Hantavirus disease illustrates the complexity of infectious diseases and puts spotlight on the need to understand the processes driving pathology and those that can prevent or mitigate disease. This Viewpoint argues that improved treatments of viral diseases will require a combination of pathogenesis-guided strategies addressing the different phases of disease, including viral replication, tissue dysfunction, and damaging immune amplification.

Memory CD8 T cells (TMEM) and exhausted CD8 T cells (TEX) are essential for host defense against infection and cancer, yet their therapeutic potential is often limited by insufficient persistence and sustained functional capacity. Strategies to enhance the longevity of both populations remain scarce. Here, we demonstrate that ablation of UBE2F, a neddylation E2 enzyme, induces a resilience program in CD8 T cells that operates across both TMEM and TEX compartments, resulting in improved viral and tumor control. This resilience state is characterized by enhanced self-renewal and survival without perturbing the conventional CD8 T cell differentiation trajectories. Mechanistically, UBE2F deficiency inhibited neddylation of CUL5, leading to accumulation of JUNB and upregulation of IL-2Rβ. The increased IL-2Rβ expression hypersensitizes CD8 T cells to physiological IL-15, thereby conferring the resilience features. Together, these findings identify the UBE2F-CUL5-JUNB-IL-2Rβ axis as a conserved posttranslational mechanism regulating CD8 T cell longevity across memory and exhausted states, providing a novel strategy for enhancing antiviral and antitumor immunity.

Macrophages in the skin reside in multiple distinct layers and perform various functions. Here, we show that CD169+ macrophages reside in the hypodermis and comprise the major skin myeloid cell population in the steady state. In a syngeneic melanoma model, CD169+ macrophages encapsulate growing melanomas and directly suppress their growth. CSF1R blockade depleted CD169+ macrophages in tumors and resulted in unrestrained growth. This local containment of tumor growth in the skin was independent of CD169+ subcapsular sinus macrophages in the tumor-draining lymph node and did not require B or T cells. Intravital imaging revealed engulfment and ingestion of live tumor cells by CD169+ macrophages. This phagocytosis did not require the phosphatidylserine receptor MERTK. CD169+ macrophages are also enriched in the hypodermis in skin biopsies from healthy human skin and melanoma. These data identify tissue-resident CD169+ macrophages as a potential cellular target to achieve innate immune containment and reinforce adaptive immune control of tumors.

In this issue of JEM, Liu et al. (https://doi.org/10.1084/jem.20251779) report PRECISE-seq, a proximity labeling platform that integrates T cell receptor specificity, functional potency, and cellular phenotype at a single-cell resolution. Using this approach, they identify an immunosuppressive Ly49+ T cell state within tumors that is alleviated by PD-1 blockade.

Microglia migrate from the yolk sac and populate the developing brain. How microglia expand rapidly to meet the microglial demand in fast-expanding human fetal brains remains uncharted. Using thick sections in 5-22-gestational week (gw) brains and super-resolution scanning, we identified a large proliferative microglial aggregate (2.129 mm2) near the lateral ganglionic eminence (>12.5 gw), expanding in Down's syndrome (DS) (4.767 mm2) and Edwards syndrome (ES) (3.437 mm2) fetal brains. Ki67+ microglia within the aggregates accounted for 26.65% (DS: 38.9%; ES: 46.3%) compared with 6.32% (DS: 6.01%; ES: 5.2%) in scattered microglia. This aggregate region contained a distinct microglial population characterized by the absence of phagocytic structures and complex processes, high CSF-1R expression, abundant IL-34+ cells, and some SPP1+ bipolar microglia. We termed this structure the secondary microglial formation center (SMFC). Chimeric microglia-human cortical organoids recapitulated the SMFC in an IL-34- and CSF-1R-dependent manner, indicating that the human SMFC may compensate for the microglial shortage during the fastest expansion period.

