Molecular interactions mediating T-B lymphocyte collaboration in human lymphoid follicles. Roles of T cell-B-cell-activating molecule (5c8 antigen) and CD40 in contact-dependent help.

In lymphoid follicles, CD4+ T lymphocytes provide contact-dependent stimuli to B cells that are critical for the generation of specific antibody responses in a process termed Th function. The CD4+ T cell-restricted surface activation protein, 5c8 Ag (T-BAM), has recently been shown to be a component of the contact-dependent helper signal to B cells. To further dissect this process, we utilized a Jurkat T cell lymphoma clone, termed D1.1, that constitutively expresses T-BAM and activates peripheral B cells to express surface CD23 in a contact-dependent mechanism that is inhibited by mAb anti-T-BAM (5c8). Similar to its effect on peripheral B cells, Jurkat D1.1 activates B cells from lymphoid organs, as well as a B cell lymphoma clone, RAMOS 266,4CN 3F10 (RAMOS 266), to up-regulate surface CD23. Interestingly, mAb to the B cell surface molecule, CD40 (mAb G28-5 and B-B20), inhibit D1.1 induced activation of RAMOS 266 and peripheral and lymphoid B cells. In contrast, mAb to CR2 or the adhesion molecules, LFA1, LFA3, or ICAM-1, have little effect. The inhibitory effect of anti-CD40 mAb on B cell activation induced by D1.1 is specific because anti-CD40 potentiates, rather than inhibits, the up-regulation of CD23 on B cells induced by rIL-4. Moreover, cross-linking CD40 molecules by anti-CD40 mAb bound to Fc gamma RII+ (CD32) L cells induces B cell CD23 expression. In vivo, T-BAM-expressing cells are CD4+ T cells that are restricted to lymphoid organs and are localized in the mantle and centrocytic zones of lymphoid follicles and the spleen periarteriolar lymphoid sheath in association with CD40+ B cells. Taken together, these data demonstrate that T-BAM on T cells and CD40 on B cells are involved in contact-dependent T-B help interactions that occur in lymphoid follicles.     

Expression of 18.6/CD23 Antigen on Human Lymphoid Progenitor Cell Lines and Phorbol 12-Myristate 13-Acetate (PMA)-Induced Microglia-Shaped Cells

The nature of lymphoid progenitors and factor(s) determining commitment to either the T- or B-lymphocyte pathway are poorly understood in the human system. In this study, we generated a monoclonal antibody (MoAb), 18.6, that recognizes a cell surface antigen on a human lymphoid progenitor cell line (FL4.4). MoAb 18.6 reacted with lymphoid progenitor lines, B lymphoid cell lines, and myelomonocytic cell lines. It did not react with any T cell or erythroid leukemic cell lines. Two color FACS analyses of normal lymphoid tissues showed that MoAb 18.6 reacted with a majority of CD20⁺ mature B cells and a minority of CD64⁺ monocytes. Molecules of 3 different sizes with MW of 34, 45, and 68 Kd were precipitated with MoAb 18.6 from the lymphoid progenitor cell line. The 18.6 antigen was not expressed on a fetal liver-derived lymphoid progenitor-like cell line, FL1.4, which has the capacity to differentiate into microglia-shaped cells upon PMA-stimulation. Stimulation of FL1.4 cells with PMA induced expression of the 18.6 antigen within 24 hr and the microglia-shaped cells stained positively with MoAb 18.6. Finally, cloning of a cDNA that encoded the 18.6 antigen revealed that the 18.6 antigen is identical to the CD23 antigen. Taken together, these data suggest that the 18.6/CD23 antigen is expressed on lymphoid precursors at a very early stage of differentiation.     

Antigens on human monocytes identified by monoclonal antibodies

Two antigens (Mo1 and Mo2) present on human peripheral blood monocytes have been defined by lytic IgM monoclonal antibodies. Both antigens are present on greater than 70% of adherent mononuclear cells (predominantly monocytes). Mo1 is expressed by monocytes, granulocytes, and Null cells, but is absent from T and B lymphocytes. Mo2, on the other hand, appears specific for peripheral blood monocytes. Neither antigen is present on Ia-positive B cell lines or on tumor cells from patients with B cell lymphoproliferative malignancies, further excluding the possibility that Mo1 and Mo2 are Ia antigens. Mo1 and Mo2 are, however, present on a significant number of blast cells from patients with monocytic leukemia (both myelomonocytic and pure monocytic variants), but relatively infrequently expressed by cells from patients with acute granulocytic leukemia. These results indicate that Mo1 and Mo2 are unique antigens that may represent distinct stages of late monocyte-granulocyte differentiation.     

