Intermediate filament proteins and actin isoforms as markers for soft tissue tumor differentiation and origin. I. Smooth muscle tumors.

A series of 3 benign and 10 malignant smooth muscle (SM) neoplasms and of 2 malignant fibrous histiocytomas was examined by light microscopy, transmission electron microscopy, two-dimensional gel electrophoresis (2D-GE) and indirect immunofluorescence, using polyclonal monospecific or monoclonal antibodies to desmin, vimentin, cytokeratin, alpha-SM and alpha-sarcomeric (alpha-SR) actins. Benign neoplasms displayed typical light-microscopic features of SM, whereas leiomyosarcomas demonstrated variations in their histologic pattern. In 6 sarcomas, light microscopy suggested a SM differentiation, whereas in the other 4, a predominant nondistinctive spindle-cell pattern was observed. By transmission electron microscopy, all 13 neoplasms showed the minimal essential features of SM differentiation. Immunofluorescence disclosed heterogeneity of cytoskeletal protein expression: 5 neoplasms (3 benign and 2 malignant well-differentiated) expressed desmin, vimentin, and alpha-SM-actin; 2 malignant neoplasms expressed desmin and vimentin; 1 malignant neoplasm expressed desmin, vimentin and alpha-SR actin; 1 malignant neoplasm expressed vimentin and alpha-SR actin; and 4 malignant neoplasms expressed vimentin alone. By 2D-GE, 3 benign and 4 malignant SM neoplasms expressed alpha, beta, and gamma actins, and the remaining expressed only beta and gamma actins. The presence of alpha-SM actin in all benign neoplasms and in 2 well-differentiated leiomyosarcomas suggests that this actin isoform reflects a high degree of cellular differentiation. In 2 leiomyosarcomas, alpha-SR actin was detected by immunofluorescence, which suggested a skeletal muscle differentiation of these neoplasms. This study supports the assumption that leiomyosarcomas represent a heterogeneous group of neoplasms and furnishes new criteria for their characterization.     

The immunophenotype of hemangiopericytomas and glomus tumors, with special reference to muscle protein expression: an immunohistochemical study and review of the literature.

Glomus tumors and hemangiopericytomas have traditionally been described as neoplasms of pericytes. Ultrastructurally, smooth muscle features have been identified in the cells of the glomus tumor, while the cells of the hemangiopericytoma have been described as more closely resembling normal pericytes. Immunocytochemical studies were performed to demonstrate the immunophenotype of these two tumors and to particularly evaluate expression of muscle-specific actin and desmin. Using the avidin-biotin immunoperoxidase method, formalin-fixed, paraffin-embedded tissue from 16 glomus tumors and 11 hemangiopericytomas was evaluated for the presence of vimentin, low-molecular-weight cytokeratins (35 beta H11), muscle actins (HHF35), desmin (clone 33), S100 protein, nerve growth factor receptor (NGFR5), myelin-associated glycoprotein (CD57), Factor VIII-related antigen, and Ulex lectin. Muscle actins were found in 14 of 16 tumors, and desmin was found in three of 16 of the glomus tumors. None of the 11 hemangiopericytomas expressed either desmin or muscle actins. Variable numbers of both tumors were positive with antibodies to CD57, with the nerve growth factor receptor, and with antibodies to S100 protein. In conclusion, these studies provide immunocytochemical evidence of smooth muscle differentiation in glomus tumors. Although muscle differentiation has been identified in the normal pericyte by expression of muscle-specific actin (HHF35), we find no evidence for analogous differentiation in the population of cells comprising hemangiopericytomas.     

Intermediate filament proteins and actin isoforms as markers for soft tissue tumor differentiation and origin. II. Rhabdomyosarcomas.

