Smooth Muscle Myosin Heavy Chain Expression in Nodal and Extranodal Follicular Dendritic Cell Sarcoma
This study was approved by our Institutional Review Board. A text search was performed of the pathology archives of the University of Michigan (Ann Arbor, MI) for all cases of FDCS. Cases without a specific diagnosis of FDCS, additional cases from the same patient and cases without available tissue blocks were excluded and the remaining cases were retrieved for review. Various neoplastic differential diagnoses were also collected to evaluate specificity. Patient age, sex, and involved site at diagnosis were reviewed. The available hematoxylin and eosin-stained slides were evaluated to assess morphology, assign a grade based on documented risk stratification criteria , , , , and determine the presence or absence of hyaline vascular Castleman disease (HVCD). The tumors were classified as a high grade if 2 or more of the following criteria were met: ≥5 cm in size, nuclear pleomorphism, >2 mitosis per mm 2 , extensive necrosis or disseminated disease; tumors meeting 1 or none of these criteria were considered low grade. These additional morphologic variables were assessed to investigate potential unique expression patterns in different settings, particularly those with cytologic atypia, which can pose diagnostic challenges in small biopsies. Sections of 4 μm thickness were deparaffinized and heat-induced epitope retrieval was performed for all antibodies on the Ventana Benchmark Ultra immunostainer for 16 minutes at 36°C using cell conditioning 1 (CC1) buffer from Ventana Medical Systems (Tucson, AZ), with the exception of CD21 which utilized Protease 1 (cat# 05266688001; Roche Diagnostics). After blocking endogenous peroxidase activity, the slides were incubated with the following antibodies: CD21 (clone 1F8; Dako, Agilent Technologies, Santa Clara, CA) for 32 minutes at 36°C, CD23 (clone SP23; Ventana, Roche Diagnostics, Indianapolis, IN) for 32 minutes at 36°C, fascin (clone 55k-2; Cell Marque) for 44 minutes at 36°C, clusterin (clone 41D; Millipore Sigma, Merck, Germany) for 36 minutes at 36°C, D2-40 (clone D2-40; BioLegend, San Diego, CA) for 32 minutes at 36°C and SMMHC (clone SMMS-1; Ventana, Roche Diagnostics) for 32 minutes at 37°C. Immunoreactivity for all antibodies was detected by the UltraView Universal DAB detection kit (Ventana Medical Systems). The staining pattern was graded as either strongly positive, weakly positive, or negative. 

Seven unique cases of nodal and extranodal FDCS were collected from over the past 22 years. The following cases were excluded: 23 cases without tissue blocks, 5 cases from pre-existing patients, and 2 cases without a definite FDCS diagnosis. The clinical and pathologic features with IHC expression patterns are summarized in Table . Clinical Features The patient's age ranged from 25 to 90 (median: 47 y), with an equal sex distribution (male-to-female ratio=1.3:1). Most cases (n=5) presented as a mass in the mediastinum (n=2) or liver, neck, or retroperitoneum (n=1 each). The remaining cases (n=2) presented as localized lymphadenopathy in the parotid gland or mediastinum. Altogether, the mediastinum was most commonly involved (n=3). All patients had localized disease measuring <5 cm. Pathologic Features Most cases (n=5) were low-grade; the 2 high-grade cases showed nuclear pleomorphism and frequent mitoses (up to 8 mitoses per mm 2 ), including atypical forms. Three patients had pre-existing HVCD involving either the mediastinum (n=2) or neck (n=1). Figure shows the range of FDCS morphology and atypia, including a case arising from HVCD. Immunohistochemical Expression Clusterin, CD21, and CD23 were the most sensitive markers studied, being positive in all cases. Clusterin was strongly expressed in all cases, while CD21 and CD23 were strongly expressed in 5 cases and weakly expressed in 2 cases each. Most cases (n=5) were positive with SMMHC, with strong expression in 3 cases and weak expression in 2 cases. Fascin was strongly positive in 3 cases and weakly positive in 1 case, and D2-40 was strongly positive in 1 case and weakly positive in 3 cases. All controls showed appropriate positive and negative staining, including similar expression in background follicular dendritic cell meshworks in benign lymph nodes and in HVCD as positive internal controls and negative expression in all various neoplastic differential diagnoses as negative controls. Fourteen histologic mimics were also evaluated, including 3 lymphoepithelial carcinomas, 2 spindle cell melanomas, 3 spindle cell (sarcomatoid) squamous cell carcinomas, 4 undifferentiated pleomorphic sarcomas, and 2 meningiomas. Figure shows the morphology and IHC stains for a single FDCS case. Figure demonstrates the varied expression patterns of SMMHC in different FDCS cases. Lastly, Figure compares appropriate staining in normal FDC meshworks and various neoplastic differential diagnoses. 

