Increased expression of the c-myc gene may be related to the aggressive transformation of human myeloma cells

Game of Bones: How Myeloma Manipulates Its Microenvironment

Multiple myeloma is a clonal disease of long-lived plasma cells and is the second most common hematological cancer behind Non-Hodgkin’s Lymphoma. Malignant transformation of plasma cells imparts the ability to proliferate, causing harmful lesions in patients. In advanced stages myeloma cells become independent of their bone marrow microenvironment and form extramedullary disease. Plasma cells depend on a rich array of signals from neighboring cells within the bone marrow for survival which myeloma cells exploit for growth and proliferation. Recent evidence suggests, however, that both the myeloma cells and the microenvironment have undergone alterations as early as during precursor stages of the disease. There are no current therapies routinely used for treating myeloma in early stages, and while recent therapeutic efforts have improved patients’ median survival, most will eventually relapse. This is due to mutations in myeloma cells that not only allow them to utilize its bone marrow niche but also facilitate autocrine pro-survival signaling loops for further progression. This review will discuss the stages of myeloma cell progression and how myeloma cells progress within and outside of the bone marrow microenvironment.

A DNA target-enrichment approach to detect mutations, copy number changes and immunoglobulin translocations in multiple myeloma

Genomic lesions are not investigated during routine diagnostic workup for multiple myeloma (MM). Cytogenetic studies are performed to assess prognosis but with limited impact on therapeutic decisions. Recently, several recurrently mutated genes have been described, but their clinical value remains to be defined. Therefore, clinical-grade strategies to investigate the genomic landscape of myeloma samples are needed to integrate new and old prognostic markers. We developed a target-enrichment strategy followed by next-generation sequencing (NGS) to streamline simultaneous analysis of gene mutations, copy number changes and immunoglobulin heavy chain (IGH) translocations in MM in a high-throughput manner, and validated it in a panel of cell lines. We identified 548 likely oncogenic mutations in 182 genes. By integrating published data sets of NGS in MM, we retrieved a list of genes with significant relevance to myeloma and found that the mutational spectrum of primary samples and MM cell lines is partially overlapping. Gains and losses of chromosomes, chromosomal segments and gene loci were identified with accuracy comparable to conventional arrays, allowing identification of lesions with known prognostic significance. Furthermore, we identified IGH translocations with high positive and negative predictive value. Our approach could allow the identification of novel biomarkers with clinical relevance in myeloma.

Cytogenetics of extramedullary manifestations in multiple myeloma

Extramedullary disease in patients with multiple myeloma is a rare event, occurring mostly in advanced disease or relapse. Outcome is poor and prognostic factors predicting the development of extramedullary disease have not been defined. We investigated cytogenetic alterations of myeloma cells in different extramedullary manifestations by adapting the fluorescence in situ hybridization (FISH) technique in combination with cytoplasmic immunoglobulin staining to study the cytogenetics of plasma cell tumours on paraffin embedded material. Thirty six patients were investigated: 19 with extramedullary disease, 11 with skeletal extramedullary disease and six with solitary extramedullary plasmacytoma. The first two groups showed the following results: del(17p13) 32% vs. 27%, del(13q14) 35% vs. 27%, MYC-overrepresentation 28% vs. 18% and t(4;14) 37% vs. 18%. We detected an overall higher incidence of del(17p13) in both groups compared to data from bone marrow samples of multiple myeloma reported to date (range 7-16%). The solitary extramedullary plasmacytomas presented overall less cytogenetic aberrations than the other groups. Most important, three patients with extramedullary disease and one with skeletal extramedullary disease presented different FISH findings in the extramedullary tumour compared to their bone marrow plasma cells. del(17p13), occurring additional in three of four cases, seems a strong marker for extramedullary progression of myeloma.

The genetic architecture of multiple myeloma

Based on the clinical features of myeloma and related malignancies of plasma cells, it has been possible to generate a model system of myeloma progression from a normal plasma cell through smouldering myeloma to myeloma and then plasma cell leukaemia. Using this model system we can study at which points the genetic alterations identified through whole-tumour molecular analyses function in the initiation and progression of myeloma. Further genetic complexity, such as intraclonal heterogeneity, and insights into the molecular evolution and intraclonal dynamics in this model system are crucial to our understandings of tumour progression, treatment resistance and the use of currently available and future treatments.