Antigens on human plasma cells identified by monoclonal antibodies

Two monoclonal antibodies that define distinct plasma cell-associated antigens, termed PCA-1 and PCA-2, were developed against human plasma cell leukemia cells. These antigens are strongly expressed on human myelomas, plasma cell leukemia, and plasmacytoma tumor cells, but are not detected on other lymphoid malignancies of B, T, null, or myeloid origin. PCA-1 and PCA-2 are not expressed on either normal T or B lymphocytes, but are weakly expressed on granulocytes and monocytes. When pokeweed mitogen is used to induce human B lymphocyte differentiation, PCA-1 is expressed when other B cell determinants are lost and plasmacytoid morphology, intracytoplasmic immunoglobulins, and surface T10 staining characteristic of plasma cells appear. In contrast, PCA-2 cannot be induced and may therefore appear later in the B cell differentiation scheme. These antigens may be of utility for the study and regulation of normal and abnormal plasma cell growth, traffic, and tissue distribution and may aid in understanding heterogeneity within plasma cell dyscrasias.     

TuJ1 (class III beta-tubulin) expression suggests dynamic redistribution of follicular dendritic cells in lymphoid tissue

Follicular dendritic cells (FDCs) play central roles in the B cell survival, proliferation, and differentiation into memory cells. Here, we show that TuJ1 (class III beta-tubulin) is expressed strongly in FDCs of human lymphoid tissue. TuJ1 has been a marker of neurons in the central and peripheral nervous systems from the early stage of neural differentiation. FDCs expressed TuJ1 protein diffusely in both light and dark zones of germinal centers in all human lymphoid tissues. In contrast, CD21 expression was relatively concentrated to the light zone, suggesting that TuJ1 was a marker for FDCs with broader spectrum than CD21. In addition to the germinal center, there were single TuJ1-expressing cells scattered in the mantle zone, blurring the border of the FDC network. In human tonsils, single scattered TuJ1-positive cells were also present in the crypt epithelium, suggesting a dynamic redistribution of FDCs among the antigen-rich epithelium, mantle zone, and germinal center. Such migration of FDCs could reflect a way of direct transport of various antigens carried on their surface to the germinal center, and a basis for the polarity of lymphoid follicles toward the epithelium in mucosa-associated lymphoid tissues. HK cells, cultured FDCs, also expressed TuJ1. The expression of TuJ1 by FDCs suggests that they may share certain biological characteristics of the neural system.     

A monoclonal antibody with reactivity restricted to normal and neoplastic plasma cells

A monoclonal antibody that defines a new and distinct plasma cell antigen, termed PC-1, was developed against human plasmacytoma cells. This antigen is strongly expressed on normal plasma cells isolated from bone marrow and on abnormal plasma cells isolated from myelomas, plasma cell leukemias, and plasmacytomas. The antigen is not detected on normal T or B lymphocytes, granulocytes, or monocytes, and with the exception of plasma cells, is absent on malignancies of B, T, or myeloid origin. Utilizing pokeweed mitogen to induce human B lymphocyte differentiation in vitro, PC-1 is expressed when B cell determinants are lost and the plasmacytoid morphology, intracytoplasmic immunoglobulin-staining, and surface PCA-1- and T10-staining characteristic of plasma cells appear. This antigen is useful for the study of the terminal stages of normal B cell differentiation to plasma cells, and may offer insight into the heterogeneity of the plasma cell dyscrasias.     

Development and distribution of a human B cell subpopulation identified by the HB-4 monoclonal antibody

