Lymphoid proliferations and lymphomas associated with gastric metaplasia, dysplasia, and carcinoma.

Gastric carcinomas are invariably accompanied by lymphoid proliferations. We studied their features in 22 resected gastric carcinomas in which the lymphoid proliferations ranged from reactive lymphoid follicles to mucosa-associated lymphoid tissue (MALT) lymphomas. In most cases, the collections of lymphocytes were abundant, which is remarkable considering the lack of lymphoid tissue in the normal stomach. They were not haphazardly located but in direct contact with the metaplastic, dysplastic, and neoplastic epithelial cells, in positions suggestive of defense barriers. They consisted of newly formed lymphoid follicles with reactive germinal centers sometimes high up in the superficial mucosa, collections of plasma cells beneath the surface epithelium, and large aggregates of B cells above and below the muscularis mucosae as well as abundant T cells. The latter, both CD4+ and CD8+, were seen within metaplastic epithelial cells as well as within carcinomatous glands that were partially destroyed, resembling apparent neoplastic lympho-epithelial lesions (LEL). In three cases, the B cells infiltrating the gastric muscular layers represented MALT-lymphomas adjacent to gastric carcinomas, as confirmed by polymerase chain reaction (PCR) analysis in two cases. In a case of lymphoepithelioma-like carcinoma, the excessive lymphoid cells were predominantly of T-CD8+ type. In this case, EBV identified by EBV-encoded RNA and latent membrane protein was present in large amounts. Helicobacter pylori was seen in only six cases in areas of chronic gastritis that were distant from carcinoma. H. pylori was not present in the areas of metaplasia, dysplasia, or carcinoma. It appears that the lymphoid proliferations accompanying these gastric changes do not arise in response to the pathogenic agent H. pylori, which caused the persistent infection leading to them yet is no longer present, but rather in response to the existence of the abnormal epithelial cells. Thus the lymphoid proliferations consistently associated with gastric metaplasia, dysplasia, and neoplasia may be regarded as immune reactions to the long-term cellular changes triggered by the initial chronic gastritis. On rare occasions, the exaggerated lymphoid proliferations may reach the end of the spectrum, resulting in MALT lymphomas coexistent with gastric carcinomas.

EBV-associated primary lymphomas in salivary glands of HIV-infected patients.

The lymph nodes within and around salivary glands are commonly involved in inflammatory processes, but rarely the site of primary lymphomas. We observed six cases of primary salivary gland lymphoma in HIV-infected patients and studied them in parallel with three cases of primary salivary gland lymphoma unrelated to HIV and three cases of HIV-related salivary gland lymphadenopathies in order to characterize this new entity. We found that all salivary gland lymphomas in HIV-infected patients were of high histologic grade while salivary gland lymphomas unrelated to HIV were predominantly of low grade MALT type. All lymphomas in both categories expressed the B-cell phenotype. Just as HIV-unrelated lymphomas frequently arise on the background of chronic inflammatory lymphoid processes, lesions characteristic of HIV-lymphadenopathy were still present in some lymphomas of HIV-infected patients. EBV RNA transcripts (EBER) were demonstrated in three, and latent membrane protein (LMP) in two of the six HIV-related and in none of the three HIV-unrelated lymphomas. The three EBER-positive lymphomas were of the histologic types known to express the virus in most cases. The presence of HIV in the form of the core protein p24 and envelope glycoprotein gp41 on the dendritic reticular cells of germinal centers was ascertained in the cases of HIV-related lymphadenopathies but also in the coexistent lymphadenopathies of lymphomas. The practical importance of diagnosing the salivary lymphadenopathies and lymphomas associated with the HIV-infection resides in avoiding their misdiagnosis and surgical removal as tumors of salivary glands.

EBV-associated anorectal lymphomas in patients with acquired immune deficiency syndrome.

