Does T-cell tolerance require a dedicated antigen-presenting cell?

Functional and Phenotypic Analysis of Thymic CD34+CD1a− Progenitor-Derived Dendritic Cells: Predominance of CD1a+ Differentiation Pathway

Whether thymic dendritic cells (DC) are phenotypically and functionally distinct from the monocyte lineage DC is an important question. Human thymic progenitors differentiate into T, NK, and DC. The latter induce clonal deletion of autoreactive thymocytes and therefore might be different from their monocyte-derived counterparts. The cytokines needed for the differentiation of DC from thymic progenitors were also questioned, particularly the need for GM-CSF. We show that various cytokine combinations with or without GM-CSF generated DC from CD34+CD1a− but not from CD34+CD1a+ thymocytes. CD34+ thymic cells generated far fewer DC than their counterparts from the cord blood. The requirement for IL-7 was strict whereas GM-CSF was dispensable but nonetheless improved the yield of DC. CD14+ monocytic intermediates were not detected in these cultures unless macrophage-CSF (M-CSF) was added. Cultures in M-CSF generated CD14−CD1a+ DC precursors but also CD14+CD1a− cells. When sorted and recultured in GM-CSF, CD14+ cells down-regulated CD14 and up-regulated CD1a. TNF-α accelerated the differentiation of progenitors into DC and augmented MHC class II transport to the membrane, resulting in improved capacity to induce MLR. The trafficking of MHC class II molecules was studied by metabolic labeling and immunoprecipitation. MHC class II molecules were transported to the membrane in association with invariant chain isoforms in CD14+ (monocyte)-derived and in CD1a+ thymic-derived DC but not in monocytes. Thus, thymic progenitors can differentiate into DC along a preferential CD1a+ pathway but have conserved a CD14+ maturation capacity under M-CSF. Finally, CD1a+-derived thymic DC and monocyte-derived DC share very close Ag-processing machinery.

Phenotype, function, and in vivo migration and survival of allogeneic dendritic cell progenitors genetically engineered to express TGF-beta

BACKGROUND

Administration of donor bone marrow (BM)-derived dendritic cell (DC) progenitors (DCp) that are major histocompatibility complex (MHC) class II+ but costimulatory molecule (CD40, CD80, CD86)-deficient can prolong mouse heart allograft survival This is associated with microchimerism and inhibition of antidonor cytotoxic T lymphocyte (CTL) activity. Genetic modification of these donor antigen-presenting cells to express an immunosuppressive molecule(s) may enhance their in vivo survival and potential tolerogenicity.

METHODS

The surface phenotype of B10(H-2b) DCp before and after gene transfer using replication-deficient adenoviral (Ad) vectors was determined by monoclonal antibody (mAb) staining and flow cytometry. Transforming growth factor-beta (TGF-beta) production was quantitated by enzyme-linked immunosorbent assay. Allostimulatory activity of the gene-transduced DCp was ascertained by mixed leukocyte reaction (MLR) and CTL induction. To assess their in vivo migratory activity and survival, the transduced cells were injected subcutaneously into one hind footpad of C3H (H-2k) mice. Tissues (draining popliteal lymph nodes [LN], spleens, and thymi) were removed 1, 2, 7, and 14 days later and stained for donor MHC class II using anti-LA(b) mAb in an immunohistochemical procedure. The mean number of IAb+ cells per unit area was determined.

RESULTS

Transduction with a control Ad vector (Ad-LacZ) at 50 multiplicity of infection slightly increased CD40 and CD86 expression and up-regulated the poor allostimulatory activity of the DCp assessed by MLR and CTL responses. These effects on function were negated in Ad-TGF-beta1-transduced cells. After their injection into mouse footpads, the gene-transduced IAb+ cells were observed in maximal numbers in the popliteal LN at day 1 and in marginal zones and T-dependent areas of spleens (peak at day 7) but were rare in thymi. Transduction with Ad-LacZ reduced the numbers of IAb+ cells identified in both LN and spleens at all time points postinjection, suggesting that the vector alone affected DC life span in allogeneic recipients. TGF-beta1 transgene expression not only fully prevented the reduction in DC induced by Ad transduction alone, but also increased numbers and prolonged the survival of donor cells in the spleen, as shown by a two-to fivefold increase in IAb+ cells at days 2-14 compared with control (Ad-LacZ-transduced) DC.

CONCLUSION

BM-derived DCp can be transduced efficiently to express TGF-beta1 using an Ad vector. They exhibit very poor allostimulatory activity and similar migration characteristics in vivo to unmodified DCp. Survival of TGF-beta gene-transduced DC, however, is enhanced significantly compared with unmodified and (especially) control Ad-LacZ gene-transduced DC. Genetic engineering of donor DC to express the immunosuppressive molecule TGF-beta promotes their survival in allogeneic hosts and may potentiate their previously reported tolerogenicity.

Retroviral delivery of viral interleukin-10 into myeloid dendritic cells markedly inhibits their allostimulatory activity and promotes the induction of T-cell hyporesponsiveness

BACKGROUND

Dendritic cells (DC) play critical roles in the initiation and modulation of immune responses and may determine the balance between tolerance and immunity. Viral interleukin-10 (vIL-10), encoded by the Epstein-Barr virus, is highly homologous to the "immunosuppressive" cytokine, mammalian IL-10. It impairs antigen-presenting cell function but lacks certain immunostimulatory properties of mammalian IL-10. We accomplished the following: (1) evaluated the effects of vIL-10 protein on DC phenotype and function, (2) transduced mouse bone marrow-derived DC to express vIL-10, and (3) assessed the impact of transgene expression on DC allostimulatory activity.

METHODS

DC progenitors propagated from bone marrow of B10 (H2b) mice in granulocyte-macrophage colony-stimulating factor plus IL-4 were repeatedly transduced by centrifugation, using retroviral supernatant obtained from the BOSC 23 ecotropic packaging cell line. To evaluate transduction efficiency, DC were transduced with the retroviral vector MFG-enhanced green fluorescence protein as a marker gene. Transgene and key cell surface molecule expression were examined by flow cytometry. The level of vIL-10 gene product in the culture supernatant was quantitated by ELISA. DC function was assessed by evaluation of the ability of DC to induce allogeneic (C3H;H2k) T-cell proliferation and cytotoxic T lymphocytes in primary mixed leukocyte reactions. Secondary mixed leukocyte reactions were used to test for T-cell hyporesponsiveness.