Monoclonal antibodies have been successfully used to identify B cell differentiation antigens, few of which mark discrete B cell subpopulations. We have produced a monoclonal antibody, HB-4, against a cell surface antigen on the human B cell line, BJAB, which has an unusual distribution on normal lymphoid cells. HB-4, an IgM antibody, was found to react with an antigen that is expressed by a subpopulation of B cells, approximately 50% of natural killer cells, and not by other types of cells in bone marrow, blood, and lymphoid tissues. In two-color immunofluorescence assays, the HB-4-reactive antigen was found on less than 5% of immature IgM+ B cells in fetal liver and bone marrow and on 25% of B cells in fetal spleen. The HB-4 antibody reacted with 40% of IgM+ cells in newborn blood and 60% of B cells in adult blood. In contrast, only 2 to 26% of IgM+ B cells in the peripheral lymphoid tissues of adults were HB-4+. HB-4+ B cells could be induced to proliferate by cross-linkage of their surface immunoglobulins but not by T cell-derived growth factors. The subpopulation of activated B cells that is responsive to T cell-derived differentiation factors was HB-4-, as were plasma cells. The HB-4 antibody was reactive with some but not all B cell malignancies and cell lines, and not with malignancies or cell lines of other lineages. The HB-4 antigen may therefore serve as a useful nonimmunoglobulin marker for the identification of a subpopulation of mature resting B cells that are present in the highest frequency in the circulation.     

Quantification of the leukocyte common antigen (CD45) in mature B-cell malignancies.

CD45 is a glycoprotein expressed on all lymphohematopoietic cells. Its expression increases during normal B-cell differentiation and remains stable on mature cells. Although it is widely known that CD45 antigen expression is decreased in B-acute lymphocytic leukemia (ALL), only scarce and contradictory information is available on CD45 expression on mature B-cell malignancies. In healthy adults (n = 15), CD45 expression on B lymphocytes was lower than that on T cells. In patients with chronic lymphocytic leukemia (CLL; n = 22), CD45 expression on malignant cells was lower than that on the whole lymphocyte population of healthy adults (n = 28) and on normal B lymphocytes (n = 15). In 6 of the 22 CLL patients, the malignant cell population could be separated from the normal lymphocyte population on the CD45-side scatter (SSC) plot. In 16 CLL patients, there was some degree of overlap between the malignant and normal cells with respect to CD45 expression. For these patients, there was an inverse correlation between CD45 expression on the whole lymphocyte population and the percentage of malignant cells in this population. In two patients with mantle cell lymphoma (MCL), CD45 expression on the malignant cells appeared lower than that on normal B cells and on the whole lymphocyte population. In six patients with hairy cell leukemia (HCL), CD45 expression on hairy cells was comparable to that on the whole lymphocyte population of healthy adults, but slightly higher than that of normal B cells. Evaluation of CD45 expression may help to characterize mature B-cell malignancies.     

Identification of human peripheral T cell antigens.

A new type of differentiation antigens on human T cells was demonstrated by using a heterologous anti-human T cell serum (ATS). This type of antigen, referred to as human peripheral T cell antigen (HPTA), was found on peripheral T cells and medullary thymocytes, but not on cortical thymocytes and B cells. The percentage of ATS-reactive lymphocytes in human peripheral lymphoid organs was correlated with that of cells rosetting with sheep erythrocytes, but contrasted with the number of B cells defined by the presence of a complement (C) receptor or by rabbit anti-human B cell serum (ABS). ATS also reacted with T cells purified by nylon fiber column filtration but ABS did not. Chronic lymphocytic leukemia cells rosetted with either sheep erythrocytes or erythrocyte-antibody-complement complexes were lysed by ATS and ABS, respectively. Mitogenic responses of blood lymphocytes to phytohemagglutinin (PHA) and concanavalin-A (Con A) were abrogated by treating them with ATS and C, whereas ABS suppressed only their response to Con A. Although numerous thymus cells rosetted with SRBC, only 14% were reactive with ATS. Quantitative absorption studies demonstrated that HPTA content of the thymus cells was much lower than that of lymph node cells. Anatomical localization of ATS-reactive lymphocytes in human lymphoid organs studied by immunofluorescence indicated that they were present in the thymus-dependent paracortical areas of lymph node and in the medullary region of thymus. ABS, on the other hand, did not stain thymocytes but reacted selectively with the cells located in the lymphoid follicles of lymph node. These data, together with that from cell suspension studies, confirmed that HPTA were shared between medullary thymocytes and peripheral T cells.     

