Functional redundancy between thymic CD8α+ and Sirpα+ conventional dendritic cells in presentation of blood-derived lysozyme by MHC class II proteins

Specificity through cooperation: BATF-IRF interactions control immune-regulatory networks

Basic leucine zipper transcription factor ATF-like (BATF), BATF2 and BATF3 belong to the activator protein 1 (AP-1) family of transcription factors, which regulate numerous cellular processes. Initially, BATF family members were thought to function only as inhibitors of AP-1-driven transcription, but recent studies have uncovered that these factors have unique, non-redundant and positive transcriptional activities in dendritic cells, B cells and T cells. The question of how BATF and BATF3 - which lack a transcriptional activation domain, unlike the AP-1 factors FOS and JUN - can exert unique positive transcriptional specificity has now been answered by the discovery that BATF molecules interact with members of the interferon-regulatory factor (IRF) family. The capacity of the BATF leucine zipper regions to mediate dimerization with AP-1 factors and also to define cooperative interactions with heterologous factors explains both the positive transcriptional activity of BATF proteins and how they activate distinct sets of target genes compared with FOS.

MARCH1-mediated MHCII ubiquitination promotes dendritic cell selection of natural regulatory T cells

Ubiquitination of MHCII molecules on dendritic cells is essential for the development of natural regulatory T cells

Membrane-associated RING-CH1 (MARCH1) is an E3 ubiquitin ligase that mediates ubiquitination of MHCII in dendritic cells (DCs). MARCH1-mediated MHCII ubiquitination in DCs is known to regulate MHCII surface expression, thereby controlling DC-mediated T cell activation in vitro. However, its role at steady state or in vivo is not clearly understood. Here, we show that MARCH1 deficiency resulted in a substantial reduction in the number of thymus-derived regulatory T cells (T reg cells) in mice. A specific ablation of MHCII ubiquitination also significantly reduced the number of thymic T reg cells. Indeed, DCs deficient in MARCH1 or MHCII ubiquitination both failed to generate antigen-specific T reg cells in vivo and in vitro, although both exhibited an increased capacity for antigen presentation in parallel with the increased surface MHCII. Thus, MARCH1-mediated MHCII ubiquitination in DCs is required for proper production of naturally occurring T reg cells, suggesting a role in balancing immunogenic and regulatory T cell development.

Novel process of intrathymic tumor-immune tolerance through CCR2-mediated recruitment of Sirpα+ dendritic cells: a murine model

Immune surveillance system can detect more efficiently secretory tumor-specific antigens, which are superior as a target for cancer immunotherapy. On the contrary, immune tolerance can be induced in the thymus when a tumor antigen is massively secreted into circulation. Thus, the secretion of tumor-specific antigen may have contradictory roles in tumor immunity in a context-dependent manner. However, it remains elusive on the precise cellular mechanism of intrathymic immune tolerance against tumor antigens. We previously demonstrated that a minor thymic conventional dendritic cell (cDC) subset, CD8α⁻Sirpα⁺ cDCs, but not the major subset, CD8α⁺Sirpα⁻ cDCs can selectively capture blood-borne antigens and crucially contribute to the self-tolerance. In the present study, we further demonstrated that Sirpα⁺ cDCs can capture a blood-borne antigen leaking inside the interlobular vascular-rich regions (IVRs). Blood-borne antigen selectively captured by Sirpα⁺ cDCs can induce antigen-specific Treg generation or negative selection, depending on the immunogenicity of the presented antigen. Furthermore, CCR2 expression by thymic Sirpα⁺ cDCs and abundant expression of its ligands, particularly, CCL2 by tumor-bearing mice prompted us to examine the function of thymic Sirpα⁺ cDCs in tumor-bearing mice. Interestingly, tumor-bearing mice deposited CCL2 inside IVRs in the thymus. Moreover, tumor formation induced the accumulation of Sirpα⁺ cDCs in IVRs under the control of CCR2-CCL2 axis and enhanced their capacity to take up antigens, resulting in the shift from Treg differentiation to negative selection. Finally, intrathymic negative selection similarly ensued in CCR2-competent mice once the tumor-specific antigen was secreted into bloodstream. Thus, we demonstrated that thymic Sirpα⁺ cDCs crucially contribute to this novel process of intrathymic tumor immune tolerance.

