The common cytokine receptor gamma chain plays an essential role in regulating lymphoid homeostasis.

In the immune system, there is a careful regulation not only of lymphoid development and proliferation, but also of the fate of activated and proliferating cells. Although the manner in which these diverse events are coordinated is incompletely understood, cytokines are known to play major roles. Whereas IL-7 is essential for lymphoid development, IL-2 and IL-4 are vital for lymphocyte proliferation. The receptors for each of these cytokines contain the common cytokine receptor gamma chain (gammac), and it was previously shown that gammac-deficient mice exhibit severely compromised development and responsiveness to IL-2, IL-4, and IL-7. Nevertheless, these mice exhibit an age-dependent accumulation of splenic CD4+ T cells, the majority of which have a phenotype typical of memory/activated cells. When gammac-deficient mice were mated to DO11.10 T cell receptor (TCR) transgenic mice, only the T cells bearing endogenous TCRs had this phenotype, suggesting that its acquisition was TCR dependent. Not only do the CD4+ T cells from gammac-deficient mice exhibit an activated phenotype and greatly enhanced incorporation of bromodeoxyuridine but, consistent with the lack of gammac-dependent survival signals, they also exhibit an augmented rate of apoptosis. However, because the CD4+ T cells accumulate, it is clear that the rate of proliferation exceeds the rate of cell death. Thus, surprisingly, although gammac-independent signals are sufficient to mediate expansion of CD4+ T cells in these mice, gammac-dependent signals are required to regulate the fate of activated CD4+ T cells, underscoring the importance of gammac-dependent signals in controlling lymphoid homeo-stasis.

Developmentally regulated expression of CD3 components independent of clonotypic T cell antigen receptor complexes on immature thymocytes.

CD3 signal transducing proteins are thought to be expressed on the surface of T cells only as part of clonotypic T cell receptor (TCR) complexes. Contrary to this paradigm, the present study describes surface expression of CD3 proteins independently of clonotypic TCR complexes, but only on immature thymocytes. Such novel clonotype-independent CD3 (CIC) complexes are composed primarily of CD3 gamma epsilon and secondarily of CD3 delta epsilon heterodimers that are independent of one another and are expressed on the cell surface in association with an unknown 90-100 kD protein termed CD3-associated protein (CD3AP). CIC complexes are expressed in normal mice on early thymocytes through the CD4+CD8+ stage of development, but not on mature peripheral T cells. Furthermore, CIC complexes are expressed by both TCR- severe combined immunodeficiency (SCID) thymocytes and thymoma cell lines, in the absence of any clonotypic chains. The isolation and biochemical characterization of surface CIC complexes provides a structural basis for the signaling effects of anti-CD3 epsilon antibody treatment in early thymocyte development.

CD5 expression is developmentally regulated by T cell receptor (TCR) signals and TCR avidity.

Recent data indicate that the cell surface glycoprotein CD5 functions as a negative regulator of T cell receptor (TCR)-mediated signaling. In this study, we examined the regulation of CD5 surface expression during normal thymocyte ontogeny and in mice with developmental and/or signal transduction defects. The results demonstrate that low level expression of CD5 on CD4(-)CD8(-) (double negative, DN) thymocytes is independent of TCR gene rearrangement; however, induction of CD5 surface expression on DN thymocytes requires engagement of the pre-TCR and is dependent upon the activity of p56(lck). At the CD4(+)CD8(+) (double positive, DP) stage, intermediate CD5 levels are maintained by low affinity TCR-major histocompatibility complex (MHC) interactions, and CD5 surface expression is proportional to both the surface level and signaling capacity of the TCR. High-level expression of CD5 on DP and CD4(+) or CD8(+) (single positive, SP) thymocytes is induced by engagement of the alpha/beta-TCR by (positively or negatively) selecting ligands. Significantly, CD5 surface expression on mature SP thymocytes and T cells was found to directly parallel the avidity or signaling intensity of the positively selecting TCR-MHC-ligand interaction. Taken together, these observations suggest that the developmental regulation of CD5 in response to TCR signaling and TCR avidity represents a mechanism for fine tuning of the TCR signaling response.

Qualitative and quantitative contributions of the T cell receptor zeta chain to mature T cell apoptosis.

