NK cells as effectors of acquired immune responses: effector CD4+ T cell-dependent activation of NK cells following vaccination

T cell regulation of natural killer cells

In light of their role in the immune response against tumors and viruses, natural killer (NK) cells represent a promising target for immunotherapy. Before this target is reached, the various mechanisms that control NK cell activity must first be identified and understood. In the past decades, studies have identified two critical processes that prevent spontaneous NK cell-mediated autoimmune activation while maximizing the efficiency of these cells during an immune response. First is the education process, whereby NK cells adapt to their environment by sensing ligands for inhibitory and activating receptors. Second is the priming phase of NK cell activation, which arms NK cells with appropriate cytotoxic molecules during inflammation. New studies now indicate that NK cell proliferation, accumulation, and activation are also under the control of regulatory T cells that restrict availability of IL-2 released by activated CD4(+) T cells. Together with other recent studies, these data highlight the importance of the adaptive immune system in the regulation of NK cell activity.

Dendritic cell derived cytokines in human natural killer cell differentiation and activation

Dendritic cells (DCs) and natural killer (NK) cells shape each other’s functions early during immune responses. DCs activate NK cells and NK cells can mature or kill DCs. In this review we will discuss which DC and NK cell subsets are mainly affected by this interaction, where these encounters might take place and which signals are exchanged. Finally, we will point out what the clinical benefit of understanding this interaction might be and how it changed our view on NK cells as innate lymphocytes.

Safety and immunogenicity of an adjuvanted protein therapeutic HIV-1 vaccine in subjects with HIV-1 infection: a randomised placebo-controlled study

The human immunodeficiency virus type-1 (HIV-1) vaccine candidate F4/AS01 has previously been shown to induce potent and persistent polyfunctional CD4(+) T-cell responses in HIV-1-seronegative volunteers. This placebo-controlled study evaluated two doses of F4/AS01 1-month apart in antiretroviral treatment (ART)-experienced and ART-naïve HIV-1-infected subjects (1:1 randomisation in each cohort). Safety, HIV-1-specific CD4(+) and CD8(+) T-cell responses, absolute CD4(+) T-cell counts and HIV-1 viral load were monitored for 12 months post-vaccination. Reactogenicity was clinically acceptable and no vaccine-related serious adverse events were reported. The frequency of HIV-1-specific CD4(+) T-cells 2 weeks post-dose 2 was significantly higher in the vaccine group than in the placebo group in both cohorts (p<0.05). Vaccine-induced HIV-1-specific CD4(+) T-cells exhibited a polyfunctional phenotype, expressing at least CD40L and IL-2. No increase in HIV-1-specific CD8(+) T-cells or change in CD8(+) T-cell activation marker expression profile was detected. Absolute CD4(+) T-cell counts were variable over time in both cohorts. Viral load remained suppressed in ART-experienced subjects. In ART-naïve subjects, a transient reduction in viral load from baseline was observed 2 weeks after the second F4/AS01 dose, which was concurrent with a higher frequency of HIV-1-specific CD4(+) T-cells expressing at least IL-2 in this cohort. In conclusion, F4/AS01 showed a clinically acceptable reactogenicity and safety profile, and induced polyfunctional HIV-1-specific CD4(+) T-cell responses in ART-experienced and ART-naïve subjects. These findings support further clinical investigation of F4/AS01 as a potential HIV-1 vaccine for therapeutic use in individuals with HIV-1 infection.

A distinct subpopulation of human NK cells restricts B cell transformation by EBV

NK cells constitute the first line of defense against pathogens and transformed cells. They mature in secondary lymphoid organs, including tonsils, where common pathogens, such as EBV, enter the host and potentially imprint differentiating cells, which then patrol the body via the blood stream. Therefore, we set out to characterize a distinct human NK cell population in tonsils that produces high amounts of the immunomodulatory and antiviral cytokine IFN-γ. We found that the tonsilar IFN-γ(high) NK cell subset is CD56(bright)NKG2A(+)CD94(+)CD54(+)CD62L(-), is present in tonsils ex vivo and is more mature than other CD56(bright) NK cells in tonsils and less mature than other NK cells in blood, shows very low plasticity even after prolonged cytokine stimulation, accumulates in tonsils of EBV carriers, and is able to potently restrict EBV-induced transformation of B cells. Thus, we characterized a distinct and stable IFN-γ(high) NK cell subpopulation that can specifically restrict malignant transformation of EBV-infected B cells. This subset should be exploited for future development of cell-based therapeutic approaches in EBV-associated malignancies.

