A subset of cytolytic dendritic cells in rat

Dendritic cells acquire tolerogenic properties at the site of sterile granulomatous inflammation

Subcutaneous implantation of polyvinyl sponges represents a suitable model for studying the mechanisms of acute and chronic inflammation, granulomatous foreign-body reaction, as well as wound healing. Using such a model in rats, we studied the phenotypic and functional characteristics of dendritic cells (DC). DC were purified from the sponge exudate using a combination of separation gradients, adherence to plastics, and immunomagnetic sorting. We have shown that the number of DC progressively increased in the sponges, reaching maximal values at day 10 after implantation, followed by their decrease thereafter. Inflammatory DC expressed MHC class II molecules and myeloid markers CD11b, CD11c, and CD68. A subset of DC expressed CD4, R-MC46, DEC-205, R-MC17, and CCR1. Compared to DC isolated in the early phase of inflammation (day 6 DC), DC in the late stage of inflammation (day 14 DC) had a lower capability to stimulate the proliferation of allogeneic lymphocytes and CD4(+) T cells. This finding correlated with the downregulation of CD80, CD86, and CD54 expression and the increased proportion of plasmacytoid MHC class II(+) His 24(+) His 48(+) DC. The suppression of allogeneic lymphocyte proliferation was abrogated by the treatment of DC with lipopolysaccharide. In addition, day 14 DC exerted tolerogenic capability in co-culture with allogenic CD4(+) T cells. These results correlated with the increased levels of IL-10 and TGF-beta in culture supernatants and the sponge exudate.

Prolongation of heart allograft survival by immature dendritic cells generated from recipient type bone marrow progenitors

Recent studies suggest that particular dendritic cells (DC) subpopulations may be tolerogenic. To test the capacity of different DC subpopulations to modulate allograft rejection, we generated two distinct populations of rat bone marrow-derived DCs (BMDC) with low doses of GM-CSF and IL-4. The non-adherent population (nBMDC), which are the 'classical' DCs was able to stimulate naive allogeneic T cells and could be induced to completely mature using various stimuli. In contrast, the adherent population (aBMDC), which displayed an immature phenotype, was unable to stimulate T cells and was more resistant to maturation. We found that syngeneic aBMDCs, injected one day before transplantation, induced significant prolongation of heart allograft survival and decreased anti-donor humoral and cellular responses. Similarly, syngeneic aBMDCs inhibited T-cell responses to KLH in the spleen but not in lymph node in a KLH immunization model without graft. This effect was not antigen specific and could be reversed using an inhibitor of inducible nitric oxide synthase. This compartmentalized inhibition could be in part explained by the fact that the majority of syngeneic adherent cells administered intravenously were found in the spleen with some of them reaching the T-cell areas. These data suggest that syngeneic aBMDCs can modulate immune responses.

Activated human umbilical cord blood dendritic cells kill tumor cells without damaging normal hematological progenitor cells

Apart from their role as antigen presenting cells, human peripheral blood monocyte and CD34+ cell-derived dendritic cells (DC), have been demonstrated to exert cytotoxicity against some tumor cells, and their tumoricidal activity can be enhanced by some stimili. However, there have been no reports concerning the tumoricidal activity of human cord blood dendritic cells (CBDC). In this article, we report that human cord blood monocyte-derived DC acquire the ability to kill hematological tumor cells, after activation with lipopolysaccharide (LPS) or gamma-interferon (IFN-gamma), associated with the enhanced TNF-alpha-related apoptosis-inducing ligand (TRAIL) expression in CBDC cytoplasm. The CD14-positive cells collected from cord blood were induced to CBDC in vitro. After activation with IFN-gamma for 12 h, CBDC exhibited cytotoxicity against HL60 and Jurkat cells, while activation with LPS induced cytotoxicity against Daudi and Jurkat cells. However, both LPS- and IFN-gamma-stimulated CBDC showed no cytotoxic activity against normal CD14-negative cord blood mononuclear cells. The formation of umbilical cord hematopoietic progenitor colonies, identified as burst-forming unit-erythroid and colony-forming unit granulocyte-macrophage, was not inhibited by stimulated or unstimulated CBDC. IFN-gamma or LPS stimulation enhanced intracellular but not cellular surface TRAIL, and neither intracellular nor cellular surface tumor necrosing factor-alpha and Fas Ligand as analyzed by flow cytometry. Our results show that activated CBDC can serve as cytotoxic cells against hematological tumor cells without damaging the normal hematopoietic progenitor cells.

