Immune Profiling Demonstrates a Common Immune Signature of Delayed Acquired Immunodeficiency in Patients With Various Etiologies of Severe Injury.

OBJECTIVES: The host response plays a central role in the pathophysiology of sepsis and severe injuries. So far, no study has comprehensively described the overtime changes of the injury-induced immune profile in a large cohort of critically ill patients with different etiologies. DESIGN: Prospective observational cohort study. SETTING: Adult ICU in a University Hospital in Lyon, France. PATIENTS: Three hundred fifty-three septic, trauma, and surgical patients and 175 healthy volunteers were included in the REAnimation Low Immune Status Marker study. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Extensive immune profiling was performed by assessing cellular phenotypes and functions, protein, and messenger RNA levels at days 1-2, 3-4, and 5-7 after inclusion using a panel of 30 standardized immune markers. Using this immunomonitoring panel, no specificity in the immune profile was observed among septic, trauma, and surgical patients. This common injury-induced immune response was characterized by an initial adaptive (i.e., physiologic) response engaging all constituents of the immune system (pro- and anti-inflammatory cytokine releases, and innate and adaptive immune responses) but not associated with increased risk of secondary infections. In contrary, the persistence in a subgroup of patients of profound immune alterations at the end of the first week after admission was associated with increased risk of secondary infections independently of exposure to invasive devices. The combined monitoring of markers of pro-/anti-inflammatory, innate, and adaptive immune responses allowed a better enrichment of patients with risk of secondary infections in the selected population. CONCLUSIONS: Using REAnimation Low Immune Status Marker immunomonitoring panel, we detected delayed injury-acquired immunodeficiency in a subgroup of severely injured patients independently of primary disease. Critically ill patients' immune status could be captured through the combined monitoring of a common panel of complementary markers of pro-/anti-inflammatory, innate, and adaptive immune responses. Such immune monitoring needs to be incorporated in larger study cohorts with more extensive immune surveillance to develop specific hypothesis allowing for identification of biological systems affecting altered immune function related to late infection in the setting of acute systemic injury.

Hallmarks of improved immunological responses in the vaccination of more physically active elderly females.

Physical inactivity is one of the leading contributors to worldwide morbidity and mortality. The elderly are particularly susceptible since the features of physical inactivity overlap with the outcomes of natural aging - including the propensity to develop cardiovascular diseases, cancer, diabetes mellitus, sarcopenia and cognitive impairment. The age-dependent loss of immune function, or immunosenescence, refers to the progressive depletion of primary immune resources and is linked to the development of many of these conditions. Immunosenescence is primarily driven by chronic immune activation and physical activity interventions have demonstrated the potential to reduce the risk of complications in the elderly by modulating inflammation and augmenting the immune system. Since poor vaccination outcome is a hallmark of immunosenescence, the assessment of vaccine efficacy provides a window to study the immunological effects of regular physical activity. Using an accelerator-based study, we demonstrate in a Singaporean Chinese cohort that elderly women (n=56) who walk more after vaccination display greater post-vaccination expansion of monocytes and plasmablasts in peripheral blood. Active elderly female participants also demonstrated lower baseline levels of IP-10 and Eotaxin, and the upregulation of genes associated with monocyte/macrophage phagocytosis. We further describe postive correlations between the monocyte response and the post-vaccination H1N1 HAI titres of participants. Finally, active elderly women reveal a higher induction of antibodies against Flu B in their 18-month second vaccination follow-up. Altogether, our data are consistent with better immunological outcomes in those who are more physically active and highlight the pertinent contribution of monocyte activity.

Clostridium difficile Toxoid Vaccine Candidate Confers Broad Protection against a Range of Prevalent Circulating Strains in a Nonclinical Setting.

