Strength of signaling by CD4 and CD8 coreceptor tails determines the number but not the lineage direction of positively selected thymocytes

The present study has assessed the impact of the intracellular domains of CD4 and CD8 on positive selection and lineage direction of MHC class I-restricted thymocytes. Contrary to current presumption, we found that the CD4 tail promotes the generation of both CD4+ and CD8+ T cells without preference for the CD4+ T cell lineage. We also found that the identity of the coreceptor tail and hence the strength of coreceptor signaling determine the number of thymocytes undergoing positive selection but not their ultimate CD4/CD8 phenotype. These findings demonstrate that the strength of coreceptor signaling has a significant quantitative but not qualitative impact on positive selection and provide a simple explanation for the greater numbers of CD4+ than CD8+ T cells selected in the normal thymus.

Synchronous deletion of Mtv-superantigen-reactive thymocytes in the CD3(medium/high) CD4(+)CD8(+) subset

Multiple model systems have demonstrated that negatively selected thymocytes can be deleted during the immature CD4(+)CD8(+) CD3(low) stage after high affinity interaction of T-cell receptors (TCRs) with antigen:major histocompatibility complex (MHC) complexes. Superantigens (SAGs) derived from endogenous mammary tumour viruses (Mtv) induce negative selection of Mtv-SAG-reactive thymocytes regardless of which peptide antigen is presented by MHC molecules. In this study, the timing of deletion of multiple subsets of Mtv-SAG-reactive CD4(+)CD8(+) thymocytes was investigated by a 4 colour flow cytometry in SJL x CBA/J cross-bred mice. Deletion of V beta 3(+), V beta 5(+), V beta 11(+), and V beta 17(+) Mtv-SAG-reactive thymocytes was found to occur synchronously in the most mature CD3(medium) and early CD3(high) subsets of CD4(+)CD8(+) thymocytes, in contrast with reports showing that the deletion of Mtv-SAG-reactive thymocytes can occur at different stages in particular model systems.

Coreceptor reversal in the thymus: signaled CD4+8+ thymocytes initially terminate CD8 transcription even when differentiating into CD8+ T cells

A central paradigm of T cell development is that CD4+8+ (DP) thymocytes differentiate into CD4+ or CD8+ T cells in response to intrathymic signals that extinguish transcription of the inappropriate coreceptor molecule. Contrary to this prevailing paradigm, we now demonstrate that signaled DP thymocytes initially terminate CD8 transcription even when differentiating into CD8+ T cells. Remarkably, thymocytes that have selectively terminated CD8 transcription can be signaled by IL-7 to differentiate into CD8+ T cells by silencing CD4 transcription and reinitiating CD8 transcription, events we refer to as "coreceptor reversal." These observations significantly alter our understanding of CD8+ T cell differentiation and lead to a new perspective ("kinetic signaling") on CD4/CD8 lineage determination in the thymus. These observations also suggest a novel mechanism by which bipotential cells throughout development can determine their appropriate cell fate.

CD8 coreceptor extinction in signaled CD4(+)CD8(+) thymocytes: coordinate roles for both transcriptional and posttranscriptional regulatory mechanisms in developing thymocytes

T-cell development in the thymus is characterized by changing expression patterns of CD4 and CD8 coreceptor molecules and by changes in CD4 and CD8 gene transcription. In response to T-cell receptor (TCR) signals, thymocytes progress through developmental transitions, such as conversion of CD4(+)CD8(+) (double-positive [DP]) thymocytes into intermediate CD4(+)CD8(-) thymocytes, that appear to require more-rapid changes in coreceptor expression than can be accomplished by transcriptional regulation alone. Consequently, we considered the possibility that TCR stimulation of DP thymocytes not only affects coreceptor gene transcription but also affects coreceptor RNA stability. Indeed, we found that TCR signals in DP thymocytes rapidly destabilized preexisting CD4 and CD8 coreceptor RNAs, resulting in their rapid elimination. Destabilization of coreceptor RNA was shown for CD8alpha to be dependent on target sequences in the noncoding region of the RNA. TCR signals also differentially affected coreceptor gene transcription in DP thymocytes, terminating CD8alpha gene transcription but only transiently reducing CD4 gene transcription. Thus, posttranscriptional and transcriptional regulatory mechanisms act coordinately in signaled DP thymocytes to promote the rapid conversion of these cells into intermediate CD4(+)CD8(-) thymocytes. We suggest that destabilization of preexisting coreceptor RNAs is a mechanism by which coreceptor expression in developing thymocytes is rapidly altered at critical points in the differentiation of these cells.

Positive selection as a developmental progression initiated by alpha beta TCR signals that fix TCR specificity prior to lineage commitment

During positive selection, immature thymocytes commit to either the CD4+ or CD8+ T cell lineage ("commitment") and convert from short-lived thymocytes into long-lived T cells ("rescue"). By formal precursor-progeny analysis, we now identify what is likely to be the initial positive selection step signaled by alpha beta TCR, which we have termed "induction". During induction, RAG mRNA expression is downregulated, but lineage commitment does not occur. Rather, lineage commitment (which depends upon the MHC class specificity of the alpha beta TCR) only occurs after downregulation of RAG expression and the consequent fixation of alpha beta TCR specificity. We propose that positive selection can be viewed as a sequence of increasingly selective developmental steps (induction-->commitment-->rescue) that are signaled by alpha beta TCR engagements of intrathymic ligands.

Developmentally regulated expression of peanut agglutinin (PNA)-specific glycans on murine thymocytes

Intrathymic maturation of T lymphocytes is characterized by variable expression of O-linked Gal beta 1,3GalNAc glycans reactive with peanut agglutinin (PNA) lectin. Recent studies on human thymocytes show that conversion from PNA+ to PNA- phenotype is correlated with increased expression of alpha 2,3 O-linked sialyltransferase (ST), which sialylates Gal beta 1,3GalNAc glycans, masking their binding sites for PNA. Interestingly, alpha 2,3 O-linked ST expression is highest within the regions of the thymus containing the most immature and most mature thymocyte subsets, suggesting that PNA-specific glycans are intermittently masked by sialylation during thymic selection processes. Here, we studied expression of PNA receptors on developing thymocytes in the murine system using thymocytes from both normal mice and transgenic mice that are genetically arrested at the early phases of T cell development. Our results confirm and extend recent findings in the human system by showing that murine T cells sequentially progress from PNAlo-->PNAhi-->PNAlo stages during their differentiation within the thymus. In addition, our data demonstrate that a similar set of polypeptides is variably masked by sialylation throughout T cell development.

Surface molecules that drive T cell development in vitro in the absence of thymic epithelium and in the absence of lineage-specific signals

Differentiation of immature double positive (DP) CD4+ CD8+ thymocytes into single positive (SP) CD4+ and CD8+ T cells is referred to as positive selection and requires physical contact with thymic cortical epithelium. We now have identified "coinducer" molecules on DP thymocytes that, together with TCR, signal DP thymocytes to differentiate into SP T cells in vitro in the absence of thymic epithelium. A remarkable number of different molecules on DP thymocytes possessed "coinducing" activity, including CD2, CD5, CD24, CD28, CD49d, CD81, and TSA-1. Interestingly, in vitro differentiation occurred in the absence of lineage-specific signals, yet resulted in the selective generation of CD4+CD8- T cells. Thus, the present study has identified surface molecules that can signal DP thymocytes to differentiate into SP T cells in the absence of thymic epithelium and has characterized a default pathway for CD4+ T cell differentiation.

Lineage commitment in the thymus: only the most differentiated (TCRhibcl-2hi) subset of CD4+CD8+ thymocytes has selectively terminated CD4 or CD8 synthesis

Lineage commitment is a developmental process by which individual CD4+CD8+ (double positive, DP) thymocytes make a decision to differentiate into either CD4+ or CD8+ T cells. However, the molecular event(s) that defines lineage commitment is controversial. We have previously proposed that lineage commitment in DP thymocytes can be molecularly defined as the selective termination of CD4 or CD8 coreceptor synthesis. The present study supports such a molecular definition by showing that termination of either CD4 or CD8 synthesis is a highly regulated event that is only evident within the most differentiated DP subset (CD5hiCD69hiTCRhibcl-2hi). In fact, essentially all cells within this DP subset actively synthesize only one coreceptor molecule. In addition, the present results identify three distinct sub-populations of DP thymocytes that define the developmental progression of the lineage commitment process and demonstrate that lineage commitment is coincident with upregulation of TCR and bcl-2. Thus, this study supports a molecular definition of lineage commitment and uniquely identifies TCRhibcl-2hi DP thymocytes as cells that are already committed to either the CD4 or CD8 T cell lineage.