RESULTS

The early addition of vIL-10 protein to cultures inhibited DC maturation and function. vIL-10 gene transfer was achieved with an approximate transduction efficiency of 35 to 40%. Transduced DC expressed vIL-10 at a level of 40 ng/10(6) cells/48 hr. In comparison with controls, vIL-10-transduced cells showed decreased surface expression of major histocompatibility complex class II and costimulatory molecules, reduced ability to stimulate T-cell proliferation and cytotoxic T lymphocyte generation, and potential to induce alloantigen-specific hyporesponsiveness.

CONCLUSIONS

DC can be effectively transduced to express vIL-10 and limit their ability to stimulate in vitro. These genetically engineered antigen-presenting cells may have therapeutic potential to inhibit undesired immune responses to allo- or autoantigens.

Efficient presentation of phagocytosed cellular fragments on the major histocompatibility complex class II products of dendritic cells

Cells from the bone marrow can present peptides that are derived from tumors, transplants, and self-tissues. Here we describe how dendritic cells (DCs) process phagocytosed cell fragments onto major histocompatibility complex (MHC) class II products with unusual efficacy. This was monitored with the Y-Ae monoclonal antibody that is specific for complexes of I-Ab MHC class II presenting a peptide derived from I-Ealpha. When immature DCs from I-Ab mice were cultured for 5-20 h with activated I-E+ B blasts, either necrotic or apoptotic, the DCs produced the epitope recognized by the Y-Ae monoclonal antibody and stimulated T cells reactive with the same MHC-peptide complex. Antigen transfer was also observed with human cells, where human histocompatibility leukocyte antigen (HLA)-DRalpha includes the same peptide sequence as mouse I-Ealpha. Antigen transfer was preceded by uptake of B cell fragments into MHC class II-rich compartments. Quantitation of the amount of I-E protein in the B cell fragments revealed that phagocytosed I-E was 1-10 thousand times more efficient in generating MHC-peptide complexes than preprocessed I-E peptide. When we injected different I-E- bearing cells into C57BL/6 mice to look for a similar phenomenon in vivo, we found that short-lived migrating DCs could be processed by most of the recipient DCs in the lymph node. The consequence of antigen transfer from migratory DCs to lymph node DCs is not yet known, but we suggest that in the steady state, i.e., in the absence of stimuli for DC maturation, this transfer leads to peripheral tolerance of the T cell repertoire to self.

Impact of Flt-3 ligand on donor-derived antigen presenting cells and alloimmune reactivity in heart graft recipients given adjuvant donor bone marrow

The influence of the haematopoietic growth factor Flt-3 ligand (FL) on the incidence and function of donor major histocompatibility complex (MHC) class II+ cells in the lymphoid tissues of noncytoablated recipients of heart allografts and donor bone marrow (BM) cells was investigated. C3H (H2k) mice received a nonvascularized B10 (H2b) heart allograft in the dorsal ear pinna, followed by an i.v. infusion of 50 x 10(6) donor BM cells. They were given FL (10 microg/day i.p., x7 days), tacrolimus (2mg/kg/day i.p., x13 days) or both agents immediately following heart transplantation (HTx) and were killed 10 or 21 days later. Their BM cells were propagated in vitro in granulocyte macrophage colony stimulating factor (GM-CSF) and IL-4 for 5 days to promote the growth of dendritic cells (DC). Donor DC were identified by immunocytochemical staining. Spleens were harvested, and donor (IAb+) cells enumerated by immunohistochemical analysis. Donor MHC class II DNA was detected in spleens and cultured BM-derived cells by reverse transcriptase-polymerase chain reaction (RT-PCR). A striking increase in donor MHC class II+ cells was noted in both the spleen and BM of the BM + tacrolimus-treated group compared to either the BM alone, or BM + FL-treated groups. Addition of FL treatment to BM + tacrolimus led to a further increase in donor cells in spleen (three-fold at 10 days, and two-fold at 21 days). The increase in donor cells at 10 days was almost 140-fold compared to that with donor BM alone. PCR analysis at this time revealed enhanced donor DNA in the BM + FL + tacrolimus group compared to that in the BM + tacrolimus group. FL treatment augmented mixed leucocyte reactions (MLR) and cytotoxic T lymphocyte (CTL) activity of host spleen cells against donor alloantigens. These effects were reversed by tacrolimus administration. Histopathology of heart grafts from tacrolimus-treated animals at 10 and 21 days showed absence or substantial reduction in cellular infiltration, and the preservation of viable myocardium. By contrast, in untreated mice, or animals given BM or BM + FL alone, there was marked cellular infiltration, and features of accelerated rejection. Donor-derived DC could be propagated in vitro from the BM of heart transplant recipients given donor BM, especially from mice that also received tacrolimus +/- FL. At day 21, donor-derived cells could only be propagated from the BM + FL + tacrolimus-treated group. These findings show that numbers of donor antigen presenting cells (APC) or their progenitors can be markedly increased in conventionally immunosuppressed organ allograft recipients given donor BM + a potent haematopoietic and DC-growth promoting cytokine. Although withdrawal of systemic immunosuppression appears to allow exhibition of the potential allostimulatory activity of these donor APC leading to rejection, the model provides a useful basis for further evaluation of the persistence and manipulation of donor haematopoietic cells and in particular, donor-derived APC, on the outcome of organ transplantation.

The evolution of self-tolerance: a new cell arises to meet the challenge of self-reactivity

Naive T cells can become either tolerant or immune as a result of their first encounter with antigen. It has been suggested that lymphoid and myeloid dendritic cells, respectively, control such decisions. Here, Barbara Fazekas de St Groth discusses evolutionary aspects of the functional distinction between these two types of dendritic cells.

Positive versus negative signaling by lymphocyte antigen receptors

Antigen receptors on lymphocytes play a central role in immune regulation by transmitting signals that positively or negatively regulate lymphocyte survival, migration, growth, and differentiation. This review focuses on how opposing positive or negative cellular responses are brought about by antigen receptor signaling. Four types of extracellular inputs shape the response to antigen: (a) the concentration of antigen; (b) the avidity with which antigen is bound; (c) the timing and duration of antigen encounter; and (d) the association of antigen with costimuli from pathogens, the innate immune system, or other lymphocytes. Intracellular signaling by antigen receptors is not an all-or-none event, and these external variables alter both the quantity and quality of signaling. Recent findings in B lymphocytes have clearly illustrated that these external inputs affect the magnitude and duration of the intracellular calcium response, which in turn contributes to differential triggering of the transcriptional regulators NF kappa B, JNK, NFAT, and ERK. The regulation of calcium responses involves a network of tyrosine kinases (e.g. lyn, syk), tyrosine or lipid phosphatases (CD45, SHP-1, SHIP), and accessory molecules (CD21/CD19, CD22, FcR gamma 2b). Understanding the biochemistry and logic behind these integrative processes will allow development of more selective and efficient pharmaceuticals that suppress, modify, or augment immune responses in autoimmunity, transplantation, allergy, vaccines, and cancer.

