Laura Surace

Cytotoxicity is a cornerstone of immune defense, critical for combating tumors and infections. This process relies on the coordinated action of granzymes and pore-forming proteins, with Granzyme B (GZMB) and Perforin ( PRF1 ) being key markers and the most widely studied molecules pertaining to cytotoxicity. However, other human granzymes and cytotoxic components remain underexplored, despite growing evidence of their distinct, context-dependent roles. Natural Killer Cell Granule Protein 7 (NKG7) has recently emerged as a crucial cytotoxicity regulator, yet its expression patterns and function are poorly understood. Using large publicly available single-cell RNA sequencing atlases, we performed a comprehensive profiling of cytotoxicity across immune subsets and tissues. Our analysis highlights NKG7 expression as a strong marker of cytotoxicity, exhibiting a strong correlation with overall cytotoxic activity (r = 0.97) and surpassing traditional markers such as Granzyme B and Perforin in reliability. Furthermore, NKG7 expression is notably consistent across diverse immune subsets and tissues, reinforcing its versatility and robustness as a cytotoxicity marker. These findings position NKG7 as an invaluable tool for evaluating immune responses and a reliable indicator of cytotoxic functionality across biological and clinical contexts.

Anorexia and fasting are host adaptations to acute infection, inducing a metabolic switch towards ketogenesis and the production of ketone bodies, including β-hydroxybutyrate (BHB) ^1-6. However, whether ketogenesis metabolically influences the immune response in pulmonary infections remains unclear. Here we report impaired production of BHB in humans with SARS-CoV-2-induced but not influenza-induced acute respiratory distress syndrome (ARDS). CD4+ T cell function is impaired in COVID-19 and BHB promotes both survival and production of Interferon-γ from CD4⁺ T cells. Using metabolic tracing analysis, we uncovered that BHB provides an alternative carbon source to fuel oxidative phosphorylation (OXPHOS) and the production of bioenergetic amino acids and glutathione, which is important for maintaining the redox balance. T cells from patients with SARS-CoV-2-induced ARDS were exhausted and skewed towards glycolysis, but can be metabolically reprogrammed by BHB to perform OXPHOS, thereby increasing their functionality. Finally, we demonstrate that ketogenic diet (KD) and delivery of BHB as ketone ester drink restores CD4+ T cell metabolism and function in respiratory infections, ultimately reducing the mortality of SARS-CoV-2 infected mice. Altogether, our data reveal BHB as alternative carbon source promoting T cell responses in pulmonary viral infections, highlighting impaired ketogenesis as a potential confounder of severe COVID-19.

Purpose of review: Innate lymphoid cells (ILCs) are specialized immune cells that rapidly sense environmental perturbations and regulate immune responses and tissue homeostasis. ILCs are mainly tissue resident and their crosstalk within tissue microenvironments influences both local and systemic metabolism. Reciprocally, metabolic status conditions ILC phenotype and effector function. In this review, we discuss the role of ILCs as metabolic sentinels and describe how ILC subset-specific activities influence homeostasis and disease. Finally, we highlight emerging challenges in the field of ILC immunometabolism. Recent findings: Accumulating evidence suggests that ILCs metabolism, phenotype, and function are shaped by signals from the tissue microenvironment. Dietary, endogenous, and microbial metabolites are sensed by ILC subsets and can impact on ILC-mediated immune responses. Recent studies have found that mitochondria are central regulators of ILC effector function. Furthermore, ILCs have emerged as crucial sensors of metabolic stress, suggesting they might act as metabolic sentinels, coordinating tissue and host metabolism. Summary: Our understanding how ILCs mechanistically regulate host metabolism and defenses is still incomplete. Unraveling critical metabolic features of ILCs may lead to novel therapeutic strategies that target these cells in the context of disease.

Group 3 innate lymphoid cells (ILC3s) are innate immune effectors that contribute to host defense. Whether ILC3 functions are stably modified after pathogen encounter is unknown. Here, we assess the impact of a time-restricted enterobacterial challenge to long-term ILC3 activation in mice. We found that intestinal ILC3s persist for months in an activated state after exposure to Citrobacter rodentium . Upon rechallenge, these “trained” ILC3s proliferate, display enhanced interleukin-22 (IL-22) responses, and have a superior capacity to control infection compared with naïve ILC3s. Metabolic changes occur in C. rodentium –exposed ILC3s, but only trained ILC3s have an enhanced proliferative capacity that contributes to increased IL-22 production. Accordingly, a limited encounter with a pathogen can promote durable phenotypic and functional changes in intestinal ILC3s that contribute to long-term mucosal defense.

