The horse was a stallion used for flat racing and was evaluated in Uruguay from August through November 2023. Its initial history included intermittent low-grade fever, mild inappetence following competitions and subtle weight loss. During the early stages of illness, the horse remained generally bright, retained its appetite and produced feces of normal consistency. Over approximately three months, it experienced recurrent fever, progressive weight loss of about 25 kilograms and persistent hypoalbuminemia, prompting an extensive diagnostic evaluation for the cause of its clinical signs.

Serial complete blood counts and serum biochemical tests showed progressive hypoproteinemia and hypoalbuminemia without leukocytosis. Parasitological examination of feces detected a low burden of strongyle-type eggs, measured at 20 eggs per gram. Thoracic and abdominal ultrasonography revealed transient splenomegaly and mild pleural irregularities, but no abdominal mass or effusion was detected. Cytology of bronchoalveolar lavage fluid showed 59% macrophages, 39% lymphocytes and 2% eosinophils. Activated macrophages and multinucleated giant cells were considered consistent with chronic inflammatory changes, while no intracellular or extracellular bacteria were identified. Urine cultures, head radiographs and repeated viral and hemoparasitic screenings were negative.

Gastroscopy was performed approximately one week before the horse’s death, following a 24-hour fast. The examination documented incomplete gastric emptying, but the gastric mucosa appeared macroscopically normal, without a visible mass, ulceration or active bleeding. The horse was treated with antimicrobial therapy, beginning with ceftiofur and later enrofloxacin and metronidazole, along with supportive care and intravenous flunixin meglumine as needed for pyrexia. On November 16, it developed a higher fever, reaching 40 degrees Celsius, mild depression and decreased feed intake. Thoracic radiographs showed a bronchial-interstitial pattern, bronchiectasis and a large, well-defined structure consistent with a pulmonary abscess. The horse was euthanized and referred for necropsy at the veterinary pathology unit in Montevideo.

At necropsy, the mass was found within ingested feed, protruding from the wall of the nonglandular stomach. It was generally oval and soft, measuring 20 by 11 by 11 centimeters. Its external surface was brown, while the cut surface was grayish white with scattered black areas. Portions of the tumor were necrotic, its base was firmly attached to the stomach wall, and the surrounding wall was thickened. A portion of the specimen was fixed in 10% buffered neutral formalin and processed for routine histological examination. Sections were stained with hematoxylin and eosin, and additional 3-micrometer serial sections were prepared for immunohistochemistry using antibodies against alpha smooth muscle actin and CD117, also called c-Kit.

Microscopic examination showed that the tumor arose in close association with the normal smooth muscle layer. In these areas, the usual smooth muscle architecture was disrupted by neoplastic cell proliferation, connective tissue expansion and interstitial edema. Neoplastic cells accompanied by inflammatory infiltrates and edema extended into the underlying serosal layer, producing serosal thickening. The part of the tumor projecting into the gastric lumen was covered by squamous epithelium, whereas much of its exposed surface showed extensive necrosis and lymphocytic, plasmacytic and histiocytic inflammatory responses. Remnants of ingesta and clusters of bacteria were present in some necrotic areas.

The neoplastic population displayed considerable pleomorphism, including round, pleomorphic and spindle-shaped cells, with cytoplasm ranging from abundant to scant. Nuclei were round, oval or irregular, and mitotic figures were infrequent. Most cells were arranged in interwoven or interlacing fascicles. In several regions, spindle-shaped cells with abundant eosinophilic cytoplasm formed clusters resembling smooth muscle differentiation. Neoplastic cells were also observed within newly formed blood vessels inside the tumor, an observation that supported classification of the lesion as malignant. On serial sections, the tumor cells showed strong cytoplasmic staining for alpha smooth muscle actin. Blood vessels with positive labeling served as an internal positive control, while the neoplastic cells were negative for CD117. Together with the tissue architecture and the tumor’s location, this immunophenotype supported the diagnosis of primary gastric leiomyosarcoma.

Immunohistochemistry was important to the diagnosis because gastrointestinal stromal tumors and smooth muscle tumors can have overlapping appearances under routine microscopy. The source report notes that morphology alone can make these tumors difficult to distinguish, including in canine gastric tumors. CD117 staining detects most gastrointestinal stromal tumors, whereas alpha smooth muscle actin expression supports smooth muscle differentiation. The combination of alpha smooth muscle actin positivity and CD117 negativity in this horse, together with direct involvement of the gastric smooth muscle layer and smooth muscle-like cellular features, supported leiomyosarcoma rather than a gastrointestinal stromal tumor. The malignant designation was based on nuclear atypia and the presence of tumor cells in intratumoral vessels. Despite that vascular finding, pathologists identified no metastatic lesions in the liver, lungs or other organs examined.