Flt-3 Ligand Increases Microchimerism But Can Prevent the Therapeutic Effect of Donor Bone Marrow in Transiently Immunosuppressed Cardiac Allograft Recipients

C3H (H2k) mice received 50 × 106 B10 (H2b) bone marrow (BM) cells either alone or with flt-3 ligand (FL) (10 μg/day), tacrolimus (2 mg/kg/day), or both agents for 7 days. Donor MHC class II+ (IAb+) cells were quantitated in spleens by immunohistochemical analysis, and donor class II DNA detected in BM by PCR. Donor cells were rare in the BM alone and BM + FL groups, whereas there was a substantial increase in chimerism in the BM + tacrolimus group. Addition of FL to BM + tacrolimus led to a further eightfold increase in donor cells and enhanced donor DNA compared with the BM + tacrolimus group. This increase in donor cells was almost 500-fold compared with BM alone. C3H recipients of B10 heart allografts given perioperative B10 BM and tacrolimus (days 0–13) exhibited a markedly extended median graft survival time (MST, 42 days) compared with those given tacrolimus alone (MST, 22 days). Addition of FL (10 μg/day; 7 days) to BM + tacrolimus prevented the beneficial effect of donor BM (MST, 18 days). BM alone or BM + FL resulted in uniform early heart graft failure (MST < 8 days). Functional studies revealed maximal antidonor MLR and CTL activities in the BM- and BM + FL-treated groups, with minimal activity in the tacrolimus-treated groups. Thus, dramatic growth factor-induced increases in chimerism achieved under cover of immunosuppression may result in augmented antidonor T cell reactivity and reduced graft survival after immunosuppressive drug withdrawal. With FL, this may reflect striking augmentation of immunostimulatory dendritic cells.

FLT3: receptor and ligand. Biology and potential clinical application

Flt3 ligand (FL) is a recently identified cytokine having a central role in the proliferation, survival and differentiation of early murine and human hematopoietic precursor/stem cells. FL acts synergistically in vitro with a number of other hematopoietic growth factors such as IL-3, IL-6, IL-11, IL-12, KIT Ligand and GM-CSF. Recently, it has been shown the in vivo administration of FL results in a significant alteration of hematopoiesis in murine bone marrow (BM), spleen, peripheral blood, liver and lymph nodes. In addition, treatment with FL resulted in a significant accumulation of functionally active dendritic cells within murine lymphoid tissues. The possible applications of FL in dendritic cell-based immunotherapies are discussed.

TNF Receptor-Deficient Mice Reveal Striking Differences Between Several Models of Thymocyte Negative Selection

Central tolerance depends upon Ag-mediated cell death in developing thymocytes. However, the mechanism of induced death is poorly understood. Among the known death-inducing proteins, TNF was previously found to be constitutively expressed in the thymus. The role of TNF in thymocyte negative selection was therefore investigated using TNF receptor (TNFR)-deficient mice containing a TCR transgene. TNFR-deficient mice displayed aberrant negative selection in two models: an in vitro system in which APC are cultured with thymocytes, and a popular in vivo system in which mice are treated with anti-CD3 Abs. In contrast, TNFR-deficient mice displayed normal thymocyte deletion in two Ag-induced in vivo models of negative selection. Current models of negative selection and the role of TNFR family members in this process are discussed in light of these results.

Dendritic cell development: multiple pathways to nature's adjuvants

Dendritic cells are a system of bone marrow-derived antigen-presenting cells specialized for interaction with T lymphocytes and essential for initiating primary T cell immune responses. Recent investigation indicates that dendritic cells are of diverse origin, with at least two types of myeloid precursors and a lymphoid precursor implicated in their generation. Mature dendritic cell subtypes, while sharing the capacity to activate T cells, show additional functional specialization. Some dendritic cells are equipped with additional mechanisms to regulate the response of the T cells they activate, while others are able to interact with B cells and modify B cell responses.

T cell receptor (TCR)-induced death of immature CD4+CD8+ thymocytes by two distinct mechanisms differing in their requirement for CD28 costimulation: implications for negative selection in the thymus

Negative selection is the process by which the developing lymphocyte receptor repertoire rids itself of autoreactive specificities. One mechanism of negative selection in developing T cells is the induction of apoptosis in immature CD4+CD8+ (DP) thymocytes, referred to as clonal deletion. Clonal deletion is necessarily T cell receptor (TCR) specific, but TCR signals alone are not lethal to purified DP thymocytes. Here, we identify two distinct mechanisms by which TCR-specific death of DP thymocytes can be induced. One mechanism requires simultaneous TCR and costimulatory signals initiated by CD28. The other mechanism is initiated by TCR signals in the absence of simultaneous costimulatory signals and is mediated by subsequent interaction with antigen-presenting cells. We propose that these mechanisms represent two distinct clonal deletion strategies that are differentially implemented during development depending on whether immature thymocytes encounter antigen in the thymic cortex or thymic medulla.

FLT3 ligand induces the generation of functionally active dendritic cells in mice

FLT3 ligand (FL) is a recently described hematopoietic growth factor that stimulates the proliferation and differentiation of hematopoietic progenitors. We have investigated the effect of FL on murine hematopoiesis and dendritic cell (DC) generation and accumulation in lymphoid tissues and liver in vivo and in vitro evaluating the morphologic, phenotypic, and functional characteristics of these DC. We have observed extramedullary hematopoiesis in the mouse spleen with all lineages of hematopoietic cells represented after the administration of FL. Injection of FL results in a time-dependent and reversible accumulation of DC in the spleen, bone marrow, lymph nodes, and liver. Both flow cytometry and immunohistochemistry revealed a significant accumulation of DC in these tissues. Results of mixed leukocyte reaction suggested that these cells, isolated from murine bone marrow or spleen, were active as antigen presenting cells. Furthermore, cultivation of splenic and marrow cells with GM-CSF and IL-4 gave rise to large numbers of functionally active mature DC. Thus, the results of this study suggest that FL is a promising growth factor that stimulates the generation of large number of DC and may be a useful cytokine for the immunotherapy of cancer.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

The role of dendritic cells in T cell activation

Dendritic cells (DC) are distinguishable from other antigen-presenting cells by their potent antigen-presenting capacity. They are not only efficient at presenting peptide antigen but can also process and present soluble protein antigen sto antigen-specific T cells and cloned T cell lines. They are very strong stimulators of both allogeneic and syngeneic mixed lymphocyte reactions and have a unique capacity to stimulate naive T cells. The potent functional capacity of DC is related to a high-level expression of major histocompatibility complex class I/II molecules and constitutive expression of costimulatory molecules, such as CD80/CD86, as well as heat stable antigen, CD40 and the leucocyte function antigen (LFA) family of adhesion molecules. Recent studies have shown that DC are also involved in regulation of the immune response via induction of both central and peripheral tolerance.

