Martin R. Goodier

Switch from inhibitory to activating NKG2 receptor expression in HIV-1 infection: lack of reversion with highly active antiretroviral therapy.

Background:HIV-1 infection is characterized by increase in inhibitory receptors and loss of activating receptors on natural killer (NK) cells, resulting in loss of cell activity. Exceptionally, for an inhibitory receptor, the proportion of NK cells bearing CD94-NKG2A decreases during HIV-1 infection. It is not understood whether HIV-1 itself or other concomitant infections drive these changes. Objectives:To investigate the relationship between HIV-1 viraemia and changes in C-type lectin-like receptor expression in NK cells and to investigate the effect of highly active antiretroviral therapy (HAART) on these changes. Methods:Three cohorts of patients were studied: (1) before, during and after treatment interruption in aviraemic and viraemic patients receiving HAART (n = 15); (2) HIV-1-positive treatment-naive individuals (n = 13); and (3) HIV-1-positive individuals receiving successful HAART for a minimum of 1 year without interruption (n = 11). Flow cytometry was used to study the expression of NKG2A before and after treatment interruption and to define expanded populations of NK cells in untreated and treated HIV-1-positive individuals. Assays were performed in vitro to assess the cytotoxicity of the expanded populations. Results:Increases in plasma HIV-1 RNA during treatment interruption in aviraemic HAART-treated individuals did not influence the proportion of NK cells carrying the complex CD94-NKG2A. Loss of NKG2A NK cells corresponded to the dramatic expansion of a distinct population of cells expressing a functional activating CD94-NKG2C receptor with skewed expression of killer cell immunoglobulin-like receptor family and natural cytotoxicity receptors. Conclusion:Changes in the NK cell repertoire during HIV-1 infection were not a result of HIV-1 viraemia alone but resembled those associated with concomitant infections.

Functional significance of CD57 expression on human NK cells and relevance to disease

Historically, human NK cells have been identified as CD3−CD56+CD16± lymphocytes. More recently it has been established that CD57 expression defines functionally discrete sub-populations of NK cells. On T cells, CD57 expression has been regarded as a marker of terminal differentiation and (perhaps wrongly) of anergy and senescence. Similarly, CD57 expression seems to identify the final stages of peripheral NK cell maturation; its expression increases with age and is associated with chronic infections, particularly human cytomegalovirus infection. However, CD57+ NK cells are highly cytotoxic and their presence seems to be beneficial in a number of non-communicable diseases. The purpose of this article is to review our current understanding of CD57 expression as a marker of NK cell function and disease prognosis, as well as to outline areas for further research.

Rapid NK cell differentiation in a population with near-universal human cytomegalovirus infection is attenuated by NKG2C deletions

Natural killer (NK) cells differentiate and mature during the human life course; human cytomegalovirus (HCMV) infection is a known driver of this process. We have explored human NK cell phenotypic and functional maturation in a rural African (Gambian) population with a high prevalence of HCMV. The effect of age on the frequency, absolute number, phenotype, and functional capacity of NK cells was monitored in 191 individuals aged from 1 to 49 years. Increasing frequencies of NK cells with age were associated with increased proportions of CD56dim cells expressing the differentiation marker CD57 and expansion of the NKG2C+ subset. Frequencies of NK cells responding to exogenous cytokines declined with age in line with a decreased proportion of CD57- cells. These changes coincided with a highly significant drop in anti-HCMV IgG titers by the age of 10 years, suggesting that HCMV infection is brought under control as NK cells differentiate (or vice versa). Deletion at the NKG2C locus was associated with a gene dose-dependent reduction in proportions of CD94+ and CD57+ NK cells. Importantly, anti-HCMV IgG titers were significantly elevated in NKG2C-/- children, suggesting that lack of expression of NKG2C may be associated with altered control of HCMV in childhood.

