B-cell-specific transgene expression using a self-inactivating retroviral vector with human CD19 promoter and viral post-transcriptional regulatory element

Retroviral gene transfer resulting in transgene expression selectively restricted to specific cell lineages would be desirable for many gene therapeutic applications. Such transcriptional targeting of retroviruses can be accomplished by employing eukaryotic control elements in self-inactivating (SIN) retroviral vectors, but use of these vectors is complicated by an accompanying reduction in viral titers. To overcome this restriction and address the influence of the post-transcriptional regulatory element of the Woodchuck hepatitis virus (WPRE) on viral titers and transgene expression, we developed SIN-vectors with and without WPRE. Using the enhancer-promoter of the Spleen Focus Forming virus (SFFV) to direct eGFP expression to multiple hematopoietic lineages, we show that WPRE significantly (>10 x) increased viral titers (>10(6) per ml of unconcentrated supernatant) and transgene expression in NIH3T3 cells in vitro. Gene expression in vivo was significantly lowered in lymphoid cells, but not in myeloid cells when WPRE was present. Furthermore, the use of WPRE in combination with the B-cell lineage-specific CD19 promoter significantly increased viral titers and allowed targeting of transgene expression by SIN-vectors specifically to B cells throughout their development in primary and secondary lymphoid organs.

Dendritic cells are sufficient to cross-present self-antigens to CD8 T cells in vivo

The mechanism of cross-presentation enables professional APCs to induce CD8 T cell-mediated immune responses against exogenous Ags. Through this mechanism, APCs can induce either immunity against infectious pathogens or tolerance against self-Ag residing in extralymphatic locations. An unanswered question in this field concerns the identity of the cross-presenting APC. All major classes of professional APCs, particularly dendritic cells, macrophages, and B cells, have previously been shown to be able to cross-present Ags in vitro. In the present study, we have created transgenic mice where MHC class I expression is driven selectively in dendritic cells and provide direct in vivo evidence that dendritic cells are sufficient to cross-present exogenous self-Ags and induce Ag-specific cell division of CD8-positive T cells.

Cellular interactions involved in Th cell memory

The cellular interactions involved in maintaining CD4+ T cell memory have hitherto not been identified. In this report, we have investigated the roles played by B cells and dendritic cells (DCs) in this process. We show that long-lasting Th cell memory depends on the presence of B cells, but that direct Ag presentation by B cells is not required. Instead, Ag presentation by DCs is critical for the survival of memory Th cells. DCs presenting specific Ag can be detected in animals long after immunization. These findings support a model in which B cells provide an environment in which Ags may be trapped and retained. This Ag is periodically presented to memory CD4+ T cells by DCs, providing an essential survival signal.

Chimeric Fv-zeta or Fv-epsilon receptors are not sufficient to induce activation or cytokine production in peripheral T cells

In current clinical trials, chimeric antibody-like receptors fused to signaling domains derived from TCR-zeta or Fc(epsilon)RIgamma-chain are tested for their ability to lyse tumor cells in vivo. In this study, the function of primary T cells expressing such receptors has been investigated in transgenic mice. These receptors cannot induce proliferation of resting T cells or trigger the production of optimal amounts of cytokines. It is further demonstrated that an initial low presence of cytokine message and protein is disappearing rather fast, whereas the triggering of endogenous TCR/CD3 in the same cells leads to normal prolonged cytokine production. The direct clinical relevance of these findings is further underlined by the increased in vivo tumor rejection by T cells expressing chimeric receptors in presence of exogenous interleukin-2. Therefore, adoptive T-cell therapy using primary T cells transfected with single chain receptors might benefit substantially from the accompanying administration of cytokines. (Blood. 2000;96:1999-2001)

Co-stimulation and selection for T-cell help for germinal centres: the role of CD28 and OX40

Given the importance of responding to infections with the right defensive strategy, much interest has focused on cytokine differentiation in CD4+ T cells. However, relatively little is known of the logistics of T-cell help for B cells. Here, Lucy Walker and colleagues propose key roles for CD28 and OX40 in coordinating the selection, expansion and migration of CD4+ T cells to B-cell follicles.