A series of 15 rhabdomyosarcomas was examined by light microscopy, transmission electron microscopy, two-dimensional gel electrophoresis (2D-GE) and indirect immunofluorescence, the latter using monoclonal or affinity-purified polyclonal antibodies to desmin, vimentin, alpha-smooth muscle and alpha-sarcomeric (alpha-sr) actins. By light microscopy, the authors diagnosed 1 botrioid, 1 alveolar, and 7 embryonal rhabdomyosarcomas, 4 pleomorphic spindle cell sarcomas, and 2 spindle cell sarcomas, one nondistinct, the other with a hemangiopericytomatous pattern. By transmission electron microscopy, 13 neoplasms disclosed rhabdomyoblastic differentiation; the remaining 2, myogenic differentiation. By immunofluorescence microscopy, all neoplasms expressed vimentin and alpha-sr actin, 12 expressed, in addition, desmin, and 1 expressed alpha-smooth muscle actin. Among the 11 neoplasms studied by means of 2D-GE, 7 demonstrated an alpha-actin spot, while 4 showed only beta and gamma spots. One tumor disclosed, in addition to alpha, beta, and gamma spots, a spot with a molecular weight corresponding to actin, but more acidic than alpha-actins. This study demonstrates that alpha-sr actin antibody represents a valuable marker for the diagnosis of rhabdomyosarcoma, because it was present in all neoplasms, including the one negative for desmin. This antibody further allowed the recognition of pleomorphic variants and morphologically atypical forms of rhabdomyosarcomas. The presence of alpha-smooth muscle actin in 1 case of rhabdomyosarcoma suggests that this actin isoform may be expressed during skeletal muscle differentiation.     

All histological types of primary human rhabdomyosarcoma express alpha-cardiac and not alpha-skeletal actin messenger RNA.

Eleven human primary rhabdomyosarcomas (RMSs), including all histological variants, were analyzed morphologically, immunohistochemically for intermediate filament proteins and actin isoforms, and by means of Northern blots with probes specific for total actin, alpha-skeletal (SK), alpha-cardiac (CARD), and alpha-smooth muscle actin messenger (m)RNAs. All tumors disclosed ultrastructural evidence of skeletal muscle features with terminal differentiation in three cases. The RMSs contained immunohistochemically the intermediate filament proteins vimentin and desmin and reacted positively with the alpha-sarcomeric actin antibody, which recognizes alpha-SK and alpha-CARD actin isoforms. All RMSs reacted with the total actin probe, recognizing at 2.1 kb cytoplasmic actin mRNAs and at 1.7 kb alpha-actin mRNAs. With the specific probes, all RMSs expressed alpha-CARD actin mRNA, four neoplasms expressed also alpha-smooth muscle actin mRNA, whereas the probe for alpha-SK actin mRNA never produced a signal except in one case, in which the tumor masses were intermingled with non-neoplastic preexistent striated muscle fibers. Because alpha-CARD and alpha-smooth muscle actins are transiently expressed during normal skeletal muscle development, RMSs seem to follow normal skeletal myogenesis without completing the final step, consisting of alpha-SK actin mRNA expression. The use of Northern blots for alpha-CARD actin as an adjunct to conventional techniques may be helpful for the precise identification of primary RMSs compared to other soft tissue neoplasms.     

Myofibroblasts from diverse pathologic settings are heterogeneous in their content of actin isoforms and intermediate filament proteins

We examined by immunofluorescence the distribution of vimentin, desmin, alpha-smooth muscle actin and alpha-sarcomeric actin in normal human soft tissues and in pathologic tissues containing myofibroblasts, including normally healing granulation tissue, hypertrophic scar, and fibromatosis. The pattern of actin isoforms was also documented biochemically by two-dimensional gel electrophoresis. Fibroblastic and/or myofibroblastic cells in each setting always expressed vimentin and never alpha-sarcomeric actin. Moreover, these cells showed an heterogeneous cytoskeletal composition which defined four phenotypes: (a) cells expressing only vimentin; (b) cells expressing vimentin, alpha-smooth muscle actin and desmin; (c) cells expressing vimentin and alpha-smooth muscle actin; and (d) cells expressing vimentin and desmin. Given this, two groups of lesions are distinguished: the first contains only vimentin cells and consists of normally healing granulation tissue, eschars and normally healed scars; the second contains vimentin cells admixed with variable proportions of vimentin, alpha-smooth muscle actin and desmin, vimentin and alpha-smooth muscle actin, and vimentin and desmin cells and consists of hypertrophic scars and fibromatoses. Immunogold electron microscopy showed that alpha-smooth muscle actin was present in a proportion of cells with ultrastructural features of myofibroblasts. Our findings suggest that contrary to myofibroblasts of normally healing granulation tissue and normally healed scars, myofibroblasts of pathologic conditions characterized by chronic retraction express always immunochemical features indicative of smooth muscle differentiation.     

