Identification of a new multigene four-transmembrane family (MS4A) related to CD20, HTm4 and beta subunit of the high-affinity IgE receptor

MS4A4B is a GITR-associated membrane adapter, expressed by regulatory T cells, which modulates T cell activation

In the aftermath of thymic negative selection, natural and adaptive regulatory T cells (Tregs) must acknowledge peripheral, "danger-free" self-Ag to ensure their sustained activity. In this paper, we show that natural and adaptive Tregs or T cells transduced with cDNA for Foxp3, just like Th1 cells, express members of the MS4A family of transmembrane molecules. Naive T cells transduced with MS4A4B become able to respond to lower levels of Ag. Using two family members, MS4A4B and MS4A6B, as baits in a yeast split-ubiquitin Treg library screen, we demonstrate their interaction with each other and with GITR, Orai1, and other surface receptors. Interaction of 4B with GITR augments GITR signaling and T cell IL-2 production in response to triggering with GITR ligand or anti-GITR Abs. This interaction provides a mechanism whereby MS4A family members, through lateral coassociation with costimulatory molecules, may amplify Ag signals. We propose that T cells preoccupied with immune defense use this MS4A family to enhance sensitivity to extrinsic Ag stimulation, ensuring its elimination, while Tregs use these adaptors to allow low level Ag signals to sustain regulatory function.

Comparative analysis of testis protein evolution in rodents

Genes expressed in testes are critical to male reproductive success, affecting spermatogenesis, sperm competition, and sperm-egg interaction. Comparing the evolution of testis proteins at different taxonomic levels can reveal which genes and functional classes are targets of natural and sexual selection and whether the same genes are targets among taxa. Here we examine the evolution of testis-expressed proteins at different levels of divergence among three rodents, mouse (Mus musculus), rat (Rattus norvegicus), and deer mouse (Peromyscus maniculatus), to identify rapidly evolving genes. Comparison of expressed sequence tags (ESTs) from testes suggests that proteins with testis-specific expression evolve more rapidly on average than proteins with maximal expression in other tissues. Genes with the highest rates of evolution have a variety of functional roles including signal transduction, DNA binding, and egg-sperm interaction. Most of these rapidly evolving genes have not been identified previously as targets of selection in comparisons among more divergent mammals. To determine if these genes are evolving rapidly among closely related species, we sequenced 11 of these genes in six Peromyscus species and found evidence for positive selection in five of them. Together, these results demonstrate rapid evolution of functionally diverse testis-expressed proteins in rodents, including the identification of amino acids under lineage-specific selection in Peromyscus. Evidence for positive selection among closely related species suggests that changes in these proteins may have consequences for reproductive isolation.

MS4A12 is a colon-selective store-operated calcium channel promoting malignant cell processes

Using a data mining approach for the discovery of new targets for antibody therapy of colon cancer, we identified MS4A12, a sequence homologue of CD20. We show that MS4A12 is a cell surface protein. Expression analysis and immunohistochemistry revealed MS4A12 to be a colonic epithelial cell lineage gene confined to the apical membrane of colonocytes with strict transcriptional repression in all other normal tissue types. Expression is maintained upon malignant transformation in 63% of colon cancers. Ca(2+) flux analyses disclosed that MS4A12 is a novel component of store-operated Ca(2+) entry in intestinal cells. Using RNAi-mediated gene silencing, we show that loss of MS4A12 in LoVo colon cancer cells attenuates epidermal growth factor receptor-mediated effects. In particular, proliferation, cell motility, and chemotactic invasion of cells are significantly impaired. Cancer cells expressing MS4A12, in contrast, are sensitized and respond to lower concentrations of epidermal growth factor. In summary, these findings have implications for both the physiology of colonic epithelium as well as for the biology and treatment of colon cancer.

A requirement for calcium in the caspase-independent killing of Burkitt lymphoma cell lines by Rituximab

The therapeutic monoclonal antibody rituximab has previously been shown to kill B cells in a caspase-independent manner. The signalling pathways underpinning this novel death pathway are unknown. The present study showed that rituximab treatment of Burkitt lymphoma cell lines induced a slow rise in intracellular calcium ([Ca(2+)](i)). This rise was only witnessed in cell lines that were killed by antibody, suggesting a critical role for Ca(2+) in mediating rituximab-driven caspase-independent cell death. Inhibition of the two main intracellular store-located Ca(2+) channels, i.e. the ryanodine and inositol-1,4,5-triphosphate receptor channels by dantrolene and xestospongen-c respectively did not prevent the rise in Ca(2+) seen with rituximab or protect cells from subsequent death. In sharp contrast, inhibition of Ca(2+) entry via plasma membrane channels with (2-aminoethoxy) diphenylborate or SKF-96365 or complete chelation of extracellular Ca(2+) with ethyleneglycol bis (aminoethylether) tetra-acetate inhibited the rise in [Ca(2+)](i) and increased cell viability. Together, these data suggest that ligation of the CD20 receptor with rituximab allows a slow sustained influx of Ca(2+) from the external environment that under certain conditions can lead to cell death.