The age of the horse makes the case unusual within the limited literature on equine gastric tumors. Gastric neoplasms have a reported incidence of 0.9% in horses, and horses with gastric neoplasms are reported to have a mean age of 18 years. Squamous cell carcinoma is the most frequently diagnosed form, while tumors derived from muscle are exceptionally rare. According to the report, only two cases of gastric leiomyoma and one earlier case of gastric leiomyosarcoma had been described in horses. The other reported gastric leiomyosarcoma occurred in a 12-year-old horse and was diagnosed from morphological findings without immunohistochemical confirmation. The present 4-year-old therefore provides a younger case documented with both histology and immunohistochemistry, but a single report cannot establish that these tumors are more common in young horses or reveal any incidence trend.

Location is another notable feature. The authors report that equine gastric smooth muscle tumors tend to arise in the nonglandular region, consistent with the site of this mass. The report also provides a cautious clinical interpretation: the authors presumed that the tumor was responsible for the horse’s persistent fever, protein-losing state and impaired gastric emptying. That explanation was not established experimentally, and the diagnostic work-up did not identify an abdominal mass on ultrasound or a visible tumor on gastroscopy. The horse’s euthanasia followed the development of a pulmonary abscess and respiratory abnormalities, rather than being an outcome directly demonstrated to result from the gastric tumor. As a single necropsy case with no separate dataset, the work cannot determine the tumor’s population frequency, risk factors or the full mechanism of systemic illness. Its central contribution is a detailed characterization of a rare gastric cancer in a young horse: a 20-centimeter smooth muscle-associated mass with alpha smooth muscle actin positivity, CD117 negativity, histologic evidence of malignancy and no detected distant metastases at necropsy.

Subject of Research: Veterinary case report of histopathological and immunohistochemical confirmation of gastric leiomyosarcoma in a 4-year-old horse.

Article Title: Case Report: Histological study of gastric leiomyosarcoma in a young horse

Article References: Guerrero, H., Varela, B., Morales, A., Larrañaga, C., Yozzi, V., Yamasaki, K., & Verdes, J. M. (2026). Case Report: Histological study of gastric leiomyosarcoma in a young horse. Frontiers in Veterinary Science, 13, 1979698. https://doi.org/10.3389/fvets.2026.1979698

Image Credits: AI Generated

DOI: 10.3389/fvets.2026.1979698

Keywords: equine oncology, gastric leiomyosarcoma, veterinary pathology, histopathology, immunohistochemistry, smooth muscle actin, CD117, horse, gastric tumor, case report, Case, Report

The study begins with a striking observation about gastric cancer’s genetic landscape. TTN, a gene encoding titin, the largest protein in the human body, is mutated in more than half of all gastric cancer cases, while TP53, the famously protective guardian-of-the-genome gene, is mutated in roughly 46 percent of patients. Despite how common these mutations are, their influence on the immune cells surrounding the tumor has remained poorly understood. That question matters because the tumor immune microenvironment—the complex ecosystem of immune cells, signaling molecules, and malignant cells within and around a tumor—can profoundly shape how cancer progresses and how patients respond to therapy.

To untangle this relationship, the research team, led by investigators at the First Hospital of Lanzhou University in China, assembled an unusually rich dataset. They combined RNA sequencing data from 431 patients in The Cancer Genome Atlas stomach adenocarcinoma cohort, known as TCGA-STAD, with single-cell transcriptomic data covering 118,383 individual cells. Single-cell transcriptomics allows scientists to read the gene activity of cells one by one, revealing subpopulations that would be invisible in bulk tissue measurements. The team then used sophisticated computational tools, including Bayesian deconvolution, the Scissor algorithm, and weighted gene co-expression network analysis, to map the TTN/TP53 mutation signatures onto specific immune cell populations.

The analysis converged on a clear result: TTN/TP53 co-mutations were strongly associated with the presence of tumor-associated atypical B cells. These cells are a terminally positioned subpopulation of B lymphocytes, the antibody-producing soldiers of the immune system, but TAABs are anything but ordinary B cells. They show elevated expression of CD27 and MS4A1, markers of activated and mature B cells, along with genes linked to cell proliferation. In other words, within the tumor environment, these B cells appear to have shifted into a distinctive, actively dividing state that sets them apart from their counterparts in healthy tissue.