Engagement of the T cell receptor (TCR) of mature T lymphocytes can lead either to activation/proliferation responses or programmed cell death. To understand the molecular regulation of these two fundamentally different outcomes of TCR signaling, we investigated the participation of various components of the TCR-CD3 complex. We found that the TCR-zeta chain, while not absolutely required, was especially effective at promoting mature T cell apoptosis compared with the CD3 epsilon, gamma, or delta chains. We also carried out mutagenesis to address the role of the immunoreceptor tyrosine-based activation motifs (ITAMs) that are the principal signaling components found three times in the TCR-zeta chain and once in each of the CD3 epsilon, gamma, or delta chains. We found that the ability of the TCR-zeta chain to promote apoptosis results both from a quantitative effect of the presence of multiple ITAMs as well as qualitatively different contributions made by individual ITAMs. Apoptosis induced by single chain chimeras revealed that the first zeta ITAM stimulated greater apoptosis than the third zeta ITAM, and the second zeta ITAM was unable to trigger apoptosis. Because microheterogeneity in the amino acid sequence of the various ITAM motifs found in the TCR-zeta and CD3 chains predicts interactions with distinct src-homology-2-domain signaling proteins, our results suggest the possibility that individual ITAM motifs might play unique roles in TCR responses by engaging specific signaling pathways.

Role of the multiple T cell receptor (TCR)-zeta chain signaling motifs in selection of the T cell repertoire.

Immature thymocytes undergo a selection process within the thymus based on their T cell antigen receptor (TCR) specificity that results either in their maturation into functionally competent, self-MHC-restricted T cells (positive selection) or their deletion (negative selection). The outcome of thymocyte selection is thought to be controlled by signals transduced by the TCR that vary in relation to the avidity of the TCR-ligand interaction. The TCR is composed of four distinct signal transducing subunits (CD3-gamma, -delta, -epsilon, and zeta) that contain either one (CD3-gamma, -delta, -epsilon) or three (-zeta) signaling motifs (ITAMs) within their intracytoplasmic domains. A possible function for multiple TCR ITAMs could be to amplify signals generated by the TCR during selection. To determine the importance of the multiple TCR-zeta chain ITAMs in thymocyte selection, transgenes encoding alpha/beta TCRs with known specificity were bred into mice in which zeta chains lacking one or more ITAMs had been genetically substituted for endogenous zeta. A direct relationship was observed between the number of zeta chain ITAMs within the TCR complex and the efficiency of both positive and negative selection. These results reveal a role for multiple TCR ITAMs in thymocyte selection and identify a function for TCR signal amplification in formation of the T cell repertoire.

Function of CD3 epsilon-mediated signals in T cell development.

The T cell antigen receptor (TCR) and pre-TCR complexes are composed of multiple signal-transducing subunits (CD3 gamma, CD3 delta, CD3 epsilon, and zeta) that each contain one or more copies of a semiconserved functional motif, the immunoreceptor tyrosine-based activation motif (ITAM). Although biochemical studies indicate that individual TCR-ITAMs may bind selectively or with different affinity to various effector molecules, data from other experiments suggest that at least some ITAMs are functionally equivalent. In this study, we examined the role of CD3straightepsilon ITAM-mediated signals in T cell development by genetically reconstituting CD3 epsilon-deficient mice with transgenes encoding either wild-type or ITAM-mutant (signaling defective) forms of the protein. The results demonstrate that signals transduced by CD3 epsilon are not specifically required for T cell maturation but instead contribute quantitatively to TCR signaling in a manner similar to that previously observed for zeta chain. Unexpectedly, analysis of TCR-transgenic/CD3 epsilon-mutant mice reveals a potential role for CD3 epsilon signals in T cell survival.

Delayed maturation of CD4- CD8- Fc gamma RII/III+ T and natural killer cell precursors in Fc epsilon RI gamma transgenic mice.

