Tumor dormancy and cell signaling. II. Antibody as an agonist in inducing dormancy of a B cell lymphoma in SCID mice

Unveiling cancer dormancy: Intrinsic mechanisms and extrinsic forces

Tumor cells disseminate in various distant organs at early stages of cancer progression. These disseminated tumor cells (DTCs) can stay dormant/quiescent without causing patient symptoms for years or decades. These dormant tumor cells survive despite curative treatments by entering growth arrest, escaping immune surveillance, and/or developing drug resistance. However, these dormant cells can reactivate to proliferate, causing metastatic progression and/or relapse, posing a threat to patients' survival. It's unclear how cancer cells maintain dormancy and what triggers their reactivation. What are better approaches to prevent metastatic progression and relapse through harnessing cancer dormancy? To answer these remaining questions, we reviewed the studies of tumor dormancy and reactivation in various types of cancer using different model systems, including the brief history of dormancy studies, the intrinsic characteristics of dormant cells, and the external cues at the cellular and molecular levels. Furthermore, we discussed future directions in the field and the strategies for manipulating dormancy to prevent metastatic progression and recurrence.

Autophagy, molecular chaperones, and unfolded protein response as promoters of tumor recurrence

Tumor recurrence is a paradoxical function of a machinery, whereby a small proportion of the cancer cell population enters a resistant, dormant state, persists long-term in this condition, and then transitions to proliferation. The dormant phenotype is typical of cancer stem cells, tumor-initiating cells, disseminated tumor cells, and drug-tolerant persisters, which all demonstrate similar or even equivalent properties. Cancer cell dormancy and its conversion to repopulation are regulated by several protein signaling systems that inhibit or induce cell proliferation and provide optimal interrelations between cancer cells and their special niche; these systems act in close connection with tumor microenvironment and immune response mechanisms. During dormancy and reawakening periods, cell proteostasis machineries, autophagy, molecular chaperones, and the unfolded protein response are recruited to protect refractory tumor cells from a wide variety of stressors and therapeutic insults. Proteostasis mechanisms functionally or even physically interfere with the main regulators of tumor relapse, and the significance of these interactions and implications in the tumor recurrence phases are discussed in this review.

Could Extracellular Vesicles Contribute to Generation or Awakening of "Sleepy" Metastatic Niches?

Pre-metastatic niches provide favorable conditions for tumor cells to disseminate, home to and grow in otherwise unfamiliar and distal microenvironments. Tumor-derived extracellular vesicles are now recognized as carriers of key messengers secreted by primary tumors, signals that induce the formation of pre-metastatic niches. Recent evidence suggests that tumor cells can disseminate from the very earliest stages of primary tumor development. However, once they reach distal sites, tumor cells can persist in a dormant state for long periods of time until their growth is reactivated and they produce metastatic lesions. In this new scenario, the question arises as to whether extracellular vesicles could influence the formation of these metastatic niches with dormant tumor cells? (here defined as "sleepy niches"). If so, what are the molecular mechanisms involved? In this perspective-review article, we discuss the possible influence of extracellular vesicles in early metastatic dissemination and whether they might play a role in tumor cell dormancy. In addition, we comment whether extracellular vesicle-mediated signals may be involved in tumor cell awakening, considering the possibility that extracellular vesicles might serve as biomarkers to detect early metastasis and/or minimal residual disease (MRD) monitoring.

The tumor-promoting effects of the adaptive immune system: a cause of hyperprogressive disease in cancer?

Adaptive antitumor immune responses, either cellular or humoral, aim at eliminating tumor cells expressing the cognate antigens. There are some instances, however, where these same immune responses have tumor-promoting effects. These effects can lead to the expansion of antigen-negative tumor cells, tumor cell proliferation and tumor growth, metastatic dissemination, resistance to antitumor therapy and apoptotic stimuli, acquisition of tumor-initiating potential and activation of various forms of survival mechanisms. We describe the basic mechanisms that underlie tumor-promoting adaptive immune responses and try to identify the variables that induce the switching of a tumor-inhibitory, cellular or humoral immune response, into a tumor-promoting one. We suggest that tumor-promoting adaptive immune responses may be at the origin of at least a fraction of hyperprogressive diseases (HPD) that are observed in cancer patients during therapy with immune checkpoint inhibitors (ICI) and, less frequently, with single-agent chemotherapy. We also propose the use of non-invasive biomarkers allowing to predict which patients may undergo HPD during ICI and other forms of antitumor therapy. Eventually, we suggest possibilities of therapeutic intervention allowing to inhibit tumor-promoting adaptive immune responses.

The current paradigm and challenges ahead for the dormancy of disseminated tumor cells

Disseminated tumor cells (DTCs) are known to enter a state of dormancy that is achieved via growth arrest of DTCs and/or a form of population equilibrium state, strongly influenced by the organ microenvironment. During this time, expansion of residual disseminated cancer is paused and DTCs survive to fuel relapse, sometimes decades later. This notion has opened a new window of opportunity for intervening and preventing relapse. Here we review recent data that have further augmented the understanding of cancer dormancy and discuss how this is leading to new strategies for monitoring and targeting dormant cancer.

Dormancy and cancer stem cells: An enigma for cancer therapeutic targeting

Dormancy occurs when cells remain viable but stop proliferating. When most of a cancer population undergoes this phenomenon, the result is called tumor dormancy, and when a single cancer cell undergoes this process, it is termed quiescence. Cancer stem cells (CSCs) share several overlapping characteristics and signaling pathways with dormant cancer cells, including therapy resistance, and an ability to metastasize and evade the immune system. Cancer cells can be broadly grouped into dormancy-competent CSCs (DCCs), cancer-repopulating cells (CRCs), dormancy-incompetent CSCs and disseminated tumor cells (DTCs). The settings in which cancer cells exploit the dormancy phase to survive and adapt are: (i) primary cancer dormancy; (ii) metastatic dormancy; (iii) therapy-induced dormancy; and (iv) immunologic dormancy. Dormancy, therapy resistance and plasticity of CSCs are fundamentally interconnected processes mediated through mechanisms involving reversible genetic alterations. Niches including metastatic, bone marrow, and perivascular are known to harbor dormant cancer cells. Mechanisms of dormancy induction are complex and multi-factorial and can involve angiogenic switching, addictive oncogene inhibition, immunoediting, anoikis, therapy, autophagy, senescence, epigenetic, and biophysical regulation. Therapy can have opposing effects on cancer cells with respect to dormancy; some therapies can induce dormancy, while others can reactivate dormant cells. There is a lack of consensus relative to the value of therapy-induced dormancy, i.e., some researchers view dormancy induction as a beneficial strategy as it can lead to metastasis inhibition, while others argue that reactivating dormant cancer cells and then eliminating them through therapy are a better approach. More focused investigations of intrinsic cell kinetics and environmental dynamics that promote and maintain cancer cells in a dormant state, and the long-term consequences of dormancy are critical for improving current therapeutic treatment outcomes.

