Natural Killer Cell Memory

Natural Killer (NK) Cells

  • NK cells are one of the members of the innate lymphoid cells (ILCs) family. NK cells share characteristics with both, innate and adaptive immune cells such as a rapid responsiveness (innate), cytotoxic potential and memory (adaptive). They are also considered as a nexus or bridge between the innate and adaptive immune system.
  • NK cell development occurs in the bone marrow (BM) from common lymphocyte progenitors (CLP) and their development depends on IL-15, which induces the expression of transcription factors such as Nfil3, Id2 and Eomesodermin (Eomes). IL-15 is also involved in their survival and innate effector functions by regulating their metabolism. After their early stages of development, NK cells continue their maturation in the BM and then in the periphery, expressing different transcription factors, surface and effector molecules. Mature NK cells can be found in lymphoid and non-lymphoid organs like peripheral blood (PB), a characteristic that distinguishes them from other ILCs.
  • Mature NK cells in mice are characterized by the expression of NK1.1+CD3– NKp46+ Tbet+ Eomes+ CD49b+. CD11b+ CD27– NK cells are considered the most mature NK cells in mice and represent the majority (~90%) of NK cells in PB. These NK cells have a high cytotoxic potential and IFN-y production after target cell recognition.

Mature NK cells in humans can be divided into 2 types:

  • CD56dimCD16+: highly cytotoxic NK cells predominantly found in PB
  • CD56brightCD16+: high cytokine production capacity, predominantly found in secondary lymphoid tissues.

 

Figure 1: Mouse NK cell development and maturation. (a) NK cells develop in the bone marrow from the CLP, and NK cell progenitors are marked by upregulation of IL-2/15Rβ. NK cell development and maturation are highly dependent on IL-15 and transcriptional programs implemented by transcription factors such as Eomes, Nfil3, Ets1, T-bet, Id2, Tox, and Tcf1. CD49b expression delineates mature NK cells, which are found in the bone marrow as well as in secondary lymphoid and non-lymphoid tissues. (b) Peripheral NK cells are heterogeneous in their maturation state, which is characterized by differential expression of surface markers and functional specialization. (c) NK cells circulate systemically and can be distinguished from tissue-resident ILC1s, including those identified in the liver. Colored and directional gradients designate varying levels of indicated proteins or functions. Abbreviations: αLP, all-lymphoid progenitor; CLP, common lymphoid progenitor; ILC1, group 1 innate lymphoid cell; iNK, immature NK cell; mNK, mature NK cell; NK, natural killer
NKP, NK cell progenitor. [Source: Adriana M. Mujal et al., 2021]
Figure 2: Model of “branched” human NK cell development. Starting from HSC in the BM (CD34+ cells), NK cell development proceeds by steps, each characterized by a peculiar surface phenotype reported for each stage. The linear pathway has been characterized in SLTs and goes through stage 2 (CD34+CD117+ cells), stage 3 (CD34–CD117+ cells), stage 4 (CD94+CD16−CD56bright NK cells), stage 5 (CD16+KIR+CD56dim NK cells), and stage 6 (the CD56dimCD57+ terminally differentiated NK cells). In addition, HSC can generate CD117+ ILCP that are localized in PB and peripheral tissues and are analogous to stage 3 NK cells in SLT. ILCPs generate NK cells and helper ILCs. Moreover, in vitro and in humanized mice a myeloid precursor has been identified (CD33+CD56+), which also generates NK cells. Finally, it was reported that a lymphoid precursor characterized by a CD34+DNAM1brightCXCR4+ phenotype residing in the BM is mobilized in the PB in chronic inflammation and is able to generate NK cells and α/β-T cells, as well as mature NKG2C+CD57+ “adaptive” NK cell. [Source: Quatrini, L. et al., 2021.]
  • Mature NK cells are involved in strong anti-tumor responses and also represent the first line defense against viral infections. The mechanisms responsible for a protective immune response brought about by NK cells are diverse and include: high cytotoxic potential, production of inflammatory cytokines  (IFN-y and TNF) and also death receptor ligation (FAS-FASL).

 

Adaptive/memory NK cells

  • Recently the concept of “trained immunity” or “innate immune memory” has been reported and it refers to the ability of innate immune system to take on characteristics of the adaptive immune system In both human and mouse models, NK cells have demonstrated  immunological memory after encountering  some viruses or inflammatory cytokine milieux.
    • Memory NK cells in mouse models
      • It has been reported that mice lacking B and T cells develop contact hypersensitivity (CHS) after hapten sensitization. CHS, however, did not develop in these mice after depletion of NK cells using NK1.1 antibodies (Ab). Also, this response has been demonstrated to be sensitization-dependent and hapten-specific.
      • Specific memory NK cells have also been reported in some virus models like mouse cytomegalovirus (MCMV)
      • RAG2-deficient mice adoptively transferred with In-vitro IL-12, IL-15 and IL-18 stimulated NK cells in absence of a defined antigen, has been able to generate NK cells that respond with sustained IFN-y production after IL-12 and/or IL-15 in-vivo stimulation.
      • The receptors driving the generation of memory NK cells in mice remain unclear.
    • Memory NK cells in humans
      • Memory NK cells have been reported in individuals who have previously encountered  human cytomegalovirus (HCMV). These memory NK cells have been defined by the expression of activation CD94-NKG2C receptor.
      • In-vitro experiments have demonstrated CD94-NKG2C+ NK cells expansion after the CD94-NKG2C receptor interacts with its ligand HLA-E. This expansion also has been reported after cell-cell contact with infected cells and soluble factors.