PD-1 Expression on Natural Killer Cells and CD8+ T Cells During Chronic HIV-1 Infection

Programmed death receptor 1 (PD-1) is an important marker of T-cell exhaustion during HIV-1 infection. Natural killer (NK) cells lose their functional capacity during HIV-1 infection, and PD-1 is expressed on NK cells during other chronic viral and bacterial infections. Here, PD-1 expression was increased on NK cells from both viremic and aviremic HIV-1-seropositive individuals, compared to seronegative controls. However, PD-1 was expressed on a small subset of NK cells and at lower frequency than that observed for CD8(+) T cells. PD-1 was also induced on a minor fraction of NK cells and CD8(+) T cells after long-term culture with IL-15. Raised levels of PD-1 were associated with limited NK cell proliferation, which may have consequences for their maintenance during chronic HIV-1 infection.

Loss of the CD56hiCD16− NK cell subset and NK cell interferon-γ production during antiretroviral therapy for HIV-1: partial recovery by human growth hormone

Previous studies have shown that human natural killer (NK) cells are lost from the periphery and are functionally suppressed during HIV‐1 infection, and that the administration of highly active antiretroviral therapy (HAART) results in a recovery of NK cell numbers in HIV‐1‐infected individuals. However, despite this recovery, interleukin (IL)‐2 + IL‐12‐driven interferon (IFN)‐γ production by NK cells has been shown to remain suppressed after HAART. Here we show that the innate immune factor IL‐15 in combination with IL‐12 is also unable to recover NK cell IFN‐γ production in HAART‐treated individuals. Furthermore, we also demonstrate an imbalance in the distribution of CD56loCD16hi and CD56hiCD16– NK subsets after successful HAART, CD56hiCD16– cells being reduced substantially in HIV‐1 patients on HAART. Treatment of patients with combined human growth hormone and antiretroviral therapy resulted in further enhancement in the absolute numbers and the proportion of NK cells in some individuals in the absence of parallel effects on CD4+ T cells. Furthermore, in these individuals HAART with growth hormone resulted in an enhancement of cytokine‐driven NK cell activation and IFN‐γ production compared to the HAART‐only baseline.

The response of gamma delta T cells to Plasmodium falciparum is dependent on activated CD4+ T cells and the recognition of MHC class I molecules.

Peripheral blood γδ T cells from non‐exposed individuals respond to antigens of the malaria parasite, Plasmodium falciparum,in vitro. This response, largely caused by T cells bearing the Vγ9+ chain of the T‐cell receptor, is stimulated by components of the parasite expressed on the schizont stage and released at schizont rupture. The response of Vγ9+ T cells to parasite components is inhibited by antibodies to major histocompatibility complex (MHC) class I and class II. However, the inhibition by anti‐MHC class II antibodies can be overcome by the addition of interleukin‐2 (IL‐2) to the cultures, suggesting that γδ T cells themselves do not recognize MHC class II molecules but require an MHC class II‐dependent response taking place in the culture. In contrast, the inhibition by anti‐class I antibodies cannot be reversed by addition of IL‐2. Since an accompanying CD4+ T‐cell response occurred in peripheral blood mononuclear cells cultured with P. falciparum antigens, it was considered that these cells provide the cytokines necessary for the subsequent activation and expansion of Vγ9+ T cells recognizing components of the parasite and MHC class I molecules. This was confirmed by reconstituting the response of enriched γδ T cells to P. falciparum schizont extract by addition of purified CD4+ T cells.

Increased proportion of CD16+ NK cells in the colonic lamina propria of inflammatory bowel disease patients, but not after azathioprine treatment

Aliment Pharmacol Ther 2011; 33: 115–126

Impaired NK Cell Responses to Pertussis and H1N1 Influenza Vaccine Antigens in Human Cytomegalovirus-Infected Individuals

