A clonotype-independent pre-TCR complex and its downstream signals can support transformation of thymic T cells

The kinase ataxia telangiectasia mutated (ATM) communicates between molecules that sense DNA damage in the form of double stranded breaks (DSB) and effector mechanisms that maintain genomic integrity. Persistent DSBs can lead to transformation and ATM-deficiency is associated with increased lymphoid malignancies in both humans and mice. ATM-deficient mice exclusively develop thymic T cell lymphomas with high penetrance. We explored the requirement for specific components of the TCR and its signaling pathway for the induction of these thymic lymphomas. We generated multiple strains of ATM-deficient mice in which mature T cell are absent by deletion of CD3ɛ, an invariant component of the pre-TCR complex; by deletion of RAG or TCRβ, in which recombining TCR variable components are deleted; or by deletion of LAT, an important component of pre-TCR and TCR signaling. These strains all showed a similar and severe developmental arrest at thymic DN3. Despite this arrest, mice deficient in RAG or TCRβ, exclusively developed thymic T cell lymphomas. In contrast, mice deficient in CD3ɛ or LAT, developed no or few T cell tumors, respectively, but developed B cell lymphomas. These findings suggest the importance of a clonotype-independent TCR complex and its downstream signals in transformation of T cells. Experiments are in progress to ask if survival of ATMKO T cell lymphomas after transformation is similarly dependent on CD3ɛ and LAT signaling.

Mechanisms of PTEN inactivation and pAKT dependence in ATM-deficient thymic T cell lymphomas

Ataxia telangiectasia mutated (ATM) is a serine-threonine kinase that is essential for maintaining genomic integrity in response to DNA double-strand breaks. AT patients and ATM-deficient mice develop lymphoid malignancies at high frequency, establishing ATM’s role as a tumor suppressor. It was previously reported that murine Atm−/− T cell lymphomas have a recurrent loss of the gene encoding phosphatase and tensin homolog (PTEN), a tumor suppressor that plays a critical role in inhibiting the PI3K/AKT/mTOR pathway. Our analyses of Atm−/− T cell lymphomas revealed that 12/13 express high levels of phospho-AKT (pAKT). This upregulation of AKT signaling is consistent with the finding that 11/12 pAKThigh tumors lack PTEN protein. One tumor was PTEN+ but failed to downregulate pAKT. Here, we study the Atm−/− tumor model in order to uncover the mechanism by which PTEN expression is ablated. Interestingly, our analyses found that 9/11 PTEN-deficient cell lines have incurred a variety of alterations including exonal deletions of pten or truncations of its mRNA, providing a basis for PTEN loss-of-function. The remaining three lines, however, possess full-length Pten exomes and transcripts. Sequencing the Pten mRNA of these cell lines is in progress to identify mechanisms responsible for the absence of PTEN protein. We have further observed that in vitro treatment of Atm−/− T cell lymphomas with the pAKT inhibitor MK2206 resulted in substantial impairment of survival, consistent with their dependence on activated AKT in the absence of regulation by PTEN. Taken together, these results demonstrate that the majority of Atm−/− T cell lymphomas lose PTEN function through multiple mechanisms and become dependent on AKT signaling for their survival.

The requirement for pre-TCR during thymic differentiation enforces a developmental pause that is essential for V-DJβ rearrangement

