Membrane region of surface IgM is not sufficient for transducing growth inhibitory signals in an immature B cell line WEHI-231

The murine B lymphoma line WEHI-231 is representative of immature B cells. Like normal immature B cells, WEHI-231 is susceptible to growth arrest following cross-linking of surface IgM (sIgM). Previously, we have shown using a WEHI-231 immunoglobulin (Ig) delta-transfectant that sIgD cross-linking failed to initiate growth arrest, in contrast to sIgM. In this report, we extend our research to investigate the structural requirement of Ig mu chain for regulating growth inhibition. Recombinant, chimeric Ig molecules delta/mu m and mu/delta m consisting of exons encoding extracellular delta and mu domains and membrane regions of different isotypes were constructed and introduced into WEHI-231 cells. A similar approach was used for sIgG2b-expressing transfectants. Our findings indicate that the mu m region is not sufficient for regulation of growth inhibition in WEHI-231 cells and suggest that additional extracellular region(s) of mu chain may be required for this response.

Egr-1 mRNA expression is independent of regulatory proliferative responses in the immature B cell line WEHI-231

We have recently reported cellular growth arrest induced following crosslinking of surface IgM (sIgM) but not surface IgD (sIgD) in the WEHI-231 cell line, representative of the immature B cell stage, and its delta heavy chain (delta) transfectant. An initial report has indicated WEHI-231.7, a subclone of WEHI-231, failed to express Egr-1 mRNA following sIgM crosslinking, in contrast to significant up-regulation found in mature B lymphocytes. The implication for linkage between selective surface immunoglobulin (sIg) signal transduction, expression of immediate/early genes and control of cellular growth imposes an attractive model for induction of immature B cell tolerance. Our investigations examined the relationships between Egr-1 mRNA expression and growth regulation in WEHI-231, WEHI-231.7 and their respective delta-transfectants (WEHI-delta, WEHI-delta 7). We report sIgM and sIgD crosslinking leads to a rapid increase of Egr-1 mRNA expression in WEHI-231 and WEHI-delta but not in the subclone WEHI-231.7 and WEHI-delta 7. Nevertheless, both WEHI-231, WEHI-231.7 and their delta-transfectants demonstrate the ability to induce growth arrest following sIgM but not sIgD crosslinking. Furthermore, we found Egr-1 expression could be achieved by direct activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) circumventing the classical sIg activated phosphatidylinositol signal transduction pathway. Our results suggest Egr-1 expression does not directly participate in growth regulation of immature B cell clones but rather is a consequence of signal transduction through sIg.

Functional heterogeneity of human T cell clones from atopic and non-atopic donors

Several distinct T helper (TH) subsets have been identified, based on the cytokines secreted. Recently, it has been demonstrated that these subsets can regulate the isotype of humoral responses. To investigate the possible differences in TH subsets between atopic donors which have elevated serum IgE and donors with normal serum IgE, we examined series of human TH cell clones. A total of 31 and 22 CD4+ T cell clones from the atopic and non-atopic donors, respectively, were characterized for the ability to help for IgE synthesis in vitro. T cell clones generated with allergen AgE from the atopic donor were autoreactive and all induced IgE synthesis. Tetanus toxoid-specific (TT) and phytohaemagglutinin clones were generated from both donors. There was significant heterogeneity between the T cells isolated with different stimuli from the same atopic donor. Also, there was a significant difference in the number of T cells generated from the atopic versus the non-atopic donor which helped for IgE, although there was no significant difference between the total number of T cells able to help for immunoglobulin synthesis of other isotypes. Most importantly, there was a higher frequency of clones able to support IgE synthesis between TT-specific T cell clones generated from the atopic versus the non-atopic donor. These results suggest that there are changes in subsets of TH cells specific for microbial antigens as well as allergens in atopics, which may have important implications for the aetiology of atopic disease.

