CD44 isoform expression mediated by alternative splicing: tissue-specific regulation in mice

CD44 is a widely distributed cell surface glycoprotein which shows heterogeneity in molecular expression as a result of post-translational modification as well as alternative splicing of CD44 mRNA. Functional studies have indicated that CD44 plays a role as an adhesion molecule and that different CD44-expressing cells differ in their capacities for CD44-dependent ligand binding. These observations have raised the possibility that structural modifications of CD44, including those resulting from alternatively spliced mRNA isoforms, are involved in the functional heterogeneity of CD44. To assess the expression of CD44 isoforms in the mouse, we examined CD44 cDNA by reverse transcription polymerase chain reaction (RT-PCR). Southern blotting of PCR products with a CD44 cDNA probe or with internal oligonucleotides revealed the expression in mouse tumor cell lines and normal tissues of multiple CD44 mRNA products which are larger than that observed in the absence of variable exon expression. Interestingly, different mouse tissues, including lymphoid cells, showed unique patterns of alternative CD44 mRNA in Southern blotting analysis. The use of exon-specific primers allowed detection of multiple alternatively spliced mRNA species involving expression of at least seven variable exons. Cloning and sequencing of these PCR products revealed sequence identity with recently identified genomic CD44 sequences and confirmed that the PCR products correspond to mature mRNA expressing alternatively spliced CD44 exons. Taken together, these findings demonstrate that the mouse expresses multiple variably spliced CD44 isoforms and that expression is regulated in a tissue- and cell-type specific manner.

CD44 expression on activated B cells. Differential capacity for CD44-dependent binding to hyaluronic acid

CD44 expression and the functional capacity for CD44-dependent binding of hyaluronic acid (HA) were analyzed on unstimulated B cells and on B cells stimulated with a variety of polyclonal B cell activators. Whereas essentially all LPS-activated and anti-IgD-dextran-activated B cells and a subpopulation of IL-5-activated B cells expressed increased levels of cell surface CD44 relative to unstimulated B cells, only IL-5-activated CD44hi B cells constitutively bound to FITC-conjugated hyaluronic acid (FITC-HA). Preincubation of LPS or anti-IgD-dextran-activated B cells with the CD44-specific mAb IRAWB14.4 (IRA) induced a high degree of FITC-HA binding in these populations; preincubation of unstimulated B cells with this CD44-specific mAb induced minimal FITC-HA binding. In contrast, preincubation with mAb IRA failed to induce FITC-HA binding by the IL-5-activated CD44lo B cell subset. Neither the amount of constitutive FITC-HA binding nor the level of IRA-inducible FITC-HA binding correlated simply with the overall level of CD44 expressed by the different B cell populations. Biochemical analysis of immunoprecipitated CD44 molecules revealed that relative to CD44 isolated from all other populations examined, CD44 isolated from IL-5-activated B cells was of a lower molecular weight. Treatment with N-Glycanase eliminated this observed difference in molecular weight, indicating that it reflected differences in N-glycosylation of CD44 on activated B cells. Polymerase chain reaction analysis of amplified cDNA showed that each B cell population expressed a common dominant CD44 mRNA. These findings suggest that post-translational modification of CD44 and/or differential association of CD44 with other cellular components plays a critical role in activation-specific ligand binding by CD44.

CD44 expression on activated B cells. Differential capacity for CD44-dependent binding to hyaluronic acid

CD44 expression and the functional capacity for CD44-dependent binding of hyaluronic acid (HA) were analyzed on unstimulated B cells and on B cells stimulated with a variety of polyclonal B cell activators. Whereas essentially all LPS-activated and anti-IgD-dextran-activated B cells and a subpopulation of IL-5-activated B cells expressed increased levels of cell surface CD44 relative to unstimulated B cells, only IL-5-activated CD44hi B cells constitutively bound to FITC-conjugated hyaluronic acid (FITC-HA). Preincubation of LPS or anti-IgD-dextran-activated B cells with the CD44-specific mAb IRAWB14.4 (IRA) induced a high degree of FITC-HA binding in these populations; preincubation of unstimulated B cells with this CD44-specific mAb induced minimal FITC-HA binding. In contrast, preincubation with mAb IRA failed to induce FITC-HA binding by the IL-5-activated CD44lo B cell subset. Neither the amount of constitutive FITC-HA binding nor the level of IRA-inducible FITC-HA binding correlated simply with the overall level of CD44 expressed by the different B cell populations. Biochemical analysis of immunoprecipitated CD44 molecules revealed that relative to CD44 isolated from all other populations examined, CD44 isolated from IL-5-activated B cells was of a lower molecular weight. Treatment with N-Glycanase eliminated this observed difference in molecular weight, indicating that it reflected differences in N-glycosylation of CD44 on activated B cells. Polymerase chain reaction analysis of amplified cDNA showed that each B cell population expressed a common dominant CD44 mRNA. These findings suggest that post-translational modification of CD44 and/or differential association of CD44 with other cellular components plays a critical role in activation-specific ligand binding by CD44.