Phenotype and function of GM-CSF independent dendritic cells generated by long-term propagation of rat bone marrow cells

GM-CSF is believed to be an essential factor for growth and differentiation of myeloid dendritic cells (DC). Employing a low-density fraction of rat bone marrow cells, we attempted to generate DC with human Flt-3/Flk-2 and IL-6. In this culture system, typical DC gradually appeared without exogenous GM-CSF supplement. Phenotypes and functions of the DC were examined. Evidence provided that the most efficient long-term outgrowth of DC progenitors was obtained by GM-CSF independent culture systems with the aid of Flt3/Flk-2 and IL-6, not with c-kit ligand and IL-6. Furthermore, CD103 (OX-62), which is widely used for rat DC separation, was found to be insufficient for enriching DC, due to the down-regulation of the marker. However, the most efficient selection of rat DC was made by CD161a (NKR-P1A), a C-type lectin family. The GM-CSF independent DC was functionally active in vitro as well as in vivo assays.

CD8alpha dendritic cells and immune protection from experimental allergic encephalomyelitis

Dendritic cells (DC) represent a phenotypically heterogeneous population endowed with two important biological functions, immunity and tolerance. Here we report that the injection of splenic CD8alpha(+) DC, derived from rats with experimental allergic encephalomyelitis (EAE), delayed the onset and suppressed the severity of EAE in Lewis rats. This was accompanied by the lack of magnetic resonance imaging (MRI) lesions in the brain and spinal cord and by reduced numbers of inflammatory cells within the central nervous system. Injection of CD8(alpha+) DC inhibited T cell proliferation that may relate to increased interferon (IFN)-gamma and nitric oxide production. Although CD8(+)CD28(-) suppressor T cells, apoptotic cells and co-stimulatory molecules were not altered, CD4(+) T cells expressing interleukin (IL)-10 were augmented in rats receiving CD8alpha(+) DC compared to rats receiving total DC or medium. These results demonstrate that rat splenic CD8alpha(+) DC could provide a cellular basis for a novel, individualized immunotherapy using autologous DC as a complement to conventional therapy in diseases with an autoimmune background such as multiple sclerosis.

Minute numbers of contaminant CD8+ T cells or CD11b+CD11c+ NK cells are the source of IFN-gamma in IL-12/IL-18-stimulated mouse macrophage populations

Macrophages were reported to be strong producers of interferon gamma (IFN-gamma) after stimulation by interleukin 12 (IL-12) plus IL-18, which gave rise to a novel concept of auto-crine macrophage activation. Here, we show that peritoneal exudate and bone marrow-derived mouse macrophages generated by conventional techniques contain small quantities of CD11b(+)CD11c(+)CD31(+)DX5(+)NK1.1(+) natural killer (NK) cells or CD3(+)CD8(+)TCRbeta(+) T cells, respectively. Intracellular cytokine staining, purification of macrophages by sorting, and the analysis of macrophages from alymphoid RAG2(-/-)gamma-chain(-/-) mice revealed that the high amount of IFN-gamma protein in the supernatants of unseparated IL-12/IL-18-stimulated macrophage populations originates exclusively from the contaminating lymphoid cells. Notably, IL-12/IL-18 still induced IFN-gamma mRNA in highly purified macrophages from wild-type mice and in macrophages from RAG2(-/-)gamma-chain(-/-) mice, whereas nuclear translocation of signal transducer and activator of transcription 4 (STAT4) and production of IFN-gamma protein were no longer detectable. These results question the concept of autocrine macrophage activation by secreted IFN-gamma, suggest differences in the expression of IFN-gamma mRNA and protein between macrophages and lymphoid cells, and illustrate that the limited purity of most myeloid cell populations (</= 98%) might lead to false conclusions.