A unique antigen expressed on myeloid cells and acute leukemia blast cells defined by a monoclonal antibody

A murine hybridoma-derived monoclonal antibody, PM-81, was obtained from a fusion of cells of the NS-1 myeloma cell line with cells from a mouse immunized with the HL-60 promyelocytic leukemia cell line. This cytotoxic IgM monoclonal antibody was specific for myeloid cells. Employing indirect immunofluorescence and flow cytometry, we determined that this antibody reacts strongly with normal human granulocytes, eosinophils, and monocytes but not lymphocytes (including phytohemagglutinin-activated lymphocytes), null cells, red blood cells, or platelets. Moreover, the PM-81 antibody reacts with leukemia cells from 19 of 22 patients with acute myelocytic leukemia of all FAB subclasses, three of three patients with common acute lymphocytic leukemia, four of four patients with chronic myelocytic leukemia (CML) in myeloid blast crisis (terminal transferase (TdT)-negative) but did not react with cells from two patients with CML in lymphoid blast crisis (TdT-positive) or five patients with chronic lymphocytic leukemia. The myeloid cell lines HL-60, K562, KG-1, and U937 were all reactive with PM-81. The lymphoid lines CCRF-CEM and Daudi did not express PM-81 but HSB-2 was positive. The PM-81 antigen was absent on myeloid and erythroid progenitor cells as determined by their insusceptibility to complement-dependent lysis. In addition, only PM-81-unreactive cells were capable of colony formation. Furthermore, the PM-81 antibody does not appear to induce modulation of the antigen to which it binds. Thus, this monoclonal antibody appears to fulfill several criteria for clinical utility in the diagnosis and treatment of both acute myelocytic and acute lymphocytic leukemia.     

A Monoclonal Antibody Defining Human B Cell Differentiation Antigen (HLB-1 Antigen)

A new monoclonal antibody specific for human B cell differentiation antigen (HLB-1) is produced by a hybridoma established by fusion of splenocytes of mice immunized with the Epstein-Barr virus (EBV)-transformed peripheral B cell line, RPMI-8057. This monoclonal, antibody designated anti-HLB-1 monoclonal antibody (anti-HLB-1), reacted with surface immunoglobulin (sIg)-positive B cells of normal peripheral blood and lymphoid tissues and sIg-positive leukemic cells. The cells of T cell leukemia, non-T non-B acute lymphoblastic leukemia (ALL) and nonlymphoid leukemia were HLB-1 negative. These data were further confirmed by studying a panel of cultured human hematopoietic cell lines. Anti-HLB-1 reacted with B cell lines derived from pre-B, Burkitt's lymphoma, B cell type ALL and EBV-transformed peripheral B cells. Anti-HLB-1 was reactive with only one of three human myeloma cell lines, and with none of the T cell, myeloid and non-T non-B ALL cell lines. This newly defined HLB-1 antigen is different from other conventional human B cell markers such as sIg, Ia antigens, and receptors for the Fc portion of Ig and complement C3.     

ALVAC-mediated gene transfer is efficient in lymphoid malignancies of T-and early B-cell origin, but not in tumors arising from mature B-cells

Natural attenuation of ALVAC virus in mammals makes it an attractive vector for cancer vaccine therapy of immunocompromised hosts, such as patients with lymphoid malignancies. However, the transduction efficiency of ALVAC constructs in lymphoid tumors has not yet been characterized. We studied a wide spectrum of human T- and B-cell leukemia and lymphomas and found significant heterogeneity of the ALVAC-mediated gene product expression in these tumors. While ALVAC-B7.1, ALVAC-B7.2, or ALVAC-luciferase vectors effectively expressed recombinant genes in malignancies arising from T- or early B-cell precursors, negative or low expression of ALVAC recombinant genes occurred in tumors arising from mature B-cells. We showed that ALVAC-encoded B7.1 or B7.2 was continuously expressed on the infected, and subsequently irradiated, leukemia cells, and only cells with ALVAC-mediated expression of costimulatory molecules (but not unmodified leukemia cells or those infected with the ALVAC-parental vector) induced significant proliferation and IFN-gamma production by alloreactive T-cells. These data provide the rationale for clinical studies using the ALVAC vector system for gene transfer into lymphoid tumors of T- and early B-cell origin to render them more immunogenic, while alternative strategies should be considered for immunotherapy of mature B-cell malignancies.     

