Neutrophils, known as the first responders of the innate immune system, rapidly infiltrate infected lung tissue during respiratory viral infections, where they exert both protective and potentially damaging effects. Until now, the scientific consensus predominantly held that these cells either proliferated within the lung microenvironment or were mobilized from the bone marrow. However, Prof. Cao’s team systematically analyzed immune cell kinetics in a SARS-CoV-2-infected golden hamster model, discovering that the spleen plays a critical and previously underappreciated role in neutrophil generation and trafficking.

Extensive single-cell RNA velocity profiling revealed a linear developmental trajectory within the lung neutrophil populations post-infection, beginning from proliferative precursors through immature and mature states to fully activated neutrophils. Yet, local proliferation in the lung failed to produce sufficient neutrophils to account for the marked infiltration observed at the infection peak. Intriguingly, a synchronous rise in splenic neutrophil numbers was recorded around days five to seven post-infection, coinciding tightly with the surge seen in the lung, suggesting a causative link.

To further unravel this inter-organ relationship, the researchers compared transcriptomic profiles of neutrophil subsets across the lung and spleen. They noted three conserved major subpopulations — proliferative, non-activated, and activated neutrophils — exhibiting strikingly similar gene expression signatures in both tissues. This powerful insight hinted strongly that splenic neutrophils, proliferating robustly during infection, were primed to migrate to the lung, replenishing its cellular repertoire and supporting immune function.

The team harnessed spatial transcriptomic techniques alongside the Redeconve deconvolution algorithm to map these neutrophils back to precise lung microenvironments. This innovative spatial analysis demonstrated that between days five and seven post-infection, splenic-origin neutrophils constituted a significant fraction of the lung’s neutrophil population — in some cases matching or surpassing that of locally derived cells. Notably, no similar migratory patterns were identified for other immune cells, underscoring the unique role of the spleen regarding neutrophil trafficking.

Temporal dynamics further illuminated the spleen-lung communication axis. Early-stage proliferative neutrophils emigrated from the spleen to the lung by day five, while subsequent influxes predominantly comprised immature and mildly activated neutrophils by day seven. This sequential migration suggests a continuous splenic contribution, supplying neutrophils at different developmental stages tailored for effective antiviral activities within the inflamed pulmonary tissue.

Mechanistically, this directional trafficking adheres to a complex chemokine-receptor signaling code. The lungs at infection peak overexpressed several neutrophil-attracting chemokines, including CXCL5, CXCL12, and CCL11, secreted respectively by epithelial cells, macrophages, and fibroblasts. Correspondingly, splenic neutrophil subsets exhibited distinct receptor profiles — immature neutrophils expressing CXCR4 primarily respond to CXCL12, while mature neutrophils upregulate CXCR2, CCR1, and CCR3, receptors for CXCL5 and CCL11. This ligand-receptor matching provides a biochemical roadmap facilitating precise, stage-specific recruitment from spleen to lung.

This discovery challenges the orthodox paradigm that views the bone marrow as the exclusive reservoir for neutrophil supply during pulmonary infections. Instead, the spleen emerges as an essential extramedullary niche harboring a dynamic pool of neutrophils ready for deployment. These findings have profound implications for understanding immune mobilization and inter-organ coordination in inflammation and infection.

Professor Cao emphasized that this spleen-lung axis elucidates the intricate systemic orchestration of innate immunity, offering novel conceptual frameworks for immunological research. The inter-organ dialogue between spleen and lung shaped by chemokine signals refines the spatial and temporal dimensions of immune responses and raises important questions about similar mechanisms in other infections and inflammatory conditions.

From a clinical perspective, modulation of this neutrophil trafficking axis may pave the way for innovative therapeutic strategies. Excessive neutrophil recruitment is a hallmark of severe viral pneumonias, including COVID-19, where it contributes to lung injury and respiratory failure. Targeting specific chemokine-receptor interactions involved in spleen-to-lung signaling may provide keener control over inflammatory cell influx, balancing protective immunity with tissue preservation.