Murine B7-2, an alternative CTLA4 counter-receptor that costimulates T cell proliferation and interleukin 2 production

The B7-1 molecule, expressed on antigen presenting cells (APC), provides a crucial costimulatory signal for T cell activation. Recent studies demonstrate the existence of alternative, non-B7-1 CTLA4 counter-receptors in mice and humans. Here, we describe the molecular cloning and demonstrate costimulatory function of the murine B7-2 (mB7-2) gene. Murine B7-2 cDNA encodes a member of the Ig supergene family that binds CTLA4-Ig and stains with the GL1 but not anti-mB7-1 mAb. Murine B7-2 costimulates the proliferation and interleukin 2 production of CD4+ T cells and this costimulation can be inhibited by either CTLA4-Ig or GL1 mAb. Identification of the B7-2 molecule will permit further manipulation of the B7:CD28/CTLA4 costimulatory pathway which has been shown to be involved in the prevention of tolerance, induction of tumor immunity, and most recently, in the pathogenesis of autoimmunity.

Uncovering of functional alternative CTLA-4 counter-receptor in B7-deficient mice

B7 delivers a costimulatory signal through CD28, resulting in interleukin-2 secretion and T cell proliferation. Blockade of this pathway results in T cell anergy. The in vivo role of B7 was evaluated with B7-deficient mice. These mice had a 70 percent decrease in costimulation of the response to alloantigen. Despite lacking B7 expression, activated B cells from these mice bound CTLA-4 and GL1 monoclonal antibody, demonstrating that alternative CTLA-4 ligand or ligands exist. These receptors are functionally important because the residual allogenic mixed lymphocyte responses were blocked by CTLA4Ig. Characterization of these CTLA-4 ligands should lead to strategies for manipulating the immune response.

Identification of an alternative CTLA-4 ligand costimulatory for T cell activation

Stimulation of T cell proliferation generally requires two signals: The first signal is provided by the T cell receptor binding to antigen, and the second signal or costimulus is provided by a different receptor-ligand interaction. In mouse and human, the CD28-B7 interaction has been identified as a source of costimulatory signals. We have identified a cell surface molecule (GL1) that is distinct from B7 and abundantly expressed on activated B cells. On activated B cells GL1, rather than B7, is the predominant ligand for the T cell-activation molecule CTLA-4. GL1 provides a critical signal for T cell-dependent responses in vitro and in vivo.

Expression of a MHC class II transgene determines both superantigenicity and susceptibility to mammary tumor virus infection

Milk-borne mouse mammary tumor virus (MMTV) is a type B retrovirus that induces mammary carcinoma. Infectious MMTV, as well as genomically integrated mouse mammary proviruses, encode superantigens that are recognized by T cells that express appropriate T cell receptor V beta products. To determine the relationship between the superantigenic property of milk-borne MMTV and its in vivo infectivity, mice which were either positive or negative for expression of a transgene-encoded E alpha E beta class II major histocompatibility complex (MHC) product were exposed to milk borne C3H MMTV. Superantigen-mediated deletion of V beta 14-expressing T cells occurred only in E alpha transgene-positive mice, indicating that the deletion was E alpha E beta dependent. When mice were analyzed for viral infection by assaying viral p28 in the milk of recipient females, significant p28 levels were found only in E alpha E beta transgene-positive mice. Similarly, the presence of C3H MMTV LTR mRNA in mammary glands, as detected by PCR, paralleled p28 levels. These findings indicate that E alpha expression or the E alpha-dependent T cell response to viral superantigen is causally related to susceptibility to MMTV infection, and that lack of a permissive class II product can protect mice from virus infection.