Primary lymphomas of the gastrointestinal tract represent 9% of all non-Hodgkin lymphomas, and of these only 3% arise in the rectum or anus. In contrast to their rare occurrence in the general population, the incidence of anorectal lymphomas in patients with acquired immune deficiency syndrome (AIDS), particularly homosexual patients, may be as high as 26% as reported in our own series of AIDS-associated lymphomas. To determine the characteristics of this entity, we studied 15 cases of primary anorectal lymphoma in AIDS patients and compared them with four cases of anorectal lymphoma unrelated to AIDS. The cases in our study were also compared with the reports of rectal lymphoma in the medical literature over the past 30 years. In the present series, the AIDS patients were all male with a median age of 34 years, human immunodeficiency virus (HIV)-positive, with homosexuality as the main risk factor. The four non-AIDS patients included a woman and had a median age of 66.5 years. Histologically, the anorectal lymphomas in AIDS patients were all high grade, predominantly immunoblastic, and polymorphous. In the non-AIDS patients, only two of four lymphomas were high grade, including one Burkitt type. All tumors were of B-cell phenotype. In the AIDS-associated anorectal lymphomas, the presence of Epstein-Barr virus (EBV) in a latent form was demonstrated by an abundance of Epstein-Barr-encoded RNA (EBER) in 14 of 15 cases and latent membrane protein (LMP) in four cases. All anorectal lymphomas unrelated to AIDS were negative for EBV. The unusual anorectal location of AIDS-associated lymphomas is explainable by the high incidence of preceding traumatic lesions and chronic infections in the area. As a result, EBV-carrying B cells may be attracted to the field providing the cell population that, under the conditions of immune deficiency, is able to give rise to high-grade lymphomas.

Lymphoid monoclonal antibodies reactive with lung tumors. Diagnostic applications.

In the course of investigating 30 monoclonal antibodies (MAbs) for their potential reactivity with 25 lung tumors of different histologic types, we found that three MAbs commonly used for their specificities for lymphoid markers were highly reactive with non-small-cell carcinomas (NSCLC) and totally nonreactive with small-cell carcinomas (SCLC). Immunostaining was performed by the standard streptavidin-biotin-peroxidase method after microwave antigen retrieval on formalin-fixed, paraffin-embedded tissue sections. LN2 (CD74), LN3 (HLA-DR), and BLA-36, which are commonly used for the identification of B-lymphocytes, strongly immunostained 19 of 25 squamous and adenocarcinomas and none of 34 small-cell carcinomas and carcinoids. Moreover, in combined tumors, these MAbs selectively stained the adenocarcinoma cells but not the adjacent small-cell carcinoma cells. A cocktail mixture of LN2, LN3, and BLA-36 assayed on 24 additional lung tumors produced similar results with even stronger and sharper stainings. Other lymphoid MAbs showed some selective staining but to a lesser degree. Among nonlymphoid MAbs, the results were as expected, with MAbs for cytokeratin (B72.3) and epithelial membrane antigen staining NSCLC but also some SCLC. The MAbs for chromogranin and neuron-specific enolase were not entirely specific, whereas some nerve-cell adhesion molecule MAbs showed good specificity for SCLC. In a field with few specific MAbs, the newly discovered ability of these lymphoid MAbs to discriminate between SCLC and NSCLC may prove useful in the immunohistochemical diagnosis of lung tumors.

Phenytoin-related immunodeficiency associated with Loeffler's syndrome.

BACKGROUND: Phenytoin is one of the most commonly prescribed drugs in the United States. Its use is associated with a myriad of adverse reactions, including: eosinophilia, selective IgA deficiency and panhypogammaglobulinemia, pseudolymphoma, Stevens-Johnson syndrome, and interstitial pneumonia. OBJECTIVE: To report a case of immunodeficiency manifest by panhypogammaglobulinemia and a low helper-to-suppressor ratio secondary to phenytoin crossreactivity with phenobarbital and carbamazepine complicated by hepatotoxicity, eosinophilia, and fleeting pulmonary infiltrates. METHODS: Case report; immunoglobulin levels, T and B cell studies, and radiologic evaluation of patient. RESULTS: A 37-year-old Oriental female taking phenytoin and phenobarbital for seizure prophylaxis after resection of a grade IV astrocytoma of the left frontal lobe, developed a rash, elevated liver function tests, and cervical lymphadenopathy with parotid gland enlargement. The abnormalities resolved with discontinuation of the drugs and the patient was discharged on carbamazepine. Eight weeks later the patient was readmitted with fever, slowed mentation, elevated liver function tests, and panhypogammaglobulinemia. Clonazepam was substituted for carbamazepine and the patient subsequently developed a rash and further elevation of her liver function tests. The clonazepam was discontinued and the patient was treated with methylprednisolone. She subsequently developed Loeffler's syndrome and a T cell deficiency with a decreased helper-to-suppressor cell ratio. She was treated with increased doses of methylprednisolone and granulocyte stimulating factor with complete resolution of her symptoms. CONCLUSIONS: Phenytoin is associated with a myriad of side effects, including, rash, eosinophilia, panhypogammaglobulinemia, pseudolymphoma, Stevens-Johnson syndrome, immunosuppression in brain tumor patients, and rarely, pulmonary complications such as Loeffler's syndrome. Cross-reactivity with other anticonvulsant agents capable of forming arene oxide intermediates occurs in the cytochrome P-450 system.