Clostridium difficile infection (CDI) is a leading cause of nosocomial and antibiotic-associated diarrhea. A vaccine, based on formalin-inactivated toxins A and B purified from anaerobic cultures of C. difficile strain VPI 10463 (toxinotype 0), has been in development for the prevention of symptomatic CDI. We evaluated the breadth of protection conferred by this C. difficile toxoid vaccine in cross-neutralization assessments using sera from vaccinated hamsters against a collection of 165 clinical isolates. Hamster antisera raised against the C. difficile toxoid vaccine neutralized the cytotoxic activity of culture supernatants from several toxinotype 0 strains and heterologous strains from 10 different toxinotypes. Further assessments performed with purified toxins confirmed that vaccine-elicited antibodies can neutralize both A and B toxins from a variety of toxinotypes. In the hamster challenge model, the vaccine conferred significant cross-protection against disease symptoms and death caused by heterologous C. difficile strains from the most common phylogenetic clades, including the most prevalent toxinotypes.

A stratification strategy to predict secondary infection in critical illness-induced immune dysfunction: the REALIST score.

BACKGROUND: Although multiple individual immune parameters have been demonstrated to predict the occurrence of secondary infection after critical illness, significant questions remain with regards to the selection, timing and clinical utility of such immune monitoring tests. RESEARCH QUESTION: As a sub-study of the REALISM study, the REALIST score was developed as a pragmatic approach to help clinicians better identify and stratify patients at high risk for secondary infection, using a simple set of relatively available and technically robust biomarkers. STUDY DESIGN AND METHODS: This is a sub-study of a single-centre prospective cohort study of immune profiling in critically ill adults admitted after severe trauma, major surgery or sepsis/septic shock. For the REALIST score, five immune parameters were pre-emptively selected based on their clinical applicability and technical robustness. Predictive power of different parameters and combinations of parameters was assessed. The main outcome of interest was the occurrence of secondary infection within 30 days. RESULTS: After excluding statistically redundant and poorly predictive parameters, three parameters remained in the REALIST score: mHLA-DR, percentage of immature (CD10- CD16-) neutrophils and serum IL-10 level. In the cohort of interest (n = 189), incidence of secondary infection at day 30 increased from 8% for patients with REALIST score of 0 to 46% in patients with a score of 3 abnormal parameters, measured ad D5-7. When adjusted for a priori identified clinical risk factors for secondary infection (SOFA score and invasive mechanical ventilation at D5-7), a higher REALIST score was independently associated with increased risk of secondary infection (42 events (22.2%), adjusted HR 3.22 (1.09-9.50), p = 0.034) and mortality (10 events (5.3%), p = 0.001). INTERPRETATION: We derived and presented the REALIST score, a simple and pragmatic stratification strategy which provides clinicians with a clear assessment of the immune status of their patients. This new tool could help optimize care of these individuals and could contribute in designing future trials of immune stimulation strategies.

SWAP-70 controls formation of the splenic marginal zone through regulating T1B-cell differentiation.

T1 and T2 transitional B cells are precursors for marginal zone B cells (MZB), which surround splenic follicles. MZB are essential for marginal zone formation, are central to the innate immune response, and contribute to adaptive immunity. Differentiation, migration, and homing of MZB and their precursors remain to be fully understood. We show that SWAP-70, a RhoGTPase-interacting and F-actin-binding protein with functions in cell polarization, migration, and adhesion regulates MZB development and marginal zone formation. The percentage of MZB in spleen of Swap70(-/-) mice was reduced to about one-third of that found in WT mice. Swap70(-/-) T1 cells accumulated in integrin ligand(high) regions of the splenic red pulp and failed to efficiently develop into T2 cells. Adoptive transfer and mixed BM chimera experiments demonstrated this to be a B-cell intrinsic phenotype. T-cell-independent antibody production was not impaired, however, and thus suggests that this process does not require correct homing of MZB precursors. B-cell adhesion through α(L)ß(2) and α(4)ß(1) integrins was hyper-activated in vitro and on tissue sections, and S1P-stimulated chemokinesis of MZB was reduced in the absence of SWAP-70. Thus, SWAP-70 acts as a regulator of the adhesion process, particularly important for differentiation control of B-cell precursors and their contribution to splenic tissue formation.

