Dynamic changes in natural killer cell subset frequencies in the absence of cytomegalovirus infection

Abstract

Individuals lacking functional natural killer (NK) cells suffer severe, recurrent infections with cytomegalovirus (CMV), highlighting the critical role of NK cells in antiviral defense. Therefore, ongoing attempts to develop an efficacious vaccine to prevent CMV infection should potentially aim to elicit NK-cell antiviral responses as an accessory to conventional T-and B-cell based approaches. In this regard, CMV infection provokes marked phenotypic and functional differentiation of the NK-cell compartment, including development of adaptive NK cells that exhibit enhanced antiviral activity. We examined longitudinal blood samples collected from 40 CMV-seronegative adolescents to ascertain whether a CMV glycoprotein B (gB) vaccine in the absence of CMV infection can stimulate differentiation or expansion of CMV-associated subsets of NK cells. Study participants uniformly lacked the CMV-dependent NKG2C + subset of NK cells, suggesting that an adjuvanted CMV gB vaccine alone is an inadequate stimulus for sustained expansion of these cells. In contrast, we observed unexpected dynamic fluctuations in the frequency of NK cells lacking FcR, EAT-2, and SYK, which were independent of vaccination or CMV infection. Whereas FcR neg NK cells in CMV infection are reported to express increased levels of the maturation marker CD57, the FcR neg NK cells observed in our CMV-negative vaccine cohort express less CD57 than their FcR + counterparts. The FcR neg NK cells in CMV-negative individuals were also functionally distinct from this subset in CMV infection, exhibiting comparable IFN- production and degranulation as FcR + NK cells in response to cytokine or antibody-dependent stimuli. These results suggest that frequencies of some NK cell subsets may increase in response to unknown environmental or inflammatory cues distinct from that which occurs after CMV infection. Greater understanding of the nature of the signals driving CMVindependent accumulation of these subsets should permit development of mechanisms to facilitate vaccine-driven expansion of CMV-reactive NK cells.Vaccine | HCMV | Innate lymphoid cell | NK cell | Memory | CD56 | CD57 | FcR | SYK | NKG2C | EAT-2

Regulation of the human NK cell compartment by pathogens and vaccines

“…12 However, expansions of NK cells lacking FceR1c, EAT-2 and Syk1 but with intermediate levels of CD57 expression are also observed in HCMV seronegative individuals, highlighting the potential for peripheral NK cell differentiation to be driven by other mechanisms. 15 The potential for a non-linear, HCMV-independent pathway for functional NK cell diversification is further supported by the observation of dynamic, intra-individual fluctuations in the frequencies of these cells over a protracted period of time in HCMV seronegative individuals. 15 HCMV-infected myeloid cells and fibroblasts are in many ways the archetypic example of pathogen-induced NK cell activation and differentiation, both directly via binding of HCMV-induced ligands on infected host cells to NK cell surface receptors including NKG2C, LILRB1 and activating KIR and indirectly through the action of pro-inflammatory cytokines, interleukin-2 and antibodies emanating from other immune cells.…”

“…15 The potential for a non-linear, HCMV-independent pathway for functional NK cell diversification is further supported by the observation of dynamic, intra-individual fluctuations in the frequencies of these cells over a protracted period of time in HCMV seronegative individuals. 15 HCMV-infected myeloid cells and fibroblasts are in many ways the archetypic example of pathogen-induced NK cell activation and differentiation, both directly via binding of HCMV-induced ligands on infected host cells to NK cell surface receptors including NKG2C, LILRB1 and activating KIR and indirectly through the action of pro-inflammatory cytokines, interleukin-2 and antibodies emanating from other immune cells. 16,17 Human 'adaptive' NK cells (defined by the expression of NKG2C or loss of PLZF/Fcer1c associated pathways and induced by cognate interaction between NKG2C and HLA-E on HCMVinfected cells) share many features of murine 'memory' NK cells induced by binding of the murine cytomegalovirus (MCMV) m157 protein to NK Ly49h receptors.…”

“…Expansions of NK cells expressing CD57 and NKG2C (a receptor recognising cognate HLA‐E stabilised on HCMV‐infected cells by peptides from the UL40 viral protein) and lacking PLZF and FcεR1γ are frequently found in bone marrow transplant patients experiencing HCMV infection or reactivation and are elevated in frequency in HCMV seropositive compared to seronegative individuals 12 . However, expansions of NK cells lacking FcεR1γ, EAT‐2 and Syk1 but with intermediate levels of CD57 expression are also observed in HCMV seronegative individuals, highlighting the potential for peripheral NK cell differentiation to be driven by other mechanisms 15 . The potential for a non‐linear, HCMV‐independent pathway for functional NK cell diversification is further supported by the observation of dynamic, intra‐individual fluctuations in the frequencies of these cells over a protracted period of time in HCMV seronegative individuals 15 …”

“…However, expansions of NK cells lacking FcεR1γ, EAT‐2 and Syk1 but with intermediate levels of CD57 expression are also observed in HCMV seronegative individuals, highlighting the potential for peripheral NK cell differentiation to be driven by other mechanisms 15 . The potential for a non‐linear, HCMV‐independent pathway for functional NK cell diversification is further supported by the observation of dynamic, intra‐individual fluctuations in the frequencies of these cells over a protracted period of time in HCMV seronegative individuals 15 …”

Natural killer cell division regulates FcεRIγ expression downstream of mTOR activity

“…In some individuals, a unique subset of mature CD56 dim CD16 + NKG2A - NKG2C high NK cells are expanded during acute HCMV infection, leading to their designation as “adaptive” NK cells 14,15 . However, NKG2C-negative CD56 dim CD16 + NKG2A + FcRγ -/low or CD56 dim CD16 + NKG2A - FcRγ -/low NK cells are also present both in HCMV-infected and uninfected individuals, indicating they may be expanded due to stimuli other than HCMV 16,17 . In addition, in some individuals, lower FcRγ levels are associated with genes such as SYK, SH2D1B (EAT2 ) , and/or ZBTB16 (PLZF) which are downregulated in adaptive CD56 dim FcRγ -/low NK cells by DNA methylation 11,12,18 .…”