It is well established that the gp120 V3 loop of T-cell-line-adapted human immunodeficiency virus type 1 (HIV-1) binds both cell-associated and soluble polyanions. Virus infectivity is increased by interactions between HIV-1 and heparan sulfate proteoglycans on some cell types, and soluble polyanions such as heparin and dextran sulfate neutralize HIV-1 in vitro. However, the analysis of gp120-polyanion interactions has been limited to T-cell-line-adapted, CXCR4-using virus and virus-derived gp120, and the polyanion binding ability of gp120 regions other than the V3 loop has not been addressed. Here we demonstrate by monoclonal-antibody inhibition, labeled heparin binding, and surface plasmon resonance studies that a second site, most probably corresponding to the newly defined, highly conserved coreceptor binding region on gp120, forms part of the polyanion binding surface. Consistent with the binding of polyanions to the coreceptor binding surface, dextran sulfate interfered with the gp120-CXCR4 association while having no detectable effect on the gp120-CD4 interaction. The interaction between polyanions and X4 or R5X4 gp120 was readily detectable, whereas weak or undetectable binding was observed with R5 gp120. Analysis of mutated forms of X4 gp120 demonstrated that the V3 loop is the major determinant for polyanion binding whereas other regions, including the V1/V2 loop structure and the NH 2 and COOH termini, exert a more subtle influence. A molecular model of the electrostatic potential of the conserved coreceptor binding region confirmed that it is basic but that the overall charge on this surface is dominated by the V3 loop. These results demonstrate a selective interaction of gp120 with polyanions and suggest that the conserved coreceptor binding surface may present a novel and conserved target for therapeutic intervention.

Toll-like receptors (TLRs) are key components of the immune system that detect microbial infection and trigger antimicrobial host defense responses. TLR5 is a sensor for monomeric flagellin, which is a component of bacterial flagella known to be a virulence factor. In this study we generated TLR5-deficient mice and investigated the role of TLR5 signaling in the detection of flagellin and antibacterial immune responses to Salmonella typhimurium and Pseudomonas aeruginosa. We found that TLR5 is essential for the recognition of bacterial flagellin both in vivo and ex vivo. TLR5 contribution to antibacterial host response to i.p. infection with S. typhimurium or intranasal administration of P. aeruginosa may be masked by TLR4 or other sensing mechanisms. By using radiation bone marrow chimera, we showed that upon i.p. injection of flagellin immune responses are mediated by lymphoid cells, whereas resident cells are required for the initiation of response upon intranasal flagellin administration. These results suggest that flagellin recognition in different organs is mediated by distinct TLR5-expressing cells and provide insights into the cooperation of the TLR5 and TLR4 signaling pathways used by the innate immune system in the recognition of bacterial pathogens.bacterial infection ͉ flagellin R ecognition of microbial infection and initiation of immune response are controlled by multiple mechanisms. Toll-like receptors (TLRs) have recently emerged as key components of the innate immune system that recognize common molecular structures detected in certain groups of microorganisms and trigger the activation of adaptive immunity (1). Each TLR detects specific microbial components. For example, TLR4 recognizes LPS, TLR2 recognizes bacterial lipoproteins and lipoteichoic acid, and TLR3 recognizes viral double-stranded RNA. All TLRs share a common intracellular domain, the Toll-IL-1 receptor homology domain, and upon activation initiate signaling cascades that lead to common responses such as the induction of inflammatory cytokines and up-regulation of costimulatory molecules. Moreover, TLRs also have specific functions as exemplified by their different ability to induce type I IFN (1). Thus, TLRs activate multiple steps in the inflammatory reactions that help to eliminate the invading pathogens and coordinate systemic defenses.Among TLRs, TLR5 is the receptor for flagellin, the major constituent of bacterial flagella and a virulence factor for Gramnegative and Gram-positive bacteria (2, 3). TLR5 engagement by bacterial flagellin activates the MyD88-dependent signaling pathway, which leads to the nuclear translocation of NF-B and the activation of the MAPKs, ultimately inducing the maturation of antigen-presenting cells and the secretion of proinflammatory cytokines and chemokines (4-8). TLR5 is expressed by a variety of cells including monocytes, dendritic cells (DCs), epithelial cells, and mast cells (5, 9-13). Interestingly, TLR5 is expressed on the basolateral side of intestinal epithelial cells, which are chronicall...

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