Kaposi's sarcoma of internal organs. A multiparameter study of 86 cases.

BACKGROUND: During the past decade, Kaposi's sarcoma (KS), one of the most common acquired immune deficiency syndrome-defining diseases, has been the subject of sustained research. However, basic questions about its etiology, histogenesis, growth, and dissemination remain unanswered. Even its nature, whether hyperplasia or neoplasia, is still controversial. Most studies and concepts to date have been based on dermatologic KS. The present study, in contrast, examines by various parameters a series of patients with KS of internal organs. MATERIALS AND METHODS: The series includes 86 cases (39 surgical specimens and 47 autopsies) of visceral and disseminated KS. The study is focused on the gross distribution of lesions, the mode of dissemination, the histologic patterns, and the cellular immunophenotypes, which are investigated with the use of 18 monoclonal antibodies. RESULTS: The involvement of various organs, multiplicity of lesions, and progression of tumors were recorded. Seven histologic patterns forming a spectrum of cellular differentiation were distinguished. Immunophenotypes characteristic for different histologic patterns were recognized. Although some cell markers such as those recognized by antibodies against Factor VIII R-Ag, Actin, and Ulex europaeus were restricted to the well differentiated KS cells, others including CD34 and CD31 demonstrated a strong affinity for the entire spectrum of KS cell differentiation. CONCLUSION: The present study of KS of internal organs revealed that poor grades of histologic and immunophenotypic differentiation correlated with invasion and dissemination, which are fundamental characteristics of malignant tumors.

Reactivity of lung tumors with lung-derived and non-lung-derived monoclonal antibodies.

The reactivity of 74 lung-derived monoclonal antibodies (MAbs) provided by the Third International Workshop on Lung Tumor Antigens and of 41 non-lung-derived commercially available MAbs against sections of 15 lung tumors of various histologic types was investigated by immunohistochemistry. Three MAbs with specificity for human neural-cell adhesion molecule (H-NCAM) and 3 MAbs with specificity for small-cell lung carcinoma (SCLC) were able to distinguish between neuro-endocrine (NE) and non-NE tumors. Fifteen MAbs stained non-small-cell carcinomas (NSCLC) but not SCLC. Neuron-specific enolase (NSE) stained all NE tumors but also some of the non-NE tumors. Two MAbs showed specificity for mesotheliomas. Carcino-embryonic MAb strongly stained all SCLC and NSCLC. Among MAbs with lymphoid-cell specificities, Leu 7 (CD57) stained SCLC, but not NSCLC. LN2 (CD45R), LN3 (HLA-DR), Leu 22 (CD43) and BLA 36 reacted with NSCLC and were non-reactive with SCLC. Some of the lung-derived MAbs showed immune staining of lymphoma and melanoma.

Cytomegalovirus, angiomatosis, and Kaposi's sarcoma: new observations of a debated relationship.

Kaposi's sarcoma (KS) encompasses a broad spectrum of lesions ranging from foci of muco-cutaneous angiomatosis to tumor masses of internal organs. Its strong association with immune deficiency and the marked differences in incidence between the various populations at risk are suggestive of an infectious etiology. The agent most often suspected of being implicated in the etiology of KS is cytomegalovirus (CMV); however, despite sustained research on this subject, its role remains controversial. The present work includes six cases of KS with a broad variety of lesions in which, with the use of light and electron microscopy, immunohistochemistry, and in situ hybridization, we investigated the presence of CMV and examined its relationship with KS. CMV was present in all six cases and showed a remarkable propensity for the KS lesions where both intranuclear and intracytoplasmic forms were not only next to but frequently within KS cells. Areas of angiomatosis, hemorrhage, and KS had usually an abundance of CMV. Herpes-like virus particles inside KS nuclei were documented by light and electron microscopy and identified as CMV by immunohistochemistry and in situ hybridization. The selective morphologic presence of this virus within the tumor cells, not previously demonstrated, indicates a strong association between CMV and KS, the significance of which remains to be established.