Identification of CD8 as a peanut agglutinin (PNA) receptor molecule on immature thymocytes

Differentiation of most T lymphocytes occurs within the thymus and is characterized by variable expression of CD4/CD8 coreceptor molecules, increased surface density of T cell antigen receptor (TCR) alpha beta proteins, and decreased expression of glycan chains recognized by the galactose-specific lectin peanut agglutinin (PNA). Although appreciated for several decades that PNA agglutination is useful for the physical separation of immature and mature thymocyte sub-populations, the identity of specific PNA-binding glycoproteins expressed on immature thymocytes remains to be determined. In the current report, we studied the expression of PNA-specific glycans on immature and mature T cells and used lectin affinity chromatography and immunoprecipitation techniques to characterize PNA-binding glycoproteins on thymocytes. Our data demonstrate that PNA-specific glycans are localized on a relatively small subset of thymocyte surface proteins, several of which were specifically identified, including CD43, CD45, and suprisingly, CD8 molecules. CD8 alpha and CD8 alpha' proteins bound to PNA in the absence of CD8 beta expression showing that O-glycans on CD8 beta glycoproteins are not necessary for PNA binding and that glycosylation of CD8 alpha and CD8 alpha' proteins proceeds effectively in the absence of CD8 beta. Finally, we demonstrate that PNA binding of CD8 is developmentally regulated by sialic acid addition as CD8 proteins from mature T cells bound to PNA only after sialidase treatment. These studies identify CD8 as a PNA receptor molecule on immature thymocytes and show that PNA binding of CD8 on immature and mature T cells is developmentally regulated by sialic acid modification.

Thymic-independent T cell regeneration occurs via antigen-driven expansion of peripheral T cells resulting in a repertoire that is limited in diversity and prone to skewing

Thymic regenerative capacity in humans decreases with age, suggesting that thymic-independent pathways of T cell regeneration may predominate during adulthood. Using a murine bone marrow transplantation model, we present evidence that thymic-independent T cell regeneration occurs primarily via expansion of peripheral T cells and is Ag driven since significant expansion of CD4+ or CD8+ transgenic (Tg+)/TCR-bearing cells occurs only in the presence of Ag specific for the TCR. Such expansion resulted in skewing of the regenerated repertoire with 40 to 65% of the regenerated CD4+ or CD8+ T cells expressing the Tg+/TCR in thymectomized hosts after bone marrow transplantation. In experiments in which nontransgenic population are used as T cell inocula, we noted decreased CD4 expansion when Class II MHC was blocked by mAb treatment in vivo, an CD8 expansion failed to occur in Class I MHC-deficient hosts providing evidence that T cell regeneration in thymic-deficient hosts largely occurs via TCR-MHC-mediated selection of peripheral T cell populations. This process results in a T cell repertoire comprised exclusively of T cells recently activated by the antigenic milieu of the host, with negligible numbers of residual "naive" cells bearing TCRs for Ags absent at the time of expansion. These findings have important implications for approached to enhance T cell regeneration in humans and provide evidence that vaccine strategies could skew the T cell repertoire toward a specific antigenic target if administered to thymic-deficient hosts during immune reconstitution.

Thymic-independent T cell regeneration occurs via antigen-driven expansion of peripheral T cells resulting in a repertoire that is limited in diversity and prone to skewing

Thymic regenerative capacity in humans decreases with age, suggesting that thymic-independent pathways of T cell regeneration may predominate during adulthood. Using a murine bone marrow transplantation model, we present evidence that thymic-independent T cell regeneration occurs primarily via expansion of peripheral T cells and is Ag driven since significant expansion of CD4+ or CD8+ transgenic (Tg+)/TCR-bearing cells occurs only in the presence of Ag specific for the TCR. Such expansion resulted in skewing of the regenerated repertoire with 40 to 65% of the regenerated CD4+ or CD8+ T cells expressing the Tg+/TCR in thymectomized hosts after bone marrow transplantation. In experiments in which nontransgenic population are used as T cell inocula, we noted decreased CD4 expansion when Class II MHC was blocked by mAb treatment in vivo, an CD8 expansion failed to occur in Class I MHC-deficient hosts providing evidence that T cell regeneration in thymic-deficient hosts largely occurs via TCR-MHC-mediated selection of peripheral T cell populations. This process results in a T cell repertoire comprised exclusively of T cells recently activated by the antigenic milieu of the host, with negligible numbers of residual "naive" cells bearing TCRs for Ags absent at the time of expansion. These findings have important implications for approached to enhance T cell regeneration in humans and provide evidence that vaccine strategies could skew the T cell repertoire toward a specific antigenic target if administered to thymic-deficient hosts during immune reconstitution.

Disruption of intrathymic CD4-Ia interactions on immature CD4+CD8+ thymocytes results in diminished TCR expression on mature CD8+ T cell progeny

Cell surface TCR expression by developing thymocytes is actively regulated during ontogeny. Whereas most immature CD4+CD8+ thymocytes express low levels of TCR alpha beta, mature CD4+ and CD8+ thymocytes express significantly higher levels of surface TCR. Low TCR expression on CD4+CD8+ thymocytes is due, at least in part, to inhibitory signals generated by CD4 interactions with MHC class II ligands in the thymus. In the present study we wished to determine whether levels of TCR expressed on mature thymocytes were also influenced by CD4-Ia interactions that had occurred on their CD4+CD8+ precursors. To do so, we examined TCR expression on mature CD8+ T cells from animals in which CD4-ligand interactions had been disrupted experimentally by in vivo administration of anti-CD4 mAb or by targeted disruption of an MHC class II gene. We found that TCR expression was significantly diminished on all mature CD8+ T cells from both anti-CD4 mAb-treated mice and MHC class II-deficient animals. These results demonstrate that TCR expression by mature CD8+ T cells, as well as that of immature CD4+CD8+ thymocytes, is regulated by CD4-mediated signals acting on CD4+CD8+ thymocytes. Because the effects of disrupting CD4-Ia interactions on TCR expression by CD8+ T cells were independent of TCR specificity, these findings directly support the concept that CD4-CD8+ T cells arise from precursor CD4+CD8+ cells.

Programmed differentiation of murine thymocytes during fetal thymus organ culture

Fetal thymus organ culture (FTOC) has become widely used to investigate the impact of immunomodulators on T cell development. However, these studies have given variable results among different laboratories. In this study, we have found that fetal tissue age and mouse strain differences can affect the development of T cell phenotypes in this system. T cell development in FTOC occurred in two 'waves', defined as peaks of cell recovery. The first wave consisted initially of CD4-CD8- double negative (DN) cells and CD4-CD8+ single positive (SP) T cells expressing gamma delta T cell receptor (TCR). CD4+CD8+ double positive (DP) cells expressing low levels of alpha beta TCR were produced soon thereafter; and these cells dominated the cultures for the balance of the first wave. Prolonged FTOC resulted in the production of another wave of T cells which were relatively enriched for CD4 or CD8 SP cells expressing high levels of alpha beta TCR, as well as DN cells and CD4-CD8+ SP T cells expressing high levels of gamma delta TCR. As defined by cell number and differentiation of alpha beta TCR SP cells, development was delayed in FTOC using fetal thymus tissue from younger fetuses relative to that observed when older fetal thymus tissue was used. The degree of development of T cells in FTOC was also strain dependent. Organ cultures derived from 14 gestation days (gd) C.B-17 scid/scid fetal thymus did not generate TCR-bearing mature SP cells, but they did produce TCR-negative CD4 and CD8 SP cells likely to be precursors of DP thymocytes. Such cultures made from 18 gd tissue did not produce SP cells. Negative selection in FTOC was also evaluated. Mtv-specific V beta 3 cells were deleted in FTOC of C3H/HeN tissue. Deletion occurred only in late FTOC, suggesting a late encounter between the Mtv deleting elements and susceptible T cells during ontogeny. These results show that while FTOC recapitulates normal thymic development by a variety of criteria, results can be influenced by the length of culture, as well as by the age and strain of fetal thymus tissue utilized.

