Debora Franceschini

An emerging player in the adaptive immune response: MicroRNA-146a is a modulator of IL-2 expression and activation-induced cell death in T lymphocytes

Activation of the T cell-mediated immune response has been associated with changes in the expression of specific microRNAs (miRNAs). However, the role of miRNAs in the development of an effective immune response is just beginning to be explored. This study focuses on the functional role of miR-146a in T lymphocyte-mediated immune response and provides interesting clues on the transcriptional regulation of miR-146a during T-cell activation. We show that miR-146a is low in human naive T cells and is abundantly expressed in human memory T cells; consistently, miR-146a is induced in human primary T lymphocytes upon T-cell receptor (TCR) stimulation. Moreover, we identified NF-kB and c-ETS binding sites as required for the induction of miR-146a transcription upon TCR engagement. Our results demonstrate that several signaling pathways, other than inflammation, are influenced by miR-146a. In particular, we provide experimental evidence that miR-146a modulates activation-induced cell death (AICD), acting as an antiapoptotic factor, and that Fas-associated death domain (FADD) is a target of miR-146a. Furthermore, miR-146a enforced expression impairs both activator protein 1 (AP-1) activity and interleukin-2 (IL-2) production induced by TCR engagement, thus suggesting a role of this miRNA in the modulation of adaptive immunity.

PD-L1 negatively regulates CD4+CD25+Foxp3+ Tregs by limiting STAT-5 phosphorylation in patients chronically infected with HCV

CD4+CD25+Foxp3+ Tregs suppress autoimmune responses. In addition, they limit T cell responses during chronic infection, thereby minimizing T cell-dependent immunopathology. We sought to investigate how Tregs are regulated in the livers of patients chronically infected with HCV, where they control the balance between an adequate protective immune response and suppression of immunopathology. We found that, despite accumulating and proliferating at sites of infection in the livers of patients chronically infected with HCV, Tregs were relatively less expanded than CD4+CD25+Foxp3- effector T cells. The relative lower expansion of intrahepatic Tregs coincided with their upregulation of programmed death-1 (PD-1). PD-1 expression inversely correlated with both Treg proliferation and clinical markers of immune suppression in vivo. Consistent with the possibility that PD-1 controls Tregs, blockade of the interaction between PD-1 and programmed death-1 ligand 1 (PD-L1) enhanced the in vitro expansion and function of Tregs isolated from the livers of patients chronically infected with HCV. Blockade of the interaction between PD-L1 and B7.1 also improved the proliferation of these cells. Interestingly, both PD-1 and phosphorylated STAT-5 were overexpressed in intrahepatic Tregs in a parallel fashion in steady disease conditions, and in an alternate-fluctuating fashion during the course of severe hepatitis reactivation. Notably, PD-L1 blockade upregulated STAT-5 phosphorylation in Tregs ex vivo. These data suggest that PD-L1 negatively regulates Tregs at sites of chronic inflammation by controlling STAT-5 phosphorylation.

miR-223 is overexpressed in T-lymphocytes of patients affected by rheumatoid arthritis

miRNAs have recently emerged as key regulators of the immune system, being involved in lymphocyte selection and proliferation, in T(reg) cells differentiation, and in hematopoiesis in general. Rheumatoid arthritis (RA) is an autoimmune pathology the etiology of which is still obscure. Although a multifactorial pathogenesis has been hypothesized, the precise mechanisms leading to the disease are still poorly understood at the molecular level. miRNA expression profile analysis highlighted that miR-223 is the only miRNA that is strikingly deregulated in peripheral T-lymphocytes from RA patients compared with healthy donors. Further analysis by quantitative reverse transcription-polymerase chain analysis confirmed that miR-223 is overexpressed in T-lymphocytes from RA patients (n = 28) compared with healthy donors (n = 10). Moreover, purification of different T-lymphocyte populations from RA patients highlights that miR-223 is expressed at higher levels in naive CD4(+) lymphocytes, whereas its expression is barely detectable in T(h)-17 cells. In summary, our data provide a first characterization of the miRNA expression profiles of peripheral T-lymphocytes of RA patients, identifying miR-223 as overexpressed in CD4(+) naive T-lymphocytes from these individuals. A deeper analysis of the biologic functions and effects of the expression of miR-223 in T-lymphocytes is needed to clarify the exact link between our observation and the disease.