Preferential V beta usage by cytotoxic T cells cross-reactive between two epitopes of HIV-1 gp160 and degenerate in class I MHC restriction

The T cell response to HIV-1 gp160 is among the most thoroughly studied immune responses to HIV-1 products. In our previous work, the MHC class I molecule Dd as well as H-2u, p, and q, were found to present P18 and HP53, two determinants of HIV-1 gp160, to CD8+ CTL in mice. We have studied the TCR V beta chain expression in CTL lines, either cross-reactive for these two peptides or specific for P18 alone, in these four different MHC haplotypes. The usage of V beta in T cells showing cross-reaction between these two peptides was remarkably conserved (primarily V beta 8 family, with some use of V beta 14) despite the extensive TCR V beta diversity of the non-cross-reactive CTL, which did not use V beta 8 or 14. This correlation of V beta usage with fine specificity was consistent in H-2d, u, and p (p < 0.01), but not in H-2q. The correlation of V beta use with peptide fine specificity independent of MHC restriction was unexpected. The strong predominance of V beta 8 family TCR was all the more surprising in view of the finding that mice bearing a genomic deletion of V beta 8 can still produce T cells with the cross-reactive phenotype, implying that other V beta chains can produce this specificity. We therefore asked whether the complexes of P18 with H-2d, p, and u are recognized as identical, and observed the surprising result that H-2d, p, and u cells mutually cross-present the peptides P18 and HP53 to allogeneic CTL lines and individual clones of each of the other haplotypes, whereas none of these cross-present to H-2q CTL, nor do H-2q targets present to CTL of the other haplotypes. This degeneracy of MHC restriction is novel for class I molecules. Moreover, the observed restriction in V beta usage occurs only in the unique set of CTL that exhibit both peptide-cross-reactive fine specificity and MHC allogeneic cross-presentation. The observation that a strain of mice in which the V beta 8 family is genomically deleted can still make CTL of this phenotype using another V beta demonstrates the plasticity of the class I MHC-restricted repertoire when the dominating receptor is not available.

Preferential V beta usage by cytotoxic T cells cross-reactive between two epitopes of HIV-1 gp160 and degenerate in class I MHC restriction

The T cell response to HIV-1 gp160 is among the most thoroughly studied immune responses to HIV-1 products. In our previous work, the MHC class I molecule Dd as well as H-2u, p, and q, were found to present P18 and HP53, two determinants of HIV-1 gp160, to CD8+ CTL in mice. We have studied the TCR V beta chain expression in CTL lines, either cross-reactive for these two peptides or specific for P18 alone, in these four different MHC haplotypes. The usage of V beta in T cells showing cross-reaction between these two peptides was remarkably conserved (primarily V beta 8 family, with some use of V beta 14) despite the extensive TCR V beta diversity of the non-cross-reactive CTL, which did not use V beta 8 or 14. This correlation of V beta usage with fine specificity was consistent in H-2d, u, and p (p < 0.01), but not in H-2q. The correlation of V beta use with peptide fine specificity independent of MHC restriction was unexpected. The strong predominance of V beta 8 family TCR was all the more surprising in view of the finding that mice bearing a genomic deletion of V beta 8 can still produce T cells with the cross-reactive phenotype, implying that other V beta chains can produce this specificity. We therefore asked whether the complexes of P18 with H-2d, p, and u are recognized as identical, and observed the surprising result that H-2d, p, and u cells mutually cross-present the peptides P18 and HP53 to allogeneic CTL lines and individual clones of each of the other haplotypes, whereas none of these cross-present to H-2q CTL, nor do H-2q targets present to CTL of the other haplotypes. This degeneracy of MHC restriction is novel for class I molecules. Moreover, the observed restriction in V beta usage occurs only in the unique set of CTL that exhibit both peptide-cross-reactive fine specificity and MHC allogeneic cross-presentation. The observation that a strain of mice in which the V beta 8 family is genomically deleted can still make CTL of this phenotype using another V beta demonstrates the plasticity of the class I MHC-restricted repertoire when the dominating receptor is not available.