CD20 homo-oligomers physically associate with the B cell antigen receptor. Dissociation upon receptor engagement and recruitment of phosphoproteins and calmodulin-binding proteins

B cell antigen receptor (BCR) signaling initiates sustained cellular calcium influx necessary for the development, differentiation, and activation of B lymphocytes. CD20 is a B cell-restricted tetraspanning protein organized in the plasma membrane as multimeric molecular complexes involved in BCR-activated calcium entry. Using coprecipitation of native CD20 with tagged or truncated forms of the molecule, we provide here direct evidence of CD20 homo-oligomerization into tetramers. Additionally, the function of CD20 was explored by examining its association with surface-labeled and intracellular proteins before and after BCR signaling. Two major surface-labeled proteins that coprecipitated with CD20 were identified as the heavy and light chains of cell surface IgM, the antigen-binding components of the BCR. After activation, BCR-CD20 complexes dissociated, and phosphoproteins and calmodulin-binding proteins were transiently recruited to CD20. These data provide new evidence of the involvement of CD20 in signaling downstream of the BCR and, together with the previously described involvement of CD20 in calcium influx, the first evidence of physical coupling of the BCR to a calcium entry pathway.

Cloning of B cell-specific membrane tetraspanning molecule BTS possessing B cell proliferation-inhibitory function

Lymphocyte proliferation is regulated by signals through antigen receptors, co-stimulatory receptors, and other positive and negative modulators. Several membrane tetraspanning molecules are also involved in the regulation of lymphocyte growth and death. We cloned a new B cell-specific tetraspanning (BTS) membrane molecule, which is similar to CD20 in terms of expression, structure and function. BTS is specifically expressed in the B cell line and its expression is increased after the pre-B cell stage. BTS is expressed in intracellular granules and on the cell surface. Overexpression of BTS in immature B cell lines induces growth retardation through inhibition of cell cycle progression and cell size increase without inducing apoptosis. This inhibitory function is mediated predominantly by the N terminus of BTS. The development of mature B cells is inhibited in transgenic mice expressing BTS, suggesting that BTS is involved in the in vivo regulation of B cells. These results indicate that BTS plays a role in the regulation of cell division and B cell growth.

Generation of rituximab polymer may cause hyper-cross-linking-induced apoptosis in non-Hodgkin's lymphomas

PURPOSE

Although Rituximab has produced significant tumor regressions in lymphoma patients, only 50% respond. Clinically, it has been shown that the major mechanism of action of Rituximab is antibody-dependent cytotoxicity requiring presentation by Fc-bearing cells. To improve the clinical efficacy of Rituximab for the treatment of CD20+ lymphomas, we now describe a new formulation of Rituximab, which, on direct binding to target, can induce apoptosis.

METHODS

In this report, enhanced apoptosis was observed by treating CD20+ lymphoma cells with a new polymer formulation of Rituximab. The polymer was produced by formation of a peptide bond using the sugar moiety of dextran (MW 6,000) to generate a clinically relevant reagent for use in vivo.

RESULTS

Comparison of Rituximab with a previously described dimer and the newly generated polymer shows that the polymer induced apoptosis more effectively in CD20+ cells as shown by the terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling assay (Rituximab, 3%; dimer, 3%; polymer, 58%). Consistent with these results, the polymer produced marked regression in CD20+ lymphoma xenografts, whereas the dimer and monomer reagents showed little effect. In addition, we were able to show that the level of apoptosis induced in human lymphoma cell lines was in accordance with the extent of both surface CD20 clustering and caspase-3 activation.

CONCLUSIONS

These data suggest that hyper-cross-linking-induced apoptosis can be simulated by the use of a dextran polymer of Rituximab, which, when used in vivo, can directly kill CD20+ lymphoma cells and improve the clinical efficacy of this important therapeutic for human B-cell lymphomas.