Natural killer (NK) cells are promising platforms for off-the-shelf immunotherapy, yet nonviral precision engineering remains limited by poor HDR efficiency, DNA toxicity, and manufacturing challenges. The aim of this study was to establish a high-yield, nonviral knock-in platform. Through extensive in-depth rational screens, we achieved ∼90% HDR insertion of therapeutic payloads while maintaining 100% postediting recovery. By hijacking endogenous transcriptional programs, we installed genetic circuits into defined genomic loci to tune transgene expression. To enable context-dependent therapeutic responses, we integrated a synthetic positive feedback circuit at the CISH locus, which enhanced NK cell persistence and drove strong expression of anti-CD22/19 dual CAR. A hypoxia-responsive IL-12 circuit gated by the PFKFB4 promoter restored cytotoxicity under environmental stress. Finally, we showed this platform is compatible with GMP manufacturing and supports clinical-scale expansion. These findings provide a scalable framework for programmable, nonviral editing of NK cell effector functions for therapeutic and research applications.

Regulatory T (Treg) cells are potent immunosuppressors of conventional αβ T cells in the tumor microenvironment, but how they may affect innate-like γδ T cells remains poorly understood. Here, we show that induced Treg depletion in mice selectively unleashes IFNγ-producing γδ T cells, which are required for tumor control in an orthotopic breast cancer model. Treg cells outcompete IFNγ+ γδ T cells for IL-2 due to increased expression of the high-affinity IL-2 receptor, thereby limiting the proliferation and effector functions of IFNγ+ γδ T cells. Consistently, in vivo neutralization of IL-2 alongside Treg depletion abrogates the induction of IFNγ+ γδ T cell responses, whereas administration of an IL-2Rβγc agonist circumvents Treg-mediated suppression and enhances tumor control. Finally, Treg cells also inhibit endogenous and expanded human γδ T-cells, which can be rescued by IL-2Rβγc agonism to enhance therapeutic responses in xenografted mice. Thus, bypassing Treg-mediated suppression may improve the outcome of γδ T cell-based immunotherapies.

Linking T cell phenotypes with antigen specificity and functional avidity is critical for understanding in vivo immune responses in infection and cancer. Here, we develop PRECISE-seq, a method that integrates multi-omics T cell analysis with contact-dependent proximity labeling for rapid screening of disease-relevant T cell repertoires, and for linking the relative TCR avidity with T cell phenotypes at single-cell resolution. PRECISE-seq accurately retrieves CMV-specific clonotypes from human peripheral blood and quantitatively measures functional avidity in physiological contexts. We find that high-potency CMV-specific T cells preferentially acquire an exhausted phenotype. In tumors, polyclonal tumor-reactive CD8+ T cells predominantly differentiate into a protumor Ly49+ regulatory state (TLy49), characterized by inhibitory killer cell lectin-like receptor expression and originating from effector memory T cells along a trajectory distinct from exhaustion. Notably, PD-1 blockade reduces TLy49 formation and promotes effector revival, which correlates with responsiveness to immunotherapy. Together, PRECISE-seq enables high-resolution mapping of TCR potency and T cell phenotype, revealing a regulatory axis shaping T cell fate in tumors.

The chemokine receptor CCR9 regulates thymus colonization by T progenitors and intestinal accumulation of TCRγδ+ intraepithelial lymphocytes (IELs). CCR9-ligand chemokine CCL25 is expressed predominantly in the thymus and the small intestine. By engineering tissue-specific CCL25-deficient mice, we demonstrate that CCL25 locally produced by thymic and intestinal epithelial cells individually supports developing T cell localization in the thymus and TCRγδ+ IEL accumulation in the small intestine, respectively. Our results also reveal that thymic epithelial CCL25 facilitates T cell positive selection in the thymic cortex. We further find that intestinal epithelial CCL25 promotes nutrient sensing in the small intestine. These results indicate that epithelial CCL25 directs thymic T cell development and intestinal homeostasis in a tissue-specific paracrine manner.

