Matthew L. Albert

Dendritic cells acquire antigen from apoptotic cells and induce class I-restricted CTLs

CD8+ cytotoxic T lymphocytes (CTLs) mediate resistance to infectious agents and tumours. Classically, CTLs recognize antigens that are localized in the cytoplasm of target cells, processed and presented as peptide complexes with class I molecules of the major histocompatibility complex (MHC). However, there is evidence for an exogenous pathway whereby antigens that are not expected to gain access to the cytoplasm are presented on MHC class I molecules. The most dramatic example is the in vivo phenomenon of cross-priming: antigens from donor cells are acquired by bone-marrow-derived host antigen-presenting cells (APCs) and presented on MHC class I molecules. Two unanswered questions concern the identity of this bone-marrow-derived cell and how such antigens are acquired. Here we show that human dendritic cells, but not macrophages, efficiently present antigen derived from apoptotic cells, stimulating class I-restricted CD8+ CTLs. Our findings suggest a mechanism by which potent APCs acquire antigens from tumours, transplants, infected cells, or even self-tissue, for stimulation or tolerization of CTLs.

Dendritic cell maturation is required for the cross-tolerization of CD8+ T cells

In vivo models have shown that tissue-restricted antigen may be captured by bone marrow–derived cells and cross-presented for the tolerization of CD8+ T cells. Although these studies have shown peripheral tolerization of CD8+ T cells, the mechanism of antigen transfer and the nature of the antigen-presenting cell (APC) remain undefined. We report here the establishment of an in vitro system for the study of cross-tolerance and show that dendritic cells (DCs) phagocytose apoptotic cells and tolerize antigen-specific CD8+ T cells when cognate CD4+ T helper cells are absent. Using this system, we directly tested the “two-signal” hypothesis for the regulation of priming versus tolerance. We found that the same CD83+ myeloid-derived DCs were required for both cross-priming and cross-tolerance. These data suggested that the current model for peripheral T cell tolerance, “signal 1 in the absence of signal 2”, requires refinement: the critical checkpoint is not DC maturation, but instead the presence of a third signal, which is active at the DC–CD4+ T cell interface.

Tumor-specific killer cells in paraneoplastic cerebellar degeneration.

Models for immune-mediated tumor regression in mice have defined an essential role for cytotoxic T lymphocytes (CTLs); however, naturally occurring tumor immunity in humans is poorly understood. Patients with paraneoplastic cerebellar degeneration (PCD) provide an opportunity to explore the mechanisms underlying tumor immunity to breast and ovarian cancer. Although tumor immunity and autoimmune neuronal degeneration in PCD correlates with a specific antibody response to the tumor and brain antigen cdr2, this humoral response has not been shown to be pathogenic. Here we present evidence for a specific cellular immune response in PCD patients. We have detected expanded populations of MHC class I-restricted cdr2-specific CTLs in the blood of 3/3 HLA-A2.1+ PCD patients, providing the first description, to our knowledge, of tumor-specific CTLs using primary human cells in a simple recall assay. Cross-presentation of apoptotic cells by dendritic cells also led to a potent CTL response. These results indicate a model whereby immature dendritic cells that engulf apoptotic tumor cells can mature and migrate to draining lymph organs where they could induce a CTL response to tissue-restricted antigens. In PCD, peripheral activation of cdr2-specific CTLs is likely to contribute to the subsequent development of the autoimmune neuronal degeneration.

A mouse model for Chikungunya: young age and inefficient type-I interferon signaling are risk factors for severe disease.

Chikungunya virus (CHIKV) is a re-emerging arbovirus responsible for a massive outbreak currently afflicting the Indian Ocean region and India. Infection from CHIKV typically induces a mild disease in humans, characterized by fever, myalgia, arthralgia, and rash. Cases of severe CHIKV infection involving the central nervous system (CNS) have recently been described in neonates as well as in adults with underlying conditions. The pathophysiology of CHIKV infection and the basis for disease severity are unknown. To address these critical issues, we have developed an animal model of CHIKV infection. We show here that whereas wild type (WT) adult mice are resistant to CHIKV infection, WT mouse neonates are susceptible and neonatal disease severity is age-dependent. Adult mice with a partially (IFN-α/βR+/−) or totally (IFN-α/βR−/−) abrogated type-I IFN pathway develop a mild or severe infection, respectively. In mice with a mild infection, after a burst of viral replication in the liver, CHIKV primarily targets muscle, joint, and skin fibroblasts, a cell and tissue tropism similar to that observed in biopsy samples of CHIKV-infected humans. In case of severe infections, CHIKV also disseminates to other tissues including the CNS, where it specifically targets the choroid plexuses and the leptomeninges. Together, these data indicate that CHIKV-associated symptoms match viral tissue and cell tropisms, and demonstrate that the fibroblast is a predominant target cell of CHIKV. These data also identify the neonatal phase and inefficient type-I IFN signaling as risk factors for severe CHIKV-associated disease. The development of a permissive small animal model will expedite the testing of future vaccines and therapeutic candidates.

Biology and pathogenesis of chikungunya virus.

Chikungunya virus (CHIKV) is a re-emerging mosquito-borne alphavirus responsible for a recent, unexpectedly severe epidemic in countries of the Indian Ocean region. Although many alphaviruses have been well studied, little was known about the biology and pathogenesis of CHIKV at the time of the 2005 outbreak. Over the past 5 years there has been a multidisciplinary effort aimed at deciphering the clinical, physiopathological, immunological and virological features of CHIKV infection. This Review highlights some of the most recent advances in our understanding of the biology of CHIKV and its interactions with the host.