Concomitant Assessment of Monocyte HLA-DR Expression and Ex Vivo TNF-α Release as Markers of Adverse Outcome after Various Injuries-Insights from the REALISM Study.

Intensive care unit (ICU) patients develop an altered host immune response after severe injuries. This response may evolve towards a state of persistent immunosuppression that is associated with adverse clinical outcomes. The expression of human leukocyte antigen DR on circulating monocytes (mHLA-DR) and ex vivo release of tumor necrosis factor α (TNF-α) by lipopolysaccharide-stimulated whole blood are two related biomarkers offered to characterize this phenomenon. The purpose of this study was to concomitantly evaluate the association between mHLA-DR and TNF-α release and adverse clinical outcome (i.e., death or secondary infection) after severe trauma, sepsis or surgery in a cohort of 353 ICU patients. mHLA-DR and TNF-α release was similarly and significantly reduced in patients whatever the type of injury. Persistent decreases in both markers at days 5-7 (post-admission) were significantly associated with adverse outcomes. Overall, mHLA-DR (measured by flow cytometry) appears to be a more robust and standardized parameter. Each marker can be used individually as a surrogate of immunosuppression, depending on center facilities. Combining these two parameters could be of interest to identify the most immunosuppressed patients presenting with a high risk of worsening. This last aspect deserves further exploration.

Monocyte Trajectories Endotypes Are Associated With Worsening in Septic Patients.

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection. The immune system plays a key role in sepsis onset and remains dysregulated over time in a heterogeneous manner. Here, we decipher the heterogeneity of the first week evolution of the monocyte HLA-DR (mHLA-DR) surface protein expression in septic patients, a key molecule for adaptive immunity onset. We found and verified four distinctive trajectories endotypes in a discovery (n = 276) and a verification cohort (n = 102). We highlight that 59% of septic patients exhibit low or decreasing mHLA-DR expression while in others mHLA-DR expression increased. This study depicts the first week behavior of mHLA-DR over time after sepsis onset and shows that initial and third day mHLA-DR expression measurements is sufficient for an early risk stratification of sepsis patients. These patients might benefit from immunomodulatory treatment to improve outcomes. Going further, our study introduces a way of deciphering heterogeneity of immune system after sepsis onset which is a first step to reach a more comprehensive landscape of sepsis.

Herpes DNAemia and TTV Viraemia in Intensive Care Unit Critically Ill Patients: A Single-Centre Prospective Longitudinal Study.

Introduction: We analysed blood DNAemia of TTV and four herpesviruses (CMV, EBV, HHV6, and HSV-1) in the REAnimation Low Immune Status Marker (REALISM) cohort of critically ill patients who had presented with either sepsis, burns, severe trauma, or major surgery. The aim was to identify common features related to virus and injury-associated pathologies and specific features linking one or several viruses to a particular pathological context. Methods: Overall and individual viral DNAemia were measured over a month using quantitative PCR assays from the 377 patients in the REALISM cohort. These patients were characterised by clinical outcomes [severity scores, mortality, Intensive Care Unit (ICU)-acquired infection (IAI)] and 48 parameters defining their host response after injury (cell populations, immune functional assays, and biomarkers). Association between viraemic event and clinical outcomes or immune markers was assessed using χ2-test or exact Fisher's test for qualitative variables and Wilcoxon test for continuous variables. Results: The cumulative incidence of viral DNAemia increased from below 4% at ICU admission to 35% for each herpesvirus during the first month. EBV, HSV1, HHV6, and CMV were detected in 18%, 12%, 10%, and 9% of patients, respectively. The incidence of high TTV viraemia (>10,000 copies/ml) increased from 11% to 15% during the same period. Herpesvirus viraemia was associated with severity at admission; CMV and HHV6 viraemia correlated with mortality during the first week and over the month. The presence of individual herpesvirus during the first month was significantly associated (p < 0.001) with the occurrence of IAI, whilst herpesvirus DNAemia coupled with high TTV viraemia during the very first week was associated with IAI. Herpesvirus viraemia was associated with a lasting exacerbated host immune response, with concurrent profound immune suppression and hyper inflammation, and delayed return to immune homeostasis. The percentage of patients presenting with herpesvirus DNAemia was significantly higher in sepsis than in all other groups. Primary infection in the hospital and high IL10 levels might favour EBV and CMV reactivation. Conclusion: In this cohort of ICU patients, phenotypic differences were observed between TTV and herpesviruses DNAemia. The higher prevalence of herpesvirus DNAemia in sepsis hints at further studies that may enable a better in vivo understanding of host determinants of herpesvirus viral reactivation. Furthermore, our data suggest that EBV and TTV may be useful as additional markers to predict clinical deterioration in ICU patients.