Dendritic cells in the T-cell areas of lymphoid organs

Substantial numbers of dendritic cells (DCs) are found in the T-cell areas of peripheral lymphoid organs such as the spleen, lymph node and Peyer's patch. By electron microscopy these DCs (also called interdigitating cells) form a network through which T-cells continually recirculate. The cytological features of DCs in the T-cell areas, as well as a number of markers detected with monoclonal antibodies, are similar to mature DCs that develop from other sites such as skin and bone marrow. Some markers that are expressed in abundance are: MHC II and the associated invariant chain, accessory molecules such as CD40 and CD86, a multilectin receptor for antigen presentation called DEC-205, the integrin CD11c, several antigens within the endocytic system that are detected by monoclonal antibodies but are as yet uncharacterized at the molecular level, and, in the human system, molecules termed S100b, CD83 and p55. DCs in the periphery can pick up antigens and migrate to the T-cell areas to initiate immunity. However, there are new observations that DCs within the T-cell areas also express high levels of self-antigens and functional fas-ligand capable of inducing CD4+ T-cell death. We speculate that there are at least 2 sets of DCs in the T-cell areas, a migratory myeloid pathway that brings in antigens from the periphery and induces immunity, and a more resident lymphoid pathway that presents self-antigens and maintains tolerance.

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.

Dramatic increase in the numbers of functionally mature dendritic cells in Flt3 ligand-treated mice: multiple dendritic cell subpopulations identified

Dendritic cells (DC) are the most efficient APC for T cells. The clinical use of DC as vectors for anti-tumor and infectious disease immunotherapy has been limited by their trace levels and accessibility in normal tissue and terminal state of differentiation. In the present study, daily injection of human Flt3 ligand (Flt3L) into mice results in a dramatic numerical increase in cells co-expressing the characteristic DC markers-class II MHC, CD11c, DEC205, and CD86. In contrast, in mice treated with either GM-CSF, GM-CSF plus IL-4, c-kit ligand (c-kitL), or G-CSF, class II+ CD11c+ cells were not significantly increased. Five distinct DC subpopulations were identified in the spleen of Flt3L-treated mice using CD8 alpha and CD11b expression. These cells exhibited veiled and dendritic processes and were as efficient as rare, mature DC isolated from the spleens of untreated mice at presenting allo-Ag or soluble Ag to T cells, or in priming an Ag-specific T cell response in vivo. Dramatic numerical increases in DC were detected in the bone marrow, gastro-intestinal lymphoid tissue (GALT), liver, lymph nodes, lung, peripheral blood, peritoneal cavity, spleen, and thymus. These results suggest that Flt3L could be used to expand the numbers of functionally mature DC in vivo for use in clinical immunotherapy.

Costimulatory molecule-deficient dendritic cell progenitors (MHC class II+, CD80dim, CD86-) prolong cardiac allograft survival in nonimmunosuppressed recipients

We have shown previously that granulocyte-macrophage colony-stimulating factor-stimulated mouse bone marrow-derived MHC class II+ dendritic cell (DC) progenitors that are deficient in cell surface expression of the costimulatory molecules B7-1 (CD80) and B7-2 (CD86) can induce alloantigen-specific T-cell anergy in vitro. To test the in vivo relevance of these findings, 2 x 10(6) B10 (H2b) mouse bone marrow-derived DC progenitors (NLDC 145+, MHC class II+, B7-1dim, B7-2-/dim) that induced T-cell hyporesponsiveness in vitro were injected systemically into normal C3H (H2k) recipients. Seven days later, the mice received heterotopic heart transplants from B10 donors. No immunosuppressive treatment was given. Median graft survival time was prolonged significantly from 9.5 to 22 days. Median graft survival time was also increased, although to a lesser extent (16.5 days), in mice that received third-party (BALB/c; H2d) DC progenitors. Ex vivo analysis of host T-cell responses to donor and third-party alloantigens 7 days after the injection of DC progenitors (the time of heart transplant) revealed minimal anti-donor mixed leukocyte reaction and cytotoxic T lymphocyte reactivity. These responses were reduced substantially compared with those of spleen cells from animals pretreated with "mature" granulocyte-macrophage colony-stimulating factor + interleukin-4-stimulated DC (MHC class IIbright, B7-1+, B7-2bright), many of which rejected their heart grafts in an accelerated fashion. Among the injected donor MHC class II+ DC progenitors that migrated to recipient secondary lymphoid tissue were cells that appeared to have up-regulated cell surface B7-1 and B7-2 molecule expression. This observation may explain, at least in part, the temporary or unstable nature of the hyporesponsiveness induced by the DC progenitors in nonimmunosuppressed recipients.

Difference in antigen presentation pathways between cortical and medullary thymic epithelial cells

Antigen presentation by thymic epithelial cells (TEC) to T cells that undergo maturation is one of the major events in the selection of the T cell repertoire. We have already reported that medullary TEC lines (mTEC) established from newborn C57BL/6 (H-2b) mice are able to present a soluble antigen, ovalbumin (OVA), to OVA-specific, I-Ab restricted helper T cell lines but cortical TEC (cTEC) lines are not (Mizuochi, T. et al., J. Exp. Med. 1992. 175: 1601). In this report, to clarify the cause of this difference, we analyzed the biochemical nature as well as the distribution of both major histocompatibility complex (MHC) class II molecules and invariant chains (Ii) in both TEC by immunoprecipitation and laser confocal scanning microscopic analysis, as well as the expression of mRNA encoding H-2Ma or H-2Mb. Our results demonstrate that cTEC and mTEC are both able to present peptide antigens to peptide-specific, I-Ab-restricted helper T cell hybridoma and are able to present class II MHC alloantigens to an I-Ab-specific T cell line, that mRNA for H-2Ma and H-2Mb are expressed in both TEC, that cTEC and mTEC apparently incorporate tetramethylrhodamine isothiocyanate-labeled OVA in the same manner, and that the SDS-stable MHC class II molecules, onto which peptides were loaded, are formed in both cTEC and mTEC. However, these molecules were more rapidly degraded in mTEC than in cTEC. In addition, two Ii-derived polypeptides of approximately 21 kDa and 10 kDa were precipitated by the anti-class II monoclonal antibody Y3P; 10-kDa polypeptides were detected in the both TEC, while 21-kDa polypeptides were detected only in cTEC. Finally, beta chains of MHC class II with less sialylated oligosaccharides were precipitated from the cell surface of cTEC. Taken together, these results suggest that there are substantial differences in the antigen-presenting pathways of cTEC and mTEC, and these difference might be responsible for T cell selection events in the thymus.