P-selectin glycoprotein ligand 1 promotes T cell lymphoma development and dissemination

P-selectin glycoprotein ligand-1 (PSGL-1) is a membrane-bound glycoprotein expressed in lymphoid and myeloid cells. It is a ligand of P-, E- and L-selectin and is involved in T cell trafficking and homing to lymphoid tissues, among other functions. PSGL-1 expression has been implicated in different lymphoid malignancies, so here we aimed to evaluate the involvement of PSGL-1 in T cell lymphomagenesis and dissemination. PSGL-1 was highly expressed at the surface of human and mouse T cell leukemia and lymphoma cell lines. To assess its impact on T cell malignancies, we stably expressed human PSGL-1 (hPSGL-1) in a mouse thymic lymphoma cell line, which expresses low levels of endogenous PSGL-1 at the cell surface. hPSGL-1-expressing lymphoma cells developed subcutaneous tumors in athymic nude mice recipients faster than control empty vector or parental cells. Moreover, the kidneys, lungs and liver of tumor-bearing mice were infiltrated by hPSGL-1-expressing malignant T cells. To evaluate the role of PSGL-1 in lymphoma cell dissemination, we injected intravenously control and hPSGL-1-expressing lymphoma cells in athymic mice. Strikingly, PSGL-1 expression facilitated disease infiltration of the kidneys, as determined by histological analysis and anti-CD3 immunohistochemistry. Together, these results indicate that PSGL-1 expression promotes T cell lymphoma development and dissemination to different organs.     

Protein expression pattern distinguishes different lymphoid neoplasms

To identify proteins associated with the histological subtypes of lymphoid neoplasms, we studied the proteomes of 42 cell lines from human lymphoid neoplasms including Hodgkin's lymphoma (HL; four cell lines), B cell malignancies (19 cell lines), T cell malignancies (16 cell lines), and natural killer (NK) cell lymphoma (three cell lines). The protein spots were sequentially selected by (i) Wilcoxon or Kruskal-Wallis tests to find the spots whose intensity was significantly (p <0.05) different among the cell line groups, (ii) by statistical-learning methods to prioritize the spots according to their contribution to the classification, and (iii) by unsupervised classification methods to validate the classification robustness by the selected spots. The selected spots discriminated (i) between HL cells and other cells, (ii) between the cells from B cell malignancies, T cell malignancies, and NK cell lymphoma cells, and (iii) between HL cells and anaplastic large cell lymphoma cells. Among the 31 informative protein spots, MS identified 24 proteins corresponding to 23 spots. Previous reports did not correlate these proteins to lymphocyte differentiation, suggesting that a proteomic study would identify the novel mechanisms responsible for the histogenesis of lymphoid neoplasms. These proteins may have potential as differential diagnostic markers for lymphoid neoplasms.     

B lymphoblast antigen (BB-1) expressed on Epstein-Barr virus-activated B cell blasts, B lymphoblastoid cell lines, and Burkitt's lymphomas

Two new cell surface antigens expressed on B lymphoblastoid cell lines (B-LCL) were defined with cytotoxic mouse monoclonal antibodies. One marker, BB-1 (for B lymphoblast antigen-1), was detected on human and nonhuman primate B-LCL, Epstein-Barr virus (EBV)-activated B cell blasts, most Burkitt's lymphomas, and Ia+ B lymphoblast-like myelomas. Polyclonal B cell activators such as pokeweed mitogen (PWM) and lipopolysaccharide (LPS) also induced the expression of BB-1 on immunoglobulin (Ig)-positive cells. In contrast, BB-1 could not be detected on normal lymphoid tissues by complement-dependent cytotoxicity and immunofluorescence (IF) assays or by analysis with a fluorescence-activated cell sorter (FACS). T cell blasts, T cell leukemias, and pre-B cell or erythroblastic leukemia cell lines were also BB-1 negative. Of particular interest was the finding that BB-1 was expressed on the Jijoye lymphoma but only marginally on a subline of Jijoye, P3HR-1, that lacks receptors for EBV and produces a defective virus incapable of transforming lymphocytes. A second lymphoblast antigen (LB-1) unlike BB-1, was present on both T and B cell blasts and virus-transformed T- and B-LCL but not on normal lymphoid tissues.     

