On the role of CD3delta chains in TCRgammadelta/CD3 complexes during assembly and membrane expression

Anti-CD3 Fab Fragments Enhance Tumor Killing by Human γδ T Cells Independent of Nck Recruitment to the γδ T Cell Antigen Receptor

T lymphocytes expressing the γδ T cell receptor (γδ TCR) can recognize antigens expressed by tumor cells and subsequently kill these cells. γδ T cells are indeed used in cancer immunotherapy clinical trials. The anti-CD3ε antibody UCHT1 enhanced the in vitro tumor killing activity of human γδ T cells by an unknown molecular mechanism. Here, we demonstrate that Fab fragments of UCHT1, which only bind monovalently to the γδ TCR, also enhanced tumor killing by expanded human Vγ9Vδ2 γδ T cells or pan-γδ T cells of the peripheral blood. The Fab fragments induced Nck recruitment to the γδ TCR, suggesting that they stabilized the γδ TCR in an active CD3ε conformation. However, blocking the Nck-CD3ε interaction in γδ T cells using the small molecule inhibitor AX-024 neither reduced the γδ T cells' natural nor the Fab-enhanced tumor killing activity. Likewise, Nck recruitment to CD3ε was not required for intracellular signaling, CD69 and CD25 up-regulation, or cytokine secretion by γδ T cells. Thus, the Nck-CD3ε interaction seems to be dispensable in γδ T cells.

Human CD3γ, but not CD3δ, haploinsufficiency differentially impairs γδ versus αβ surface TCR expression

Background

The T cell antigen receptors (TCR) of αβ and γδ T lymphocytes are believed to assemble in a similar fashion in humans. Firstly, αβ or γδ TCR chains incorporate a CD3δε dimer, then a CD3γε dimer and finally a ζζ homodimer, resulting in TCR complexes with the same CD3 dimer stoichiometry. Partial reduction in the expression of the highly homologous CD3γ and CD3δ proteins would thus be expected to have a similar impact in the assembly and surface expression of both TCR isotypes. To test this hypothesis, we compared the surface TCR expression of primary αβ and γδ T cells from healthy donors carrying a single null or leaky mutation in CD3G (γ^+/−) or CD3D (δ^+/−, δ^+/leaky) with that of normal controls.

Results

Although the partial reduction in the intracellular availability of CD3γ or CD3δ proteins was comparable as a consequence of the mutations, surface TCR expression measured with anti-CD3ε antibodies was significantly more decreased in γδ than in αβ T lymphocytes in CD3γ^+/− individuals, whereas CD3δ^+/− and CD3δ^+/leaky donors showed a similar decrease of surface TCR in both T cell lineages. Therefore, surface γδ TCR expression was more dependent on available CD3γ than surface αβ TCR expression.

Conclusions

The results support the existence of differential structural constraints in the two human TCR isotypes regarding the incorporation of CD3γε and CD3δε dimers, as revealed by their discordant surface expression behaviour when confronted with reduced amounts of CD3γ, but not of the homologous CD3δ chain. A modified version of the prevailing TCR assembly model is proposed to accommodate these new data.

A leaky mutation in CD3D differentially affects αβ and γδ T cells and leads to a Tαβ-Tγδ+B+NK+ human SCID

T cells recognize antigens via their cell surface TCR and are classified as either αβ or γδ depending on the variable chains in their TCR, α and β or γ and δ, respectively. Both αβ and γδ TCRs also contain several invariant chains, including CD3δ, which support surface TCR expression and transduce the TCR signal. Mutations in variable chains would be expected to affect a single T cell lineage, while mutations in the invariant chains would affect all T cells. Consistent with this, all CD3δ-deficient patients described to date showed a complete block in T cell development. However, CD3δ-KO mice have an αβ T cell-specific defect. Here, we report 2 unrelated cases of SCID with a selective block in αβ but not in γδ T cell development, associated with a new splicing mutation in the CD3D gene. The patients' T cells showed reduced CD3D transcripts, CD3δ proteins, surface TCR, and early TCR signaling. Their lymph nodes showed severe T cell depletion, recent thymus emigrants in peripheral blood were strongly decreased, and the scant αβ T cells were oligoclonal. T cell-dependent B cell functions were also impaired, despite the presence of normal B cell numbers. Strikingly, despite the specific loss of αβ T cells, surface TCR expression was more reduced in γδ than in αβ T cells. Analysis of individuals with this CD3D mutation thus demonstrates the contrasting CD3δ requirements for αβ versus γδ T cell development and TCR expression in humans and highlights the diagnostic and clinical relevance of studying both TCR isotypes when a T cell defect is suspected.

