Establishment in continuous culture of a T-lymphoid cell line (HD-Mar) from a patient with Hodgkin's lymphoma

Hodgkin lymphoma cell lines: to separate the wheat from the chaff

Characteristic components of Hodgkin lymphoma (HL) tissue are the mono- or multinucleated Hodgkin-Reed-Sternberg (HRS) cells. Given the challenges of isolating these rare malignant cells and the difficulty in culturing cells from patients, many investigators have tried to establish cell lines in efforts to develop cellular tools for in vitro studies. A limited number of HL cell lines exist and have provided valuable insights into HL pathobiology. A literature survey indicated that 35 cell lines derived from HL patients have been published. To determine whether all these alleged HL cell lines hold up to scrutiny, we examined the available data and also put some of these cell lines to the test of hierarchical clustering, providing additional information regarding assignment to cell line type and tissue derivation. Hierarchical clustering separated the bona fide (classical) HL cell lines completely from cell lines derived from other lymphoma categories and proved conclusively that HL cell lines represent a distinct entity, irrespective of the cellular origin of the HRS cells. We conclude by pointing out the need for an intensified search for new cell culture avenues in order to develop a new generation of informative HL cell lines covering more widely the spectrum of HL stages and subtypes.

Overexpression of novel short isoforms of Helios in a patient with T-cell acute lymphoblastic leukemia

OBJECTIVE

In previous studies, we demonstrated overexpression of the dominant-negative isoform of the transcription factor Ikaros, Ik-6, in patients with blast crisis of chronic myelogenous leukemia and B-cell acute lymphoblastic leukemia. In the present study, we analyzed expression of the Ikaros family genes Ikaros, Aiolos, and Helios in a panel of human T-cell leukemia/lymphoma cell lines and bone marrow samples of patients with T-cell acute lymphoblastic leukemia.

MATERIALS AND METHODS

We performed reverse transcriptase polymerase chain reaction, sequencing analysis, immunoblotting, and Southern blotting.

RESULTS

We found overexpression of novel short isoforms of Helios (Hel-5 and Hel-6) in the HD-Mar cell line. Southern blot analysis suggested that there might be a small deletion in the Helios locus of HD-Mar. In addition, we observed decreased expression of more than one Ikaros family gene in 3 of 9 patients with T-cell acute lymphoblastic leukemia. Moreover, one of the patients overexpressed novel short isoforms of Helios (Hel-7 and Hel-8).

CONCLUSION

This study provides the first evidence of an Ikaros family member (other than Ikaros) of which novel short isoforms become overexpressed in human leukemia.

Preferential cytotoxic effect of Newcastle disease virus on lymphoma cells

Susceptibility of lymphoma cells (Daudi, HD-Mar) to Newcastle disease virus toxicity was found to be higher than that of lymphoblastoid cells (Milstein) and of resting peripheral blood lymphocyte (PBL). Phytohemagglutinin- and/or pokeweed-mitogen-activated PBL however, exhibited, elevated sensitivity, similar to that of lymphoma cells. The level of cytotoxicity was monitored by cell viability, inhibition of DNA synthesis and release of 51Cr. When Daudi cells were mixed with PBL they were significantly more sensitive to the killing effect of the virus (70% mortality compared to 30% 30 h after infection, P < 0.05). The degree of sensitivity to viral cytotoxicity was unrelated to the efficacy of adsorption, which was similar for all cell lines as shown by immunofluorescent staining and flow cytometry. Also an influenza strain A/PR/8/34 (H1N1) adsorbed but did not affect the viability of any of the cells tested. Our results demonstrate that New-castle disease virus caused preferential damage to lymphoma cells as compared to non-cancerous normal cells.

Recent results on the biology of Hodgkin and Reed-Sternberg cells. II. Continuous cell lines