Patterns of expression, membrane localization, and effects of ectopic expression suggest a function for MS4a4B, a CD20 homolog in Th1 T cells

The membrane-spanning 4A (MS4A) family of proteins includes CD20, Fc epsilonRIbeta, and HTm4, whose genes are grouped in a chromosomal location that is associated with increased susceptibility to allergy and atopic asthma. One family member, Chandra/MS4a4B, was reported to be expressed in T helper 1 (Th1) T cells but not Th2 T cells. In the present study, Ms4a4b was isolated in a screen of genes differentially expressed during thymocyte development. MS4a4B was detected in immature CD4- CD8- CD44+ CD25- thymocytes, turned off during further stages of thymocyte development and reexpressed in mature single-positive thymocytes. MS4a4B expression was found in naive CD8+ and CD4+ peripheral T cells and natural killer (NK) cells but not in B cells. MS4a4B is expressed at the cell surface with its C-terminus located in the cytoplasm. When expressed in a T-cell hybridoma by retroviral vector, MS4a4B protein constitutively associated with lipid raft microdomains, whereas in primary T cells endogenous MS4a4B protein became enriched in rafts after T-cell activation. Overexpression of MS4a4B in primary CD4+ T-cell blasts enhanced T-cell receptor (TCR)-induced Th1 cytokine production. These results suggest that MS4a4B expression is tightly regulated during T-cell development and that MS4a4B expression promotes Th1 function and/or differentiation.

Isolation and Characterization of the B-Cell Marker CD20

The integral membrane protein CD20 has been identified as an important therapeutic target in the treatment of non-Hodgkin's lymphoma (NHL). CD20 binding of many antibodies including the therapeutic antibody, rituximab, has been shown to be critically dependent upon the conformation of a loop structure between the third and fourth helical transmembrane regions. In this work, human and murine CD20 proteins expressed in Escherichia coli are shown to be localized with the cell membrane and are purified in nondenaturing detergent solutions. The purified human and murine CD20 proteins have a substantial helical structure as measured by circular dichroism spectroscopy. Only small changes in the secondary structure are observed following the reduction of CD20, with the addition of SDS, or after heating. The rituximab antibody is shown to bind to purified human CD20 with nanomolar affinity. Rituximab binding is abolished by reduction and alkylation of CD20, with data consistent with the proposed antibody epitope being within the disulfide-bonded loop formed between cysteine residues 167 and 183. Disulfide-bond-dependent antibody binding is partially recovered following reoxidation of reduced CD20. Antibody binding is unaffected by mutations of cysteines proposed to be in the intracellular domain of CD20. The affinities of intact rituximab and its Fab fragment to the isolated and purified CD20 are similar to the observed affinity of rituximab Fab for CD20 on the surface of B cells. However, the intact rituximab antibody shows much higher affinity for CD20 on B cells. This suggests that B cells display CD20 in such a way that allows for marked avidity effects to be observed, perhaps through cross-linking of CD20 monomers into lipid rafts, which limits receptor diffusion in the membrane. Such cross-linking may play a role in partitioning CD20 into lipid rafts and in enhancing antibody-dependent B-cell depletion activities of rituximab and other therapeutic anti-CD20 antibodies.

Identification of a new member of the CD20/FcepsilonRIbeta family overexpressed in tolerated allografts

We identified a novel rat gene specifically overexpressed in tolerated heart allografts in a model of tolerance induced by donor-specific blood transfusion (DST). We named this gene TORID, for tolerance-related and induced transcript. We show that TORID expression can be attributed to non-T cells infiltrating tolerated grafts. Interestingly, TORID overexpression was also observed in long-term grafts from a different model of tolerance in which chronic rejection does not occur. Comparison of the predicted amino acid sequence of TORID and of its human counterpart LR8 showed an homology with the four-transmembrane CD20/FcepsilonRIbeta family proteins. We investigated TORID expression in naive rat immune cells and lymphoid tissues. TORID was found to be preferentially expressed in cells of the myeloid lineage such as macrophages and dendritic cells (DCs). Its expression dramatically decreased following activation/maturation. Similar results were obtained in human monocyte-derived DCs. Interestingly, TORID overexpression in bone marrow-derived DCs alters expression of MHC II and CD86 and production of IL12p40 following activation. These results suggest that TORID may be involved in the control of DC maturation and may, therefore, play a role in the induction or maintenance of allograft tolerance.

