Analysis of thymic subpopulations expressing the activation antigen GL7. Expression, genetics, and function

Previously, we reported an mAb, GL7, that defines an activation Ag expressed by in vitro-stimulated B and T cells as well as by a subpopulation of thymocytes. The current study analyzes the GL7-expressing populations of adult and fetal thymus and demonstrates that: 1) The majority of GL7+ adult thymocytes are CD4+CD8- cells that are CD3 epsilon high, TCR-alpha beta high, HSAlow, and bimodal for CD69 expression. The 3G11-6C10- subset of CD4+CD8- thymocytes is enriched in GL7-expressing cells. 2) Strain differences exist in the expression of GL7 on adult CD4+CD8- thymocytes; 21.9 +/- 5.9% of BALB/c CD4+CD8- thymocytes are GL7+, whereas 4.4 +/- 1.7% of C57BL/6 CD4+CD8- thymocytes are GL7+. The low GL7 expression phenotype is dominant in CB6F1 thymocytes (7.0 +/- 2.0%), and analysis of BALB/c x CB6F1 mice suggests that low GL7 expression is determined by multiple genes. 3) CD4+CD8- GL7+ thymocytes from BALB/c mice, but not C57BL/6 mice, are skewed toward a high proportion of V beta 8+ cells. 4) Adult GL7+ CD4+CD8- thymocytes can be activated by TCR-specific stimuli to proliferate and to secrete high amounts of IL-4. 5) Fetal thymocytes contain GL7+ cells, which are predominantly CD4-CD8-, HSAlow, CD69-, and bimodal for TCR-gamma delta. Thus, GL7 expression defines a subpopulation of functionally competent TCR-alpha beta+ CD4+CD8- thymocytes as well as TCR-gamma delta+ and TCR- subpopulations of fetal CD4-CD8- thymocytes.

Analysis of thymic subpopulations expressing the activation antigen GL7. Expression, genetics, and function

Previously, we reported an mAb, GL7, that defines an activation Ag expressed by in vitro-stimulated B and T cells as well as by a subpopulation of thymocytes. The current study analyzes the GL7-expressing populations of adult and fetal thymus and demonstrates that: 1) The majority of GL7+ adult thymocytes are CD4+CD8- cells that are CD3 epsilon high, TCR-alpha beta high, HSAlow, and bimodal for CD69 expression. The 3G11-6C10- subset of CD4+CD8- thymocytes is enriched in GL7-expressing cells. 2) Strain differences exist in the expression of GL7 on adult CD4+CD8- thymocytes; 21.9 +/- 5.9% of BALB/c CD4+CD8- thymocytes are GL7+, whereas 4.4 +/- 1.7% of C57BL/6 CD4+CD8- thymocytes are GL7+. The low GL7 expression phenotype is dominant in CB6F1 thymocytes (7.0 +/- 2.0%), and analysis of BALB/c x CB6F1 mice suggests that low GL7 expression is determined by multiple genes. 3) CD4+CD8- GL7+ thymocytes from BALB/c mice, but not C57BL/6 mice, are skewed toward a high proportion of V beta 8+ cells. 4) Adult GL7+ CD4+CD8- thymocytes can be activated by TCR-specific stimuli to proliferate and to secrete high amounts of IL-4. 5) Fetal thymocytes contain GL7+ cells, which are predominantly CD4-CD8-, HSAlow, CD69-, and bimodal for TCR-gamma delta. Thus, GL7 expression defines a subpopulation of functionally competent TCR-alpha beta+ CD4+CD8- thymocytes as well as TCR-gamma delta+ and TCR- subpopulations of fetal CD4-CD8- thymocytes.

The tissue distribution of the B7-2 costimulator in mice: abundant expression on dendritic cells in situ and during maturation in vitro