Functional comparison of thymic B cells and dendritic cells in vivo

In this report we present a transgenic mouse model in which we targeted gene expression specifically to B-lymphocytes. Using the human CD19 promoter, we expressed major histocompatibility complex class II I-E molecules specifically on B cells of all tissues, but not on other cell types. If only B cells expressed I-E in a class II-deficient background, positive selection of CD4(+) T cells could not be observed. A comparison of the frequencies of I-E reactive Vbeta5(+) and Vbeta11(+) T cells shows that I-E expression on thymic B cells is sufficient to negatively select I-E reactive CD4(+) T cells partially, but not CD8(+) T cells. Thus partial negative but no positive selection events can be induced by B-lymphocytes in vivo. (Blood. 2000;95:2610-2616)

Developmental dissociation of thymic dendritic cell and thymocyte lineages revealed in growth factor receptor mutant mice

Thymocytes and thymic dendritic cell (DC) lineages develop simultaneously and may originate from a common intrathymic progenitor. Mice deficient for two growth factor receptor molecules [c-kit and the common cytokine receptor gamma chain (gamma(c))] lack all thymocytes including T cell progenitors. Despite this lack of pro-T cells, thymic DC compartments were identified in c-kit(-)gamma(c)(-) mice. Thus, c-kit- and gamma(c)-mediated signals are not essential to generate thymic DCs. In addition, pro-T cells do not appear to be obligatory progenitors of thymic DCs, because DC development is dissociated from the generation of thymocytes in these mice. Thymic DCs in c-kit(-)gamma(c)(-) mice are phenotypically and functionally normal. In contrast to wild-type mice, however, thymic DCs in c-kit(-)gamma(c)(-) and, notably, in RAG-2-deficient mice are CD8alpha(neg/low), indicating that CD8alpha expression on thymic DCs is not independent of thymocytes developing beyond the "RAG-block."

Role of dendritic cells in the immune response induced by mouse mammary tumor virus superantigen

After mouse mammary tumor virus (MMTV) infection, B lymphocytes present a superantigen (Sag) and receive help from the unlimited number of CD4(+) T cells expressing Sag-specific T-cell receptor Vbeta elements. The infected B cells divide and differentiate, similarly to what occurs in classical B-cell responses. The amplification of Sag-reactive T cells can be considered a primary immune response. Since B cells are usually not efficient in the activation of naive T cells, we addressed the question of whether professional antigen-presenting cells such as dendritic cells (DCs) are responsible for T-cell priming. We show here, using MMTV(SIM), a viral isolate which requires major histocompatibility complex class II I-E expression to induce a strong Sag response in vivo, that transgenic mice expressing I-E exclusively on DCs (I-EalphaDC tg) reveal a strong Sag response. This Sag response was dependent on the presence of B cells, as indicated by the absence of stimulation in I-EalphaDC tg mice lacking B cells (I-EalphaDC tg muMT(-/-)), even if these B cells lack I-E expression. Furthermore, the involvement of either residual transgene expression by B cells or transfer of I-E from DCs to B cells was excluded by the use of mixed bone marrow chimeras. Our results indicate that after priming by DCs in the context of I-E, the MMTV(SIM) Sag can be recognized on the surface of B cells in the context of I-A. The most likely physiological relevance of the lowering of the antigen threshold required for T-cell/B-cell collaboration after DC priming is to allow B cells with a low affinity for antigen to receive T-cell help in a primary immune response.

The role of dendritic cells in T cell selection and survival

Dendritic cells (DC) not only play an important role in the induction of immunity, but also have major functions in thymus and periphery in the absence of nominal antigen. In order to study these and other DC-functions in vivo, we expressed MHC class II I-E molecules under the control of the DC-specific CD11c promoter in C57B1/6 mice. With this targeted expression we were able to investigate DC functions in vivo without further manipulation of DC. We describe our findings with respect to the two major thymic events of negative and positive selection and demonstrate that DC are able and sufficient to induce negative selection of autoreactive CD4+ thymocytes by clonal deletion but do not induce positive selection. We further demonstrate in thymus-transplant experiments that DC play a major role in inducing survival of peripheral CD4+ mature T cells. This transgenic model therefore seems to be a valuable tool for the investigation of DC functions in vivo.

Developmental regulation of dendritic cell function

1998 saw key advances in our understanding of the molecular mechanisms whereby immature dendritic cells recognise foreign pathogens in tissues and are induced to migrate to secondary lymphoid organs. In particular, there have been some key insights into how dendritic cells subsequently direct the evolution of immune responses by differential expression of co-stimulatory molecules.