Immunotherapy of cancer: targeting cancer during active disease or during dormancy?

Immunotherapeutic targeting of advanced stage cancers has prolonged the survival of cancer patients, yet its curative efficacy is limited due to tumor immunoediting and escape. On the other hand, human vaccines have been able to eradicate smallpox and control several other infectious diseases. The success has resulted from the administration of vaccines in prophylactic settings, or during latency periods in order to protect an individual during future exposure to the disease rather than curing an established disease. Therefore, administration of immunotherapy at the right time is the key to success. However, instead of focusing on the prevention of cancer, current cancer immunotherapies are often being used in a therapeutic setting with the goal of eliminating tumor cells. The present review of evidence related to cancer immunotherapeutics suggests that immunotherapeutic targeting of tumor dormancy could be more promising than targeting of advanced stage disease to achieve a cure for cancer.

A history of exploring cancer in context

The concept that progression of cancer is regulated by interactions of cancer cells with their microenvironment was postulated by Stephen Paget over a century ago. Contemporary tumour microenvironment (TME) research focuses on the identification of tumour-interacting microenvironmental constituents, such as resident or infiltrating non-tumour cells, soluble factors and extracellular matrix components, and the large variety of mechanisms by which these constituents regulate and shape the malignant phenotype of tumour cells. In this Timeline article, we review the developmental phases of the TME paradigm since its initial description. While illuminating controversies, we discuss the importance of interactions between various microenvironmental components and tumour cells and provide an overview and assessment of therapeutic opportunities and modalities by which the TME can be targeted.

The Role of Regulatory B Cell-Like Malignant Cells and Treg Cells in the Mouse Model of BCL1 Tumor Dormancy

Cancer dormancy is a clinical state in which residual tumor cells persist for long periods of time but do not cause detectable disease. In the mouse B cell lymphoma model (BCL1), dormancy can be induced and maintained by immunizing mice with a soluble form of the IgM expressed on the surface of the tumor cells. Immunization induces an anti-idiotype antibody response that maintains dormancy. Mice with dormant tumor have low numbers of BCL1 cells in their spleens that divide and are killed at the same rate. When the anti-Id antibodies wane, the tumor cells grow rapidly and kill the host. Spleens from tumor-bearing mice contain both effector (CD4⁺ and CD8⁺) and regulatory T cells (T_regs). In other tumor models, it has been reported that T_regs promote tumor progression by preventing effector cells from killing the tumor. In this report, we demonstrate that the tumor site with rapidly dividing BCL1 cells has fewer T_regs than the tumor site harboring dormant BCL1 cells. In both cases, the T_regs were equally suppressive in vitro. In spleens from mice with actively growing tumor, CD8⁺ but not CD4⁺ T cells were virtually absent. In vitro analysis demonstrated a tumor-mediated elimination of CD8⁺ T cells that was contact dependent and involved the caspase-3 pathway. Most importantly, we found that the BCL1 cells expressed characteristics of B10 regulatory B cells, i.e., they were CD1d^hiCD5⁺ and secreted high levels of IL-10. These BCL1 tumor cells can inhibit anti-tumor immune responses by depleting CD8⁺ effector T cells.

Tumour dormancy and clinical implications in breast cancer

The aim of adjuvant therapy in breast cancer is to reduce the risk of recurrence. Some patients develop metastases many years after apparently successful treatment of their primary cancer. Tumour dormancy may explain the long time between initial diagnosis and treatment of cancer, and occurrence of relapse. The regulation of the switch from clinical dormancy to cancer regrowth in locoregional and distant sites is poorly understood. In this review, we report some data supporting the existence of various factors that may explain cancer dormancy including genetic and epigenetic changes, angiogenic switch, microenvironment, and immunosurveillance. A better definition and understanding of these factors should allow the identification of patients at high risk of relapse and to develop new therapeutic strategies in order to improve prognosis.

Idiotypes as immunogens: facing the challenge of inducing strong therapeutic immune responses against the variable region of immunoglobulins

Idiotype (Id)-based immunotherapy has been exploited as cancer treatment option. Conceived as therapy for malignancies bearing idiotypic antigens, it has been also extended to solid tumors because of the capacity of anti-idiotypic antibodies to mimic Id-unrelated antigens. In both these two settings, efforts are being made to overcome the poor immune responsiveness often experienced when using self immunoglobulins as immunogens. Despite bearing a unique gene combination, and thus particular epitopes, it is normally difficult to stimulate the immune response against antibody variable regions. Different strategies are currently used to strengthen Id immunogenicity, such as concomitant use of immune-stimulating molecules, design of Id-containing immunogenic recombinant proteins, specific targeting of relevant immune cells, and genetic immunization. This review focuses on the role of anti-Id vaccination in cancer management and on the current developments used to foster anti-idiotypic B and T cell responses.

Controversies in clinical cancer dormancy

Clinical cancer dormancy is defined as an unusually long time between removal of the primary tumor and subsequent relapse in a patient who has been clinically disease-free. The condition is frequently observed in certain carcinomas (e.g., breast cancer), B-cell lymphoma, and melanoma, with relapse occurring 5-25 y later. Clinical data suggest that a majority of breast cancer survivors have cancer cells for decades but can remain clinically cancer-free for their lifetime. Thus, there is a major effort to characterize the molecular mechanisms responsible for inducing tumor cell dormancy using experimental models or studying the early phases of cancer growth in humans. Many molecules and signaling pathways have been characterized and have led to concepts that dominate the field, such as the possible role of innate and adaptive immunity in immune surveillance and initiation and maintenance of dormancy. However, recent clinical data do not support many of these concepts. Several areas need further study to determine their relevance to clinical cancer dormancy. We suggest hypotheses that may contribute to elucidation of the mechanisms underlying the dormant state.