Early molecular events induced by T cell receptor (TCR) signaling in immature CD4+ CD8+ thymocytes: increased synthesis of TCR-alpha protein is an early response to TCR signaling that compensates for TCR-alpha instability, improves TCR assembly, and parallels other indicators of positive selection

Differentiation of immature CD4+ CD8+ thymocytes into mature CD4+ or CD8+ T cells occurs within the thymus and is dependent upon expression of antigen receptor complexes (T cell receptor [TCR]) containing clonotypic alpha/beta proteins. We have recently found that CD4+ CD8+ thymocytes express low levels of surface TCR because of limitations placed on TCR assembly by the instability of nascent TCR-alpha proteins within the endoplasmic reticulum (ER) of immature thymocytes. Because TCR-alpha/beta expression increases during development, a molecular mechanism must exist for increasing the number of assembled TCR complexes present in immature CD4+ CD8+ thymocytes that have been signaled to differentiate into mature T cells, although no such mechanism has yet been described. In the current report we have examined the molecular consequences of intracellular signals generated by engagement of surface TCR complexes on immature CD4+ CD8+ thymocytes. Isolated TCR engagement generated signals that increased TCR-alpha RNA levels and increased synthesis of TCR-alpha proteins, which, in turn, significantly increased assembly of complete TCR-alpha/beta complexes in CD4+ CD8+ thymocytes. Increased TCR-alpha protein levels in TCR-signaled CD4+ CD8+ thymocytes was the result of increased synthesis and not increased stability of TCR-alpha proteins, indicating that TCR engagement compensates for, but does not correct, the inherent instability of TCR-alpha proteins in the ER of immature thymocytes. Consistent with the delivery by TCR engagement of a positive selection signal, TCR engagement also increased CD5 expression, decreased RAG-1 expression, and decreased CD4/CD8 coreceptor expression in immature CD4+ CD8+ thymocytes. These data identify amplified TCR-alpha expression as an initial response of immature CD4+ CD8+ thymocytes to TCR-mediated positive selection signals and provide a molecular basis for increased surface TCR density on developing thymocytes undergoing selection events within the thymus.

Engagement of the external domains of CD45 tyrosine phosphatase can regulate the differentiation of immature CD4+CD8+ thymocytes into mature T cells

Immature precursor cells are induced in the thymus to express clonotypic T-cell antigen receptors (TCRs) and to differentiate into mature T cells. Perhaps the least understood event which occurs during intrathymic development is the positive selection of immature CD4+CD8+ thymocytes for differentiation into mature CD4+ and CD8+ T cells based on the TCR specificity individual thymocytes express. TCR expression by CD4+CD8+ thymocytes is quantitatively regulated by CD4-mediated activation of p56lck protein-tyrosine kinase whose activity can in turn be regulated by the membrane-bound protein-tyrosine-phosphatase CD45. Here we show that antibody engagement of CD45 external domains enhances Lck tyrosine kinase activity in CD4+CD8+ thymocytes, inhibits TCR expression, and inhibits differentiation of immature CD4+CD8+ thymocytes into mature T cells. Thus, engagement of the external domains of CD45 tyrosine phosphatase can regulate the ability of immature CD4+CD8+ thymocytes to undergo positive selection, suggesting an important regulatory role for intrathymic ligands that are capable of engaging CD45 within the thymus.

Quantitative differences in cell surface expression of class I MHC antigens on murine epidermal Langerhans cells

Epidermal Langerhans cells are derived from cells of bone marrow origin and, as the primary APC population in the skin, are responsible for initiation of many immune responses. Consequently, cell surface expression of MHC Ags by Langerhans cells is central to their function. Although murine Langerhans cells express class II MHC Ags at high levels, their level of expression of class I MHC has been controversial. In this study, cell surface expression of multiple individual class I MHC Ags on murine epidermal Langerhans cells was analyzed using quantitative immunofluorescence. It was found that Langerhans cells differentially express products of distinct class I genes. Langerhans cells expressed low cell surface amounts of H-2K and Qa-2, whereas expression of surface H-2D and H-2L by the same cells was high. Murine epidermal Langerhans cells therefore express low cell surface amounts of some but not all class I MHC Ags. Differential surface expression of products of distinct class I MHC genes by Langerhans cells may have a profound effect on cutaneous immune responses.

Quantitative differences in cell surface expression of class I MHC antigens on murine epidermal Langerhans cells

Epidermal Langerhans cells are derived from cells of bone marrow origin and, as the primary APC population in the skin, are responsible for initiation of many immune responses. Consequently, cell surface expression of MHC Ags by Langerhans cells is central to their function. Although murine Langerhans cells express class II MHC Ags at high levels, their level of expression of class I MHC has been controversial. In this study, cell surface expression of multiple individual class I MHC Ags on murine epidermal Langerhans cells was analyzed using quantitative immunofluorescence. It was found that Langerhans cells differentially express products of distinct class I genes. Langerhans cells expressed low cell surface amounts of H-2K and Qa-2, whereas expression of surface H-2D and H-2L by the same cells was high. Murine epidermal Langerhans cells therefore express low cell surface amounts of some but not all class I MHC Ags. Differential surface expression of products of distinct class I MHC genes by Langerhans cells may have a profound effect on cutaneous immune responses.

Expression of the homotypic adhesion molecule E-cadherin by immature murine thymocytes and thymic epithelial cells

Cadherins mediate homotypic adhesion between lineage-related cells in epithelia and other tissues. One cadherin, E-cadherin, is also responsible for adhesion of murine epidermal Langerhans cells to keratinocytes in vitro, and may play a role in the localization of Langerhans cells in epidermis. The thymus is another tissue in which important adhesive interactions between bone marrow-derived cells and keratinizing epithelia occur. To determine whether cadherins might be involved in interactions between thymocytes and thymic epithelial cells, we examined thymocytes from C57BL/6 mice of various gestational ages for cadherin expression. Most day 14 (D14) and essentially all D16 isolated fetal thymocytes expressed cell surface E-cadherin. After D16, the proportion of fetal thymocytes expressing E-cadherin and the level of E-cadherin expressed by individual thymocytes decreased with increasing gestational age. A minority of neonatal thymocytes and very few adult thymocytes expressed E-cadherin. E-cadherin was maximally expressed by the least mature (CD4-CD8-, HSA (J11d)high, CD5 (Ly-1)low, CD25 (IL-2R alpha)+) thymocytes. P-cadherin, another epithelial cadherin, was not detected on thymocytes at any stage of development. Immunohistologic studies revealed that thymic epithelial cells also expressed E-cadherin. Similar levels of E-cadherin were expressed by neonatal and adult thymic epithelial cells in situ, and E-cadherin was easily demonstrable on the thymic epithelial cell line, TE-71. In contrast, P-cadherin was transiently expressed by thymic epithelial cells in situ, and only small amounts of P-cadherin were detected on TE-71 cells. These studies demonstrate that thymocytes and thymic epithelial cells each have the capacity to express the homotypic adhesion molecule E-cadherin. E-cadherin may play a role in developmentally regulated interactions between early thymocytes and thymic stromal cells.