Differential requirements of cellular and humoral immune responses for Fv2-associated resistance to erythroleukemia and for regulation of retrovirus-induced myeloid leukemia development

To assess the possible contribution of host immune responses to the exertion of Fv2-associated resistance to Friend virus (FV)-induced disease development, we inoculated C57BL/6 (B6) mice that lacked various subsets of lymphocytes with FV containing no lactate dehydrogenase-elevating virus. Fv2(r) B6 mice lacking CD4(+) T cells developed early polycythemia and fatal erythroleukemia, while B6 mice lacking CD8(+) T cells remained resistant. Erythroid progenitor cells infected with spleen focus-forming virus (SFFV) were eliminated, and no polycythemia was observed in B cell-deficient B6 mice, but they later developed myeloid leukemia associated with oligoclonal integration of ecotropic Friend murine leukemia virus. Additional depletion of natural killer and/or CD8(+) T cells from B cell-deficient B6 mice resulted in the expansion of SFFV proviruses and the development of polycythemia, indicating that SFFV-infected erythroid cells are not only restricted in their growth but are actively eliminated in Fv2(r) mice through cellular immune responses.

Impaired CD4⁺ T cell stimulation of NK cell anti-fibrotic activity may contribute to accelerated liver fibrosis progression in HIV/HCV patients

BACKGROUND & AIMS

HIV/HCV co-infection is characterized by a faster progression to liver fibrosis compared to HCV mono-infection. Epidemiologic studies found an association between low CD4(+) T cell counts and advanced stages of liver fibrosis. However, the mechanisms underlying this association remain unclear. CD4(+) T cells critically modulate NK cell activity. Of note, NK cells have been shown to display anti-fibrotic activity via killing of activated hepatic stellate cells (HSC). Thus, we speculated that CD4(+) T cells might modulate fibrosis progression by interacting with NK cells.

METHODS

NK cells from HCV(+) (n=35), HIV(+)/HCV(+) (n=28), HIV(+) (n=8) patients, and healthy controls (n=30) were used in this study. NK cells were cultured in the presence or absence of supernatants from CD3/CD28-stimulated CD4(+) cells. Then, NK cells were co-incubated with activated HSC and studied for degranulation, IFN-γ secretion, and induction of HSC apoptosis.

RESULTS

Following incubation with CD4(+) T cell supernatants, NK cells displayed a significantly increased activity against primary HSC as compared to unstimulated NK cells. This effect was, at least in part, mediated via an IL-2 dependent upregulation of NKG2D expression. HCV/HIV co-infection was associated with an impaired IL-2 secretion of CD4(+) T cells resulting in an ineffective stimulation of anti-fibrotic NK cell function.

CONCLUSIONS

Here, we show that CD4(+) T cells are able to stimulate anti-fibrotic NK cell activity via IL-2 mediated upregulation of NKG2D. HIV-induced loss of CD4(+) T cells together with an impaired activity of CD4(+) T cells may contribute to accelerate progression of liver fibrosis observed in co-infection.

Deciphering serology to understand the ecology of infectious diseases in wildlife

The ecology of infectious disease in wildlife has become a pivotal theme in animal and public health. Studies of infectious disease ecology rely on robust surveillance of pathogens in reservoir hosts, often based on serology, which is the detection of specific antibodies in the blood and is used to infer infection history. However, serological data can be inaccurate for inference to infection history for a variety of reasons. Two major aspects in any serological test can substantially impact results and interpretation of antibody prevalence data: cross-reactivity and cut-off thresholds used to discriminate positive and negative reactions. Given the ubiquitous use of serology as a tool for surveillance and epidemiological modeling of wildlife diseases, it is imperative to consider the strengths and limitations of serological test methodologies and interpretation of results, particularly when using data that may affect management and policy for the prevention and control of infectious diseases in wildlife. Greater consideration of population age structure and cohort representation, serological test suitability and standardized sample collection protocols can ensure that reliable data are obtained for downstream modeling applications to characterize, and evaluate interventions for, wildlife disease systems.

Target-induced natural killer cell loss as a measure of NK cell responses

Natural killer (NK) cells are an important effector cell of innate immunity. Their interaction with susceptible target cells triggers NK cell cytotoxicity and the release of cytokines. Immunofluorescence flow cytometry-based assays are now the preferred methods for measuring NK cell responses. For these assays, assessment is made on NK cells (CD3(-)CD56(+) CD16(+)) within the viable lymphocyte gate, defined by the parameters of size (FSC) and granularity (SSC). Accordingly, NK cells that have not dissociated from target cells at the time of measurement, or that have undergone target cell-induced apoptosis, are excluded from the viable lymphocyte gate and therefore from analysis. This unit describes a protocol for assessing NK cell function in response to various target cells (natural killing, antibody-dependent cell cytotoxicity, and NK cell alloreactivity) based on the loss of NK cells from the lymphocyte gate. This target-induced NK loss (TINKL) should provide a sensitive measure of NK cell responses in a clinical laboratory setting.