Acquired immunodeficiency syndrome-associated lymphomas: clinical, pathologic, immunologic, and viral characteristics of 111 cases.

The incidence of lymphomas in individuals infected with the human immunodeficiency virus has increased progressively since the beginning of the acquired immunodeficiency syndrome (AIDS) epidemic. The present series includes 111 patients, all diagnosed and studied at one hospital in New York City. There were 108 men and three women; the average age was 39 years and male homosexuality was the predominant risk factor. The materials examined originated from 138 surgical specimens and 24 autopsies. There were 11 cases of Hodgkin's lymphoma and 100 cases of non-Hodgkin's lymphomas (NHL), a proportion strongly skewed in favor of the latter. Hodgkin's lymphoma in AIDS patients was characterized by advanced clinical stage, high histologic grade, and frequent bone marrow involvement. Non-Hodgkin's lymphoma in AIDS patients, in contrast to the general population, originated predominantly in extranodal locations (61 cases) versus locations in which the lymph nodes were the site of the primary tumors (39 cases). In the digestive tract, the unusual oral and anal primary locations were often noted and were possibly related to specific risk factors. There were 15 cases of NHL of the central nervous system, an incidence 14 times greater than that recorded in the general population. The majority of NHLs were of high histologic grade, Burkitt's and large cell immunoblastic, representing most of the cerebral and gastrointestinal tumors. All NHLs were of B-cell immunophenotype. Lymphadenopathies with the histologic features of human immunodeficiency virus infection, particularly of the late stage (type C), often preceded NHL. Probing for Epstein-Barr virus genome was more frequently positive in Hodgkin's lymphoma than in NHL. Immunologic evaluations showed severely depressed T cell counts and CD4 to CD8 cell ratios as well as markedly increased levels of antilymphocyte antibodies. Reflecting the background of profound immune deficiency, the AIDS-associated lymphomas were characterized by high aggressiveness, early tendency to generalization, frequent post-treatment relapse, and short periods of survival.

Presence and prognostic significance of antilymphocyte antibodies in symptomatic and asymptomatic human immunodeficiency virus infection.

The presence of antilymphocyte antibody (ALA) in patients with the acquired immunodeficiency syndrome (AIDS), identified in a previous study, was confirmed by testing a population of 200 patients with AIDS. Of these, 88% had significant levels of ALA vs only 8% of a control group of patients with non-AIDS-related diseases. In a prospective study, the levels of ALA were determined in 61 patients with AIDS-related complex who were followed up for 18 to 30 months. During this interval, 31 (67%) of 46 patients with significant elevation of ALA levels developed AIDS, while none of 15 patients without elevation of ALA levels progressed to AIDS. In a group of 85 apparently healthy homosexual men, also followed up for 18 to 30 months, a significant number of those with high levels of ALA developed clinically apparent disease, while those with low levels did not. These results show that the amount of ALA correlates with the present clinical status as well as the future risk of developing immune deficiency.

Ovarian cancer-associated antibodies recovered from ascites: their use for the isolation of ovarian cancer-associated antigen to produce monoclonal antibodies.

Immune complexes (ICs) were recovered from the ascites of a patient with stage IV endometrioid ovarian cancer by sequential precipitation with 33% saturated ammonium sulfate and 2.5% polyethylene glycol 6000 (PEG 6000), followed by affinity chromatography on protein A-Sepharose CL-4B. The IgG-containing ICs were dissociated using 8 M urea, separated by ion-exchange chromatography on Sephadex QAE-50, and subsequently analyzed for purity by immunoelectrophoresis (IEP) and radial immunodiffusion (RID). Recovered antibody was tested for reactivity by immunohistologic techniques against paraffin-embedded tumor tissue and acetone-fixed cell suspensions of epithelial tumors. The antibody which demonstrated ovarian cancer-associated activity was absorbed with antigen extracts of breast, colon, and lung cancers as well as keratin to reduce cross-reactivity. The absorbed endometrioid ovarian cancer-associated antibody (OCAAb) was used to produce an immunoadsorbent column for the recovery of tumor-associated antigens. A mouse monoclonal antibody designated FEN-1 was produced using this antigen-containing fraction, and preliminary screening has demonstrated ovarian tumor-associated reactivity. The use of autologous ICs as reagents for preparing tumor antigen-rich immunogens may provide a valuable tool in the search for tumor-associated antigens.