Teleost B7 expressed on monocytes regulates T cell responses

In mammals, professional APCs induce adaptive immunity via the activation of T cells. During this process, B7 family molecules present upon APCs are known to play crucial roles in optimal T cell stimulation. In contrast, the confirmation of APCs in a nonmammalian vertebrate has yet to be achieved. To obtain further insights into the evolutionary origin of APCs, we have identified three members of the B7 family in the teleost Takifugu rubripes (fugu): B7-H1/DC, B7-H3, and B7-H4. The three fugu B7s were expressed on the surface of blood monocytes. The B7(+) monocytes, which are composed of at least two distinct populations, expressed the MHC class II component gene. The fugu B7 molecules bound to activated T cells, indicating that putative B7 receptors were expressed upon T cells. Fugu B7-H1/DC inhibited T cell proliferation concomitant with increasing levels of both IL-10 and IFN-gamma expression, whereas both B7-H3 and B7-H4 promoted T cell growth following IL-2 induction and the suppression of IL-10. These observations indicate that fugu B7s regulate T cell responses via receptors upon T cells. We suggest that fish B7(+) monocytes are APCs and that a costimulatory system has already developed in fish via the evolutionary process.

Different composition of the human and the mouse gammadelta T cell receptor explains different phenotypes of CD3gamma and CD3delta immunodeficiencies

The γδ T cell receptor for antigen (TCR) comprises the clonotypic TCRγδ, the CD3 (CD3γε and/or CD3δε), and the ζζ dimers. γδ T cells do not develop in CD3γ-deficient mice, whereas human patients lacking CD3γ have abundant peripheral blood γδ T cells expressing high γδ TCR levels. In an attempt to identify the molecular basis for these discordant phenotypes, we determined the stoichiometries of mouse and human γδ TCRs using blue native polyacrylamide gel electrophoresis and anti-TCR–specific antibodies. The γδ TCR isolated in digitonin from primary and cultured human γδ T cells includes CD3δ, with a TCRγδCD3ε₂δγζ₂ stoichiometry. In CD3γ-deficient patients, this may allow substitution of CD3γ by the CD3δ chain and thereby support γδ T cell development. In contrast, the mouse γδ TCR does not incorporate CD3δ and has a TCRγδCD3ε₂γ₂ζ₂ stoichiometry. CD3γ-deficient mice exhibit a block in γδ T cell development. A human, but not a mouse, CD3δ transgene rescues γδ T cell development in mice lacking both mouse CD3δ and CD3γ chains. This suggests important structural and/or functional differences between human and mouse CD3δ chains during γδ T cell development. Collectively, our results indicate that the different γδ T cell phenotypes between CD3γ-deficient humans and mice can be explained by differences in their γδ TCR composition.

Stoichiometry of the murine gammadelta T cell receptor

The T cell receptor for antigen (TCR) complex is organized into two functional domains: the antigen-binding clonotypic heterodimer and the signal-transducing invariant CD3 and TCRzeta chains. In most vertebrates, there are two different clonotypic heterodimers (TCRalphabeta and TCRgammadelta) that define the alphabeta and gammadelta T cell lineages, respectively. alphabeta- and gammadeltaTCRs also differ in their invariant chain subunit composition, in that alphabetaTCRs contain CD3gammaepsilon and CD3deltaepsilon dimers, whereas gammadeltaTCRs contain only CD3gammaepsilon dimers. This difference in subunit composition of the alphabeta- and gammadeltaTCRs raises the question of whether the stoichiometries of these receptor complexes are different. As the stoichiometry of the murine gammadeltaTCR has not been previously investigated, we used two quantitative immunofluorescent approaches to determine the valency of TCRgammadelta heterodimers and CD3gammaepsilon dimers in surface murine gammadeltaTCR complexes. Our results support a model of murine gammadeltaTCR stoichiometry in which there are two CD3gammaepsilon dimers for every TCRgammadelta heterodimer.