NK cells contribute to postvaccination immune responses after activation by IL-2 from Ag-specific memory T cells or by cross-linking of the low-affinity IgG receptor, CD16, by Ag–Ab immune complexes. Sensitivity of NK cells to these signals from the adaptive immune system is heterogeneous and influenced by their stage of differentiation. CD56dimCD57+ NK cells are less responsive to IL-2 and produce less IFN-γ in response to T cell–mediated activation than do CD56bright or CD56dimCD57− NK cells. Conversely, NK cell cytotoxicity, as measured by degranulation, is maintained across the CD56dim subsets. Human CMV (HCMV), a highly prevalent herpes virus causing lifelong, usually latent, infections, drives the expansion of the CD56dimCD57+NKG2C+ NK cell population, skewing the NK cell repertoire in favor of cytotoxic responses at the expense of cytokine-driven responses. We hypothesized, therefore, that HCMV seropositivity would be associated with altered NK cell responses to vaccine Ags. In a cross-sectional study of 152 U.K. adults, with HCMV seroprevalence rate of 36%, we find that HCMV seropositivity is associated with lower NK cell IFN-γ production and degranulation after in vitro restimulation with pertussis or H1N1 influenza vaccine Ags. Higher expression of CD57/NKG2C and lower expression of IL-18Rα on NK cells from HCMV seropositive subjects do not fully explain these impaired responses, which are likely the result of multiple receptor–ligand interactions. This study demonstrates for the first time, to our knowledge, that HCMV serostatus influences NK cell contributions to adaptive immunity and raises important questions regarding the impact of HCMV infection on vaccine efficacy.

Differential activation of CD57-defined natural killer cell subsets during recall responses to vaccine antigens

Natural killer (NK) cells contribute to the effector phase of vaccine‐induced adaptive immune responses, secreting cytokines and releasing cytotoxic granules. The proportion of responding NK cells varies between individuals and by vaccine, suggesting that functionally discrete subsets of NK cells with different activation requirements may be involved. Here, we have used responses to individual components of the DTP vaccine [tetanus toxoid (TT), diphtheria toxoid (DT), whole cell inactivated pertussis] to characterize the NK cell subsets involved in interleukin‐2‐dependent recall responses. Culture with TT, DT or pertussis induced NK cell CD25 expression and interferon‐γ production in previously vaccinated individuals. Responses were the most robust against whole cell pertussis, with responses to TT being particularly low. Functional analysis of discrete NK cell subsets revealed that transition from CD56bright to CD56dim correlated with increased responsiveness to CD16 cross‐linking, whereas increasing CD57 expression correlated with a loss of responsiveness to cytokines. A higher frequency of CD56dim CD57− NK cells expressed CD25 and interferon‐γ following stimulation with vaccine antigen compared with CD56dim CD57+ NK cells and made the largest overall contribution to this response. CD56dim CD57int NK cells represent an intermediate functional phenotype in response to vaccine‐induced and receptor‐mediated stimuli. These findings have implications for the ability of NK cells to contribute to the effector response after vaccination and for vaccine‐induced immunity in older individuals.

Depletion of natural killer cells in the colonic lamina propria of viraemic HIV-1-infected individuals.

Background:HIV-1 infection is known to have a detrimental impact on peripheral blood natural killer cell phenotype and function. Chronic HIV-1 also causes a substantial depletion of CD4+ T cells in the gastrointestinal tract and the blood. Objective:To investigate the impact of chronic HIV-1 infection with on natural killer cell populations in the gastrointestinal tract and the effect of suppression of plasma viraemia with antiretroviral therapy. Methods:Lymphocyte populations were extracted from the lamina propria of biopsies taken from the sigmoid colon of HIV-1-infected and uninfected individuals. The proportions of natural killer cell subsets were compared in viraemic (n = 15) and aviraemic HIV-1-positive, HAART-treated individuals (n = 27) and HIV-1 negative control individuals (n = 26) using flow cytometry on gated subsets. Results:Natural killer cells are depleted in colonic biopsies from HIV-1-infected individuals with detectable plasma virus in comparison with HIV-1-negative individuals. A significant increase in the proportion of both natural killer and CD4+ T cells in the colonic lamina propria is observed in aviraemic individuals compared to viraemic individuals. Conclusions:Chronic HIV-1 infection results in depletion of both natural killer cells and CD4+ T cells in colonic tissue and antiretroviral therapy results in a recovery of these subsets in individuals with undetectable plasma viral load.