Cancer dormancy: lessons from a B cell lymphoma and adenocarcinoma of the prostate

Cancer dormancy delineates a situation in which residual tumor cells persist in a patient with no apparent clinical symptoms. Although the precise mechanisms underlying cancer dormancy have not been explained, experimental models have provided some insights into the factors that might be involved in the induction and maintenance of a tumor dormant state. The authors of the present chapter studied a murine B cell lymphoma that can be made dormant when interacting with antibodies directed against the idiotype on its immunoglobulin Ig receptor. This experimental model of antibody-induced dormancy enabled the isolation and characterization of dormant lymphoma cells. The results indicated that anti-Ig antibodies activate growth-inhibiting signals that induced cycle arrest and apoptosis. This process appeared to be balanced by the growth of the tumor cells such that the tumor did not expand. In contrast, antibodies against HER-2expressed on prostate adenocarcinoma (PAC) cells were not growth inhibitory. However, an immunotoxin (IT) prepared by conjugating HER-2 to the A-chain of ricin (RTA) was internalized by PAC cells, followed by induction of cycle arrest and apoptotic death. Infusion of HER-2-specific IT into PAC-bearing immunodeficient mice did not eradicate the tumor but retained it dormant over an extended period of time. Hence, certain aspects of signaling receptors expressed on cancer can be manipulated by antibodies to induce and maintain a tumor dormant state.

Mathematical models of cancer dormancy

The objective of this paper is to present preliminary mathematical models of the interaction between tumor and antibody for the murine BCL1 lymphoma and illustrate how this interaction leads to dormancy of the tumor. We explicitly model the induction by the immune response of cell cycle arrest and apoptosis of the tumor cells. In the absence of large amounts of quantitative data and because the models are preliminary, they are deliberately simple. We neglect, for example, spatial effects on this lymphoid tumor and the synergistic effect of antigen-specific T cells. A comparison of alternative models shows that, although vaccination is necessary to stimulate a sufficient immune response to control tumor growth, boosting of the antibody response by the tumor itself is vital to the mechanisms that maintain dormancy. We determine parameters that control the size of the dormant tumors, and the fraction of proliferating cells. Finally, we discuss the implications for tumor immunotherapy.

Tumor cell dormancy: implications for the biology and treatment of breast cancer

Despite progress made in the therapy of solid tumors such as breast cancer, the prognosis of patients even with small primary tumors is still limited by metastatic relapse often long after removal of the primary tumor. Therefore, it has been hypothesized that primary tumors shed tumor cells already at an early stage into the blood circulation. A subset of these disseminated tumor cells may persist in a state of so-called "dormancy". Based on cell culture and animal models, dormancy can occur at two different stages. Single dormant cells are defined as cells with a lack of proliferation and apoptosis with the cells undergoing cell cycle arrest. The micrometastasis model defines tumor cell dormancy as a state of balanced apoptosis and proliferation of micrometastasis resulting in no net increase of tumor mass. Mechanisms leading to a growth activation of dormant tumor cells and the outgrowth of manifest metastases are not completely understood. Genetic predisposition of the dormant cells as well as immunological and angiogenetic influences of the surrounding environment may contribute to this phenomenon. In this review, we summarize findings on different factors for tumor cell dormancy and potential therapeutic implications that should help to reduce metastatic relapse in cancer patients.

Tumor dormancy of primary and secondary cancers

Tumor dormancy is a phenomenon whereby cancer cells persist below the threshold of diagnostic detection for months to decades. This condition may arise due to either cell cycle arrest or a dynamic equilibrium state in which cell proliferation is in balance with cells undergoing apoptosis. Tumor dormancy is usually a reference to occult cancer cells that persist for an extended period of time after treatment, but primary cancers can also exhibit extended growth plateaus below the limits of detection. For example, autopsies of individuals who died of trauma reveal that most individuals harbor microscopic primary cancers. Mechanisms that operate independently or successively may restrict tumor expansion throughout tumor progression from incipiency to late-stage cancer. Proposed mechanisms include cell cycle withdrawal, immune surveillance, and blocked angiogenesis. The precise mechanisms underlying dormancy remain to be established, and relevant models will have an important impact on diagnostic and therapeutic strategies for treating cancer. This review summarizes the phenomenon of tumor dormancy, experimental models, and potential mechanisms.

Cancer vaccines and tumor dormancy: a long-term struggle between host antitumor immunity and persistent cancer cells?

Tumor dormancy is a phenomenon characterized by the persistence of residual cancer cells for long periods in the host. Evidence has emerged that a balance exists between the immune response and dormant tumor cells. This review presents our current understanding of the immune relationship between host and dormant tumor cells and the mechanism developed by these cells to escape host antitumor immunity. Implications of this immune escape for cancer vaccine strategy are considered.

The pattern of clinical breast cancer metastasis correlates with a single nucleotide polymorphism in the C1qA component of complement

Complement is one of primary defense mechanisms against intravascular microorganisms and could play a role in the immune response to malignancy and hence its clinical behavior. We evaluated if the sole coding polymorphism of C1qA associates with outcome in patients with breast carcinoma. Genotyping for C1qA[276A/G] was performed in 63 breast cancer subjects with localized tumor and compared with that in 38 breast cancer subjects with metastasis. Established risk factors for clinical outcome were considered and evaluated in multivariable analysis. Breast cancer subjects with heterozygous or homozygous C1qA[276G] genotype had a higher rate of metastasis than subjects with the homozygous C1qA[276A] genotype [hazard ratio (HR) 2.4, 95% confidence interval (CI) 1.1-4.1]. This association was stronger when only metastatic sites associated with hematogenous spread, i.e., to the bone, liver, and brain, were considered (HR 3.5, 95% CI 1.4-5.6) and remained statistically significant after adjustment for the number of positive lymph nodes, estrogen receptor status, and progesterone receptor status. There was no statistical difference in the C1qA[276A/G] allelic distribution between all subjects with breast cancer and controls. These results suggest there could be an association of a single nucleotide polymorphism at position 276 of the C1qA component of complement with breast cancer metastasis to sites linked to hematogenous spread of disease. The C1qA polymorphism associated with decreased distant metastasis has also been correlated with an increased incidence of subcutaneous systemic lupus and C1q deficiencies, suggesting that an altered immune response may play a role in the observed association.

Mathematical models of the fate of lymphoma B cells after antigen receptor ligation with specific antibodies

We formulate models of the mechanism(s) by which B cell lymphoma cells stimulated with an antibody specific to the B cell receptor (IgM) become quiescent or apoptotic. In particular, we aim to reproduce experimental results by Marches et al. according to which the fate of the targeted cells (Daudi) depends on the levels of expression of p21(Waf1) (p21) cell-cycle inhibitor. A simple model is formulated in which the basic ingredients are p21 and caspase activity, and their mutual inhibition. We show that this model does not reproduce the experimental results and that further refinement is needed. A second model successfully reproduces the experimental observations, for a given set of parameter values, indicating a critical role for Myc in the fate decision process. We use bifurcation analysis and objective sensitivity analysis to assess the robustness of our results. Importantly, this analysis yields experimentally testable predictions on the role of Myc, which could have therapeutic implications.