Immunohistochemical characterization of a monoclonal antibody detecting an endometrioid ovarian cancer-associated antigen.

Murine monoclonal antibody FEN-1 was derived by immunizing Balb/c mice with an affinity-purified endometrioid ovarian cancer-associated antigen recovered from ascites-derived immune complexes. Splenic lymphocytes from the immunized mouse were fused with the myeloma cells SP2/0-AG14 in the presence of PEG 1500. The hybrid cultures were screened for production of immunoglobulins reactive with an extract preparation of an endometrioid ovarian tumor by enzyme-linked immunosorbent assay and flow cytometry. One of the hybrids secretes a monoclonal antibody of the IgG3 subtype designated FEN-1, which reacts with 100% of endometrioid ovarian cancer containing adenoacanthoma by indirect immunoperoxidase on paraffin-embedded tissue. No detectable levels of antigen were found in squamous metaplasia associated with nonendometrioid tumors, and no reactivity occurred against endometrial adenocarcinomas, endometriosis, or normal ovary and endometrium. The antibody does not cross-react with mucinous tumors, nonepithelial tumors of the ovary, or gastrointestinal tissue. This antibody may be used as an aid in the diagnosis of nonmucinous ovarian carcinomas by immunohistology.

Description of an endometrioid ovarian cancer cell line.

A human cell line, designated L-1, has been established from the ascites of an untreated patient with stage IV (FIGO) endometrioid ovarian cancer. This cell line initially grew uninterupted for 6 months without fibroblast contamination and contact inhibition, and has been subcultured weekly for the past 7 years. L-1 does not contain steroid hormone receptors nor does it demonstrate the presence of oncofetal antigens by immunohistochemical techniques. The doubling time of L-1 is 11.8 hr. Flow cytometric analysis reveals an aneuploid DNA peak, and an abnormal karyotype demonstrates hyperdiploidy, translocations, and deletions. Morphology, growth patterns, cytogenetic analysis, and other features of L-1 are characterized.

Lymphocyte membrane antigens in glycol methacrylate embedded tissue.

The use of formalin or Michel's solution either alone or in combination with acetone, and acetone, methanol or ethanol alone as fixatives, and glycol methacrylate as embedding medium were evaluated for their suitability in procedures to detect lymphocyte membrane antigens by OKT and Leu monoclonal antibodies in human tonsils. No staining was detected in sections fixed in 70% or absolute ethanol and embedded in glycol methacrylate with either the direct immunofluorescence or avidin-biotin methods. Fixation in Michel's solutions plus acetone at room temperature revealed staining by both. Neither method resulted in staining after fixation in Michel's solution plus acetone at 4 C presumably due to the slow action of the fixative. Staining was enhanced using a combination of primary and secondary biotinylated antibodies. Dual staining allowed concurrent detection of two antigens in the same section. Glycol methacrylate embedding is a possible replacement for ultracold storage in the preservation of tissue for immunofluorescent staining.

Anti-lymphocyte antibodies in patients with the acquired immune deficiency syndrome.

Human lymphotropic retroviruses lymphadenopathy-associated virus/human T lymphoma virus III have been recently implicated in the pathogenesis of the acquired immune deficiency syndrome (AIDS). The mechanisms leading to the complex immune deregulations of this disease, however, are still largely unknown. To investigate the possible presence of anti-lymphocyte antibodies, lymphocytes from a normal donor were incubated with serum samples from patients with AIDS. Substantial increases of up to 75 percent in the number of surface immunoglobulin-positive lymphocytes resulted from incubation with serum of patients with AIDS and AIDS-related complex but not with serum of patients with non-AIDS-related diseases or of normal control subjects. Monoclonal antibodies to OKT11, OKT4, and OKT8 in conjunction with a double-labeling technique were then used to identify the type of surface immunoglobulin-positive lymphocytes. These experiments showed that binding of immunoglobulins to lymphocytes did not occur at random but was directed against OKT4- or OKT11-positive cells whereas OKT8-positive cells showed no detectable reactivity. The results of these studies indicate that patients with AIDS and AIDS-related complex have circulating antibodies capable of reacting selectively with a population of T cells that is predominantly composed of helper cells and does not include suppressor cells. The augmentation of surface immunoglobulin-positive lymphocytes in the patients studied consistently paralleled the marked decreases of the helper/suppressor cell ratios and the presence of circulating anti-human T lymphoma virus III antibodies. Binding of antibodies to the surface of helper T cells may be a determining event in the pathogenesis of this disease.