Equally significant, this research introduces potential diagnostic biomarkers identifying splenic neutrophil activation states or chemokine gradients that could predict disease progression or therapeutic responses. The integration of single-cell and spatial multi-omics represents a powerful technology platform to discern such immune axes across diseases, offering a path toward precision medicine in infectious and inflammatory lung disorders.

In summary, this pioneering study uncovers a critical spleen-to-lung neutrophil axis instrumental in orchestrating antiviral defense during respiratory infection. The convergence of cutting-edge single-cell transcriptomics, spatial mapping, and functional analysis reshapes the understanding of systemic immune cell trafficking, providing exciting opportunities for both fundamental research and clinical translation in pulmonary medicine.

Subject of Research: Animals

Article Title: Single-cell spatiotemporal mapping reveals a spleen-to-lung neutrophil axis in antiviral defense

News Publication Date: 11-Mar-2026

References: DOI: 10.1007/s44466-026-00030-8

Image Credits: Professor Xuetao Cao, Chinese Academy of Medical Sciences, Beijing, China

Keywords

Neutrophil trafficking, spleen-to-lung axis, antiviral immunity, single-cell RNA velocity, spatial transcriptomics, SARS-CoV-2 infection, chemokine signaling, innate immunity, pulmonary inflammation, immune cell migration, extramedullary hematopoiesis, immunotherapy.

The analysis reveals that neutrophils and macrophages congregate along non-epithelialized surfaces of the fistula, demarcating zones of superficial granulation tissue. These zones are dominated by SPP1-positive macrophages that produce a suite of chemokines, including CXCL5, CCL3, CCL4, and CXCL2, establishing a feed-forward loop for sustained recruitment of inflammatory cells. Such chemokine-mediated signaling orchestrates an inflammatory milieu that perpetuates tissue damage and complicates resolution, confirming the centrality of innate immune components in early fistula pathology.

Beyond the superficial layers, the study identifies spatially defined strata rich in FAS-LAZ and FAS-ALC fibroblasts, cohabited by MMP9-expressing macrophages with dual remodeling and immunoregulatory capacities. These macrophages express genes such as LYZ, IDO1, C1QA/B, MRC1, and STAT1, indicating a complex phenotype that simultaneously reshapes the extracellular matrix (ECM) and modulates immune responses. Notably, another macrophage cluster in this region produces T cell-attracting chemokines (CXCL9, CXCL10, and CXCL11), suggesting a critical role for these cells in bridging innate and adaptive immunity within fistula tracts.

This chemokine production correlates with the presence of diverse adaptive immune subsets, including CD8+, CD4+, regulatory T cells, dendritic cells, and B cells. The occasional formation of follicle-like aggregates alongside FAS-LOC fibroblasts hints at the emergence of tertiary lymphoid structures, an indication of chronic immune activation and local antigen-driven responses. Such organized lymphoid assemblies could contribute to persistent inflammation and resistance to healing, underscoring the multifaceted nature of immune involvement in fistula evolution.

In more distal zones enriched in FAS-FOZ fibroblasts, immune cell infiltration diminishes markedly, replaced instead by proliferative signatures in endothelial cells and pericytes. This observation testifies to ongoing angiogenesis and vascular remodeling in these regions, processes that are essential for supporting the expanding tissue mass of the fistula tract. The interplay between fibroblast niches and vascular components thus appears to orchestrate the spatial heterogeneity within the lesion, balancing inflammation with tissue reconstruction.

The spatial intercellular signaling landscape of fistulae, as revealed in this study, is defined by robust cytokine–chemokine networks and pathways underpinning angiogenesis, ECM remodeling, and cell adhesion. Fibroblast–macrophage communication emerges as a key axis, featuring molecular interactions such as LRP1-MMP9 and SERPINE1 that regulate ECM turnover, integrin-TGFB1 and SPP1-mediated fibrotic signaling, alongside PDGFRB-driven proliferation signals. The engagement of SPP1-CD44 pairs highlights mechanisms of fibroblast activation critical for the persistent fibrotic state characteristic of fistula tracts.