BCL1 lymphoma protection induced by idiotype DNA vaccination is entirely dependent on anti-idiotypic antibodies

DNA vaccination with the idiotype (Id) of tumour B-cell membrane immunoglobulins (Ig) is a validated strategy to induce tumour protection to several mouse lymphomas. The relative contribution of anti-Id antibodies and T lymphocytes to tumour rejection is still debated. Previous studies in the BCL1 lymphoma model showed that scFv DNA immunisation induces a polyclonal antibody response restricted to conformational epitopes formed by the parental V(L)/V(H) association. We implemented a system based on this specificity to investigate the mechanism of BCL1 lymphoma protection induced by DNA immunisation. Antibody response and survival of mice immunised with the tumour Id scFv were compared with those of mice immunised simultaneously with two chimeric scFvs, containing either the tumour-derived V(L) or V(H) paired to an irrelevant V(H) or V(L) domain, respectively. Animals vaccinated with one or both chimeric constructs were not protected, despite the exposure to all putative tumour Id-derived MHC class I and class II T-cell epitopes. In addition, conformational antibodies induced by DNA vaccination caused tumour cells apoptosis and cell cycle arrest in vitro and transferred protection in vivo. Therefore, lymphoma rejection appears to be completely dependent on the induction of anti-Id antibodies.

In a model of tumor dormancy, long-term persistent leukemic cells have increased B7-H1 and B7.1 expression and resist CTL-mediated lysis

In tumor dormancy, tumor cells persist in the host over a long period of time but do not grow. We investigated in the DA1-3b mouse model of acute myeloid leukemia how leukemic cells could persist for months in spite of an effective antileukemic immune response. Mice were immunized with irradiated interleukin 12 (IL12)- or CD154-transduced DA1-3b cells, challenged with wild-type DA1-3b cells, and randomly killed during 1-year follow-up. Quantification of residual disease 1 year after challenge showed that persistent leukemic cells represented less than 0.02% of spleen cells in most animals. These residual cells were still able to kill naive hosts, even when isolated after 1 year of persistence. Persistent leukemic cells were more resistant to specific cytotoxic T-cell (CTL)-mediated killing and had enhanced B7-H1 and B7.1 expression proportional to the time they had persisted in the host. Blocking B7-H1 or B7.1/cytotoxic T-lymphocyte-associated antigen (CTLA-4) interaction enhanced CTL-mediated killing of the persistent cells, and blocking B7-H1, B7.1, or CTLA-4 in vivo prolonged survival of naive mice injected with persistent leukemic cells. Thus, escape of leukemic cells from tumor immunity via overexpression of B7-H1 or B7.1 might represent a new mechanism of tumor dormancy in acute leukemia.

Activation of the Syk tyrosine kinase is insufficient for downstream signal transduction in B lymphocytes

BACKGROUND

Immature B lymphocytes and certain B cell lymphomas undergo apoptotic cell death following activation of the B cell antigen receptor (BCR) signal transduction pathway. Several biochemical changes occur in response to BCR engagement, including activation of the Syk tyrosine kinase. Although Syk activation appears to be necessary for some downstream biochemical and cellular responses, the signaling events that precede Syk activation remain ill defined. In addition, the requirements for complete activation of the Syk-dependent signaling step remain to be elucidated.

RESULTS

A mutant form of Syk carrying a combination of a K395A substitution in the kinase domain and substitutions of three phenylalanines (3F) for the three C-terminal tyrosines was expressed in a murine B cell lymphoma cell line, BCL1.3B3 to interfere with normal Syk regulation as a means to examine the Syk activation step in BCR signaling. Introduction of this kinase-inactive mutant led to the constitutive activation of the endogenous wildtype Syk enzyme in the absence of receptor engagement through a 'dominant-positive' effect. Under these conditions, Syk kinase activation occurred in the absence of phosphorylation on Syk tyrosine residues. Although Syk appears to be required for BCR-induced apoptosis in several systems, no increase in spontaneous cell death was observed in these cells. Surprisingly, although the endogenous Syk kinase was enzymatically active, no enhancement in the phosphorylation of cytoplasmic proteins, including phospholipase Cgamma2 (PLCgamma2), a direct Syk target, was observed.

CONCLUSION

These data indicate that activation of Syk kinase enzymatic activity is insufficient for Syk-dependent signal transduction. This observation suggests that other events are required for efficient signaling. We speculate that localization of the active enzyme to a receptor complex specifically assembled for signal transduction may be the missing event.

Dormancy in a model of murine B cell lymphoma

A B cell lymphoma model of dormancy in mice was established by prior immunization to the B cell membrane immunoglobulin idiotype. The antibody to the idiotype was the major factor in inducing and maintaining dormancy and acted primarily as an agonist rather than via effector functions. CD8+ T cells synergized with anti-Id in inducing dormancy by secreting IFN-gamma. Cycling in the dormant population was reduced 3-5 fold, but each mouse contained approximately 10(6) tumor cells in its spleen, some of which were cycling, during the 1.5 years of observation. Thus, replication is balanced by cell death.

Thioether-bonded constructs of Fab'gamma and Fc gamma modules utilizing differential reduction of interchain disulfide bonds

We describe a two-stage preparation of chemically engineered Ab constructs, employing as modules Fab'gamma from mAb or rAb, and Fc from human normal IgG1. A multivalent, optionally multispecific F(ab')(n) core is formed in stage one, and one or more Fc modules added in stage two. Examples include bispecific Fab(2)Fc(2) (for simplicity, primes and Greek letters are omitted from names of final constructs) and trivalent Fab(3)Fc(2), which are designed to kill neoplastic cells. An essential element in the construction is the availability of the Fab' in two reduced forms, Fab'(-sulfhydryl (SH))(5) and Fab'-SH. The first is obtained by full reduction of the interchain disulfide bonds (SS) in the F(ab')(2) fragment of IgG. Fab'-SH is obtained by disulfide-interchange reactions on Fab'(-SH)(5), whereby the gamma-light SS is reconstituted, an unusual intrachain SS forms in the gamma-chain hinge, and one hinge SH remains. F(ab')(2) and F(ab')(3) cores are built using partially reduced modules, being given intermodular thioether links that resist reduction. These cores are then fully reduced, making available SH groups for addition of the Fcgamma modules. In the final constructs, all intermodular links embody tandem thioether bonds arising at hinge-region cysteines. Cytotoxic activities of representative constructs, and some enhancements deriving from multiple modules, are assessed. In guinea pigs, catabolism of Fab(2)Fc(2) yielded a t(1/2) similar to that of human IgG1, although the serum Fab(2)Fc(2) revealed some proteolytic breakdown not shown by the IgG1. Immunotherapy of a guinea-pig leukemia confirmed the ability of these constructs to kill target cells in vivo.

