Mohamed Emara

Interaction of TIA-1/TIAR with West Nile and dengue virus products in infected cells interferes with stress granule formation and processing body assembly

The West Nile virus minus-strand 3′ terminal stem loop (SL) RNA was previously shown to bind specifically to cellular stress granule (SG) components, T cell intracellular antigen-1 (TIA-1) and the related protein TIAR. In vitro TIAR binding was 10 times more efficient than TIA-1. The 3′(−)SL functions as the promoter for genomic RNA synthesis. Colocalization of TIAR and TIA-1 with the viral replication complex components dsRNA and NS3 was observed in the perinuclear regions of West Nile virus- and dengue virus-infected cells. The kinetics of accumulation of TIAR in the perinuclear region was similar to those of genomic RNA synthesis. In contrast, relocation of TIA-1 to the perinuclear region began only after maximal levels of RNA synthesis had been achieved, except when TIAR was absent. Virus infection did not induce SGs and progressive resistance to SG induction by arsenite developed coincident with TIAR relocation. A progressive decrease in the number of processing bodies was secondarily observed in infected cells. These data suggest that the interaction of TIAR with viral components facilitates flavivirus genome RNA synthesis and inhibits SG formation, which prevents the shutoff of host translation.

Cell Proteins TIA-1 and TIAR Interact with the 3′ Stem-Loop of the West Nile Virus Complementary Minus-Strand RNA and Facilitate Virus Replication

ABSTRACT It was reported previously that four baby hamster kidney (BHK) proteins with molecular masses of 108, 60, 50, and 42 kDa bind specifically to the 3′-terminal stem-loop of the West Nile virus minus-stand RNA [WNV 3′(−) SL RNA] (P. Y. Shi, W. Li, and M. A. Brinton, J. Virol. 70:6278-6287, 1996). In this study, p42 was purified using an RNA affinity column and identified as TIAR by peptide sequencing. A 42-kDa UV-cross-linked viral RNA-cell protein complex formed in BHK cytoplasmic extracts incubated with the WNV 3′(−) SL RNA was immunoprecipitated by anti-TIAR antibody. Both TIAR and the closely related protein TIA-1 are members of the RNA recognition motif (RRM) family of RNA binding proteins. TIA-1 also binds to the WNV 3′(−) SL RNA. The specificity of these viral RNA-cell protein interactions was demonstrated using recombinant proteins in competition gel mobility shift assays. The binding site for the WNV 3′(−) SL RNA was mapped to RRM2 on both TIAR and TIA-1. However, the dissociation constant (Kd) for the interaction between TIAR RRM2 and the WNV 3′(−) SL RNA was 1.5 × 10−8, while that for TIA-1 RRM2 was 1.12 × 10−7. WNV growth was less efficient in murine TIAR knockout cell lines than in control cells. This effect was not observed for two other types of RNA viruses or two types of DNA viruses. Reconstitution of the TIAR knockout cells with TIAR increased the efficiency of WNV growth, but neither the level of TIAR nor WNV replication was as high as in control cells. These data suggest a functional role for TIAR and possibly also for TIA-1 during WNV replication.

The mannose receptor mediates the uptake of diverse native allergens by dendritic cells and determines allergen-induced T cell polarization through modulation of IDO activity.

The mannose receptor (MR) is a C-type lectin expressed by dendritic cells (DCs). We have investigated the ability of MR to recognize glycosylated allergens. Using a gene silencing strategy, we have specifically inhibited the expression of MR on human monocyte-derived DCs. We show that MR mediates internalization of diverse allergens from mite (Der p 1 and Der p 2), dog (Can f 1), cockroach (Bla g 2), and peanut (Ara h 1) through their carbohydrate moieties. All of these allergens bind to the C-type lectin-like carbohydrate recognition domains 4–7 of MR. We have also assessed the contribution of MR to T cell polarization after allergen exposure. We show that silencing MR expression on monocyte-derived DCs reverses the Th2 cell polarization bias, driven by Der p 1 allergen exposure, through upregulation of IDO activity. In conclusion, our work demonstrates a major role for MR in glycoallergen recognition and in the development of Th2 responses.