T cell development occurs in the thymus and is critically dependent on productive TCRβ rearrangement and pre-TCR expression in DN3 cells. The requirement for pre-TCR expression results in the arrest of thymocytes at the DN3 stage (β checkpoint), which is uniquely permissive for V-DJβ recombination; only cells expressing pre-TCR survive and develop beyond the DN3 stage. In addition, the requirement for TCRβ rearrangement and pre-TCR expression enforces suppression of TCRβ rearrangement on a second allele, allelic exclusion, thus ensuring that each T cell expresses only a single TCRβ product. However, it is not known whether pre-TCR expression is essential for allelic exclusion or alternatively if allelic exclusion is enforced by developmental changes that can occur in the absence of pre-TCR. We asked if thymocytes that were differentiated without pre-TCR expression, and therefore without pause at the β checkpoint, would suppress all V-DJβ rearrangement. We previously reported that premature CD28 signaling in murine CD4(-)CD8(-) (DN) thymocytes supports differentiation of CD4(+)CD8(+) (DP) cells in the absence of pre-TCR expression. The present study uses this model to define requirements for TCRβ rearrangement and allelic exclusion. We demonstrate that if cells exit the DN3 developmental stage before TCRβ rearrangement occurs, V-DJβ rearrangement never occurs, even in DP cells that are permissive for D-Jβ and TCRα rearrangement. These results demonstrate that pre-TCR expression is not essential for thymic differentiation to DP cells or for V-DJβ suppression. However, the requirement for pre-TCR signals and the exclusion of alternative stimuli such as CD28 enforce a developmental "pause" in early DN3 cells that is essential for productive TCRβ rearrangement to occur.

Both telomeric and non-telomeric DNA damage are determinants of mammalian cellular senescence

Background

Cellular senescence is a state reached by normal mammalian cells after a finite number of cell divisions and is characterized by morphological and physiological changes including terminal cell-cycle arrest. The limits on cell division imposed by senescence may play an important role in both organismal aging and in preventing tumorigenesis. Cellular senescence and organismal aging are both accompanied by increased DNA damage, seen as the formation of γ-H2AX foci (γ-foci), which may be found on uncapped telomeres or at non-telomeric sites of DNA damage. However, the relative importance of telomere- and non-telomere-associated DNA damage to inducing senescence has never been demonstrated. Here we present a new approach to determine accurately the chromosomal location of γ-foci and quantify the number of telomeric versus non-telomeric γ-foci associated with senescence in both human and mouse cells. This approach enables researchers to obtain accurate values and to avoid various possible misestimates inherent in earlier methods.

Results

Using combined immunofluorescence and telomere fluorescence in situ hybridization on metaphase chromosomes, we show that human cellular senescence is not solely determined by telomeric DNA damage. In addition, mouse cellular senescence is not solely determined by non-telomeric DNA damage. By comparing cells from different generations of telomerase-null mice with human cells, we show that cells from late generation telomerase-null mice, which have substantially short telomeres, contain mostly telomeric γ-foci. Most notably, we report that, as human and mouse cells approach senescence, all cells exhibit similar numbers of total γ-foci per cell, irrespective of chromosomal locations.

Conclusion

Our results suggest that the chromosome location of senescence-related γ-foci is determined by the telomere length rather than species differences per se. In addition, our data indicate that both telomeric and non-telomeric DNA damage responses play equivalent roles in signaling the initiation of cellular senescence and organismal aging. These data have important implications in the study of mechanisms to induce or delay cellular senescence in different species.

Depletion of accessory cells by adherence to Sephadex G-10

Accessory cells such as macrophages and other antigen-presenting cells (APC) in suspensions of mouse spleen and lymph node are removed by means of their preferential adherence to Sephadex G-10 polymers. Unlike the use of nylon wool columns to remove B and accessory cells (UNIT 3.2), Sephadex G-10 columns are employed when the main purpose is to yield functional populations of T cells and B cells (by depleting mostly macrophages and other APC). The following technique is simple, fast, and reproducible; however, it results in some retention of B cells on the resin. The basic protocol in this unit involves assembling the column, loading and equilibrating the sterile Sephadex slurry, and passing the cells through the column followed by collection and analysis of eluate. Two support protocols describe preparation of the Sephadex G-10 slurry for use in the column by fining and sterilization, plus recycling resin from used columns.

