Aksoy E, Taboubi S, Torres D, Delbauve S, Hachani A, Whitehead MA, Pearce WP, Berenjeno-Martin I, Nock G, Filloux A, Beyaert R, Flamand V, Vanhaesebroeck B. The p110δ isoform of the kinase PI(3)K controls the subcellular compartmentalization of TLR4 signaling and protects from endotoxic shock. Nat Immunol 2012; 13: 1045-1054.

Lipopolysaccharide activates plasma-membrane signaling and endosomal signaling by Toll-like receptor 4 (TLR4) through the TIRAP-MyD88 and TRAM-TRIF adaptor complexes, respectively, but it is unclear how the signaling switch between these cell compartments is coordinated. In dendritic cells, we found that the p110δ isoform of phosphatidylinositol-3-OH kinase (PI(3)K) induced internalization of TLR4 and dissociation of TIRAP from the plasma membrane, followed by calpain-mediated degradation of TIRAP. Accordingly, inactivation of p110δ prolonged TIRAP-mediated signaling from the plasma membrane, which augmented proinflammatory cytokine production while decreasing TRAM-dependent endosomal signaling that generated anti-inflammatory cytokines (interleukin 10 and interferon-β). In line with that altered signaling output, p110δ-deficient mice showed enhanced endotoxin-induced death. Thus, by controlling the 'topology' of TLR4 signaling complexes, p110δ balances overall homeostasis in the TLR4 pathway.

Antoine P, Olislagers V, Huygens A, Lecomte S, Liesnard C, Donner C, Marchant A. Functional exhaustion of CD4+ T lymphocytes during primary cytomegalovirus infection. J Immunol 2012; 189: 2665-2672.

Human CMV establishes lifelong persistence after primary infection. Chronic CMV infection is associated with intermittent viral reactivation inducing high frequencies of CD4⁺ T lymphocytes with potent antiviral and helper properties. Primary CMV infection is characterized by an intense viral replication lasting for several months. The impact of this prolonged exposure to high Ag loads on the functionality of CD4⁺ T cells remains incompletely understood. In pregnant women with primary CMV infection, we observed that CMV-specific CD4⁺ T lymphocytes had a decreased capacity to proliferate and to produce IL-2. A very large proportion of CMV-specific CD4⁺ T cells had downregulated the expression of CD28, a costimulatory molecule centrally involved in the production of IL-2. Unexpectedly, both CD28⁻ and CD28⁺CD4⁺ T cells produced low levels of IL-2. This defective production of IL-2 was part of a larger downregulation of cytokine production. Indeed, CMV-specific CD4⁺ T cells produced lower amounts of IFN-γ and TNF-α and showed lower functional avidity during primary as compared with chronic infection. Increased programmed death-1 expression was observed in CD28⁺ CMV-specific CD4⁺ T cells, and programmed death-1 inhibition increased proliferative responses. These results indicate that primary CMV infection is associated with the exhaustion of CMV-specific CD4⁺ T cells displaying low functional avidity for viral Ags.

Charbonnier LM, Vokaer B, Lemaître PH, Field KA, Leo O, Le Moine A. CTLA4-Ig restores rejection of MHC class-II mismatched allografts by disabling IL-2-expanded regulatory T cells. Am J Transplant 2012; 12: 2313-2321

Allograft acceptance and tolerance can be achieved by different approaches including inhibition of effector T cell responses through CD28-dependent costimulatory blockade and induction of peripheral regulatory T cells (Tregs). The observation that Tregs rely upon CD28-dependent signals for development and peripheral expansion, raises the intriguing possibility of a counterproductive consequence of CTLA4-Ig administration on tolerance induction. We have investigated the possible negative effect of CTLA4-Ig on Treg-mediated tolerance induction using a mouse model of single MHC class II-mismatched skin grafts in which long-term acceptance was achieved by short-term administration of IL-2/anti-IL-2 complex. CTLA4-Ig treatment was found to abolish Treg-dependent acceptance in this model, restoring skin allograft rejection and Th1 alloreactivity. CTLA4-Ig inhibited IL-2-driven Treg expansion, and prevented in particular the occurrence of ICOS⁺ Tregs endowed with potent suppressive capacities. Restoring CD28 signaling was sufficient to counteract the deleterious effect of CTLA4-Ig on Treg expansion and functionality, in keeping with the hypothesis that costimulatory blockade inhibits Treg expansion and function by limiting the delivery of essential CD28-dependent signals. Inhibition of regulatory T cell function should therefore be taken into account when designing tolerance protocols based on costimulatory blockade.

Welsby I, Hutin D, Leo O. Complex roles of members of the ADP-ribosyl transferase super family in immune defences: Looking beyond PARP1. Biochem Pharmacol 2012; 84: 11-20.