Preferential expansion and activation of V beta 5+ CD4+ T cells in murine acquired immunodeficiency syndrome

Infection of B6 mice with LP-BM5 MuLV results in a syndrome characterized by progressive and profound immunodeficiency, termed murine acquired immunodeficiency syndrome (MAIDS). In this report we show that preferential expansion and activation of V beta 5+ CD4+ spleen cells occurs early in the course of disease, which then progresses to more widespread polyclonal activation of CD4+ T cells. V beta 5 expansion occurs only on infection with MAIDS-associated MuLV and is not observed after infection with other MuLV. The possible role of V beta 5 activation and expansion as a necessary early step in the pathogenesis of MAIDS is discussed.

Preferential expansion and activation of V beta 5+ CD4+ T cells in murine acquired immunodeficiency syndrome

Infection of B6 mice with LP-BM5 MuLV results in a syndrome characterized by progressive and profound immunodeficiency, termed murine acquired immunodeficiency syndrome (MAIDS). In this report we show that preferential expansion and activation of V beta 5+ CD4+ spleen cells occurs early in the course of disease, which then progresses to more widespread polyclonal activation of CD4+ T cells. V beta 5 expansion occurs only on infection with MAIDS-associated MuLV and is not observed after infection with other MuLV. The possible role of V beta 5 activation and expansion as a necessary early step in the pathogenesis of MAIDS is discussed.

T cell receptor V alpha-V beta combinatorial selection in the expressed T cell repertoire

This study has evaluated whether preferential pairing occurs between TCR alpha- and beta-chains expressing specific V alpha and V beta gene products in the mature peripheral T cell population, as a result of either thymic selection or of structural constraints on chain pairing. The association of specific V alpha products with specific V beta products on individual T cells was found, in multiple instances, to be highly selective. Moreover, patterns of preferential V alpha-V beta association were highly strain-specific and were independently expressed in CD4+ and CD8+ T cell subsets. Although these findings do not exclude the possibility that structural constraints may limit V alpha-V beta pairing in other instances, they indicate that the observed instances of skewed expression are not caused by structural constraints in chain pairing. Rather, they suggest that strain-specific selective events alter the expressed V alpha V beta repertoire as a result of recognition of self or environmental Ag during T cell repertoire selection.

T cell receptor V alpha-V beta combinatorial selection in the expressed T cell repertoire

This study has evaluated whether preferential pairing occurs between TCR alpha- and beta-chains expressing specific V alpha and V beta gene products in the mature peripheral T cell population, as a result of either thymic selection or of structural constraints on chain pairing. The association of specific V alpha products with specific V beta products on individual T cells was found, in multiple instances, to be highly selective. Moreover, patterns of preferential V alpha-V beta association were highly strain-specific and were independently expressed in CD4+ and CD8+ T cell subsets. Although these findings do not exclude the possibility that structural constraints may limit V alpha-V beta pairing in other instances, they indicate that the observed instances of skewed expression are not caused by structural constraints in chain pairing. Rather, they suggest that strain-specific selective events alter the expressed V alpha V beta repertoire as a result of recognition of self or environmental Ag during T cell repertoire selection.

Characterization of a novel cell-surface molecule expressed on subpopulations of activated T and B cells

A mAb, GL7, is described that reacts with a 35-kDa protein on subsets of activated mouse B cells as well as activated CD4+ and CD8+ peripheral T cells. In normal mice analyzed by flow cytometry, GL7 bound at low surface density to 0 to 9% of splenic B cells and 0 to 1% of splenic T cells. In contrast, GL7 bound at high density to a subpopulation comprising approximately 20% of TCR-bright thymocytes, and to B220+ cells in the bone marrow. The activation of B cells by various stimuli resulted in high levels of expression of the surface molecule identified by GL7 on up to 70% of B cells after 48 h; the remaining B cells expressed low or undetectable levels of this molecule, despite evidence of other activation-specific changes in cell-surface phenotype. The GL7-positive population of B cells induced by IL-5 stimulation exhibited high levels of both proliferative and IgM secretory activity, whereas the GL7-negative population showed little of either activity. Activation of splenic T cells with Con A for 48 h resulted in the expression of this determinant at high density on both CD4+ and CD8+ cells. GL7 thus appears to identify a previously uncharacterized cell-surface molecule expressed selectively on subpopulations of activated B and T cells as well as on discrete subpopulations of T and B lineage cells in vivo.