Glycosylation of surface Ig creates a functional bridge between human follicular lymphoma and microenvironmental lectins

Surface Ig (sIg) of follicular lymphoma (FL) is vital for tumor cell survival. We found previously that the Ig in FL is unusual, because the variable region genes carry sequence motifs for N-glycan addition. These are introduced by somatic mutation and are tumor specific. Unexpectedly, added glycans terminate at high mannose, suggesting a potentially important interaction of FL cells with mannose-binding lectins of the innate immune system. We have now identified mannosylated IgM at the surface of primary lymphoma cells. Recombinant lectin domains of the mannose receptor (MR) or DC-SIGN bind mannosylated Igs in vitro and bind to FL cells, signaling sIgM-associated increases in intracellular Ca2+. Lectins also bind to normal B cells but fail to signal. In contrast, anti-Ig signaled similarly in both FL and normal B cells. Mannosylation patterns were mimicked by FL Ig-derived single-chain Fvs (scFv), providing probes for potential receptors. Mannosylated scFv bound specifically to the lectin domains of the MR and DC-SIGN and blocked signaling. Mannosylated scFv also bound to DC-SIGN on the surface of dendritic cells. This unique lymphoma-specific interaction of sIg with lectins of innate immunity reveals a potential route for microenvironmental support of tumor cells, mediated via the key B-cell receptor.

Recognition of the major cat allergen Fel d 1 through the cysteine-rich domain of the mannose receptor determines its allergenicity.

Dendritic cells are professional antigen-presenting cells that are specialized in antigen uptake and presentation. Allergy to cat has increased substantially in recent years and has been shown to be positively associated with asthma. We have recently shown that the mannose receptor (MR), a C-type lectin expressed by dendritic cells, recognizes various glycoallergens from diverse sources and is involved in promoting allergic responses to a major house dust mite allergen in vitro. Here we investigated the potential role of MR in allergic responses to Fel d 1, a major cat allergen. Fel d 1 binding to MR was confirmed by ELISA. Using blocking, gene silencing (siRNA) experiments, and MR knock-out (MR−/−) cells, we have demonstrated that MR plays a major role in internalization of Fel d 1 by human and mouse antigen-presenting cells. Intriguingly, unlike other glycoallergens, recognition of Fel d 1 by MR is mediated by the cysteine-rich domain, which correlates with the presence of sulfated carbohydrates in natural Fel d 1. WT and MR−/− mice were used to study the role of MR in allergic sensitization to Fel d 1 in vivo. MR−/− mice sensitized with cat dander extract and Fel d 1 produced significantly lower levels of total IgE, Fel d 1-specific-IgE and IgG1, the hallmarks of allergic response, compared with WT mice. Our data show for the first time that Fel d 1 is a novel ligand of the cysteine-rich domain of MR and that MR is likely to play a pivotal role in allergic sensitization to airborne allergens in vivo.

Retagging Identifies Dendritic Cell-specific Intercellular Adhesion Molecule-3 (ICAM3)-grabbing Non-integrin (DC-SIGN) Protein as a Novel Receptor for a Major Allergen from House Dust Mite

Background: Allergen uptake by DCs is central to allergic sensitization. Results: DC-SIGN recognizes major allergens from house dust mite and dog. However, silencing DC-SIGN leads to Th2 differentiation. Conclusion: DC-SIGN is a newly identified receptor for Der p 1 and Can f 1 that appears to support Th1 cell differentiation. Significance: Understanding of how allergic responses are selected and propagated is essential for developing novel therapies. Dendritic cells (DCs) have been shown to play a key role in the initiation and maintenance of immune responses to microbial pathogens as well as to allergens, but the exact mechanisms of their involvement in allergic responses and Th2 cell differentiation have remained elusive. Using retagging, we identified DC-SIGN as a novel receptor involved in the initial recognition and uptake of the major house dust mite and dog allergens Der p 1 and Can f 1, respectively. To confirm this, we used gene silencing to specifically inhibit DC-SIGN expression by DCs followed by allergen uptake studies. Binding and uptake of Der p 1 and Can f 1 allergens was assessed by ELISA and flow cytometry. Intriguingly, our data showed that silencing DC-SIGN on DCs promotes a Th2 phenotype in DC/T cell co-cultures. These findings should lead to better understanding of the molecular basis of allergen-induced Th2 cell polarization and in doing so paves the way for the rational design of novel intervention strategies by targeting allergen receptors on innate immune cells or their carbohydrate counterstructures on allergens.