Crucially, the study found that TAABs carried prognostic significance, meaning their abundance was associated with patient outcomes—and, notably, in a favorable direction. The cells also preferentially accumulated in both primary tumors and metastatic tissues, suggesting they are not passive bystanders but active participants in the cancer’s evolution. Using cell-cell communication networks, the researchers found that TAABs appeared to act as potential coordinators in their interactions with CD4-positive T cells, the helper cells that orchestrate much of the immune response. The key channel of this communication was TNF-alpha signaling, a pathway best known for driving inflammation but increasingly recognized as a modulator of anti-tumor immunity.

The team also probed the molecular machinery that might govern TAAB behavior. Through transcription factor profiling, they identified CUX1 as a candidate regulator that may be associated with controlling the TAAB state. Transcription factors are proteins that switch genes on and off, and identifying a candidate upstream regulator provides a concrete molecular handle for future experiments. If validated, CUX1 could help explain how gastric tumors with TTN/TP53 co-mutations recruit or shape this unusual B cell population, and why that population correlates with better prognosis.

Perhaps the most clinically intriguing part of the study concerns metastasis. Gastric cancer does not spread randomly; it shows marked organ preference, with the liver and the peritoneum—the membrane lining the abdominal cavity—being two of the most common destinations. When the researchers compared the molecular environments of liver metastases and peritoneal metastases, they found fundamentally different biology at each site. Liver metastases displayed increased TNF-alpha signaling along with heightened activity of the epithelial-mesenchymal transition, or EMT, a developmental program that cancer cells hijack to become mobile and invasive. Peritoneal metastases, by contrast, were dominated by metabolic pathways, pointing to a different set of survival strategies in that environment.

Within this site-specific landscape, the MIF-CD74-CXCR4 axis emerged as a prominent ligand-receptor feature of the interactions between TAABs and tumor cells. MIF, or macrophage migration inhibitory factor, is a pleiotropic inflammatory cytokine; CD74 and CXCR4 are receptors on cell surfaces that can receive its signal. This axis is increasingly implicated in chronic inflammation and tumor progression, and its prominence in TAAB-tumor communication suggests a possible mechanism by which these atypical B cells and malignant cells influence one another. In silico predictions further suggested a potential association between this signaling and BCL6 activity, a transcription factor central to B cell biology, as well as EMT-related processes in the tumor cells.

The authors are careful to frame their conclusions appropriately. Because the work is entirely computational—built on publicly available sequencing data and algorithmic inference rather than laboratory experiments—the findings are presented as testable hypotheses rather than established mechanisms. The association between TTN/TP53 co-mutations and TAABs, the role of TNF-alpha signaling in TAAB-CD4 T cell crosstalk, and the divergent pathway activities of liver versus peritoneal metastases all now await experimental validation in the laboratory and the clinic. Still, the study’s scale and multi-method design give the hypotheses considerable weight, and the identification of a prognostically favorable B cell population opens a potential new direction for immunotherapy research in gastric cancer, a disease where immune-based treatments have so far delivered uneven benefits.

For patients and clinicians, the implications are tantalizing even at this early stage. If TAAB abundance can be reliably measured, it might eventually serve as a biomarker to help stratify patients with TTN/TP53-mutant gastric cancer, identifying those whose immune microenvironments are more favorable. Understanding why liver metastases lean on inflammatory and EMT programs while peritoneal metastases rely on metabolic rewiring could also inform site-specific treatment strategies, an idea that resonates with the growing field of organotropic metastasis research. And the MIF-CD74-CXCR4 axis, already the target of drug development efforts in other contexts, offers a concrete molecular thread connecting B cell behavior, tumor cell plasticity, and metastatic pattern. The road from computational correlation to validated biology is long, but this study maps the terrain with unusual clarity, and it hands gastric cancer researchers a set of well-defined, experimentally addressable questions about one of medicine’s most stubborn cancers.

Subject of Research: The association of tumor-associated atypical B cells with TTN/TP53 mutations, TNF-alpha signaling, and site-specific metastatic patterns in gastric cancer

Article Title: TAABs are associated with TNF-α signaling and site-specific metastatic patterns in TTN/TP53-mutant gastric cancer

Article References: Li, J., Qin, Z., Li, W., Guan, Q., & Ma, J. (2026). TAABs are associated with TNF-α signaling and site-specific metastatic patterns in TTN/TP53-mutant gastric cancer. Journal of Translational Medicine. https://doi.org/10.1186/s12967-026-08923-3

Image Credits: AI Generated

DOI: 10.1186/s12967-026-08923-3

Keywords: gastric cancer, TAABs, TTN mutation, TP53 mutation, TNF-alpha signaling, tumor immune microenvironment, single-cell transcriptomics, metastasis, liver metastases, peritoneal metastases, MIF-CD74-CXCR4 axis, EMT