Expression of the homotypic adhesion molecule E-cadherin by immature murine thymocytes and thymic epithelial cells

Cadherins mediate homotypic adhesion between lineage-related cells in epithelia and other tissues. One cadherin, E-cadherin, is also responsible for adhesion of murine epidermal Langerhans cells to keratinocytes in vitro, and may play a role in the localization of Langerhans cells in epidermis. The thymus is another tissue in which important adhesive interactions between bone marrow-derived cells and keratinizing epithelia occur. To determine whether cadherins might be involved in interactions between thymocytes and thymic epithelial cells, we examined thymocytes from C57BL/6 mice of various gestational ages for cadherin expression. Most day 14 (D14) and essentially all D16 isolated fetal thymocytes expressed cell surface E-cadherin. After D16, the proportion of fetal thymocytes expressing E-cadherin and the level of E-cadherin expressed by individual thymocytes decreased with increasing gestational age. A minority of neonatal thymocytes and very few adult thymocytes expressed E-cadherin. E-cadherin was maximally expressed by the least mature (CD4-CD8-, HSA (J11d)high, CD5 (Ly-1)low, CD25 (IL-2R alpha)+) thymocytes. P-cadherin, another epithelial cadherin, was not detected on thymocytes at any stage of development. Immunohistologic studies revealed that thymic epithelial cells also expressed E-cadherin. Similar levels of E-cadherin were expressed by neonatal and adult thymic epithelial cells in situ, and E-cadherin was easily demonstrable on the thymic epithelial cell line, TE-71. In contrast, P-cadherin was transiently expressed by thymic epithelial cells in situ, and only small amounts of P-cadherin were detected on TE-71 cells. These studies demonstrate that thymocytes and thymic epithelial cells each have the capacity to express the homotypic adhesion molecule E-cadherin. E-cadherin may play a role in developmentally regulated interactions between early thymocytes and thymic stromal cells.

Negative selection of CD4+CD8+ thymocytes by T cell receptor-induced apoptosis requires a costimulatory signal that can be provided by CD28

CD4+CD8+ thymocytes expressing self-reactive T cell antigen receptors (TCR) are deleted in the thymus as a consequence of TCR/self-antigen/major histocompatibility complex interactions. However, the signals that are necessary to initiate clonal deletion have not yet been clarified. Here we demonstrate that TCR engagement does not efficiently induce apoptosis of CD4+CD8+ thymocytes, although it generates signals that increase expression of CD5, a thymocyte differentiation marker. In fact, TCR signals fail to induce thymocyte apoptosis even when augmented by simultaneous engagement with CD4 or lymphocyte function 1-associated molecules. In marked contrast, signals generated by engagement of both TCR and the costimulatory molecule CD28 potently induce apoptosis of CD4+CD8+ thymocytes. Thus, the present results define a requirement for both TCR and costimulatory signals for thymocyte apoptosis and identify CD28 as one molecule that is capable of providing the necessary costimulus. These results provide a molecular basis for differences among cell types in their ability to mediate negative selection of developing thymocytes.

Murine dendritic epidermal T cells (DETC) express the homophilic adhesion molecule E-cadherin

Cadherins mediate homotypic intercellular adhesion in epidermis and other epithelia. E-cadherin is also involved in interactions between murine epidermal Langerhans cells (LC) and keratinocytes (KC). Dendritic epidermal T cells (DETC) comprise another subpopulation of epidermal leukocytes. Using flow cytometry, we determined that DETC expressed levels of E-cadherin similar to those expressed by LC and KC. DETC also adhered congruent to three-fold better to KC and E-cadherin-transfected fibroblasts than to normal fibroblasts. Treatment of DETC with trypsin in the absence of calcium caused a loss of E-cadherin and resulted in an congruent to 80% decrease in DETC-KC adhesion whereas treatment of DETC with trypsin in the presence of calcium did not significantly affect E-cadherin expression or DETC-KC binding. Thus, E-cadherin may be involved in adhesion of DETC to KC. DETC are derived from TCR V gamma 3+ thymocytes that transiently populate embryonic murine thymus. We determined that TCR V gamma 3+ thymocytes as well as other early (fetal day 16) TCR gamma/delta+ thymocytes expressed E-cadherin; TCR gamma/delta+ (TCR V gamma 3-) thymocytes that developed later did not. These results indicate that cells of the T cell lineage can express E-cadherin, and suggest that E-cadherin may play a role in adhesion of DETC (and/or DETC precursors) to KC.

Antigenic heterogeneity of Qa-2 antigens in C57BL/6

The Qa-2 antigens are class I-like molecules encoded by genes mapped telomeric to the H-2D region on chromosome 17 in the mouse. A panel of 8 new monoclonal anti-Qa-2 antibodies derived from a C3H.KBR anti-C3H. SW immunization was studied. Immunoprecipitation of 125I-labeled C57BL/6 splenocyte antigens showed that all of these antibodies precipitated 40 kDa molecules which could be completely precleared by the monoclonal antibody 20-8-4, which had previously been shown to crossreact with Qa-2. One of the monoclonal antibodies (1-12-1), however, was found not to completely preclear Qa-2 antigens precipitable by the other 7 antibodies or by 20-8-4, suggesting the existence of at least two different species of Qa-2 molecules. Cell lines transfected with Q7 or Q9 genes were reactive with all 9 antibodies and the Qa-2 antigens expressed on surface membranes of these cells were completely precleared by both 20-8-4 and 1-12-1. Therefore, the observed heterogeneity of these molecules cannot be explained by an antigenic difference between the Q7 and Q9 gene products. 2D gel analyses showed identical pI spectra between Qa-2 molecules precipitated with 20-8-4 and 1-12-1. In addition, all of the monoclonal antibodies reacted with labeled antigen preparations following treatment with Endo F or neuraminidase, indicating that carbohydrate moieties are probably not responsible for the antigenic difference between the two species of Qa-2 antigen.

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

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

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

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

Differential expression of VLA-alpha 4 and VLA-beta 1 discriminates multiple subsets of CD4+CD45R0+ "memory" T cells

Given the importance of adhesion in T cell development, we have undertaken systematic flow cytometric analysis of CD4 T cells to determine relationships between the developmentally regulated marker CD45R0 and adhesion receptors (five VLA integrin chains). The most important findings are that: 1) expression of alpha 3, alpha 5, and alpha 6 are closely coregulated with beta 1 on CD4 cells, while regulation of VLA-alpha 4 is quite discordant. 2) CD45R0- cells, generally understood to be naive cells, have low homogeneous expression of VLA-alpha 3, VLA-alpha 4, VLA-alpha 5, VLA-alpha 6, and beta 1 integrin chains; studies of cord blood CD4 cells confirm the low homogeneous expression of alpha 4 and beta 1 on naive cells. 3) In marked contrast, CD45R0+ cells, generally understood to be memory cells, show not only an overall increase in expression of these integrins (relative to CD45R0- cells) but also heterogeneity. Dramatic heterogeneity is revealed when the markers VLA-alpha 4 and beta 1 are analyzed together. Many CD45R0+ cells show increased levels of both VLA-alpha 4 and VLA-beta 1; however, some have increased levels principally of either VLA-beta 1 or VLA-alpha 4. We hypothesize that T cells becoming memory cells in different microenvironments specialize their integrin phenotype, thereby acquiring distinctive functional and homing capacities; in this process, VLA-4 (CD49d) appears to play a unique role.

Differential expression of VLA-alpha 4 and VLA-beta 1 discriminates multiple subsets of CD4+CD45R0+ "memory" T cells

Given the importance of adhesion in T cell development, we have undertaken systematic flow cytometric analysis of CD4 T cells to determine relationships between the developmentally regulated marker CD45R0 and adhesion receptors (five VLA integrin chains). The most important findings are that: 1) expression of alpha 3, alpha 5, and alpha 6 are closely coregulated with beta 1 on CD4 cells, while regulation of VLA-alpha 4 is quite discordant. 2) CD45R0- cells, generally understood to be naive cells, have low homogeneous expression of VLA-alpha 3, VLA-alpha 4, VLA-alpha 5, VLA-alpha 6, and beta 1 integrin chains; studies of cord blood CD4 cells confirm the low homogeneous expression of alpha 4 and beta 1 on naive cells. 3) In marked contrast, CD45R0+ cells, generally understood to be memory cells, show not only an overall increase in expression of these integrins (relative to CD45R0- cells) but also heterogeneity. Dramatic heterogeneity is revealed when the markers VLA-alpha 4 and beta 1 are analyzed together. Many CD45R0+ cells show increased levels of both VLA-alpha 4 and VLA-beta 1; however, some have increased levels principally of either VLA-beta 1 or VLA-alpha 4. We hypothesize that T cells becoming memory cells in different microenvironments specialize their integrin phenotype, thereby acquiring distinctive functional and homing capacities; in this process, VLA-4 (CD49d) appears to play a unique role.