Fc epsilon RI gamma (gamma) is a member of a group of related proteins (the zeta-family dimers) that function as signal-transducing components of both Fc receptors and the T cell antigen receptor (TCR). Analysis of gamma expression during fetal thymus ontogeny revealed that it is expressed in early thymocytes, before the initiation of clonotypic TCR-alpha and TCR-beta gene rearrangement but is down-regulated in most adult thymocytes. To explore a possible role for gamma in thymocyte development, we generated transgenic mice in which this protein was overexpressed at all stages of ontogeny. Overexpression of gamma inhibited the maturation of T cells as well as natural killer (NK) cells. The developmental effects were transgene dose related and correlated with markedly delayed maturation of fetal CD4-CD8- FcRII/III+ thymocytes, cells thought to include the progenitors of both T and NK cells. These results suggest that the zeta and gamma chains serve distinctive functions in thymocyte development and indicate that Fc receptor(s) may play an important role in regulating the differentiation of early progenitor cells within the thymus.

Differential effects of zeta and eta transgenes on early alpha/beta T cell development.

The zeta-family dimers (zeta, eta, and gamma) are a group of structurally and functionally related proteins that are expressed in developing thymocytes and function as signal transducing subunits of the T cell antigen receptor (TCR) and certain Ig Fc receptors. Zeta, eta, and gamma each contain one or more copies of a conserved tyrosine-based activation motif (TAM) that is known to be required for signal transduction. To examine the developmental importance of multiple or individual TAM elements we generated transgenic mice that express: (a) full-length (FL) zeta-chain (3 TAMs); (b) eta-chain, a naturally occurring variant of zeta that is derived from alternative splicing (2 TAMs); or (c) truncated zeta-chain (CT108; 1 TAM), under the control of the human CD2 promoter and regulatory elements. Unexpectedly, we found that overexpression of the FL zeta chain caused premature termination of RAG-1 and RAG-2 expression, prevented productive rearrangement of the TCR-alpha and TCR-beta genes and blocked entry of thymocytes into the CD4/CD8 developmental pathway. In contrast, we found that overexpression of eta or CT108 had no effect on normal thymocyte maturation. These results suggest that an early signaling pathway exists in precursor TCR- thymocytes that can regulate RAG-1 and RAG-2 expression and is differentially responsive to individual members of the zeta-family dimers.

T cell development in mice lacking all T cell receptor zeta family members (Zeta, eta, and FcepsilonRIgamma).

The zeta family includes zeta, eta, and FcepsilonRIgamma (Fcgamma). Dimers of the zeta family proteins function as signal transducing subunits of the T cell antigen receptor (TCR), the pre-TCR, and a subset of Fc receptors. In mice lacking zeta/eta chains, T cell development is impaired, yet low numbers of CD4+ and CD8+ T cells develop. This finding suggests either that pre-TCR and TCR complexes lacking a zeta family dimer can promote T cell maturation, or that in the absence of zeta/eta, Fcgamma serves as a subunit in TCR complexes. To elucidate the role of zeta family dimers in T cell development, we generated mice lacking expression of all of these proteins and compared their phenotype to mice lacking only zeta/eta or Fcgamma. The data reveal that surface complexes that are expressed in the absence of zeta family dimers are capable of transducing signals required for alpha/beta-T cell development. Strikingly, T cells generated in both zeta/eta-/- and zeta/eta-/--Fcgamma-/- mice exhibit a memory phenotype and elaborate interferon gamma. Finally, examination of different T cell populations reveals that zeta/eta and Fcgamma have distinct expression patterns that correlate with their thymus dependency. A possible function for the differential expression of zeta family proteins may be to impart distinctive signaling properties to TCR complexes expressed on specific T cell populations.

Mice deficient in nuclear factor (NF)-kappa B/p52 present with defects in humoral responses, germinal center reactions, and splenic microarchitecture.

p52 is a subunit of nuclear factor (NF)-kappa B transcription factors, most closely related to p50. Previously, we have shown that p52, but not p50 homodimers can form transactivating complexes when associated with Bcl-3, an unusual member of the I kappa B family. To determine nonredundant physiologic roles of p52, we generated mice deficient in p52. Null mutant mice were impaired in their ability to generate antibodies to T-dependent antigens, consistent with an absence of B cell follicles and follicular dendritic cell networks in secondary lymphoid organs, and an inability to form germinal centers. Furthermore, the splenic marginal zone was disrupted. These phenotypes are largely overlapping with those observed in Bcl-3 knockout animals, but distinct from those of p50 knockouts, supporting the notion of a physiologically relevant complex of p52 homodimers and Bcl-3. Adoptive transfer experiments further suggest that such a complex may be critical in accessory cell functions during antigen-specific immune reactions. Possible roles of p52 and Bcl-3 are discussed that may underlie the oncogenic potential of these proteins, as evidenced by recurrent chromosomal translocations of their genes in lymphoid tumors.