B7-2 is a recently discovered, second ligand for the CTLA-4/CD28, T cell signaling system. Using the GL-1 rat monoclonal antibody (mAb), we monitored expression of B7-2 on mouse leukocytes with an emphasis on dendritic cells. By cytofluorography, little or no B7-2 was detected on most cell types isolated from spleen, thymus, peritoneal cavity, skin, marrow, and blood. However, expression of B7-2 could be upregulated in culture. In the case of epidermal and spleen dendritic cells, which become highly immunostimulatory for T cells during a short period of culture, the upregulation of B7-2 was dramatic and did not require added stimuli. Lipopolysaccharide did not upregulate B7-2 levels on dendritic cells, in contrast to macrophages and B cells. By indirect immunolabeling, the level of staining with GL-1 mAb exceeded that seen with rat mAbs to several other surface molecules including intercellular adhesion molecule 1, B7-1, CD44, and CD45, as well as new hamster mAbs to CD40, CD48, and B7-1/CD80. Of these accessory molecules, B7-2 was a major species that increased in culture, implying a key role for B7-2 in the functional maturation of dendritic cells. B7-2 was the main (> 90%) CTLA-4 ligand on mouse dendritic cells. When we applied GL-1 to tissue sections of a dozen different organs, clear-cut staining with B7-2 antigen was found in many. B7-2 staining was noted on liver Kupffer cells, interstitial cells of heart and lung, and profiles in the submucosa of the esophagus. B7-2 staining was minimal in the kidney and in the nonlymphoid regions of the gut, and was not observed at all in the brain. In the tongue, only rare dendritic cells in the oral epithelium were B7-2+, but reactive cells were scattered about the interstitial spaces of the muscle. In all lymphoid tissues, Gl-1 strongly stained certain distinct regions that are occupied by dendritic cells and by macrophages. For dendritic cells, these include the thymic medulla, splenic periarterial sheaths, and lymph node deep cortex; for macrophages, the B7-2-rich regions included the splenic marginal zone and lymph node subcapsular cortex. Splenic B7-2+ cells were accessible to labeling with GL-1 mAb given intravenously. Dendritic cell stimulation of T cells (DNA synthesis) during the mixed leukocyte reaction was significantly (35-65%) blocked by GL-1.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative analysis of B7-1 and B7-2 costimulatory ligands: expression and function

Antigen-specific T cell activation requires the engagement of the T cell receptor (TCR) with antigen as well as the engagement of appropriate costimulatory molecules. The most extensively characterized pathway of costimulation has been that involving the interaction of CD28 and CTLA4 on the T cell with B7 (now termed B7-1) on antigen presenting cells. Recently, B7-2 a second costimulatory ligand for CTLA4, was described, demonstrating the potential complexity of costimulatory interactions. This report examines and compares the expression and function of B7-1 and B7-2. Overall these results indicate that (a) B7-1 and B7-2 can be expressed by multiple cell types, including B cells, T cells, macrophages, and dendritic cells, all of which are therefore candidate populations for delivering costimulatory signals mediated by these molecules; (b) stimulating B cells with either LPS or anti-IgD-dextran induced expression of both B7-1 and B7-2, and peak expression of both costimulatory molecules occurred after 18-42 h of culture. Expression of B7-2 on these B cell populations was significantly higher than expression of B7-1 at all times assayed after stimulation; (c) blocking of B7-2 costimulatory activity inhibited TCR-dependent T cell proliferation and cytokine production, without affecting early consequences of TCR signaling such as induction of CD69 or interleukin 2 receptor alpha (IL-2R alpha); and (d) expression of B7-1 and of B7-2 can be regulated by a variety of stimuli. Moreover, expression of B7-1 and B7-2 can be independently regulated by the same stimulus, providing an additional complexity in the mechanisms available for regulating costimulation and hence immune response.

Regulation of immunostimulatory function and costimulatory molecule (B7-1 and B7-2) expression on murine dendritic cells

Dendritic cells (DC) play a critical role in the initiation of T cell-mediated immune responses, and express costimulatory molecules that are required for optimal activation of unprimed T cells. Studies on the regulation of the costimulatory molecules on DC have produced evidence from several systems that GM-CSF can up-regulate expression of CTLA4 counter receptor (CTLA4-CR) (but not intercellular adhesion molecule 1 (ICAM-1) and heat stable Ag (HsAg)) on DC. This is demonstrated on splenic DC, Langerhans cells, kidney DC in culture, and in a skin-explant culture system, in which the increased expression of CTLA4-CR on Langerhans cells (LC) occurs concomitantly with their migration out of skin. Interestingly, despite the ability of both GM-CSF and IFN-gamma to increase CTLA4-CR and maintain similar levels of ICAM-1, HsAg, and MHC molecule expression, the functional consequences of these cytokines on splenic DC are distinctly different. GM-CSF enhances the ability of DC to stimulate both T cell proliferation and cytokine release, whereas IFN-gamma causes no increase in immunostimulatory function. Further analysis of the CTLA4-CR on these cell populations by using the GL-1 and IG10 mAbs has shown that GM-CSF-cultured DC express high levels of both B7-1 and B7-2, whereas IFN-gamma-cultured DC express increased levels of only B7-2. These results suggest that optimal stimulation of unprimed T cells to proliferate and release cytokines may require participation of both of these CTLA4 counter receptors, and confirm the importance of GM-CSF for the maturation of DC into potent stimulators of T cell activation.