Dichotomy of glutathione regulation of the activation of resting and preactivated lymphocytes

The present study has examined the effect of GSH on two lines of IL-2-dependent activated killer cells, LAK cells and alpha CD3-activated killer (CD3-AK) cells. We found that GSH added during first 24 hr decreased the generation of LAK and CD3-AK cells from resting lymphocytes, whereas after 48 hr of activation, the addition of GSH increased the killer cell activity. In addition, BSO, an inhibitor of GSH biosynthesis, decreased the proliferation and cytotoxic activities of activated killer cells, and the inhibitory effect was reversed by GSH. These results indicate that GSH downregulates the generation of LAK or CD3-AK cells from resting lymphocytes, but it upregulates the further differentiation of preactivated killer cells. The effect of GSH thus varied with the state of activation of the killer cells. Culturing CD3-AK cells in GSH did not change the distribution of T cell subsets, did not affect the cells' ability to produce lymphokine (IL-2), and did not induce suppressor cells. One striking change as revealed by flow cytometry analysis was that the levels of IL-2 receptor and TCR (alpha/beta)-CD3 were reduced by 80 and 30%, respectively, after 48 hr culturing in GSH. Determination of the mRNA of IL-2 receptor suggests that a post-transcriptional block existed. It appears that the negative effect of GSH on the function of surface IL-2 receptors or T cell receptors on resting lymphocytes severely affected the signal transduction through these receptors and thus abrogated or reduced LAK or CD3-AK cell response. In contrast, for preactivated killer cells, upregulation by intracellular GSH of IL-2 utilization is a dominant effect, thus allowing further differentiation of these killer cells. Our results indicate that the balance between the activation signal (IL-2 or alpha CD3) and the immunoregulatory signal (induced by GSH) may determine the outcome of the immune response.

MONOCLONAL ANTIBODIES AGAINST HUMAN T CELL ADHESION MOLECULES—MODULATION OF IMMUNE FUNCTION IN NONHUMAN PRIMATES

The cytotoxic T cell is thought to be a primary effector of allograft rejection. In vitro studies have demonstrated that the interaction between cytotoxic T cells and target cells involves cell surface adhesion molecules that result in conjugate formation, with subsequent antigen recognition, T cell activation, and target cell lysis. Experiments have also demonstrated the ability of monoclonal antibodies with specificity for two human T cell adhesion molecules, lymphocyte function associated (LFA) antigen-1 (LFA-1, CD11a, alpha-chain/CD18, beta-chain) and LFA-2 (CD2), to inhibit conjugate formation in vitro. Studies in a nonhuman primate model were undertaken to determine whether the in vivo administration of monoclonal antibodies with specificity for the alpha chain of LFA-1 (CD11a) or with specificity for CD2 could modulate in vivo T cell function. Cynomolgus monkeys (Macaca fascicularis) received 10 daily intravenous infusions of either anti-CD11a, anti-CD2 or both anti-CD11a and anti-CD2 monoclonal antibodies. Antibody administration was well tolerated and resulted in high levels of circulating murine monoclonal antibody in the peripheral circulation. Nearly all the animals generated antimurine antibodies that were specific for both idiotypic and nonidiotypic determinants of the infused mouse protein. Circulating lymphocytes and T cells were not depleted by treatment with anti-CD11a or anti-CD2 mAbs; in fact, treatment with the combination of anti-CD11a plus anti-CD2 or anti-CD11a alone led to increased numbers of circulating lymphocytes and T cells. Modulation of the LFA-1 molecule on circulating T cells occurred as a result of treatment with anti-CD11a (or the combination of anti-CD11a plus anti-CD2), whereas treatment with anti-CD2 (or anti-CD11a plus anti-CD2) did not result in modulation of the CD2 antigen despite detectable levels of circulating anti-CD2 mAb. In vivo T cell function was assessed by placement of skin allografts. As compared with treatment with saline or a control mAb, allograft survival was significantly prolonged in animals treated with anti-CD11a or combination treatment but not in animals receiving anti-CD2 alone. We conclude that the in vivo administration of anti-LFA-1 mAb may be useful for the blockade of effector T cell activity during allograft rejection, that saturation of antigen and antigen modulation may be important for efficacy of such antibody effects in vivo, and that monoclonal antibodies with specificity for functionally important T cell surface molecules may alter T cell function in vivo without lymphocyte depletion.

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.

Expression of an unusual T cell receptor (TCR)-V beta repertoire by Ly-6C+ subpopulations of CD4+ and/or CD8+ thymocytes. Evidence for a developmental relationship between Ly-6C+ thymocytes and CD4-CD8-TCR-alpha beta+ thymocytes

A novel thymocyte subpopulation expressing an unusual TCR repertoire was identified by high surface expression of the Ly-6C Ag. Ly-6C+ thymocytes were distributed among all four CD4/CD8 thymocyte subsets, and represented a readily identifiable subpopulation within each one. Ly-6C+ thymocytes express TCR-alpha beta, arise late in ontogeny, and appear in the CD4/CD8 developmental pathway after birth in a sequence that resembles that followed by conventional Ly-6C- cells during fetal ontogeny. Most interestingly, adult Ly-6C+ thymocytes express an unusual TCR-V beta repertoire that is identical to that expressed by CD4-CD8-TCR-alpha beta+ thymocytes in its overexpression of TCR-V beta 8 and in its expression of some potentially autoreactive TCR-V beta specificities. This unusual TCR-V beta repertoire was even expressed by Ly-6C+ thymocytes contained within the CD4+ CD8- 'single positive' thymocyte subset. Thus, expression of this unusual TCR-V beta repertoire is not limited to CD4-CD8-thymocytes, and is unlikely to be a consequence of their double negative phenotype. Rather, we think that Ly-6C+TCR-alpha beta+ thymocytes and CD4-CD8-TCR-alpha beta+ are developmentally interrelated, a conclusion supported by several lines of evidence including the selective failure of both Ly-6C+ and CD4-CD8-TCR-alpha beta+ thymocyte subsets to appear in TCR-beta transgenic mice. In contrast, peripheral Ly-6C+ T cells are developmentally distinct from Ly-6C+ thymocytes in that peripheral Ly-6C+ T cells expressed a conventional TCR-V beta repertoire and developed normally in TCR-beta transgenic mice in which Ly-6C+ thymocytes failed to arise. We conclude that: 1) expression of a skewed TCR-V beta repertoire is a characteristic of Ly-6C+TCR-alpha beta+ thymocytes as well as CD4-CD8-TCR-alpha beta+ thymocytes, and is not unique to thymocytes expressing neither CD4 nor CD8 accessory molecules; and 2) Ly-6C+ thymocytes are developmentally linked to CD4-CD8-TCR-alpha beta+ thymocytes, but not to Ly-6C+ peripheral T cells. We suggest that Ly-6C+TCR-alpha beta+ thymocytes are not the developmental precursors of Ly-6C+ peripheral T cells, but rather may be the developmental precursors of CD4-CD8-TCR-alpha beta+ thymocytes.