Inverse Association Between IgG–Anti-κ and Antierythrocyte Autoantibodies in Patients With Cold Agglutination

It has been known for a long time that IgG–anti-F(ab′)2 antibodies (Abs) are able to suppress the B-cell response. We showed that natural IgG-anti–F(ab′)2 autoantibodies appear in the serum of patients with cold agglutination. If the anti-F(ab′)2 Ab suppresses cold agglutinin (CA)-producing B cells, one would expect an inverse correlation between the titers of these two Abs. Our study confirmed this correlation. Subsequent experiments showed that some anti-F(ab′)2 Abs bind to the hinge region of IgG. It was difficult to explain how this Ab suppresses CA-producing B cells, which are of IgM isotype. Here we show that patients with cold agglutination have an IgG–anti-κ light chain autoantibody in their serum. This is another member of the anti-F(ab′)2 Ab group. Because the vast majority of CAs are IgM-κ Abs, the anti-κ Ab might suppress CA-producing B cells. If this is the case, there should be an inverse association between the titer of anti-κ Ab and CA. In a group of 302 patients, we found that high titers of the anti-κ Ab correlate with low titers of CA and vice versa (P= .009). Interestingly, this association is found only in patients whose disease is caused by noninfectious agents, including mainly B-cell proliferations (P = .0058). Our data show that the inverse correlation is not confined to a particular CA autoantibody specificity. The results are discussed in the light of recent findings showing that anti-IgM Abs may either inactivate or kill tumoral B cells by apoptosis.

Inverse Association Between IgG–Anti-κ and Antierythrocyte Autoantibodies in Patients With Cold Agglutination

It has been known for a long time that IgG–anti-F(ab′)2 antibodies (Abs) are able to suppress the B-cell response. We showed that natural IgG-anti–F(ab′)2 autoantibodies appear in the serum of patients with cold agglutination. If the anti-F(ab′)2 Ab suppresses cold agglutinin (CA)-producing B cells, one would expect an inverse correlation between the titers of these two Abs. Our study confirmed this correlation. Subsequent experiments showed that some anti-F(ab′)2 Abs bind to the hinge region of IgG. It was difficult to explain how this Ab suppresses CA-producing B cells, which are of IgM isotype. Here we show that patients with cold agglutination have an IgG–anti-κ light chain autoantibody in their serum. This is another member of the anti-F(ab′)2 Ab group. Because the vast majority of CAs are IgM-κ Abs, the anti-κ Ab might suppress CA-producing B cells. If this is the case, there should be an inverse association between the titer of anti-κ Ab and CA. In a group of 302 patients, we found that high titers of the anti-κ Ab correlate with low titers of CA and vice versa (P= .009). Interestingly, this association is found only in patients whose disease is caused by noninfectious agents, including mainly B-cell proliferations (P = .0058). Our data show that the inverse correlation is not confined to a particular CA autoantibody specificity. The results are discussed in the light of recent findings showing that anti-IgM Abs may either inactivate or kill tumoral B cells by apoptosis.

Expression of costimulatory molecules in low-grade mucosa-associated lymphoid tissue-type lymphomas in vivo

B-cell lymphomas of mucosa-associated lymphoid tissue (MALT) type develop against a background of chronic inflammation and have functional autoantigen receptors. Because they respond to environmental factors in vivo, the expression of costimulatory molecules, which play a key role in the differentiation of normal B-lymphocytes and in T-/B-cell interaction, may be critical in early MALT-type lymphoma pathogenesis until further chromosomal aberration leads to progression. We found a high number of tumor-infiltrating T-lymphocytes (TITLs) in all low-grade MALT-type lymphomas. The TITLs in low-grade lymphomas were activated and expressed a memory and immunocompetent phenotype. Reverse transcriptase-polymerase chain reaction analyses and immunohistochemistry confirmed the presence of CD40-ligand and Fas-ligand in 80% of low-grade lymphomas. In contrast to the TITLs, the tumor B cells did not express CD40-ligand or Fas-ligand in vivo or in vitro. Moreover, the cytokine profile in vivo suggested a Th2/Th3-weighted profile (interleukin-10, interleukin-13, transforming growth factor beta(1)) rather than Th1-weighted (interferon-gamma, interleukin-2). By interphase fluorescence in situ hybridization analysis the translocation t(11;18)(q21;q21) was found in four of nine (44%) cases studied. Interestingly, there was a four times higher proliferation and survival rate of purified t(11;18)-positive tumor B cells in vitro, although there were no significant profile differences from the TITLs in vivo. The finding of essential costimulating molecules in low-grade MALT-type lymphomas in vivo indicates a locally directed cognate T-/B-cell interaction. Consequently, a potentially equipped inflammatory background may not only determine the fate of autoreactive B-cells, but is also crucial to lymphoma maintenance and progression.

Annexin V staining due to loss of membrane asymmetry can be reversible and precede commitment to apoptotic death

Signal-induced apoptosis is a normal phenomenon in which cells respond to changes in their environment through a cascade of intracellular biochemical changes culminating in cell death. However, it is not clear at what point in this process the cell becomes committed to die. An early biochemical change characteristic of cells undergoing apoptosis is the loss of plasma membrane asymmetry, such that high levels of phosphatidylserine become exposed on the outside cell surface. These cells can be recognized by staining with Annexin V, which binds to phosphatidylserine with high affinity. To investigate the mechanisms controlling signal-induced apoptosis we have examined the response of a B cell lymphoma to crosslinking of the membrane immunoglobulin (mIg) receptor. We have found that many of the cells that stain positive for Annexin V are viable and can resume growth and reestablish phospholipid asymmetry once the signal is removed. These results indicate that Annexin V staining, and thus loss of membrane asymmetry, precedes commitment to apoptotic death in this system.