The relative scarcity of Hodgkin (H) and Reed-Sternberg (RS) cells within biopsies from cases with Hodgkin's disease (HD) is an impediment to the analysis of the nature and function of these cells. Continuous cell lines as uniform and permanently available sources of cells provide a valid alternative. Development of HD cell lines has proven to be rather difficult when compared with the results on leukemia and Non-Hodgkin lymphoma cells. Only a few cell lines containing cells that resemble in-vivo H-RS cells have been established. Because the in-vitro culture conditions favor the self-propagation of residual normal cells, e.g. Epstein-Barr virus transformed B-lymphoblastoid cells or monocyte/macrophage monolayers, early attempts at culturing HD tissue resulted mainly in the generation of such cell lines. Even for the bona fide HD cell lines it is difficult to prove that the immortalized cells originated from an H-RS cell. These 13 HD cell lines have been extensively characterized in a large variety of aspects. These data have resulted in widely varying conclusions about the nature of the cell lines. It is apparent that all HD cell lines are unique among hematopoietic cell lines and are also different from one another. No conclusive evidence towards the origin of the cells has been obtained for some cell lines, while others could be operationally, albeit not always unequivocally, assigned to the T- or B-cell or monocyte-macrophage lineages. The overall phenotypes are often not concordant with those of normal hematopoietic cells; some cell lines show clearly mixed lineage attributes. The artifactual expansion of non-HRS cells in culture and the acquisition or loss of certain properties during the adaptation to culture systems cannot be excluded. There was also a bias for the establishment of cell lines from cases with advanced clinical stages, nodular sclerosing subtype and pleural effusions. The extensive analysis of a few cell lines has provided a wealth of information useful for the understanding of the biology of H-RS cells. The striking heterogeneity could be reflective of a biologically heterogeneous disease.

Establishment of a human T-acute lymphoblastic leukemia cell line with a (16;20) chromosome translocation

A new T-cell line, Loucy, was established from the peripheral blood of a patient with T-cell acute lymphoblastic leukemia (T-ALL). The surface marker analysis of the cell line is OKT3+, OKT4+, THB4+, J5 +/-, OKT6-, TdT-, and HLA-DR-, indicating stage IV in T-cell lineage. Karyotype analysis revealed 45,X,5q-,t(16;20)(p12;q13). The translocation between chromosomes 16 and 20 has not been previously detected in ALL. This cell line may be of value in evaluating the role of t(16;20) in the etiology of T-ALL.

Specific binding of placental acidic isoferritin to cells of the T-cell line HD-MAR

Acidic placental isoferritin inhibited the blastogenic response of peripheral human lymphocytes to T-cell activating lectins. We measured specific binding of radioiodinated placental isoferritin to cells of the T-cell line HD-MAR and found specific high-affinity binding. Binding was faster and more ferritin was bound at 37 degrees C than at 4 degrees C. Displacement experiments indicated that most of the binding occurred at the cell surface. Acidic placental ferritin and isolated H-type ferritin subunits but not isolated L-type subunits, competed for the binding. Scatchard plot analysis showed characteristics of a single binding species with a dissociation constant (Kd) of 1.3-4.4 x 10(-11) M. The results suggest the presence of receptors for acidic isoferritin on T-lymphocytes and thus, a regulatory role for the acidic ferritin H-type subunit in T-cell function.

Cytogenetic studies of Hodgkin's disease. Analysis of involved lymph nodes from 12 patients

Cytogenetic studies were performed on 12 involved lymph nodes from Hodgkin's disease patients utilizing conditioned medium from 12-O-tetradecanoylphorbol-13-acetate-staphylococcus enterotoxin A induced mononuclear cells. The majority of cells analyzed had a normal karyotype. An unusually high rate of nonclonal karyotypic abnormalities was noted in most cultures. Clonal abnormalities involving chromosomes 3 and 21 were noted in two patients. Cytogenetic analysis of cultures stimulated with conditioned medium or specific growth factors may lead to a better understanding of the genetic mechanisms involved in Hodgkin's disease.

Interaction of soybean agglutinin with human leukemia-lymphoma lines at various stages of differentiation

Human leukemia-lymphoma cell lines reflecting hematopoietic clones at various stages of differentiation were examined for reactivity with soybean agglutinin (SBA). The binding and redistribution pattern of soybean surface receptors was determined with fluorescein-isothiocyanate conjugated SBA (F-SBA) by ultraviolet microscopy, and with a fluorescent activated cell sorter (FACS). The results indicate that there is a correlation between SBA labelling--distribution and the stage of lymphoid cell differentiation. The SBA labelling on the membrane of null lines was undetectable by U.V. microscopy and flow cytometry. A gradual increase in SBA labelling correlating with the stage of differentiation was observed on cell lines of both B and T origin. However the maximal fluorescence intensity of the T lines was lower than the B lines. The redistribution pattern of SBA on the membrane of T lines was rings and mild patches, whereas that on the B lines was moderate to large patches. The reactivity of the lymphoid lines with SBA was not affected by growth conditions. The binding of SBA to normal lymphoblastoid lines was generally low and the fluorescence intensity weak. The reactivity of these lines with SBA was not associated with their origin or "age". It is suggested that the differences in the reactivity of SBA with human hematopoietic lines at various stages of maturation may be of value in future understanding the differences in structure and function of the surface membrane between normal and malignant cells, and the relation to normal and abnormal cell differentiation.