Negative selection of precursor thymocytes before their differentiation into CD4+CD8+ cells.

Thymic selection of the developing T cell repertoire is thought to occur at the CD4+CD8+ stage of differentiation and to be determined by the specificity of the T cell receptors (TCRs) that CD4+CD8+ thymocytes express. However, TCR signals can inhibit the differentiation of precursor thymocytes into CD4+CD8+ cells, which suggests that selection might occur earlier than thought. Indeed, in a negatively selecting male thymus, CD4-CD8lo precursor thymocytes that express a transgenic TCR to male antigen are developmentally arrested as a consequence of antigen encounter and fail to become CD4+CD8+. Thus, negative selection can occur before the CD4+CD8+ stage of differentiation.

In vivo calcium elevations in thymocytes with T cell receptors that are specific for self ligands.

Selection of the T cell receptor (TCR) repertoire in the thymus probably involves TCR-mediated signals transduced in developing thymocytes after interaction with thymic stromal cells bearing self ligands. TCR-transduced signals should have identifiable consequences that would distinguish thymocytes whose TCRs have been engaged by self ligands from those whose TCRs have not. Among thymocytes expressing a transgenic TCR of defined specificity, a large number had elevated intracellular calcium concentrations but only when resident in a negatively selecting thymus in which their self ligand was expressed. Thus, developing thymocytes are stimulated by endogenous ligands in vivo to mobilize intracellular calcium, and increased intracellular calcium concentrations may reflect the consequences of intrathymic signaling associated with thymic negative selection.

Role of TCR zeta chain in T cell development and selection.

Signals mediated by the T cell receptor (TCR) are required for thymocyte maturation and selection. To examine the role of TCR zeta chain signals in development, TCR expression was restored in zeta-deficient mice with transgenic zeta chains that partially or completely lacked sequences required for signal transduction. The zeta chain played a role in thymic development by promoting TCR surface expression, but zeta-mediated signals were not essential because TCRs that contained signaling-deficient zeta chains promoted T cell maturation and transduced signals associated with thymic selection.

T cell development in mice that lack the zeta chain of the T cell antigen receptor complex.

The zeta subunit of the T cell antigen receptor complex is required for targeting nascent receptor complexes to the cell surface and for receptor-mediated signal transduction. To examine the significance of the zeta subunit in T cell development, mice deficient for zeta expression were generated by gene targeting. These zeta-/- mice had few CD4+CD8+ thymocytes, and the generation of CD4+ and CD8+ single positive T cells was impaired but not completely abrogated. Peripheral T cells were present but were unusual in that they expressed small amounts of CD5 and few T cell receptors. Thus, zeta chain expression influences thymocyte differentiation but is not absolutely required for the generation of single positive T cells.

TCR zeta chain in T cell development and selection.

Current data suggest that an important function of the multimeric structure of the TCR is to enable the assembly of structurally and functionally different forms of the TCR, the pre-TCR and alphabetaTCR complexes, at different stages in development. Four distinct TCR subunits (the CD3gamma, delta, and epsilon chains and the zeta chain) contain signal transducing motifs; however, the zeta chain is notable for containing three of these elements. These motifs, especially those within the zeta chain, function to amplify signals generated by the TCR, and this property is especially critical during thymocyte selection. The results of several recent experiments argue that positive and negative selection of thymocytes may involve activation of distinct downstream signaling pathways. The outcome of thymocyte selection can also be influenced, however, by quantitative effects such as changes in ligand concentration or direct alteration of the TCR signaling potential. Recent studies pertaining to the kinetics of TCR-ligand interactions may provide insight into how signaling through the TCR can be regulated either quantitatively or qualitatively.

Murine txk: a protein tyrosine kinase gene regulated by T cell activation.