Mutation of Mapped TIA-1/TIAR Binding Sites in the 3′ Terminal Stem-Loop of West Nile Virus Minus-Strand RNA in an Infectious Clone Negatively Affects Genomic RNA Amplification

ABSTRACT Previous data showed that the cellular proteins TIA-1 and TIAR bound specifically to the West Nile virus 3′ minus-strand stem-loop [WNV3′(−)SL] RNA (37) and colocalized with flavivirus replication complexes in WNV- and dengue virus-infected cells (21). In the present study, the sites on the WNV3′(−)SL RNA required for efficient in vitro T-cell intracellular antigen-related (TIAR) and T-cell intracellular antigen-1 (TIA-1) protein binding were mapped to short AU sequences (UAAUU) located in two internal loops of the WNV3′(−)SL RNA structure. Infectious clone RNAs with all or most of the binding site nucleotides in one of the 3′ (−)SL loops deleted or substituted did not produce detectable virus after transfection or subsequent passage. With one exception, deletion/mutation of a single terminal nucleotide in one of the binding sequences had little effect on the efficiency of protein binding or virus production, but mutation of a nucleotide in the middle of a binding sequence reduced both the in vitro protein binding efficiency and virus production. Plaque size, intracellular genomic RNA levels, and virus production progressively decreased with decreasing in vitro TIAR/TIA-1 binding activity, but the translation efficiency of the various mutant RNAs was similar to that of the parental RNA. Several of the mutant RNAs that inefficiently interacted with TIAR/TIA-1 in vitro rapidly reverted in vivo, indicating that they could replicate at a low level and suggesting that an interaction between TIAR/TIA-1 and the viral 3′(−)SL RNA is not required for initial low-level symmetric RNA replication but instead facilitates the subsequent asymmetric amplification of genome RNA from the minus-strand template.

Laminin and Fibronectin Treatment Leads to Generation of Dendritic Cells with Superior Endocytic Capacity

Background Sampling the microenvironment at sites of microbial exposure by dendritic cells (DC) and their subsequent interaction with T cells in the paracortical area of lymph nodes are key events for initiating immune responses. Most of our knowledge of such events in human is based on in vitro studies performed in the absence of extracellular matrix (ECM) proteins. ECM in basement membranes and interstitial spaces of different tissues, including lymphoid organs, plays an important role in controlling specific cellular functions such as migration, intracellular signalling and differentiation. The aim of this study was, therefore, to investigate the impact of two abundant ECM components, fibronectin and laminin, on the phenotypical and functional properties of DC and how that might influence DC induced T-cell differentiation. Methodology/Principal Findings Human monocyte derived DC were treated with laminin and fibronectin for up to 48 hours and their morphology and phenotype was analyzed using scanning electron microscopy, flow cytometry and real time PCR. The endocytic ability of DC was determined using flow cytometry. Furthermore, co-culture of DC and T cells were established and T cell proliferation and cytokine profile was measured using H3-thymidine incorporation and ELISA respectively. Finally, we assessed formation of DC-T cell conjugates using different cell trackers and flow cytometry. Our data show that in the presence of ECM, DC maintain a ‘more immature’ phenotype and express higher levels of key endocytic receptors, and as a result become significantly better endocytic cells, but still fully able to mature in response to stimulation as evidenced by their superior ability to induce antigen-specific T cell differentiation. Conclusion These studies underline the importance of including ECM components in in vitro studies investigating DC biology and DC-T cell interaction. Within the context of antigen specific DC induced T cell proliferation, inclusion of ECM proteins could lead to development of more sensitive assays.

An immunoglobulin E‐reactive chimeric human immunoglobulin G1 anti‐idiotype inhibits basophil degranulation through cross‐linking of FcɛRI with FcγRIIb

Background IgE binds to mast cells and basophils via its high‐affinity receptor, FcɛRI, and cross‐linking of FcɛRI‐bound IgE molecules by allergen leads to the release of allergic mediators characteristic of type I hypersensitivity reactions. Previous work has shown that cross‐linking of FcɛRI with FcγRIIb, an ITIM‐containing IgG receptor, leads to inhibition of basophil triggering. 2G10, a chimeric human IgG1 anti‐idiotype, has broad reactivity with human IgE and as such has the potential to bind simultaneously to FcɛRI‐bound IgE, via its Fab regions, and the negative regulatory receptor, FcγRIIb, via its Fc region.