Expression of an unusual T cell receptor (TCR)-V beta repertoire by Ly-6C+ subpopulations of CD4+ and/or CD8+ thymocytes. Evidence for a developmental relationship between Ly-6C+ thymocytes and CD4-CD8-TCR-alpha beta+ thymocytes

A novel thymocyte subpopulation expressing an unusual TCR repertoire was identified by high surface expression of the Ly-6C Ag. Ly-6C+ thymocytes were distributed among all four CD4/CD8 thymocyte subsets, and represented a readily identifiable subpopulation within each one. Ly-6C+ thymocytes express TCR-alpha beta, arise late in ontogeny, and appear in the CD4/CD8 developmental pathway after birth in a sequence that resembles that followed by conventional Ly-6C- cells during fetal ontogeny. Most interestingly, adult Ly-6C+ thymocytes express an unusual TCR-V beta repertoire that is identical to that expressed by CD4-CD8-TCR-alpha beta+ thymocytes in its overexpression of TCR-V beta 8 and in its expression of some potentially autoreactive TCR-V beta specificities. This unusual TCR-V beta repertoire was even expressed by Ly-6C+ thymocytes contained within the CD4+ CD8- 'single positive' thymocyte subset. Thus, expression of this unusual TCR-V beta repertoire is not limited to CD4-CD8-thymocytes, and is unlikely to be a consequence of their double negative phenotype. Rather, we think that Ly-6C+TCR-alpha beta+ thymocytes and CD4-CD8-TCR-alpha beta+ are developmentally interrelated, a conclusion supported by several lines of evidence including the selective failure of both Ly-6C+ and CD4-CD8-TCR-alpha beta+ thymocyte subsets to appear in TCR-beta transgenic mice. In contrast, peripheral Ly-6C+ T cells are developmentally distinct from Ly-6C+ thymocytes in that peripheral Ly-6C+ T cells expressed a conventional TCR-V beta repertoire and developed normally in TCR-beta transgenic mice in which Ly-6C+ thymocytes failed to arise. We conclude that: 1) expression of a skewed TCR-V beta repertoire is a characteristic of Ly-6C+TCR-alpha beta+ thymocytes as well as CD4-CD8-TCR-alpha beta+ thymocytes, and is not unique to thymocytes expressing neither CD4 nor CD8 accessory molecules; and 2) Ly-6C+ thymocytes are developmentally linked to CD4-CD8-TCR-alpha beta+ thymocytes, but not to Ly-6C+ peripheral T cells. We suggest that Ly-6C+TCR-alpha beta+ thymocytes are not the developmental precursors of Ly-6C+ peripheral T cells, but rather may be the developmental precursors of CD4-CD8-TCR-alpha beta+ thymocytes.

Repopulation of host lymphohematopoietic systems by donor cells during graft-versus-host reaction in unirradiated adult F1 mice injected with parental lymphocytes

The graft-vs-host reaction (GVHR) generated by the injection of parental lymphocytes into unirradiated immune-competent F1 hosts is characterized by an acute loss of immune functions, an attack on host tissues, and a gradual recovery of function. Flow cytometric analysis of the donor- and host-derived splenic populations during the course of acute dysfunction and gradual recovery revealed a complex pattern of changes in lymphoid and myeloid populations that resulted in the repopulation of the host with donor-derived cells. Initially, donor-derived T cell populations expanded, particularly CD8+ T cells. Next, host T cell and B cell populations disappeared. Finally, donor-derived cells repopulated the lymphohematopoietic system in the sequence myeloid populations, B cells, and, after a protracted period, T cells. The recovery of immune functions following GVHR-induced immune deficiency was associated with this repopulation of the spleen by donor-derived cells. Donor repopulation of the host lymphohematopoietic system required the presence of both CD4 and CD8 cells in the original donor inoculum. Depletion of donor CD4 populations precluded development of GVHR or any donor engraftment; depletion of CD8 cells resulted in engraftment solely of donor CD4 populations.

Repopulation of host lymphohematopoietic systems by donor cells during graft-versus-host reaction in unirradiated adult F1 mice injected with parental lymphocytes

The graft-vs-host reaction (GVHR) generated by the injection of parental lymphocytes into unirradiated immune-competent F1 hosts is characterized by an acute loss of immune functions, an attack on host tissues, and a gradual recovery of function. Flow cytometric analysis of the donor- and host-derived splenic populations during the course of acute dysfunction and gradual recovery revealed a complex pattern of changes in lymphoid and myeloid populations that resulted in the repopulation of the host with donor-derived cells. Initially, donor-derived T cell populations expanded, particularly CD8+ T cells. Next, host T cell and B cell populations disappeared. Finally, donor-derived cells repopulated the lymphohematopoietic system in the sequence myeloid populations, B cells, and, after a protracted period, T cells. The recovery of immune functions following GVHR-induced immune deficiency was associated with this repopulation of the spleen by donor-derived cells. Donor repopulation of the host lymphohematopoietic system required the presence of both CD4 and CD8 cells in the original donor inoculum. Depletion of donor CD4 populations precluded development of GVHR or any donor engraftment; depletion of CD8 cells resulted in engraftment solely of donor CD4 populations.

Presence of CD4 and CD8 determinants on CD4-CD8- murine thymocytes: passive acquisition of CD8 accessory molecules

In the present study we have examined the possibility that CD4 and CD8 accessory molecules can be passively acquired by thymocytes. We initially observed that most thymocytes contained within the CD4-CD8- subset actually possess low levels of CD4 and CD8 on their cell surface. However, the detection of CD4 and CD8 on CD4-CD8- cells was dependent on the presence of other CD4+/CD8+ thymocytes which were actively synthesizing CD4 and CD8. These initial findings suggested that the appearance of CD4/CD8 on "double-negative" thymocytes was due to the passive acquisition of these accessory molecules from CD4+/CD8+ cells present within the thymus. To investigate this possibility directly, we made both in vivo and in vitro mixes of thymocytes possessing different alleles of CD8 (Ly-2.1 and Ly-2.2). Under these experimental conditions, we detected Ly-2.2 on the surface of thymocytes that were genetically Ly-2.1+ and incapable of synthesizing Ly-2.2. These data indicate that thymocytes can express cell surface CD8 molecules which they have not produced but have acquired from other cells in their environment. Thus, the present study indicates that low-level surface expression of cell surface CD4/CD8 differentiation molecules does not necessarily identify distinct thymocyte subpopulations.

Subpopulations of fetal thymocytes defined by expression of T cell receptor/CD3 and IL-2 receptor. CD3 and IL-2 receptor alpha-chain are expressed on reciprocal cell populations

The expression of the TCR/CD3 complex and the IL-2R alpha chain (p55) on fetal thymocytes has been analyzed by flow cytometry (FCM). Two-parameter immunofluorescence identified three subpopulations which were respectively IL-2R alpha-/CD3+, IL-2R alpha+/CD3-, or IL-2R alpha-/CD3-; no detectable population of IL-2R alpha+/CD3+ cells was found in unstimulated fetal thymocytes. Fractionation by "panning" and by sterile flow cytometric separation was used to characterize the functional responsiveness of these three subpopulations to a variety of stimuli. All three populations proliferated in response to PMA + ionomycin + rIL-2. In contrast, stimulation with anti-CD3 + IL-2 induced proliferation in IL-2R alpha-/CD3+ and IL-2R alpha-/CD3- but not in IL-2R alpha+/CD3- thymocytes. IL-2R alpha- cells, including sorted IL-2R alpha-/CD3- thymocytes, underwent a phenotypic change in response to in vitro stimulation with anti-CD3 + IL-2, resulting in the appearance of an IL-2R alpha+/CD3+ population that was not detected in freshly isolated thymocytes. The ability of fractionated fetal thymocytes to produce lymphokine in response to PMA + ionomycin was also evaluated. Only the IL-2R alpha-/CD3- fraction generated detectable IL-2. These findings demonstrate for the first time that CD3 and IL-2R alpha are expressed in a mutually exclusive fashion in fetal thymocytes and define three subpopulations of thymocytes that differ significantly in their proliferative and differentiative responses to TCR-mediated, IL-2R-mediated, and pharmacologic stimulation.

Subpopulations of fetal thymocytes defined by expression of T cell receptor/CD3 and IL-2 receptor. CD3 and IL-2 receptor alpha-chain are expressed on reciprocal cell populations

The expression of the TCR/CD3 complex and the IL-2R alpha chain (p55) on fetal thymocytes has been analyzed by flow cytometry (FCM). Two-parameter immunofluorescence identified three subpopulations which were respectively IL-2R alpha-/CD3+, IL-2R alpha+/CD3-, or IL-2R alpha-/CD3-; no detectable population of IL-2R alpha+/CD3+ cells was found in unstimulated fetal thymocytes. Fractionation by "panning" and by sterile flow cytometric separation was used to characterize the functional responsiveness of these three subpopulations to a variety of stimuli. All three populations proliferated in response to PMA + ionomycin + rIL-2. In contrast, stimulation with anti-CD3 + IL-2 induced proliferation in IL-2R alpha-/CD3+ and IL-2R alpha-/CD3- but not in IL-2R alpha+/CD3- thymocytes. IL-2R alpha- cells, including sorted IL-2R alpha-/CD3- thymocytes, underwent a phenotypic change in response to in vitro stimulation with anti-CD3 + IL-2, resulting in the appearance of an IL-2R alpha+/CD3+ population that was not detected in freshly isolated thymocytes. The ability of fractionated fetal thymocytes to produce lymphokine in response to PMA + ionomycin was also evaluated. Only the IL-2R alpha-/CD3- fraction generated detectable IL-2. These findings demonstrate for the first time that CD3 and IL-2R alpha are expressed in a mutually exclusive fashion in fetal thymocytes and define three subpopulations of thymocytes that differ significantly in their proliferative and differentiative responses to TCR-mediated, IL-2R-mediated, and pharmacologic stimulation.