 

Figure 3: Pathways for the generation of memory NK cells. a | Following sensitization of mice with haptens, hapten-specific memory natural killer (NK) cells are detected in the liver. The generation of these memory NK cells is dependent on the cytokines interleukin-12 (IL-12), interferon-γ (IFNγ) and IFNα. Antibody-mediated blockade of the NK cell receptor natural killer group 2, member D (NKG2D) or CXC-chemokine receptor 6 (CXCR6), or molecules that are involved in NK cell trafficking (such as CD18 and P- and E-selectin) prevents the development of contact hypersensitivity (CHS) responses in the ear after hapten challenge. b | During mouse cytomegalovirus (MCMV)-infection, naive NK cells expressing the LY49H receptor expand with contribution of signalling mediated by the DNAX accessory molecule 1 (DNAM1) receptor and the inflammatory cytokines IL-12, IL-18 and IL-33. Inflammatory cytokines drive the expression of zinc finger and BTB domain-containing 32 (ZBTB32) and the microRNA miR-155, which are involved in the expansion of ‘effector’ NK cells. Following the elimination of the virus, the BIM and autophagy pathways regulate the contraction of the expanded populations of NK cells, giving rise to a population of MCMV-specific memory NK cells. Although MCMV-specific memory NK cells distribute systemically in mice, memory NK cells specific for vaccinia virus reside in the liver, and influenza virus-specific memory NK cells are found in the liver and lungs (not shown). In non-human primates, simian immunodeficiency virus-specific memory NK cells reside in the spleen and liver (not shown). c | In vitro, the brief exposure of NK cells to the cytokines IL-12, IL-15 and IL-18 results in the upregulation of IFNγ, perforin and granzymes, and the production of high levels of CD25, the high-affinity α-chain of the IL-2 receptor, is also induced. After adoptive transfer, these cytokine-induced memory NK cells persist long term, and their ability to produce abundant cytokines and express perforin and granzymes is maintained. The presence of IL-2 (or IL-15) further increases NK cell numbers and their ability to express IFNγ, perforin and granzymes after adoptive transfer. [Source: Cerwenka A. et al., 2016.]
Figure 4: NK cell memory in HCMV-seropositive donors. a | In human cytomegalovirus (HCMV)-seropositive donors, a population of CD94–NKG2C (natural killer group 2, member C)-positive NK cells expands on interaction of CD94–NKG2C with HLA-E that is upregulated on fibroblasts during HCMV infection. Monocytes producing interleukin-12 (IL-12) have an essential role in supporting the expansion of these NK cell populations. NK cell expansion in response to virus-infected fibroblasts can also occur in NKG2C-deficient individuals, but the relevant receptors and ligands that are involved in this setting are poorly understood. b | A population of CD94–NKG2C+ high-affinity IgE receptor subunit-γ (FcεRIγ)-negative NK cells has been identified in the sera of HCMV-seropositive individuals that express low amounts of SYK and/or EWS/FLI1-activated transcript 2 (EAT2) and can be expanded by co-culture with HCMV-infected fibroblasts coated with HCMV-specific antibodies. LILRB1, leukocyte immunoglobulin-like receptor subfamily B member 1. [Source: Cerwenka A. et al., 2016.]
  • As has been mentioned “innate immune memory” cells are characterized by their ability to rapidly expand and respond by increasing production of effector molecules after a re-encounter with a pathogen. Memory NK cells have been demonstrated to produce higher levels of IFN-y in comparison to  naïve NK cells after MCMV re-infection but not with heterologous pathogens in mouse models. Another characteristic that distinguishes memory NK cells from naïve NK cells are the stable epigenetic changes. The expansion of CD94-NKG2C human NK cells and their robust cytotoxic response has been reported in HCMV-seropositive individuals. These cells also exhibited epigenetic changes such as a demethylated CNS1 region in the IFNG locus.

 

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References
  • Quatrini, L., Della Chiesa, M., Sivori, S., Mingari, M.C., Pende, D. and Moretta, L. (2021), Human NK cells, their receptors and function. Eur. J. Immunol., 51: 1566-1579. https://doi.org/10.1002/eji.202049028
  • Cerwenka A, Lanier LL. Natural killer cell memory in infection, inflammation and cancer. Nat Rev Immunol. 2016 Feb;16(2):112-23. doi: 10.1038/nri.2015.9. Epub 2016 Jan 25. PMID: 26806484.
  • Mujal A, Delconte R, Sun J. Natural Killer Cells: From Innate to Adaptive Features. Annual Review of Immunology 2021 39:1, 417-447