Endoplasmic reticulum stress response and bile acid signatures associate with multi-strain seroresponsiveness during elderly influenza vaccination.

The elderly are an important target for influenza vaccination, and the determination of factors that underlie immune responsiveness is clinically valuable. We evaluated the immune and metabolic profiles of 205 elderly Singaporeans administered with Vaxigrip. Despite high seroprotection rates, we observed heterogeneity in the response. We stratified the cohort into complete (CR) or incomplete responders (IR), where IR exhibited signs of accelerated T cell aging. We found a higher upregulation of genes associated with the B-cell endoplasmic-reticulum stress response in CR, where XBP-1 acts as a key upstream regulator. B-cells from IR were incapable of matching the level of XBP-1 upregulation observed in CR after inducing ER stress with tunicamycin in vitro. Metabolic signatures also distinguished CR and IR - as CR presented with a greater diversity of bile acids. Our findings suggest that the ER-stress pathway activation could improve influenza vaccination in the elderly.

Deciphering the domain specificity of C. difficile toxin neutralizing antibodies.

Clostridium difficile infection (CDI) is the principal cause of nosocomial diarrhea and pseudomembranous colitis associated with antibiotic therapy. The pathological effects of CDI are primarily attributed to toxins A (TcdA) and B (TcdB). Adequate toxin-specific antibody responses are associated with asymptomatic carriage, whereas insufficient humoral responses are associated with recurrent CDI. While the data supporting the importance of anti-toxin antibodies are substantial, clarity about the toxin domain specificity of these antibodies is more limited. To investigate this matter, combinations of human mAbs targeting multiple domains of TcdB were assessed using toxin neutralization assays. These data revealed that a combination of mAbs specific to all major toxin domains had improved neutralizing potency when compared to equivalent concentrations of a single mAb or a combination of mAbs against one or two domains. The function and toxin domain binding specificity of serum antibodies elicited by immunization of hamsters with a toxoid vaccine candidate was also assessed. Immunization with a toxoid vaccine candidate provoked toxin neutralizing antibodies specific to multiple domains of both TcdA and TcdB. When assessed in a toxin neutralization assay, polyclonal sera displayed greater activity against elevated concentrations of toxins than equivalent concentrations of individual mAbs. These data suggest a potential benefit of any antibody based therapeutic or prophylactic treatment that targets multiple toxin domains.

The REAnimation Low Immune Status Markers (REALISM) project: a protocol for broad characterisation and follow-up of injury-induced immunosuppression in intensive care unit (ICU) critically ill patients.