Construction of human-mouse T cell hybrids that express human T cell-associated surface antigens and allow the chromosomal localization of these antigens

We have constructed somatic cell hybrids between the murine T cell line BW5147 and cells from patients suffering from T cell acute lymphoblastic leukemia. The obtained hybrid clones were analyzed for expression of human T cell antigens and presence of human chromosomes. T cell hybrids derived from fusion between the BW5147 cell line and bone marrow cells from a patient with pre-T acute lymphoblastic leukemia (TdT+/HLA-DR+/Tp41+/T11+/T1-/T6-/T4-/T8-/T3-) appeared to express the human T cell antigen Tp41, which can be recognized by the monoclonal antibodies 3A1 and WT1. Although this panel of hybrid cells contained all human chromosomes, no other T cell antigens were expressed. Fusion of the BW5147 cell line with peripheral blood cells from a patient with a more mature T cell acute lymphoblastic leukemia (TdT+/HLA-DR+/Tp41+/T11+/T1+/T6-/T4+/T8+/T3-) resulted in a panel of hybrid clones that expressed not only the Tp41 antigen, but also the human T cell antigens T1 and T4; two hybrids even expressed the T3 antigen. This panel of hybrids also contained the whole human genome. The two panels of human-mouse T cell hybrids allowed us to assign the genes coding for the human T cell antigens Tp41, T1, and T4 to human chromosomes 17, 11, and 12, respectively. Furthermore, these data support our previous suggestion that the expression of human lymphoid differentiation antigens in human-mouse lymphoid hybrids is influenced by the differentiation stage of the fusion partners.     

Establishment of a Ph1 chromosome-positive cell line from chronic myelogenous leukemia in blast crisis

A new hematopoietic cell line, designated KCL-22, was established in vitro by cultivation of pleural effusion cells obtained from a woman with chronic myelogenous leukemia in blast crisis. KCL-22 grew in suspension culture with a doubling time of 24 h and consisted of immature undifferentiated cells which were positive for periodic acid-Schiff and acid phosphatase staining. Chromosome analysis of the KCL-22 line showed a female karyotype with double Ph1 chromosomes and additional chromosome abnormalities. Its representative karyotype was 52,XX, + 1p-,+6,+8,+8,+8, t(9q+;22q-), +22q-. This cell line possessed receptors for the Fc portion of IgG, but lacked lymphoid cell characteristics and the Epstein-Barr virus-associated nuclear antigen. These results indicate that the KCL-22 cells were derived from chronic myelogenous leukemia cells. This cell line should prove useful for research involving various aspects of chronic myelogenous leukemia.     

Similarity and difference of the response of Peyer's patches and mesenteric lymph nodes of rats to polychemotherapy]

The structure of the Peyer's patches and mesenteric lymph nodes of rats at different periods after polychemotherapy was investigated by light microscopy. After the use of antitumor drugs the number of the blasts and mytoses in the lymphoid follicles with the mesenteric lymph node bright centres and in the Peyer's patch follicles lowered, that along with the decrease of the size of the mantle zone in the lymph node follicles and the decrease of the area of the bright centres in the follicles of the lympoid formations in the intestinal wall was evident of the proliferation inhibition and B cells differentiation in the lymphoid organs. After the polychemotherapy the size of the germinative centres of the Payer's patch follicles decreased while the size of the mantle zone remained unchanged. The size of the mantle zone in the follicles of the mesenteric lymph nodes decreased while the size of the germinative centres did not change. The different responses of the lymphoid organs could be associated with some remote location of the lymph nodes with respect to the antigen source (damaged epithelium and intestinal lumen contents).     

A monoclonal antibody defining a lymphoma-associated antigen in man

A monoclonal antibody (Ab 89) was developed against a lymphoma-associated antigen on the tumor cells of a patient (N.B.) with a B cell, poorly differentiated lymphocytic lymphoma (D-PDL). By indirect immunofluorescence, Ab 89 was shown to react only with N.B. lymphoma cells and was unreactive with normal N.B. lymphoid cells, normal fractionated peripheral blood cells, other normal lymphoid tissues, fetal tissues, and non-lymphoid malignant cells. In addition, Ab 89 was unreactive with conventional immunoglobulins, private and public HLA antigens, or Ia-like antigens. More importantly, Ab 89 was reactive with some B cell lymphoid malignancies. Of the 57 B cell lymphomas tested, it was found that Ab 89 reacted with approximately 10% of B cell D-PDL and B cell chronic lymphocytic leukemia (CLL). Of interest was the finding that N.B. serum contained a circulating antigen which could specifically block the reactivity of Ab 89. The data obtained suggests that Ab 89 defines a tumor-associated antigen shared by a unique subgroup of B cell lymphomas. The group of patients reactive with Ab 89 did not fall into any histopathologic classification system. These data support the notion that there is greater heterogeneity of B cell lymphomas than may have been previously recognized.