Human monoclonal IgG selection of Plasmodium falciparum for the expression of placental malaria-specific variant surface antigens.

Pregnancy-associated Plasmodium falciparum malaria (PAM) is a major cause of morbidity and mortality in African women and their offspring. PAM is characterized by accumulation of infected erythrocytes (IEs) that adhere to chondroitin sulphate A (CSA) in the placental intervillous space. We show here that human monoclonal IgG antibodies with specificity for variant surface antigens (VSA) specifically expressed by CSA-adhering IEs (VSAPAM) can be used in vitro to select parasites from nonpregnant donors to express VSAPAM and that this selection for VSAPAM expression results in preferential transcription of var2csa. The results corroborate current efforts to develop PAM-specific vaccines based on VAR2CSA.

Distinct expression pattern of IFN-alpha and TNF-alpha in juvenile idiopathic arthritis synovial tissue.

OBJECTIVES: Recent laboratory and clinical data suggest that two prototype autoimmune diseases, systemic lupus erythematosus and rheumatoid arthritis are mainly driven by distinct cytokines, interferon (IFN)-alpha and tumour necrosis factor (TNF)-alpha, respectively. We here investigated the presence and characteristics of natural type I IFN-producing cells (IPCs), as well as IFN-alpha and TNF-alpha expression at sites of inflammation in juvenile idiopathic arthritis (JIA). METHODS: Peripheral blood (PB) and synovial fluid (SF) mononuclear cells (MNCs) (n = 25 each) from JIA patients with active disease were studied. IPCs were identified as BCDA-2(+)CD123(+)HLA-DR(+)CD45RA(+) cells, and dendritic cells (DCs) as CD11c(+)CD14(-/low)lin(-) cells by flow cytometry. IPCs and DCs were analysed for Toll-like receptor-7 and -9 mRNA expression by real-time polymerase chain reaction. IFN-alpha was measured by enzyme-linked immunosorbent assay in serum, SF and in supernatants of influenza virus-infected, cultured IPCs. Synovial tissues of n = 6 additional JIA patients were analysed by immunohistochemistry using mAbs against CD123, IFN-alpha, TNF-alpha, CD3, CD19 and CD138. RESULTS: IPCs were enriched in SF MNCs compared with PB MNCs in all JIA patients. Influenza-induced, but no spontaneous IFN-alpha release was detected from SF IPCs, and serum and SF IFN-alpha levels were not elevated. Nonetheless, in synovial tissue IFN-alpha producing cells accumulated at inflammatory lymph-follicular-like structures, while TNF-alpha producing cells were mostly found at the lining and sublining layers. CONCLUSIONS: These data suggest that besides TNF-alpha-expressing cells, IFN-alpha-producing IPCs are involved in initiation, maintenance or regulation of the inflammatory response in JIA.

Cytokine-driven proliferation and differentiation of human naïve, central memory and effector memory CD4+ T cells.

Memory T lymphocytes divide in vivo in the absence of antigen maintaining a pool of central memory (T(CM)) and effector memory cells (T(EM)) with distinct effector function and homing capacity. We compared human CD4+ naïve T, T(CM) and T(EM) cells for their capacity to proliferate in response to cytokines, which have been implicated in T cell homeostasis. Interleukin (IL)-7 and IL-15 expanded with very high efficiency T(EM), while T(CM) were less responsive and naïve T cells did not respond at all. Dendritic cell (DC)-derived cytokines allowed naïve T cells to respond selectively to IL-4 and potently boosted the response of T(CM) to IL-7 and IL-15 by increasing the expression of the IL-2/IL-15Rss and the common gamma chain (gamma(c)). The ERK and the p38 MAP kinases were selectively required for TCR and cytokine-driven proliferation, respectively. Importantly, in cytokine-driven cultures T(CM) proliferated and some of the proliferating cells acquired effector function and non-lymphoid tissue homing capacity. Ex vivo BrdU incorporation experiments showed that both T(CM) and T(EM) proliferated under steady state conditions in vivo. Altogether these results provide a plausible mechanism for the maintenance of a polyclonal and functionally diverse repertoire of human CD4+ memory T cells and for a sustained antigen-independent generation of T(EM) from a pool of T(CM) cells.

Specialization and complementarity in microbial molecule recognition by human myeloid and plasmacytoid dendritic cells.