INTRODUCTION: The host response to septic shock is dynamic and complex. A sepsis-induced immunosuppression phase has recently been acknowledged and linked to bad outcomes and increased healthcare costs. Moreover, a marked suppression of the immune response has also been partially described in patients hospitalized in intensive care unit (ICU) for severe trauma or burns. It has been hypothesized that immune monitoring could enable identification of patients who might most benefit from novel, adjunctive immune-stimulating therapies. However, there is currently neither a clear definition for such injury-induced immunosuppression nor a stratification biomarker compatible with clinical constraints. METHODS AND ANALYSIS: We set up a prospective, longitudinal single-centre clinical study to determine the incidence, severity and persistency of innate and adaptive immune alterations in ICU patients. We optimized a workflow to describe and follow the immunoinflammatory status of 550 patients (septic shock, severe trauma/burn and major surgery) during the first 2 months after their initial injury. On each time point, two immune functional tests will be performed to determine whole-blood TNF-α production in response to ex vivo lipopolysaccharide stimulation and the T lymphocyte proliferation in response to phytohaemagglutinin. In addition, a complete immunophenotyping using flow cytometry including monocyte HLA-DR expression and lymphocyte subsets will be obtained. New markers (ie, levels of expression of host mRNA and viral reactivation) will be also evaluated. Reference intervals will be determined from a cohort of 150 age-matched healthy volunteers. This clinical study will provide, for the first time, data describing the immune status of severe ICU patients over time. ETHICS AND DISSEMINATION: Ethical approval has been obtained from the institutional review board (no 69HCL15_0379) and the French National Security agency for drugs and health-related products. Results will be disseminated through presentations at scientific meetings and publications in peer-reviewed journals. TRIAL REGISTRATION NUMBER: Clinicaltrials.gov Registration number: NCT02638779. Pre-results.

Control of proliferation by Bcl-2 family members.

The anti-proliferative effect of Bcl-2 acts mainly at the level of the G0/G1 phase of the cell cycle. Deletions and point mutations in the bcl-2 gene show that the anti-proliferative activity of Bcl-2, can in some cases, be dissociated from its anti-apoptotic function. This indicates that the effect of Bcl-2 on cell cycle progression can be a direct effect and not only a consequence of its anti-apoptotic activity. Bcl-2 appears to mediate its anti-proliferative effect by acting on both signal transduction pathways (NFAT, ERK) and on specific cell cycle regulators (p27, p130).

Immunosuppressive antimetabolites inhibit induction of contact hypersensitivity while lymphoablative drugs also prevent its expression.

Contact hypersensitivity is one of the most common skin diseases and its pharmacological control is an important clinical issue. We investigated the control of contact hypersensitivity by immunosuppressive drugs administered during sensitization or challenge. Mycophenolate mofetil, methotrexate and 5-fluorouracil completely inhibited contact hypersensitivity when administered during sensitization whereas they did not decrease inflammatory reaction when administered during challenge. Conversely, mitoxantrone, and cyclophosphamide, given as a single injection at the time of sensitization or challenge, completely inhibited the reaction, a property associated with T and B cell depletion. The data indicate that antimetabolites which are cell cycle dependent inhibit clonal expansion and subsequent differentiation of cytotoxic CD8+ T cells. Their lack of effect at the time of challenge indicates that T cell proliferation is not required for the expression of effector or regulatory T cell activation. Conversely lymphoablative drugs can inactivate or destroy differentiated cytotoxic T cells with rapid kinetics.

SWAP-70 deficiency causes high-affinity plasma cell generation despite impaired germinal center formation.

Germinal centers (GCs) are lymphoid tissue structures central to the generation of long-lived, high-affinity, antibody-forming B cells. However, induction, maintenance, and regulation of GCs are not sufficiently understood. The F-actin-binding, Rac-interacting protein SWAP-70 is strongly expressed in activated B cells like those in B follicles. Recent work suggests that SWAP-70 is involved in B-cell activation, migration, and homing. Therefore, we investigated the role of SWAP-70 in the T-dependent immune response, in GC formation, and in differentiation into plasma and memory B cells. Compared with wt, sheep red blood cell (SRBC)-, or NP-KLH-immunized SWAP-70(-/-) mice have strongly reduced numbers of GCs and GC-specific B cells. However, SWAP-70(-/-) NP-specific B cells accumulate outside of the B follicles, and SWAP-70(-/-) mice show more plasma cells in the red pulp and in the bone marrow, and increased NP-specific Ig and antibody-forming B cells. Yet the memory response is impaired. Thus, SWAP-70 deficiency uncouples GC formation from T-dependent antibody and long-lived plasma cell production and causes extrafollicular generation of high-affinity plasma cells, but does not adequately support the memory response.

