Development of a DNA damage-induced senescence model in osteoarthritic chondrocytes.

Senescent cells (SnCs) have been described to accumulate in osteoarthritis (OA) joint tissues in response to injury, thereby participating in OA development and progression. However, clinical therapeutic approaches targeting SnCs using senolysis, although promising in preclinical OA models, have not yet proven their efficacy in patients with knee OA. This pitfall may be due to the lack of understanding of the mechanisms underlying chondrocyte senescence. Therefore, our study aimed to generate models of chondrocyte senescence. This study used etoposide, to induce DNA damage-related senescence or chronic exposure to IL-1ß to entail inflammation-related senescence in human OA chondrocytes. Several hallmarks of cellular senescence, such as cell cycle arrest, expression of cyclin-dependent kinase inhibitors, DNA damages, and senescence-associated secretory profile were evaluated. Chronic exposure to IL-1ß induces only partial expression of senescence markers and does not allow us to conclude on its ability to induce senescence in chondrocytes. On the other hand, etoposide treatment reliably induces DNA damage-related senescence in human articular chondrocytes evidenced by loss of proliferative capacity, DNA damage accumulation, and expression of some SASP components. Etoposide-induced senescence model may help investigate the initiation of cellular senescence in chondrocytes, and provide a useful model to develop therapeutic approaches to target senescence in OA.

The tuberculous granuloma: an unsuccessful host defence mechanism providing a safety shelter for the bacteria?

One of the main features of the immune response to M. Tuberculosis is the formation of an organized structure called granuloma. It consists mainly in the recruitment at the infectious stage of macrophages, highly differentiated cells such as multinucleated giant cells, epithelioid cells and Foamy cells, all these cells being surrounded by a rim of lymphocytes. Although in the first instance the granuloma acts to constrain the infection, some bacilli can actually survive inside these structures for a long time in a dormant state. For some reasons, which are still unclear, the bacilli will reactivate in 10% of the latently infected individuals, escape the granuloma and spread throughout the body, thus giving rise to clinical disease, and are finally disseminated throughout the environment. In this review we examine the process leading to the formation of the granulomatous structures and the different cell types that have been shown to be part of this inflammatory reaction. We also discuss the different in vivo and in vitro models available to study this fascinating immune structure.

First human model of in vitro Candida albicans persistence within granuloma for the reliable study of host-fungi interactions.

BACKGROUND: The balance between human innate immune system and Candida albicans virulence signaling mechanisms ultimately dictates the outcome of fungal invasiveness and its pathology. To better understand the pathophysiology and to identify fungal virulence-associated factors in the context of persistence in humans, complex models are indispensable. Although fungal virulence factors have been extensively studied in vitro and in vivo using different immune cell subsets and cell lines, it is unclear how C. albicans survives inside complex tissue granulomas. METHODOLOGY/PRINCIPAL FINDING: We developed an original model of in vitro human granuloma, reproducing the natural granulomatous response to C. albicans. Persistent granulomas were obtained when the ratio of phagocytes to fungi was high. This in vitro fungal granuloma mimics natural granulomas, with infected macrophages surrounded by helper and cytotoxic T lymphocytes. A small proportion of granulomas exhibited C. albicans hyphae. Histological and time-lapse analysis showed that C. albicans blastoconidia were located within the granulomas before hyphae formation. Using staining techniques, fungal load calculations, as well as confocal and scanning electron microscopy, we describe the kinetics of fungal granuloma formation. We provide the first direct evidence that C. albicans are not eliminated by immunocompetent cells inside in vitro human granulomas. In fact, after an initial candicidal period, the remaining yeast proliferate and persist under very complex immune responses. CONCLUSIONS/SIGNIFICANCE: Using an original in vitro model of human fungal granuloma, we herein present the evidence that C. albicans persist and grow into immunocompetent granulomatous structures. These results will guide us towards a better understanding of fungal invasiveness and, henceforth, will also help in the development of better strategies for its control in human physiological conditions.

Foamy macrophages and the progression of the human tuberculosis granuloma.