Following encounter with pathogens, dendritic cells (DC) mature and migrate from peripheral tissues to the T cell areas of secondary lymphoid organs, where they produce regulatory cytokines and prime naive T lymphocytes. We investigated in two subsets of human peripheral blood DC the expression of Toll-like receptors (TLR1 through TLR9) and the regulation of chemokine receptors and cytokine production in response to different maturation stimuli. Myeloid DC express all TLR except TLR7 and TLR9, which are selectively expressed by plasmacytoid DC. Myeloid and plasmacytoid DC respond to pathogen-associated molecular patterns according to their TLR expression. In response to the appropriate stimuli both DC types up-regulate CCR7, a receptor that drives DC migration to the T cell areas. Type I IFN was produced only by plasmacytoid DC and at early time points after stimulation. Furthermore, its production was elicited by some of the maturation stimuli tested. These results reveal a remarkable specialization and complementarity in microbial molecule recognition as well as a flexibility in effector function among myeloid and plasmacytoid DC.

Mouse pre-immunocytes as non-proliferating multipotent precursors of macrophages, interferon-producing cells, CD8alpha(+) and CD8alpha(-) dendritic cells.

In this study we characterize in mouse bone marrow and peripheral blood a homogeneous cell subset expressing Ly6C, CD31 and CD11c, that can give rise to multiple cell types involved in the immune response. Under the aegis of M-CSF or GM-CSF these cells rapidly differentiate without division to either macrophages or immature dendritic cells, which can be further induced to mature by LPS stimulation. In fetal thymic organ cultures the same cells generate both CD8alpha(+) and CD8alpha(-) dendritic cells in comparable proportion as found in normal thymus. The Ly6C(+), CD31(+) and CD11c(+) cells express not only TLR2 and TLR4, which are characteristic of myeloid dendritic cells, but also TLR7 and TLR9, which conversely are characteristic of human interferon-producing cells. Moreover, following stimulation with influenza virus, they rapidly express high levels of IFN-alpha mRNA. Finally these precursors are increased in bone marrow and peripheral blood during systemic inflammation. These cells are defined as "pre-immunocytes" to underline the fact that they serve in a flexible fashion multiple, and often divergent, functions required for the immune response to pathogens.

Cytokine-driven proliferation and differentiation of human naive, central memory, and effector memory CD4(+) T cells.

Memory T lymphocytes proliferate in vivo in the absence of antigen maintaining a pool of central memory T cells (T(CM)) and effector memory T cells (T(EM)) with distinct effector function and homing capacity. We compared human CD4(+) naive T, T(CM), and T(EM) cells for their capacity to proliferate in response to cytokines, that have been implicated in T cell homeostasis. Interleukin (IL)-7 and IL-15 expanded with very high efficiency T(EM), while T(CM) were less responsive and naive T cells failed to respond. Dendritic cells (DCs) and DC-derived cytokines allowed naive T cells to proliferate selectively in response to IL-4, and potently boosted the response of T(CM) to IL-7 and IL-15 by increasing the expression of the IL-2/IL-15Rbeta and the common gamma chain (gamma(c)). The extracellular signal regulated kinase and the p38 mitogen-activated protein (MAP) kinases were selectively required for TCR and cytokine-driven proliferation, respectively. Importantly, in cytokine-driven cultures, some of the proliferating T(CM) differentiated to T(EM)-like cells acquiring effector function and switching chemokine receptor expression from CCR7 to CCR5. The sustained antigen-independent generation of T(EM) from a pool of T(CM) cells provides a plausible mechanism for the maintenance of a polyclonal and functionally diverse repertoire of human CD4(+) memory T cells.

BDCA-2, a novel plasmacytoid dendritic cell-specific type II C-type lectin, mediates antigen capture and is a potent inhibitor of interferon alpha/beta induction.