Distribution and modulation of surface charges of cells from human leukemia-lymphoma lines at various stages of differentiation

Untreated and retinoic acid (RA) treated human leukemia-lymphoma cell lines reflecting hematopoietic cells at various stages of differentiation, were examined electron microscopically for their surface negative charge distribution using cationized ferritin (CF), an electron dense label of anionic sites. The results indicate that there is a correlation between the CF labeling density/distribution and the stage of lymphoid cell differentiation. Viable unfixed null cell lines show a low CF labeling density with few and small CF patches. A gradual increase in CF labeling density and increase in size and number of CF patches correlates with the stage of differentiation on cell lines of both T or B origin. Treatment of viable unfixed cells with 10(-5) MRA for 10 days seems to prevent the CF-induced formation of CF patches, resulting in a continuous and even distribution of the CF label, similar to that observed on the surface of cells fixed before CF labeling. Some correlation between the distribution of surface anionic sites and the malignant potential of the human leukemic lines could be detected.

Growth of human mammary carcinoma cells from biopsy specimens in serum-free medium on extracellular matrix

A method developed for the initiation and maintenance in primary culture of human normal mammary epithelial cells was adopted for the growth of epithelial cells from 45 primary human breast tumors. The cells were grown on a naturally produced extracellular matrix (ECM) or on regular tissue culture plastic in a serum-free medium containing growth supplements and high-density lipoprotein (HDL). Successful enzymatic dissociation of the tumor biopsy into organoid structures and cell aggregates was crucial for subsequent cell attachment and growth. Fifty-five percent of the biopsy specimens were successfully dissociated and 87% of these gave rise to actively dividing epithelial cells forming monolayer cultures. In contrast, only 21% of the biopsies which were not optimally dissociated yielded growing cultures. Variations in sample size, duration of enzymatic digestion, and tumor composition affected the outcome of tumor dissociation. Omission of serum from the culture medium prevented the growth of fibroblasts, while plating on ECM greatly improved and in some cases was essential for cell attachment and subsequent outgrowth. The epithelial nature of the cells was verified by their cuboidal and closely apposed morphology and positive staining with antikeratin antibodies. The growth and subculture requirements and the expression of the B38.1 tumor marker were compared in human mammary epithelial cells derived from solid tumors, pleural effusion and normal breast tissue.

Generation of a monoclonal anti-GP70 antibody and characterization of its reactivity with various human cell lines and cells from leukemia-lymphoma patients

Monoclonal anti GP-70 antibodies (BI) were generated in mice and used for screening of various malignant and non-malignant cell lines. The reactivity of these monoclonal antibodies was compared with that obtained with the polyclonal anti GP-70 antibody described in earlier studies [1-3]. The results indicated complete similarity in reactivity of both of the antibodies used. Furthermore, the reactivity of BI antibodies with cell samples obtained from a variety of leukemia and lymphoma patients and with peripheral blood samples from healthy blood donors was also very similar to the pattern of specificity described in earlier reports for the polyclonal preparation. From these studies we conclude that the monoclonal antibodies can substitute the polyclonal anti GP-70 antibodies in the diagnosis and subtyping of B-type leukemias and lymphomas.

Characterization of a novel Burkitt's lymphoma-associated antigen: GP70. Reactivity of anti-GP70 antibodies with various malignant and non-malignant cell lines

A glycoprotein of 70 kDa (GP70) was isolated from sera of Burkitt's lymphoma (BL) patients and used to immunize rabbits. Anti-GP70 antibodies at a high titer were obtained and used for screening of cancer cells of various origin by the indirect immunofluorescence test. Thus, 66% of BL-cell lines tested were positive to GP70. On the other hand, all lymphoblastoid cell lines tested were negative. Moreover, all peripheral blood cells and mononuclear cells from tonsils were negative, indicating specificity of antibodies to malignant transformation. Comparison between positively stained BL-cell lines indicated no correlation between the presence of GP70 and EBNA. Positive stain (1-5%) obtained with bone marrow cells might indicate that anti-GP70 antibodies are directed against a surface membrane differentiation glycoprotein.

