Microenvironment tailors nTreg structure and function.

Natural regulatory T cells (nTregs) ensure the control of self-tolerance and are currently used in clinical trials to alleviate autoimmune diseases and graft-versus-host disease after hematopoietic stem cell transfer. Based on CD39/CD26 markers, blood nTreg analysis revealed the presence of five different cell subsets, each representing a distinct stage of maturation. Ex vivo added microenvironmental factors, including IL-2, TGFß, and PGE2, direct the conversion from naive precursor to immature memory and finally from immature to mature memory cells, the latest being a no-return stage. Phenotypic and genetic characteristics of the subsets illustrate the structural parental maturation between subsets, which further correlates with the expression of regulatory factors. Regarding nTreg functional plasticity, both maturation stage and microenvironmental cytokines condition nTreg activities, which include blockade of autoreactive immune cells by cell-cell contact, Th17 and IL-10 Tr1-like activities, or activation of TCR-stimulating dendritic cell tolerization. Importantly, blood nTreg CD39/CD26 profile remained constant over a 2-y period in healthy persons but varied from person to person. Preliminary data on patients with autoimmune diseases or acute myelogenous leukemia illustrate the potential use of the nTreg CD39/CD26 profile as a blood biomarker to monitor chronic inflammatory diseases. Finally, we confirmed that naive conventional CD4 T cells, TCR-stimulated under a tolerogenic conditioned medium, could be ex vivo reprogrammed to FOXP3 lineage Tregs, and further found that these cells were exclusively committed to suppressive function under all microenvironmental contexts.

Mature IgM-expressing plasma cells sense antigen and develop competence for cytokine production upon antigenic challenge.

Dogma holds that plasma cells, as opposed to B cells, cannot bind antigen because they have switched from expression of membrane-bound immunoglobulins (Ig) that constitute the B-cell receptor (BCR) to production of the secreted form of immunoglobulins. Here we compare the phenotypical and functional attributes of plasma cells generated by the T-cell-dependent and T-cell-independent forms of the hapten NP. We show that the nature of the secreted Ig isotype, rather than the chemical structure of the immunizing antigen, defines two functionally distinct populations of plasma cells. Fully mature IgM-expressing plasma cells resident in the bone marrow retain expression of a functional BCR, whereas their IgG+ counterparts do not. Antigen boost modifies the gene expression profile of IgM+ plasma cells and initiates a cytokine production program, characterized by upregulation of CCL5 and IL-10. Our results demonstrate that IgM-expressing plasma cells can sense antigen and acquire competence for cytokine production upon antigenic challenge.

Follicular B Lymphomas Generate Regulatory T Cells via the ICOS/ICOSL Pathway and Are Susceptible to Treatment by Anti-ICOS/ICOSL Therapy.

The prognosis of follicular lymphoma (FL) patients is suspected to be influenced by tumor-infiltrating regulatory T cells (Treg). The mechanism of Treg enrichment in FL and their impact on malignant FL B cells remains to be elucidated. We analyzed 46 fresh lymph node biopsy samples, including FL (n = 20), diffuse large B-cell lymphoma (n = 10), classical Hodgkin lymphoma (n = 9), and reactive lymphadenitis (n = 7). Using multicolor flow cytometry and cell sorting, we observed an accumulation of CD25(high)CD127(low/neg) Tregs in FL tissues. These Tregs comprised activated ICOS(+) Tregs that were able to suppress not only conventional T cells, but also FL B cells. These FL B cells were able to express ICOSL in vitro and to generate CD25(high)FoxP3(high) Tregs expressing ICOS. Treg generation was associated with ICOS/ICOSL engagement and was abrogated by antagonist anti-ICOS and anti-ICOSL antibodies. Interactions between Tregs and FL B cells resulted in ICOSL downregulation on FL B cells. Our results highlight a key role for Tregs in FL pathogenesis and suggest that targeting the ICOS/ICOSL pathway may be a promising immunotherapy for FL treatment. Cancer Res; 76(16); 4648-60. ©2016 AACR.

Free HTLV-1 induces TLR7-dependent innate immune response and TRAIL relocalization in killer plasmacytoid dendritic cells.