Ectopic lymphokine gene expression in human peripheral blood lymphocytes in vivo

An animal model to study the effects of ectopic expression of cytokines involved in cell growth and differentiation has been established. Retrovirus vectors containing the human interleukin 6 cDNA were used to produce high titer virus-producing lines. Human peripheral blood lymphocytes (hPBLs) were successfully infected with the retrovirus and engrafted into severe combined immunodeficient mice. The majority of the animals were engrafted with hPBLs, as determined by the presence of human glucose phosphate isomerase. Furthermore, six of seven mice engrafted with hPBLs infected with high titer virus and detectable hPBLs present in the spleen expressed the retroviral human interleukin 6 gene. Importantly, human interleukin 6 protein was expressed at physiologically significant levels in these mice. These results demonstrate that models for human disease and immunotherapy involving retrovirus-mediated gene transfer into human cells can be developed in mice.

Comparative biodistribution and antibody-dependent cellular cytotoxicity of native and heavy chain chimeric antibody

We have recently chimerized the heavy chain of the pan-carcinoma monoclonal antibody (mAb) B72.3. Studies were undertaken to compare the IgG1 chimeric antibody, B72.3-1-3 with native murine B72.3 (nB72.3). Using fluorescence-activated cell sorting analysis, B72.3-1-3 demonstrated specific binding to fresh LS174T tumor cells. Biodistribution of 131I B72.3-1-3 was similar to 131I nB72.3 in nude mice bearing LS174T xenografts. Peak radiolocalization indices were noted on day 6 for B72.3-1-3 and day 8 for nB72.3. Both antibodies were capable of imaging LS174T tumors by radioimmunoscintigraphy. Antibody-dependent cellular cytotoxicity of LS174T by human peripheral blood lymphocytes was tested in 8h 51Cr release assays. With either no antibody or nB72.3, lymphocytes were not capable of killing LS174T cells. However, B72.3-1-3 at a concentration of 5 and 50 micrograms/ml mediated significant lysis of tumor cells by human lymphocytes. These results suggest that chimeric antibodies retain their binding properties to tumor cells and display biodistribution patterns similar to their unmodified counterparts. Such modifications may reduce the deleterious human antimouse antibody response to murine mAbs as well as augment antibody-dependent cellular cytotoxicity of tumor cells by human effectors.

Amino acid residues in the T cell receptor CDR3 determine the antigenic reactivity patterns of insulin-reactive hybridomas

We examined TCR gene usage in a panel of beef insulin/I-Ad-restricted T cell hybrids obtained from BALB/c mice. These hybrids demonstrated several distinct patterns of reactivity defined by their ability to respond to species variants of insulin. Correlation of TCR-alpha and -beta-gene usage with these patterns of reactivity demonstrated that TCR gene usage was restricted within Ag reactivity groups. In particular, V-J junctional regions (CDR3 equivalent) were restricted with conserved junctional amino acid motifs present in both TCR-alpha- and -beta-chains. Comparison of TCR gene usage in hybrids expressing identical V alpha and V beta gene segments but demonstrating different patterns of reactivity revealed that changes in either J alpha and/or J beta gene segment usage could alter antigenic reactivity. Indeed, single or limited amino acid differences within the CDR3 region were sufficient to markedly alter fine specificity. These data demonstrate the critical role for CDR3 in determining antigenic reactivity in beef insulin-reactive hybrids and are compatible with the current model of TCR/peptide/MHC interaction.

Amino acid residues in the T cell receptor CDR3 determine the antigenic reactivity patterns of insulin-reactive hybridomas

We examined TCR gene usage in a panel of beef insulin/I-Ad-restricted T cell hybrids obtained from BALB/c mice. These hybrids demonstrated several distinct patterns of reactivity defined by their ability to respond to species variants of insulin. Correlation of TCR-alpha and -beta-gene usage with these patterns of reactivity demonstrated that TCR gene usage was restricted within Ag reactivity groups. In particular, V-J junctional regions (CDR3 equivalent) were restricted with conserved junctional amino acid motifs present in both TCR-alpha- and -beta-chains. Comparison of TCR gene usage in hybrids expressing identical V alpha and V beta gene segments but demonstrating different patterns of reactivity revealed that changes in either J alpha and/or J beta gene segment usage could alter antigenic reactivity. Indeed, single or limited amino acid differences within the CDR3 region were sufficient to markedly alter fine specificity. These data demonstrate the critical role for CDR3 in determining antigenic reactivity in beef insulin-reactive hybrids and are compatible with the current model of TCR/peptide/MHC interaction.