Characterization of a novel cell-surface molecule expressed on subpopulations of activated T and B cells

A mAb, GL7, is described that reacts with a 35-kDa protein on subsets of activated mouse B cells as well as activated CD4+ and CD8+ peripheral T cells. In normal mice analyzed by flow cytometry, GL7 bound at low surface density to 0 to 9% of splenic B cells and 0 to 1% of splenic T cells. In contrast, GL7 bound at high density to a subpopulation comprising approximately 20% of TCR-bright thymocytes, and to B220+ cells in the bone marrow. The activation of B cells by various stimuli resulted in high levels of expression of the surface molecule identified by GL7 on up to 70% of B cells after 48 h; the remaining B cells expressed low or undetectable levels of this molecule, despite evidence of other activation-specific changes in cell-surface phenotype. The GL7-positive population of B cells induced by IL-5 stimulation exhibited high levels of both proliferative and IgM secretory activity, whereas the GL7-negative population showed little of either activity. Activation of splenic T cells with Con A for 48 h resulted in the expression of this determinant at high density on both CD4+ and CD8+ cells. GL7 thus appears to identify a previously uncharacterized cell-surface molecule expressed selectively on subpopulations of activated B and T cells as well as on discrete subpopulations of T and B lineage cells in vivo.

CD45 expression by murine B cells and T cells: alteration of CD45 isoforms in subpopulations of activated B cells

The CD45 family of high molecular weight cell surface glycoproteins is abundantly expressed by virtually all hematopoietic cells. CD45 molecules exist as multiple isoforms whose extracellular portions vary in protein structure and carbohydrate content but whose intracellular portions are highly conserved and possess tyrosine phosphatase activity. In this review we summarize current studies describing CD45 isoform expression on peripheral and thymic lymphocytes. Further, we analyze changes in CD45 isoform expression by selective populations of activated B cells.

Association of a V beta 2-specific superantigen with a tumorigenic milk-borne mouse mammary tumor virus

A number of endogenous mouse mammary tumor virus (MMTV) proviruses encode superantigens that have the property of stimulating mature T lymphocytes in a TCR V beta-specific fashion and of mediating V beta-specific clonal deletion in developing T cells. The tumorigenic milk-borne MMTV carried by C3H and GR mice also have superantigen properties in vivo, and it has been proposed that this superantigenic function is critical to the infectivity and/or tumorigenicity of the virus. To test the requirement for superantigen properties in tumorigenic MMTV, a highly tumorigenic strain of MMTV isolated from BALB/c mice (BALB/cV virus) was analyzed for its effect on TCR V beta expression. It was found that exposure of newborn mice to milk-borne virus results in marked deletion of V beta 2-expressing CD4+ peripheral T cells. This deletion is detected in mature TCRhigh thymocytes as well as in peripheral T cells from virus-exposed mice. Deletion is dependent on expression of a permissive MHC type in mice exposed to virus. Subcutaneous injection of adult mice with virus-containing milk induces a substantial increase in V beta 2+ CD4+ cells in draining lymph nodes within 4 days. Thus, tumorigenic BALB/cV is associated with V beta 2-specific superantigen activity capable of mediating both T cell expansion and clonal deletion in vivo. These findings are consistent with a critical role of superantigen-mediated T cell activation in MMTV infection and tumorigenesis.

Association of a V beta 2-specific superantigen with a tumorigenic milk-borne mouse mammary tumor virus