Transcriptional responses in mouse lungs induced by vaccination with Mycobacterium bovis BCG and infection with Mycobacterium tuberculosis

Transcriptome analyses enable the assessment of signature alterations in whole tissues and organs undergoing pathological processes. We analyzed gene expression profiles of lungs from mice infected with Mycobacterium tuberculosis or vaccinated with Mycobacterium bovis bacille Calmette-Guérin (BCG). We compared high-dose systemic and low-dose aerosol M. tuberculosis infections as well as systemic BCG vaccination. Expression profiles in lungs were analyzed at day (d) 1 and d 30 post infection / vaccination using a custom tailored 'in situ' synthesized 60-mer oligonucleotide microarray with focus on immunologically relevant genes. At d 1, a small number of genes were differentially regulated, whereas at d 30, a discrete expression pattern was identified in the lung. Differential gene expression profiles between M. tuberculosis infection and BCG vaccination indicate differences in naturally and vaccine induced pulmonary immune responses. The shared signature of systemic and aerosol M. tuberculosis infection revealed dominance of genes related to or controlled by interferon gamma (IFN-gamma). We assume that differential gene expression profiles after M. tuberculosis infection are strongly influenced by differences in cellular composition of the lung due to migration of immune cells, primarily neutrophils, basophils, eosinophils and monocytes to the site of infection.

Rituxan (anti-CD20 antibody)-induced translocation of CD20 into lipid rafts is crucial for calcium influx and apoptosis

Rituxan, a chimeric anti-CD20 antibody, is the first antibody approved for immunotherapy in non-Hodgkin's B-cell lymphoma and other B-cell lymphoproliferative disorders. Additionally, efficacy of Rituxan treatment has been reported in nonmalignant autoimmune diseases such as rheumatoid arthritis. Crosslinking of CD20 molecules by Rituxan induces therapeutic B-cell depletion. CD20 is a B-lymphocyte specific integral membrane protein, proposed to function as a store-operated calcium channel, which is activated upon receptor-stimulated calcium depletion of intracellular stores. Crosslinking of CD20 by antibodies has been reported to induce a redistribution of CD20 molecules to specialized microdomains at the plasma membrane known as lipid rafts. Here, we report that in the absence of Rituxan, CD20 exhibits a low affinity to lipid rafts. However, binding of Rituxan significantly increases the affinity of CD20 for lipid rafts resulting in its redistribution to a fraction resistant to Triton X-100 solubilization. Furthermore, we demonstrate that disturbing the raft integrity by cholesterol extraction results in dissociation of CD20 from a Triton X-100 resistant fraction followed by complete inhibition of Rituxan-induced calcium entry and apoptosis. The integrity of lipid rafts seems to play a crucial role for CD20-induced caspase activation. These data show, for the first time, that Rituxan-induced translocation of CD20 to lipid rafts is important for increased intracellular Ca(2+) levels and downstream apoptotic signalling.

Binding of HTm4 to Cyclin-dependent Kinase (Cdk)-associated Phosphatase (KAP)·Cdk2·Cyclin A Complex Enhances the Phosphatase Activity of KAP, Dissociates Cyclin A, and Facilitates KAP Dephosphorylation of Cdk2

Cyclin-dependent kinase 2 (cdk2) activation requires phosphorylation of Thr160 and dissociation from cyclin A. The T-loop of cdk2 contains a regulatory phosphorylation site at Thr160. An interaction between cdc-associated phosphatase (KAP) and cdk2 compromises the interaction between cdk2 and cyclin A, which permits access of KAP, a Thr160-directed phosphatase, to its substrate, cdk2. We have reported that KAP is bound and activated by a nuclear membrane protein, HTm4. Here, we present in vitro data showing the direct interaction between the HTm4 C terminus and KAP Tyr141. We show that this interaction not only facilitates access of KAP to Thr160 and accelerates KAP kinetics, but also forces exclusion of cyclin A from the KAP.cdk2 complex.

The cell cycle associated protein, HTm4, is expressed in differentiating cellsof the hematopoietic and central nervous system in mice

HTm4 is a member of a newly defined family of human and murine proteins, the MS4 (membrane-spanning four) protein group, which has a distinctive four-transmembrane structure. MS4 protein functions include roles as cell surface signaling receptors and intracellular adapter proteins. We have previously demonstrated that HTm4 regulates the function of the KAP phosphatase, a key regulator of cell cycle progression. In humans, the expression of HTm4 is largely restricted to cells of the hematopoietic lineage, possibly reflecting a causal role for this molecule in differentiation/proliferation of hematopoietic lineage cells. In this study, we show that, like the human homologue, murine HTm4 is also predominantly a hematopoietic protein with distinctive expression patterns in developing murine embryos and in adult animals. In addition, we observed that murine HTm4 is highly expressed in the developing and adult murine nervous system, suggesting a previously unrecognized role in central and peripheral nervous system development.

