Myofibroblast differentiation in a cat with eosinophilic sclerosing fibroplasia involving mesenteric lymph nodes and liver

Abstract

Feline eosinophilic sclerosing fibroplasia (FESF) is a unique fibroproliferative disease of cats marked by eosinophilic inflammation and extensive tissue fibroplasia that affects the gastrointestinal tract predominantly. Biopsy specimens were examined from an 11-y-old, spayed female, domestic shorthair cat with a 1–2-mo history of vomiting and anorexia, abdominal lymphadenopathy, and multiple hepatic nodules. Microscopically, the liver nodules and enlarged mesenteric lymph node had profound eosinophilic inflammation and sclerosis characteristic of FESF; the stomach, duodenum, jejunum, and ileum were unremarkable. Immunohistochemistry (IHC) was performed to further characterize the cell populations. Increased intralesional mesenchymal cells, interpreted as reactive fibroblasts, were positive with antibodies against smooth muscle actin (SMA), indicating a myofibroblast phenotype. Abundant intralesional macrophages were ionized calcium–binding adapter molecule 1 (IBA1) immunolabeled. Dual IHC of SMA and IBA1 revealed several double-positive mesenchymal cells, suggesting macrophage-to-myofibroblast transition (MMT). Our findings underscore the important role of macrophages not only in chronic inflammation, but also in tissue repair and fibrosis. Our case was a unique presentation of FESF with primary liver and mesenteric lymph node involvement, and without overt gastrointestinal lesions. Additionally, to our knowledge, myofibroblast phenoconversion and MMT have not been reported previously in cats, giving new insights to the pathogenesis of this poorly understood disease entity.

An 11-y-old, spayed female, domestic shorthair cat was presented to the Garden State Veterinary Services Internal Medicine department (Woodbridge, NJ, USA) with a 1–2-mo history of vomiting, inappetence, and loose stools. Ultrasonographic examination of the abdomen revealed severe multifocal abdominal lymphadenopathy with associated hyperechoic mesentery and multifocal thickening of the jejunal wall. Fine-needle aspirates of the jejunal wall revealed small numbers of neutrophils with a few eosinophils; the cytologic examination of mesenteric lymph nodes was unremarkable. CBC and serum chemistry results were within RIs. Given the nonspecific nature of lesions and concern for a potential neoplastic process, prednisolone (1 mg/kg/d) treatment was initiated. One month later, the patient revisited because of acute progression of the aforementioned clinical signs. Abdominal ultrasound at this time revealed improvement of lymphadenopathy except for one mesenteric lymph node, static jejunal wall thickening, and a novel lesion involving the liver. Laparotomy was performed to collect biopsy specimens. During the surgery, the liver was firm with 3–6-mm white nodules throughout multiple liver lobes. The right medial liver lobe was firm with rounded edges and expanded by a mass-like 47 × 35 × 15-mm lesion. One mesenteric lymph node was markedly enlarged to 4.0 × 2.5 cm. Poorly delineated regions of duodenum and jejunum were subjectively thickened. Representative lesions of liver and mesenteric lymph node were biopsied. Full-thickness punch biopsies were also taken from the stomach, duodenum, jejunum, and ileum.

Formalin-fixed biopsy specimens, including liver, mesenteric lymph node, stomach, duodenum, jejunum, and ileum, were submitted to the Cornell University–Animal Health Diagnostic Center (Ithaca, NY, USA). The submitted tissues were trimmed and processed routinely; slides were stained with H&E according to standard operating procedures. Histologic examination of the liver, specifically the right medial lobe, revealed multifocal-to-coalescing fibroplasia encircling and expanding the inflamed portal tracts (Fig. 1A). Periportal fibrosis and inflammatory infiltrates extended through the limiting plates into the hepatic parenchyma. The hepatic nodules noted at surgery were composed of dense bundles of collagen and inflammatory cells that replaced the hepatocellular cords (Fig. 1B). Adjacent to the lesions, the remaining hepatocytes had cytoplasmic rarefaction and vacuolar degeneration with many brown cytoplasmic granules interpreted as lipofuscin (Fig. 1B). Hyperplasia of small bile ducts and moderate numbers of eosinophils, histiocytes, and lymphocytes were commonly noted between the collagen bundles (Fig. 1C).