Inhibition of T cell receptor expression and function in immature CD4+CD8+ cells by CD4

Most immature CD4+CD8+ thymocytes express only a small number of T cell receptor (TCR) molecules on their surface, and the TCR molecules they do express are only marginally capable of transducing intracellular signals. TCR expression and function was not intrinsically low in immature CD4+CD8+ thymocytes, but was found to be actively inhibited by CD4-mediated signals. Indeed, release of CD4+CD8+ thymocytes from CD4-mediated signals resulted in significant increases in both TCR expression and signaling function. These results suggest that, in CD4+CD8+ cells developing in the thymus, increased TCR expression and function requires release from CD4-mediated inhibition.

Clonal deletion and clonal anergy in the thymus induced by cellular elements with different radiation sensitivities

The present study demonstrates that immune tolerance can be achieved in the thymus both by clonal deletion and by clonal inactivation, but that the two tolerant states are induced by cellular elements with different radiation sensitivities. TCR engagement of self antigens on bone marrow-derived, radiation-sensitive (presumably dendritic) cells induces clonal deletion of developing thymocytes, whereas TCR engagement of self antigens on radiation-resistant cellular elements, such as thymic epithelium, induces clonal anergy. The nondeleted, anergic thymocytes can express IL-2-Rs but are unable to proliferate in response to either specific antigen or anti-TCR antibodies, and do develop into phenotypically mature cells that emigrate out of the thymus and into the periphery.

T cell receptor-negative thymocytes from SCID mice can be induced to enter the CD4/CD8 differentiation pathway

In order to investigate the role of T cell receptor (TcR) expression in thymocyte maturation, we have analyzed thymocytes from C.B-17/SCID mice, which are unable to productively rearrange their antigen receptor genes and fail to express TcR. Despite this defect, SCID thymocytes are functional as they produce lymphokines and proliferate in response to a variety of stimuli. Phenotypic analysis revealed that thymocyte populations from young adult SCID mice resemble thymocyte populations from normal embryonic mice in that they are large, Thy-1.2+, CD4-, CD8-, TcR- and enriched in CD5lo, IL2R+ and Pgp1+ cells. However, other TcR- populations normally present in adult mice (i.e., CD4-CD8+ cells and CD4+CD8+ cells) are absent from the thymus of TcR- adult SCID mice. To understand the basis of the developmental arrest of TcR- SCID thymocytes at the CD4-CD8- stage of differentiation, we analyzed thymi from the occasional "leaky" SCID mouse which possesses small numbers of TcR+ thymocytes. We found that the presence of TcR+ cells within a SCID thymus was invariably associated with the presence of CD4+ and/or CD8+ SCID thymocytes. Interestingly, however, the CD4+/CD8+ SCID thymocytes were not themselves necessarily TcR+. That is, emergence of SCID thymocytes expressing CD4/CD8 was tightly linked to the presence of TcR+ cells within that SCID thymus, but the SCID thymocytes that expressed CD4/CD8 were not necessarily the same cells that expressed TcR. Finally, we found that the introduction into TcR- SCID mice of normal bone marrow cells that give rise to TcR+ cells within the SCID thymus promoted the differentiation of SCID thymocytes into CD4-CD8+ and CD4+CD8+ TcR- cells. These data indicate that TcR+ cells within the thymic milieu provide critical signals which promote entry of CD4-CD8-TcR- precursor T cells into the CD4/CD8 differentiation pathway. When applied to differentiation of normal thymocytes, these findings may imply a critical role for early appearing CD4-CD8- TcR (gamma/delta)+ cells in initiating normal thymic ontogeny.

Failure of T cell receptor V beta negative selection in an athymic environment

The mature T cell receptor (TCR) repertoire is the result of selection events during T cell development. Previous assessment of TCR beta-chain selection with serologic and molecular probes demonstrated both positive and negative selection. Although this work suggested a critical role for the thymus, no direct assessment has been made of the requirement for a thymus in TCR V beta selection. A comparison of TCR V beta expression in four different congenic pairs of normal and nu/nu (athymic) mice indicated that the normal V beta deletions associated with tolerance to self minor lymphocyte stimulating (Mlsc) antigens or to self major histocompatibility complex (MHC)-encoded E alpha E beta products did not occur in most athymic mice. Thus, the thymus has a critical role in mediating self tolerance by negative selection.

Effect of cyclosporin A on lymphopoiesis. II. Developmental defects of immature and mature thymocytes in fetal thymus organ cultures treated with cyclosporin A

The effect of cyclosporin A (CsA) on early T cell development was studied by two-color flow cytometric and biochemical analyses using the fetal thymus organ culture system. Addition of CsA to organ culture resulted in a decreased cell yield and complete inhibition of the appearance of TCR-alpha beta-bearing, single positive thymocytes (both CD4+CD8- and CD4-CD8+). Furthermore, the generation of CD4+CD8+ thymocytes was markedly inhibited by CsA treatment, whereas the development of CD3-, CD4-CD8+ thymocytes and TCR-gamma delta-bearing, CD4-CD8- thymocytes was not affected. These results suggest that CsA induces a maturational arrest of T cells entirely within the thymic environment, and indicate that CsA-induced inhibition occurs at more than one stage of intrathymic T cell development.

Effect of cyclosporin A on lymphopoiesis. II. Developmental defects of immature and mature thymocytes in fetal thymus organ cultures treated with cyclosporin A

The effect of cyclosporin A (CsA) on early T cell development was studied by two-color flow cytometric and biochemical analyses using the fetal thymus organ culture system. Addition of CsA to organ culture resulted in a decreased cell yield and complete inhibition of the appearance of TCR-alpha beta-bearing, single positive thymocytes (both CD4+CD8- and CD4-CD8+). Furthermore, the generation of CD4+CD8+ thymocytes was markedly inhibited by CsA treatment, whereas the development of CD3-, CD4-CD8+ thymocytes and TCR-gamma delta-bearing, CD4-CD8- thymocytes was not affected. These results suggest that CsA induces a maturational arrest of T cells entirely within the thymic environment, and indicate that CsA-induced inhibition occurs at more than one stage of intrathymic T cell development.

Cardiac allograft survival across major histocompatibility complex barriers in the rhesus monkey following T lymphocyte-depleted autologous marrow transplantation. IV. Immune reconstitution

Studies of postmyeloablative immune reconstitution have been reported for allogeneic bone marrow transplantation and also for non-T cell-depleted autologous/syngeneic BMT. However, there is a paucity of information regarding immune recovery following T cell-depleted autologous/syngeneic BMT. We have developed a primate transplantation tolerance model in which rhesus monkeys were conditioned with total-body irradiation and extensively T cell-depleted autologous BMT and given a major histocompatibility complex-mismatched heterotopic cardiac allograft. This model provided an opportunity to study peripheral immune recovery following T cell-depleted autologous BMT. Limiting dilution analysis was used to quantify marrow T cells following depletion (2.8% to 25.6% marrow T cells predepletion, 0.00014% to 0.036% residual marrow T cells postdepletion). We found that (1) hematopoietic engraftment was prompt despite extensive marrow T cell depletion, (2) reconstitution of CD4+ helper T cells and CD8+ cytotoxic T cells were substantially delayed (6-12 months) compared with the recovery of CD8+ suppressor T cells, CD16+ NK cells, and CD20+ B cells, (3) distinction between CD8+ cytotoxic T cells and CD8+ suppressor T cells by the CD28 marker was critical in revealing the markedly discrepant recoveries of those subsets, and (4) immune reconstitution resembled that observed in recipients of T cell-depleted allogeneic and non-T cell-depleted autologous/syngeneic BMT, suggesting that the pattern of immune recovery following BMT is not substantially influenced by either allogeneic effects or the number of transferred T cells over a range of values.