The hematopoietic development of dendritic cells: a distinct pathway for myeloid differentiation

Dendritic cells (DC) are leukocytes that are specialized to capture antigens and initiate T cell-mediated immune responses. Because DC can prime animals in the absence of any other adjuvant, they have been termed 'nature's adjuvant'. DC express high levels of antigen presenting major histocompatibility complex (MHC) products (HLA-DP, DQ, DR; HLA-A, B, C) as well as several accessory molecules (e.g., B7-1, B7-2, LFA-3, ICAM-1, ICAM-3, CD40) that mediate T cell binding and costimulation. This review outlines some of the ways in which DC are distinguished from two other myeloid lineages, macrophages and granulocytes. Recent data regarding DC development from class II MHC-negative precursors in the mouse, as well as unselected and selected CD34+ progenitors in human bone marrow and peripheral and cord blood, are reviewed. Additional pathways via post-colony-forming units, intermediate cell types have also become evident in suspension cultures where the cytokine milieu can alter terminal differentiation. The availability of larger numbers of DC is opening new avenues for immune therapy that use this physiologic adjuvant.

Danger - pathogen on the premises! Immunological tolerance

Recent results show that immune responses can be induced in neonatal mice. Do they really refute the traditional view that the ability to discriminate between 'self' and 'non-self' is a fundamental property of the immune system?

Tracing interactions of thymocytes with individual stromal cell partners

Cellular interactions in T cell development can be analyzed using thymus chimeras prepared in vitro, in which stromal cells and T cell precursors are manipulated separately. In an earlier study, we showed that for optimal T cell maturation most--if not all--stromal cells must display appropriate (selecting) major histocompatibility complex (MHC) molecules: the substitution of selecting by nonselecting stromal cells leads to a proportional decrease in mature T cell production. These data imply that the availability of selecting stromal micro-environments is rate limiting for positive selection, and that in positive selection each thymocyte engages only one (rather than multiple) stromal cell partners. To test this hypothesis, we developed a tracing system for thymocyte/stromal cell interactions, based on the acquisition by thymocytes of stroma-derived MHC class II determinants. When MHC class II-deficient precursors are placed in H-2b x k F1 environments (where all stromal cells co-express H-2b and H-2k), individual thymocytes acquire class II determinants of both haplotypes. In striking contrast, when placed in mosaic stromal environments (where stromal cells express either H-2b or H-2k evenly interspersed), individual thymocytes preferentially acquire MHC class II determinants of one or the other haplotypes, but rarely both. This provides strong evidence that thymocytes have intimate interactions with individual stromal cells: having engaged one stromal cell niche, thymocytes do not (or only rarely) have promiscuous liaisons with others.

The anatomy of T-cell activation and tolerance

The mammalian immune system must specifically recognize and eliminate foreign invaders but refrain from damaging the host. This task is accomplished in part by the production of a large number of T lymphocytes, each bearing a different antigen receptor to match the enormous variety of antigens present in the microbial world. However, because antigen receptor diversity is generated by a random mechanism, the immune system must tolerate the function of T lymphocytes that by chance express a self-reactive antigen receptor. Therefore, during early development, T cells that are specific for antigens expressed in the thymus are physically deleted. The population of T cells that leaves the thymus and seeds the secondary lymphoid organs contains helpful cells that are specific for antigens from microbes but also potentially dangerous T cells that are specific for innocuous extrathymic self antigens. The outcome of an encounter by a peripheral T cell with these two types of antigens is to a great extent determined by the inability of naive T cells to enter nonlymphoid tissues or to be productively activated in the absence of inflammation.

Additional monoclonal antibody (mAB) injections can replace thymic irradiation to allow induction of mixed chimerism and tolerance in mice receiving bone marrow transplantation after conditioning with anti-T cell mABs and 3-Gy whole body irradiation

While allogeneic bone marrow transplantation (BMT) has long been known to be capable of inducing donor-specific tolerance and hence permitting allograft acceptance without immunosuppressive pharmacotherapy, the toxicity of conditioning regimens required to achieve marrow engraftment has precluded the application of this approach to clinical organ transplantation. A relatively nontoxic method of conditioning mice that allows allogeneic bone marrow engraftment and induction of donor-specific skin allograft tolerance has recently been described. This regimen included anti-CD4 and anti-CD8 mAbs administered on day -5, followed by 3-Gy whole body irradiation (WBI) and 7-Gy thymic irradiation (TI) on day 0. To further reduce the potential toxicity of this regimen, we have now attempted to overcome the requirement for TI by administering additional mAb injections before and after BMT. Mixed chimerism and prolonged donor-specific skin graft acceptance were induced in 90% of B10 mice conditioned with anti-CD4 and -CD8 mAbs on days -6 and -1 and 3-Gy WBI on day 0 without TI. Despite long-term acceptance of donor-specific skin grafts, however, some of these animals showed a gradual decline in donor-type hematopoietic repopulation, and 2 of 10 mice regrafted with a second donor-type skin graft 5-9 months after BMT rejected the second and/or the original graft. This rejection after repeat donor-specific skin grafting correlated with a decline in the percentage of donor-type T cells between 6 and 12 weeks after BMT. In contrast, all animals receiving additional mAb injections 7 and 14 days following BMT after conditioning with mAbs on days -6 and -1 and 3-Gy WBI showed stable chimerism and accepted both primary and secondary donor-specific skin grafts. Animals receiving TI in addition to mAb and 3-Gy WBI also showed stable chimerism and long-term acceptance of initial (at 7 weeks) and later repeat donor-specific grafts. In contrast, the majority of mice receiving mAbs only on day -5 or on day -1 only, followed by 3-Gy WBI on day 0 without TI, did not accept initial donor-specific skin grafts, and showed only transient chimerism. Thus, the requirement for thymic irradiation to allow permanent mixed chimerism and donor-specific tolerance induction can be overcome by the administration of additional T cell-depleting mAb injections. These results establish a less toxic method of inducing donor-specific tolerance, thus increasing the potential clinical applicability of this approach to inducing organ allograft acceptance without chronic immunosuppressive therapy.