Glomus tumor of the stomach. Light and electron microscopic study with literature review of related tumors.

A gastric tumor from a 27-year-old female showed the light microscopic features of glomus tumor. Ultrastructurally there were two types of tumor cells, i.e. smooth muscle and possible pericytes. From the literature review of ultrastructure of glomus tumor, hemangiopericytomas (HPC), leiomyoblastomas and leiomyomas, the glomus tumor appeared to exist as an intermediate form between HPC and leiomyomas which were considered to be in a broad spectrum of smooth muscle tumors.     

h-Caldesmon in leiomyosarcoma and tumors with smooth muscle cell-like differentiation: its specific expression in the smooth muscle cell tumor

h-Caldesmon (h-CD) is a protein combined with actin and tropomyosin that regulates cellular contraction. h-CD has been thought to be expressed exclusively in vascular and visceral smooth muscle cells (SMC). We examined h-CD expression immunohistochemically in tumors with SMC and SMC-like differentiation to clarify whether h-CD is specifically expressed in SMC tumors. The tumors examined in this study were six leiomyomas (LM), two angioleiomyomas (ALM), six leiomyosarcomas (LMS), eight rhabdomyosarcomas (RMS), eight malignant fibrous histiocytomas (MFH), four desmoids, three glomus tumors (GT), and two inflammatory myofibroblastic pseudotumors (IMP) of urinary bladder. We found that LM, ALM, LMS, and GT showed intense and extensive immunoreactivity for h-CD, whereas other tumors were completely negative for h-CD. In addition, h-CD was not present in the vascular pericytes and myofibroblasts, in contrast to actin. Although myoepithelial cells were immunopositive for h-CD, neoplastic myoepithelial cells of myoepithelial tumors and mixed tumors of the salivary gland and skin were all negative. These findings indicate that h-CD is a specific marker of both SMC and its neoplasms and that immunohistochemical detection of h-CD may facilitate the differential diagnosis between LMS and other tumors with SMC-like differentiation, including myofibroblastic tumors.     

Thick (myosin) filaments in a glomus tumor

An otherwise classic digital glomus tumor is presented with the unusual ultrastructural finding of cytoplasmic thick (myosin) filaments together with thin (actin) filaments in many of the cells. In places, sarcomere-like orientation was seen. It is little appreciated among diagnostic pathologists, but is well-established, that thick (myosin) filaments occur in smooth muscle type cells. They are present in vivo and can be demonstrated ultrastructurally if rather stringent preparative conditions are met. Whether or not the contractile process in smooth muscle is analogous to skeletal muscle is a debated issue. In the context of diagnostic electron microscopy, it is stressed that thick filaments are not, as often stated, pathognomonic of skeletal muscle neoplasms, and may potentially be found in smooth muscle neoplasms and neoplasms of related cell type (glomus tumors, hemangiopericytomas, tumors of myofibroblasts, etc.).     

GLOMUS TUMOR OF THE STOMACH

A gastric tumor from a 27-year-old female showed the light microscopic features of glomus tumor. Ultrastructurally there were two types of tumor cells, i.e. smooth muscle and possible pericytes. From the literature review of ultrastructure of glomus tumor, hemangiopericytomas (HPC), leiomyo-blastomas and leiomyomas, the elomus tumor appeared to exist as an intermediate form between HPC and leiomyomas which were considered to be in a broad spectrum of smooth muscle tumors. ACTA PATH. JAP. 27: 533–539, 1977.     