T cell enrichment by nonadherence to nylon

Differential adherence properties of T cells, B cells, and accessory cells such as macrophages or other APC (antigen-presenting cells) can be employed to enrich for T cells. While the basis for this adherence is not understood, either nylon wool or Sephadex G-10 can be used as column matrices to which B cells and accessory cells will more readily adhere than T cells. Such procedures offer a convenient, although imprecise, means of enriching T cells through removal of accessory and B cells; use of nylon wool is preferred if both of the latter subsets are to be removed, while Sephadex is used when the goal is primarily to remove accessory cells.

T cell depletion by cytotoxic elimination

This unit describes the complete removal of T cells from lymphocyte preparations based on the presence of the glycoprotein Thy-1 on the cell surface of T lymphocytes. As in UNIT 3.3, cytotoxic elimination is employed; however, Thy-1-specific antibodies are used rather than MHC class II-specific antibodies so that T cells are eliminated rather than B cells and accessory cells. Murine T cells can be further fractionated into subpopulations based on their expression of cell differentiation markers CD4 (Leu3, L3T4, T4) and CD8 (Leu2, Lyt2,3). A protocol in this unit describes removal of T cell subsets by cytotoxic elimination using CD-specific antibodies. An alternate but technically more demanding method of enriching T cell subsets is the panning procedure described in UNIT 3.5.

Persistence of tumor infiltrating lymphocytes in adoptive immunotherapy correlates with telomere length

Transfer of autologous tumor-specific tumor infiltrating lymphocytes (TILs) in adoptive immunotherapy can mediate the regression of tumor in patients with metastatic melanoma. In this procedure, TILs from resected tumors are expanded in vitro, then administered to patients and further stimulated to proliferate in vivo by the administration of high dose IL-2. After in vitro expansion, TILs are often dominated by a few specific clonotypes, and recently it was reported that the persistence in vivo of one or more of these clonotypes correlated with positive therapeutic response. We and others have previously shown that repeated in vitro stimulation and clonal expansion of normal human T lymphocytes results in progressive decrease in telomerase activity and shortening of telomeres, ultimately resulting in replicative senescence. In the studies reported here, we therefore compared telomerase activity and telomere length in persistent and nonpersistent TIL clonotypes before transfer in vivo, and found a correlation between telomere length and clonal persistence. We also observed that TILs proliferate extensively in vivo in the days after transfer, but fail to induce substantial telomerase activity, and undergo rapid decreases in telomere length within days after transfer. Thus, in vivo loss of telomeres by clonotypes that have the shortest telomeres at the time of administration may drive these clones to replicative senescence, whereas cells with longer telomeres are able to persist and mediate antitumor effects. These findings are relevant both to predicting effectiveness of adoptive immunotherapy and in deriving strategies for improving effectiveness by sustaining telomere length.

Abnormal differentiation of memory T cells in systemic lupus erythematosus

OBJECTIVE

The chemokine receptor CCR7 and the tumor necrosis factor receptor family member CD27 define 3 distinct, progressively more differentiated maturational stages of CD4 memory subpopulations in healthy individuals: the CCR7+, CD27+, the CCR7-, CD27+, and the CCR7-, CD27- populations. The goal of this study was to examine maturational disturbances in CD4 T cell differentiation in systemic lupus erythematosus (SLE), using these phenotypic markers.

METHODS

Phenotypic analysis by flow cytometry, in vitro stimulation experiments, telomere length measurement, and determination of inducible telomerase were carried out. RESULTS. In SLE patients, significant increases of CCR7-, CD27- and CCR7-, CD27+ and a reduction of CCR7+, CD27+ CD4 memory T cells were found. In vitro stimulation of SLE T cells showed a stepwise differentiation from naive to CCR7+, CD27+ to CCR7-, CD27+ to CCR7-, CD27-; telomere length and inducible telomerase decreased in these subsets in the same progressive sequence. The in vitro proliferative response of these populations progressively declined as their susceptibility to apoptosis increased. Interestingly, a significant reduction in inducible telomerase was noted in SLE naive and CCR7+, CD27+ CD4+ memory T cells. Additionally, SLE CCR7-, CD27+ and CCR7-, CD27- CD4 memory T cells proliferated poorly in response to in vitro stimulation and underwent significantly more apoptosis than their normal counterparts. Finally, expression of CXCR4 was significantly reduced in all SLE subsets compared with normal.