Isolation of lung carcinoma-associated antibodies from immune complexes and production of heterologous antisera.

Immune complexes isolated from pleural effusions of lung carcinomas were dissociated by ion exchange chromatography in the presence of 8 M urea. The antibodies thus obtained from 2 lung adenocarcinomas and 2 squamous cell carcinomas were assayed by indirect immunofluorescence against a variety of target cells in fresh suspensions, tissue cultures, and paraffin-embedded sections. Strong cytoplasmic fluorescence was obtained with greater than 90% of the cell populations of all major histological types of lung carcinomas, and negative fluorescence was obtained with the cells of normal lung tissues and most of the nonpulmonary carcinomas. Antigen fractions prepared from the same lung carcinoma-associated immune complexes were used to immunize rabbits for the production of antisera directed against human lung carcinoma. The rabbit antibody preparations after proper absorptions were assayed in indirect immunofluorescence showing patterns of reactivity with normal and tumor cells of various derivations similar to those obtained with the human antibodies. Complete blocking of immunofluorescence staining with both allogeneic and xenogeneic antibody preparations was achieved by prior absorption with lung tumor tissue extracts or with various lung tumor antigen preparations. The present study demonstrates the isolation from lung tumor effusion immune complexes of antibodies with high affinity for lung carcinoma cells and of antigens that can be used to produce tumor-directed heterologous antibodies. The immunofluorescence staining of lung carcinoma-associated antigens in situ on paraffin-embedded sections provides new, topographical information.

Immunologic aspects of gynecologic cancer.

All vertebrates have a defense mechanism, the immune defense system, that protects them from disease-causing microorganisms. Its deliberate exploitation has conquered many infectious diseases and has been a major achievement of medical science in preventing suffering and saving lives. At the beginning of this century, hope was held that dissimilarities between normal and neoplastic cells could be demonstrated by immunologic methods and that vaccination against cancer might become possible. When it was recognized that the many claims of tumor=specific antigenicity were based on experiments in which an immunity to normal alloantigens, rather than tumor-specific antigens, had been demonstrated, the field of tumor immunology came into disrepute. The work of Gross in 1943 and Prehn and Main in 1957 rekindled interest in tumor immunology. Many contributions have advanced the concept of tumor immunology. They are the following: (1) an abundant supply of highly inbred (syngeneic) animals; (2) extensive work on experimental transplantable tumors; (3) an understanding of the mechanism causing rejection of grafted normal and cancerous tissues in animals; (4) identification of the function of the humoral and cell-mediated mechanisms following organ transplantation; (5) the observation that cancers do arouse a specific immune response in the organisms in which they appear; (6) antigenic differences represent the first known qualitative distinction between cancer cells and their normal counterparts; (7) the application of improved technology--columns, use of fluorescein-tagged antibodies, urea as a chaotropic agent, nephelometer; (8) hybridoma to produce a supply of monoclonal antibodies; (9) new vaccines directed against invasive tumors, and (10) the exploration of the role of immune complexes in oncology. The areas of promise and the future of cancer immunology have once again challenged the minds of scientists.

Identification of prekeratin by immunofluorescence staining in the differential diagnosis of tumors.

Fibrillar proteins with a role in cellular shape and motility are present in the cytoplasm of most animal cells. They vary greatly in size, organization, and reactivity according to cell type and can be separately identified by the use of recently developed monospecific antisera and indirect immunofluorescence staining. Prekeratin, a structural protein in the form of intermediate sized filaments is present exclusively in cells of epithelial origin. In the present study 41 human tumors of various organs and their normal tissue counterparts were reacted with prekeratin antiserum and examined by immunofluorescence staining in paraffin embedded sections. Prekeratin was identified in all epithelial cells of the squamous type, which gave the strongest staining reaction, and in smaller amounts in epithelial cells of other histologic types. Cells of lymphoid, melanic, neural, and connective tissue origin were not stained. Thus, combining the specificity of antiprekeratin sera with the selectivity of immunofluorescence staining resulted in a new method of identifying tissues that is applicable to the differential diagnosis of tumors.