Antibody specific to muscle actins in the diagnosis and classification of soft tissue tumors

A series of soft tissue tumors, melanomas, carcinomas, and lymphomas were studied immunohistochemically for the presence of muscle actins (MA) with the monoclonal antibody HHF-35, and for the presence of desmin for comparison. In nonneoplastic tissues, MA immunoreactivity was present in skeletal and smooth muscle cells, in the pericytes of small vessels, and in the myoepithelial cells. Desmin immunoreactivity had a similar distribution, except that the pericytes of small vessels and myoepithelial cells were negative. All 17 rhabdomyosarcomas were positive for both MA and desmin. Of leiomyosarcomas, 31/32 were positive for MA, and 29/32 for desmin. In pleomorphic undifferentiated sarcomas (malignant fibrous histiocytomas) MA and desmin-positive cells were present in 9/35 and 5/35 cases, respectively. Three of five pleomorphic liposarcomas showed MA-positive tumor cells, which were also desmin-positive in one case. Desmoid tumors often showed a moderate number of both desmin- and MA-positive cells. Hemangiopericytoma, Kaposi's sarcoma, and endometrial stromal sarcoma showed MA-positive staining only in the pericytes and not in the neoplastic cells. In various types of carcinomas, melanomas, and lymphomas, MA- or desmin-positive neoplastic cells were not identified. MA, but not desmin, was present in the desmoplastic stroma in many carcinomas. Both MA and desmin are good markers for muscle differentiation and especially serve to identify rhabdomyosarcomas and leiomyosarcomas. These markers are also present in some sarcomas currently regarded as nonmuscle tumors. This may suggest that some of these tumors have differentiation properties related to true myosarcomas. The absence of muscle actin, a pericytic marker, in hemangiopericytoma does not confirm the concept of pericytic nature of this tumor.     

h-Caldesmon as a specific marker for smooth muscle tumors. Comparison with other smooth muscle markers in bone tumors

Caldesmon is a protein widely distributed in smooth and non-smooth muscle cells and is thought to regulate cellular contraction. Its isoform, high-molecular-weight caldesmon (h-CD), was demonstrated to be specific for smooth muscle cells and smooth muscle tumors of the soft tissue and to never be expressed in myofibroblasts. We performed an immunohistochemical study to examine h-CD expression in the following bone tumors: conventional and non-conventional osteosarcoma, 13; malignant fibrous histiocytoma of bone, 5; giant cell tumors of bone, 5; chondroblastoma, 3; metastatic leiomyosarcoma, 2; and rhabdomyosarcoma, 1. Frequent immunoreactivity for muscle actin (alpha-smooth muscle actin or muscle-specific actin) was seen in 11 of 13 osteosarcomas and all other tumors, whereas h-CD was expressed intensely only in 2 leiomyosarcomas. h-CD is considered a specific and useful marker to distinguish smooth muscle tumor from bone tumors with myoid differentiation.     

Calponin and h-caldesmon in soft tissue tumors: consistent h-caldesmon immunoreactivity in gastrointestinal stromal tumors indicates traits of smooth muscle differentiation.