B cell-driven lymphangiogenesis in inflamed lymph nodes enhances dendritic cell mobilization.

Dendritic cell (DC) migration from the periphery to lymph nodes is regulated by the pattern of genes expressed by DCs themselves and by signals within the surrounding peripheral environment. Here, we report that DC mobilization can also be regulated by signals initiated within the downstream lymph nodes, particularly when lymph nodes enlarge as a consequence of immunization. Lymph node B lymphocytes orchestrate expansion of the lymphatic network within the immunized lymph node. This expanded network in turn supports increased DC migration from the periphery. These results reveal unique relationships between B cells, lymphatic vessels, and migratory DCs. Knowledge that DC migration from the periphery is augmented by B cell-dependent signals reveals new potential strategies to increase DC migration during vaccination.

Caspase-independent phosphatidylserine exposure during apoptosis of primary T lymphocytes.

Exposure of phosphatidylserine (PS) on the outer leaflet of the plasma membrane is a key feature of apoptosis. As the signals underlying these phenomena are unknown, it is generally assumed that PS exposure is a consequence of caspase activation, another hallmark of apoptosis. In this study we investigated the role of caspases in PS externalization during apoptosis of activated PBL triggered by drugs (etoposide, staurosporine), CD95 engagement, or IL-2 withdrawal. Anti-CD95 mAb induces a rapid activation of caspases, followed by PS exposure and mitochondrial transmembrane potential (DeltaPsim) disruption. In contrast, etoposide (ETO), staurosporine (STS), or IL-2 withdrawal triggers concomitant caspase activation, PS exposure, and DeltaPsim disruption. Such kinetics suggest that PS exposure could be independent of caspase activation. As expected, in activated PBL treated by anti-CD95 mAb, the pan-caspase inhibitor Cbz-Val-Ala-Asp(OMe)-fluoromethylketone and the caspase-8 inhibitor Cbz-Leu-Glu-Thr-Asp(OMe)-fluoromethylketone, but not the caspase-9 inhibitor Cbz-Leu-Glu-His-Asp(OMe)-fluoromethylketone, inhibit PS externalization and DeltaPsim disruption. Surprisingly, during apoptosis induced by ETO, STS, or IL-2 withdrawal, none of those caspase inhibitors prevents PS externalization or DeltaPsim disruption, whereas they all inhibit DNA fragmentation as well as the morphological features of nuclear apoptosis. In Jurkat and H9 T cell lines, as opposed to activated PBL, PS exposure is inhibited by Cbz-Val-Ala-Asp(OMe)-fluoromethylketone during apoptosis induced by CD95 engagement, ETO, or STS. Thus, caspase-independent PS exposure occurs in primary T cells during apoptosis induced by stimuli that do not trigger death receptors.

Differential control of cell cycle, proliferation, and survival of primary T lymphocytes by purine and pyrimidine nucleotides.