To identify genes involved in signal transduction pathways that regulate T cell activation and development, murine fetal thymocytes were screened for expression of protein tyrosine kinase family members by the polymerase chain reaction. Using this approach, a non-receptor protein tyrosine kinase, txk, was identified and cloned. Tsk is expressed in thymocytes as early as fetal day 13.5 and its expression at the mRNA level continues throughout development. Txk transcripts are present in thymocytes, peripheral T cells and mast cell lines, but are not detectable in B cell macrophage/monocyte cell lines or in non-hematopoietic fetal or adult tissues. In both thymocytes and T cells, txk transcripts are down-regulated after activation with PMA and ionomycin, concanavalin A or T cell receptor cross-linking. Sequence analysis indicates that txk contains SH2, SH3 and kinase catalytic domains and belongs to the tec family of cytoplasmic protein tyrosine kinases which includes tec, itk and btk. Its unique N-terminus contains a proline-rich region, but unlike the other tec family members, does not contain a pleckstrin homology domain. The restricted expression pattern of txk and its regulation by T cell activation make it an excellent candidate for involvement in signal transduction during thymocyte development.

Insights into T cell development and signal transduction provided by TCR-zeta chain deficient mice.

The T cell antigen receptor (TCR) transduces signals that mediate different responses depending on the stage of development of the T cell and the nature of the ligand it engages. The presence of multiple signal transducing subunits (CD3-gamma-delta,-epsilon and zeta chain) suggests the potential to control these responses by altering the subunit composition of the TCR. zeta chain represents an especially important signalling molecule as it contains multiple signalling motifs within its cytoplasmic tail. The generation and analysis of zeta deficient (zeta-/-) and zeta-transgenic mice has provided insight into the role of zeta as well as the CD3 subunits in TCR surface expression, T cell activation and thymocyte development. Herein, we discuss the results from such experiments which suggest distinct roles for zeta chain and the CD3 components at different stages of T cell development.

Critical relationship between TCR signaling potential and TCR affinity during thymocyte selection.

Whether a developing thymocyte becomes positively or negatively selected is thought to be determined by the affinity/avidity of its TCR for MHC/peptide ligands expressed in the thymus. Presumably, differences in affinity translate into differences in the potency of the ensuing TCR-mediated signals, and these differences in signal strength determine the outcome of thymocyte selection. However, there is little direct evidence establishing a relationship between TCR-ligand affinity and signal strength during positive and negative selection. The TCR complex contains multiple signaling motifs, known as immunoreceptor tyrosine-based activation motifs (ITAMs) that are required for T cell activation. To examine the effects of TCR signal strength on selection, the signaling potential of the TCR was modified by substituting transgenic TCR zeta-chains containing either three, one, or zero ITAMs for endogenous (3-ITAM) zeta-chain. These zeta-chain variants were then bred into different alphabetaTCR transgenic backgrounds. We report that reductions in TCR signaling potential have distinct effects on the selection of thymocytes expressing different TCRs, and that the requirement for zeta-chain ITAMs critically depends upon the specificity and apparently, affinity, of the TCR for its selecting ligand(s).

Maturation of medullary thymic epithelium requires thymocytes expressing fully assembled CD3-TCR complexes.

Unlike medullary thymic epithelial cells (TEC) of normal mice, medullary TEC of TCR- SCID mice are immature and disorganized. In order to assess directly the role of TCR+ cells in the development of medullary TEC, we bred mice which co-expressed the SCID genetic defect and transgenes encoding clonotypic TCR chains. Immunohistologic examination revealed that medullary thymic epithelial cells from TCR beta transgenic SCID mice, whose thymocytes only express TCR beta chains that inefficiently associate with CD3 and zeta components, remained immature and disorganized. In contrast, medullary TEC from TCR alpha beta transgenic SCID mice, whose thymocytes express fully assembled CD3-zeta-TCR alpha beta complexes were mature and organized. Interestingly, the ability of TCR alpha beta(+)-zeta(+)-CD3+ thymocytes to induce maturation of medullary TEC appeared not to be related to the antigen specificity of the TCR as thymi from positively selecting, negatively selecting and non-selecting TCR alpha beta transgenic SCID mice all possessed induced medullary thymic epithelial cells. In addition, we found that induction of medullary TEC cells was associated with the presence of medullary thymocytes, including those of the CD4-CD8- TCR alpha beta+ phenotype. The present findings demonstrate that fully assembled CD3-zeta-TCR complexes are required to induce maturation of medullary thymic epithelial cells and indicate that thymocyte induction of medullary thymic epithelial cells may result from signaling independently of their clonotypic TCR chains.