Surface T-cell antigen receptor expression and availability for long-term antigenic signaling

It is important to understand how T-cell antigen receptor (TCR) engagement and signaling are regulated throughout an immune response. This review examines the dynamics of surface TCR expression and signaling capacity during thymic and effector T-cell development. Although the TCR can undergo vast changes in surface expression, T cells remain capable of sustaining TCR engagement for long periods of time. This may be achieved by a combination of mechanisms that involve (a) controlling the quantity of surface TCR available for ligand interaction and (b) controlling the quality of surface TCR expression during T-cell activation. TCR signaling itself appears to be one of the main quantitative modulators of surface TCR expression, and it can cause both downregulation and upregulation at different times of T-cell activation. Recent studies indicate that the degree of upregulation is tunable by the strength of antigenic stimulation. There is evidence that qualitatively distinct forms of the TCR exist, and their potential role in sustained antigenic signaling is also discussed. A goal of future studies will be to better characterize these modulations in surface TCR expression and to clarify their impact on the regulation of immune responses.

Gammadelta T cells and interleukin-6 levels could provide information regarding the progression of human renal allograft

We have determined the percentage of alphabeta and gammadelta T cells by flow cytometry as well as serum interleukin-6 (IL-6) and soluble interleukin-6 receptor (sIL-6R) levels by enzyme-linked immunosorbent assay in kidney allograft recipients with acute, chronic or stable graft evolution. The percentage of CD4 and CD8 T cells in transplanted patients was lower than in the control group (P < 0.001) with the exception of CD8 gammadelta T cells from patients with stable evolution (P > 0.05). The serum levels of IL-6 and sIL-6R in acute and chronic rejection were higher than in the controls (P < 0.05). No differences in IL-6 levels were observed between the stable evolution and the control groups (P > 0.05). The levels of sIL-6R were higher in stable evolution patients than in the controls (P < 0.05) and no differences were observed between the chronic and stable evolution patients (P > 0.05). IL-6 decreased in patients with a favourable evolution, increased in those with an increased renal dysfunction and was maintained when the renal dysfunction was not modified. These results suggest that gammadelta T cells could participate in renal allograft maintenance and that IL-6 but not sIL-6R serum levels may provide a prognostic marker for measuring the evolution of kidney allograft.

Structure of gammadelta T cell receptors and their recognition of non-peptide antigens

The gammadelta T cell receptors (TCRs) and alphabeta TCRs are similar in both sequence and structure; however, gammadelta+ and alphabeta+ T cells are not merely similar lymphocytes with subtly different receptors. These cell types differ in several ways, including the types of antigens recognized, the mechanism of antigen presentation and recognition and the mechanism and kinetics of downstream signaling events. gammadelta TCRs can directly recognize antigens in the form of intact proteins or non-peptidic compounds, unlike alphabeta TCRs which recognize peptide antigens bound to major histocompatibility complex molecules (MHC). One of the major classes of human gammadelta+ T cells expresses Vgamma9Vdelta2 TCRs which recognize pyrophosphomonoester, alkylamine and aminobisphosphonate antigens. This review focuses on the recently determined structure of a Vgamma9Vdelta2 TCR, with emphasis on antigen recognition and receptor signaling.

Some hints concerning the shape of T-cell receptor structures

Several models are proposed for T-cell antigen receptor (TCR) assembly and structure. However, there is little experimental data favouring directly either one or the other(s). The minimal complex appears to be composed of a TCRalphabeta/CD3deltaepsilon,gammaepsilon/zeta2 structure but at the cell membrane, multimers of this minimal structure may be formed. Quantitative cytofluometry has suggested three CD3epsilon chains for two TCRbeta (or TCRdelta) chains/complex. Such data should be repeated with monoclonal antibodies (MoAb) against extracellular (EC) parts of CD3delta or CD3gamma chains. In the present review, we have compared the TCR/CD3 assembly of pre-TCR, TCRgammadelta and TCRalphabeta containing complexes, and analysed the reactivity of antibodies (Abs) against the EC part of CD3delta chains. Our data suggest an alternative assembly pathway and structure of TCR/CD3 complexes.