Fc receptor-mediated immune responses: new tools but increased complexity in HIV prevention

The modest success of the RV144 HIV vaccine trial in Thailand and the ensuing suggestion that a Fc-receptormediated antibody activity might have played a role in the protection observed have intensified investigations on Fcrelated immune responses. HIV neutralizing antibodies have been and continue to be the focal point of research into humoral immune protection. However, recent knowledge that their protective efficacy can be augmented by Fc-FcR interactions has increased the complexity of identifying immune correlates of protection. If anything, continued studies of both humoral and cellular immune mechanisms point to the lack of a single protective anti-HIV immune response. Here we focus on humoral immunity, analyzing the role played by Fc receptor-related responses and discussing how new knowledge of their interactions requires further investigation, but may also spur novel vaccination approaches. We initially address classical Fc-receptor mediated anti-viral mechanisms including antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cell mediated viral inhibition (ADCVI), and antibody-dependent cellular phagocytosis (ADCP), as well as the effector cells that mediate these functions. Next, we summarize key aspects of FcR-Fc interactions that are important for potential control of HIV/SIV such as FcR polymorphisms and post-transcriptional modifications. Finally we discuss less commonly studied non-mechanistic anti-HIV immune functions: antibody avidity and envelopespecific B cell memory. Overall, a spectrum of immune responses, reflecting the immune system's redundancy, will likely be needed to prevent HIV infection and/or disease progression. Aside from elicitation of critical immune mechanisms, a successful vaccine will need to induce mature B cell responses and long-lasting immune memory.

Differential recruitment and activation of natural killer cell sub-populations by Mycobacterium bovis-infected dendritic cells

Through complex interplay with APCs, subsets of NK cells play an important role in shaping adaptive immune responses. Bovine tuberculosis, caused by Mycobacterium bovis, is increasing in incidence and detailed knowledge of host-pathogen interactions in the natural host is essential to facilitate disease control. We investigated the interactions of NK-cell sub-populations and M. bovis-infected DCs to determine early innate mechanisms in the response to infection. A sub-population of NK cells (NKp46(+) CD2(-) ) selectively expressing lymphoid homing and inflammatory chemokine receptors were induced to migrate towards M. bovis-infected DCs. This migration was associated with increased expression of chemokines CCL3, 4, 5, 20 and CXCL8 by M. bovis-infected DCs. Activation of NKp46(+) CD2(-) NK cells and secretion of IFN-γ was observed, a response reliant on localised IL-12 release and direct cellular interaction. In a reciprocal manner, NKp46(+) CD2(-) cells induced an increase in the intensity of cell surface MHC class II expression on DCs. In contrast, NKp46(+) CD2(+) NK cells were unable to secrete IFN-γ and did not reciprocally affect DCs. This study provides novel evidence to demonstrate distinct effector responses between bovine NK-cell subsets during mycobacterial infection.

HIV-1 Vaccine Trials: Evolving Concepts and Designs

An effective prophylactic HIV-1 vaccine is needed to eradicate the HIV/AIDS pandemic but designing such a vaccine is a challenge. Despite many advances in vaccine technology and approaches to generate both humoral and cellular immune responses, major phase-II and -III vaccine trials against HIV/AIDS have resulted in only moderate successes. The modest achievement of the phase-III RV144 prime-boost trial in Thailand re-emphasized the importance of generating robust humoral and cellular responses against HIV. While antibody-directed approaches are being pursued by some groups, others are attempting to develop vaccines targeting cell-mediated immunity, since evidence show CTLs to be important for the control of HIV replication. Phase-I and -IIa multi-epitope vaccine trials have already been conducted with vaccine immunogens consisting of known CTL epitopes conserved across HIV subtypes, but have so far fallen short of inducing robust and consistent anti-HIV CTL responses. The concepts leading to the development of T-cell epitope-based vaccines, the outcomes of related clinical vaccine trials and efforts to enhance the immunogenicity of cell-mediated approaches are summarized in this review. Moreover, we describe a novel approach based on the identification of SIV and FIV antigens which contain conserved HIV-specific T-cell epitopes and represent an alternative method for developing an effective HIV vaccine against global HIV isolates.

Protective Effector Cells of the Recombinant Asp f3 Anti-Aspergillosis Vaccine

An Aspergillus fumigatus vaccine based on recombinant Asp f3-protein has the potential to prevent aspergillosis in humans, a devastating fungal disease that is the prime obstacle to the success of hematopoietic cell transplantation. This vaccine protects cortisone acetate (CA)-immunosuppressed mice from invasive pulmonary aspergillosis via CD4(+) T cell mediators. Aside from these mediators, the nature of downstream fungicidal effectors is not well understood. Neutrophils and macrophages protect immunocompetent individuals from invasive fungal infections, and selective neutrophil depletion rendered mice susceptible to aspergillosis whereas macrophage depletion failed to increase fungal susceptibility. We investigated the effect of neutrophil depletion on rAsp f3-vaccine protection, and explored differences in pathophysiology and susceptibility between CA-immunosuppression and neutrophil depletion. In addition to being protective under CA-immunosuppression, the vaccine also had a protective effect in neutrophil-depleted mice. However, in non-immunized mice, a 10-fold higher conidial dose was required to induce similar susceptibility to infection with neutrophil depletion than with CA-immunosuppression. The lungs of non-immunized neutrophil-depleted mice became invaded by a patchy dense mycelium with highly branched hyphae, and the peribronchial inflammatory infiltrate consisted mainly of CD3(+) T cells and largely lacked macrophages. In contrast, lungs of non-immunized CA-immunosuppressed mice were more evenly scattered with short hyphal elements. With rAsp f3-vaccination, the lungs were largely clear of fungal burden under either immunosuppressive condition. We conclude that neutrophils, although important for innate antifungal protection of immunocompetent hosts, are not the relevant effectors for rAsp f3-vaccine derived protection of immunosuppressed hosts. It is therefore more likely that macrophages represent the crucial effectors of the rAsp f3-based vaccine.