Currently, the immunohistochemical evaluation of smooth muscle differentiation is usually based on desmin, which also reacts with skeletal muscle and is not present in all smooth muscle tumors, and alpha-smooth muscle actin, which reacts with myoepithelial cells. Neither marker typically reacts with gastrointestinal stromal tumors (GISTs), previously classified as smooth muscle tumors or presently often classified as smooth muscle/stromal tumors. Two cytoskeleton-associated actin-binding proteins, calponin (CALP) and h-caldesmon (HCD), are putative smooth muscle markers that also react with myoepithelia. These markers are of particular interest in the immunohistochemical analysis of tumors; neither of them has been extensively documented in soft tissue tumors. In this study, we evaluated selected normal and reactive tissues and more than 250 mesenchymal tumors for CALP and HCD. Both markers were expressed in parenchymal and vascular smooth muscle cells in various organs and in myoepithelial cells. CALP also reacted with myofibroblasts of desmoplastic stroma. All of our 25 benign smooth muscle tumors from various locations were positive for CALP and HCD, as were most of the retroperitoneal and uterine leiomyosarcomas. HCD was more specific, because CALP also reacted with myofibroblastic lesions. The common reactivity of malignant fibrous histiocytomas with CALP and HCD suggests a combination of myofibroblastic and smooth muscle differentiation in these tumors. The GISTs (c-kit positive, usually actin negative) showed nearly consistent HCD reactivity, suggesting traits of smooth muscle differentiation. GISTs were usually CALP negative and showed a CALP expression pattern similar to that of alpha-smooth muscle actin. Although nonmuscle, nonmyofibroblastic tumors were negative for CALP and HCD, synovial sarcomas showed streaks of CALP-positive cells of unknown significance. CALP and HCD should be explored as markers to identify myofibroblastic and smooth muscle cell differentiation in mesenchymal tumors.     

Cytoskeletal properties of alveolar soft part sarcoma

The immunohistochemical expression of cytoskeletal proteins in alveolar soft part sarcoma (ASPS) was studied by light and electron microscopy. Of the five cases examined by the avidinbiotin-peroxidase complex method, variable numbers of immunoreactive cells for desmin were found in three, for vimentin in two, for muscle-specific actins in three, and for alpha-smooth muscle actin in four. Immunoelectron microscopic study demonstrated that desmin and vimentin were localized on whorled bundles of intermediate filaments in the perinuclear cytoplasm. In addition, a few dispersed intermediate filaments became evident in specimens treated with saponin and fixed with tannic acid. These immunohistochemical results indicate that a few tumor cells of ASPS may express some properties of the cytoskeleton of smooth muscle cells in addition to those of skeletal muscle cells. Considering the discrepancies reported in the actin isoforms demonstrated in myogenic tumors, we conclude that ASPS is probably a peculiar, primitive myogenic tumor that does not show any distinctive features of rhabdomyogenic or leiomyogenic differentiation.     

Malignant glomus tumor of the lung

Primary malignant glomus tumors of the lung are extremely rare, and to our knowledge, only three cases have been described to date. We report one such case in a 53-year-old man who presented with a persistent dry cough. Chest computed tomography scans demonstrated an irregularly shaped mass in the right lower lobe of the lung. Many small nodules were distributed from the main tumor to the periphery, along with bronchovascular bundles. Right lower lobectomy was performed under the diagnosis of lung tumor. The tumor was located in the proximal portion of the right lower lobe and extended along the pulmonary arteries. Histological examination revealed a sheet-like proliferation of epithelioid glomus cells and fascicles of spindle cells. The presence of increased mitotic activity, tumor necrosis and prominent intravascular invasion suggested malignancy. The tumor cells were immunoreactive for vimentin, calponin, h-caldesmon, and alpha-smooth muscle actin, which indicated definitive smooth muscle differentiation. We believe that this is the fourth reported case of malignant glomus tumor of the lung.     

Intermediate and fine cytofilaments in cutaneous and subcutaneous leiomyosarcomas.