Establishment of the Amsalem T-cell line from a patient with acute lymphoblastic leukemia. Expression of E-receptor-associated antigens in cells incapable of forming E-rosettes

A new T-cell line (Amsalem) was established from the peripheral blood of a patient with pre-T leukemia. Amsalem cells are unique in that they possess antigenic determinants associated with the E-receptor, yet fail to form rosettes with sheep red blood cells (SRBC). Amsalem cells were found to possess morphological and cytochemical features characteristic of T-lymphocytes, and were sensitive to the cytotoxic effect of rabbit antisera specific for T-cell antigens. In immunofluorescent tests with monoclonal antibodies, Amsalem cells showed a strong reactivity with the OKT-11A and A-22 antibodies, specific for the E-receptor. The cells were reactive with OKT-4 and showed a very weak reactivity with OKT-6 and OKT-8. No reactivity was found with the OKT-3, Leu 7, Leu 11, and OKM1 antibodies. Amsalem cells failed to form rosettes with SRBC; however, mouse anti-Amsalem serum inhibited the formation of E-rosettes. It is concluded that the Amsalem cell line is a line of pre-T leukemia cells characterized by a dissociation between its inability to form E-rosettes and the presence of antigenic constituents of the E-receptor.

On the natural biology of the malignant cells in Hodgkin's disease

The biological attributes and cellular origin of the malignant cells in Hodgkin's disease have been subject to numerous investigations both on fresh biopsy material and from long-term in vitro cultures of cells derived from Hodgkin's disease lesions. Studies on fresh biopsy material have led to suggestions that Hodgkin's cells are derived from T-lymphocytes, B-lymphocytes, or monocytes/macrophages. However, all these studies are hampered by the low amounts of Reed-Sternberg cells in the neoplastic lesions as well as some technical difficulties. It has consequently been desirable to establish long-term cell cultures of the malignant Hodgkin's cells, and the technical problems as well as the experimental outcome of these studies are reviewed. Among the technical, the potential and limitations in the use of monoclonal antibodies are described in particular. A number of cell lines from Hodgkin's disease lesions have been described, but a critical appraisal of these studies reveal that most of the published cell lines can be dismissed as being non-representative for Hodgkin's and Reed-Sternberg cells; most of the cell lines are thus EBV-transformed lymphocytes and in some cases even non-human cell lines. Two cell lines (L428 and SU/RH-HD-1), however, seem to be derived from the malignant cell population in Hodgkin's disease. The phenotypic analyses of these lines--and in particular the SU/RH-HD-1 line--strongly indicate that these cells have monocyte/macrophage attributes, and it is concluded that Hodgkin's disease is a neoplasm of cells in the monocyte/macrophage lineage.

Utilization of monoclonal antibodies and immuno-scanning electron microscopy for the positive identification of human leukemic cells

Monoclonal antibodies generated against normal and leukemic human leukocytes were tested for their differential reactivity with leukemia and lymphoma cell lines as well as with circulating lymphoid and myeloid leukemic cells by means of immuno-scanning electron microscopy (immuno-SEM). Anti-T (OKT3), anti-mu-chain, anti-CALLA (J5), anti-BA-1, anti-BA-2, and anti-nonlymphoid (Mol) monoclonal antibodies were covalently conjugated to polystyrene latex microspheres (immunolatex), using a two-step glutaraldehyde reaction, and subsequently incubated with the various cell types. Cultured B-type Burkitt lymphoma cells (Daudi) and chronic lymphocytic leukemia (CLL) cells displayed extensive labeling with monoclonal anti-mu, anti-B1, and anti-BA-1 immunolatex conjugates, while cultured malignant T cells (HD-Mar) showed positive labeling with OKT3 immunolatex alone. Cultured myelomonocytic cells (GDM-1) and cells obtained from patients with acute myeloblastic (AML) and monoblastic leukemia (AMoL) labeled only with anti-Mol immunolatex, while cultured promyelocytic cells (HL-60) displayed far less labeling with this conjugate. Common-type acute lymphoblastic leukemia (C/ALL) cells were labeled predominantly with the J5 (anti-CALLA) and anti-BA-2 immunolatex conjugates. Evidence is presented indicating that immuno-SEM employing monoclonal antibodies is a reproducible technique which may be used in the study of leukocyte maturation and may provide additional information in the classification of poorly differentiated leukemias.