NK and CD4+ T cell cooperative immune responses correlate with control of disease in a macaque simian immunodeficiency virus infection model

Control of infectious disease may be accomplished by successful vaccination or by complex immunologic and genetic factors favoring Ag-specific multicellular immune responses. Using a rhesus macaque model, we evaluated Ag-specific T cell-dependent NK cell immune responses in SIV-infected macaques, designated "controlling" or "noncontrolling" based on long-term chronic viremia levels, to determine whether NK cell effector functions contribute to control of SIV infection. We observed that Gag stimulation of macaque PBMCs induced subset-specific NK cell responses in SIV-controlling but not SIV-noncontrolling animals, as well as that circulatory NK cell responses were dependent on Ag-specific IL-2 production by CD4(+) central memory T cells. NK cell activation was blocked by anti-IL-2-neutralizing Ab and by CD4(+) T cell depletion, which abrogated the Gag-specific responses. Among tissue-resident cells, splenic and circulatory NK cells displayed similar activation profiles, whereas liver and mucosal NK cells displayed a decreased activation profile, similar in SIV-controlling and -noncontrolling macaques. Lack of T cell-dependent NK cell function was rescued in SIV-noncontrolling macaques through drug-mediated control of viremia. Our results indicate that control of disease progression in SIV-controlling macaques is associated with cooperation between Ag-specific CD4(+) T cells and NK cell effector function, which highlight the importance of such cell-to-cell cooperativity in adaptive immunity and suggest that this interaction should be further investigated in HIV vaccine development and other prophylactic vaccine approaches.

An African horse sickness virus serotype 4 recombinant canarypox virus vaccine elicits specific cell-mediated immune responses in horses

A recombinant canarypox virus vectored vaccine co-expressing synthetic genes encoding outer capsid proteins, VP2 and VP5, of African horse sickness virus (AHSV) serotype 4 (ALVAC(®)-AHSV4) has been demonstrated to fully protect horses against homologous challenge with virulent field virus. Guthrie et al. (2009) detected weak and variable titres of neutralizing antibody (ranging from <10 to 40) 8 weeks after vaccination leading us to hypothesize that there could be a participation of cell mediated immunity (CMI) in protection against AHSV4. The present study aimed at characterizing the CMI induced by the experimental ALVAC(®)-AHSV4 vaccine. Six horses received two vaccinations twenty-eight days apart and three horses remained unvaccinated. The detection of VP2/VP5 specific IFN-γ responses was assessed by enzyme linked immune spot (ELISpot) assay and clearly demonstrated that all ALVAC(®)-AHSV4 vaccinated horses developed significant IFN-γ production compared to unvaccinated horses. More detailed immune responses obtained by flow cytometry demonstrated that ALVAC(®)-AHSV4 vaccinations induced immune cells, mainly CD8(+) T cells, able to recognize multiple T-epitopes through all VP2 and only the N-terminus sequence of VP5. Neither VP2 nor VP5 specific IFN-γ responses were detected in unvaccinated horses. Overall, our data demonstrated that an experimental recombinant canarypox based vaccine induced significant CMI specific for both VP2 and VP5 proteins of AHSV4.

Improved CD4⁺ T cell responses to Mycobacterium tuberculosis in PPD-negative adults by M72/AS01 as compared to the M72/AS02 and Mtb72F/AS02 tuberculosis candidate vaccine formulations: a randomized trial

BACKGROUND

The Bacille Calmette-Guérin (BCG) tuberculosis (TB) vaccine provides incomplete protection, necessitating development of an effective vaccine against TB disease. The Mtb72F/AS02 candidate vaccine was previously shown to be clinically well tolerated and immunogenic in Purified Protein Derivative (PPD)-negative adults. To improve the stability of Mtb72F, a point mutation was introduced into a putative serine protease site to give the final M72 construct. AS01 is an Adjuvant System that can potentially improve both humoral and cellular immune responses compared to the AS02 Adjuvant System or unadjuvanted vaccine. This study evaluated the safety and immunogenicity in Mtb-naïve adults of vaccines containing 40 μg of the M72 antigen with AS02 or AS01 and compared the results with Mtb72F/AS02 vaccine (40 μg dose), M72 in saline (40 μg dose) and AS01 alone.

METHODS

In this Phase I/II observer-blind controlled trial, 110 participants were randomized (4:4:1:1:1) to receive M72/AS01, M72/AS02, Mtb72F/AS02, M72/saline or AS01, following a 0, 1-month schedule. Subjects receiving the adjuvanted M72 vaccines were followed up until 3 years post vaccination. Evaluation of the immune response and safety/reactogenicity was performed.