Regulation of immunostimulatory function and costimulatory molecule (B7-1 and B7-2) expression on murine dendritic cells

Dendritic cells (DC) play a critical role in the initiation of T cell-mediated immune responses, and express costimulatory molecules that are required for optimal activation of unprimed T cells. Studies on the regulation of the costimulatory molecules on DC have produced evidence from several systems that GM-CSF can up-regulate expression of CTLA4 counter receptor (CTLA4-CR) (but not intercellular adhesion molecule 1 (ICAM-1) and heat stable Ag (HsAg)) on DC. This is demonstrated on splenic DC, Langerhans cells, kidney DC in culture, and in a skin-explant culture system, in which the increased expression of CTLA4-CR on Langerhans cells (LC) occurs concomitantly with their migration out of skin. Interestingly, despite the ability of both GM-CSF and IFN-gamma to increase CTLA4-CR and maintain similar levels of ICAM-1, HsAg, and MHC molecule expression, the functional consequences of these cytokines on splenic DC are distinctly different. GM-CSF enhances the ability of DC to stimulate both T cell proliferation and cytokine release, whereas IFN-gamma causes no increase in immunostimulatory function. Further analysis of the CTLA4-CR on these cell populations by using the GL-1 and IG10 mAbs has shown that GM-CSF-cultured DC express high levels of both B7-1 and B7-2, whereas IFN-gamma-cultured DC express increased levels of only B7-2. These results suggest that optimal stimulation of unprimed T cells to proliferate and release cytokines may require participation of both of these CTLA4 counter receptors, and confirm the importance of GM-CSF for the maturation of DC into potent stimulators of T cell activation.

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.

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.

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.

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.

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.

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.

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.

Antigen-specific helper function of cell-free T cell products bearing TCR V beta 8 determinants

Although the T cell receptor (TCR) alpha beta heterodimer and its encoding genes have been characterized, a cell-free form of this receptor, which is needed for the study of functional or ligand-binding properties of the receptor, has not previously been isolated. When the cell-free supernatant products of activated cloned T helper (TH) cells were found to mediate helper activity with antigen specificity identical to that of intact T cells, experiments were carried out to determine whether this functional activity was mediated by a cell-free form of TCR-related material. A disulfide-linked dimer indistinguishable from the T cell surface alpha beta heterodimer was precipitated from cell-free supernatants of cloned TH cells with F23.1, a monoclonal antibody specific for a TCR V beta 8 determinant. Moreover, when cell-free TH products were bound to and eluted from immobilized F23.1, these affinity-purified materials had antigen-specific and major histocompatibility complex-restricted helper activity that synergized with recombinant lymphokines in the generation of B cell antibody responses. These findings suggest that the factor isolated from T cell supernatants is a cell-free form of the TCR alpha beta dimer.

Activation of Lyt-2+ (CD8+) and L3T4+ (CD4+) T cell subsets by anti-receptor antibody

The mAb F23.1, specific for V beta 8-related determinants on the TCR, was used to study the requirements for TCR cross-linking and for accessory cells (AC) in the induction of proliferation or IL-2 responsiveness in L3T4+ (CD4+) and Lyt-2+ (CD8+) T cells. T cells were exposed in vitro to soluble native F23.1 antibody, to heteroconjugates composed of the Fab fragments of F23.1 linked to Fab fragments of antibodies specific for Ia determinants on AC, or to F23.1 immobilized on an insoluble matrix. Soluble F23.1 antibody-induced proliferation in naive T cells only in the presence of both AC and exogenous IL-2, and these responses were confined to Lyt-2+ T cells. In contrast, heteroconjugates capable of crosslinking F23.1+ TCR to AC surface Ia determinants were capable of inducing proliferation in both L3T4+ and Lyt-2+ T cells in the absence of added lymphokine. Moreover, binding to and presumably multi-valent crosslinking of the TCR by immobilized F23.1 was sufficient to induce proliferation in both Lyt-2+ and L3T4+ T cells in the absence of AC or exogenous IL-2. Further, it was found that the conditions necessary for T cell growth factor secretion paralleled closely those required for induction of T cell proliferation in the absence of added lymphokine, suggesting that production of endogenous lymphokine might be the limiting process for triggering of T cell proliferation. Taken together, these findings suggest that under optimal conditions of TCR cross-linking, TCR occupancy and cross-linking is sufficient to deliver all of the signals necessary to initiate proliferation in naive populations of both L3T4+ and Lyt-2+ T cells. However, when conditions for TCR signaling are suboptimal, as may be the case for normal Ag-mediated stimulation, a role for second signals delivered by AC or exogenous lymphokines can become critical for T cell activation.