The patient's age ranged from 25 to 90 (median: 47 y), with an equal sex distribution (male-to-female ratio=1.3:1). Most cases (n=5) presented as a mass in the mediastinum (n=2) or liver, neck, or retroperitoneum (n=1 each). The remaining cases (n=2) presented as localized lymphadenopathy in the parotid gland or mediastinum. Altogether, the mediastinum was most commonly involved (n=3). All patients had localized disease measuring <5 cm. 

Most cases (n=5) were low-grade; the 2 high-grade cases showed nuclear pleomorphism and frequent mitoses (up to 8 mitoses per mm 2 ), including atypical forms. Three patients had pre-existing HVCD involving either the mediastinum (n=2) or neck (n=1). Figure shows the range of FDCS morphology and atypia, including a case arising from HVCD. 

Clusterin, CD21, and CD23 were the most sensitive markers studied, being positive in all cases. Clusterin was strongly expressed in all cases, while CD21 and CD23 were strongly expressed in 5 cases and weakly expressed in 2 cases each. Most cases (n=5) were positive with SMMHC, with strong expression in 3 cases and weak expression in 2 cases. Fascin was strongly positive in 3 cases and weakly positive in 1 case, and D2-40 was strongly positive in 1 case and weakly positive in 3 cases. All controls showed appropriate positive and negative staining, including similar expression in background follicular dendritic cell meshworks in benign lymph nodes and in HVCD as positive internal controls and negative expression in all various neoplastic differential diagnoses as negative controls. Fourteen histologic mimics were also evaluated, including 3 lymphoepithelial carcinomas, 2 spindle cell melanomas, 3 spindle cell (sarcomatoid) squamous cell carcinomas, 4 undifferentiated pleomorphic sarcomas, and 2 meningiomas. Figure shows the morphology and IHC stains for a single FDCS case. Figure demonstrates the varied expression patterns of SMMHC in different FDCS cases. Lastly, Figure compares appropriate staining in normal FDC meshworks and various neoplastic differential diagnoses. 