Cancer dormancy and cell signaling: induction of p21(waf1) initiated by membrane IgM engagement increases survival of B lymphoma cells

The p21(WAF1) (p21) cyclin-dependent kinase inhibitor plays a major role in regulating cell cycle arrest. It was recently reported that the p53-independent elevation of p21 protein levels is essential in mediating the G(1) arrest resulting from signal transduction events initiated by the crosslinking of membrane IgM on Daudi Burkitt lymphoma cells. Although the role of p21 in cell cycle regulation is well documented, there is little information concerning its role in antibody-mediated apoptosis. In the present study, we examined the involvement of p21 in the regulation of apoptosis by suppressing its induction in anti-IgM-treated Daudi cells through a p21 antisense expression construct approach. Reduction in induced p21 protein levels resulted in diminished G(1) arrest and increased apoptosis. The increased susceptibility to anti-IgM-mediated apoptosis was associated with increased caspase-3-like activity and poly-(ADP)ribose polymerase cleavage. These data suggest that p21 may directly interfere with the caspase cascade, thus playing a dual role in regulating both cell cycle progression and apoptosis.

The Importance of Antibody-Specificity in Determining Successful Radioimmunotherapy of B-Cell Lymphoma

We report the radioimmunotherapy of mouse B-cell lymphoma, BCL1, using a panel of anti–B-cell monoclonal antibodies (MoAb) (anti-CD19, anti-CD22, anti-major histocompatibility complex (MHC) II, and anti-idiotype (Id) radiolabeled with 131-iodine. When administered early in disease (day 4), the 131I–anti-MHCII MoAb cured tumors as a result of targeted irradiation alone, the unlabeled MoAb being nontherapeutic. In contrast,131I–anti-Id, despite targeting irradiation and having therapeutic activity as an unconjugated antibody, protected mice for only 30 days; 131I–anti-CD19 and anti-CD22 were therapeutically inactive. Binding and biodistribution studies showed that the anti-Id, unlike anti-MHCII, MoAb was cleared from target cells in vivo and delivered 4 times less irradiation to splenic tumor. Treating later in the disease (day 14) increased tumor load and produced the expected reduction in therapeutic activity with the anti-MHCII, but surprisingly, allowed 131I–anti-Id to cure most mice. This unexpected potency of 131I–anti-Id late in the disease appeared to result from the direct cytotoxicity of the anti-Id MoAb, which was more active in established disease, in combination with targeted irradiation. We believe the ability of targeted irradiation and certain cytotoxic MoAb to work cooperatively against tumor in this way has important implications for the selection of reagents in radioimmunotherapy of B-cell lymphoma.

Cancer Dormancy. VII. A Regulatory Role for CD8+ T Cells and IFN-γ in Establishing and Maintaining the Tumor-Dormant State

Dormant tumor cells resistant to ablative cancer therapy represent a significant clinical obstacle due to later relapse. Experimentally, the murine B cell lymphoma (BCL1) is used as a model of tumor dormancy in mice vaccinated with the BCL1 Ig. Here, we used this model to explore the cellular mechanisms underlying dormancy. Our previous studies have demonstrated that T cell-mediated immunity is an important component in the regulation of tumor dormancy because Id-immune T cells adoptively transferred into passively immunized SCID mice challenged with BCL1 cells significantly increased the incidence and duration of the dormant state. We have extended these observations and demonstrate that CD8+, but not CD4+, T cells are required for the maintenance of dormancy in BCL1 Ig-immunized BALB/c mice. In parallel studies, the transfer of Id-immune CD8+ cells, but not Id-immune CD4+ cells, conferred significant protection to SCID mice passively immunized with nonprotective levels of polyclonal anti-Id and then challenged with BCL1 cells. Furthermore, the ability of CD8+ T cells to induce a state of dormancy in passively immunized SCID mice was completely abrogated by treatment with neutralizing α-IFN-γ mAbs in vivo. In vitro studies demonstrated that IFN-γ alone or in combination with reagents to cross-link the surface Ig induced both cell cycle arrest and apoptosis in a BCL1 cell line. Collectively, these data demonstrate a role for CD8+ T cells via endogenous production of IFN-γ in collaboration with humoral immunity to both induce and maintain a state of tumor dormancy.

Analysis of the interaction of monoclonal antibodies with surface IgM on neoplastic B-cells

In vitro studies identified three Burkitts lymphoma cell lines, Ramos, MUTU-I and Daudi, that were growth inhibited by anti-IgM antibody. However, only Ramos and MUTU-I were sensitive to monoclonal antibodies (mAb) recognizing the Fc region of surface IgM (anti-Fc mu). Experiments using anti-Fc mu mAb (single or non-crossblocking pairs), polyclonal anti-mu Ab, and hyper-crosslinking with a secondary layer of Ab, showed that growth inhibition of B-cell lines was highly dependent on the extent of IgM crosslinking. This was confirmed by using Fab', F(ab')2 and F(ab')3 derivatives from anti-Fc mu mAb, where increasing valency caused corresponding increases in growth arrest and apoptosis, presumably as a result of more efficient BCR-crosslinking on the cell surface. The ability of a single mAb to induce growth arrest was highly dependent on epitope specificity, with mAb specific for the Fc region (C mu2-C mu4 domains) being much more effective than those recognizing the Fab region (anti-L chain, anti-Id and anti-Fd mu, or C mu1). Only when hyper-crosslinked with polyclonal anti-mouse IgG did the latter result in appreciable growth inhibition. Binding studies showed that these differences in function were not related to differences in the affinity, but probably related to intrinsic crosslinking capacity of mAb.

Monoclonal Antibody Therapy of B Cell Lymphoma: Signaling Activity on Tumor Cells Appears More Important Than Recruitment of Effectors

Despite the recent success of mAb in the treatment of certain malignancies, there is still considerable uncertainty about the mechanism of action of anti-cancer Abs. Here, a panel of rat anti-mouse B cell mAb, including Ab directed at surface IgM Id, CD19, CD22, CD40, CD74, and MHC class II, has been investigated in the treatment of two syngeneic mouse B cell lymphomas, BCL1 and A31. Only three mAb were therapeutically active in vivo, anti-Id, anti-CD19, and anti-CD40. mAb to the other Ags showed little or no therapeutic activity in either model despite giving good levels of surface binding and activity in Ag-dependent cellular cytotoxicity and complement assays, and in some cases inhibiting cell growth in vitro. We conclude that the activity of mAb in vitro does not predict therapeutic performance in vivo. Furthermore, in vivo tracking experiments using fluorescently tagged cells showed that anti-Id and anti-CD40 mAb probably operate via different mechanisms: the anti-Id mAb cause growth arrest that is almost immediate and does not eliminate cells over a period of 5 or 6 days, and the anti-CD40 mAb have a delayed effect that allows tumor to grow normally for 3 days, but then abruptly eradicates lymphoma cells. This work supports the belief that mAb specificity is critical to therapeutic success in lymphoma and that, in addition to any effector-recruiting activity they may possess, in vivo mAb operate via mechanisms that involve cross-linking and signaling of key cellular receptors.