The expression of fine and intermediate cytofilaments in 10 cutaneous and seven subcutaneous leiomyosarcomas was studied immunohistochemically. All the tumors contained tumor cells which showed a positive immunoreactivity for desmin in formaldehyde-fixed and paraffin-embedded sections, but none of the seven anti-desmin antibodies used alone produced a distinct positive staining in all the tumors. A lack of correspondence in terms of immunoreactivity between tumor cells and the supposed muscle of origin was observed, especially in the subcutaneous leiomyosarcomas. In all cases, antibodies to muscle-specific and smooth muscle-specific actin were found to produce a positive staining in both the tumors and the supposed muscle of origin. Vimentin was detected in 8/10 cutaneous and 4/7 subcutaneous leiomyosarcomas, while the supposed muscle of origin was positive in 3/10 and 7/7 cases, respectively. Four of the cutaneous and three of the subcutaneous leiomyosarcomas contained tumor cells which stained positively for cytokeratins, while the supposed muscle of origin showed no positivity. It thus appears that a phenotypic shift in terms of vimentin and cytokeratin expression occurs in the tumor cells of cutaneous and subcutaneous leiomyosarcomas compared with the supposed muscle of origin. It is recommended that more than one monoclonal anti-desmin antibody is used to characterize these tumor entities. It is also concluded that the immunoreactivity for muscle-specific actins in superficial leiomyosarcomas is more constant, although less specific, than that of desmin and that the demonstration of the simultaneous expression of muscle-specific actins and desmin is helpful.     

Pathological situations characterized by altered actin isoform expression

Modulation of actin isoform expression is a well-established feature of developmental phenomena. As one might expect, it is also characteristic of several pathological situations that are the subject of the present review. alpha-Smooth muscle actin has proven to be a reliable marker for identifying (a) vascular smooth muscle cells during vascular development and vascular diseases, and (b) myofibroblasts during wound healing, fibrocontractive diseases, and stromal reaction to epithelial tumours. The hallmark of a differentiated myofibroblast relies on the acquisition of an organized contractile apparatus characterized by alpha-smooth muscle actin-expressing stress fibres. More and more data suggest that alpha-smooth muscle actin plays a direct role in myofibroblast contractile activity through its N-terminal domain AcEEED. Newly developed antibodies against alpha-skeletal and alpha-cardiac actins have allowed the detection of subpopulations of alpha-skeletal positive cardiomyocytes in adult, hypertrophic, and failing heart. These antibodies have also permitted us to identify the differentiation degree of malignant cells in tumours such as rhabdomyosarcoma. Whether the differential expression of actin isoforms in human diseases is functionally relevant is not yet fully established, although studies on human actin mutations, actin null mice, and the N-terminal end of alpha-smooth muscle actin support this possibility.     

Malignant islet cell tumor with sarcomatous differentiation.

Malignant mesenchymal neoplasms of the pancreas are rare and malignant islet cell tumors with sarcomatous dedifferentiation are rarer still. We present a case of malignant islet cell tumor with sarcomatous differentiation, which to our knowledge is only the second reported case showing such a combination of morphologic features. Clinically, the neoplasm was not hormonally active and immunohistochemical staining was negative for gastrin, glucagon, insulin and somatostatin. The sarcomatous component strongly reacted with an antibody directed against vimentin, and a minority of cells stained strongly with antisera directed against desmin and smooth muscle actin. The spindle cell component was nonreactive with antibodies directed against Factor VIII. The myogenous direction of differentiation in the present tumor is similar to that seen in the prior case report of malignant islet cell tumor with rhabdomyosarcomatous differentiation.     

Liposarcoma. An ultrastructural study of 15 cases

Fifteen liposarcomas from 13 patients were examined by electron microscopy. These included nine primary tumors, four recurrent tumors after primary surgery or irradiation, and two metastatic lesions. Twelve of the liposarcomas were located in the thigh, and 11 were of the myxoid variety. All neoplasms were composed of cells having the ultrastructural characteristics of some stage of lipoblastic differentiation, i.e., lipid droplets, micropinocytotic vesicles, glycogen, external lamina, intermediate filaments, Golgi apparatuses, rough and smooth endoplasmic reticulum, and mitochondria. The nuclear-cytoplasmic ratio and nuclear pleomorphism were related inversely to the size and number of lipid droplets. Lipoblasts were frequently in close association with capillaries and pericytes, and in four cases lipid droplets were found in pericytes. Multivacuolated, mitochondria-rich lipoblasts, resembling brown fat cells, also were seen. Most tumors contained lipid-free, poorly differentiated mesenchymal cells that showed a continuum of morphologic differentiation to cells that closely resembled early lipoblasts that contained nonmembrane-bound lipid vacuoles. Fibrolipoblasts, cells containing lipid droplets and abundant rough endoplasmic reticulum, were observed only in well-differentiated liposarcomas. Some soft tissue sarcomas contain vacuolated cells that simulate lipoblasts by light microscopy but prove to be reactive or malignant fibroblasts, histiocytes, or smooth muscle cells ultrastructurally. Therefore, use of electron microscopy may be necessary to establish the line of differentiation in these neoplasms.     