RESULTS

For all vaccines, solicited adverse events (AEs) were predominantly mild to moderate and transient. No vaccine-related serious AEs occurred and no subject withdrew due to an AE. Immune responses induced by Mtb72F and M72 antigens combined with AS02 were similar. M72/AS01 and M72/AS02 induced robust polyfunctional M72-specific CD4(+) T cell and antibody responses persisting at 3 years, with the highest CD4(+) T cell responses found with M72/AS01.

CONCLUSION

This first clinical study with M72/AS01 and M72/AS02 showed that both vaccines were clinically well tolerated and induced high magnitude and persistent cell-mediated and humoral immune responses. The Mtb72F/AS02 and M72/AS02 vaccines were comparably immunogenic with significantly higher immune responses compared to the M72/saline control. Of the formulations tested, M72/AS01 demonstrated significantly higher vaccine specific Th1 CD4(+) T cell responses supporting its further clinical evaluation.

Long-term proliferation of functional human NK cells, with conversion of CD56(dim) NK cells to a CD56 (bright) phenotype, induced by carcinoma cells co-expressing 4-1BBL and IL-12

4-1BB ligation co-stimulates T cell activation, and agonistic antibodies have entered clinical trials. Natural killer (NK) cells also express 4-1BB following activation and are implicated in the anti-tumour efficacy of 4-1BB stimulation in mice; however, the response of human NK cells to 4-1BB stimulation is not clearly defined. Stimulation of non-adherent PBMC with OVCAR-3 cells expressing 4-1BB ligand (4-1BBL) or IL-12 resulted in preferential expansion of the NK cell population, while the combination 4-1BBL + IL-12 was superior for the activation and proliferation of functional NK cells from healthy donors and patients with renal cell or ovarian carcinoma, supporting long-term (21 day) NK cell proliferation. The expanded NK cells are predominantly CD56(bright), and we show that isolated CD56(dim)CD16(+) NK cells can switch to a CD56(bright)CD16(-) phenotype and proliferate in response to 4-1BBL + IL-12. Whereas 4-1BB upregulation on NK cells in response to 4-1BBL required 'help' from other PBMC, it could be induced on isolated NK cells by IL-12, but only in the presence of target (OVCAR-3) cells. Following primary stimulation with OVCAR-3 cells expressing 4-1BBL + IL-12 and subsequent resting until day 21, NK cells remained predominantly CD56(bright) and retained both high cytotoxic capability against K562 targets and enhanced ability to produce IFNγ relative to NK cells in PBMC. These data support the concept that NK cells could contribute to anti-tumour activity of 4-1BB agonists in humans and suggest that combining 4-1BB-stimulation with IL-12 could be beneficial for ex vivo or in vivo expansion and activation of NK cells for cancer immunotherapy.

Antigen-specific IL-2 secretion correlates with NK cell responses after immunization of Tanzanian children with the RTS,S/AS01 malaria vaccine

RTS,S/AS01, a vaccine targeting pre-erythrocytic stages of Plasmodium falciparum, is undergoing clinical trials. We report an analysis of cellular immune response to component Ags of RTS,S-hepatitis B surface Ag (HBs) and P. falciparum circumsporozoite (CS) protein-among Tanzanian children in a phase IIb RTS,S/AS01(E) trial. RTS,S/AS01 (E) vaccinees make stronger T cell IFN-γ, CD69, and CD25 responses to HBs peptides than do controls, indicating that RTS,S boosts pre-existing HBs responses. T cell CD69 and CD25 responses to CS and CS-specific secreted IL-2 were augmented by RTS,S vaccination. Importantly, more than 50% of peptide-induced IFN-γ(+) lymphocytes were NK cells, and the magnitude of the NK cell CD69 response to HBs peptides correlated with secreted IL-2 concentration. CD69 and CD25 expression and IL-2 secretion may represent sensitive markers of RTS,S-induced, CS-specific T cells. The potential for T cell-derived IL-2 to augment NK cell activation in RTS,S-vaccinated individuals, and the relevance of this for protection, needs to be explored further.

Genital HSV-2 infection induces short-term NK cell memory

NK cells are known as innate immune cells that lack immunological memory. Recently, it has been shown that NK cells remember encounters with chemical haptens that induce contact hypersensitivity and cytomegalovirus infection. Here, we show the existence of NK cell memory following HSV-2 infection. Stimulation with HSV-2 Ags led to higher IFNγ production in NK cells that were exposed 30 days previously to HSV-2, compared to NK cells from naïve mice. More importantly, this increased production of IFNγ in NK cells was independent of B- and T- lymphocytes and specific for the HSV-2 Ags. We also showed that previously exposed NK cells in a B- and T-lymphocyte free environment mediate protection against HSV-2 infection and they are necessary for the protection of mice against HSV-2 infection. Collectively, NK cells remember prior HSV-2 encounters independent of B- and T- lymphocytes leading to protection against HSV-2 mediated morbidity and mortality upon re-exposure.