Surface morphology and membrane phenotype of cultured human leukemia-lymphoma cells. A scanning electron microscopic study of 36 cell lines

Scanning electron microscopy and immunologic methods, to detect the expression of a variety of surface markers, were performed on cells from 36 established human leukemia-lymphoid cell lines. Attempts were made to correlate the surface morphologic findings with the membrane phenotype as determined by the presence or absence of a number of specific antigens and B- or T-cell markers. Thirteen of the cell lines were of the T-lymphoid type, 15 B-derived, and eight were defined as non-B non-T in nature. All the lines derived from patients with acute lymphoblastic leukemia (ALL) had similar surface topographies and generally displayed relatively smooth surfaces with few microvilli, while in some a proportion of moderately villous cells was evident. Burkitt's lymphoma cells tended to show more villous surfaces but, similar to circulating B-ALL cells, variable numbers of microvilli were frequently seen making consistent distinctions between this and other lymphoid leukemias difficult in individual cases. Two of the non-B non-T lines are known to be of erythroid (K-562) and myeloid origin (HL-60), respectively. In both these lines, cells with relatively few microprojections dominated; however, some showed transverse ridge-like profiles, a feature frequently encountered on circulating leukemic cells of myeloid type.

The reaction of xenogeneic and monoclonal antisera with Reed-Sternberg cells

Xenogeneic antisera have been prepared against human monocytes, B lymphocytes and T lymphocytes. The reactivity of these antisera against a variety of normal and neoplastic cells is described. Reed-Sternberg cells from three cases of Hodgkin's disease failed to react with xenogeneic anti-T and anti-monocyte sera, but reacted with an anti-B serum. A further three cases were examined with monoclonal antibodies. Negative results were obtained with monoclonal anti-T and positive results were given by monoclonal antibodies directed against DR and HLA antigens.

Human monoclonal autoimmune antibody produced in vitro: rheumatoid factor generated by Epstein-Barr virus-transformed cell line

The infection of selected lymphocytes from a rheumatoid arthritis patient with Epstein-Barr virus resulted in an immortalized cell line that secretes a monoclonal rheumatoid factor (RF). The cloned line has been growing for more than 24 months, and constantly produces a monoclonal IgM, lambda, 19S, RF (1-2 micrograms/ml/106 cells). The RF agglutinates human and rabbit IgG (but not IgM) and also protein A-coated erythrocytes, but fails to do so to mouse, goat and swine IgG-coated erythrocytes. When bound to immune complexes, this monoclonal RF dose not bind complement. In the cell supernatant RF is the only immunoglobulin and it comprises approximately 5% of the total proteins. The affinity of RF to aggregated human IgG, as detected in inhibition experiments, is higher than that of Fc receptors found on human non-T lymphocytes, K562 and Daudi cell lines.

Induction of Epstein-Barr virus nuclear antigen and DNA synthesis in a human epithelial cell line after Epstein-Barr virus infection

The association of Epstein-Barr virus (EBV) with nasopharyngeal carcinoma is supported by the presence of EBV genomes in the epithelial elements of the tumor and by elevated antibody titers to EBV-specific antigens in the patients; the levels of these titers are related to the clinical course of the disease. However, since most laboratory data suggest that EBV is a B-lymphotropic virus, it is unclear how the virus becomes associated with the epithelial elements of the nasopharynx. The purpose of the present work was to find a human model system to study this association. A human epithelial line (U) was found that could be directly infected by EBV, and viral functions, the induction of EBV nuclear antigen and cellular DNA synthesis, were demonstrated. The U line was established in 1957 by the late H. J. Van Kooten (Kok-Doorschodt at the University of Utrecht), and although it is no longer diploid, it exhibits density inhibition. When U cells were infected with EBV, EBV nuclear antigen was expressed in 6 to 16% of the cells, 1 and 2 days after infection with B95-8 virus, but not with the P3HR-1 strain. No evidence for virus replication was obtained; immunofluorescence staining for early antigens and virus capsid antigens gave negative results. Quantitative adsorption experiments for EBV indicated that the adsorption capacity of U cells is significant (60% of Raji cells). The present results also demonstrated that infection with the virus overcomes block(s) in cellular DNA synthesis caused by 5-fluorodeoxyuridine. The induction of DNA synthesis was determined by increased incorporation of [3H]thymidine into the cells. The highest level of isotope incorporation was observed at about 15 h after infection and thereafter decreased. Analysis of the induced DNA indicated that it was of cellular origin.