CD4 T cell traffic control: in vivo evidence that ligation of OX40 on CD4 T cells by OX40-ligand expressed on dendritic cells leads to the accumulation of CD4 T cells in B follicles

We report here that CD40- but not lipopolysaccharide (LPS)-activated murine dendritic cells (DC) express OX40-ligand (OX40L) as has been reported in humans. To understand how OX40 ligation affects differentiation of CD4 T cells at the time of priming, we constitutively expressed OX40L on DC using the DC-specific promoter of CD11c. Transgenic mice showed greatly increased numbers of CD4 but not CD8 T cells in their B cell areas. This effect was to a great extent immunization dependent, as spleen and lymphoid tissue with no germinal center reactions from mice which had not been deliberately immunized did not show marked CD4 T cell accumulation. The increased numbers of CD4+ CD62low cells in transgenic mice suggest that it is activated CD4 T cells that accumulate within B cell follicles. These data are consistent with the notion that physiological engagement of OX40 (CD134) on activated CD4 T cells either initiates their migration into or causes them to be retained in B follicles. In contrast, LPS-treated CD did not up-regulate OX40L expression. This dichotomy provides a molecular explanation of how DC might integrate environmental and accessory signals to control cytokine differentiation and migration in CD4 effector cells.

Mutations affecting either generation or survival of cells influence the pool size of mature B cells

The mature B cell compartment of MHC class II-deficient B6 I-Aalpha(-/-) and the btk-defective CBA/N mouse strain is 4- to 5-fold smaller than in wild-type B6 mice. The defect in B6 I-Aalpha(-/-) mice is intrinsic to B cells and due to a 4- to 5-fold reduced lifespan, which however can be normalized by an I-Ealpha(d) transgene, but only when expressed early during B cell development. The reduced number of mature B cells in the btk-defective CBA/N mouse is due to a 4- to 5-fold lower number of immature splenic B cells entering the mature compartment. The combined defects of reduced lifespan and impaired generation in double mutant mice result in a severe deficiency in the mature B cell pool.

CD45 and RPTPalpha display different protein tyrosine phosphatase activities in T lymphocytes

To examine the substrate specificity and function of two receptor protein tyrosine phosphatases, CD45 and RPTPalpha, RPTPalpha was expressed in a CD45(-), T-cell receptor (TCR)+, BW5147 T-lymphoma cell. High levels of expression of RPTPalpha did not fully restore either proximal or distal TCR-mediated signalling events. RPTPalpha was unable to reconstitute the phosphorylation of CD3zeta and did not increase the expression of the activation marker, CD69, on stimulation with TCR/CD3. RPTPalpha did not significantly alter the phosphorylation state or kinase activity of two CD45 substrates, p56(lck) or p59(fyn), suggesting that RPTPalpha does not have the same specificity or function as CD45 in T-cells. Further comparison of the two phosphatases indicated that immunoprecipitated RPTPalpha was approx. one-seventh to one-tenth as active as CD45 when tested against artificial substrates. This difference in activity was also observed in vitro with purified recombinant enzymes at physiological pH. Additional analysis with Src family phosphopeptides and recombinant p56(lck) as substrates indicated that CD45 was consistently more active than RPTPalpha, having both higher Vmax and lower Km values. Thus CD45 is intrinsically a much more active phosphatase than RPTPalpha, which provides one reason why RPTPalpha cannot effectively dephosphorylate p56(lck) and substitute for CD45 in T-cells. This work establishes that these two related protein tyrosine phosphatases are not interchangeable in T-cells and that this is due, at least in part, to quantitative differences in phosphatase activity.

Survival of mature CD4 T lymphocytes is dependent on major histocompatibility complex class II-expressing dendritic cells

Thymic T cell development is controlled by T cell receptor (TCR)-major histocompatibility complex (MHC) interactions, whereas a further dependence of peripheral mature T cells on TCR-MHC contact has not been described so far. To study this question, CD4 T cell survival was surveyed in mice lacking MHC class II expression and in mice expressing MHC class II exclusively on dendritic cells. Since neither of these mice positively select CD4 T cells in the thymus, they were grafted with MHC class II-positive embryonic thymic tissue, which had been depleted of bone marrow derived cells. Although the thymus grafts in both hosts were repopulated with host origin thymocytes of identical phenotype and numbers, an accumulation of CD4+ T cells in peripheral lymphoid organs could only be observed in mice expressing MHC class II on dendritic cells, but not in mice that were completely MHC class II deficient. As assessed by histology, the accumulating peripheral CD4 T cells were found to be in close contact with MHC class II+ dendritic cells, suggesting that CD4 T cells need peripheral MHC class II expression for survival and that class II+ dendritic cells might play an important role for the longevity of CD4 T cells.