Rat plasmacytoid dendritic cells are an abundant subset of MHC class II+ CD4+CD11b-OX62- and type I IFN-producing cells that exhibit selective expression of Toll-like receptors 7 and 9 and strong responsiveness to CpG

We have identified in the rat a new subset of MHC class II(+) CD4(+)CD3(-)CD11b(-) leukocytes that produce high amounts of type I IFN upon viral stimulation and that appeared homologous to plasmacytoid DC (pDC) previously described in humans and mice. These cells exhibited the following phenotype: CD5(+),CD90(+),CD45R(+),CD45RC(+),CD11c(-),CD161a(+),CD200(+),CD172a(+),CD32(+),CD86(+). Rat pDC did not express the DC-specific marker OX62 and were more abundant in the spleen than the classical CD4(+) and CD4(-) subsets of OX62(+)CD11b(+) DC we previously described that produced very little, if any, type I IFN. Spleen pDC exhibited an undifferentiated morphology and rapidly died in vitro, but showed extensive dendrite formation, survival, maturation, and moderate type I IFN production upon stimulation by oligonucleotides containing type B CpG motifs (CpG ODN). Type A CpG ODN and CD40 ligand induced pDC to produce large amounts of type I IFN, but did not promote maturation. CpG ODN and CD40 ligand, but not influenza virus, induced IL-12p40 and IL-6 secretion. Spleen pDC did not produce IL-12p70, TNF-alpha, IL-1beta, or IL-10 using these stimulation conditions. Correlating with their strong responsiveness to virus and CpG ODN, rat pDC specifically expressed Toll-like receptor 7 and 9 mRNA. Fresh spleen pDC were poor stimulators of allogenic CD4(+) and CD8(+) T cells, but became potent inducers of allogenic T cell proliferation as well as Th1 differentiation after stimulation by type B CpG. Therefore, rat pDC appear very similar to human pDC, indicating that the specific phenotype and functions of pDC have been highly conserved between species.

Defective costimulatory function is a striking feature of antigen-presenting cells in an HLA-B27-transgenic rat model of spondylarthropathy

OBJECTIVE

A disease resembling the human spondylarthropathies develops in HLA-B27-transgenic rats. This disease in rats is mediated by CD4+ T cells, but antigen-presenting cells (APCs) may also play a role. Dendritic cells (DCs) have been reported to be defective in allogeneic mixed lymphocyte culture in this model. Here, we further investigated the functional defect of APCs.

METHODS

DCs and B cells from nontransgenic, HLA-B27 (33-3)-transgenic, and HLA-B7 (120-4)-transgenic rats were used to stimulate T cells. Surface expression of HLA-B transgene and rat molecules on APCs and the formation of conjugates between DCs and T cells were monitored by flow cytometry.

RESULTS

We observed a strikingly defective stimulation of allogeneic and syngeneic T lymphocytes by APCs from HLA-B27 but not HLA-B7 rats, even if stimulation was driven in the presence of anti-T cell receptor (TCR) antibody. We found no evidence that HLA-B27 DCs were immature, lacked production of some diffusible factor, or produced an inhibitory factor for T cells. When comparing the levels of expression of class II major histocompatibility complex, CD2, intercellular adhesion molecule 1, lymphocyte function-associated antigen 1, B7, and CD40 molecules at the surface of DCs from 33-3, 120-4, and nontransgenic rats, we found little difference. However, HLA-B27-transgenic DCs formed fewer conjugates with T cells than did nontransgenic DCs. Furthermore, the proportion of conjugates formed between DCs and T cells, as well as the difference between nontransgenic and HLA-B27-transgenic DCs, were in large part reduced by blocking CD86 on DCs.

CONCLUSION

We confirmed defective stimulation of T cells by APCs in HLA-B27 rats, the mechanism of which appears to implicate APC/T cell contact, independent of TCR engagement. In addition, decreased use of the CD86 costimulatory molecule by B27 DCs was observed. Impaired costimulatory function could result in a loss of tolerance toward microbial flora in this model.