Purine and pyrimidine nucleotides play critical roles in DNA and RNA synthesis as well as in membrane lipid biosynthesis and protein glycosylation. They are necessary for the development and survival of mature T lymphocytes. Activation of T lymphocytes is associated with an increase of purine and pyrimidine pools. However, the question of how purine vs pyrimidine nucleotides regulate proliferation, cell cycle, and survival of primary T lymphocytes following activation has not yet been specifically addressed. This was investigated in the present study by using well-known purine (mycophenolic acid, 6-mercaptopurine) and pyrimidine (methotrexate, 5-fluorouracil) inhibitors, which are used in neoplastic diseases or as immunosuppressive agents. The effect of these inhibitors was analyzed according to their time of addition with respect to the initiation of mitogenic activation. We showed that synthesis of both purine and pyrimidine nucleotides is required for T cell proliferation. However, purine and pyrimidine nucleotides differentially regulate the cell cycle since purines control both G(1) to S phase transition and progression through the S phase, whereas pyrimidines only control progression from early to intermediate S phase. Furthermore, inhibition of pyrimidine synthesis induces apoptosis whatever the time of inhibitor addition whereas inhibition of purine nucleotides induces apoptosis only when applied to already cycling T cells, suggesting that both purine and pyrimidine nucleotides are required for survival of cells committed into S phase. These findings reveal a hitherto unknown role of purine and pyrimidine de novo synthesis in regulating cell cycle progression and maintaining survival of activated T lymphocytes.

Restriction of de novo nucleotide biosynthesis interferes with clonal expansion and differentiation into effector and memory CD8 T cells.

Nucleotide synthesis inhibitors are currently used in neoplastic diseases or as immunosuppressive agents for the prevention of acute rejection in organ transplantation and the treatment of autoimmune disorders. We have previously described that these inhibitors interfere with proliferation and survival of primary T cells in vitro. However, the precise effects of nucleotide restriction on effector and memory functions have not been elucidated. In this study, we investigated the impact of nucleotide synthesis inhibition on CD8 T cell differentiation by using TCR transgenic mice (F5) specific for the influenza virus nucleoprotein 68 peptide presented on the H-2Db molecule. Our results show that methotrexate and 5-fluorouracil prevent the acquisition of effector functions, such as IFN-gamma, granzyme B expression, and cytotoxic function following antigenic stimulation of naive cells. Surprisingly, in the presence of mycophenolate mofetil, activated F5 cells are still able to produce granzyme B and to kill target cells but to a lesser extent compared with control. All three inhibitors interfere with the differentiation of naive cells into memory CD8 T cells. In contrast, the drugs are unable to inhibit the development of improved cytotoxic functions displayed by memory CD8 T cells.

Mycophenolic acid inhibits IL-2-dependent T cell proliferation, but not IL-2-dependent survival and sensitization to apoptosis.

Mycophenolic acid (MPA), the active metabolite of the immunosuppressive drug mycophenolate mofetil, is a selective inhibitor of inosine 5'-monophosphate dehydrogenase type II, a de novo purine nucleotide synthesis enzyme expressed in T and B lymphocytes and up-regulated upon cell activation. In this study, we report that the blockade of guanosine nucleotide synthesis by MPA inhibits mitogen-induced proliferation of PBL, an effect fully reversed by addition of guanosine and shared with mizoribine, another inhibitor of inosine 5'-monophosphate dehydrogenase. Because MPA does not inhibit early TCR-mediated activation events, such as CD25 expression and IL-2 synthesis, we investigated how it interferes with cytokine-dependent proliferation and survival. In activated lymphoblasts that are dependent on IL-2 or IL-15 for their proliferation, MPA does not impair signaling events such as of the extracellular signal-regulated kinase 2 and Stat5 phosphorylation, but inhibits down-regulation of the cyclin-dependent kinase inhibitor p27(Kip1). Therefore, in activated lymphoblasts, MPA specifically interferes with cytokine-dependent signals that control cell cycle and blocks activated T cells in the mid-G(1) phase of the cell cycle. Although it blocks IL-2-mediated proliferation, MPA does not inhibit cell survival and Bcl-x(L) up-regulation by IL-2 or other cytokines whose receptors share the common gamma-chain (CD132). Finally, MPA does not interfere with IL-2-dependent acquisition of susceptibility to CD95-mediated apoptosis and degradation of cellular FLIP. Therefore, MPA has unique functional properties not shared by other immunosuppressive drugs interfering with IL-2R signaling events such as rapamycin and CD25 mAbs.