Innate lymphoid cells (ILCs) include T-bet-dependent NK and ILC1 cells, GATA-3-dependent ILC2 cells, and RORγt-dependent ILC3 cells. Their functional and developmental regulation at the posttranscriptional level remains elusive. The CCR4-NOT complex plays a central role in mRNA decay by mediating deadenylation. To explore the overall impact of mRNA decay on ILCs, we conditionally deleted Cnot3, an essential subunit of the CCR4-NOT complex. Loss of CNOT3 in ILC2 cells led to aberrant expression of T-bet and RORγt, accompanied by upregulation of type 1 and type 3 signature genes. Mechanistically, CNOT3 targeted the 3' untranslated regions of Tbx21 and Rorc mRNAs through interactions with Roquin and ZFP36L1, respectively. Elevated T-bet expression in CNOT3-deficient ILC2 cells suppressed GATA-3 levels, thereby impairing type 2 immune responses in models of airway allergy and helminth infection. Thus, our findings reveal that CNOT3 maintains ILC2 differentiation and function by restricting type 1 and type 3 transcriptional programs.

Memory B cell (Bmem) survival is essential for guarding against reinfection, yet processes ensuring their longevity remain unclear. As decay-accelerating factor (DAF, CD55), a negative regulator of complement activation, is requisitely downregulated on germinal center B cells and is reexpressed on Bmem, we investigated the effects of deleting DAF on murine (B1-8hi) Bmem in competitive settings. Kinetic analysis showed a progressive reduction in DAF-/- Bmem numbers over 6 wk, without affecting Bmem production, pool size, or their ability to respond to rechallenge. Following transfer into unimmunized hosts, wild-type Bmem proliferated to maintain stable Bmem pool sizes, outcompeting DAF-/- Bmem, reflecting homeostatic proliferation. Reduced proliferation and increased cell death in DAF-/- Bmem associated with transcriptional differences in metabolism and migration pathways. Wild-type Bmem proliferation increased in C3-/- hosts, and vaccination with a heterologous antigen, which induces local complement activation, locally inhibited bystander B1-8hi Bmem proliferation. Thus, complement-dependent regulation of Bmem homeostatic proliferation influences Bmem longevity and repertoire composition in mice.

Repeated tumor contact leaves more behind than simple CAR-T exhaustion. This study by Gu et al. (https://doi.org/10.1084/jem.20252564) shows that chronic antigen exposure impairs a Rab5-dependent endocytic program, allowing trogocytosed antigen to accumulate, functional CAR to decline, and fratricide to increase.

In this issue, Luff et al. (https://doi.org/10.1084/jem.20251308) show that cell-intrinsic signals from TACI drive marginal zone B cell development from T2 B cells through a mechanism involving activation of the PI3K-AKT pathway and inhibition of the FOXO1-KLF2 axis.

We show continuous tumor exposure results in a loss of chimeric antigen receptor (CAR) T cell (CART) endocytic activity due to downregulation of Rab5. Loss of endocytic activity exacerbates the effects of trogocytosis, the bidirectional transfer of tumor target antigens and CARs between malignant cells and CARTs, resulting in CART dysfunction and fratricide. Constitutive expression of Rab5 within the CARTs reduced fratricide by reducing the amount of trogocytosed antigens on the cell surface, while simultaneously enhancing CAR availability through dissociation of CAR from target, recycling unbound CAR back to the plasma membrane, and limiting CAR capture by tumor cells. Rab5-expressing CARTs exhibited superior antitumor activity in both BCMA-CARTs isolated from the bone marrow of treated patients and mesothelin-specific CARTs in a solid tumor model. These studies uncover an unexpected relationship between endocytosis and CART function and suggest that pairing Rab5 with CAR expression could improve the clinical efficacy of CART therapy.

The mature B cell compartment consists of follicular and marginal zone (MZ) B cells, which develop from transitional type 2 (T2) B cells. TACI, a member of the TNF receptor superfamily, is expressed on all mature B cells, with highest levels on MZ B cells and plasma cells. Previous studies reported that TACI is a negative regulator of B cell survival. However, this conclusion is confounded by elevated levels of BAFF, a cytokine that supports B cell survival, in TACI-deficient mice. We now show that TACI does not directly regulate B cell survival in mice but rather has a cell-intrinsic role in MZ B cell development. Loss of TACI leads to reduced MZ B cell numbers and an impaired T-independent antibody response. Mechanistically, we show that TACI is required for MZ B cell development from T2 B cell precursors via activation of the PI3K-AKT pathway and subsequent inhibition of the FOXO1 transcription factor.