The role of the FcepsilonRI beta-chain in allergic diseases

The high affinity receptor for IgE, FcepsilonRI, is a multimeric surface receptor that is expressed exclusively as a tetramer on rodent cells, but exists as a tetramer or trimer on human cells. The tetrameric form is expressed on effector cells of allergic responses such as mast cells and basophils and is composed of an IgE-binding alpha-subunit, a beta-subunit and a gamma-subunit dimer. Complexes lacking the beta-subunit are found on human antigen-presenting cells. On mast cells and basophils, FcepsilonRI is essential for IgE-mediated acute allergic reactions. Crosslinking of FcepsilonRI by IgE and multivalent antigen induces a signaling cascade that culminates in the release of preformed mediators and the synthesis of lipid mediators and cytokines. The beta-subunit functions as an amplifier of FcepsilonRI expression and signaling. As a consequence, strongly enhanced mast cell effector functions and in vivo allergic reactions can be observed in the presence of FcepsilonRIbeta. In contrast, a truncated beta-isoform (betaT) that is produced by alternative splicing acts as an inhibitor of FcepsilonRI surface expression. Thus, by producing two proteins with antagonistic functions, the FcepsilonRIbeta gene could serve as a potent regulator of allergic responses. In addition, the genomic region encompassing the beta-chain has been linked to atopy and a number of polymorphisms within the FcepsilonRIbeta gene are associated with various atopic diseases. It remains to be elucidated how these polymorphisms might affect the allergic phenotype. These functions of the beta-chain together with the described genetic linkages to atopy make it a candidate for a role in the pathophysiology of allergic diseases.

Store-operated cation entry mediated by CD20 in membrane rafts

B cell activation requires sustained elevation of cytoplasmic free calcium, achieved by influx through store-operated calcium (SOC) channels. The molecular identity of these channels is not known. Ectopic expression of the raft-associated tetraspan protein CD20 in Chinese hamster ovary cells introduced a novel SOC entry pathway that was permeable to strontium as well as to calcium. The activity of this SOC pathway was abolished by deletion of a cytoplasmic sequence in CD20 essential for its efficient raft localization. Strontium-permeable SOC channels were detected in B cells, and B cell receptor-stimulated influx was significantly reduced by downregulation of CD20 expression using short interfering RNA and also by cholesterol depletion. This is the first evidence that raft-associated CD20 constitutes a component of a SOC entry pathway activated by the B cell receptor.

Identification of transcription coactivator OCA-B-dependent genes involved in antigen-dependent B cell differentiation by cDNA array analyses

The tissue-specific transcriptional coactivator OCA-B is required for antigen-dependent B cell differentiation events, including germinal center formation. However, the identity of OCA-B target genes involved in this process is unknown. This study has used large-scale cDNA arrays to monitor changes in gene expression patterns that accompany mature B cell differentiation. B cell receptor ligation alone induces many genes involved in B cell expansion, whereas B cell receptor and helper T cell costimulation induce genes associated with B cell effector function. OCA-B expression is induced by both B cell receptor ligation alone and helper T cell costimulation, suggesting that OCA-B is involved in B cell expansion as well as B cell function. Accordingly, several genes involved in cell proliferation and signaling, such as Lck, Kcnn4, Cdc37, cyclin D3, B4galt1, and Ms4a11, have been identified as OCA-B-dependent genes. Further studies on the roles played by these genes in B cells will contribute to an understanding of B cell differentiation.

CD20-mediated apoptosis: signalling through lipid rafts

CD20 is an effective target for therapeutic B-cell depletion with monoclonal antibodies. One proposed mechanism of action is direct cytotoxicity mediated via tyrosine kinase-dependent signalling pathways activated upon CD20 cross-linking. The association of CD20 with membrane microdomains known as lipid rafts, enriched in src-family tyrosine kinases and other signalling effectors, suggests an indirect mechanism of anti-CD20-induced apoptosis in which activation of src-family kinases occurs as a consequence of lipid raft clustering.