Figure 1.

Eosinophilic sclerosing fibroplasia in a cat. H&E. A. Prominent periportal fibrosis (arrowheads) and inflammatory infiltrates in right medial lobe of the liver. Bar = 1 mm. B. Anastomosing trabeculae of collagen, and inflammation replacing the hepatic parenchyma. Note the vacuolar degeneration and lipofuscin pigments in the hepatocytes in the upper right corner. Bar = 200 µm. C. Infiltrating inflammatory cells are mainly eosinophils, histiocytes, and lymphocytes. Also note the biliary hyperplasia (arrowheads). Bar = 100 µm. D. Mesenteric lymph node is replaced by increased mesenchymal cells and collagen with layers of collagen trabeculae deposited on the capsule. Note the peripheralized lymphoid follicles (asterisks). Bar = 1 mm. E. The collagen trabeculae are interspersed with well-vascularized fibrous connective tissue. Bar = 100 µm. F. Lymph node architecture is effaced by the increased mesenchymal cells. Note the eosinophilic and histiocytic inflammatory infiltrates and the residual lymphoid follicle. Bar = 100 µm.

The capsule of the mesenteric lymph node was markedly expanded by dense collagen bundles forming anastomosing trabeculae (Fig. 1D), interspersed with well-vascularized fibrous connective tissue (Fig. 1E). The preexisting lymphoid follicles were peripheralized and partially or completely effaced by inflammation and fibrous tissue. The nodal parenchyma was replaced by increased mesenchymal cells intermingled with collagen bundles and mixed inflammatory cells, including eosinophils, histiocytes, and lymphocytes (Fig. 1F). The mesenchymal cells were spindle-shaped with indistinct cell borders, had moderate amounts of pale eosinophilic cytoplasm, plump fusiform-to-oval nuclei, finely stippled-to-vesicular chromatin, and 1–2 conspicuous nucleoli with moderate anisokaryosis (Fig. 1F). These spindle cells were closely associated with collagen and small-caliber blood vessels, interpreted as reactive fibroblasts with neovascularization. Eosinophils dispersed throughout the lesions of liver and mesenteric lymph node were highlighted by Luna staining (Suppl. Fig. 1). The stomach, duodenum, jejunum, and ileum had no significant histologic changes except for minimal lymphoplasmacytic and neutrophilic infiltrates in the lamina propria (data not shown).

A series of antibodies were applied to the affected tissues for immunohistochemistry (IHC; Table 1). Smooth muscle actin (SMA) immunolabeling highlighted the increased, disorganized mesenchymal cells within the mesenteric lymph node and liver, phenotypically consistent with myofibroblasts (Fig. 2A, 2B). Immunolabeling of ionized calcium–binding adapter molecule 1 (IBA1) highlighted large numbers of polygonal-to-spindle cells scattered throughout the fibrotic lesions (Fig. 2C, 2D). Dual immunolabeling of SMA and IBA1 revealed frequent double-positive, spindle-to-stellate mesenchymal cells (Fig. 2E, 2F) predominantly at the periphery of the sclerotic nodular lesions. CD31 immunolabeling highlighted many small-caliber, tortuous blood vessels throughout the lesions, indicating neovascularization (Suppl. Fig. 2A). Both CD117 (c-kit) IHC (Suppl. Fig. 2B) and Giemsa histochemical stains (Suppl. Fig. 2C, 2D) were performed, but only small numbers of mast cells were found scattered throughout the lesions. Our morphologic diagnoses were eosinophilic and histiocytic periportal hepatitis, with fibrosis, sclerosis, and biliary hyperplasia, and eosinophilic and histiocytic lymphadenitis, with extensive fibrosis, sclerosis, and granulation tissue with myofibroblast differentiation.

Table 1.

Antibodies used in immunohistochemistry in our study of eosinophilic sclerosing fibroplasia in a cat.