Targeted expression of major histocompatibility complex (MHC) class II molecules demonstrates that dendritic cells can induce negative but not positive selection of thymocytes in vivo

It is well established that lymphoid dendritic cells (DC) play an important role in the immune system. Beside their role as potent inducers of primary T cell responses, DC seem to play a crucial part as major histocompatibility complex (MHC) class II+ "interdigitating cells" in the thymus during thymocyte development. Thymic DC have been implicated in tolerance induction and also by some authors in inducing major histocompatibility complex restriction of thymocytes. Most of our knowledge about thymic DC was obtained using highly invasive and manipulatory experimental protocols such as thymus reaggregation cultures, suspension cultures, thymus grafting, and bone marrow reconstitution experiments. The DC used in those studies had to go through extensive isolation procedures or were cultured with recombinant growth factors. Since the functions of DC after these in vitro manipulations have been reported to be not identical to those of DC in vivo, we intended to establish a system that would allow us to investigate DC function avoiding artificial interferences due to handling. Here we present a transgenic mouse model in which we targeted gene expression specifically to DC. Using the CD 11c promoter we expressed MHC class II I-E molecules specifically on DC of all tissues, but not on other cell types. We report that I-E expression on thymic DC is sufficient to negatively select I-E reactive CD4+ T cells, and to a less complete extent, CD8+ T cells. In contrast, it only DC expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. Thus negative, but not positive, selection events can be induced by DC in vivo.

CD45 up-regulation during lymphocyte maturation

CD4+ CD8+ double-positive thymocytes differentiate into CD4+ and CD8+ single-positive T cells during thymic positive selection. This process requires the interaction between the TCR and self MHC molecules. In this context we have analyzed the expression of CD45, an abundant transmembrane protein tyrosine phosphatase, and describe here its differential surface expression during T cell maturation. Using four-color FACS analysis of thymocytes from normal as well as TCR-transgenic mice we demonstrate that CD45 is up-regulated only during positive selection concomitantly with the TCR-CD3 complex and the transient early activation marker CD69, but that this up-regulation precedes heat stable antigen down-regulation. The tight linkage of the up-regulation of the TCR-CD3 complex and CD45 may be required because the CD45 tyrosine phosphatase plays a role in modulating signal transduction by the TCR-CD3 complex during positive selection. In addition, our findings argue for a regulation mechanism that adapts the CD45 levels to increasing antigen receptor levels on mature T cells and B cells.

Redirecting the complete T cell receptor/CD3 signaling machinery towards native antigen via modified T cell receptor

We show that a chimeric T cell receptor (TCR) beta chain consisting of a single-chain Fv portion derived from a monoclonal antibody and the full TCR beta chain is able to assemble functionally with endogenous TCR/CD3 components and transfer the antibody specificity as well as the TCR specificity into TCR beta- as well as into TCR beta+ T cells. This allows the incorporation new non-major histocompatibility complex-restricted ligand specificities into the intact TCR/CD3 complex which can exploit the full range of biological activities of the endogenous TCR signaling machinery. This approach can provide wider opportunities to redirect T cells to virus or tumor antigen-bearing cells.

Signals through T cell receptor-zeta chain alone are insufficient to prime resting T lymphocytes

Activation studies performed with transfected T cell hybridomas and tumors revealed that chimeric molecules containing the CD3 epsilon or zeta chain intracytoplasmic portions can induce the complete effector functions normally seen only when the complete T cell receptor (TCR)/CD3 complexes of T lymphocytes are triggered. Therefore, the zeta chain, with its three antigen recognition activation motives, is thought to connect the antigen-binding Ti chains with the intracellular signaling machinery of the T cell. Here we demonstrate that the cytoplasmic portion of the TCR-zeta chain is not sufficient to activate resting T lymphocytes when cells from transgenic mice expressing a chimeric zeta receptor are used. However, after (in vivo and in vitro) activation through their endogenous TCR/CD3 complexes, the preactivated T lymphocytes could be triggered through the zeta chimera to the same extent as when they were activated through their endogenous TCR/CD3 complexes. They were able to proliferate and elicit cytotoxic functions when triggered through their zeta chimeras. These results suggest that the triggering requirements for effector functions seem to be different in resting than in activated T cells.

