successfully to screen human sera for ADCC activity directed against HIV-1 gp120 Env.

Anti-HIV antibody-dependent activation of NK cells impairs NKp46 expression

There is much interest in the potential of Ab-dependent cellular cytotoxicity (ADCC) to slow disease progression following HIV infection. Despite several studies demonstrating a positive association between ADCC and slower disease progression, it is possible that continued stimulation of NK cells by ADCC during chronic HIV infection could render these cells dysfunctional. Indeed, activation of NK cells by ADCC results in matrix metalloproteinase-induced reductions in CD16 expression and activation refractory periods. In addition, ex vivo analyses of NK cells from HIV-infected individuals revealed other alterations in phenotype, such as decreased expression of the activating NKp46 receptor that is essential for NK-mediated antitumor responses and immunity from infection. Because NKp46 shares a signaling pathway with CD16, we hypothesized that activation-induced downregulation of both receptors could be controlled by a common mechanism. We found that activation of NK cells by anti-HIV or anti-CD16 Abs resulted in NKp46 downregulation. The addition of a matrix metalloproteinase inhibitor attenuated NKp46 downregulation following NK cell activation by anti-HIV Abs. Consequently, these results suggest that continued stimulation through CD16 has the potential to impair natural cytotoxicity via attenuation of NKp46-dependent signals.

KIR2DS4 allelic variants: Differential effects on in utero and intrapartum HIV-1 mother-to-child transmission

KIR2DS4 is the only activating gene within the A haplotype, and alleles of KIR2DS4 can encode either functional (KIR2DS4-f) or non-functional (KIR2DS4-v) variants. To establish the role of KIR2DS4 in the context of HIV-1 mother-to-child transmission, we KIR genotyped 145 HIV-1 non-transmitting mothers (NT) and their exposed uninfected infants (EU), and 72 HIV-1 transmitting mothers (TR) and their infected infants [intrapartum (IP), in utero (IU) or IU2 (an IU-enriched infected group)]. The frequency of KIR2DS4-v was significantly higher in IU2 infants compared to EU infants (P=0.022, OR=2.88); this association was more significant amongst AA haplotypes (P=0.004, OR=18.4). Possession of KIR2DS4-f in the mother with absence in the infant (M+I- discordance) was associated with a higher risk of IP transmission (P=0.005, OR=3.84); whilst in M-I+ discordance, infant possession of KIR2DS4-v was associated with increased risk of IU acquisition (P=0.002; OR=6.40). This study highlights the importance of KIR2DS4 in HIV-1 transmission/acquisition.