Antibody	Host	Source	Clone	Antigen retrieval (min)	Dilution	Chromogen	Positive control
SMA	Mouse	Leica, PA0943-U	αsm-1	None	RTU	DAB	Intestine (canine)
IBA1	Rabbit	Wako, 018-28523	6A4	H2 (10)	1:3,000	DAB	Spleen, lymph node, tonsil (canine)
CD117 (c-kit)	Rabbit	Dako, A4502	Polyclonal	H2 (10)	1:800	DAB	Cutaneous mast cell tumor (canine)
CD31	Mouse	Dako, M0823	JC70A1	H2 (10)	1:100	DAB	Lymph node, spleen, intestine (canine)
Vimentin	Mouse	Leica, PA0640-U	V9	H2 (20)	RTU	DAB	Skin (canine)
IBA1 and SMA (dual stain)	—	—	—	None	1:3,000, RTU	DAB (IBA1) RED (SMA)	Spleen, lymph node, tonsil (canine)

All immunohistochemical stains were performed on a Leica Bond-Max autostainer. DAB = 3,3′-diaminobenzidine; H2 = Bond epitope retrieval solution 2, pH 8.9–9.1 at 25°C (minutes the tissue slides were exposed to the solution); IBA1 = ionized calcium–binding adapter molecule 1; RTU = ready-to-use; RED = Bond Polymer Refine Red; SMA = smooth muscle actin; — = same antibodies as listed above.

Figure 2.

Immunohistochemistry (IHC) of eosinophilic sclerosing fibroplasia in a cat. A. Myofibroblasts in the hepatic nodule with moderate-to-strong cytoplasmic expression of smooth muscle actin (SMA). These myofibroblasts are mainly at the periphery of the lesion, extending into adjacent hepatic parenchyma (arrowheads). Bar = 200 µm. B. In mesenteric lymph node, the myofibroblasts have robust cytoplasmic expression of SMA. Tunica media of many small-caliber blood vessels are also highlighted. Bar = 200 µm. C. In the hepatic nodule, many macrophages are highlighted by strong cytoplasmic expression of ionized calcium–binding adapter molecule 1 (IBA1). Note that macrophages are also present within the collagen bundles (asterisks). Bar = 200 µm. D. In mesenteric lymph node, many macrophages are highlighted by strong cytoplasmic expression of IBA1. Note that macrophages are closely associated with the proliferating spindle cells. Bar = 200 µm. E, F. Dual IHC of SMA (red) and IBA1 (brown) demonstrates double-positive cytoplasmic expressions in the spindle-to-stellate mesenchymal cells (arrowheads; Fig. 2F) within the liver lesion. Note the oval-to-reniform nuclei of those double-positive cells. Bar = 50 µm (Fig. 2E). Bar = 20 µm (Fig. 2F).

Ours is an unusual case of eosinophilic sclerosing fibroplasia with involvement of the hepatic parenchyma in a cat. Feline gastrointestinal eosinophilic sclerosing fibroplasia (FGESF) is an emerging, poorly understood disease associated with chronic inflammation in domestic cats and, rarely, exotic felids.^3,5,11,12 It has a distinctive histomorphology of eosinophil-rich inflammatory responses, extensive fibroplasia, and deposition of dense collagen trabeculae (i.e., sclerosis) in affected tissues. ³ Several case series and many sporadic case reports of FGESF have been published.^2,3,11,19 Affected cats typically have a single or multiple intrabdominal mass(es), and the lesions are largely confined to the gastrointestinal tract, most commonly in the pylorus, ileocecocolic junction, and associated draining lymph nodes.^2,3,11,19 Clinically, FGESF should be considered for cats with nodular abdominal masses and peripheral blood eosinophilia.

Clinical differentiation of FGESF from other disease entities, such as fungal granuloma, feline infectious peritonitis, and solid neoplasms involving the abdominal organs, can be difficult. Exploratory laparotomy and histopathology are usually required for a definitive diagnosis. ¹¹ Notably, FGESF-like lesions have also been reported in tissues outside of the gastrointestinal tract, including the pancreas, ³ mesentery, ⁸ retroperitoneum, ¹⁷ and retropharyngeal and submandibular lymph nodes. ²⁰ Rarely, involvement of the common bile duct and hepatic parenchyma^9,19 were mentioned in cats diagnosed with FGESF, which were presumed to be extensions of the intramural gastrointestinal lesions. However, the histomorphology and description of the hepatic lesions are quite limited in the literature. Our case had nodular FGESF-like lesions in the liver and mesenteric lymph node with no overt histopathologic changes in the gastrointestinal tract. Considering the overall pattern, a more inclusive term, feline eosinophilic sclerosing fibroplasia (FESF), as proposed by others, ²⁰ would be more appropriate for our case.