Adenovirus type 35, but not type 5, stimulates NK cell activation via plasmacytoid dendritic cells and TLR9 signaling

In hematopoietic stem cell transplant (HSCT) recipients, disseminated adenoviral infections during the first two months after HSCT can lead to severe complications and fatal outcome. Since NK cells are usually the first lymphocytes to reconstitute after HSCT and have been implicated in the clearance of adenovirus-infected cells, it was investigated whether NK cells are activated by adenovirus in vitro. Exposure of PBMC to human adenovirus type 5 (HAdV5) or HAdV35 resulted in the up-regulation of the activation marker CD69 on NK cells and enhanced the cytolytic activity of NK cells. HAdV5-induced NK cell activation relied on the contribution of T cells as the depletion of T cells from PBMC abolished NK cell activation. In contrast, NK cell activation in response to HAdV35 occurred in the absence of T cells. Plasmacytoid dendritic cells (pDC) were necessary and sufficient to mediate NK cell activation. HAdV35 induced significantly more interferon-α (IFN-α) production by pDC than HAdV5. The increased IFN-α production and NK cell activation correlated with a higher infection efficiency of viruses with the type 35 fiber. The IFN-α response of pDC was enhanced by the presence of NK cells, suggesting a reciprocal interaction between pDC and NK cells. Incubation with a TLR9 antagonist impaired the IFN-α production by pDC as well as NK cell activation, implying that TLR9 signaling is critically involved in the IFN-α response of pDC and NK cell activation after HAdV35 exposure. In conclusion, two human adenovirus serotypes from two different species differ considerably in their capacity to stimulate pDC and NK cells.

Activation of natural killer cells during microbial infections

Natural killer (NK) cells are large granular lymphocytes that express a diverse array of germline encoded inhibitory and activating receptors for MHC Class I and Class I-like molecules, classical co-stimulatory ligands, and cytokines. The ability of NK cells to be very rapidly activated by inflammatory cytokines, to secrete effector cytokines, and to kill infected or stressed host cells, suggests that they may be among the very early responders during infection. Recent studies have also identified a small number of pathogen-derived ligands that can bind to NK cell surface receptors and directly induce their activation. Here we review recent studies that have begun to elucidate the various pathways by which viral, bacterial, and parasite pathogens activate NK cells. We also consider two emerging themes of NK cell–pathogen interactions, namely their contribution to adaptive immune responses and their potential to take on regulatory and immunomodulatory functions.

Comparison of the immunogenicity of the human papillomavirus (HPV)-16/18 vaccine and the HPV-6/11/16/18 vaccine for oncogenic non-vaccine types HPV-31 and HPV-45 in healthy women aged 18-45 years

Protection against oncogenic non-vaccine types (cross-protection) offered by human papillomavirus (HPV) vaccines may provide a significant medical benefit. Available clinical efficacy data suggest the two licensed vaccines (HPV-16/18 vaccine, GlaxoSmithKline Biologicals (GSK), and HPV-6/11/16/18 vaccine, Merck & Co., Inc.) differ in terms of protection against oncogenic non-vaccine HPV types -31/45. The immune responses induced by the two vaccines against these two non-vaccine HPV types (cross-reactivity) was compared in an observer-blind study up to Month 24 (18 mo post-vaccination), in women HPV DNA-negative and seronegative prior to vaccination for the HPV type analyzed (HPV-010 [NCT00423046]). Geometric mean antibody titers (GMTs) measured by pseudovirion-based neutralization assay (PBNA) and enzyme-linked immunosorbent assay (ELISA) were similar between vaccines for HPV-31/45. Seropositivity rates for HPV-31 were also similar between vaccines; however, there was a trend for higher seropositivity with the HPV-16/18 vaccine (13.0-16.7%) versus the HPV-6/11/16/18 vaccine (0.0-5.0%) for HPV-45 with PBNA, but not ELISA. HPV-31/45 cross-reactive memory B-cell responses were comparable between vaccines. Circulating antigen-specific CD4+ T-cell frequencies were higher for the HPV-16/18 vaccine than the HPV-6/11/16/18 vaccine (HPV-31 [geometric mean ratio [GMR] =2.0; p=0.0002] and HPV-45 [GMR=2.6; p=0.0092]), as were the proportion of T-cell responders (HPV-31, p=0.0009; HPV-45, p=0.0793). In conclusion, immune response to oncogenic non-vaccine HPV types -31/45 was generally similar for both vaccines with the exception of T-cell response which was higher with the HPV-16/18 vaccine. Considering the differences in cross-protective efficacy between the two vaccines, the results might provide insights into the underlying mechanism(s) of protection.

The contribution of Plasmodium chabaudi to our understanding of malaria

Malaria kills close to a million people every year, mostly children under the age of five. In the drive towards the development of an effective vaccine and new chemotherapeutic targets for malaria, field-based studies on human malaria infection and laboratory-based studies using animal models of malaria offer complementary opportunities to further our understanding of the mechanisms behind malaria infection and pathology. We outline here the parallels between the Plasmodium chabaudi mouse model of malaria and human malaria. We will highlight the contribution of P. chabaudi to our understanding of malaria in particular, how the immune response in malaria infection is initiated and regulated, its role in pathology, and how immunological memory is maintained. We will also discuss areas where new tools have opened up potential areas of exploration using this invaluable model system.

