IntroductionT helper (Th) cells may differentiate into distinct functional subsets according to the circumstances in which naive precursors encounter the nominal antigen for which they are specific. In the presence of interleukin-12 (IL-12), Th precursor cells become Th1 and high producers of interferon-␥ (IFN-␥), whereas in the presence of IL-4 they become Th2 and high producers of IL-4 and IL-5. These subsets have fundamentally different effector functions, in particular for their capacity to induce monocytes activation. 1 Recently, a distinct Th subset has been characterized in mice 2-6 and humans [7][8][9][10][11] for its high production of IL-17, the use of retinoid-related orphan receptor ␥t (ROR␥t) as master transcription factor, 12 its role in protection against extracellular bacteria and fungi, and in the pathogenesis of several autoimmune conditions, such as collageninduced arthritis, experimental autoimmune encephalomyelitis, and an animal model of inflammatory bowel disease. 4,13,14 Of interest, the generation of Th17 from naive precursors may follow different requirements in mice and humans. In mice, commitment to the Th17 lineage is dependent on transforming growth factor-␤ (TGF-␤), IL-1␤, and IL-6, whereas in humans IL-1␤ and IL-6 but not TGF-␤ appear to be required. 10,[15][16][17][18] The role of IL-23 in Th17 differentiation and effector function is still debated. IL-23 is a member of the IL-6 family of cytokines that shares with IL-12 the p40 subunit and has a unique p19 subunit. 19 IL-23 promotes Th17 responses in vivo but may be more important for the survival and population expansion of Th17 cells than for Th17 lineage commitment. 3,5,16 However, in conjunction with IL-1, IL-23 is sufficient for inducing naive human T cells to produce IL-17A, IL-17F, IL-22, IL-26, IFN-␥, C-C chemokine ligand 20 (CCL20)/ macrophage inflammatory protein 3␣ (MIP-3␣), and the transcription factor ROR␥t. 10 Moreover, mice lacking IL-23 are fully resistant to experimental autoimmune encephalomyelitis, collageninduced arthritis, and inflammatory bowel disease.The pattern of chemokine receptors expressed varies according to the differentiation stage and effector function of T cells. 20 The preferential expression of C-C chemokine receptor 6 (CCR6) distinguishes human Th17 from other Th subsets. 7,8,21,22 It should be stressed, however, that expression of CXC chemokine receptor 3 (CXCR3) within CCR6 ϩ T cells identifies T cells with preferential IFN-␥ production in response to purified protein derivative, whereas IL-17 production in response to Candida albicans hyphae is restricted to CCR6 ϩ CCR4 ϩ cells. 7 IL-17, when overexpressed in murine knee joint, causes inflammation and bone erosion. 23 Likewise, erosion observed in streptococcal cell wall-induced arthritis is highly reduced in IL-17R Ϫ/Ϫ and is associated with impaired expression of matrix metalloproteinases (MMPs). 24 T-cell clones producing IL-17 were generated from the synovium and synovial fluid of rheumatoid arthritis patients, 25 and IL-17 was shown t...

Natural killer (NK) cells play a crucial role in the immune response to micro-organisms and tumours. Recent evidence suggests that NK cells also regulate the adaptive T-cell response and that it might be possible to exploit this ability to eliminate autoreactive T cells in autoimmune disease and alloreactive T cells in transplantation. Mature NK cells consist of a highly diverse population of cells that expresses different receptors to facilitate recognition of diseased cells and possibly pathogens themselves. Ex vivo culture of NK cells with cytokines such as IL-2 and IL-15 is an approach that permits significant expansion of the NK cell subpopulations, which are likely to have potent antitumour, antiviral, or immunomodulatory effects in autoimmunity. Our data indicate that the addition of IL-21 has a synergistic effect by increasing the numbers of NK cells on a large scale. IL-2 and IL-15 may induce the expression of killer cell immunoglobulin-like receptors (KIRs) in KIRnegative populations, the c-lectin receptor NKG2D and the natural cytotoxic receptor NKp44. The addition of IL-21 to IL-15 or IL-2 can modify the pattern of the KIR receptors and inhibit NKp44 expression by reducing the expression of the adaptor DAP-12. IL-21 also preserved the production of interferon-γ and enhanced the cytotoxic properties of NK cells. Our findings indicate that the proinflammatory cytokines IL-2, IL-15 and IL-21 can modify the peripheral repertoire of NK cells. These properties may be used to endow subpopulations of NK cells with specific phenotypes, which may be used in ex vivo cellular immunotherapy strategies.