A recent report demonstrated that free human T-cell leukemia virus 1 (HTLV-1) could infect plasmacytoid dendritic cells (pDCs). The major role of pDCs is to secrete massive levels of interferon-alpha (IFN-alpha) upon virus exposure; however, the induction of IFN-alpha by HTLV-1 remains unknown. We demonstrate here that cell-free HTLV-1 generated a pDC innate immune response by producing massive levels of IFN-alpha that were inhibited by anti-HTLV-1 antibodies. HTLV-1 induced costimulatory molecules and rapid expression of the apoptotic ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Furthermore, HTLV-1 stimulated pDC-induced apoptosis of CD4(+) T cells expressing DR5, transforming pDCs into IFN-producing killer pDCs. We also observed that an endosomal acidification inhibitor and a Toll-like receptor-7 (TLR7)-specific blocker drastically inhibited pDC response to HTLV-1. Three-dimensional microscopy analysis revealed that unstimulated pDCs were "dormant" IFN-producing killer pDCs with high levels of intracellular TRAIL that could be rapidly mobilized to the surface in response to TLR7 activation. Inhibition of viral degradation in endosomes by chloroquine maintained viral integrity, allowing virus detection by 3-dimensional microscopy. We demonstrate that pDCs respond to cell-free HTLV-1 by producing high levels of IFN-alpha and by mobilizing TRAIL on cell surface after TLR7 triggering. This is the first demonstration of an innate immune response induced by free HTLV-1.

Critical role for asparagine endopeptidase in endocytic Toll-like receptor signaling in dendritic cells.

Intracellular Toll-like receptor 3 (TLR3), TLR7, and TLR9 localize in endosomes and recognize single-stranded RNA and nucleotides from viruses and bacteria. This interaction induces their conformational changes resulting in the production of proinflammatory cytokines and upregulation of cell surface molecules. TLR9 requires a proteolytic cleavage for its signaling. Here, we report that myeloid and plasmacytoid dendritic cells (DCs) deficient for the asparagine endopeptidase (AEP), a cysteine lysosomal protease, showed a decrease in the secretion of proinflammatory cytokines in response to TLR9 stimulation in vitro and in vivo. Upon stimulation, full-length TLR9 was cleaved into a 72 kDa fragment and this processing was strongly reduced in DCs lacking AEP. Processed TLR9 coeluted with the adaptor molecule MyD88 and AEP after size exclusion chromatography. When expressed in AEP-deficient DCs, the 72 kDa proteolytic fragment restored TLR9 signaling. Thus, our results identify an endocytic protease playing a critical role in TLR processing and signaling in DCs.

Cancer cells regulate lymphocyte recruitment and leukocyte-endothelium interactions in the tumor-draining lymph node.

The physiologic function of the secondary lymphoid organs to recruit large numbers of naïve lymphocytes increases the probability that antigens encounter their rare, sometimes unique, specific T lymphocytes and initiate a specific immune response. In peripheral lymph nodes (LNs), this recruitment is a multistep process, initiated predominantly within the high endothelial venules (HEVs), beginning with rolling and chemokine-dependent firm adhesion of the lymphocytes on the venular endothelium surface. We report here that, in C57BL/6 mice, the recruitment of naïve lymphocytes is impaired in LNs draining a B16 melanoma tumor. Intravital microscopy analysis of the tumor-draining LNs revealed that this effect is associated with an important defect in lymphocyte adhesion in the HEVs and a progressive decrease in the expression of the LN chemokine CCL21. In parallel with these effects, the tumor up-regulated, essentially through a P-selectin-dependent mechanism, the rolling and sticking of circulating polymorphonuclear cells within the LN low-order venules where few rolling and sticking events are usually observed. These effects of the tumor were independent of the presence of metastasis into the LN and occurred as long as the tumor developed. Together, these results indicate that the tumor proximity disturbs the LN physiology by modifying the molecular, spatial, and cellular rules that usually control leukocyte-endothelium interactions into the peripheral LNs. In addition, they emphasize a new role for the low-order venules of the peripheral LNs, which compared with the HEVs, seem to be the preferential port of entry for cells linked to inflammatory processes.