Mechanisms of NK cell activation: CD4(+) T cells enter the scene

Natural killer (NK) cells are innate lymphocytes involved in immunosurveillance through their cytotoxic activity and their capacity to secrete inflammatory cytokines. NK cell activation is necessary to initiate effector functions and results from a complex series of molecular and cellular events. We review here the signals that trigger NK cells and discuss recent findings showing that, besides antigen-presenting cells, T cells can play a central role in the initiation of NK cell activation in lymph nodes.

NK cells: immune cross-talk and therapeutic implications

Increased evidence of cross-talk between NK cells and other immune cells has enhanced the possibilities of exploiting the interplay between the activation and inhibition of NK cells for immunotherapeutic purposes. The battery of receptors possessed by NK cells help them to efficiently detect aberrant and infected cells and embark on the signaling pathways necessary to eliminate them. Endogenous expansion of NK cells and their effector mechanisms are under exploration for enhancing adoptive immunotherapy prospects in combination with immunostimulatory and cell-death-sensitizing treatments against cancer, viral infections and other pathophysiological autoimmune conditions. Various modes of NK cell manipulation are being undertaken to overcome issues such as relapse and graft rejections associated with adoptive immunotherapy. While tracing the remarkable properties of NK cells and the major developments in this field, we highlight the role of immune cooperativity in the betterment of current immunotherapeutic approaches.

Target-induced natural killer cell loss as a measure of NK cell responses

The interaction of natural killer cells with susceptible target cells triggers NK cell activation, eliciting not only NK cell cytotoxicity and cytokine secretion, but also NK cell death. This study shows that following target cell interaction there is a substantial loss of NK cells, the extent of which correlates with measures of NK cell cytotoxicity assessed by the target cell release of (51)Cr and by the externalisation of the lysosomal marker LAMP-1 (CD107a) which is assessed on the remaining NK cells. This is the case for the killing of K562 (natural killing) and the CD20 mAb (Rituximab)-mediated killing of RAJI cells and autologous B cells (antibody-dependent cell cytotoxicity). This target-induced NK loss (TINKL) provides a sensitive and specific measure of NK cell responses appropriate to a clinical laboratory setting.

Cooperativity of adaptive and innate immunity: implications for cancer therapy

The dichotomy of immunology into innate and adaptive immunity has created conceptual barriers in appreciating the intrinsic two-way interaction between immune cells. An emerging body of evidence in various models of immune rejection, including cancer, indicates an indispensable regulation of innate effector functions by adaptive immune cells. This bidirectional cooperativity in innate and adaptive immune functions has broad implications for immune responses in general and for regulating the tumor-associated inflammation that overrides the protective antitumor immunity. Mechanistic understanding of this two-way immune cross-talk could provide insights into novel strategies for designing better immunotherapy approaches against cancer and other diseases that normally defy immune control.

A distinct subset of human NK cells expressing HLA-DR expand in response to IL-2 and can aid immune responses to BCG

Subsets of NK cells can have distinct functions. Here, we report that >25% of human peripheral blood NK cells express HLA-DR after culture with IL-2. This can be driven by an expansion of a small subset of NK cells expressing HLA-DR, in contrast to previous assumptions that HLA-DR is upregulated on previously negative cells. HLA-DR-expressing NK cells showed enhanced degranulation to susceptible target cells and expressed chemokine receptor CXCR3, which facilitated their enrichment following exposure to CXCL11/I-TAC. Suggesting HLA-DR-expressing NK cells have an important role in an immune response, stimulation of PBMCs with Mycobacterium bovis BCG (BCG) triggered expansion of this subset. Importantly, the magnitude of an individual's NK cell IFN-γ response triggered by BCG was associated with the initial frequency of HLA-DR-expressing NK cells in PBMCs. More directly indicating the importance of HLA-DR-expressing NK cells, enriching the frequency of this subset in PBMCs substantially augmented the IFN-γ response to BCG. Thus, HLA-DR expression marks a distinct subset of NK cells, present at low frequency in circulating blood but readily expanded by IL-2, that can play an important role during immune responses to BCG.

KIR/HLA interactions and pathogen immunity

The innate immune system is the first line of defence in response to pathogen infection. Natural killer (NK) cells perform a vital role in this response with the ability to directly kill infected cells, produce cytokines, and cross-talk with the adaptive immune system. These effector functions are dependent on activation of NK cells which is determined by surface receptor interactions with ligands on target cells. Of these receptors, the polymorphic killer immunoglobulin-like receptors (KIRs), which interact with MHC class 1 (also highly polymorphic), are largely inhibitory, and exhibit substantial genetic diversity. The result is a significant variation of NK cell repertoire between individuals and also between populations, with a multitude of possible KIR:HLA combinations. As each KIR:ligand interaction may have differential effects on NK cell activation and inhibition, this diversity has important potential influences on the host response to infections. Genetic studies have demonstrated associations between specific KIR:ligand combinations and the outcome of viral (and other) infections, in particular hepatitis C and HIV infection. Detailed functional studies are not required to define the mechanisms underpinning these disease associations.