Establishment and characterization of a new permanent cell line (GDM-1) from a patient with myelomonoblastic leukemia

The GDM-1 permanent cell line was established from the peripheral blood of a patient with a Philadelphia chromosome negative myeloproliferative disorder, after transformation to acute myelomonoblastic leukemia. The GDM-1 cells exhibited the same characteristics as those isolated from the peripheral blood of the patient prior to death: cells contained non-specific esterase sensitive to fluoride, myeloperoxidase, lysozyme (muramidase), and exhibited both Fc and complement (C3) receptors but lacked B- and T-cell surface markers including T-associated antigens. E-rosetting capacity, surface and intracytoplasmic immunoglobulins and EBV determined nuclear antigen (EBNA). The GDM-1 cells bore the 1a receptor and the myeloid leukemia antigen (M-1). The karyotype of the cultured leukemic cells showed the same specific chromosomal abnormalities present in the monoblasts obtained from the peripheral blood prior to death, indicating that the cell line was derived from the original leukemic cells.

Scanning immuno-electron microscopy of human leukaemia and lymphoma cells: a comparative study of techniques using immunolatex spheres as marker

In this study scanning immuno-electron microscopic (SIEM) techniques were used to identify human leukaemia-lymphoma cells. Monodispersed polystyrene (latex) beads were conjugated to specific antisera using glutaraldehyde, in an attempt to detect surface antigenic components on a variety of cells of known origin. Antisera, mostly immunoglobulin fractions, against human thymus (T) derived cells, common type acute lymphoblastic leukaemia cells (C/ALL) and surface immunoglobulin (sIg) bearing cells were used to coat latex spheres, while rabbit anti-mouse Thy-1 antiserum or whole human-IgG (gamma-globulin) bound to latex were used as controls in some experiments. The use of SIEM techniques in the direct mode as a simple and sensitive method for labelling surface antigens is described. The disadvantages of the SIEM methodology are also summarized while the requirements for optimal cell preparation using this technique are stressed. The experiments were designed to ascertain whether prolonged fixation of cells could be used prior to incubation of the cells with the marker. In this respect, repeated neutralization of the glutaraldehyde with glycine is essential. SIEM labelling of cells is random and unreliable without adequate quenching with glycine. The heteroantisera used in this study proved to be adequate and insignificant non-specific attachment and cross reactivity were seen. SIEM adds a further dimension to ultrastructural aspects of immunology and is a potentially useful tool in the study and identification of leukaemia and lymphoma cells.

Positive identification of human leukaemic cells with scanning immuno-electron microscopy, using antibody coated polystyrene (latex) beads as markers

Scanning immuno-electron microscopic labelling techniques were successfully used to identify a variety of leukaemia-lymphoma cells. Anti-T, -B and -common type acute lymphoblastic leukaemia (ALL) antisera were covalently conjugated to polystyrene latex spheres by means of glutaraldehyde. Peripheral blood mononuclear cells obtained from healthy individuals and from patients with T-type ALL, B-type chronic lymphocytic leukaemia (CLL), acute and chronic non-lymphoblastic leukaemias (ANLL) and cultured leukaemia-lymphoma cells were incubated with the above antisera-latex conjugates. Positively labelled cells were easily identified by the presence of latex spheres on their surfaces. Cross reactivity of the antisera used and non-specific attachment of the markers were infrequently encountered. The results of this study confirm earlier scanning electron microscopic reports indicating that different leukaemic cells have characteristic surface features and show that scanning immuno-electron microscopy (SIEM) can readily be applied to the study of human leukaemic cells.

Human prothymocytes. Membrane properties, differentiation patterns, glucocorticoid sensitivity, and ultrastructural features

Thymic precursor cells (prothymocytes) comprise a large proportion of the fetal thymic cell population, but are less frequently encountered in the postnatal thymus, where they compose < 1% of the entire population. In the present study we attempted to characterize a number of properties of the prothymocytes obtained from human fetal thymic tissues after depletion of the E-rosette thymocyes on a Ficoll-Hypaque gradient. The prothymocytes are larger than the thymocytes and show a different nuclear chromatin pattern. This subset of cells lacks the E-rosetting and natural-attachment capacities and, unlike thymocytes, does not bind the lectin peanut agglutinin. Human prothymocytes are highly sensitive to the in vitro cytolytic effect of hydrocortisone, whereas the thymocytes are resistant. Long-term in vitro culture of prothymocytes resulted in the expression of thymocyte characteristics together with a burst of mitotic activity. Results of this study indicate that the rate of the prothymocyte proliferation is regulated by the small thymocytes present in the same suspension.