A number of endogenous mouse mammary tumor virus (MMTV) proviruses encode superantigens that have the property of stimulating mature T lymphocytes in a TCR V beta-specific fashion and of mediating V beta-specific clonal deletion in developing T cells. The tumorigenic milk-borne MMTV carried by C3H and GR mice also have superantigen properties in vivo, and it has been proposed that this superantigenic function is critical to the infectivity and/or tumorigenicity of the virus. To test the requirement for superantigen properties in tumorigenic MMTV, a highly tumorigenic strain of MMTV isolated from BALB/c mice (BALB/cV virus) was analyzed for its effect on TCR V beta expression. It was found that exposure of newborn mice to milk-borne virus results in marked deletion of V beta 2-expressing CD4+ peripheral T cells. This deletion is detected in mature TCRhigh thymocytes as well as in peripheral T cells from virus-exposed mice. Deletion is dependent on expression of a permissive MHC type in mice exposed to virus. Subcutaneous injection of adult mice with virus-containing milk induces a substantial increase in V beta 2+ CD4+ cells in draining lymph nodes within 4 days. Thus, tumorigenic BALB/cV is associated with V beta 2-specific superantigen activity capable of mediating both T cell expansion and clonal deletion in vivo. These findings are consistent with a critical role of superantigen-mediated T cell activation in MMTV infection and tumorigenesis.

CD45 expression by B cells. Expression of different CD45 isoforms by subpopulations of activated B cells

To determine the effect of distinct activation stimuli on CD45 expression by B cells, we have examined the expression of CD45 molecules on murine B cells stimulated with LPS or the Th cell cytokine IL-5. Analysis of CD45 by flow cytometry revealed that unstimulated and stimulated B cells expressed homogeneous amounts of total CD45 but that stimulation with IL-5 resulted in a CD44hi, hyaluronate-adherent subpopulation of activated B cells that expressed a markedly altered pattern of expression of exon-specific CD45R or B220 determinants. The predominant CD45 immunoprecipitated from either unstimulated or LPS-stimulated B cells was of the high molecular mass form (approximately 220 kDa) usually associated with B cells. In contrast, the CD45 proteins immunoprecipitated from the hyaluronate-adherent subpopulation of IL-5-activated B cells were predominantly lower m.w. forms. PCR analysis of amplified CD45 cDNA also showed distinct expression profiles characteristic of each B cell population. The highest molecular size PCR product, corresponding to expression of all three variably expressed CD45 exons (A, B, and C) was prominent in resting B cells and in LPS-activated B cells but was selectively reduced in hyaluronate-adherent IL-5-activated B cells, where lower molecular size PCR products predominated, corresponding to expression of one or two of the variable exons. In contrast, LPS-activated B cells expressed reduced levels of these one- or two-exon forms. In addition, all B cell populations expressed a lower m.w. PCR product corresponding in size to the product expected when exons A, B, and C are spliced out of CD45 mRNA. Thus, analysis of alternative splicing of CD45 mRNA, as well as cell surface expression of CD45 provides a novel parameter for analysis of B cell activation by different stimuli.

T cell recognition of Mls-like superantigens: analysis of TCR requirements, superantigenic ligands, and signal transduction

Mls-like endogenous superantigens have been reported to be unique, differing from conventional antigens in the T cell receptor (TCR) requirements for their recognition, in the signal transduction events which they elicit in T cells, and in the identity of the stimulatory ligand. This review describes recent findings indicating that TCR V alpha as well as V beta products play a role in Mls recognition. Data are also summarized which indicate that T cell recognition of Mls leads to increased intracellular [Ca2+] and phosphatidyl inosotol hydrolysis. Finally, genetic linkage between mouse mammary tumor virus (MMTV) proviruses and endogenous superantigens is analyzed, and the possible role of non-MMTV products is examined.

CD45 expression by B cells. Expression of different CD45 isoforms by subpopulations of activated B cells

To determine the effect of distinct activation stimuli on CD45 expression by B cells, we have examined the expression of CD45 molecules on murine B cells stimulated with LPS or the Th cell cytokine IL-5. Analysis of CD45 by flow cytometry revealed that unstimulated and stimulated B cells expressed homogeneous amounts of total CD45 but that stimulation with IL-5 resulted in a CD44hi, hyaluronate-adherent subpopulation of activated B cells that expressed a markedly altered pattern of expression of exon-specific CD45R or B220 determinants. The predominant CD45 immunoprecipitated from either unstimulated or LPS-stimulated B cells was of the high molecular mass form (approximately 220 kDa) usually associated with B cells. In contrast, the CD45 proteins immunoprecipitated from the hyaluronate-adherent subpopulation of IL-5-activated B cells were predominantly lower m.w. forms. PCR analysis of amplified CD45 cDNA also showed distinct expression profiles characteristic of each B cell population. The highest molecular size PCR product, corresponding to expression of all three variably expressed CD45 exons (A, B, and C) was prominent in resting B cells and in LPS-activated B cells but was selectively reduced in hyaluronate-adherent IL-5-activated B cells, where lower molecular size PCR products predominated, corresponding to expression of one or two of the variable exons. In contrast, LPS-activated B cells expressed reduced levels of these one- or two-exon forms. In addition, all B cell populations expressed a lower m.w. PCR product corresponding in size to the product expected when exons A, B, and C are spliced out of CD45 mRNA. Thus, analysis of alternative splicing of CD45 mRNA, as well as cell surface expression of CD45 provides a novel parameter for analysis of B cell activation by different stimuli.

