Endi K. Santosa

Natural killer (NK) cells are innate lymphocytes that play a critical role in host defense against viral infection. In addition to rapid effector cytokine production and direct cytotoxicity, NK cells exhibit features of adaptive immunity, including the capacity to undergo robust antigen-specific clonal proliferation and to generate immunological memory^1-3. However, the transcriptional programs and regulators governing dynamic NK cell responses to viral infection have not been fully uncovered. In this study, we identified transcription factor 19 (TCF19) as a key driver of NK cell proliferation and calcium signaling in the context of mouse cytomegalovirus infection. Ablation of TCF19 was detrimental to NK cell clonal expansion and host protection against viral infection. Tcf19^-/- NK cells were also unable to properly mobilize calcium downstream of antigen signaling to mediate cytotoxicity. Altogether, we find that TCF19 drives a transcriptional program that coordinates the innate and adaptive NK cell responses against viral infection.

Latent viral infections rely on a precise coordination of the immune response to control sporadic viral reactivation. CD8+ T cells play a crucial role in controlling viral latency by generating diverse memory responses in an epitope-specific manner. Among these distinct responses, conventional and inflationary memory responses have been described during herpesvirus infections. Using a newly generated TCR transgenic mouse strain, we investigated the transcriptomic and epigenetic remodeling of distinct epitope-specific CD8+ T cells during CMV infection across tissues at both population and single-cell levels. Our findings reveal that whereas the transcriptomic and epigenetic landscapes of conventional and inflationary memory responses diverge in the spleen and liver, these molecular programs converge in the salivary gland, a site of CMV persistence. Thus, we provide evidence that the dynamics of memory CD8+ T cell responses are distinct between tissues.

Natural killer (NK) cells are innate lymphocytes that exhibit adaptive traits particularly evident during cytomegalovirus (CMV) infection. Following mouse CMV (MCMV) infection, NK cells upregulate the transcription factors IRF4 and IRF8, which are indispensable for their survival and proliferation upon viral infection. However, it is unclear whether these factors are expressed within the same individual cell and whether deficiency in one could be compensated by the other. In this study, we observed that a subset of NK cells co-express high levels of IRF4 and IRF8 in an NFκB-dependent manner. These IRF4HighIRF8High NK cells are specifically enriched for activated but immature cells with high proliferative potential during MCMV infection. Functionally, NK cells lacking both IRF4 and IRF8 develop normally, but experience a more severe expansion defect during virus exposure compared to NK cells deficient in a single factor. Thus, our study reveals a cooperative interplay between IRF4- and IRF8-dependent transcriptional networks in regulating NK-cell antiviral responses.

The ability of vertebrates to ‘remember’ previous infections had once been attributed exclusively to adaptive immunity. We now appreciate that innate lymphocytes also possess memory properties akin to those of adaptive immune cells. In this Review, we draw parallels from T cell biology to explore the key features of immune memory in innate lymphocytes, including quantity, quality, and location. We discuss the signals that trigger clonal or clonal-like expansion in innate lymphocytes, and highlight recent studies that shed light on the complex cellular and molecular crosstalk between metabolism, epigenetics, and transcription responsible for differentiating innate lymphocyte responses towards a memory fate. Additionally, we explore emerging evidence that activated innate lymphocytes relocate and establish themselves in specific peripheral tissues during infection, which may facilitate an accelerated response program akin to those of tissue-resident memory T cells.

Natural killer (NK) cells are innate cytotoxic lymphocytes with adaptive immune features, including antigen specificity, clonal expansion and memory. As such, NK cells share many transcriptional and epigenetic programs with their adaptive CD8⁺ T cell siblings. Various signals ranging from antigen, co-stimulation and proinflammatory cytokines are required for optimal NK cell responses in mice and humans during virus infection; however, the integration of these signals remains unclear. In this study, we identified that the transcription factor IRF4 integrates signals to coordinate the NK cell response during mouse cytomegalovirus infection. Loss of IRF4 was detrimental to the expansion and differentiation of virus-specific NK cells. This defect was partially attributed to the inability of IRF4-deficient NK cells to uptake nutrients required for survival and memory generation. Altogether, these data suggest that IRF4 is a signal integrator that acts as a secondary metabolic checkpoint to orchestrate the adaptive response of NK cells during viral infection.

Immune cells responding to pathogens undergo molecular changes that are intimately linked to genome organization. Recent work has demonstrated that natural killer (NK) and CD8 ⁺ T cells experience substantial transcriptomic and epigenetic rewiring during their differentiation from naïve to effector to memory cells. Whether these molecular adaptations are accompanied by changes in three-dimensional (3D) chromatin architecture is unknown. In this study, we combine histone profiling, ATAC-seq, RNA-seq and high-throughput chromatin capture (HiC) assay to investigate the dynamics of one-dimensional (1D) and 3D chromatin during the differentiation of innate and adaptive lymphocytes. To this end, we discovered a coordinated 1D and 3D epigenetic remodeling during innate immune memory differentiation, and demonstrate that effector CD8 ⁺ T cells adopt an NK-like architectural program that is maintained in memory cells. Altogether, our study reveals the dynamic nature of the 1D and 3D genome during the formation of innate and adaptive immunological memory.