CONCLUSION

Together these data indicate an increased degree of in vivo T cell stimulation in SLE, resulting in the accumulation of terminally differentiated memory T cells with a decreased proliferative capacity and an increased tendency to undergo apoptosis upon stimulation.

Regulated costimulation in the thymus is critical for T cell development: dysregulated CD28 costimulation can bypass the pre-TCR checkpoint

Expression of CD28 is highly regulated during thymic development, with CD28 levels extremely low on immature thymocytes but increasing dramatically as CD4- CD8- cells initiate expression of TCRbeta. B7-1 and B7-2, the ligands for CD28, have a restricted distribution in the thymic cortex where immature thymocytes reside and are more highly expressed in the medulla where the most mature thymocytes are located. To determine the importance of this regulated CD28/B7 expression for T cell development, we examined the effect of induced CD28 signaling of immature thymocytes in CD28/B7-2 double-transgenic mice. Strikingly, we found that differentiation to the CD4+ CD8+ stage in CD28/B7-2 transgenics proceeds independent of the requirement for TCRbeta expression manifest in wild-type thymocytes, occurring even in Rag- or CD3epsilon- knockouts. These findings indicate that signaling of immature thymocytes through CD28 in the absence of TCR- or pre-TCR-derived signals can promote an aberrant pathway of T cell differentiation and highlight the importance of finely regulated physiologic expression of CD28 and B7 in maintaining integrity of the "beta" checkpoint for pre-TCR/TCR-dependent thymic differentiation.

Stepwise Differentiation of CD4 Memory T Cells Defined by Expression of CCR7 and CD27

To study the steps in the differentiation of human memory CD4 T cells, we characterized the functional and lineage relationships of three distinct memory CD4 subpopulations distinguished by their expression of the cysteine chemokine receptor CCR7 and the TNFR family member CD27. Using the combination of these phenotypic markers, three populations were defined: the CCR7+CD27+, the CCR7-CD27+, and the CCR7-CD27- population. In vitro stimulation led to a stepwise differentiation from naive to CCR7+CD27+ to CCR7-CD27+ to CCR7-CD27-. Telomere length in these subsets differed significantly (CCR7+CD27+ > CCR7-CD27+ > CCR7-CD27-), suggesting that these subsets constituted a differentiative pathway with progressive telomere shortening reflecting antecedent in vivo proliferation. The in vitro proliferative response of these populations declined, and their susceptibility to apoptosis increased progressively along this differentiation pathway. Cytokine secretion showed a differential functional capacity of these subsets. High production of IL-10 was only observed in CCR7+CD27+, whereas IFN-gamma was produced by CCR7-CD27+ and to a slightly lesser extent by CCR7-CD27- T cells. IL-4 secretion was predominantly conducted by CCR7-CD27- memory CD4 T cells. Thus, by using both CCR7 and CD27, distinct maturational stages of CD4 memory T cells with different functional activities were defined.

In vivo regulation of telomerase activity and telomere length

Telomeres are specialized DNA-protein structures found at the ends of all linear chromosomes. In mammalian cells, they consist of hexanucleotide (TTAGGG) repeats and multiple associated proteins. Telomeres protect the ends of chromosomes and prevent their recognition as DNA breaks. Loss of functional telomere length below a critical threshold can activate programs leading to cell senescence or death. Telomere length represents a balance between the loss of terminal telomeric repeats, which occurs during cell division with incomplete DNA replication, and the addition of telomeric repeats by the unique RNA-dependent DNA polymerase telomerase. Although most somatic cells do not express telomerase, telomerase is induced in lymphocytes at critical stages of development and activation. Telomerase expression thus may prolong the replicative capacity of lymphocytes and thereby enhance their function in immune responses. We have used murine model systems to address two broadly defined questions about lymphocyte telomere biology: how is telomerase physiologically regulated in T cells responding to antigen challenge, and what is the effect of transcriptionally altered telomerase expression on telomere length and, consequently, on immune function?