Seven unique cases of FDCS were collected from a large institutional archive to evaluate SMMHC expression in nodal and extranodal FDCS. The expression patterns of SMMHC were compared directly with established FDC markers for each case and among various neoplastic mimics, showing similar sensitivity and high specificity. Clinicopathologic Features The patients had a wide age range at diagnosis (25 to 90 y) with a median of 47 years. The sex distribution was roughly equal. These findings are similar to published data, which show that most patients are older (median: 50 y) with no sex predilection. – , , , FDCS classically presents as a slow-growing, painless mass. It most often arises in extranodal sites, including the abdomen and upper aerodigestive tract. , , , , This was also similar to our findings, in which all cases involved the abdomen and upper aerodigestive tract, with the mediastinum as the most frequent site of disease. A small subset of reported cases (<20%) occur in the setting of HVCD, – , , , while a larger subset of our cases arose from HVCD (43%). This is likely confounded by collection at a tertiary care center. Nonetheless, the association of FDCS with HVCD is consistent with the evidence suggesting that HVCD arises from clonal FDC proliferations, which have the potential to become neoplastic. , , , , Immunohistochemical Features FDCS diagnosis requires the correlation of morphology and immunohistochemistry, including at least 2 FDC markers, to demonstrate FDC origin. Clusterin (apolipoprotein J) is the most sensitive marker (reported to be positive in up to 100% of cases), while the complement and chemokine receptors CD21 (CR2), CD23 (FcεRII), CD35 (CR1), and CXCL13 (B-cell chemokine) are the most specific markers for FDCs. Our results are similar to these published findings. For example, clusterin was strongly positive in all of our cases, while CD21 and CD23 were both either strongly or weakly positive in all cases. D2-40 tended to show weaker positivity in our study, in contrast to some published studies that show strong expression in all or nearly all cases, , but nonetheless showed overall similar sensitivity as our other studied markers. SMMHC complexes with myosin light chain in myoepithelial and smooth muscle cells to produce contractility in coordination with smooth muscle actin (SMA), analogous to skeletal muscle. In hematopathology, the gene encoding SMMHC (MYH11) has long been established as a recurring genetic abnormality in acute myeloid leukemia with inv(16)(p13.1q22) or t(16;16)(p13.1;q22), resulting in CBFB:MYH11 rearrangement. , SMMHC immunostaining was originally studied in vascular walls and used as a marker for vascular lesions, including atherosclerosis, aneurysm, dissection, patent ductus arteriosus, and remodeling. – More recently, SMMHC has been used in other smooth muscle disorders, such as diverticular disease and breast cancer. The specificity of SMMHC is also well reported, , – including possible artifactual staining in the desmoplastic stroma, which did not occur in our study. Ioannidis and Laurini recently showed that SMMHC is a sensitive marker for normal FDCs in both benign and neoplastic B-cell and T-cell nodal proliferations. Their study evaluated 44 cases, including follicular hyperplasia, follicular lymphoma, diffuse large B-cell lymphoma, peripheral T-cell lymphoma, nodular sclerosis Hodgkin lymphoma, and chronic lymphocytic leukemia/small lymphocytic lymphoma. However, the expression of SMMHC in extranodal sites and in FDCS were not studied and have not previously been reported. Our study is the first to report the expression of SMMHC in both nodal and extranodal FDCS, confirming similar findings as Ioannidis and Laurini for benign nodal FDCs. While grading is not performed in the diagnosis of FDCS, grading criteria are used in risk stratification models. , , , , We assigned grades based on the aforementioned criteria to attempt to evaluate for possible differences in expression between low-grade and high-grade cases. For example, SMMHC is more likely to be used in the initial workup for challenging cases, such as those with higher grade features, and awareness of differing expression patterns would be important for proper interpretation. Interestingly, SMMHC was either weakly positive or negative in both high-grade cases and weakly positive in 2 of the 3 cases arising from HVCD; however, the significance of this finding in HVCD is uncertain with the limited number of cases in these subgroups. Expression of SMMHC in other cases arising from HVCD were excluded, as no unstained slides or tissue blocks were available. The association of HVCD and FDCS was also unable to be further examined as the remaining cases without available unstained slides or tissue blocks were either internal cases over a decade old or outside cases without additional history. Nonetheless, we believe that weaker or negative staining for SMMHC may be seen in these unusual scenarios, and that any staining positivity should prompt consideration of FDCS in the differential. Differential Diagnosis FDCS notoriously mimics a variety of neoplasms, including carcinomas, sarcomas, and melanomas. , , Since FDCs are nonhematopoietic stromal cells, they are negative for the leukocyte common antigen (LCA), CD45, – which can cause those who are unfamiliar with FDC histogenesis to exclude erroneously this neoplasm if CD45 is used as a screening marker. Many case reports also reveal immunophenotypic variability in FDCS, including nonspecific staining with CD4, CD30, CD31, CD45, CD68, CD163, claudin-4, EMA, lysozyme, S100, and vimentin; rare cases have also shown expression with CD20, cytokeratins, TTF-1, and renin. – , , – Keen interpretation of the staining pattern can be useful in these cases, such as noticing cytokeratin positivity in the dendritic processes of FDCS. However, we found that SMMHC was negative in all 14 examined cases of neoplastic mimics, further supporting its utility in the diagnostic evaluation of these particularly challenging cases. The key to correctly identifying FDCS relies on combining the morphology of normal FDCs with the immunophenotype of classic FDC markers. Summary We confirm that SMMHC, an IHC marker already used in many laboratories, is also expressed in nodal and extranodal FDCS with a high specificity and similar sensitivity to commonly used FDC markers. SMMHC may be included in a broad IHC evaluation of challenging small biopsies, and any positive expression should prompt FDCS in the differential diagnosis. FDCS is a rare neoplasm and may mimic other neoplasms, requiring a high index of suspicion with close attention to morphology and immunophenotype. 

The patients had a wide age range at diagnosis (25 to 90 y) with a median of 47 years. The sex distribution was roughly equal. These findings are similar to published data, which show that most patients are older (median: 50 y) with no sex predilection. – , , , FDCS classically presents as a slow-growing, painless mass. It most often arises in extranodal sites, including the abdomen and upper aerodigestive tract. , , , , This was also similar to our findings, in which all cases involved the abdomen and upper aerodigestive tract, with the mediastinum as the most frequent site of disease. A small subset of reported cases (<20%) occur in the setting of HVCD, – , , , while a larger subset of our cases arose from HVCD (43%). This is likely confounded by collection at a tertiary care center. Nonetheless, the association of FDCS with HVCD is consistent with the evidence suggesting that HVCD arises from clonal FDC proliferations, which have the potential to become neoplastic. , , , , 