Modification in framework region I results in a decreased affinity of chimeric anti-TAG72 antibody

A mouse/human chimeric B72.3P-1-6 antibody was produced by construction of a novel expression vector mpSV2neo-EP2-V-Crl containing the same gene fragments as the expression vector mpSV2neo-EP1-V-Crl (Xiang J., Roder J. and Hozunni N., submitted to Molec. Immun., 1991) except the promoter (P2) fragment in which the translation start codon ATG is retained. The expression vector was transfected into a heavy chain loss mutant cell line, B72.3M1. The translation of the chimeric heavy chain may start at the exogenous start codon ATG within the P2 fragment, which is 27 base pairs upstream of the endogenous start codon ATG in B72.3 heavy chain V region cDNA fragment, leading to an alteration in leader sequence cleavage sites and the formation of chimeric heavy chain with an elongation in the FR1 region. Chimeric B72.3P-1-6 antibody retained binding specificity to TAG72 antigen, but showed an eight-fold decrease in binding affinity to TAG72 compared with chimeric B72.3-1-3 antibody. This suggests that residues in FRI contribute to the correct folding of the antibody binding region of the B72.3 antibody.

Long-term expression of a T-cell receptor beta-chain gene in mice reconstituted with retrovirus-infected hematopoietic stem cells

To determine the feasibility of retrovirus-mediated gene transfer into stem cells for studying T-cell development, we constructed a high-titer retrovirus vector containing the neomycin phosphotransferase (neo) gene and a murine T-cell receptor (TCR) beta-chain gene with the V beta 6 variable segment. The TCR gene was placed under the control of the human beta-actin promoter and enhancer. Bone marrow cells pretreated with 5-fluorouracil were infected by coculturing with psi-2 virus-producing cells in the presence of recombinant interleukins 1, 2, 4, and 6 as well as interleukin 3 from WEHI-3 conditioned medium. The infected cells were transplanted into irradiated mice, and expression of the exogenous V beta 6 gene was examined with a V beta 6-specific monoclonal antibody, RNase protection, and polymerase chain reaction amplification. Three of seven mice expressed the retroviral TCR gene on the surface of a significant proportion of mature T cells 5-6 months after transplantation. In mice analyzed less than 1 month after transplantation, up to 30% of mature T cells expressed V beta 6 TCRs, an increase of at least 20% above the level of endogenous V beta 6 expression. DNA analysis revealed that pluripotent hematopoietic stem cells were infected by the retroviral vector in a long-term reconstituted mouse that showed increased V beta 6 expression.

Parameters that govern the regulation of immunoglobulin delta heavy-chain gene expression

The mu and delta immunoglobulin heavy-chain genes comprise a complex transcriptional unit in which a single mRNA precursor gives rise to mu- and delta-specific transcripts. During the immature B-cell stage, posttranscriptional processing events involving alternate splicing and cleavage-polyadenylation site selection give rise to mu- but not delta-encoding transcripts. In terminally differentiated B cells, delta mRNA is not synthesized because of a transcription termination event occurring upstream of the delta-gene locus. In an attempt to gain insight into the respective contributions of alternate splicing and cleavage-polyadenylation in the control of delta mRNA synthesis, we have constructed a set of plasmids in which membrane mu (mu m)-delta intergenic sequences containing the mu m poly(A) site but differing in splicing capacity were inserted in between a VH and delta gene. The mu m-delta insertion vectors were transfected into a B lymphoma line representative of an immature stage, and proximal mu m poly(A) site usage and delta mRNA synthesis were assessed. To determine unequivocally whether the mu m-delta intergenic region can regulate termination, the insertion vectors were also transfected into a B myeloma line, and transcription through the region was measured. In immature B-cell transfectants, splicing site selection was found to have a key role in determining poly(A) site utilization and concomitant delta mRNA expression. Mature delta mRNA synthesis was blocked by an upstream cleavage-polyadenylation event only when the proximal poly(A) site was associated with appropriate splicing signals. Furthermore, in vitro transcription assays revealed that the mu m-delta intergenic region is sufficient to regulate transcription termination within a 1,2430-base-pair region containing the mu m poly(A) site in myeloma transfectants. The mu m-delta insertion vectors provide an excellent model system for studying the regulatory aspects of this transcription termination event.