Proteins of intermediate filaments. An immunohistochemical and biochemical approach to the classification of soft tissue tumors

The intermediate filament cytoskeleton of various types of human soft tissue tumors was analyzed by immunofluorescence microscopy with the use of specific antibodies against cytokeratins, vimentin, and desmin, as well as by one- and two-dimensional gel electrophoresis of high-salt buffer- and detergent-resistant cytoskeletal preparations. All leiomyomas as well as a leiomyosarcoma contained desmin. Leiomyomas of both gastrointestinal and uterine derivation and the retroperitoneal leiomyosarcoma showed strong reaction for desmin in the smooth muscle cells, but the latter two exhibited also vimentin staining. In embryonal rhabdomyosarcomas, desmin prevailed in the large, apparently well-differentiated rhabdomyoblasts; whereas the smaller, less differentiated tumor cells preferentially contained vimentin. Cells of malignant fibrous histiocytomas were characterized by their content of vimentin as the only intermediate filament protein present. In alveolar soft part sarcoma, a rare tumor of hitherto unknown histogenesis, vimentin and desmin co-existed within the same tumor cells, indicating, together with chemical determinations, the myogenic derivation of this neoplasm. The results show that immunologic and biochemical analysis of proteins associated with the intermediate filament cytoskeleton is a useful adjunct in the diagnosis of diverse neoplasms, particularly those with equivocal histologic features, and thus aids in the histogenetic classification of soft tissue tumors.     

Distribution of actin isoforms in sarcomas: An immunohistochemical study

The actin immunophenotype of eight benign mesenchymal tumors, 14 nonsarcomatoid tumors, and 46 sarcomatoid tumors was studied, using monoclonal antibodies (MoAb) specific for alpha-smooth muscle actin (clone 1A4), alpha- and gamma-smooth muscle actin (designated CGA7), and muscle actin (designated HHF35) on frozen sections. Tumor cells of nonsarcomatoid tissues were not reactive, but all leiomyomas and five of the seven leiomyosarcomas reacted with the three MoAbs. One leiomyosarcoma was immunoreactive for the MoAb 1A4 only. One of the six malignant schwannomas showed staining for muscle actin (HHF35). The 22 malignant fibrous histocytomas (MFH) expressed these actin isoforms in various degrees. One case immunoreacted with all three MoAbs, three reacted with 1A4 only, seven reacted with CGA7 and HHF35, and two reacted with HHF35 only. Nine MFHs were not immunoreactive for any of the MoAbs specific for (smooth) muscle and actin. In addition, the expression of desmin and collagen type IV was investigated for the group of leiomyosarcomas and MFHs. Desmin was found in five leiomyosarcomas and in two MFHs. Collagen type IV was seen in all leiomyosarcomas, and was seen weakly in a few small areas in four MFHs. When we take into account the expression of all markers tested [( smooth] muscle actin, desmin, and collagen type IV), then six of the 22 MFHs were unreactive for all these markers. Five of these six tumors were located intramuscularly, whereas only half of the total number of MFH cases had an intramuscular location. The fact that 15 of 22 MFHs displayed one or more markers linked with (smooth) muscle differentiation suggests that some of the MFHs may be classified as poorly differentiated leiomyosarcomas, and that MFH is not a unique entity.