The modulation of thymosin alpha 1 in the maturation, differentiation and function of murine bone marrow-derived dendritic cells in the absence or presence of tumor necrosis factor-alpha

Thymosin alpha 1 (Talpha1) has immunomodulatory effects on T-cells, NK-cells and macrophages, but its action on dendritic cells (DCs), which are recognized as the sole professional antigen presenting cells (APCs) capable of priming naïve T-cells, is poorly understood. In this study, the effect of Talpha1 in vitro on murine bone marrow-derived DCs (BMDCs) maturation, differentiation, and function with or without tumor necrosis factor-alpha (TNF-alpha), which is one of the important inflammatory parameters, has been investigated. We have shown, through flow cytometry, ELISA and mixed leukocyte reaction (MLR), that Talpha1 promoted CD4-expressed DC differentiation and the expression of activation markers, but did not influence IL-12 production and T cell-stimulatory capacity of DCs in the absence of TNFalpha during BMDCs maturation. Furthermore, in the presence of TNF-alpha, Talpha1 has been shown not only to promote the expression of CD4 on MHC class II+ DCs and enhance the up-regulated levels of mature markers induced by TNF-alpha, but also to suppress the up-regulated IL-12 production. Particularly, thus effects seen were obvious at pharmacological Talpha1 concentrations. However, Talpha1 did not inhibit TNF-alpha-induced T-cell stimulating function of DCs. This is the first reported example of a direct Talpha1-DC interaction and suggests a mechanism by which Talpha1 may in part affect T-cell responses by acting at the DC level and it may play an important role in the modulation of the local inflammatory responses in vivo.

Immunobiological Characterization of N-Nitrosomethylurea-Induced Rat Breast Carcinomas: Tumoral IL-10 Expression as a Possible Immune Escape Mechanism

Improvement of immunotherapy-based protocols in cancer requires a better understanding of tumor microenvironment and tumor-host interaction. Stromal and immune cells and molecules such as cytokines, chemokines, growth factors and metalloproteases mediate tumor-host interaction determining, at least in part, tumor development. In the present study, we used an immunohistochemical approach to explore leukocyte sub-populations, cytokine profiles and costimulatory molecule expression in rat N -Nitrosomethylurea (NMU)-induced breast tumors. Our results show a strong leukocyte infiltration mainly composed of macrophages and TCR alphabeta positive T cells. We observed a weak expression of costimulatory molecules (CD80, CD86) and an absence of inflammatory cytokines (IFNgamma, TNFalpha, IP-10) and lymphocyte activation markers (CD25). Interestingly, this immunosuppressed status could be a consequence of IL-10 expression by malignant cells, as demonstrated by immunohistology and western blot analysis, which seems to be an early event during mammary carcinogenesis. Analysis of a cell line derived from an NMU-induced rat breast tumor showed that this cell line also expresses IL-10. This study shows that the NMU model of rat breast cancer could be used to evaluate different immune based therapies as well as to study the role of IL-10 in breast cancer. Furthermore, this rat breast cancer model shows an immunohistological profile similar to that found in human cancer and the fact that it develops like spontaneously arising malignancies make it interesting as a cancer model in immunobiology.