Plasmacytoid dendritic cells are present in lymphoid and nonlymphoid tissue and contribute substantially to both innate and adaptive immunity. Recently, we have described several monoclonal antibodies that recognize a plasmacytoid dendritic cell-specific antigen, which we have termed BDCA-2. Molecular cloning of BDCA-2 revealed that BDCA-2 is a novel type II C-type lectin, which shows 50.7% sequence identity at the amino acid level to its putative murine ortholog, the murine dendritic cell-associated C-type lectin 2. Anti-BDCA-2 monoclonal antibodies are rapidly internalized and efficiently presented to T cells, indicating that BDCA-2 could play a role in ligand internalization and presentation. Furthermore, ligation of BDCA-2 potently suppresses induction of interferon alpha/beta production in plasmacytoid dendritic cells, presumably by a mechanism dependent on calcium mobilization and protein-tyrosine phosphorylation by src-family protein-tyrosine kinases. Inasmuch as production of interferon alpha/beta by plasmacytoid dendritic cells is considered to be a major pathophysiological factor in systemic lupus erythematosus, triggering of BDCA-2 should be evaluated as therapeutic strategy for blocking production of interferon alpha/beta in systemic lupus erythematosus patients.

Molecular regulation of CC-chemokine receptor 3 expression in human T helper 2 cells.

In developing T helper 1 (Th1) and Th2 cells the acquisition of effector function is intimately connected with the acquisition of new migratory capacities, as exemplified by differential expression of chemokine receptors. This study investigates the molecular mechanisms responsible for Th2-restricted expression of the CC-chemokine receptor 3 (CCR3). The minimal promoter in T cells was identified in the -149 base pair (bp) upstream sequence that contains a positive regulatory element. A strong negative element was also localized in the flanking intronic sequence. The study further investigates the role of chromatin remodeling in the regulation of this Th2-specific gene. Drugs that affect the chromatin structure facilitate CCR3 expression in T cells. Furthermore, in differentiating Th2 cells, selected regions are associated with acetylated-H3 histones and become more accessible to DNase I. These results suggest that in Th2 cells both cytokine production and migratory capacity are regulated through a similar mechanism involving chromatin remodeling.

The instructive role of dendritic cells on T cell responses: lineages, plasticity and kinetics.

Dendritic cells are responsible for directing different types of T cell responses, from thymic negative selection to the generation of effector and memory cells and the induction of peripheral tolerance. Recent studies indicate that the dendritic cell lineage, the extent of recruitment into inflamed tissues and migration to lymph nodes, the nature of maturation stimuli and the kinetics of activation have a quantitative and qualitative impact on T cell stimulation, thus exerting an instructive control on T cell responses.

Antigen decoding by T lymphocytes: from synapses to fate determination.

Naïve T lymphocytes sense foreign antigens by establishing contacts with dendritic cells (DCs). At the immunological synapse between the T cell and a DC, T cell receptors (TCRs) are serially engaged and triggered by specific ligands. The amount and duration of TCR triggering and the efficiency of signal amplification determine T cell commitment to proliferation and differentiation. The nature and availability of DCs bearing antigen and costimulatory molecules shape the T cell response, giving rise to distinct functional outputs such as effector and memory T cell generation or T cell tolerance.

Migration and function of antigen-primed nonpolarized T lymphocytes in vivo.

Upon antigenic stimulation, naive T lymphocytes proliferate and a fraction of the activated cells acquire a T helper cell type 1 (Th1) or Th2 phenotype as well as the capacity to migrate to inflamed tissues. However, the antigen-primed T cells that receive a short T cell receptor (TCR) stimulation do not acquire effector function and remain in a nonpolarized state. Using TCR transgenic CD4(+) T cells in an adoptive transfer system, we compared the in vivo migratory capacities of naive, nonpolarized, Th1 or Th2 cells. Although all cell types migrated to the spleen, only naive and nonpolarized T cells efficiently migrated to lymph nodes. In addition Th1, but not Th2, migrated to inflamed tissues. In the lymph nodes, nonpolarized T cells proliferated and acquired effector function in response to antigenic stimulation, displaying lower activation threshold and faster kinetics compared with naive T cells. These results suggest that nonpolarized T cells are in an intermediate state of differentiation characterized by lymph node homing capacity and increased responsiveness that allows them to mount a prompt and effective secondary response.

Organization of plasma membrane functional rafts upon T cell activation.