Inflammation as a tumor promoter in cancer induction

Opposing effects of inflammation on cancer have been described. Acute inflammation usually counteracts cancer development, while chronic inflammation promotes cancer development. Just as inactivation of the p53 pathway may be universal in the neoplasia, the activation of the NFkappaB pathway may, conversely, be frequent in carcinogenesis, and a requirement for inflammation and promotion. TNF, a key pro-inflammatory cytokine when binding to TNF receptor 1 (TNFR1), may cause survival or apoptosis, dependent on biochemical modifications that determine the type of complex formed; one complex causes NFkappaB activation and gives a cell survival signal (pro-oncogenic), while the other (modified) complex recruits caspases and causes apoptosis (anti-oncogenic). Fas-ligand (FasL)-Fas interaction can also result in opposing effects on carcinogenesis due to similar mechanisms. While IL-6 counteracts apoptosis and can promote cancer development, interferons can increase DNA repair and stabilize p53, thereby be anti-oncogenic.

Occurrence of dysregulated oncogenes in primary plasma cells representing consecutive stages of myeloma pathogenesis: indications for different disease entities

This study investigated the expression pattern in primary plasma cells (PCs) of putative oncogenes suggested to be involved in multiple myeloma (MM) development. cDNA archives were generated by global reverse transcription polymerase chain reaction from CD38++/CD19-/CD56-/++ aberrant PCs of a prospective cohort of 96 subjects, including healthy individuals, patients with monoclonal gammopathies of undetermined significance (MGUS), MM and MM with extramedullary manifestations (ExMM). The cDNA archives were analysed quantitatively for expression of the cyclin D1, fibroblast growth factor receptor 3 (FGFR3), C-MYC, C-MAF and cyclin D3 oncogenes. In addition, all patients were screened for IGH-MMSET hybrid transcripts. None of the analysed oncogenes was randomly distributed. C-MYC and cyclin D3 expression increased at the extramedullary transformation stage. Furthermore, C-MYC and cyclin D3 expression in CD56+ MM was similar to MGUS, whereas CD56- MM was similar to ExMM. FGFR3/IGH-MMSET was only observed among CD56+ MM patients, whereas an increased frequency of C-MAF dysregulation was seen among CD56- MM. High cyclin D1 expression levels were identified at similar frequencies at all stages, whereas the frequency of patients with low cyclin D1 levels increased during MM development. These data support the stepwise transformation model accumulating genetic alterations and proliferative capacity during MM initiation and development resulting in different clinical entities.

Fine-needle aspiration cytology of a case of HIV-associated anaplastic myeloma

Plasma cell neoplasia occurs much less frequently than high-grade B-cell non-Hodgkins lymphoma in HIV-infected patients, but is nevertheless an AIDS-associated malignancy. In this report, we describe the fine-needle aspiration (FNA) findings of a mass in the left parotid region with plasmablastic features that occurred in a 41-yr-old HIV-infected homosexual man whom we diagnosed as having anaplastic myeloma. The patient had normochromic, normocytic anemia with a hematocrit of 21%, a white blood count of 2.2 x 10(9)/l with 76% neutrophils, and a CD4 count of 31%. He also had elevated levels of calcium (13.2 mg/dl), alkaline phosphatase (25,400 IU/l), blood urea nitrogen (2,600 mg/dl), and creatinine (2.5 mg/dl). Serum protein electrophoresis showed polyclonal hypergammaglobulinemia without any monoclonal component. A bone survey revealed multiple punched-out lytic lesions. FNA smears showed large plasmacytoid cells with eccentrically placed nuclei, prominent nucleoli, and moderate amounts of basophilic cytoplasm. By immunocytochemical staining, tumor cells were negative for CD19, CD20, and leukocyte-common antigen (LCA), but strongly positive for CD38 and kappa light chain. Anaplastic myeloma and plasmablastic lymphoma were considered in the differential diagnosis. Although the cytomorphologic and immunophenotypic findings of our case overlapped with those of plasmablastic lymphoma, the pattern of bone involvement with punched-out lytic lesions and absence of localization of the tumor to the mucosa of the oral cavity led us to a diagnosis of anaplastic myeloma. The patient initially received antiretroviral therapy followed by thalidomide and pulse dexamethasome therapy, but his response was poor. His HIV load increased, and his malignancy rapidly progressed with the development of multiple vertebral lesions, extraosseous extension, and eventually cord compression. He died of the disease less than 2 mo after presentation.