New simplified molecular design for functional T cell receptor

We have produced a chimeric single-chain T cell receptor (TcR) that combines the specific antibody recognition function and TcR/CD3 signaling properties within the same polypeptide chain. This hybrid molecule consisted of a single-chain antibody combining site that was connected over a short spacer to the transmembrane and cytoplasmic region of CD3 zeta. When expressed on TcR- or TcR+ T cell hybridomas it could mediate recognition of relevant target cells and subsequent production of lymphokines; i.e. it could functionally replace the TcR/CD3 complex. Therefore, the single-chain TcR model presented here represents an interesting and useful means for the creation of T cells with new specificities.

Soluble CD40 ligand can replace the normal T cell-derived CD40 ligand signal to B cells in T cell-dependent activation

We have constructed a soluble chimeric fusion protein between the mouse CD8 alpha chain and the mouse CD40 T cell ligand. This protein binds to both human and mouse B cells. By itself it induced a modest degree of B cell proliferation, but together with anti-immunoglobulin (anti-Ig) antibody it greatly stimulated B cell proliferation, as determined by both [3H]thymidine uptake and increase in cell numbers. These data are evidence that the CD40 ligand on T cells provides a signal that drives B cell proliferation. This signal is synergistic with that delivered by anti-Ig antibody.

The T cell receptor/CD3 complex is composed of at least two autonomous transduction modules

Recent studies have demonstrated that the CD3-zeta subunit of the T cell antigen receptor (TCR) complex is involved in signal transduction. However, the function of the remaining invariant subunits, CD3-gamma, -delta, and epsilon, is still poorly understood. To examine their role in TCR function, we have constructed TCR/CD3 complexes devoid of functional zeta subunit and showed that they are still able to trigger the production of interleukin-2 in response to antigen or superantigen. These data, together with previous results, indicate that the TCR/CD3 complex is composed of at least two parallel transducing units, made of the gamma delta epsilon and zeta chains, respectively. Furthermore, the analysis of partially truncated zeta chains has led us to individualize a functional domain that may have constituted the building block of most of the transducing subunits associated with antigen receptors and some Fc receptors.

DY determinants, possibly associated with novel class II molecules, stimulate autoreactive CD4+ T cells with suppressive activity

A set of T cell clones (TCC) isolated from HLA-DR-, Dw-, DQ-matched allogeneic MLCs was found to proliferate autonomously when stimulated with cells carrying a wide range of class I or II specificities. This apparently unrestricted proliferation was relatively weak, and only low levels of IL-2 were present in the supernatants of stimulated cells. Autologous as well as allogeneic PBMC and B lymphoblastoid cell lines (B-LCL) were capable of stimulating such clones, which were also restimulated by suppressive, but not by helper, TCC. Moreover, such clones displayed the unusual property of autostimulation. mAb inhibition experiments suggested that class II- or class II-restricted antigens were involved in stimulation. Thus, certain "broad" mAbs (TU39, SG520) reacting with multiple locus products inhibited activation of these reagents, but none of those reacting more specifically with DR (TU34, TU37, L243, Q2/70, SG157), DQ (TU22, SPV-L3, Leu 10), or DP (B7/21), or mixtures of these mAbs, were able to do so. Evidence from sequential immunoprecipitation experiments suggested that mAb TU39 bound class II-like molecules other than DR, DQ, and DP on TCC and B-LCL, and it is therefore proposed that such putative novel class II-like molecules may carry the stimulating determinants for these autoreactive clones. DY-reactive clones lacked helper activity for B cells but mediated potent suppressive activity on T cell proliferative responses that was not restricted by the HLA type of the responding cells. Suppressive activity was induced in normal PBMC by such clones, as well as by independent suppressive clones, which was also inhibited only by mAb TU39. These findings lead to the proposal that DY-reactive autostimulatory cells may constitute a self-maintaining suppressive circuit, the level of activity of which would be regulated primarily by the availability of IL-2 in the microenvironment.