Activation of Lyt-2+ (CD8+) and L3T4+ (CD4+) T cell subsets by anti-receptor antibody

The mAb F23.1, specific for V beta 8-related determinants on the TCR, was used to study the requirements for TCR cross-linking and for accessory cells (AC) in the induction of proliferation or IL-2 responsiveness in L3T4+ (CD4+) and Lyt-2+ (CD8+) T cells. T cells were exposed in vitro to soluble native F23.1 antibody, to heteroconjugates composed of the Fab fragments of F23.1 linked to Fab fragments of antibodies specific for Ia determinants on AC, or to F23.1 immobilized on an insoluble matrix. Soluble F23.1 antibody-induced proliferation in naive T cells only in the presence of both AC and exogenous IL-2, and these responses were confined to Lyt-2+ T cells. In contrast, heteroconjugates capable of crosslinking F23.1+ TCR to AC surface Ia determinants were capable of inducing proliferation in both L3T4+ and Lyt-2+ T cells in the absence of added lymphokine. Moreover, binding to and presumably multi-valent crosslinking of the TCR by immobilized F23.1 was sufficient to induce proliferation in both Lyt-2+ and L3T4+ T cells in the absence of AC or exogenous IL-2. Further, it was found that the conditions necessary for T cell growth factor secretion paralleled closely those required for induction of T cell proliferation in the absence of added lymphokine, suggesting that production of endogenous lymphokine might be the limiting process for triggering of T cell proliferation. Taken together, these findings suggest that under optimal conditions of TCR cross-linking, TCR occupancy and cross-linking is sufficient to deliver all of the signals necessary to initiate proliferation in naive populations of both L3T4+ and Lyt-2+ T cells. However, when conditions for TCR signaling are suboptimal, as may be the case for normal Ag-mediated stimulation, a role for second signals delivered by AC or exogenous lymphokines can become critical for T cell activation.

Influence of helper T cells on the expression of a murine intrastrain crossreactive idiotype

The requirement for idiotype-specific helper T (Th) cells in the generation of a major intrastrain crossreactive idiotype was investigated. This idiotype, designated CRIA, is associated with a large proportion of anti-p-azobenzenearsonate (anti-Ar) antibodies in A/J mice. Secondary in vitro responses were studied. Using carrier-primed heterogeneous Th-cell populations, it was found that CRIA expression is determined by the mouse strain that provides the responding B cells and is independent of the strain of the Th cells functioning in vitro. Thus, A/J or A.BY (Ighe) B-plus-accessory-cell populations, primed in vivo to keyhole limpet hemocyanin-Ar (KLH-Ar), generated CRIA-dominant responses in vitro in the presence of KLH-Ar regardless of whether the KLH-primed Th cells were derived from CRIA+ strains (A/J or A.BY, Ighe) or CRIA- strains (B10.A or C57BL/10, Ighb). Further, when major histocompatibility complex-restricted, KLH-specific Th-cell clones were used, the CRIA dominance of the Ar-specific responses was again determined by the strain providing B plus accessory cells. Similar levels of expression of CRIA in Ar-specific antibodies were generated in the presence of heterogeneous or cloned Th cells. The results suggest that there is no absolute requirement for idiotype-specific Th cells in generating an Ar-specific secondary antibody response in vitro.

Helper T cell requirements for T15 idiotype expression on phosphorylcholine-specific antibodies

The requirement for idiotype-specific T cells was investigated in the T15 idiotype-dominant T cell-dependent response of unprimed BALB/c and (BALB/c X C57BL/6)F1 B cells to phosphorylcholine (PC). It was first demonstrated that cloned keyhole limpet hemocyanin (KLH)-specific, major histocompatibility complex (MHC)-restricted T helper (Th) cells as well as heterogeneous KLH-primed Th populations were capable of generating PC-specific antibody responses in T-depleted unprimed B cell populations cultured in the presence of PC-KLH. The PC-binding antibody responses generated under these conditions were indistinguishable when assayed for carrier-hapten linkage requirements, immunoglobulin isotype (predominantly IgM) or PC affinity. Further, it was observed that the PC-binding antibodies which were generated in the presence of these two T cell populations expressed equivalently high levels of T15 idiotype. Assaying antibody and idiotype by either enzyme-linked immunosorbent assay or plaque-forming cell assay yielded similar results. Since monoclonal MHC-restricted, KLH-specific Th cells presumably lack any additional T cell populations, these results argue against an absolute requirement for anti-idiotypic Th cells in the generation of T15-dominant antibody responses.