Molecular aspects of multiple myeloma

Multiple myeloma (MM) is a B-cell neoplasm characterized by bone marrow infiltration with malignant plasma cells, which synthesize and secrete monoclonal immunoglobulin (Ig) fragments. Despite the considerable progress in the understanding of MM biology, the molecular basis of the disease remains elusive. The initial transformation is thought to occur in a postgerminal center B-lineage cell, carrying a somatically hypermutated Ig heavy chain (IGH) gene. This plasmablastic precursor cell colonizes the bone marrow, propagates clonally and differentiates into a slowly proliferating myeloma cell population, all under the influence of specific cell adhesion molecules and cytokines. Production of interleukin-6 by stromal cells, osteoblasts and, in some cases, neoplastic cells is an essential element of myeloma cell growth, with the cytokine stimulus being delivered intracellularly via the Jack-STAT and ras signaling pathways. While karyotypic changes have been identified in up to 50% of MM patients, recent molecular cytogenetic techniques have revealed chromosomal abnormalities in the vast majority of examined cases. Translocations mostly involve illegal switch rearrangements of the IGH locus with various partner genes (CCND1, FGFR3, c-maf). Such events have been assigned a critical role in MM development. Mutations in coding and regulatory regions, as well as aberrant expression patterns of several oncogenes (c-myc, ras) and tumor suppressor genes (p16, p15) have been reported. Key regulators of programmed cell death (BCL-2, Fas), tumor expansion (metalloproteinases) and drug responsiveness (topoisomerase II alpha) have also been implicated in the pathogenesis of this hematologic malignancy. A tumorigenic role for human herpesvirus 8 (HHV8) was postulated recently, following the detection of viral sequences in bone marrow dendritic cells of MM patients. However, since several research groups were unable to confirm this observation, the role of HHV8 remains unclear. Translation of the advances in MM molecular biology into novel therapeutic strategies is essential in order to improve disease prognosis.

Diverse karyotypic abnormalities of the c-myc locus associated with c-myc dysregulation and tumor progression in multiple myeloma

Translocations involving c-myc and an Ig locus have been reported rarely in human multiple myeloma (MM). Using specific fluorescence in situ hybridization probes, we show complex karyotypic abnormalities of the c-myc or L-myc locus in 19 of 20 MM cell lines and approximately 50% of advanced primary MM tumors. These abnormalities include unusual and complex translocations and insertions that often juxtapose myc with an IgH or IgL locus. For two advanced primary MM tumors, some tumor cells contain a karyotypic abnormality of the c-myc locus, whereas other tumor cells do not, indicating that this karyotypic abnormality of c-myc occurs as a late event. All informative MM cell lines show monoallelic expression of c-myc. For Burkitt's lymphoma and mouse plasmacytoma tumors, balanced translocation that juxtaposes c-myc with one of the Ig loci is an early, invariant event that is mediated by B cell-specific DNA modification mechanisms. By contrast, for MM, dysregulation of c-myc apparently is caused principally by complex genomic rearrangements that occur during late stages of MM progression and do not involve B cell-specific DNA modification mechanisms.

A variant Burkitt-type translocation (8;22)(q24;q11) in multiple myeloma. Report of a new case and review of the literature

We report here a new case of multiple myeloma (IgG, kappa, stage IIIA) with a variant Burkitt-type translocation (8;22)(q24;q11). Bone marrow plasma cells were morphologically immature with fine nuclear chromatin and nucleoli. Chromosome analysis showed complex aberrations; that is, 53,XX, der(1)add(1)(p11)dup(1)(q12q32),+3,+5,t(8;22)(q24;q11),+9,add(10)( p13), +11,+15,add(19)(q13),+21. Fluorescence in situ hybridization analysis with the yeast artificial chromosome (YAC) clone I2 containing the C-MYC gene at 8q24 and the chromosome-22-specific DNA library pBS22 revealed that 12 was located on the der(8)t(8;22). A fusion signal derived from I2 and the YAC clone B99E1 containing the BCR gene at 22q11 was also observed on the der(8)t(8;22). Our results indicate that the breakpoint at 8q24 in this patient was located far downstream of the C-MYC gene. This breakpoint site is similar to Burkitt lymphoma with t(8;22)(q24;q11). A review of eight cases in the literature and the present case of multiple myeloma with t(8;22)(q24;q11) showed that most of them were of advanced stage and had an immature phenotype. It is suggested that the C-MYC gene may be activated by t(8;22)(q24;q11) and implicated in disease progression in multiple myeloma.