Immunoglobulin (Ig) mu, kappa transgenic mice express transgenic idiotype on endogenously rearranged IgM and IgA molecules by secretion of chimeric molecules

The sera of C57BL/6 mice transgenic for a mu a allotype heavy (H) chain and kappa light chain gene contained endogenous nontransgene immunoglobulin (IgM) (mu b allotype) and IgA molecules which carried the idiotype expressed by the transgenically encoded IgM (mu a) molecule. Serological analysis demonstrated that the presence of the transgenic idiotype on endogenous IgM and IgA was caused by the secretion of chimeric molecules that carried both chains encoded by the mu a transgene and products of endogenously rearranged Ig mu b or alpha genes. These and other results suggest that allelic exclusion of Ig gene rearrangement in mu, kappa transgenic mice is not absolute, that B cells can secrete Igs composed of more than a single (H) chain type, and that endogenous isotype switching does not result in a complete silencing of transgene expression.

Influence of T cell receptor V alpha expression on Mlsa superantigen-specific T cell responses

Recognition of conventional foreign antigen by T cells is determined by the expression of multiple variable regions of both alpha and beta chains of the T cell receptor (TCR) alpha/beta heterodimer. In contrast, there exists a class of antigens that appears to interact with the TCR alpha/beta heterodimer through the variable region on the beta chain (V beta), independent of other TCR components, a property that has led to their designation as superantigens. The goal of the present study was to analyze V alpha use in V beta 6+ T cells responsive to the superantigen, Mlaa. Results indicate that while deletion of T cells expressing V beta 6 in Mlsa-expressing mice is essentially complete and therefore appears to occur regardless of V alpha usage, in vitro Mlsa stimulation of T cells from Mlsa-negative mice results in significant skewing of V alpha use among responding V beta 6+ T cells. This indicates that V alpha expression influences recognition of the superantigen, Mlsa by mature peripheral T cells.

Expression of variable exon A-, B-, and C-specific CD45 determinants on peripheral and thymic T cell populations

A mAb (I/24) has been generated that is specific for a determinant on mouse CD45 molecules. Reactivity of this mAb with a panel of CD45 transfected cell lines demonstrated that the determinant recognized is dependent upon expression of one or more CD45 variable exons and that exon C is sufficient for its expression. The exon C-specific epitope detected by I/24 is expressed at high density on essentially all B lymphocytes and at an intermediate density on the vast majority of CD8+ splenic T cells. Two distinct subpopulations of CD4+ splenic T cells were detected, a minor subpopulation that expresses this exon determinant at high density and a major subpopulation that expresses it at a much lower density. This first identification of a CD45RC-specific reagent allowed a comparison of the expression of exon A-, exon B-, and exon C-specific determinants on peripheral and thymic lymphoid populations. When splenic lymphocytes were analyzed for expression of CD45RA (reactive with mAb 14.8), CD45RB (reactive with mAb 23G2 or mAb 16.A), and CD45RC (reactive with mAb I/24) determinants, it was found that each of these CD45 determinants had a distinct pattern of expression on CD4+ and CD8+ T cells and B cells. CD45RB and RC epitopes were also detected at high density on a small proportion (0.7 to 4.1%) of thymocytes. Both CD45RB and RC epitopes were found predominantly on CD4-CD8- and CD4-CD8+ thymocytes but were also found on small numbers of CD4+CD8+ and CD4+CD8- cells. The population of thymocytes that expressed CD45RB and CD45RC determinants displayed a novel TCR CD3 phenotype characterized by a level of expression that was intermediate between that seen in the larger CD3 bright and CD3 dull populations of thymocytes.