Raft microdomains have been shown to play a key role in T cell activation. We found that in human T lymphocytes the formation of functional rafts at the plasma membrane was induced by T cell priming. In resting T cells from peripheral blood Lck and the raft glycosphingolipid GM1 resided in intracellular membranes. T cell activation induced synthesis of GM1 and effector cells showed very high levels of this lipid, which became predominantly plasma membrane associated. TCR triggering also induced targeting of the cytosolic Lck to the plasma membrane. Thus, effector cells acquire an improved signaling machinery by increasing the amount of rafts at the plasma membrane. The fact that, when compared with naive T cells, memory T cells showed higher GM1 levels suggests that raft lipid synthesis may be developmentally regulated and tune T cell responsiveness.

A shift in the phenotype of melan-A-specific CTL identifies melanoma patients with an active tumor-specific immune response.

In a significant proportion of melanoma patients, CTL specific for the melan-A(26/7-35) epitope can be detected in peripheral blood using HLA-A2/peptide tetramers. However, the functional capacity of these CTL has been controversial, since although they prove to be effective killers after in vitro expansion, in some patients they have blunted activation responses ex vivo. We used phenotypic markers to characterize melan-A tetramer(+) cells in both normal individuals and melanoma patients, and correlated these markers with ex vivo assays of CTL function. Melanoma patients with detectable melan-A tetramer(+) cells in peripheral blood fell into two groups. Seven of thirteen patients had a CCR7(+) CD45R0(-) CD45RA(+) phenotype, the same as that found in some healthy controls, and this phenotype was associated with a lack of response to melan-A peptide ex vivo. In the remaining six patients, melan-A tetramer(+) cells were shifted toward a CCR7(-) CD45R0(+) CD45RA(-) phenotype, and responses to melan-A peptide could be readily demonstrated ex vivo. When lymph nodes infiltrated by melan-A-expressing melanoma cells were examined, a similar dichotomy emerged. These findings demonstrate that activation of melan-A-specific CTL occurs in only some patients with malignant melanoma, and that only patients with such active immune responses are capable of responding to Ag in ex vivo assays.

Dynamics of T lymphocyte responses: intermediates, effectors, and memory cells.

The immune response is initiated in organized lymphoid tissues where antigen-loaded dendritic cells (DCs) encounter antigen-specific T cells. DCs function as packets of information that must be decoded by the T cell before an appropriate immune response can be mounted. We discuss how the dynamics of DC-T cell encounter and the mechanism of T cell differentiation make the decoding of this information stochastic rather than determinate. This results in the generation of both terminally differentiated effector cells and intermediates that play distinctive roles in protection, immunoregulation, and immunological memory.

Cholera toxin induces maturation of human dendritic cells and licences them for Th2 priming.

Cholera toxin (CT) is a potent mucosal adjuvant that amplifies B and T cell responses to mucosally co-administered antigens, stimulating predominant Th2-type responses. However, little is known about the mechanism of adjuvanticity of CT and on the influence this toxin may have on Th2 cell development during the priming of an immune response. We analyzed the effect of CT on dendritic cells (DC), which are responsible for the priming of immune responses at the systemic as well as at the mucosal level. We found that CT induces phenotypic and functional maturation of blood monocyte-derived DC. Indeed, CT-treated DC up-regulate expression of HLA-DR molecules, B7. 1 and B7.2 co-stimulatory molecules, and are able to prime naive CD4(+)CD45RA(+) T cells in vitro, driving their polarization towards the Th2 phenotype. Furthermore, CT-matured DC express functional chemokine receptors CCR7 and CXCR4 which may render them responsive to migratory stimuli towards secondary lymphoid organs. Interestingly, the maturation program induced by CT is unique since CT does not induce but rather inhibits cytokine (IL-12p70 and TNF-alpha) and chemokine (RANTES, MIP-1alpha and MIP-1beta) secretion by lipopolysaccharide- or CD40 ligand-activated DC. Our results help to elucidate the mechanism of action of CT as an adjuvant and highlight a new stimulus of bacterial origin that promotes maturation of DC.

The role of chemokine receptors in primary, effector, and memory immune responses.

The immune system is composed of single cells, and its function is entirely dependent on the capacity of these cells to traffic, localize within tissues, and interact with each other in a precisely coordinated fashion. There is growing evidence that the large families of chemokines and chemokine receptors provide a flexible code for regulating cell traffic and positioning in both homeostatic and inflammatory conditions. The regulation of chemokine receptor expression during development and following cell activation explains the complex migratory pathways taken by dendritic cells, T and B lymphocytes, providing new insights into the mechanisms that control priming, effector function, and memory responses.