Multiple myeloma and the translocation t(11;14)(q13;q32): a report on 13 cases

Complex cytogenetic abnormalities have been described in patients with multiple myeloma (MM). To better understand the significance of the most frequent translocation observed in MM, we studied the clinical characteristics of patients with MM and the t(11;14)(q13;q32) abnormality. A search of the cytogenetic database at the Mayo Clinic identified patients with MM and t(11;14)(q13;q32). The medical records were reviewed for the clinical characteristics of these patients. We identified 13 patients with MM and t(11;14)(q13;q32) determined by standard cytogenetic analysis: in 10 patients the abnormality was detected at the time of relapse (three with previously normal results of cytogenetic examination). At the time the translocation was detected, plasma cell (PC) leukaemia was clinically diagnosed in two patients. The median number of circulating PCs, as determined by the cytoplasmic immunofluorescence of T-cell-depleted peripheral blood mononuclear cells, was 1.1x10(9)/l (mean 1.74; range 0.0017-6.26x10(9)/l). On linear regression analysis there was a strong correlation between the number of circulating PCs and the number of bone marrow PCs. The median survival after demonstration of the translocation was 8.1 months. Of all patients, 10 died of disease progression and three were alive. Patients with MM who have t(11;14)(q13;q32) seem to have an aggressive clinical course, even when the abnormality is detected at the time of diagnosis, with evidence of many circulating PCs.

Disease progression in patients with multiple myeloma is associated with a concurrent alteration in the expression of both oncogenes and tumour suppressor genes and can be monitored by the oncoprotein phenotype

The dysregulation of specific oncogenes due to either mutation or activation has previously been reported in a small number of patients with myeloma but the extent of oncogene dysregulation during the course of the disease is not known. The oncoprotein phenotype of plasma cells in 146 bone marrow samples from 81 patients with multiple myeloma was determined by dual colour flow cytometry using a predetermined panel of 8 monoclonal antibodies. High intensity CD38 expression was used to distinguish the plasma cell population and the cells were permeabilised to detect intracellular antigen expression. In situ hybridization using biotinylated cDNA probes for c-myc and bcl-2 was used to determine mRNA expression and to validate the flow cytometric assay. The normal range of expression for each of 6 oncoproteins (c-myc, c-fos, c-neu, bcl-2, p-ras, p53 mutant) and 2 tumour suppressor gene products (p53 wild and Rb) was determined in plasma cells from 33 normal bone marrows. Disease progression was associated with the concurrent abnormal expression of at least one oncogene and one tumour suppressor gene where as stable disease was associated with a normal expression of at least one or both (chi2 = 34.1; p < 0.001). At diagnosis there was a correlation between serum beta2 microglobulin and the concurrent overexpression of both an oncoprotein and a tumour suppressor gene product. Longitudinal studies of 33 different patients over 4 years, suggests that the progressive evolution of myeloma is a multistep process of genomic instability producing ongoing alterations in the expression of both oncogenes and tumour suppressor genes.

The SCID mouse as a model for multiple myeloma

The SCID mouse was investigated as a potential animal model for human multiple myeloma (MM). Duplicate samples of bone marrow mononuclear cells (BMMC) and/or peripheral blood mononuclear cells (PBMC) of six MM patients in different clinical phases and one patient with monoclonal gammapathy of undetermined significance (MGUS) were injected intraperitoneally into SCID mice. Human immunoglobulins (Ig) in the SCID sera were quantified with a light-chain isotype-specific ELISA, and their monoclonality biochemically characterized, using a sensitive immunoblotting technique after agar gel electrophoresis. Successful transplantation of bone marrow derived-tumour cells in SCID mice was obtained with BMCC of two MM patients with progressive disease. Human plasma cells were detected in the mesenteric fat tissue around the pancreas and the spleen. This model in SCID mice may facilitate studies on processes involved in tumour progression and provides a new tool for therapeutic approaches in MM.

Increasing multiple myeloma mortality among the elderly: a manifestation of aging and differential survival

Increasing multiple myeloma incidence and mortality among the elderly in industrialized nations has been attributed to associated environmental carcinogens. Age-specific multiple myeloma mortality rates in the United States from 1968 to 1989 were analyzed using the Strehler-Mildvan modification of the Gompertz relationship between aging and mortality. The results suggest that worsening environmental influences are not responsible for increasing multiple myeloma mortality among the elderly. Differential survival, a concept originally popularized by Charles Darwin, and its effect upon the surviving gene pool in an aging population is an alternative explanation for increasing multiple myeloma incidence and mortality in the elderly.