IFN‐α‐conditioned dendritic cells are highly efficient in inducing cross‐priming CD8+ T cells against exogenous viral antigens

Dendritic cells (DC) generated after a short‐term exposure of monocytes to IFN‐α and GM‐CSF (IFN‐DC) are highly effective in inducing cross‐priming of CD8+ T cells against viral antigens. We have investigated the mechanisms responsible for the special attitude of these DC and compared their activity with that of reference DC. Antigen uptake and endosomal processing capabilities were similar for IFN‐DC and IL‐4‐derived DC. Both DC types efficiently cross‐presented soluble HCV NS3 protein to the specific CD8+ T cell clone, even though IFN‐DC were superior in cross‐presenting low amounts of viral antigens. Moreover, when DC were pulsed with inactivated HIV‐1 and injected into hu‐PBL‐SCID mice, the generation of virus‐specific CD8+ T cells was markedly higher in animals immunized with IFN‐DC than in mice immunized with CD40L‐matured IL‐4‐DC. Of interest, in experiments with purified CD8+ T cells, IFN‐DC were superior with respect to CD40L‐matured IL‐4‐DC in inducing in vitro cross‐priming of HIV‐specific CD8+ T cells. This property correlated with enhanced potential to express the specific subunits of the IL‐23 and IL‐27 cytokines. These results suggest that IFN‐DC are directly licensed for an efficient CD8+ T cell priming by mechanisms likely involving enhanced antigen presentation and special attitude to produce IL‐12 family cytokines.

Cross-presentation of caspase-cleaved apoptotic self antigens in HIV infection

We found that the proteome of apoptotic T cells includes prominent fragments of cellular proteins generated by caspases and that a high proportion of distinct T cell epitopes in these fragments is recognized by CD8+ T cells during HIV infection. The frequencies of effector CD8+ T cells that are specific for apoptosis-dependent epitopes correlate with the frequency of circulating apoptotic CD4+ T cells in HIV-1–infected individuals. We propose that these self-reactive effector CD8+ T cells may contribute to the systemic immune activation during chronic HIV infection. The caspase-dependent cleavage of proteins associated with apoptotic cells has a key role in the induction of self-reactive CD8+ T cell responses, as the caspase-cleaved fragments are efficiently targeted to the processing machinery and are cross-presented by dendritic cells. These findings demonstrate a previously undescribed role for caspases in immunopathology.

Increased CD8+ T cell responses to apoptotic T cell-associated antigens in multiple sclerosis

BackgroundHere, we evaluated the hypothesis that CD8+ T cell responses to caspase-cleaved antigens derived from effector T cells undergoing apoptosis, may contribute to multiple sclerosis (MS) immunopathology.MethodsThe percentage of autoreactive CD8+ T effector cells specific for various apoptotic T cell-associated self-epitopes (apoptotic epitopes) were detected in the peripheral blood and cerebrospinal fluid (CSF) by both enzyme-linked immunospot and dextramers of class I molecules complexed with relevant apoptotic epitopes. Moreover, the capacity of dextramer+ CD8+ T cells to produce interferon (IFN)-γ and/or interleukin (IL)-17 in response to the relevant apoptotic epitopes was evaluated by the intracellular cytokine staining. Cross-presentation assay of apoptotic T cells by dendritic cells was also evaluated ex vivo.ResultsWe found that polyfunctional (IFN-γ and/or IL-17 producing) autoreactive CD8+ T cells specific for apoptotic epitopes were represented in MS patients with frequencies significantly higher than in healthy donors. These autoreactive CD8+ T cells with a strong potential to produce IFN-γ or IL-17 in response to the relevant apoptotic epitopes were significantly accumulated in the CSF from the same patients. In addition, the frequencies of these autoreactive CD8+ T cells correlated with the disease disability. Cross-presentation assay revealed that caspase-cleaved cellular proteins are required to activate apoptotic epitope-specific CD8+ T cells ex vivo.ConclusionTaken together, these data indicate that apoptotic epitope-specific CD8+ T cells with strong inflammatory potential are recruited at the level of the inflammatory site, where they may be involved in MS immunopathology through the production of high levels of inflammatory cytokines.

Polyfunctional Type-1, -2, and -17 CD8+ T Cell Responses to Apoptotic Self-Antigens Correlate with the Chronic Evolution of Hepatitis C Virus Infection

Caspase-dependent cleavage of antigens associated with apoptotic cells plays a prominent role in the generation of CD8+ T cell responses in various infectious diseases. We found that the emergence of a large population of autoreactive CD8+ T effector cells specific for apoptotic T cell-associated self-epitopes exceeds the antiviral responses in patients with acute hepatitis C virus infection. Importantly, they endow mixed polyfunctional type-1, type-2 and type-17 responses and correlate with the chronic progression of infection. This evolution is related to the selection of autoreactive CD8+ T cells with higher T cell receptor avidity, whereas those with lower avidity undergo prompt contraction in patients who clear infection. These findings demonstrate a previously undescribed strict link between the emergence of high frequencies of mixed autoreactive CD8+ T cells producing a broad array of cytokines (IFN-γ, IL-17, IL-4, IL-2…) and the progression toward chronic disease in a human model of acute infection.