FESF in domestic cats does not have an apparent age or sex predisposition. Although some studies indicate a predilection in middle-aged cats, a wide age distribution is often recorded. Although neutered males seem to slightly outnumber others in several studies, FESF is equally reported in males and females.^2,3,9,11,19 Vomiting, diarrhea, chronic weight loss, lethargy, and anorexia are frequently reported signs in clinically ill cats with FESF,^2,11 likely secondary to extensive inflammation and fibroplasia of the gastrointestinal tract. Eosinophilia, hypoalbuminemia, and hyperglobulinemia are also commonly noted hematologic abnormalities. In our case, peripheral blood eosinophilia was not observed; however, lack of eosinophilia is also not uncommon for cats diagnosed with FESF.^2,11

The initial cytology examination did not provide a specific diagnosis, likely because FESF lesions are mainly composed of tightly bound, poorly exfoliated mesenchymal tissue with mixed inflammatory cell populations, as shown by histology. The signalment and clinical presentations of our case are consistent with previously reported FESF cats,^2,3,11 except that the gastrointestinal tract appears to have been spared from the inflammation and sclerotic lesions. Based on the clinical history, our case may initially have had an undefined lesion involving the small intestines and mesenteric lymph nodes, which ceased or resolved following the corticosteroid treatment. At the time of biopsy, FESF lesions were only observed in one mesenteric lymph node and extended throughout the liver. However, we cannot exclude the possibility that the lack of gastrointestinal change was simply a result of the limited, nonrepresentative biopsy samples examined.

Corticosteroids appear to play an important role in mitigating inflammation and disease progression in cats diagnosed with FESF.^2,3,11 In general, cats affected by FESF have a more favorable prognosis and longer survival time compared to those diagnosed with neoplasia, especially with immunomodulatory and antibiotic treatments following surgical resection of the nodular masses.^2,3,11 In our case, due to progressive anorexia, the cat was euthanized ~2 mo after initial presentation and the initiation of corticosteroid therapy. The patient was doing well initially when receiving an appropriate dosage of prednisolone consistently, but started to decline following difficulty administering medication. The role of corticosteroids in the clinical progression of our case is difficult to interpret because of the inconsistent prednisolone dosages and the relatively short disease course available for assessment before euthanasia. Unfortunately, the body was not available for autopsy.

Histologically, FESF shares some features with other disease entities, and has in the past been misinterpreted as sclerosing mast cell tumor (sMCT), fibrosarcoma, and extra-skeletal osteosarcoma.^3,11 Differentiating FESF from sMCT can be challenging in some cases, ¹⁴ especially in surgical biopsy specimens. Our case was initially interpreted as sMCT, but the subsequent histochemical stains and IHC revealed only minimal numbers of scattered mast cells within the lesions (Suppl. Fig. 2B–D), making the diagnosis of sMCT unlikely. Interestingly, a 2025 study reported that FESF lesions were observed in cats with T-cell/natural killer T-cell lymphoma and proposed the term eosinophilic sclerosing lymphoma. ⁹ However, unfortunately, cell–cell interactions between FESF lesions and lymphoma, such as regulating factors, cytokines, cell surface receptors, and cell signaling pathways, were not characterized in that study, therefore, the pathogenesis remained to be determined.