Cell surface comodulation of CD4 and T cell receptor by anti-CD4 monoclonal antibody

In our study we have used anti-CD4 mAb to investigate the cell surface association between CD4 and the Ag-specific TCR complex on mature peripheral T cells. Anti-CD4 mAb was administered in vivo and in vitro and its effects on CD4 and CD3 cell surface expression were determined. In vivo, anti-CD4 mAb reduced cell surface expression of its ligand, CD4, and secondarily also reduced cell surface expression of CD3/TCR on CD4+ splenic T cells. In vitro, multivalent cross-linking of CD4 by anti-CD4 mAb and either FcR+ cells or anti-Ig mAb also resulted in decreased surface expression of CD4 and specific comodulation of CD3/TCR. The secondary reduction in cell surface CD3/TCR expression induced by CD4 cross-linking could be pharmacologically disrupted by high doses of PMA, indicating that the comodulation of CD3 with CD4 was dependent upon intracellular mediators, possibly including protein kinase C. These results demonstrate that, in the presence of anti-CD4 mAb, CD4 is functionally associated with the CD3/TCR complex, and that this association is dependent upon the activity of intracellular mediators. Such intracellular mediators might induce the coordinate down-modulation of physically unassociated CD4 and CD3/TCR molecules, or, alternatively, might promote a physical interaction between CD4 and CD3/TCR molecules.

Cell surface comodulation of CD4 and T cell receptor by anti-CD4 monoclonal antibody

In our study we have used anti-CD4 mAb to investigate the cell surface association between CD4 and the Ag-specific TCR complex on mature peripheral T cells. Anti-CD4 mAb was administered in vivo and in vitro and its effects on CD4 and CD3 cell surface expression were determined. In vivo, anti-CD4 mAb reduced cell surface expression of its ligand, CD4, and secondarily also reduced cell surface expression of CD3/TCR on CD4+ splenic T cells. In vitro, multivalent cross-linking of CD4 by anti-CD4 mAb and either FcR+ cells or anti-Ig mAb also resulted in decreased surface expression of CD4 and specific comodulation of CD3/TCR. The secondary reduction in cell surface CD3/TCR expression induced by CD4 cross-linking could be pharmacologically disrupted by high doses of PMA, indicating that the comodulation of CD3 with CD4 was dependent upon intracellular mediators, possibly including protein kinase C. These results demonstrate that, in the presence of anti-CD4 mAb, CD4 is functionally associated with the CD3/TCR complex, and that this association is dependent upon the activity of intracellular mediators. Such intracellular mediators might induce the coordinate down-modulation of physically unassociated CD4 and CD3/TCR molecules, or, alternatively, might promote a physical interaction between CD4 and CD3/TCR molecules.

Expression of a class I MHC transgene: effects of in vivo alpha/beta-interferon treatment

Transgenic mice containing a swine class I major histocompatibility complex (MHC) gene, PD1, express swine MHC (SLA) antigen. The tissue distribution of PD1 RNA parallels that observed in the swine, indicating that the expression of PD1 is regulated and that trans-acting factors involved in this regulation have been conserved between the species. Although PD1 RNA levels were much greater in transgenic spleen than in thymus, no difference in the chromatin organization of the PD1 gene was detected. In both tissues, a single DNase I hypersensitive site mapped within the 5' flanking region. In vivo treatment of the transgenics with mouse alpha, beta-interferon increases PD1 expression in a number of tissues. In the spleen, this increase parallels that observed for the endogenous transplantation antigen, Kb, but differs markedly from the differentiation antigen, Qa-2. Increases in cell surface expression of both PD1 and Kb occurred equally in splenic T- and B-cell populations following alpha, beta-interferon treatment. In contrast, Qa-2 expression in B cells was enhanced by alpha, beta-interferon, whereas it was unaffected in T cells and thymocytes.

Phenotypic and functional analysis of murine CD3+,CD4-,CD8- TCR-gamma delta-expressing peripheral T cells

Murine CD3+,CD4-,CD8- peripheral T cells, which express various forms of the TCR-gamma delta on their cell surface, have been characterized in terms of their cell-surface phenotype, proliferative and lytic potential, and lymphokine-producing capabilities. Three-color flow cytofluorometric analysis demonstrated that freshly isolated CD3+,CD4-, CD8- TCR-gamma delta lymph node cells were predominantly Thy-1+,CD5dull,IL-2R-,HSA-,B220-, and approximately 70% Ly-6C+ and 70% Pgp-1+. After CD3+,CD4-,CD8-splenocytes were expanded for 7 days in vitro with anti-CD3-epsilon mAb (145-2C11) and IL-2, the majority of the TCR-gamma delta cells expressed B220 and IL-2R, and 10 to 20% were CD8+. In comparison to CD8+ TCR-alpha beta T cells, the population of CD8+ TCR-gamma delta-bearing T cells exhibited reduced levels of CD8, and about 70% of the CD8+ TCR-gamma delta cells did not express Lyt-3 on the cell surface. Functional studies demonstrated that splenic TCR-gamma delta cells proliferated when stimulated with mAb directed against CD3-epsilon, Thy-1, and Ly-6C, but not when incubated with an anti-TCR V beta 8 mAb, consistent with the lack of TCR-alpha beta expression. In addition, activated CD3+,CD4-,CD8- peripheral murine TCR-gamma delta cells were capable of lysing syngeneic FcR-bearing targets in the presence of anti-CD3-epsilon mAb and the NK-sensitive cell line, YAC-1, in the absence of anti-CD3-epsilon mAb. Finally, activated CD3+, CD4-,CD8-,TCR-gamma delta+ splenocytes were also capable of producing IL-2, IL-3, IFN-gamma, and TNF when stimulated in vitro with anti-CD3-epsilon mAb.

Phenotypic and functional analysis of murine CD3+,CD4-,CD8- TCR-gamma delta-expressing peripheral T cells

Murine CD3+,CD4-,CD8- peripheral T cells, which express various forms of the TCR-gamma delta on their cell surface, have been characterized in terms of their cell-surface phenotype, proliferative and lytic potential, and lymphokine-producing capabilities. Three-color flow cytofluorometric analysis demonstrated that freshly isolated CD3+,CD4-, CD8- TCR-gamma delta lymph node cells were predominantly Thy-1+,CD5dull,IL-2R-,HSA-,B220-, and approximately 70% Ly-6C+ and 70% Pgp-1+. After CD3+,CD4-,CD8-splenocytes were expanded for 7 days in vitro with anti-CD3-epsilon mAb (145-2C11) and IL-2, the majority of the TCR-gamma delta cells expressed B220 and IL-2R, and 10 to 20% were CD8+. In comparison to CD8+ TCR-alpha beta T cells, the population of CD8+ TCR-gamma delta-bearing T cells exhibited reduced levels of CD8, and about 70% of the CD8+ TCR-gamma delta cells did not express Lyt-3 on the cell surface. Functional studies demonstrated that splenic TCR-gamma delta cells proliferated when stimulated with mAb directed against CD3-epsilon, Thy-1, and Ly-6C, but not when incubated with an anti-TCR V beta 8 mAb, consistent with the lack of TCR-alpha beta expression. In addition, activated CD3+,CD4-,CD8- peripheral murine TCR-gamma delta cells were capable of lysing syngeneic FcR-bearing targets in the presence of anti-CD3-epsilon mAb and the NK-sensitive cell line, YAC-1, in the absence of anti-CD3-epsilon mAb. Finally, activated CD3+, CD4-,CD8-,TCR-gamma delta+ splenocytes were also capable of producing IL-2, IL-3, IFN-gamma, and TNF when stimulated in vitro with anti-CD3-epsilon mAb.