The oncoprotein phenotype of plasma cells from patients with multiple myeloma

The expression of 6 different oncoproteins and 2 tumour suppressor gene products in the plasma cells of 63 bone marrow samples was used to determine a profile of the oncogenic phenotype of patients with multiple myeloma. Dual label flow cytometry after periodatelysine paraformaldehyde fixation was used to detect cell surface phenotype and intracellular protein expression simultaneously. The normal range for both the incidence and intensity of expression was determined for each protein by analysing plasma cells (high CD38 intensity) in 22 normal bone marrow samples. The percentage of myeloma patients with a greater than normal incidence of plasma cells expressing these proteins was 53% for c-myc, 28% for Rb, 28% for bcl-2, 27% for c-fos, 24% for p53 wild, 22% for p53 mutant, 13% for c-neu and 13% for pan-ras. When a panel of 8 antibodies was used, 82% of the samples (n = 28) had an increased incidence of expression by at least one oncoprotein or tumour suppressor gene product. The 5 patients with a normal incidence of expression of all 8 proteins were in plateau stage and 4 had not received chemotherapy for more than 12 months. The number of patients with an increased incidence of expression by 2 or more oncoproteins was significantly greater (X2 = 9.0; p < 0.005) in progressive disease (55%) than in stable disease (14%) but there was no specific phenotype pattern associated with progressive disease. All 6 oncoproteins and both tumour suppressor gene products had a greater incidence and intensity of expression in progressive than in stable disease. The expression of c-myc oncoprotein correlated with c-myc mRNA expression in the same samples (n = 10) but c-myc did not correlate with either the plasma cell labelling index (r = -0.15) nor serum thymidine kinase (r = 0.10). Our results suggest that there is a heterogeneous, non-systematic but almost universal presence of activated oncogenes and tumour suppressor genes in the plasma cells of patients with multiple myeloma and that disease progression is associated with the accumulation of a variety of secondary genetic changes which confer increased malignant behaviour.

Prognosis in myeloma

OBJECTIVE

To review biologic factors that may be useful in determining prognosis in patients with myeloma.

DESIGN

The currently available clinical variables and staging systems were assessed for their predictive value in myeloma, and newly proposed prognostic factors were summarized.

RESULTS

Because some patients with myeloma may potentially benefit by new, more intensive therapies such as stem cell transplantation or may have prolonged survival with standard therapy, determining factors that would distinguish such patients is important. Independent prognostic factors such as the plasma cell labeling index and beta 2-microglobulin have repeatedly outperformed other prognostic clinical variables and can be combined into a system that identifies poor, intermediate, and good prognostic groups of patients with myeloma. New factors are needed to improve on the predictive value of these two variables. Studies of the biologic features of myelomas have led to the discovery of soluble interleukin 6 receptor (sIL 6R) in the serum. sIL 6R enhances the myeloma cell response to interleukin 6, a central growth factor that affects myeloma cell proliferation. Preliminary studies by the Eastern Cooperative Oncology Group show that sIL 6R predicts survival independent of the plasma cell labeling index and beta 2-microglobulin. Other proposed independent prognostic factors include cytokines and their soluble receptors, regulatory T cells, circulating myeloma cells, myeloma cell precursors, and markers of proliferation, apoptosis, and drug resistance.

CONCLUSION

Because of wide variation in duration of survival, the prognosis of patients with myeloma, a fatal bone marrow plasma cell neoplasm, cannot yet be adequately predicted. Better prognostic factors are needed for stratification in clinical trials, counseling of patients, and designing of new therapeutic trials.

Multiple myeloma in the elderly

While multiple myeloma is an incurable disease for nearly all patients, current chemotherapy and supportive care can result in significant disease control and improved duration of survival and quality of life. With the average age of patients about 70, most of the high-dose curative strategies exclude the bulk of patients affected by the disease. Recent advances in understanding the biology and pathophysiology of myeloma have led to novel therapies aimed at altering drug resistance, improving duration of the plateau phase, interrupting the cytokine growth stimulation, and improving management of common complications including infections, anemia, and bone lesions. These latter approaches are not restricted to younger patients, and early evidence indicates that elderly patients are also likely to benefit.

Mechanisms of the escape phase of myeloma

In multiple myeloma the duration of plateau is an important clinical and biological determinant of quality of life and survival. During plateau phase, the tumour is in an indolent state, as manifested by a low labelling index of plasma cells and other proliferative markers, e.g. the thymidine kinase level. The mechanism by which plasma cells escape from this indolent phase to a more aggressive phase of this disease is unknown, but a number of possible mechanisms have been postulated. These include loss of immunoregulation, clonal evolution, cytokine dysfunction and oncogene activation or tumour suppressor gene dysfunction. As current chemotherapy protocols do not appear to be able to eradicate the malignant clone, understanding the nature of the indolent phase of the malignant clone and the reasons for its escape from this phase are very important and may provide new options for disease control.