Pancreatic islet allograft immunity and tolerance: the two-signal hypothesis revisited

The principle assumption of this discussion is that costimulation (CoS) forms the primary stimulus that compels T cells to mount a response to their specific antigen. However, this response can be either positive or negative, depending on the developmental stage of the T cell and the microenvironment in which the antigen and CoS are received. Thus, both immunity and tolerance may represent different outcomes of a two-signal process. We would emphasize that CoS is a functional term and not a strict molecular definition. While many molecular interactions have been described as providing CoS activity, notably those involving the B-7 family of cell surface molecules, it is not yet clear what combination(s) of non-antigen-specific signals may fulfil this function. This point is important because many studies have achieved tolerance through strategies designed to inhibit specific CoS molecules. However, it may be that differential signaling through distinct CoS molecules, rather than a global inhibition of CoS per se, plays a role in the generation of active tolerance in such studies (Bluestone 1995). A corollary of this notion is that antigen (signal 1) delivery to T cells is a null event and so is not an inherently paralysing signal. Of course, if signal 1 is not itself a tolerogenic signal, then other mechanisms are necessary to explain many empirical observations of tolerance to allogeneic or self antigens. This is best illustrated by those forms of functional tolerance to either alloantigens or self antigens that do not appear to be the result of clonal deletion/inactivation. It would be relatively simple to invoke a model of tolerance whereby the relevant tissue-destructive cell is eliminated or inactivated; such a model would preclude the necessity to suggest active regulatory mechanisms of tolerance. However, in several model systems, including our own observations concerning tolerance induction to APC-depleted islet allografts, tissue-destructive T cells can persist in recipients tolerant to allogeneic or self antigens. Furthermore, there are key examples in which tolerance demonstrates a dominant phenotype; that is, tolerant cells can regulate the activity of naive, non-tolerant cells. This latter observation points to the function of an active, regulatory form of tolerance. As such, we would emphasize that tolerance should not be defined as unresponsiveness since the tolerant state is the consequence of very active immune reactions.

CD4-negative cytotoxic T cells with a T cell receptor alpha/beta intermediate expression in CD8-deficient mice

Targeted disruption of the CD8 gene results in a profound block in cytotoxic T cell (CTL) development. Since CTL are major histocompatibility complex (MHC) class I restricted, we addressed the question of whether CD8-/- mice can reject MHC class I-disparate allografts. Studies have previously shown that skin allografts are rejected exclusively by T cells. We therefore used the skin allograft model to answer our question and grafted CD8-/- mice with skins from allogeneic mice deficient in MHC class II or in MHC class I (MHC-I or MHC-II-disparate, respectively). CD8-/- mice rejected MHC-I-disparate skin rapidly even if they were depleted of CD4+ cells in vivo (and were thus deficient in CD4+ and CD8+ T cells). By contrast, CD8+/+ controls depleted of CD4+ and CD8+ T cells in vivo accepted the MHC-I-disparate skin. Following MHC-I, but not MHC-II stimulation, allograft-specific cytotoxic activity was detected in CD8-/- mice even after CD4 depletion. A population expanded in both the lymph nodes and the thymus of grafted CD8-/- animals which displayed a CD4-8-3intermediateTCR alpha/betaintermediate phenotype. Indeed its T cell receptor (TCR) density was lower than that of CD4+ cells in CD8-/- mice or of CD8+ cells in CD8+/+ mice. Our data suggest that this CD4-8- T cell population is responsible for the CTL function we have observed. Therefore, MHC class I-restricted CTL can be generated in CD8-/- mice following priming with MHC class I antigens in vivo. The data also suggest that CD8 is needed to up-regulate TCR density during thymic maturation. Thus, although CD8 plays a major role in the generation of CTL, it is not absolutely required.

Microchimerism, dendritic cell progenitors and transplantation tolerance

The recent discovery of multilineage donor leukocyte microchimerism in allograft recipients up to three decades after organ transplantation implies the migration and survival of donor stem cells within the host. It has been postulated that in chimeric graft recipients, reciprocal modulation of immune responsiveness between donor and recipient leukocytes may lead, eventually, to the induction of mutual immunologic nonreactivity (tolerance). A prominent donor leukocyte, both in human organ transplant recipients and in animals, has invariably been the bone marrow-derived dendritic cell (DC). These cells have been classically perceived as the most potent antigen-presenting cells but evidence also exists for their tolerogenicity. The liver, despite its comparatively heavy leukocyte content, is the whole organ that is most capable of inducing tolerance. We have observed that DC progenitors propagated from normal mouse liver in response to GM-CSF express only low levels of major histocompatibility complex (MHC) class II antigen and little or no cell surface B7 family T cell costimulatory molecules. They fail to activate resting naive allogeneic T cells. When injected into normal allogeneic recipients, these DC progenitors migrate to T-dependent areas of host lymphoid tissue, where some at least upregulate cell surface MHC class II. These donor-derived cells persist indefinitely, recapitulating the behavior pattern of donor leukocytes after the successful transplantation of all whole organs, but most dramatically after the orthotopic (replacement) engraftment of the liver. A key finding is that in mice, progeny of these donor-derived DC progenitors can be propagated ex vivo from the bone marrow and other lymphoid tissues of nonimmunosuppressed spontaneously tolerant liver allograft recipients. In humans, donor DC can also be grown from the blood of organ allograft recipients whose organ-source chimerism is augmented with donor bone marrow infusion. DC progenitors cannot, however, be propagated from the lymphoid tissue of nonimmunosuppressed cardiac-allografted mice that reject their grafts. These findings are congruent with the possibility that bidirectional leukocyte migration and donor cell chimerism play key roles in acquired transplantation tolerance. Although the cell interactions are undoubtedly complex, a discrete role can be identified for DC under well-defined experimental conditions. Bone marrow-derived DC progenitors (MHC class II+, B7-1dim, B7-2-) induce alloantigen-specific hyporesponsiveness (anergy) in naive T cells in vitro. Moreover, costimulatory molecule-deficient DC progenitors administered systemically prolong the survival of mouse heart or pancreatic islet allografts. How the regulation of donor DC phenotype and function relates to the balance between the immunogenicity and tolerogenicity of organ allografts remains to be determined.

Maturation and migration of cutaneous dendritic cells

Dendritic cells have been isolated from the epidermis, dermis, and lymphatics of skin. Cells from each cutaneous compartment can exhibit the distinct morphology, surface phenotype, and strong T-cell-stimulating activity of dendritic cells that are isolated from other organs. Of importance are the mechanisms by which the maturation and movement of dendritic cells are regulated within intact tissues. Epidermal dendritic cells turn over slowly in the steady state. Stimuli, including contact allergens and transplantation, perhaps by inducing a release of cytokines such as granulocyte macrophage-colony-stimulating factor, mobilize these dendritic cells into the dermis and lymph. This migration is accompanied by the maturation of dendritic cell functions; e.g., antigen-presenting major histocompatibility complex molecules and B7 costimulators increase markedly. On the other hand, there is a sizable, steady-state flux of dendritic cells in afferent lymph draining the skin, which suggests a constant traffic through the dermis that is independent of sessile epidermal dendritic cells. When explants of skin are placed in organ culture, dendritic cells emigrate into the medium for 1-3 d. The dendritic cells are mature and can bind tightly to small memory T cells that also migrate in these cultures. The emigrated mixtures of dendritic cells and T cells should be useful in the study of many clinical states. This is illustrated by recent experiments showing that migratory skin cells are readily infected with human immunodeficiency virus (HIV)-1. A strong productive infection takes place in the absence of exogenous cytokines, foreign sera, or mitogens or antigens. The dendritic cell-T-cell conjugates are the essential site for infection. This cellular milieu may model events during the sexual transmission of HIV-1, where relevant mucosal surfaces are covered by skin-like epithelia. The capture of CD4+ memory T cells by dendritic cells may explain the chronic drain of immune memory in HIV infection.