Differential sensitivity of specific and nonspecific antigen-presentation by B cells to a protein synthesis inhibitor

Specific and nonspecific Ag-presentation by B cells was examined for the sensitivity to the treatment with emetin, an irreversible protein synthesis inhibitor. For this aim, A20-HL B lymphoma cells expressing surface IgM receptors specific for TNP were used as APC. OVA and TNP-OVA were used as nonspecific and specific Ag, respectively. The treatment with emetin greatly impaired the ability of A20-HL cells to present specific Ag, but not nonspecific Ag, to 42-6A cloned T cells specific for OVA. The ability of the emetin-treated A20-HL cells to present nonspecific Ag indicates that the treated cells are able to process nonspecific Ag and to present processed Ag. Ag binding and the internalization by A20-HL cells through surface receptors were not affected by the emetin treatment. A20-HL cells took up specific Ag for stimulation of 42-6A cells in the presence of cycloheximide, a reversible protein synthesis inhibitor. These results suggest that the action of emetin is localized to the intracellular processing of specific Ag, not of nonspecific Ag. Thus, the processing pathway for specific Ag seems to be different from that for nonspecific Ag.

Differential sensitivity of specific and nonspecific antigen-presentation by B cells to a protein synthesis inhibitor

Specific and nonspecific Ag-presentation by B cells was examined for the sensitivity to the treatment with emetin, an irreversible protein synthesis inhibitor. For this aim, A20-HL B lymphoma cells expressing surface IgM receptors specific for TNP were used as APC. OVA and TNP-OVA were used as nonspecific and specific Ag, respectively. The treatment with emetin greatly impaired the ability of A20-HL cells to present specific Ag, but not nonspecific Ag, to 42-6A cloned T cells specific for OVA. The ability of the emetin-treated A20-HL cells to present nonspecific Ag indicates that the treated cells are able to process nonspecific Ag and to present processed Ag. Ag binding and the internalization by A20-HL cells through surface receptors were not affected by the emetin treatment. A20-HL cells took up specific Ag for stimulation of 42-6A cells in the presence of cycloheximide, a reversible protein synthesis inhibitor. These results suggest that the action of emetin is localized to the intracellular processing of specific Ag, not of nonspecific Ag. Thus, the processing pathway for specific Ag seems to be different from that for nonspecific Ag.

Production of murine V-human Cr1 chimeric anti-TAG72 antibody using V region cDNA amplified by PCR

A mouse/human chimeric B72.3-1-3 antibody was produced by construction of a novel expression vector mpSV2neo-EP1-V-Cr1. This vector contains the neo gene as a selection marker, the murine immunoglobulin heavy chain promoter and enhancer, the murine V region cDNA containing mRNA splicing joint sequences, amplified and cloned by the PCR technique directly from the B72.3 hybridoma RNA, and the human genomic Cr1 region. The expression vector containing the murine/human chimeric immunoglobulin heavy chain gene was transfected into heavy chain loss mutant cell line, B72.3Ml. Chimeric B72.3-1-3 antibody was produced at 2 micrograms/ml and retained full binding reactivity to TAG72 compared to the murine B72.3 parental antibody. Using this method, chimeric immunoglobulin molecules can be produced rapidly in comparison with the cDNA and genomic cloning techniques.