Telomere biology and immune system

Extract: In the 1960s Hayflick reported that normal human somatic cells have a finite replicative capacity and that, after a limited number of cell divisions, cells no longer divide and enter a state termed senescence. Since this observation was made, considerable research energy has focused on identifying molecular pathways that regulate the proliferative capacity of normal somatic (body) cells. One molecular mechanism that has been implicated in the regulation of somatic cell proliferation is mediated by telomeres -- specialized DNA-protein structures that cap the ends of all linear chromosomes. In mammalian cells, telomeres are composed of hexanucleotide repeats (TTAGGG) and a variety of associated proteins. Although all mammalian telomeres are composed of these (TTAGGG) repeats, telomere length varies substantially between different species. The higher order chromatin structure formed by telomeres functions to protect chromosomes ends from degradation and activation of DNA-repair pathways. In the absence of compensatory mechanisms, telomeres shorten progressively with successive rounds of cell division as a result of incomplete replication of telomeric termini, until they reach a critically short length that is no longer protective. Cells that lack protective telomeres fail to proliferate, and they undergo senescence or apoptosis.

Expression of telomerase RNA template, but not telomerase reverse transcriptase, is limiting for telomere length maintenance in vivo

Telomerase consists of two essential components, the telomerase RNA template (TR) and telomerase reverse transcriptase (TERT). The haplo-insufficiency of TR was recently shown to cause one form of human dyskeratosis congenita, an inherited disease marked by abnormal telomere shortening. Consistent with this finding, we recently reported that mice heterozygous for inactivation of mouse TR exhibit a similar haplo-insufficiency and are deficient in the ability to elongate telomeres in vivo. To further assess the genetic regulation of telomerase activity, we have compared the abilities of TR-deficient and TERT-deficient mice to maintain or elongate telomeres in interspecies crosses. Homozygous TERT knockout mice had no telomerase activity and failed to maintain telomere length. In contrast, TERT(+/-) heterozygotes had no detectable defect in telomere elongation compared to wild-type controls, whereas TR(+/-) heterozygotes were deficient in telomere elongation. Levels of TERT mRNA in heterozygous mice were one-third to one-half the levels expressed in wild-type mice, similar to the reductions in telomerase RNA observed in TR heterozygotes. These findings indicate that both TR and TERT are essential for telomere maintenance and elongation but that gene copy number and transcriptional regulation of TR, but not TERT, are limiting for telomerase activity under the in vivo conditions analyzed.

Induction of telomerase activity and maintenance of telomere length in virus-specific effector and memory CD8+ T cells

Acute viral infections induce extensive proliferation and differentiation of virus-specific CD8+ T cells. One mechanism reported to regulate the proliferative capacity of activated lymphocytes is mediated by the effect of telomerase in maintaining the length of telomeres in proliferating cells. We examined the regulation of telomerase activity and telomere length in naive CD8+ T cells and in virus-specific CD8+ T cells isolated from mice infected with lymphocytic choriomeningitis virus. These studies reveal that, compared with naive CD8+ T cells, which express little or no telomerase activity, Ag-specific effector and long-lived memory CD8+ T cells express high levels of telomerase activity. Despite the extensive clonal expansion that occurs during acute lymphocytic choriomeningitis virus infection, telomere length is maintained in both effector and memory CD8+ T cells. These results suggest that induction of telomerase activity in Ag-specific effector and memory CD8+ T cells is important for the extensive clonal expansion of both primary and secondary effector cells and for the maintenance and longevity of the memory CD8+ T cell population.

