Elizabeth W. Shores

Defective lymphoid development in mice lacking expression of the common cytokine receptor γ chain

The common gamma chain (gamma c) of the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors is defective in humans with XSCID. Mice lacking gamma c expression had hypoplastic thymuses; the thymocytes responded to gamma c-independent mitogens, but not gamma c-dependent stimuli. Splenic T cells were diminished at 3 weeks of age, but CD4+ T cells markedly increased by 4 weeks. B cells were greatly diminished in contrast with the situation in XSCID. NK cells, gamma delta intestinal intraepithelial lymphocytes, dendritic epidermal T cells, peripheral lymph nodes, and gut-associated lymphoid tissue were absent. These findings underscore the importance of gamma c in lymphoid development. Moreover, differences in humans and mice lacking gamma c expression indicate species-specific differences in the roles of gamma c-dependent cytokines or in the existence of redundant pathways. These mice provide an important model for studying the pathophysiology provide an important model for studying the pathophysiology of and gene therapy for human XSCID.

Fine Tuning of TCR Signaling by CD5

Current data indicate that CD5 functions as an inhibitor of TCR signal transduction. Consistent with this role, thymocyte selection in TCR transgenic/CD5−/− mice is altered in a manner suggestive of enhanced TCR signaling. However, the impact of CD5 deletion on thymocyte selection varies depending on the transgenic TCR analyzed, ranging from a slight to a marked shift from positive toward negative selection. An explanation for the variable effect of CD5 on selection is suggested by the observation that CD5 surface expression is regulated by TCR signal intensity during development and CD5 surface levels on mature thymocytes and T cells parallel the avidity of the positively selecting TCR/MHC/ligand interaction. In this study, we generated mice that overexpress CD5 during thymocyte development (CD5-tg), and then examined the effect of CD5 overexpression or CD5 deletion (CD5−/−) on selection of thymocytes that express the same TCR transgenes. The results demonstrate that the effect on thymocyte selection of altering CD5 expression depends on the avidity of the selecting interaction and, consequently, the level of basal (endogenous) CD5 surface expression. Substitution of endogenous CD5 with a transgene encoding a truncated form of the protein failed to rescue the CD5−/− phenotype, demonstrating that the cytoplasmic domain of CD5 is required for its inhibitory function. Together, these results indicate that inducible regulation of CD5 surface expression during thymocyte selection functions to fine tune the TCR signaling response.

Role of the Common Cytokine Receptor γ Chain in Cytokine Signaling and Lymphoid Development

To examine the role of gamma c in lymphoid development, we have analyzed mice in which the gamma c gene was specifically inactivated by homologous recombination. These mice also serve as an animal model of human X-linked severe combined immunodeficiency (XSCID). Interestingly, gamma c knockout mice exhibited a somewhat different phenotype than humans with XSCID. Absolute T-cell numbers are greatly diminished in young gamma c-/Y mice, but accumulate with age. gamma delta T cells and NK cells are absent in gamma c-/Y mice and conventional B cells are greatly diminished, yet substantial numbers of peritoneal B-1 cells are present. Since humans with XSCID have essentially no mature T cells, it is especially striking that T cells are readily apparent in gamma c-/Y mice. This observation indicates that in these mice, the gamma c-dependent block in T cell development is less severe than it is in humans. It is possible but unproven that thymic stromal derived lymphopoietin, TSLP, contributes to thymocyte development in these mice. Since B-cell numbers are normal in humans with XSCID, it is also striking that gamma c-/Y mice paradoxically exhibit greatly diminished numbers of B cells. This likely indicates that IL-7 signaling plays a critical role in pre-B cell maturation in mice but is less important in humans. Thus, the abnormalities observed in gamma c-/Y mice have provided clues to assist in dissecting the role of cytokines and their receptors in lymphoid development and have also identified interesting differences in the regulation of this process in mice and humans.

Antiproliferative action of interferon-alpha requires components of T-cell-receptor signalling

Signal transduction through both cytokine and lymphocyte antigen receptors shares some common pathways by which they initiate cellular responses, such as activation of mitogen-activated protein kinase(s),. However, other signalling components appear to be uniquely coupled to each receptor. For example, the interferon receptors transduce regulatory signals through the JAK/STAT pathway, resulting in an inhibition of growth and of antiviral effects, whereas this pathway apparently plays no role in T-cell-receptor (TCR)-dependent gene expression,. Conversely, signal transduction through the TCR requires the tyrosine kinases Lck and ZAP-70 and the tyrosine phosphatase CD45 (ref. 5). Here we show that, unexpectedly, transmission of growth-inhibitory signals by interferon-α (IFN-α) in T cells requires the expression and association of CD45, Lck and ZAP-70 with the IFN-α-receptor signalling complex.

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 ζ-chains containing either three, one, or zero ITAMs for endogenous (3-ITAM) ζ-chain. These ζ-chain variants were then bred into different αβTCR 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 ζ-chain ITAMs critically depends upon the specificity and apparently, affinity, of the TCR for its selecting ligand(s).

An architectural perspective on signaling by the pre-, αβ and γδ T cell receptors

The T cell antigen receptor (TCR) is a multimeric complex composed of an antigen‐binding clonotypic heterodimer and a signal transducing complex consisting of the CD3 dimers (CD3γε and CD3δε) and a TCR‐ζ homodimer. In all jawed vertebrates there are two T cell lineages, αβ and γδ, distinguished by the clonotypic subunits contained within their TCRs (TCR‐α and ‐β or TCR‐γ and ‐δ, respectively). A third receptor complex, the preTCR, is only expressed on immature T cells. The preTCR, which contains the invariant pre‐Tα (pTα) chain in lieu of TCR‐α, plays a critical role in the early development of αβ lineage cells. The subunit composition of the signal transducing complexes of the pre‐, αβ‐ and γδTCRs was previously thought to be identical. However, recent data demonstrate that there are significant differences in the signal transducing complexes of these three TCRs. For example, αβTCRs contain both CD3γε and CD3δε dimers, whereas γδTCRs contain only CD3γε dimers. Moreover, preTCR function appears to be unaffected in the absence of CD3δ, suggesting that CD3δε dimers are dispensable for pre‐TCR assembly. In this review, we summarize current data relating to the subunit composition of the pre‐, αβ‐ and γδTCRs and discuss how these structural differences may impact receptor signaling and αβ/γδ lineage determination.

TCR ζ chain in T cell development and selection

Abstract Current data suggest that an important function of the multimericstructure of the TCR is to enable the assembly of structurally and functionally different forms of the TCR, the pre-TCR and αβTCR complexes, at different stages in development. Four distinct TCR subunits (the CD3γ, δ, and e chains and the ζ chain) contain signal transducing motifs; however, the ζ chain is notable for containing three of these elements. These motifs, especially those within the ζ 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.

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.