Strikingly, developmental morphogen pathways also appear hijacked within these niches. The expression of WNT family members WNT2, WNT4, and WNT5A, along with Frizzled receptors, is markedly upregulated in FAS fibroblast subsets. Of particular note is the enrichment of WNT4 and planar cell polarity (PCP) components such as CELSR1 and DVL1 at the invasive leading edges of fistula tracts. This aberrant activation of PCP and related morphogen signaling links directly to invasiveness and proliferative expansion characteristic of pathogenic fibroblast populations.

The identification of actively cycling MKI67-positive fibroblasts at these leading edges further supports the idea that dysregulated morphogen signaling fuels cellular proliferation and tissue invasion, promoting fistula persistence. These findings provide a compelling mechanistic framework that connects developmental signaling pathways, immune activation, and stromal remodeling in a spatially resolved manner, offering new avenues for targeted therapeutic intervention.

Collectively, the data portray a dynamic, multicellular ecosystem within Crohn’s fistulae, where immune-stromal cross-talk is not merely a reaction to injury but a driving force shaping lesion architecture and chronicity. The integrated use of spatial transcriptomics combined with detailed cellular phenotyping uncovers the emergent properties of these niches that cannot be discerned through bulk analyses, marking a significant leap forward in understanding fistula pathogenesis.

Future therapeutic strategies informed by these insights might aim to disrupt harmful immune-fibroblast signaling loops, modulate aberrant morphogen pathways such as WNT-PCP, and restore normal ECM remodeling dynamics. The spatially delineated checkpoints of cellular interaction emerging from this study provide multiple potential molecular targets to halt fistula progression and promote resolution, moving towards precision medicine in inflammatory bowel disease complications.

This landmark research not only deciphers the microenvironmental complexity of Crohn’s fistulae but also underscores the critical importance of spatial context in disease biology. By capturing the interplay between innate and adaptive immunity, fibroblast heterogeneity, and vascular remodeling within an anatomically defined framework, the study sets the stage for next-generation diagnostics and therapies tailored to the intricate cellular topography of chronic lesions.

As Crohn’s disease continues to impose significant clinical burdens worldwide, these revelations offer renewed hope that unraveling spatial tissue niches will lead to breakthroughs in managing fistula-associated morbidity. The intersection of immune dysregulation, stromal plasticity, and developmental pathway misappropriation now emerges as a cardinal theme in fistula biology, spotlighting the need for integrated, spatially informed approaches in inflammatory disease research.

Subject of Research:
Spatial fibroblast niches and immune-stromal interactions in Crohn’s disease-associated fistulae.

Article Title:
Spatial fibroblast niches define Crohn’s fistulae.

Article References:
McGregor, C., Qin, X., Jagielowicz, M. et al. Spatial fibroblast niches define Crohn’s fistulae. Nature (2025). https://doi.org/10.1038/s41586-025-09744-y

Image Credits:
AI Generated

DOI:
https://doi.org/10.1038/s41586-025-09744-y

Keywords:
Crohn’s disease, fistula, spatial transcriptomics, fibroblast niches, immune-macrophage interaction, chemokines, morphogen signaling, WNT-PCP pathway, extracellular matrix remodeling, angiogenesis, adaptive immunity, inflammatory bowel disease

Macrophages, which are integral components of the innate immune system, typically serve as scavengers, engulfing pathogens and apoptotic cells while orchestrating inflammatory responses to combat infection. However, the landscape within prostate tumors presents a paradox: rather than executing their protective functions, certain macrophage subsets become reprogrammed, adopting an immune-suppressive phenotype that actively promotes tumor progression. This study uncovers the molecular identity of one such detrimental macrophage subtype, characterized by the expression of the proteins SPP1 and TREM2, which congregates within tumor cores and correlates with enhanced angiogenesis, impaired immune surveillance, and metastatic potential.

Employing cutting-edge technologies including single-cell RNA sequencing and spatial transcriptomics, the research team meticulously mapped cellular interactions and gene expression profiles at an unprecedented resolution. These spatially resolved transcriptomic analyses unveiled a striking spatial segregation within the tumor microenvironment: macrophages exhibiting pro-inflammatory, potentially anti-tumor activities were predominantly located outside tumor boundaries, whereas the SPP1/TREM2-positive macrophages deeply infiltrated the tumor mass, intimately associated with malignant cells. This spatial distribution underscores the sophisticated tumor strategy to shield itself from immune-mediated destruction.