Telomeres in T and B cells

Telomeres are the structures at the ends of linear chromosomes. In mammalian cells, they consist of hexanucleotide (TTAGGG) repeats, together with many associated proteins. In the absence of a compensatory mechanism, dividing cells undergo gradual telomere erosion until a critical degree of shortening results in chromosomal abnormalities and cell death or senescence. For T and B cells, the ability to undergo extensive cell division and clonal expansion is crucial for effective immune function. This article describes our current understanding of telomere-length regulation in lymphocytes and its implications for immune function.

Haploinsufficiency of mTR results in defects in telomere elongation

Telomeres are usually maintained about an equilibrium length, and the set point for this equilibrium differs between species and between strains of a given species. To examine the requirement for telomerase in mediating establishment of a new telomere length equilibrium, we generated interspecies crosses with telomerase mTR knockout mice. In crosses between C57BL/6J (B6) and either of two unrelated mouse species, CAST/Ei and SPRET/Ei, telomerase mediated establishment of a new telomere length equilibrium in wild-type mTR(+/+) mice. This new equilibrium was characterized by elongation of the short telomeres of CAST/Ei or SPRET/Ei origin. In contrast, mTR(-/-) offspring of interspecies crosses failed to elongate telomeres. Unexpectedly, haploinsufficiency was observed in mTR(+/-) heterozygous interspecies mice, which had an impaired ability to elongate short SPRET/Ei or CAST/Ei telomeres to the new equilibrium set point that was achieved in wild-type mTR(+/+) mice. These results demonstrate that elongation of telomeres to a new telomere set point requires telomerase and indicate that telomerase RNA may be limiting in vivo.

Blockade of costimulation through B7/CD28 inhibits experimental autoimmune uveoretinitis, but does not induce long-term tolerance

It has been reported that costimulation blockade can result in T cell anergy. We investigated the effects of blocking costimulatory molecules in vivo on the development of experimental autoimmune uveoretinitis (EAU), a model for autoimmune uveitis in humans that is induced in mice by immunization with the retinal Ag interphotoreceptor retinoid binding protein. B10.A mice immunized with a uveitogenic regimen of interphotoreceptor retinoid-binding protein were treated with Abs to B7.1 and B7.2 for 2 wk. Evaluation of EAU and immunological responses 1 wk later showed that disease had been abrogated, and cellular responses were suppressed. To determine whether the costimulation blockade resulted in tolerance, adult-thymectomized mice immunized for uveitis and treated with anti-B7 or anti-CD28 were rechallenged for uveitis induction 5 wk after the initial immunization. Although confirmed to be disease free after the initial immunization, both anti-B7- and anti-CD28-treated mice developed severe EAU and elevated cellular responses after the rechallenge, equivalent to those of control mice. We conclude that in this model costimulatory blockade in vivo prevents the development of autoimmune disease, but does not result in long-term tolerance. The data are compatible with the interpretation that B7/CD28 blockade prevents generation of effector, but not of memory, T cells.

Telomere length regulation in mice is linked to a novel chromosome locus

Little is known about the mechanisms that regulate species-specific telomere length, particularly in mammalian species. The genetic regulation of telomere length was therefore investigated by using two inter-fertile species of mice, which differ in their telomere length. Mus musculus (telomere length >25 kb) and Mus spretus (telomere length 5-15 kb) were used to generate F1 crosses and reciprocal backcrosses, which were then analyzed for regulation of telomere length. This analysis indicated that a dominant and trans-acting mechanism exists capable of extensive elongation of telomeres in somatic cells after fusion of parental germline cells with discrepant telomere lengths. A genome wide screen of interspecific crosses, using M. spretus as the recurrent parent, identified a 5-centimorgan region on distal chromosome 2 that predominantly controls the observed species-specific telomere length regulation. This locus is distinct from candidate genes encoding known telomere-binding proteins or telomerase components. These results demonstrate that an unidentified gene(s) mapped to distal chromosome 2 regulates telomere length in the mouse.