Phenotype and function of rat dendritic cell subsets

Dendritic cells (DC) comprise phenotypically-distinct subsets that sub-serve distinct functions in immune induction. Understanding the biology of DC subsets in vivo is crucial for the understanding of immune regulation and its perturbations in disease. This review focuses on the phenotype and functions of rat DC subsets and compares these with subsets identified in other species. Our research has concentrated on DC migrating in lymph. DC migrate constitutively from peripheral tissues to draining nodes, probably to induce/maintain tolerance to self- or harmless foreign antigens. After removal of mesenteric lymph nodes (MLN) in the rat, healing of afferent and efferent lymphatics permits migrating intestinal DC (iLDC) to be collected from the thoracic duct. We have shown that iLDC consist of least two subsets that differ in phenotype, in situ distribution and function. CD4+/SIRPalpha+ iLDC are highly immunostimulatory, but are excluded from T cell areas of MLN. In contrast, CD4-/SIRPalpha- iLDC are less potent stimulators of T cells, but carry material from apoptotic enterocytes to T cell areas of MLN. Similar subsets exist in both lymph nodes and spleen. It has been shown that phenotypically-similar subsets migrate in skin-draining lymph in cattle and sheep. We and others have shown that splenic CD4-/SIRPalpha- DC can phagocytose allogeneic cells in vitro, are poor stimulators of CD8+ T cells, and can lyse NK-sensitive target cells. Although some of our data suggest that rat CD4-/SIRPalpha- DC may equate to murine CD8+ DC, there is at present insufficient evidence to be confident of this.

Active suppression of allogeneic proliferative responses by dendritic cells after induction of long-term allograft survival by CTLA4Ig

Costimulatory blockade using cytotoxic T lymphocyte-associated antigen 4 immunoglobulin (CTLA4Ig) efficiently down-regulates immune responses in animal models and is currently used in autoimmune and transplantation clinical trials, but the precise cellular and molecular mechanisms involved remain unclear. Rats that received allogeneic heart transplants and were treated with adenoviruses coding for CTLA4Ig show long-term allograft survival. The immune mechanisms regulating induction of long-term allograft acceptance were analyzed in splenocytes using mixed leukocyte reactions (MLRs). MLRs of splenocytes but not purified T cells from CTLA4Ig-treated rats showed higher than 75% inhibition compared with controls. Splenocytes from CTLA4Ig-treated rats inhibited proliferation of naive and allogeneically primed splenocytes or T cells. MLR suppression was dependent on soluble secreted product(s). Production of soluble inhibitory product(s) was triggered by a donor antigen-specific stimulation and inhibited proliferation in an antigen-nonspecific manner. CTLA4Ig levels in the culture supernatant were undetectable and neither interleukin-10 (IL-10), transforming growth factor beta 1 (TGF beta 1), IL-4, nor IL-13 were responsible for suppression of MLRs. Inhibition of nitric oxide (NO) production or addition of IL-2 could not restore proliferation independently, but the combined treatment synergistically induced proliferation comparable with controls. Stimulation of APCs using tumor necrosis factor (TNF)-related activation-induced cytokine (TRANCE) or CD40L and addition of IL-2 normalized MLRs of CTLA4Ig-treated splenocytes. Finally, dendritic cells (DCs), but not T cells, from CTLA4Ig-treated rats inhibited naive MLRs. Altogether, these results provide evidence that after in vivo CTLA4Ig treatment, splenocytes, and in particular DCs, can inhibit alloantigen-induced proliferative responses through secretion of inhibitory products, thus promoting alloantigen-specific tolerance in vivo.

Two phenotypically distinct subsets of spleen dendritic cells in rats exhibit different cytokine production and T cell stimulatory activity

We recently reported that splenic dendritic cells (DC) in rats can be separated into CD4(+) and CD4(-) subsets and that the CD4(-) subset exhibited a natural cytotoxic activity in vitro against tumor cells. Moreover, a recent report suggests that CD4(-) DC could have tolerogenic properties in vivo. In this study, we have analyzed the phenotype and in vitro T cell stimulatory activity of freshly isolated splenic DC subsets. Unlike the CD4(-) subset, CD4(+) splenic DC expressed CD5, CD90, and signal regulatory protein alpha molecules. Both fresh CD4(-) and CD4(+) DC displayed an immature phenotype, although CD4(+) cells constitutively expressed moderate levels of CD80. The half-life of the CD4(-), but not CD4(+) DC in vitro was extremely short but cells could be rescued from death by CD40 ligand, IL-3, or GM-CSF. The CD4(-) DC produced large amounts of the proinflammatory cytokines IL-12 and TNF-alpha and induced Th1 responses in allogeneic CD4(+) T cells, whereas the CD4(+) DC produced low amounts of IL-12 and no TNF-alpha, but induced Th1 and Th2 responses. As compared with the CD4(+) DC that strongly stimulated the proliferation of purified CD8(+) T cells, the CD4(-) DC exhibited a poor CD8(+) T cell stimulatory capacity that was substantially increased by CD40 stimulation. Therefore, as previously shown in mice and humans, we have identified the existence of a high IL-12-producing DC subset in the rat that induces Th1 responses. The fact that both the CD4(+) and CD4(-) DC subsets produced low amounts of IFN-alpha upon viral infection suggests that they are not related to plasmacytoid DC.

