Peta J. O'Connell

Immature and mature CD8α+ dendritic cells prolong the survival of vascularized heart allografts

CD8α+ and CD8α− dendritic cells (DCs) arise from committed bone marrow progenitors and can induce or regulate immune reactivity. Previously, the maturational status of CD8α− (myeloid) DCs has been shown to influence allogeneic T cell responses and allograft survival. Although CD8α+ DCs have been implicated in central tolerance and found to modulate peripheral T cell function, their influence on the outcome of organ transplantation has not been examined. Consistent with their equivalent high surface expression of MHC and costimulatory molecules, sorted mature C57BL/10J (B10; H2b) DCs of either subset primed naive, allogeneic C3H/HeJ (C3H; H2k) recipients for Th1 responses. Paradoxically and in contrast to their CD8α− counterparts, mature CD8α+ B10 DCs given systemically 7 days before transplant markedly prolonged B10 heart graft survival in C3H recipients. This effect was associated with specific impairment of ex vivo antidonor T cell proliferative responses, which was not reversed by exogenous IL-2. Further analyses of possible underlying mechanisms indicated that neither immune deviation nor induction of regulatory cells was a significant contributory factor. In contrast to the differential capacity of the mature DC subsets to affect graft outcome, immature CD8α+ and CD8α− DCs administered under the same experimental conditions significantly prolonged transplant survival. These observations demonstrate for the first time the innate capacity of CD8α+ DCs to regulate alloimmune reactivity and transplant survival, independent of their maturation status. Mobilization of such a donor DC subset with capacity to modulate antidonor immunity may have significant implications for the therapy of allograft rejection.

Hepatic dendritic cells: immunobiology and role in liver transplantation.

Traditional investigations of hepatic dendritic cells (DC) have focused on immunohistochemical studies of these cells within normal and pathological liver tissue. The recent availability of reagents for the improved characterization of DC, together with cytokine‐based methods for the expansion of liver DC both in vivo and in vitro have begun to provide new insight into the immunobiology of these important antigen‐presenting cells. Hepatic DC probably play a key role in the host response to blood‐borne pathogens, and in the pathogenesis of infectious and autoimmune liver diseases. They appear to be important in determining the balance between liver transplant tolerance and rejection. Their possible role in oral and portal venous tolerance remains to be defined. In this article, we focus on emerging aspects of hepatic DC immunobiology, with particular reference to liver transplantation. J. Leukoc. Biol. 66: 322–330; 1999.

Immunobiology of liver dendritic cells.

Dendritic cells (DC) are rare, bone marrow‐derived antigen‐presenting cells that play a critical role in the induction and regulation of immune reactivity. In this article, we review the identification and characterization of liver DC, their ontogenic development, in vivo mobilization and population dynamics. In addition, we discuss the functions of DC isolated from liver tissue or celiac lymph, or propagated in vitro from liver‐resident haemopoietic stem/progenitor cells. Evidence concerning the role of DC in viral hepatitis, liver tumours, autoimmune liver diseases, granulomatous inflammation and the outcome of liver transplantation is also discussed.

Identification of functional type 1 ryanodine receptors in mouse dendritic cells.

Ca2+ signaling plays an important role in the function of dendritic cells (DC), the specialized antigen‐presenting cells of the immune system. Here we describe functional ryanodine receptor (RyR) Ca2+ release channels in murine, bone marrow‐derived DC. RT‐PCR analysis identified selective expression of the type 1 RyR, with higher levels detected in immature rather than mature DC. The RyR activators caffeine, FK506, ryanodine and 4‐chloro‐m‐cresol mobilized Ca2+ in DC, and responses to 4‐chloro‐m‐cresol were inhibited by dantrolene. Furthermore, activation of RyRs both inhibited subsequent inositol trisphosphate‐mediated Ca2+ release and provoked store‐operated Ca2+ entry, suggesting a functional interaction between these intracellular Ca2+ channels. Thus, the RyR1 channel may play an intrinsic role in Ca2+ signaling in DC.

Rare-event analysis of circulating human dendritic cell subsets and their presumptive mouse counterparts.

Background. Considerable interest has focused recently on murine CD8&agr;− and CD8&agr;+ dendritic cell (DC) subsets, because of their roles in initiating and regulating immune responses. Attention has also centered on their presumed human counterparts, DC1 and DC2, respectively, and their precursors. Identification and quantification of these subsets in the blood may be crucial to understanding and monitoring of their immunologic significance, particularly in humans, where blood may be the only tissue readily or routinely available. Methods. Leukocytes were isolated from anticoagulated human or mouse (C57BL/10J) blood using conventional procedures. Four-color, rare-event, flow cytometric analysis was used to identify DC1 precursors (pDC1; lineage [lin]− CD4+ CD11c+ HLA-DR+) or DC2 precursors (pDC2; lin− CD4+ CD11c− CD123hi [IL-3R&agr;hi] HLA-DR+) in normal humans. In mice, CD8&agr;+ (CD11blo, CD11c+) and CD8&agr;− (CD11bhi, CD11c+) DC subsets were identified both in normal animals and after administration of the potent DC growth factor, fms-like tyrosine kinase 3 ligand (Flt3L). Results. All human subjects examined had discrete populations of pDC1 and pDC2 comprising approximately 0.6% and 0.1% of blood mononuclear cells. CD8&agr;− and CD8&agr;+ DC constituted approximately 0.75% and 0.2%, respectively, of blood mononuclear cells in normal mice, and 12% and 0.5%, respectively, in Flt3L-treated animals. Flt3L administration substantially increased the absolute numbers of circulating CD11c+ DC by approximately 200-fold. Conclusions. In addition to pDC1 and CD8&agr;− DC, pDC2 and CD8&agr;+ DC can be identified in normal human or mouse blood, respectively. Monitoring and isolation or characterization of these cells may provide novel insights into their functional significance in transplantation and other clinical conditions.