Lesion-associated bacterial, fungal, or other infectious agents were not identified in our case. A variety of pathogens, including bacteria,^2,3,11 fungi,^1,7,19 Toxoplasma gondii, ¹¹ feline immunodeficiency virus ³ in domestic cats, and Cylicospirura spp. nematodes in exotic felids,^5,12 have been reported in association with FESF. However, whether these infectious agents represent the true inciting cause of FESF, or secondary opportunistic infection, remains undetermined; there is no available animal model that can consistently reproduce FESF lesions to fulfill the Koch postulates. Eosinophils are one of the key components of FESF lesions,^3,9 and they play a complex immunomodulatory role in inflammation by enhancing Th2 responses and eliciting and balancing Th1 immune responses.^4,16 In addition to the potential association with infectious agents, FESF may represent another feline-specific, eosinophil-rich inflammatory disease associated with chronic antigenic stimulation, similar to the well-known eosinophilic granuloma complex in cats.^{2,3,9,11,13,19,20}

Myofibroblast differentiation has been reported in FESF, ³ but the link between myofibroblast differentiation and extensive tissue fibrosis was not discussed. Eosinophils are known to play an important role in wound healing and fibrosis by promoting fibroblast proliferation, differentiation, and collagen production.^4,16 Activated eosinophils elaborate several potent fibrogenic cytokines such as transforming growth factor–β (TGF-β) and interleukin 1β (IL1β). ⁴ TGF-β is secreted as an inactive form associated with latency peptides that are subsequently cleaved and activated by proteases, cell surface integrins, and specialized matrix proteins. ⁶ Once activated, TGF-β induces a profibrotic microenvironment and preserves the extracellular matrix (ECM) through activating resident fibroblasts and fibroblast progenitor cells.^6,15 Activated fibroblasts become myofibroblasts and can produce excessive ECM, a process known as fibroblast-to-myofibroblast transition (FMT), which is recognized as the hallmark of fibrosis in many physiologic and pathologic conditions.^6,15 In our case, we demonstrated the pronounced myofibroblast differentiation in FESF lesions by IHC. The myofibroblast phenoconversion in FESF is not a neoplastic change based on the disease progression and clinical outcomes of the affected cats,^2,3,11 but instead, is likely a critical step that leads to profibrotic status and ultimately to extensive fibrosis and sclerosis. Future studies are warranted to verify the correlation between eosinophilic inflammation and myofibroblast phenoconversion.

TGF-β expression in FESF has been observed in fibroblasts, tissue macrophages, and intestinal crypt epithelial cells based on IHC and cell morphology. ¹³ As the central regulator of fibrosis, TGF-β can be secreted by many different cell types, including macrophages, lymphocytes, platelets, fibroblasts, and epithelial cells,^6,15 and TGF-β facilitates FMT and fibrosis. Furthermore, many studies have shown that myofibroblasts can originate from circulating monocytes and tissue macrophages, a phenomenon often referred to as macrophage-to-myofibroblast transition (MMT), which could contribute to fibrotic conditions and diseases.^10,18 We performed dual IHC, which highlights SMA (myofibroblast phenotype) and IBA1 (macrophage phenotype) double-positive myofibroblasts within FESF lesions, suggesting the contribution of MMT to this disease entity. This finding supports the active involvement of macrophages in eosinophilic inflammation, which potentially amplifies the fibrogenic process through self-transformation into myofibroblasts in FESF. In a search of PubMed, Google Scholar, Web of Science, and Scopus, we retrieved no results using the search terms “macrophage to myofibroblast transition” and “cats”, suggesting that MMT has not been reported previously in cats. In humans, therapeutic agents targeting TGF-β signaling, FMT, and MMT have emerged as promising candidates for treating fibrotic diseases such as scleroderma, systemic sclerosis, and cancer-associated desmoplasia.^10,15,18 Likewise, our case highlights the potential of immunomodulatory therapies targeting TGF-β signaling, FMT, and MMT for treating FESF in cats. However, further investigation into the pathogenesis is required to reach a solid conclusion.

Supplemental Material

sj-pdf-1-vdi-10.1177_10406387251339813 – Supplemental material for Myofibroblast differentiation in a cat with eosinophilic sclerosing fibroplasia involving mesenteric lymph nodes and liver

Supplemental material, sj-pdf-1-vdi-10.1177_10406387251339813 for Myofibroblast differentiation in a cat with eosinophilic sclerosing fibroplasia involving mesenteric lymph nodes and liver by Darian L. Giannino, Mason C. Jager, Hannah Brodlie and Rory C. Chien in Journal of Veterinary Diagnostic Investigation