FDCS diagnosis requires the correlation of morphology and immunohistochemistry, including at least 2 FDC markers, to demonstrate FDC origin. Clusterin (apolipoprotein J) is the most sensitive marker (reported to be positive in up to 100% of cases), while the complement and chemokine receptors CD21 (CR2), CD23 (FcεRII), CD35 (CR1), and CXCL13 (B-cell chemokine) are the most specific markers for FDCs. Our results are similar to these published findings. For example, clusterin was strongly positive in all of our cases, while CD21 and CD23 were both either strongly or weakly positive in all cases. D2-40 tended to show weaker positivity in our study, in contrast to some published studies that show strong expression in all or nearly all cases, , but nonetheless showed overall similar sensitivity as our other studied markers. SMMHC complexes with myosin light chain in myoepithelial and smooth muscle cells to produce contractility in coordination with smooth muscle actin (SMA), analogous to skeletal muscle. In hematopathology, the gene encoding SMMHC (MYH11) has long been established as a recurring genetic abnormality in acute myeloid leukemia with inv(16)(p13.1q22) or t(16;16)(p13.1;q22), resulting in CBFB:MYH11 rearrangement. , SMMHC immunostaining was originally studied in vascular walls and used as a marker for vascular lesions, including atherosclerosis, aneurysm, dissection, patent ductus arteriosus, and remodeling. – More recently, SMMHC has been used in other smooth muscle disorders, such as diverticular disease and breast cancer. The specificity of SMMHC is also well reported, , – including possible artifactual staining in the desmoplastic stroma, which did not occur in our study. Ioannidis and Laurini recently showed that SMMHC is a sensitive marker for normal FDCs in both benign and neoplastic B-cell and T-cell nodal proliferations. Their study evaluated 44 cases, including follicular hyperplasia, follicular lymphoma, diffuse large B-cell lymphoma, peripheral T-cell lymphoma, nodular sclerosis Hodgkin lymphoma, and chronic lymphocytic leukemia/small lymphocytic lymphoma. However, the expression of SMMHC in extranodal sites and in FDCS were not studied and have not previously been reported. Our study is the first to report the expression of SMMHC in both nodal and extranodal FDCS, confirming similar findings as Ioannidis and Laurini for benign nodal FDCs. While grading is not performed in the diagnosis of FDCS, grading criteria are used in risk stratification models. , , , , We assigned grades based on the aforementioned criteria to attempt to evaluate for possible differences in expression between low-grade and high-grade cases. For example, SMMHC is more likely to be used in the initial workup for challenging cases, such as those with higher grade features, and awareness of differing expression patterns would be important for proper interpretation. Interestingly, SMMHC was either weakly positive or negative in both high-grade cases and weakly positive in 2 of the 3 cases arising from HVCD; however, the significance of this finding in HVCD is uncertain with the limited number of cases in these subgroups. Expression of SMMHC in other cases arising from HVCD were excluded, as no unstained slides or tissue blocks were available. The association of HVCD and FDCS was also unable to be further examined as the remaining cases without available unstained slides or tissue blocks were either internal cases over a decade old or outside cases without additional history. Nonetheless, we believe that weaker or negative staining for SMMHC may be seen in these unusual scenarios, and that any staining positivity should prompt consideration of FDCS in the differential. 

FDCS notoriously mimics a variety of neoplasms, including carcinomas, sarcomas, and melanomas. , , Since FDCs are nonhematopoietic stromal cells, they are negative for the leukocyte common antigen (LCA), CD45, – which can cause those who are unfamiliar with FDC histogenesis to exclude erroneously this neoplasm if CD45 is used as a screening marker. Many case reports also reveal immunophenotypic variability in FDCS, including nonspecific staining with CD4, CD30, CD31, CD45, CD68, CD163, claudin-4, EMA, lysozyme, S100, and vimentin; rare cases have also shown expression with CD20, cytokeratins, TTF-1, and renin. – , , – Keen interpretation of the staining pattern can be useful in these cases, such as noticing cytokeratin positivity in the dendritic processes of FDCS. However, we found that SMMHC was negative in all 14 examined cases of neoplastic mimics, further supporting its utility in the diagnostic evaluation of these particularly challenging cases. The key to correctly identifying FDCS relies on combining the morphology of normal FDCs with the immunophenotype of classic FDC markers. 

We confirm that SMMHC, an IHC marker already used in many laboratories, is also expressed in nodal and extranodal FDCS with a high specificity and similar sensitivity to commonly used FDC markers. SMMHC may be included in a broad IHC evaluation of challenging small biopsies, and any positive expression should prompt FDCS in the differential diagnosis. FDCS is a rare neoplasm and may mimic other neoplasms, requiring a high index of suspicion with close attention to morphology and immunophenotype. 