Systemic effector and regulatory immune responses to chlamydial antigens in trachomatous trichiasis

Trachomatous trichiasis (TT) caused by repeated or chronic ocular infection with Chlamydia trachomatis is the result of a pro-fibrotic ocular immune response. At the conjunctiva, the increased expression of both inflammatory (IL1B, TNF) and regulatory cytokines (IL10) have been associated with adverse clinical outcomes. We measured in vitro immune responses of peripheral blood to a number of chlamydial antigens. Peripheral blood effector cells (CD4, CD69, IFNγ, IL-10) and regulatory cells (CD4, CD25, FOXP3, CTLA4/GITR) were readily stimulated by C. trachomatis antigens but neither the magnitude (frequency or stimulation index) or the breadth and amount of cytokines produced in vitro [IL-5, IL-10, IL-12 (p70), IL-13, IFNγ, and TNFα] were significantly different between TT cases and their non-diseased controls. Interestingly we observed that CD4+ T cells account for <50% of the IFNγ positive cells induced following stimulation. Further investigation in individuals selected from communities where exposure to ocular infection with C. trachomatis is endemic indicated that CD3-CD56+ (classical natural killer cells) were a major early source of IFNγ production in response to C. trachomatis elementary body stimulation and that the magnitude of this response increased with age. Future efforts to unravel the contribution of the adaptive immune response to conjunctival fibrosis should focus on the early events following infection and the interaction with innate immune mediated mechanisms of inflammation in the conjunctiva.

Short-lived IFN-γ effector responses, but long-lived IL-10 memory responses, to malaria in an area of low malaria endemicity

Immunity to malaria is widely believed to wane in the absence of reinfection, but direct evidence for the presence or absence of durable immunological memory to malaria is limited. Here, we analysed malaria-specific CD4+ T cell responses of individuals living in an area of low malaria transmission in northern Thailand, who had had a documented clinical attack of P. falciparum and/or P. vivax in the past 6 years. CD4+ T cell effector memory (CD45RO+) IFN-γ (24 hours ex vivo restimulation) and cultured IL-10 (6 day secretion into culture supernatant) responses to malaria schizont antigens were detected only in malaria-exposed subjects and were more prominent in subjects with long-lived antibodies or memory B cells specific to malaria antigens. The number of IFN-γ-producing effector memory T cells declined significantly over the 12 months of the study, and with time since last documented malaria infection, with an estimated half life of the response of 3.3 (95% CI 1.9-10.3) years. In sharp contrast, IL-10 responses were sustained for many years after last known malaria infection with no significant decline over at least 6 years. The observations have clear implications for understanding the immunoepidemiology of naturally acquired malaria infections and for malaria vaccine development.

Impact of blood processing variations on natural killer cell frequency, activation, chemokine receptor expression and function

Understanding the role of natural killer (NK) cells in human disease pathogenesis is crucial and necessitates study of patient samples directly ex vivo. Manipulation of whole blood by density gradient centrifugation or delays in sample processing due to shipping, however, may lead to artifactual changes in immune response measures. Here, we assessed the impact of density gradient centrifugation and delayed processing of both whole blood and peripheral blood mononuclear cells (PBMC) at multiple timepoints (2-24 h) on flow cytometric measures of NK cell frequency, activation status, chemokine receptor expression, and effector functions. We found that density gradient centrifugation activated the NK cells and modified the chemokine receptor expression. Delays in processing beyond 8h activated NK cells in PBMC but not in whole blood. Likewise, processing delays decreased chemokine receptor (CCR4 and CCR7) expression in both PBMC and whole blood. Finally, delays in processing PBMC were associated with a decreased ability of NK cells to degranulate (as measured by CD107a expression) or secrete cytokines (IFN-γ and TNF-α). In summary, our findings suggest that density gradient centrifugation and delayed processing of PBMC can alter measures of clinically relevant NK cell characteristics including effector functions; and therefore should be taken into account in designing clinical research studies.

Human conjunctival transcriptome analysis reveals the prominence of innate defense in Chlamydia trachomatis infection

Trachoma is the leading infectious cause of blindness and is endemic in 52 countries. There is a critical need to further our understanding of the host response during disease and infection, as millions of individuals are still at risk of developing blinding sequelae. Infection of the conjunctival epithelial cells by the causative bacterium, Chlamydia trachomatis, stimulates an acute host response. The main clinical feature is a follicular conjunctivitis that is incompletely defined at the tissue-specific gene expression and molecular levels. To explore the features of disease and the response to infection, we measured host gene expression in conjunctival samples from Gambian children with active trachoma and healthy controls. Genome-wide expression and transcription network analysis identified signatures characteristic of the expected infiltrating immune cell populations, such as neutrophils and T/B lymphocytes. The expression signatures were also significantly enriched for genes in pathways which regulate NK cell activation and cytotoxicity, antigen processing and presentation, chemokines, cytokines, and cytokine receptors. The data suggest that in addition to polymorph and adaptive cellular responses, NK cells may contribute to a significant component of the conjunctival inflammatory response to chlamydial infection.