αvβ5 integrin recruits the CrkII-Dock180-rac1 complex for phagocytosis of apoptotic cells

Integrin receptors are important for the phagocytosis of apoptotic cells. However, little is known about their function in mediating internalization, as previous studies used blocking antibodies for the inhibition of binding. Here we show that the αvβ5 receptor mediates both binding and internalization of apoptotic cells. Internalization is dependent upon signalling through the β5 cytoplasmic tail, and engagement of the αvβ5 heterodimer results in recruitment of the p130cas–CrkII–Dock180 molecular complex, which in turn triggers Rac1 activation and phagosome formation. In addition to defining integrin-receptor signalling as critical for the internalization of apoptotic material, our results also constitute the first evidence in human cells that the CED-2–CED-5–CED-10 complex defined in Caenorhabditis elegans is functionally analagous to the CrkII–Dock180–Rac1 molecular complex in mammalian cells. By linking the αvβ5 receptor to this molecular switch, we reveal an evolutionarily conserved signalling pathway that is responsible for the recognition and internalization of apoptotic cells by both professional and non-professional phagocytes.

Characterization of Reemerging Chikungunya Virus

An unprecedented epidemic of chikungunya virus (CHIKV) infection recently started in countries of the Indian Ocean area, causing an acute and painful syndrome with strong fever, asthenia, skin rash, polyarthritis, and lethal cases of encephalitis. The basis for chikungunya disease and the tropism of CHIKV remain unknown. Here, we describe the replication characteristics of recent clinical CHIKV strains. Human epithelial and endothelial cells, primary fibroblasts and, to a lesser extent, monocyte-derived macrophages, were susceptible to infection and allowed viral production. In contrast, CHIKV did not replicate in lymphoid and monocytoid cell lines, primary lymphocytes and monocytes, or monocyte-derived dendritic cells. CHIKV replication was cytopathic and associated with an induction of apoptosis in infected cells. Chloroquine, bafilomycin-A1, and short hairpin RNAs against dynamin-2 inhibited viral production, indicating that viral entry occurs through pH-dependent endocytosis. CHIKV was highly sensitive to the antiviral activity of type I and II interferons. These results provide a general insight into the interaction between CHIKV and its mammalian host.

Death-defying immunity: do apoptotic cells influence antigen processing and presentation?

The clearance of apoptotic cells has been paid much attention for its role not only in tissue homeostasis, but also as a source of antigen for immune tolerance and activation. The complexity of this process has been borne out by the many receptor families and signalling pathways involved; however, an important aspect of the biology has so far been overlooked. This article explores the possible immunological instructions that are delivered by dying cells, as influenced by the specific execution pathways that are active during programmed cell death.

Detection and treatment of activated T cells in the cerebrospinal fluid of patients with paraneoplastic cerebellar degeneration

Patients with paraneoplastic cerebellar degeneration (PCD) offer the opportunity to explore the mechanisms underlying tumor immunity and immune‐mediated neuronal degeneration. Cytotoxic T lymphocytes (CTLs) specific for the PCD onconeural antigen cdr2 found in the blood of patients with PCD are likely to be effectors of PCD tumor immunity. Here, we suggest a role for CTLs in the autoimmune destruction of Purkinje neurons. More than 75% of the cells obtained from the cerebrospinal fluid (CSF) of PCD patients were CD3+ αβ T cells. In patients with active/progressive disease, 20% to 40% of CSF cells were activated T cells, and the CD4+ helper cells were Th1‐type cells. Three PCD patients were given tacrolimus, a specific inhibitor of activated T cells, which markedly reduced these cells in the CSF. Tacrolimus also reduced the number of activated cdr2‐specific CTLs in the peripheral blood, but did not lead to tumor recurrence. We suggest that activated cdr2‐specific CTLs in the CSF contribute to Purkinje degeneration in PCD, and that tacrolimus therapy may benefit patients with paraneoplastic neurological disease and other T cell–mediated autoimmune neurological disorders. Ann Neurol 2000; 47: 9–17

Type I IFN controls chikungunya virus via its action on nonhematopoietic cells.

Chikungunya virus (CHIKV) is the causative agent of an outbreak that began in La Réunion in 2005 and remains a major public health concern in India, Southeast Asia, and southern Europe. CHIKV is transmitted to humans by mosquitoes and the associated disease is characterized by fever, myalgia, arthralgia, and rash. As viral load in infected patients declines before the appearance of neutralizing antibodies, we studied the role of type I interferon (IFN) in CHIKV pathogenesis. Based on human studies and mouse experimentation, we show that CHIKV does not directly stimulate type I IFN production in immune cells. Instead, infected nonhematopoietic cells sense viral RNA in a Cardif-dependent manner and participate in the control of infection through their production of type I IFNs. Although the Cardif signaling pathway contributes to the immune response, we also find evidence for a MyD88-dependent sensor that is critical for preventing viral dissemination. Moreover, we demonstrate that IFN-α/β receptor (IFNAR) expression is required in the periphery but not on immune cells, as IFNAR−/−→WT bone marrow chimeras are capable of clearing the infection, whereas WT→IFNAR−/− chimeras succumb. This study defines an essential role for type I IFN, produced via cooperation between multiple host sensors and acting directly on nonhematopoietic cells, in the control of CHIKV.