Multistep transformation in low-grade lymphoproliferative diseases

BACKGROUND

Carcinogenesis, the formation of solid tumors, is now widely accepted to represent a multistep process. Several genetic events, activation of proto-oncogenes and inactivation of tumor suppressor genes, are involved.

DESIGN

Review of the literature for evidence that the concept of multistep transformation has relevance also for the formation of low-grade lymphoproliferative diseases.

RESULTS AND CONCLUSION

The common translocations in low-grade lymphoid tumors are probably early events, predominantly involved in the activation of oncogenes, leading to growth stimulation or prolonged cell survival. As a result 'monoclonal lymphoproliferative disorders of undetermined significance (MLDUS)' occur, undetermined, because some translocations may not always led to tumor formation. For progression to full malignancy, additional genetic events are required besides sequential selection of variant subpopulations within the neoplastic clone. Recent data indicate that mutations and deletions of putative tumor suppressor genes, including the P53 and retinoblastoma genes, are also involved in the progression of lymphoproliferative disorders. A list of lymphoproliferative diseases stressing this concept of multistep transformation is presented in this article.

Methylation status within exon 3 of the c-myc gene as a prognostic marker in myeloma and leukaemia

The third exon of the c-myc gene contains a CpG site which has been implicated as a regulatory region. When this site is methylated it has protein binding properties and binds a different set of proteins in normal and neoplastic cells. Recent work using myeloma cell lines indicates a correlation between hypomethylation at this site and enhanced expression of the myc protein. We investigated the methylation of this site in 10 cases of myeloma but found that there was no change from the high degree of methylation found in normal cells. Therefore, methylation status at this site is unlikely to serve as a prognosticator in myelomatosis. However, methylation changes at this site were observed in DNA from two cases of CMML, in which hypomethylation was observed and in three AML cases, which were completely methylated at this site.

Molecular analysis of clonality in plasma cell dyscrasias

It has been suggested that multiple myeloma, generally considered a neoplastic disorder of mature plasma cells, may arise from a pluripotent haemopoietic stem cell. The possibility that circulating lymphocytes derive from the same neoplastic progenitor has been tested in a large number of studies in the past few years, as proof of the interest that this subject is raising among scientists, and also of its elusiveness. We studied a group of 29 patients with plasma cell dyscrasias in order to evaluate clonality of haemopoietic cell populations. The X-linked markers hypoxantine phosphoribosyltransferase (HPRT) and phosphoglycerate kinase (PGK) disclosed no monoclonal component in seven heterozygous women. Analysis of immunoglobulin gene rearrangement with four probes showed a germline configuration in samples from 25/29 patients. Only four bone marrow samples from subjects with aggressive disease had rearranged C mu sequence; one had rearrangement of JH and C mu.

Homotypic cell aggregations of human myeloma cells with ICAM-1 and LFA-1 molecules

Some myeloma cells freshly isolated from bone marrow aspirates in human myelomas and some myeloma cell lines formed spontaneous cell aggregations in vitro (homotypic cell aggregations). In order to clarify the surface molecules involved in homotypic cell aggregations and physiological roles of these cell aggregations, we investigated the expressions of intercellular adhesion molecule 1 (ICAM-1) and lymphocyte function-associated antigen 1 (LFA-1) on 20 samples of freshly isolated myeloma cells and three myeloma cell lines and the effect of anti-ICAM-1 and anti-LFA-1 alpha antibodies on myeloma cell proliferation in vitro. All myeloma cells that we tested expressed ICAM-1 on their surface. Among them, myeloma cells that strongly coexpressed LFA-1 alpha, formed homotypic cell aggregates in vitro. These spontaneous cell aggregations were completely released by adding either anti-ICAM-1 or anti-LFA-1 alpha antibody. During short-term culture, spontaneous proliferation of myeloma cells in vitro and their proliferative responses to recombinant interleukin-6 (rIL-6) were not affected by pretreatment of myeloma cells with anti-ICAM-1 or anti-LFA-1 alpha antibody. Therefore these data suggest that homotypic cell aggregation of myeloma cells is mediated by ICAM-1 and LFA-1 molecules, but myeloma cell proliferation may not be modulated by these adhesion molecules during short-term cultures.