Anti-Idiotype Antibodies Can Induce Long-Term Complete Remissions in Non-Hodgkin’s Lymphoma Without Eradicating the Malignant Clone

The immunoglobulin on the surface of B-cell lymphomas can be a tumor-specific target for monoclonal antibody therapy. Between 1981 and 1993, 45 individuals with low grade B-cell lymphoma were treated with 52 courses of custom-made anti-idiotype antibodies. The antibodies were used either alone or in combination with -interferon, chlorambucil, or interleukin-2 (IL-2). The majority of these patients responded to treatment, with a 66% overall and 18% complete response rate. Six patients (13%) experienced prolonged complete remissions, five of which are ongoing from 4 to 10 years after therapy and are the subject of this report. We asked whether residual lymphoma could be found in these patients with prolonged remissions. We performed enzyme-linked immunosorbent assay (ELISA) assays for idiotype protein or anti-idiotype antibodies in serum. Blood and bone marrow samples were examined by flow cytometry for idiotype positive cells, and by polymerase chain reaction (PCR) for clonal gene rearrangements of immunoglobulin CDR3 sequences or t(14;18) translocations. Using these sensitive and specific tests it was possible to detect very low levels of residual lymphoma in five of these patients who had been in clinical remission for 3 to 8 years before this evaluation. These five have continued without recurrence for up to 3 years since. Thus, we have found a pattern of residual inactive disease in patients treated with anti-idiotype antibodies. The biology of follicular lymphoma evidently includes the potential for tumor dormancy after therapies with varied mechanisms of action, resulting in clinical inactivity for many years. Thus, long-term control of the disease is possible at a clinical level despite persistence of the malignant clone.

© 1998 by The American Society of Hematology.

Anti-Idiotype Antibodies Can Induce Long-Term Complete Remissions in Non-Hodgkin’s Lymphoma Without Eradicating the Malignant Clone

The immunoglobulin on the surface of B-cell lymphomas can be a tumor-specific target for monoclonal antibody therapy. Between 1981 and 1993, 45 individuals with low grade B-cell lymphoma were treated with 52 courses of custom-made anti-idiotype antibodies. The antibodies were used either alone or in combination with -interferon, chlorambucil, or interleukin-2 (IL-2). The majority of these patients responded to treatment, with a 66% overall and 18% complete response rate. Six patients (13%) experienced prolonged complete remissions, five of which are ongoing from 4 to 10 years after therapy and are the subject of this report. We asked whether residual lymphoma could be found in these patients with prolonged remissions. We performed enzyme-linked immunosorbent assay (ELISA) assays for idiotype protein or anti-idiotype antibodies in serum. Blood and bone marrow samples were examined by flow cytometry for idiotype positive cells, and by polymerase chain reaction (PCR) for clonal gene rearrangements of immunoglobulin CDR3 sequences or t(14;18) translocations. Using these sensitive and specific tests it was possible to detect very low levels of residual lymphoma in five of these patients who had been in clinical remission for 3 to 8 years before this evaluation. These five have continued without recurrence for up to 3 years since. Thus, we have found a pattern of residual inactive disease in patients treated with anti-idiotype antibodies. The biology of follicular lymphoma evidently includes the potential for tumor dormancy after therapies with varied mechanisms of action, resulting in clinical inactivity for many years. Thus, long-term control of the disease is possible at a clinical level despite persistence of the malignant clone.

© 1998 by The American Society of Hematology.

Tumor Dormancy and Cell Signaling. V. Regrowth of the BCL1 Tumor After Dormancy Is Established

The majority of BALB/c mice immunized with the BCL1 lymphoma-derived idiotype (Id+) IgM and subsequently challenged with BCL1 tumor cells develop a state of tumor dormancy. The vast majority of dormant lymphoma cells are in cell cycle arrest, but there are also residual replicating cells. In the present studies, we attempted to define features of both the dormant lymphoma cells and the host that lead to escape from dormancy. Escape from dormancy occurs at a steady rate over a 2-year period, suggesting that it is a stochastic process. We found that, in the majority of mice, escape was due to the emergence of genetic variants that were no longer susceptible to the anti-Id–mediated induction of dormancy. Ten percent of these variants were Id−; the remainder were Id+ but could grow in the presence of anti-Id antibodies, suggesting that there were mutations in molecules involved in one or more mIg-mediated negative-signaling pathways. In two of five such escapees, alterations in either Syk, HS1, and/or Lyn were observed. In a small percentage of mice, a low titer of circulating anti-Id antibody before tumor challenge correlated with a subsequent, more rapid loss of dormancy.

Inverse correlation between IgG-antihinge region and antierythrocyte autoantibody in chronic benign and malignant cold agglutination

Previous reports provided evidence of an immunosuppressive role of natural anti-F(ab')2 antibodies. If suppressive anti-F(ab')2 antibodies also regulated the autoantibody production in cold agglutination, one would expect high titers of anti-F(ab')2 to be associated with low titers of cold agglutinins. Indeed, our previous studies revealed an inverse correlation between IgG-anti-F(ab')2 and cold agglutinins. Many previous experiments focused on anti-F(ab')2 of an antiidiotypic nature. Recent epitope mapping showed that anti-F(ab')2 of healthy persons is not an antiidiotype but recognizes a hinge region sequence. We attempted to answer the question whether this IgG-antihinge antibody is responsible for the previously described association between anti-F(ab')2 and cold agglutinins. IgG-antihinge and IgG-anti-F(ab')2 antibody was determined and statistically analyzed in the serum of 334 patients with cold agglutination. Our experiments revealed a strong correlation between the concentrations of antihinge and the previously described anti-F(ab')2 antibody. The anti-F(ab')2 activity was competitively inhibited by a synthetic hinge peptide. Moreover, patients with high antihinge titers had low cold agglutinin titers, and vice versa. A stratification according to cold agglutinin specificity and disease etiology showed that the inverse correlation is present only in anti-I and anti-i patients suffering from monoclonal B-lymphocyte proliferation. In conclusion, our results confirm the correlation previously described for anti-F(ab')2 antibody and antierythrocyte autoantibody and define for the first time an association between an idiotype-independent anti-IgG autoantibody and cold agglutinin.