The progression of tuberculosis from a latent, subclinical infection to active disease that culminates in the transmission of infectious bacilli is determined locally at the level of the granuloma. This progression takes place even in the face of a robust immune response that, although it contains infection, is unable to eliminate the bacterium. The factors or environmental conditions that influence this progression remain to be determined. Recent advances have indicated that pathogen-induced dysregulation of host lipid synthesis and sequestration serves a critical role in this transition. The foamy macrophage seems to be a key participant in both sustaining persistent bacteria and contributing to the tissue pathology that leads to cavitation and the release of infectious bacilli.

Early triggering of exclusive IFN-gamma responses of human Vgamma9Vdelta2 T cells by TLR-activated myeloid and plasmacytoid dendritic cells.

gammadelta T cells, a major innate-like T cell subset, are thought to play in vivo an important role in innate and adaptive immune responses to various infection agents like parasites, bacteria, or viruses but the mechanisms contributing to this immune process remain ill defined. Owing to their ability to recognize a broad set of microbial molecular patterns, TLRs represent a major innate pathway through which pathogens induce dendritic cells (DC) maturation and acquisition of immunostimulatory functions. In this study, we studied the effects of various TLR ligands on the activation of human Vgamma9Vdelta2 T cells, a main human gammadelta PBL subset, which has been recently involved in the licensing of mycobacteria-infected DC. Both TLR3 and TLR4, but not TLR2 ligands, induced a rapid, strong, and exclusive IFN-gamma production by Vgamma9Vdelta2 T cells. This gammadelta subset contributed to a large extent to the overall PBL IFN-gamma response induced after short-term TLR stimulation of human PBMC. Importantly, this phenomenon primarily depended on type I IFN, but not IL-12, produced by monocytic DC upon TLR engagement. Vgamma9Vdelta2 T cells were similarly activated by plasmacytoid DC upon TLR8/9 activation or Yellow Fever virus infection. Moreover TLR-induced Vgamma9Vdelta2 IFN-gamma noncytolytic response led to efficient DC polarization into IL-12p70-producing cells. Our results support an adjuvant role played by Vgamma9Vdelta2 T cells along microbial infections through a particular cross-talk with pathogen-associated molecular patterns-activated DC. Moreover they provide new insights into the mechanisms underlying functional activation of this unique peripheral innate-like T cell subset during viral infections.

Bridging innate and adaptive immunity through gammadelta T-dendritic cell crosstalk.

Like Natural Killer cells, gammadelta T cells and Natural Killer T cells display several innate-like features that confer them a broad reactivity against tumors and pathogens. By recognizing stress-induced conserved antigens upregulated a wide array of physiopathological contexts, these lymphoid subsets develop strong and early responses to a broad set of targets. One of the most exciting roles possibly played in vivo by non-conventional T lymphocytes, which exhibit a biased natural memory phenotype, is active regulation of adaptive immune responses through interactions with antigen presenting cells (APCs), such as dendritic cells. Here we will review recent studies reporting functional interactions between gammadelta T cells and APC and a possible involvement of these lymphocytes in bridging innate and adaptative immunity along infections and tumor development. Our discussion will focus on human gammadelta T cells and more specifically on Vgamma9Vdelta2 T cells, a major subset found in human peripheral blood.

CD4 engagement by CD1d potentiates activation of CD4+ invariant NKT cells.

The CD4 coreceptor is crucial in the activation of major histocompatibility complex (MHC) class II restricted CD4 (+) T lymphocytes by binding the same MHC class as the T-cell receptor (TCR) and by potentiating TCR-dependent signaling. CD4 is also expressed by invariant natural killer T cells (iNKT), which recognize natural and synthetic lipid antigens, such as alpha-galactosyl ceramide (alpha-GalCer), in association with the MHC class I-like CD1d molecule. Human iNKT cells can be divided into 2 major subsets depending on CD4 expression: CD4 (+) iNKT preferentially produce T-helper (Th)0/Th2 cytokines, whereas CD4(-) iNKT cells produce Th1 cytokines after antigenic activation. Cytokines produced by iNKT may have immunomodulatory roles in various physiopathologic contexts, but their mode of regulation by iNKT cells remains ill-defined. Using blocking reagents neutralizing CD4 binding, experimental systems where MHC class II molecules are absent and recombinant alpha-GalCer/CD1d complexes, we show that CD4 potentiates human iNKT cell activation by engaging CD1d molecules. These results indicate that the CD4 coreceptors may contribute to the fine tuning of iNKT cells reactivity.