The study’s integrative approach combined advanced molecular profiling with the analysis of extensive datasets from hundreds of human prostate cancer samples, validating the universality of their findings across clinical stages and models. This multi-institutional collaboration incorporated expertise from premier institutions including Harvard Medical School, Massachusetts General Hospital, the University of Chicago, and Sweden’s Karolinska Institute, enabling a comprehensive investigation into the cellular ecology of prostate cancer metastasis, particularly within the bone microenvironment where treatment options remain limited and prognosis poor.

Of particular therapeutic interest, the researchers demonstrated through in vivo experiments that blocking SPP1 in murine models of prostate cancer markedly enhanced the efficacy of immunotherapy. While immune checkpoint inhibitors have revolutionized treatment for many cancers, their success in prostate cancer has been notably limited. In this context, inhibiting the suppressive macrophage subset via an anti-SPP1 antibody not only reinstated immune activation but also facilitated the infiltration of cytotoxic T cells—the frontline effectors in tumor eradication—ultimately decelerating tumor growth and dissemination.

This revelation provides compelling evidence that targeting tumor-supportive macrophages can transform a previously refractory tumor microenvironment into one amenable to immunotherapeutic intervention. Shenglin Mei emphasizes that “although macrophages are often our allies in fighting cancer, certain specialized subtypes craft an immune-suppressive niche that thwarts the body’s natural defenses.” By reversing this immunosuppression, the study highlights an exploitable vulnerability in prostate cancer’s armor.

Prostate cancer remains a formidable global health challenge as the second most commonly diagnosed cancer among men, with nearly 1.5 million new cases worldwide recorded in 2022. Decoding the tumor microenvironment’s complex cellular players is critical for improving clinical outcomes, especially in advanced stages where metastatic spread, particularly to bone, is the primary cause of mortality. This research significantly advances that understanding by linking a discrete macrophage population to specific pathological features such as neovascularization and immune evasion.

The team’s approach leverages high-dimensional single-cell technologies alongside NanoString’s digital spatial profiling to attain both transcriptomic depth and spatial context—a methodological synergy that unveils cellular dynamics impossible to discern through traditional bulk analyses. This analytic rigor not only confirms the pathological role of the SPP1/TREM2 macrophages but also delineates their precise localization and interactions within the tumor milieu.

Furthermore, the study builds on Mei’s prior work, which mapped immunosuppressive microenvironments in bone metastases and primary prostate tumors, further expanding the atlas of tumor-immune cell interplay. These cumulative insights pave the way for novel therapeutic strategies aimed at modulating macrophage phenotypes and dismantling the protective niches that cancers engineer for themselves.

The broader implications of this work resonate beyond prostate cancer, suggesting that a nuanced understanding of immune cell subtypes and their spatial arrangement is paramount for the rational design of next-generation cancer immunotherapies. Chris Hourigan, director of the Fralin Biomedical Research Institute Cancer Research Center, underscores this sentiment, noting that “integrating cancer genomics with computational oncology is essential not just for fundamental biological insight but for unlocking actionable treatment paradigms.”

In summary, the identification of the SPP1/TREM2-expressing tumor-associated macrophage subpopulation elucidates a critical mechanism by which prostate cancer orchestrates immune evasion and metastasis. By illuminating this intricate cellular crosstalk and providing a tangible target for therapeutic intervention, this study opens promising avenues for enhancing the effectiveness of immunotherapy in one of the most challenging cancer types. As precision medicine continues to evolve, such interdisciplinary and collaborative efforts exemplify the transformative potential of modern cancer research.

Subject of Research: Cells
Article Title: Single-Cell and Spatial Transcriptomics Reveal a Tumor-Associated Macrophage Subpopulation that Mediates Prostate Cancer Progression and Metastasis
News Publication Date: July 2, 2025
Web References: Molecular Cancer Research Article
References: DOI: 10.1158/1541-7786.MCR-24-0791
Image Credits: Journal cover by Molecular Cancer Research; photo by Virginia Tech
Keywords: Cancer, Prostate cancer, Metastasis, Health care