Group 2 innate lymphoid cells (ILC2s) represent innate homologs of type 2 helper T cells (T _H 2) that participate in immune defense and tissue homeostasis through production of type 2 cytokines. While T lymphocytes metabolically adapt to microenvironmental changes, knowledge of human ILC2 metabolism is limited, and its key regulators are unknown. Here, we show that circulating ‘naive’ ILC2s have an unexpected metabolic profile with a higher level of oxidative phosphorylation (OXPHOS) than natural killer (NK) cells. Accordingly, ILC2s are severely reduced in individuals with mitochondrial disease (MD) and impaired OXPHOS. Metabolomic and nutrient receptor analysis revealed ILC2 uptake of amino acids to sustain OXPHOS at steady state. Following activation with interleukin-33 (IL-33), ILC2s became highly proliferative, relying on glycolysis and mammalian target of rapamycin (mTOR) to produce IL-13 while continuing to fuel OXPHOS with amino acids to maintain cellular fitness and proliferation. Our results suggest that proliferation and function are metabolically uncoupled in human ILC2s, offering new strategies to target ILC2s in disease settings.

Distinct metabolic demands accompany lymphocyte differentiation into short-lived effector and long-lived memory cells. How bioenergetics processes are structured in innate natural killer (NK) cells remains unclear. We demonstrate that circulating human CD56Dim (NKDim) cells have fused mitochondria and enhanced metabolism compared with CD56Br (NKBr) cells. Upon activation, these 2 subsets showed a dichotomous response, with further mitochondrial potentiation in NKBr cells vs paradoxical mitochondrial fission and depolarization in NKDim cells. The latter effect impaired interferon-γ production, but rescue was possible by inhibiting mitochondrial fragmentation, implicating mitochondrial polarization as a central regulator of NK cell function. NKDim cells are heterogeneous, and mitochondrial polarization was associated with enhanced survival and function in mature NKDim cells, including memory-like human cytomegalovirus–dependent CD57+NKG2C+ subsets. In contrast, patients with genetic defects in mitochondrial fusion had a deficiency in adaptive NK cells, which had poor survival in culture. These results support mitochondrial polarization as a central regulator of mature NK cell fitness.

Hundreds of patients with autosomal recessive, complete IL-12p40 or IL-12Rβ1 deficiency have been diagnosed over the last 20 years. They typically suffer from invasive mycobacteriosis and, occasionally, from mucocutaneous candidiasis. Susceptibility to these infections is thought to be due to impairments of IL-12–dependent IFN-γ immunity and IL-23–dependent IL-17A/IL-17F immunity, respectively. We report here patients with autosomal recessive, complete IL-12Rβ2 or IL-23R deficiency, lacking responses to IL-12 or IL-23 only, all of whom, unexpectedly, display mycobacteriosis without candidiasis. We show that αβ T, γδ T, B, NK, ILC1, and ILC2 cells from healthy donors preferentially produce IFN-γ in response to IL-12, whereas NKT cells and MAIT cells preferentially produce IFN-γ in response to IL-23. We also show that the development of IFN-γ–producing CD4⁺ T cells, including, in particular, mycobacterium-specific T_H1* cells (CD45RA⁻CCR6⁺), is dependent on both IL-12 and IL-23. Last, we show that IL12RB1, IL12RB2, and IL23R have similar frequencies of deleterious variants in the general population. The comparative rarity of symptomatic patients with IL-12Rβ2 or IL-23R deficiency, relative to IL-12Rβ1 deficiency, is, therefore, due to lower clinical penetrance. There are fewer symptomatic IL-23R– and IL-12Rβ2–deficient than IL-12Rβ1–deficient patients, not because these genetic disorders are rarer, but because the isolated absence of IL-12 or IL-23 is, in part, compensated by the other cytokine for the production of IFN-γ, thereby providing some protection against mycobacteria. These experiments of nature show that human IL-12 and IL-23 are both required for optimal IFN-γ–dependent immunity to mycobacteria, both individually and much more so cooperatively.