Type-1 polarized nature of mouse liver CD8α– and CD8α+ dendritic cells: tissue-dependent differences offset CD8α-related dendritic cell heterogeneity

Interleukin‐12 p70 (IL‐12p70) is a major dendritic cell (DC)‐produced cytokine known to support type‐1 T helper (Th1) cells and inflammatory‐type immunity. While the ability of DC to produce bioactive IL‐12p70 depends on both the DC subtype and the microenvironmental conditions of DC development, the relative contribution of each of these factors remains unclear. Here, we report that in contrast to spleen CD8α+ and CD8α– DC that show strong differences in their respective IL‐12p70‐producing capacities, CD8α+ and CD8α– DC isolated from the liver, a non‐lymphoid organ, both efficiently produce IL‐12p70 in amounts comparable to spleen CD8α+ DC. The IL‐12p70‐producing capacity of CD8α+ and CD8α– DC from either location is greatly increased following their overnight culture in the presence of granulocyte‐macrophage colony‐stimulating factor. The elevated production of IL‐12p70 by short‐term cultured DC correlates with their enhanced expression of CD40 and other costimulatory molecules, and elevated T cell‐stimulatory capacity. These data indicate that low IL‐12‐producing capacity is not an intrinsic property of the CD8α– DC subtype, and support the hypothesis that factors such as the site of DC development and maturation stage play a dominant role in defining DC function.

Donor dendritic cell persistence in organ allograft recipients in the absence of immunosuppression

Donor‐derived leukocytes are known to persist in the peripheral blood of organ allograft recipients after withdrawal of all immunosuppressive drug therapy and can exert a donor‐specific veto effect. Antigen‐presenting cells (APC), in particular dendritic cells (DC), have been proposed as a candidate for this veto leukocyte. Myeloid DC were derived from the peripheral blood of two non‐compliant organ transplant recipients: D. S., a heart transplant recipient, and J. M., a liver transplant recipient. Donor‐specific signal was enriched in the cultured DC fraction relative to whole blood for both patients. The clinical outcome in each patient was different: D. S. lost his heart allograft due to biopsy‐proven acute and chronic rejection 2.5 years after discontinuing anti‐rejection medication; J. M. continues to maintain adequate liver function. The results have important implications for the planned withdrawal of immunosuppression in tolerance protocols as DC may play a role either in the maintenance of tolerance or immune activation. J. Leukoc. Biol. 66: 301–305; 1999.

IP3Rs are sufficient for dendritic cell Ca2+ signaling in the absence of RyR1

Calcium (Ca2+) signaling plays a pivotal role in the function of dendritic cells (DC). The Type 1 ryanodine receptor (RyR), a major intracellular Ca2+ channel, is highly expressed in immature DC. We therefore investigated whether RyR1 plays a role in DC development and function by studying properties of DC derived from wild‐type (WT) and RyR1 null [knockout (KO)] mice. Fetal liver cells from WT and RyR1 KO mice retained full hematopoietic competence. Adoptive transfer of these cells into congenic hosts resulted in the generation of functionally equivalent DC populations. WT and RyR1 KO DC exhibited a similar capacity to mature in response to inflammatory and/or activation stimuli, to endocytose antigen, and to stimulate T cell proliferation. Moreover, the absence of RyR1 did not lead to de novo expression of RyR2 or RyR3. WT and RyR KO DC express all three isoforms of inositol 1,4,5‐trisphosphate receptor (IP3R), although Type 3 IP3R gene transcripts are predominant. Further, IP3‐mediated Ca2+ transients proceed normally after inhibition of RyRs with dantrolene. Signaling via IP3R may therefore be sufficient to drive essential DC Ca2+ signaling processes in the absence of RyR expression or function.

T lymphocytes possess the machinery for 5-HT synthesis, storage, degradation and release

Although activated T lymphocytes express tryptophan hydroxylase 1 and produce 5‐HT, the metabolic fate and cellular handling of this 5‐HT is unclear. Here, we investigated key proteins in T cells linked to 5‐HT metabolism and storage and compare differences in 5‐HT synthesis and metabolism between T‐cell subsets.