Cytomegalovirus (CMV) infection is the most common viral complication after solid organ transplantation (SOT). Whilst current immunosuppression is knownto impair antiviral-specific T-cell immunity in SOT, a potential role for natural killer (NK) cells not affected by immunosuppressive therapy remains to be determined. To address this, we compared the genotype of the NK immunoglobulin-like receptor (KIR) genes and their HLA cognate ligands to the rate of CMV infection in 196 kidney transplant recipients. We have shown that the absence of the HLA-C ligand for inhibitory KIR and the presence of activating KIR genes in the recipients were both associated with a lower rate of CMV infection after transplantation. In a cohort of 17 recipients with acute CMV infection, NK cells were phenotyped over a period of time after diagnosis by their expression profile of C-type lectin receptors and capacity to secrete IFN-c . The increased expression of the activating C-type lectin receptors NKG2C and NKG2D was paralleled by the decreased IFN-c secretion during the early phase of CMV infection. In conclusion, our findings suggest that KIR/HLA genotype and expression of NKG2C and NKG2D might play a significant role in regulating NK cell function and anti-CMV immunity after kidney transplantation.

Neural progenitor cells (NPC) of foetal origin or derived from human embryonic stem cells (HESC) have the potential to differentiate into mature neurons after transplantation into the central nervous system, opening the possibility of cell therapy for neurodegenerative disorders. In most cases, the transplanted NPC are genetically unrelated to the recipient, leading to potential rejection of the transplanted cells. Very few data provide reliable information as to the potential immune response of allogeneic neural progenitors derived from HESC. In this study, we analyzed in vitro the allogeneic immune response of T lymphocytes and natural killer (NK) cells to NPC derived from HESC or of foetal origin. We demonstrate that NPC induce T-cell stimulation and a strong NK cytotoxic response. NK-cell activity is unrelated to MHC-I expression but driven by the activating NKG2D receptor. Cyclosporine and dexamethasone previously used in clinical studies with foetal NPC did not only fail to prevent NK alloreactivity but strongly inhibited the terminal maturation from NPC into mature neurons. We conclude that allogenic transplantation of NPC in the central nervous system will most likely require an immunosuppressive regimen targeting allogenic T and NK cells, whereas possible interference with the differentiation of NPC needs to be carefully evaluated.

Great hopes have been placed on human pluripotent stem (hPS) cells for therapy. Tissues or organs derived from hPS cells could be the best solution to cure many different human diseases, especially those who do not respond to standard medication or drugs, such as neurodegenerative diseases, heart failure, or diabetes. The origin of hPS is critical and the idea of creating a bank of well-characterized hPS cells has emerged, like the one that already exists for cord blood. However, the main obstacle in transplantation is the rejection of tissues or organ by the receiver, due to the three main immunological barriers: the human leukocyte antigen (HLA), the ABO blood group, and minor antigens. The problem could be circumvented by using autologous stem cells, like induced pluripotent stem (iPS) cells, derived directly from the patient. But iPS cells have limitations, especially regarding the disease of the recipient and possible difficulties to handle or prepare autologous iPS cells. Finally, reaching standards of good clinical or manufacturing practices could be challenging. That is why well-characterized and universal hPS cells could be a better solution. In this review, we will discuss the interest and the feasibility to establish hPS cells bank, as well as some economics and ethical issues.