ADP ribosylation has been recently recognised as an important posttranslational modification regulating numerous cellular processes. This enzymatic activity is shared by two major families of enzymes, the extracellular ADP-ribosyl-transferases, or ecto-ARTS and the poly-ADP-ribosyltranferases, whose denomination derives from the capacity of its founding member, PARP1, to synthesise large linear or branched polymers of ADP-ribose on target proteins. This latter post-translational modification has recently attracted much interest based on its role in the cellular response to genotoxic and oxidative stress. Accordingly, a series of PARP-specific pharmacological inhibitors have demonstrated cell survival and anti-inflammatory properties in vivo, promoting a renewed interest in the potential immunoregulatory role of this gene family. More recently, the role of ADP-ribosylation in regulating several aspects of intracellular signalling and gene transcription has been uncovered, in particular within cells of the immune system, revealing the potential immunomodulatory role of several members of this family in addition to PARP1. We review herein the experimental evidence illustrating the complex role played by this gene family in regulating multiple aspects of the immune response, including cell survival, cytokine gene transcription and antiviral innate defences. In particular, the unexpected potential anti-inflammatory role of members of this family (including in particular PARP5a, 5b and PARP14) will be briefly discussed, raising some concern on the use of pan-specific PARP inhibitors to treat chronic inflammatory diseases.

Debock I, Delbauve S, Dubois A, Pétein M, Leo O, Goldman M, Flamand V. Th17 Alloimmunity Prevents Neonatal Establishment of Lymphoid Chimerism in IL-4-Deprived Mice. Am J Transplant 2012; 12: 81-89.

Immune responses in newborn mice are known to be biased toward the helper type 2 phenotype. This may account for their propensity to develop tolerance. Herein, we evaluated the effects of IL-4 deprivation on CD4(+) T-cell activities elicited by neonatal exposure to allogeneic spleen cells. We showed that chimerism, Th2-type polarization and pathology, as well as skin allograft acceptance were inhibited in BALB/c mice immunized at birth with (A/J x BALB/c) F(1) spleen cells upon in vivo IL-4 neutralization. While IL-4 neutralization inhibited the development of Th2 cells in this model, it led to the accumulation of IL-17A, IL-17F, IL-22, IL-6 and RORγt mRNA in the spleen or graft tissues. Moreover, IL-4 deprivation led to the differentiation of donor-specific Th17 cells with a concomitant Th1 response characterized by IFN-γ production. The Th17-type response emerging in IL-4-deprived mice was found to mediate both intragraft neutrophil infiltration and the abrogation of B-cell chimerism. Neutralization of this Th17 response failed however to restore functional skin graft acceptance. Collectively, our observations indicate that the neonatal Th2 response opposes the development of Th17 cells, and that Th17 cells are responsible for controlling lymphoid chimerism in mice neonatally injected with semiallogeneic cells.

Charbonnier LM, Le Moine A. Rapamycin as immunosuppressant in murine transplantation model. Methods Mol Biol 2012; 821: 435-445.

The potent immunosuppressive action of rapamycin has been described in many different mouse models of transplantation. In these models, rapamycin prevent or delay allograft rejection. In several models, rapamycin allowed mixed donor-recipient hematopoietic chimerism to develop facilitating tolerance induction. In our own experience, we observed that rapamycin synergized with CD8(+) T cell depletion and coreceptor/costimulation blockade to induce long-term survival of Balb/C to C57Bl/6 heterotopic limb allograft. Herein, we describe immunosuppression cocktails containing rapamycin and methods to evaluate several read outs associated with tolerance induction such as mixed donor-recipient hematopoietic chimerism and in vitro or in vivo recipient alloreactivity.

Wilmar A, Lonez C, Vermeersch M, Andrianne M, Pérez-Morga D, Ruysschaert JM, Vandenbranden M, Leo O, Temmerman S. The cationic lipid, diC14 amidine, extends the adjuvant properties of aluminum salts through a TLR-4- and caspase-1-independent mechanism. Vaccine 2012; 30 :414-424.

Adjuvant efficiency is critical for inducing a protective and long-lasting immune response against weak immunogenic antigens. Discovered more than 70 years ago, aluminum salts remain the most widely used adjuvant in human vaccine. Prone to induce a strong humoral response, alum fails to drive a cell-mediated immunity, which is essential to fight against intracellular pathogens. Adjuvant systems that contain more than one component may represent an excellent alternative for completing the lack of T cell immunity associated with the injection of alum-based vaccine. In this work, we demonstrated that the adjuvant effects of alum strongly benefited from combining with a cationic lipid, the diC14 amidine. Indeed, we measured a significant improvement of alum-driven IL-1β release when human macrophages were co-cultured with a mixed suspension of alum and the diC14 amidine. Morphological analysis suggested that diC14 amidine improved the alum uptake by phagocytes. Furthermore, the addition of diC14 amidine to alum efficiently enhanced antigen processing and cross-presentation by antigen presenting cells. The biological relevance of these in vitro data was assessed by measuring the in vivo development of a cytotoxic activity and the enhanced synthesis of antigen-specific immunoglobulins after immunization with alum combined to diC14 amidine. Mechanistically, we demonstrated that diC14 amidine supported the alum adjuvanticity independently of the TLR-4 and caspase-1 agonist activities of the cationic lipid. Based on our findings, we conclude that diC14 amidine works synergistically with alum to achieve higher immune protection after vaccination.