Dendritic cell appearance and differentiation during early and late stages of rat stomach carcinogenesis

Dendritic cell appearance and differentiation during early and late stages of rat stomach carcinogenesis were studied in the pyloric mucosa. Young male rats were given drinking water with or without N-methyl-N'-nitro-N-nitrosoguanidine (MNNG; 100 mg/liter) for 14 days. Use of competitive RT-PCR and northern blotting showed that MNNG exposure induced 3- to 4-fold greater expression of the genes for integrin beta7 and integrin alphaE2 (identical with antigen OX-62, a dendritic cell marker), as well as three cytokines, IL-4, GM-CSF and TNFalpha, in the stomach pyloric mucosa of resistant Buffalo rats compared to sensitive ACI rats. These genes were minimally expressed in control animals. The results confirm the appearance of dendritic cells in the target pyloric mucosa and suggest the possibility that dendritic cell differentiation and maturation are induced by various cytokines, at least in Buffalo rats. Competitive RT-PCR showed expression of integrin alphaE2 and beta7, MHC class II-associated invariant chain (Ii), MHC class II, B7-1, CD28, GM-CSF and TNFalpha genes in all 12 examined stomach adenocarcinomas and adenomas induced in male Lewis and WKY rats with 30 weeks' MNNG exposure, suggesting the presence of dendritic cells in tumors. OX-62 staining and western blotting for OX-62 also confirmed the presence of dendritic cells in tumors. However, the population of dendritic cells in tumors was less than that in the pyloric mucosa after 14 days' MNNG exposure. The present results suggest that immune defense involving dendritic cells is marshaled from the very early initiation stage during rat stomach cancer development, but is downgraded in developed tumors.

CD40L blockade prevents autoimmune diabetes by induction of bitypic NK/DC regulatory cells

Systemic treatment with antibody to CD40 ligand (aCD40L) can prevent autoimmunity and transplant rejection in several animal models and is currently under evaluation in clinical trials. While it is known that aCD40L administration inhibits expansion and effector functions of aggressive T cells, it is still unclear whether additional regulatory mechanisms are operative. Here we demonstrate that a single episode of CD40L blockade during development of the autoaggressive immune response completely prevented autoimmune disease in the RIP-LCMV mouse model for virally induced type 1 diabetes. Interestingly, protection could be transferred by a highly potent, bitypic cell population sharing phenotypic and functional properties of both natural killer (NK) and dendritic cells (DC). Furthermore, protection of prediabetic recipients was autoantigen specific and did not result in generalized immunosuppression. The origin, function, and therapeutic potential of these bitypic NK/DC regulatory cells is discussed.

Tumoricidal activity of monocyte-derived dendritic cells: evidence for a caspase-8-dependent, Fas-associated death domain-independent mechanism

Monocyte-derived dendritic cells (DC) were found to be cytotoxic for several tumor cell lines including Jurkat cells, which were killed through a calcium-independent pathway. K562 cells were resistant, excluding a NK cell-like activity. DC-mediated apoptosis did not involve classical death receptors because it was not reversed by blocking TNF/TNFR, CD95/CD95 ligand, or TNF-related apoptosis-inducing ligand/TNF-related apoptosis-inducing ligand receptor interactions. Fas-associated death domain-deficient, but not caspase-8 deficient, Jurkat cells were killed by DC. Indeed, caspase-8 cleavage was demonstrated in Jurkat cells cocultured with DC, and the use of specific caspase inhibitors confirmed that apoptosis triggered by DC was caspase-8 dependent. Furthermore, the involvement of Bcl-2 family members in the control of DC-mediated apoptosis was demonstrated by Bid cleavage in Jurkat cells cocultured with DC and resistance of Jurkat cells overexpressing Bcl-2 to DC-mediated cytotoxicity. Overall, these data indicate that monocyte-derived DC exert a caspase-8-dependent, Fas associated death domain-independent tumoricidal activity, a finding that could be relevant to their therapeutic use in cancer.