T-cell tolerance: central and peripheral

Somatic recombination of TCR genes in immature thymocytes results in some cells with useful TCR specificities, but also many with useless or potentially self-reactive specificities. Thus thymic selection mechanisms operate to shape the T-cell repertoire. Thymocytes that have a TCR with low affinity for self-peptide-MHC complexes are positively selected to further differentiate and function in adaptive immunity, whereas useless ones die by neglect. Clonal deletion and clonal diversion (Treg differentiation) are the major processes in the thymus that eliminate or control self-reactive T cells. Although these processes are thought to be efficient, they fail to control self-reactivity in all circumstances. Thus, peripheral tolerance processes exist wherein self-reactive T cells become functionally unresponsive (anergy) or are deleted after encountering self-antigens outside of the thymus. Recent advances in mechanistic studies of central and peripheral T-cell tolerance are promoting the development of therapeutic strategies to treat autoimmune disease and cancer and improve transplantation outcome.

Dendritic cells and regulation of graft-versus-host disease and graft-versus-leukemia activity

Hematopoietic stem cell transplantation is the only curative treatment for many malignant hematologic diseases, with an often critical graft-versus-leukemia effect. Despite peritransplant prophylaxis, GVHD remains a significant cause of posthematopoietic stem cell transplantation morbidity and mortality. Traditional therapies have targeted T cells, yet immunostimulatory dendritic cells (DCs) are critical in the pathogenesis of GVHD. Furthermore, DCs also have tolerogenic properties. Monitoring of DC characteristics may be predictive of outcome, and therapies that target DCs are innovative and promising. DCs may be targeted in vivo or tolerogenic (tol) DCs may be generated in vitro and given in the peritransplant period. Other cellular therapies, notably regulatory T cells (T(reg)) and mesenchymal stem cells, mediate important effects through DCs and show promise for the prevention and treatment of GVHD in early human studies. Therapies are likely to be more effective if they have synergistic effects or target both DCs and T cells in vivo, such as tolDCs or T(reg). Given the effectiveness of tolDCs in experimental models of GVHD and their safety in early human studies for type 1 diabetes, it is crucial that tolDCs be investigated in the prevention and treatment of human GVHD while ensuring conservation of graft-versus-leukemia effects.

The earliest intrathymic precursors of CD8α(+) thymic dendritic cells correspond to myeloid-type double-negative 1c cells

The dendritic cells (DCs) present in lymphoid and non-lymphoid organs are generated from progenitors with myeloid-restricted potential. However, in the thymus a major subset of DCs expressing CD8α and langerin (CD207) appears to stand apart from all other DCs in that it is thought to derive from progenitors with lymphoid potential. Using mice expressing a fluorescent reporter and a diphtheria toxin receptor under the control of the cd207 gene, we demonstrated that CD207(+) CD8α(+) thymic DCs do not share a common origin with T cells but originate from intrathymic precursors that express markers that are normally present on all (CD11c(+) and MHCII molecules) or on some (CD207, CD135, CD8α, CX3CR1) DC subsets. Those intrathymic myeloid-type precursors correspond to CD44(+) CD25(-) double-negative 1c (DN1c) cells and are continuously renewed from bone marrow-derived canonical DC precursors. In conclusion, our results demonstrate that the earliest intrathymic precursors of CD8α(+) thymic DCs correspond to myeloid-type DN1c cells and support the view that under physiological conditions myeloid-restricted progenitors generate the whole constellation of DCs present in the body including the thymus.