The receptor DEC-205 expressed by dendritic cells and thymic epithelial cells is involved in antigen processing

Dendritic cells and thymic epithelial cells perform important immunoregulatory functions by presenting antigens in the form of peptides bound to cell-surface major histocompatibility complex (MHC) molecules to T cells. Whereas B cells are known to present specific antigens efficiently through their surface immunoglobins, a comparable mechanism for the capture and efficient presentation of diverse antigens by dendritic cells and thymic epithelial cells has not previously been described. We show here that their antigen-presentation function is associated with the high-level expression of DEC-205, an integral membrane protein homologous to the macrophage mannose receptor and related receptors which are able to bind carbohydrates and mediate endocytosis. DEC-205 is rapidly taken up by means of coated pits and vesicles, and is delivered to a multivesicular endosomal compartment that resembles the MHC class II-containing vesicles implicated in antigen presentation. Rabbit antibodies that bind DEC-205 are presented to reactive T-cell hybridomas 100-fold more efficiently than rabbit antibodies that do not bind DEC-205. Thus DEC-205 is a novel endocytic receptor that can be used by dendritic cells and thymic epithelial cells to direct captured antigens from the extracellular space to a specialized antigen-processing compartment.

Cytotoxic effects of irradiation and deoxyguanosine on fetal thymus

Effects of irradiation and deoxyguanosine on the fetal thymus were examined both in vitro and in vivo. Fetal thymi (gestation day 15) of C57BL/6 mice that had been irradiated (0-25 Gy) or treated with various doses of deoxyguanosine (dGuo) were engrafted under the renal capsules of BALB/c nu/nu mice, and the differentiation of T cells was investigated in the engrafted thymi or spleens of these mice. After in vitro treatment of fetal thymi with 1.35 mM dGuo (which was previously reported to be an optimal dose), T cell precursors still remained in some cultures, whereas 1.80 mM dGuo was highly cytotoxic not only to T cell precursors but also to thymic epithelial cells. In contrast, 25 Gy irradiation totally eliminated the T cell precursors from the fetal thymi, though the capacity of epithelial cells to induce T cell differentiation was retained. Although irradiated thymi had the capacity to induce T cell differentiation when assayed in an in vitro organ culture system, long-term observation of thymi engrafted into BALB/c nu/nu mice revealed that, if they had been irradiated (9.5 Gy or 25 Gy), the thymi became scarred by 12 wks after their transplantation. Furthermore, the expression of cell interaction molecules such as ICAM-1 and MHC class II on the thymus stromal cells decreased after irradiation. The interaction molecules decreased 3 wks after 25 Gy irradiation and 7 wks after 9.5 Gy irradiation. The alteration in T cell subsets in the thymus (decreases in both double- and single- positive cells and an increase in double-negative cells) correlated with the decreases in the interaction molecules. This indicates that irradiation (even 9.5 Gy) impairs the T cell-induction capacity of the thymus stromal cells, resulting in an alteration of the T cell subsets followed by a change in the T cell counts in the thymus. Therefore, the long-term effects of irradiation of the thymus should be considered in cases of fetal thymus grafts or total body irradiation before bone marrow transplantation, particularly in the newborn.

Central tolerance of T cells

The immune system is constructed to tolerate self antigens but give vigorous responses to foreign antigens. How this state of self/nonself discrimination is maintained is controversial. In the case of T cells, many self antigens are transported to the thymus via the bloodstream and induce tolerance (clonal deletion) of self-reactive thymocytes in situ. Although such central tolerance in the thymus is well documented, it is often argued that full induction of tolerance requires peripheral mechanisms such as suppression or induction of anergy. This article proposes that steady-state tolerance of T cells to self components is due solely to central tolerance to circulating self antigens combined with sequestration of tissue-specific antigens. Backup mechanisms for tolerance do exist but such immunoregulation only operates when self tolerance breaks. This scheme allows the immune system to give unrestricted primary responses to foreign antigens.

Evidence for a single-niche model of positive selection

Thymocyte maturation depends on interactions with thymic stromal elements expressing major histocompatibility complex (MHC) molecules. Mutant mouse strains lacking MHC class I (beta 2-microglobulin-null) or class II (A beta-null) expression fail to generate normal CD8 or CD4 T-cell populations and provide model systems for reconstitution experiments. We have constructed in vitro chimeras between normal and MHC-deficient thymi to evaluate the efficiency of positive selection. Unexpectedly, the generation of mature single-positive thymocytes was proportional to the fraction of wild-type (i.e., MHC-expressing) stroma over a wide range of chimerism. Similar results were obtained for the development of T-cell receptor-transgenic thymocytes in graded chimeras expressing selecting and nonselecting MHC alleles. These findings are best explained by hypothesizing that positive selection involves a rate-limiting step at which each thymocyte can interact with only one stromal cell niche.

In vitro negative selection of alpha beta T cell receptor transgenic thymocytes by conditionally immortalized thymic cortical epithelial cell lines and dendritic cells

We have established conditionally immortalized thymic cortical epithelial cell lines from transgenic mice carrying a temperature-sensitive SV40 large T antigen. One of these cell lines expresses cortical markers and produces IL-1 alpha, IL-6, IL-7, and TGF-beta 1. These cells express class I major histocompatibility complex (MHC) constitutively and class II MHC upon induction with IFN-gamma. The cells appear to have a normal class I antigen presenting pathway since messages for both peptide transporter genes (TAP1, TAP2) were detected. The ability of these cortical epithelial cells to present peptide antigen was compared to that of thymic dendritic cells. In suspension culture with alpha beta T cell receptor (TcR) transgenic thymocytes, these epithelial cells and dendritic cells (pre-pulsed with peptide cognate for the transgenic TcR) caused down-regulation of CD4, CD8, and TcR in an antigen dose-dependent and MHC-restricted manner. CD4dullCD8dull cells were taken as evidence for negative selection because these cells contained apoptotic DNA. Concentration of peptide required for negative selection of thymocytes was similar between dendritic cells and cortical epithelial cells. In contrast, alpha beta TcR transgenic spleen cells were activated only by dendritic cells but not by cortical epithelial cells.