Cutting Edge: Antigen-Dependent Regulation of Telomerase Activity in Murine T Cells

Telomeres, structures on the ends of linear chromosomes, function to maintain chromosomal integrity. Telomere shortening occurs with cell division and provides a mechanism for limiting the replicative potential of normal human somatic cells. Telomerase, a ribonucleoprotein enzyme, synthesizes telomeric repeats on chromosomal termini, potentially extending the capacity for cell division. The present study demonstrates that resting T cells express little/no activity, and optimal Ag-specific induction of telomerase activity in vitro requires both TCR and CD28-B7 costimulatory signals. Regulation of telomerase in T cells during in vivo Ag-dependent activation was also assessed by adoptive transfer of TCR transgenic T cells and subsequent Ag challenge. Under these conditions, telomerase was induced in transgenic T cells coincident with a phase of extensive clonal expansion. These findings suggest that telomerase may represent an adoptive response that functions to preserve replicative potential in Ag-reactive lymphocytes.

Cutting edge: antigen-dependent regulation of telomerase activity in murine T cells

Telomeres, structures on the ends of linear chromosomes, function to maintain chromosomal integrity. Telomere shortening occurs with cell division and provides a mechanism for limiting the replicative potential of normal human somatic cells. Telomerase, a ribonucleoprotein enzyme, synthesizes telomeric repeats on chromosomal termini, potentially extending the capacity for cell division. The present study demonstrates that resting T cells express little/no activity, and optimal Ag-specific induction of telomerase activity in vitro requires both TCR and CD28-B7 costimulatory signals. Regulation of telomerase in T cells during in vivo Ag-dependent activation was also assessed by adoptive transfer of TCR transgenic T cells and subsequent Ag challenge. Under these conditions, telomerase was induced in transgenic T cells coincident with a phase of extensive clonal expansion. These findings suggest that telomerase may represent an adoptive response that functions to preserve replicative potential in Ag-reactive lymphocytes.

Inhibition of transplant rejection following treatment with anti-B7-2 and anti-B7-1 antibodies

Antigen-specific T cell activation depends initially on the interaction of the T cell receptor (TCR) with peptide/MHC. In addition, a costimulatory signal, mediated by distinct cell surface accessory molecules, is required for complete T cell activation leading to lymphokine production and proliferation. CD28 has been implicated as the major receptor on T cells responsible for delivering the costimulatory signal. Although two distinct ligands for CD28, B7-1 and B7-2, have been identified on antigen-presenting cells (APC), the co-stimulatory role of each molecule during a physiological immune response remains unresolved. In the present study, the relative roles of B7-1 and B7-2 interactions were evaluated in an allogeneic pancreatic islet transplant setting. In isolation, anti-B7-2 mAbs and, to a much lesser degree, anti-B7-1 mAbs suppressed T cell proliferative responses to allogeneic islets or splenic APC in vitro. Maximal inhibition of the allogeneic response was observed using a combination of the anti-B7-1 and anti-B7-2 mAbs. Administration of anti-B7-2 but not anti-B7-1 mAbs prolonged C3H allograft survival in B6 recipients, with a combination of both mAbs significantly prolonging rejection beyond either mAb alone. The immunosuppressive effects of the in vivo mAb treatment were not manifested in in vitro analyses as T cells isolated from suppressed mice responded normally to allogeneic stimuli in terms of both proliferation and lymphokine production. However, combined mAb therapy in vivo selectively delayed CD4+ T lymphocyte infiltration into the graft. These data suggest that both B7-1 and B7-2 costimulatory molecules are active in vivo, although B7-2 plays a clearly dominant role in this allograft model. The mechanism of immune suppression in vivo remains unresolved but may occur at sites distinct from the allograft.

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.