Cytotoxic activity of human dendritic cells is differentially regulated by double-stranded RNA and CD40 ligand

The main function of dendritic cells (DCs) is to induce adaptive immune response through Ag presentation and specific T lymphocyte activation. However, IFN-alpha- or IFN-gamma-stimulated CD11c+ blood DCs and IFN-beta-stimulated monocyte-derived DCs were recently reported to express functional TNF-related apoptosis-inducing ligand (TRAIL), suggesting that DCs may become cytotoxic effector cells of innate immunity upon appropriate stimulation. In this study, we investigate whether dsRNA and CD40 ligand (CD40L), that were characterized as potent inducers of DC maturation, could also stimulate or modulate DC cytotoxicity toward tumoral cells. We observed that dsRNA, but not CD40L, is a potent inducer of TRAIL expression in human monocyte-derived DCs. As revealed by cytotoxicity assays, DCs acquire the ability to kill tumoral cells via the TRAIL pathway when treated with dsRNA. More precisely, dsRNA is shown to induce IFN-beta synthesis that consecutively mediates TRAIL expression by the DCs. In contrast, we demonstrate that TRAIL expression in dsRNA- or IFN-alpha-treated DCs is potently inhibited after CD40L stimulation. Unexpectedly, CD40L-activated DCs still developed cytotoxicity toward tumoral cells. This latter appeared to be partly mediated by TNF-alpha induction and a yet unidentified pathway. Altogether, these results demonstrate that dsRNA and CD40L, that were originally characterized as maturation signals for DCs, also stimulate their cytotoxicity that is mediated through TRAIL-dependent or -independent mechanisms.

Delineation of intrathymic T, NK, and dendritic cell (DC) progenitors in fetal and adult rats: demonstration of a bipotent T/DC intermediate precursor

We previously published study results stating that the early rat fetal liver contains a high frequency of T/dendritic cells (DCs), but rarely T/NK bipotent common progenitors. Now, by using xenogenic rat/SCID mouse fetal thymic organ cultures, we extend these observations to the thymus, in which conflicting data have been published in human and mouse. On the one hand, enriched adult intrathymic CD45+CD2- triple negative for CD8, CD4, and CD3 Ag cell progenitors, which contained both rearranged TCRbeta chain and pre-Talpha chain transcripts, completely lacked NKR-P1A expressing cells, and upon limiting dilution conditions, generated T- and T/DC-containing lobes, but no T/NK or NK ones were found. On the other hand, the CD45+CD2- triple negative for CD8, CD4, and CD3 Ags cell population obtained from 15- and 16-day-old fetal rat thymus can be divided into NKR-P1A- and NKR-P1A(low) cell subpopulations that differ in several aspects. Both cell subsets expressed pre-TCRalpha chain transcripts, but only the former contained fully rearranged TCRbeta chain transcripts. Upon limiting dilution, T cell-committed progenitors were only found in the NKR-P1A- cell population, whereas NK-committed progenitors were present in the NKR-P1A(low) population. More importantly, bipotential T/NK progenitors were very rare and were found only in the NKR-P1A(low) cell population, whereas bipotential T/DC progenitors, only previously suggested in the adult mouse thymus, were observed frequently in the NKR-P1A-CD2- cell subpopulation. Our results demonstrate, therefore, that a common intrathymic T/DC intermediate represents the main T cell developmental pathway in rat thymus.

Advances in transplant immunobiology

The present review briefly addresses the most recent knowledge acquired in the field of transplant immunology. A particular emphasis is placed on articles published during the past 12-18 months that have focused on allorecognition, dendritic cells and tolerance.