Programmed cell death of T lymphocytes during acute viral infection: a mechanism for virus-induced immune deficiency

Acute viral infections induce immune deficiencies, as shown by unresponsiveness to mitogens and unrelated antigens. T lymphocytes isolated from mice acutely infected with lymphocytic choriomeningitis virus (LCMV) were found in this study to undergo activation-induced apoptosis upon signalling through the T-cell receptor (TcR)-CD3 complex. Kinetic studies demonstrated that this sensitivity to apoptosis directly correlated with the induction of immune deficiency, as measured by impaired proliferation in response to anti-CD3 antibody or to concanavalin A. Cell cycling in interleukin-2 (IL-2) alone stimulated proliferation of LCMV-induced T cells without inducing apoptosis, but preculturing of T cells from acutely infected mice in IL-2 accelerated apoptosis upon subsequent TcR-CD3 cross-linking. T lymphocytes isolated from mice after the acute infection were less responsive to IL-2, but those T cells, presumably memory T cells, responding to IL-2 were primed in each case to die a rapid apoptotic death upon TcR-CD3 cross-linking. These results indicate that virus infection-induced unresponsiveness to T-cell mitogens is due to apoptosis of the activated lymphocytes and suggest that the sensitization of memory cells by IL-2 induced during infection will cause them to die upon antigen recognition, thereby impairing specific responses to nonviral antigens.

The immune system victorious: selective preservation of self

How the body successfully distinguishes its own tissue cells from those that are foreign and genetically nonidentical to it has been a focus of much research. Clonal deletion maintains that immune system cells with the potential to injure self constituents are eliminated during development, thereby neutralizing their capacity to induce self injury. Selected self-reactive maturing T cell clones undergo deletion in the thymus. A two-step selection process affects immature T cells that enter the thymus. Positive selection makes certain that all surviving cells are able to identify major histocompatibility complex (MHC) proteins present on all body cells. These MHC proteins interact with antigens and present them to T lymphocytes. Negative selection is essential for self-tolerance. It eliminates potentially injurious self-reactive T cells by placing them in contact with a mixture of self antigens in the thymus. Clonal anergy might act together with clonal deletion to maintain self tolerance. Self-reactive T cells in the blood of healthy subjects could represent cells whose affinities for antigen are too weak to initiate an immunologic disease. The fate of T cells reacting to a specific antigen has been traced in transgenic mice. Class I MHC molecules present peptides manufactured within the cell, whereas class II MHC molecules present peptides from extracellular proteins. Interaction of a T cell receptor with its homologous antigen associated with MHC molecules leads to proliferation of that T cell in the presence of costimulatory signals. Investigations elucidating the role of T cell receptors, MHC molecules and antigen peptides in self-nonself discrimination are discussed. The article concludes with an introductory summary of the remaining articles in the issue that address selected topics in self-nonself discrimination.

Intrathymic and extrathymic tolerance in bone marrow chimeras

Parent-->F1 bone marrow (BM) chimeras provide a useful model for studying self tolerance induction. When prepared with supralethal irradiation (1300 cGy) and conditioned with anti-T cell antibodies, parent-->F1 BM chimeras are devoid of host BM-derived cells; host H-2 expression is apparent in both the intrathymic and extrathymic environments but is limited to non BM-derived cells. When parent-->F1 chimeras are injected with T cells from normal parental strain mice, the expression of host H-2 antigens on nonprofessional APC might be expected to induce tolerance through induction of clonal anergy. In practice, this does not occur. Instead, a small proportion of the injected T cells is induced to proliferate and differentiate into effector cells. Tolerance is not seen. Similarly, tolerance is not apparent when thymectomized parent-->F1 chimeras are given parental strain thymus grafts. These findings suggest that the expression of host H-2 antigens in the post-thymic environment of chimeras is not intrinsically tolerogenic for mature T cells or recent thymic emigrants. Interestingly, post-thymic tolerance does occur when parental strain T cells differentiate in the endogenous thymus of chimeras. Thus, when mature CD8+ cells are prepared from thymus vs lymph nodes (LN) of parent-->F1 chimeras, tolerance to host class I antigens is more marked in LN than thymus; this applies to cytotoxic T lymphocyte (CTL) precursors, generated by limiting dilution analysis. It would appear therefore that many of the host-reactive CTL precursors generated in the thymus of chimeras undergo tolerance induction (deletion or irreversible inactivation) in the post-thymic environment. We suggest that such tolerance is a reflection of a covert form of tolerance induced in the thymus: intrathymic contact with host antigens on thymic epithelial cells (TEC) in chimeras does not delete typical CTL precursors, but these cells are rendered "semi-tolerant". When cultured in vitro in the presence of lymphokines, the cells are able to recover and differentiate into CTL. In vivo, however, the cells recognize antigen in the periphery in the relative absence of lymphokines and the cells die. Although host class I expression on TEC in chimeras deletes only a small proportion of CTL precursors, contact with TEC induces strong tolerance of CD8+ cells in terms of helper-independent proliferative responses in vitro and induction of lethal graft-versus-host disease in vivo. We postulate that these latter responses are controlled by high-affinity T cells, whereas typical CTL generated in LDA are predominantly low-affinity cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Granulocytes, macrophages, and dendritic cells arise from a common major histocompatibility complex class II-negative progenitor in mouse bone marrow

The developmental origin of dendritic cells, a specialized system of major histocompatibility complex (MHC) class II-rich antigen-presenting cells for T-cell immunity and tolerance, is not well characterized. Granulocyte-macrophage colony-stimulating factor (GM-CSF) is known to stimulate dendritic cells, including growth and development from MHC class II-negative precursors in suspension cultures of mouse bone marrow. Here we studied colony formation in semi-solid methylcellulose cultures, a classical bioassay system in which GM-CSF induces the formation of mixed granulocyte-macrophage colonies. When colonies were induced from MHC class II-negative precursors, a small subset (1-2%) of typical dendritic cells developed alongside macrophages and granulocytes. The dendritic cells were distinguished by their cytologic features, high levels of MHC class II products, and distinct intracellular granule antigens. By using differential adherence to plastic, enriched populations of the various myeloid cell types were isolated from colonies. Only the dendritic cells stimulated a primary T-cell immune response, the mixed leukocyte reaction, and the potency was comparable to typical dendritic cells isolated from spleen. Macrophages from mixed or pure colonies were inactive as stimulator cells. Therefore, three distinct pathways of myeloid development--granulocytes, macrophages, and dendritic cells--can develop from a common MHC class II-negative progenitor under the aegis of GM-CSF.