Expression of ras oncogene leads to down-regulation of protein kinase C

The effect of mutated c-Ha-ras expression on Ca2+ and phospholipid-dependent protein kinase C (PKC) activity during the process of transformation was analysed using an inducible metallothionein-ras hybrid oncogene system. A close correlation was found between the timing of ras expression and the loss of PKC enzymatic activity measured in a cell-free system. Examination of the subcellular distribution of the enzyme in inducible and constitutive ras-transformants revealed that expression of ras was associated with an apparent translocation of PKC to the plasma membrane concomitant with down-regulation of PKC enzymatic activity in particulate as well as cytosolic fractions. Quantitation of PKC protein utilizing a PKC-specific antiserum showed that ras expression was associated with a decrease in the total amount of PKC protein present in the cell. We conclude that transformation by c-Ha-ras is accompanied by down-regulation of PKC activity and that the basis of this effect may, to a large extent, lie in the down-regulation of the amount of PKC protein.

CD4+ cytolytic T cell clones that recognize polymorphism of HLA-DR beta 3 chains

We have investigated HLA-DR beta 3-associated functional polymorphism using selected Epstein-Barr virus (EBV) specific human T cell clones and EBV-transformed B cell (EBV-B) lines. To study the relationship between T cell recognition and the gene products of the three alleles of the DR beta 3 locus, Dw24, 25 and 26 (these were previously called DRw52a, b and c, respectively), CD4+ cytolytic T cell clones (CD4+ CTL) were isolated by repeated stimulation of peripheral blood mononuclear cells (HLA A2 A24; B8 B27; DRw17, Dw24, DRw2) with autologous EBV-B. Clone no. 32 proliferated strongly in response to HLA-Dw24 EBV-B, but not to Dw25 or Dw26 EBV-B. Furthermore, clones no. 32 and no. 45 both lysed HLA-Dw24 EBV-B but not Dw25 or Dw26 EBV-B. In addition, cold target inhibition studies showed that the cytolytic activity of both clones was blocked by unlabelled HLA-Dw24 EBV-B, but not by Dw25 or Dw26 EBV-B. Clones no. 32 and no. 45, therefore, could distinguish between the three allelic products of DR beta 3 haplotypes.

Class I (H-2Kb) gene transfection reduces susceptibility of YAC-1 lymphoma targets to natural killer cells

A "hybrid gene" (MTKb) comprised of the human metallothionein IIA promoter ligated to the genomic sequence of the major histocompatibility complex class I (H-2Kb) gene was subcloned into the expression vector pSV2neo and transfected into the natural killer (NK) cell-sensitive YAC-1 lymphoma. The Kb gene product was readily detectable on the cell surface of G418-resistant transfectants using both Kb-specific monoclonal antibodies and H-2b-specific cytolytic T cells. Unlike control pSV2neo transfectants, MTKb-pSV2neo transfectants were relatively resistant to lysis by NK cells from H-2a, H-2b, H-2k or H-2 (a x b)F1 haplotype mice. These data strongly suggest that the effects of MHC expression on susceptibility to NK cells can be mediated by a single and well-defined class I molecule, Kb.

CD4+ cytolytic T cell clones restricted to HLA class II, DR beta I, and DR beta III chains

We have investigated the functional polymorphism of HLA class II antigens using CD4+ CTL clones. Seven CD4+ CTL clones were isolated from a healthy donor (HLA A2 A24; B8 B27; DRw17 DRw52a) by repeated stimulation with irradiated autologous EBV-transformed B cell lines (EBV-B). According to the HLA restriction specificity we divided CD4+ CTL clones into three subgroups: (i) DRw17-restricted CD4+ CTL clones; (ii) DRw52a-restricted CD4+ CTL clones; and (iii) the CD4+ CTL clones, of which the restriction specificity could not be assigned to products of a single HLA locus. Interestingly, DRw17-restricted CD4+ CTL clones distinguished between DRw17 and DRw18. Similarly, DRw52a-restricted CD4+ CTL clones distinguished between DRw52a, w52b, and w52c. There are four amino acids which differ between DRw17 and DRw18, whereas five differ between DRw52a and the other two alleles (DRw52b and DRw52c). The recent elucidation of the crystal structure of a human class I MHC molecule has identified the probable peptide binding site to be a cleft on the outer surface of the molecule, between two alpha-helices. On the basis of the theoretical model for HLA class II molecules, amino acid positions 26 and 28 (DRw17 vs DRw18) and amino acid positions 26, 28, and 74 (DRw52a vs the other two alleles) lie within the "cleft." We propose that amino acid positions 26 and 28 are very important sites with regard to the recognition of antigen-MHC complex by the TCR.