Colocalization of the B cell receptor and CD20 followed by activation-dependent dissociation in distinct lipid rafts

The B cell Ag receptor (BCR) and CD20, a putative calcium channel, inducibly associate with cholesterol-dependent membrane microdomains known as lipid rafts. A functional association between the BCR and CD20 is suggested by the effects of CD20-specific mAbs, which can modulate cell cycle transitions elicited by BCR signaling. Using immunofluorescence microscopy we show here that the BCR and CD20 colocalize after receptor ligation and then rapidly dissociate at the cell surface before endocytosis of the BCR. After separation, surface BCR and CD20 were detected in distinct lipid rafts isolated as low density, detergent-resistant membrane fragments. Pretreatment with methyl-beta-cyclodextrin, which we have previously shown to enhance receptor-mediated calcium mobilization, did not prevent colocalization of the BCR and CD20, but slowed their dissociation. The data demonstrate rapid dynamics of the BCR in relation to CD20 at the cell surface. Activation-dependent dissociation of the BCR from CD20 occurs before receptor endocytosis and appears to require in part the integrity of lipid rafts.

Identification of the genes differentially expressed in human dendritic cell subsets by cDNA subtraction and microarray analysis

Recent studies on dendritic cell (DC)–associated genes have been performed using monocyte-derived DCs (MoDCs) in different maturation stages. In our approach, to uncover the novel DC-associated genes and their expression profiles among the different DC subsets, we constructed a subtracted DC-cDNA library from CD1a+, CD14+, and CD11c− DCs by subtracting the genes shared with T cells, B cells, and monocytes, and we then screened the libraries with the aid of microarray technique. The genes showing remarkable specificity to DCs in the microarray analysis were selected and confirmed by semiquantitative reverse transcriptase–polymerase chain reaction. Our investigations revealed the following: (1) Genes highly expressed in myeloid DCs are those involved in antigen uptake/processing/presentation, cell metamorphosis, or chemotaxis. (2) Most of the genes previously identified in MoDCs, such as TARC, ferritin L-chain, lysosomal acid lipase, α- and β-tubulin, osteopontin (Eta-1), and others, are not markedly expressed in CD11c− DCs regardless of their maturation status. On the other hand, specific transcription factors and MHC class II molecules, such as interferon regulatory factor-4 (IRF4) and HLA-DR, are similarly expressed in both DC subsets. (3) CD14+ DCs retain unique features of tissue DCs, as evidenced by the gene expression profile of “no CCR7 but more CCR1” and “no TARC but abundant MCP1 and Eta-1.” (4) The genes for immunoglobulin (Ig) superfamily Z39Ig, CD20-like precursor, glycoprotein NMB (GPNMB), transforming growth factorβ (TGF-β)–induced protein (TGFBI), myeloid DAP12-associated lectin (MDL-1), and 6 novel genes are newly identified as being associated with the phenotypic expression of the DC subsets. These identifications provide important molecular information for further functional studies of the DC subsets.

Alanine-170 and proline-172 are critical determinants for extracellular CD20 epitopes; heterogeneity in the fine specificity of CD20 monoclonal antibodies is defined by additional requirements imposed by both amino acid sequence and quaternary structure

In vivo ablation of malignant B cells can be achieved using antibodies directed against the CD20 antigen. Fine specificity differences among CD20 monoclonal antibodies (mAbs) are assumed not to be a factor in determining their efficacy because evidence from antibody-blocking studies indicates limited epitope diversity with only 2 overlapping extracellular CD20 epitopes. However, in this report a high degree of heterogeneity among antihuman CD20 mAbs is demonstrated. Mutation of alanine and proline at positions 170 and 172 (AxP) (single-letter amino acid codes; x indicates the identical amino acid at the same position in the murine and human CD20 sequences) in human CD20 abrogated the binding of all CD20 mAbs tested. Introduction of AxP into the equivalent positions in the murine sequence, which is not otherwise recognized by antihuman CD20 mAbs, fully reconstituted the epitope recognized by B1, the prototypic anti-CD20 mAb. 2H7, a mAb previously thought to recognize the same epitope as B1, did not recognize the murine AxP mutant. Reconstitution of the 2H7 epitope was achieved with additional mutations replacing VDxxD in the murine sequence for INxxN (positions 162-166 in the human sequence). The integrity of the 2H7 epitope, unlike that of B1, further depends on the maintenance of CD20 in an oligomeric complex. The majority of 16 antihuman CD20 mAbs tested, including rituximab, bound to murine CD20 containing the AxP mutations. Heterogeneity in the fine specificity of these antibodies was indicated by marked differences in their ability to induce homotypic cellular aggregation and translocation of CD20 to a detergent-insoluble membrane compartment previously identified as lipid rafts.