Recognition and response to alloantigens in vivo. II. Priming with accessory cell-depleted donor allogeneic splenocytes: induction of specific unresponsiveness to foreign major histocompatibility complex determinants

Although the role of non-T, non-B Ia+ accessory cells as the cells that stimulate alloreactive T cells in vitro has been carefully examined, the capacity of such accessory cells to trigger host T cells in vivo has been considerably less well studied. Therefore, to address the latter issue, this investigation compared the ability of accessory cell-containing and accessory cell-depleted donor cells to mediate in vivo negative and positive selection of mixed lymphocyte reaction (MLR)-responsive peripheral blood lymphocytes (PBL). It was observed that both unseparated and accessory cell-depleted (Sephadex G-10-passed) allogeneic splenocytes, which expressed similar levels of Ia antigens as detected by flow cytometry, were equally efficient in inducing temporary specific unresponsiveness to alloantigens in host PBL 24 hr after donor cell injection. In contrast, 4 days after priming with unseparated major histocompatibility complex (MHC)-incompatible splenocytes, specific MLR hyperresponsiveness was detected in the PBL of these recipient mice, whereas specific hyporesponsiveness was consistently noted in animals injected with accessory-cell depleted splenocytes. PBL obtained from this latter group of mice continued to be specifically reduced in MLR proliferative capacity throughout the culture period and for at least 13 days after administration of accessory cell-depleted allogeneic splenocytes. Mixture of these MLR hyporesponsive and hyperresponsive PBL did not identify suppressor cells as responsible for the specifically lowered proliferative potential. These findings are discussed in the context of discrete signals required for T cell recognition and T cell activation.

Recognition and response to alloantigens in vivo. II. Priming with accessory cell-depleted donor allogeneic splenocytes: induction of specific unresponsiveness to foreign major histocompatibility complex determinants

Although the role of non-T, non-B Ia+ accessory cells as the cells that stimulate alloreactive T cells in vitro has been carefully examined, the capacity of such accessory cells to trigger host T cells in vivo has been considerably less well studied. Therefore, to address the latter issue, this investigation compared the ability of accessory cell-containing and accessory cell-depleted donor cells to mediate in vivo negative and positive selection of mixed lymphocyte reaction (MLR)-responsive peripheral blood lymphocytes (PBL). It was observed that both unseparated and accessory cell-depleted (Sephadex G-10-passed) allogeneic splenocytes, which expressed similar levels of Ia antigens as detected by flow cytometry, were equally efficient in inducing temporary specific unresponsiveness to alloantigens in host PBL 24 hr after donor cell injection. In contrast, 4 days after priming with unseparated major histocompatibility complex (MHC)-incompatible splenocytes, specific MLR hyperresponsiveness was detected in the PBL of these recipient mice, whereas specific hyporesponsiveness was consistently noted in animals injected with accessory-cell depleted splenocytes. PBL obtained from this latter group of mice continued to be specifically reduced in MLR proliferative capacity throughout the culture period and for at least 13 days after administration of accessory cell-depleted allogeneic splenocytes. Mixture of these MLR hyporesponsive and hyperresponsive PBL did not identify suppressor cells as responsible for the specifically lowered proliferative potential. These findings are discussed in the context of discrete signals required for T cell recognition and T cell activation.

Regulatory mechanisms in cell-mediated immune response. V. Distinct Lyt subsets mediate antigen-specific and antigen-nonspecific suppression

These studies were undertaken to assess the T cell subpopulations mediating antigen-specific as well as antigen-nonspecific suppression of cytotoxic T lymphocyte (CTL) responses to alloantigens. It was first demonstrated that generation of the CTL response itself requires Lyt 1+2+ T cells. Subsequent studies then characterized two distinct suppressor pathways that regulate the CTL response. Antigen nonspecific suppression was mediated by Lyt 1+2-Ts cells that were generated during in vitro culture from an Lyt 1+2- precursor population. In a second pathway, antigen-specific suppression was mediated predominantly by Lyt 1-2+ Ts cells that were activated during the in vitro allosensitization of Lyt 1+2+ precursors. Thus, the in vitro CTL response to alloantigens is modulated by two different pathways of T-cell-mediated suppression, and these pathways are mediated by distinct Lyt-defined T cell subpopulations.