Murine lymphocytes with natural killer activity express CTL-derived serine protease genes

Serine protease genes (C11, B10 and HF) derived from activated cytolytic T lymphocytes have been shown to be important in CTL-mediated cytotoxicity. In this study, we examined the expression of these genes in fresh natural killer (NK) cells from severe combined immunodeficiency (SCID) and athymic nude mice, as well as in T-cell lines with NK activity. All of these serine protease genes were expressed in NK cells freshly isolated from SCID and athymic nude mice. In addition, all lines showed similar strong levels of expression of C11 and B10 genes, but not the HF gene. However, levels of expression in the T-cell lines did not correlate with levels of NK-like cytotoxicity. These results suggest that C11, B10 and HF serine protease genes are necessary but not sufficient for NK-like cytotoxicity.

Influence of the valency and hydrophobicity of an antigen on its efficiency of processing and presentation by antigen-specific B cells

We examined the influence of antigen structure on the efficiency of antigen-specific B-cell processing and presentation. The ability of untransfected and anti-TNP surface IgM (sIgM) bearing transfected TA3 B-hybridoma cells to process and present various conjugates of 2,4,6-trinitrophenyl-pork insulin (TNP-PI) to PI/I-Ad-specific T cells was investigated. Similar antigen concentrations were required for presentation of underivatized antigen by untransfected and transfected cells (antigen-non-specific presentation). Transfected cells present TNP-PI at about a 50-fold lower concentration and TNP-ovalbumin (TNP-OVA), at a 600-fold lower concentration than do untransfected cells (antigen-specific presentation). PI was derivatized at 3 possible residues, A1, B1 and B29, and the different TNP-PI conjugates obtained were separated by hydrophobic interaction high-pressure liquid chromatography. All TNP1-PI conjugates were presented equally by transfected and untransfected cells. Transfected TA3 cells presented a TNP2-PI conjugate derivatized at A1 and B29, TNP2(A1,B29)-PI, at about a 50-fold lower concentration than was required by untrasfected cells. Another TNP2-PI conjugate, derivatized at residues A1 and B1, TNP2(A1,B1)-PI, and a TNP3(A1,B1,B29)-PI conjugate were presented at about the same concentration by transfected vs untransfected B cells. Of the various TNP conjugates tested, only TNP2(A1,B29)-PI, which is more hydrophobic than any of the other TNP-PI conjugates, was processed more efficiently by transfected anti-TNP-specific TA3 cells vs untransfected cells. The efficiency of presentation of these TNP-PI conjugates was directly proportional to their rate of processing. Our data demonstrate that the valency and relative hydrophobicity of an antigen control its binding to sIgM and the efficiency of processing and presentation by B cells.

The cell biology of ras-induced transformation: insights from studies utilizing an inducible hybrid oncogene system