The antigen-binding domain of a human IgG-anti-F(ab')2 autoantibody

Recent studies revealed an immunoregulatory role of natural IgG-anti-F(ab')2 antibodies in both healthy individuals and patients with certain diseases. The implication of anti-F(ab')2 antibodies in the pathogenesis of diseases prompted us to study the gene segment structure of their antigen-binding domains and their binding characteristics. cDNA was prepared from the lymphocytes of a patient with a high IgG-anti-F(ab')2 serum titer. Variable heavy and light gene segments were amplified by PCR and inserted into a phagemid surface expression vector. Single-chain antibodies displayed on the phage surface were screened for binding to F(ab')2 fragments. The subsequent analysis of 95 single clones demonstrated that they all bound specifically to F(ab')2. Sequence analyses of 12 clones showed that 11 were identical and 1 contained a silent point mutation in the heavy chain and three amino acid exchanges in the light chain. The heavy chains belonged to the V(H)3 and the light chains to the V(kappa)2 gene family. The 11 identical light-chain genes were completely homologous to a germ-line sequence (DPK-15). Binding assays showed that the single-chain antibodies bind to F(ab')2, but not to Fab, Fc, or intact IgG. This binding pattern was confirmed by surface plasmon resonance studies, which revealed a relatively high affinity (Ka = 2.8 x 10(7) M(-1)). The strong binding capacity was further demonstrated by competitive inhibition of the serum anti-IgG antibody's interaction with antigen. The present study defines for the first time to our knowledge the gene segment structure of the antigen-binding domain of two human IgG-anti-F(ab')2 autoantibody clones and describes the binding kinetics of the purified monomeric fragments.

Synergy between an antibody and CD8+ cells in eliminating an established tumor

We investigated the effector mechanisms operating during the rejection of a transplantable solid lymphoma E.G7 (H-2b) which expresses the gene encoding chicken ovalbumin (OVA). Anti-OVA cytotoxic T lymphocytes (CTL) completely and specifically protected animals from the onset of, but could not eradicate established, E.G7 tumors. The growth of the same lymphoma was also effectively prevented by the antibody GK1.5, whose target molecule, CD4, was expressed on E.G7 cells in vivo. Furthermore, GK1.5 was able to eradicate established solid E.G7 tumors. GK1.5-mediated tumor elimination was due to its antitumor activity, and not to the elimination of regulatory CD4+ cells, based on unimpaired tumor growth in the absence of GK1.5 in animals that genetically lack CD4 T cells. In vitro, GK1.5 did not kill tumor cells: complement activation or apoptosis induction were not evident. In vivo, GK1.5-mediated tumor regression did not depend on natural killer cells, but it absolutely required CD8+ cells and intact Fcgamma receptor. We conclude that, in the E.G7 model, the collaboration of antibody and CTL immunity was crucial for the successful immunotherapy of established tumors. The mechanism of this collaboration is discussed.

Vaccine strategies in non-Hodgkin's lymphomas

As described above, the most recent advances in anti-idiotype vaccination strategies have gone hand in hand with recent developments in molecular biology and other forms of cancer therapy. The techniques that are currently available in antibody engineering will greatly facilitate protein production and purification and will reduce the time and effort needed to produce the patient specific vaccines. Cytokine (gene) therapy has extensively been studied in cancer treatment and cancer vaccination and some therapeutic strategies are currently being evaluated in clinical trials (Bubenik, 1996). Combination therapy of idiotypic vaccination with cytokine therapy has recently been explored with promising results. The main focus so far has been on GM-CSF and IL-2, although other cytokines might prove to be efficient in stimulating different effector arms of the immune system. The nature of the immune response mounted by the host against the tumour and the mechanisms by which the tumour cells escape the effector functions of the immune system are not yet fully known. A better knowledge of the nature of B-cell lymphomas and the relation to the patient's immune system will therefore benefit the further development of the therapeutic strategies. Further research will provide us with a better view of how to break the immune tolerance and of which components of the immune system have to be targeted in order to obtain optimal therapeutic results.

Circulating clonotypic B cells in the biology of multiple myeloma: speculations on the origin of myeloma

The population of circulating B cells in myeloma patients includes an apparently large but variable subset with the IgH VDJ rearrangement diagnostic for the malignant clone of plasma cells in individual myeloma patients. Although the biological significance is at present unknown, it is likely that they include both malignant and non-malignant clonal relatives of the myeloma plasma cells. This article presents speculations on the significance of these cells in the origin of myeloma and the relationship between monoclonal gammopathy of undetermined significance (MGUS) and frank myeloma. MGUS appears to represent the establishment of clonal dominance probably by a chronically antigen-stimulated B cell clone. It seems likely that malignant transformation event(s) occurring in a clonal daughter cell give rise to myeloma. If correct, this implies that in a myeloma patient, non-malignant antigen-responsive B cells expressing the patient-specific IgH rearrangement coexist in the circulation and probably all lymphoid tissues, with their malignant antigen-independent relatives. However, the significance one attributes to the clonotypic B cells detected in the blood of myeloma patients depends in part on the view one takes of the progression from MGUS to myeloma. An alternative perspective is that MGUS represents a dormant state of malignancy held in check by controlled apoptosis, arrested cell cycling, and/or by immunoregulatory networks. Although lacking in experimental support, if this interpretation were correct, myeloma would occur when the regulatory mechanisms fail, allowing uncontrolled malignant cell renewal. This alternative view would imply that the majority of circulating clonotypic B cells might be malignant. Thus, an analysis of the biology of these clonotypic circulating B cells, with an emphasis on measures of malignancy, is likely to shed considerable light on the events underlying myeloma genesis, progression and spread.

Tumor dormancy and cell signaling: anti-mu-induced apoptosis in human B-lymphoma cells is not caused by an APO-1-APO-1 ligand interaction

Signal transduction initiated by crosslinking of antigen-specific receptors on T- and B-lymphoma cells induces apoptosis. In T-lymphoma cells, such crosslinking results in upregulation of the APO-1 ligand, which then interacts with induced or constitutively expressed APO-1, thereby triggering apoptosis. Here we show that crosslinking the membrane immunoglobulin on human lymphoma cells (Daudi) (that constitutively express APO-1) does not induce synthesis of APO-1 ligand. Further, a noncytotoxic fragment of anti-APO-1 antibody that blocks T-cell-receptor-mediated apoptosis in T-lymphoma cells does not block anti-mu-induced apoptosis. Hence, in B-lymphoma cells, apoptosis induced by signaling via membrane IgM is not mediated by the APO-1 ligand.