The c-Ha-ras oncogene has been implicated as a causative agent in the development of tumors in humans as well as mice. The molecular nature of the ras-induced tumorigenic process remains unclear, however. To address this question directly we have constructed a cell line which carries a zinc-inducible metallothionein-ras hybrid oncogene, transformant 212. Upon exposure to zinc for 24-48 hr, 212 cells assume a highly transformed morphology, concomitant with the induction of ras-expression. Natural killer cells constitute a subpopulation of lymphoid effector cells which have for a long time been hypothesized to be involved in the earliest stages of antitumor surveillance. Central to this hypothesis is the prediction that NK sensitivity arises during cellular transformation. By carrying out cytotoxicity assays against the 212 transformant, we showed that, indeed, increased sensitivity to NK-mediated lysis correlated with expression of the ras oncogene, which is consistent with the above hypothesis. We then addressed the question of the biochemical mechanism of ras-induced transformation. Owing to their similarity to G proteins, regulatory elements interposed between cell-surface receptors and their effector enzymes, it has been postulated that p21, the ras oncogene protein, mediates its transforming effects by constitutive activation of proliferative signal transduction pathways. We studied the effect of ras expression on the regulation of adenylate cyclase (A.C.), key enzyme of one such major pathway. We found that ras expression correlated with a dampening of responsiveness of A.C. to several stimuli, including hormones such as isoproterenol and other agents such as GMP-PNP, forskolin and fluoride-ion. Accumulation of cAMP as measured by RIA in intact cells, as basal or in response to stimulation of A.C. activity with forskolin, was also decreased (approximately 10-fold) with ras expression. Because the regulation of calcium, another important second messenger is dependent, in part, upon cAMP and GTP-binding proteins, we investigated the possible influence of ras expression on the intracellular concentration of calcium. Steady-state intracellular free Ca2+ concentration, as measured by fluorimetry, was indeed increased by approximately 50-125% in association with ras expression. Finally, we studied the possible influence of p21ras on protein kinase C (PKC), which is a key enzyme in the important signal transduction pathway of phosphatidylinositol lipid turnover. We assessed PKC activity directly, in a cell-free system, by measuring the ability of the enzyme to transfer radiolabelled phosphate from gamma-32P-ATP to histone, and exogenous substrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional differences between immunoglobulins M and D expressed on the surface of an immature B-cell line

Crosslinked IgM molecules expressed on the surface of immature B cells mediate responses that inhibit further development, in contrast to the activational and proliferative events that follow crosslinking of the mu heavy chain in mature B cells. Concomitant with this change in IgM signaling capacity is the appearance of surface IgD, which has been proposed to modulate the response elicited by the mu heavy chain. In an attempt to gain insight into the mechanism(s) by which surface IgM is able to generate such disparate responses, delta heavy chain gene transfectants of the murine B-cell lymphoma line WEHI-231 were established. WEHI-231 cells resemble phenotypically immature B cells, in addition to being highly susceptible to the growth-inhibitory effect of surface IgM cross-linking. Endogenous mu and exogenous delta heavy chains expressed on the surface of the transfectants were compared for their role in cell proliferation and on gene expression. Our results indicate that the growth-inhibitory response is associated only with the mu heavy chain and that surface IgD does not mediate such a response. Furthermore, in contrast to IgM, IgD molecules appear to have an inductive effect on the expression of Myc and the endogenous mu and exogenous delta Ig heavy chain genes but not on the expression of the housekeeping gene encoding beta 2-microglobulin. These findings suggest that IgM and IgD are functionally distinct when expressed on the surface of an immature B cell.

Rejection of B16 melanoma induced by expression of a transfected major histocompatibility complex class I gene

Transfection of a functional major histocompatibility complex class I gene into certain tumor cells, induced by oncogenic viruses or chemical carcinogens, can effectively abrogate their tumorigenic activity. Since experimentally induced tumors possess strong tumor-specific transplantation antigens, expression of cell surface class I antigens may present the tumor cells to appropriate immune effector cells. Most spontaneously arising tumors do not possess tumor-specific transplantation antigens, and their tumorigenicity may not be affected by the expression of a transfected class I gene. We demonstrate that the poorly immunogenic B16-BL6 melanoma can be rendered nontumorigenic in syngeneic mice by the expression of the class I H-2K antigen but not the class II I-A antigen. Furthermore, the poorly tumorigenic, class I-expressing B16-BL6-transfected cells can effectively immunize syngeneic C57BL/6 mice against the highly tumorigenic, class I-deficient B16-BL6 parental cells. Our success in experimentally manipulating the tumorigenicity of a spontaneously derived neoplasm offers hope for a potential modality for the effective treatment of human cancer.