During T cell–dependent responses, B cells can either differentiate extrafollicularly into short-lived plasma cells or enter follicles to form germinal centers (GCs). Interactions with T follicular helper (Tfh) cells are required for GC formation and for selection of somatically mutated GC B cells. Interleukin (IL)-21 has been reported to play a role in Tfh cell formation and in B cell growth, survival, and isotype switching. To date, it is unclear whether the effect of IL-21 on GC formation is predominantly a consequence of this cytokine acting directly on the Tfh cells or if IL-21 directly influences GC B cells. We show that IL-21 acts in a B cell–intrinsic fashion to control GC B cell formation. Mixed bone marrow chimeras identified a significant B cell–autonomous effect of IL-21 receptor (R) signaling throughout all stages of the GC response. IL-21 deficiency profoundly impaired affinity maturation and reduced the proportion of IgG1+ GC B cells but did not affect formation of early memory B cells. IL-21R was required on GC B cells for maximal expression of Bcl-6. In contrast to the requirement for IL-21 in the follicular response to sheep red blood cells, a purely extrafollicular antibody response to Salmonella dominated by IgG2a was intact in the absence of IL-21.

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Shigella flexneri is a gram-negative bacterium which causes the most communicable of bacterial dysenteries, shigellosis. Shigellosis causes 1.1 million deaths and over 164 million cases each year, with the majority of cases occurring in the children of developing nations. The pathogenesis of S. flexneri is based on the bacteria's ability to invade and replicate within the colonic epithelium, which results in severe inflammation and epithelial destruction. The molecular mechanisms used by S. flexneri to cross the epithelial barrier, evade the host's immune response and enter epithelial cells have been studied extensively in both in vitro and in vivo models. Consequently, numerous virulence factors essential to bacterial invasion, intercellular spread and the induction of inflammation have been identified in S. flexneri. The inflammation produced by the host has been implicated in both the destruction of the colonic epithelium and in controlling and containing the Shigella infection. The host's humoral response to S. flexneri also appears to be important in protecting the host, whilst the role of the cellular immune response remains unclear. The host's immune response to shigellosis is serotype-specific and protective against reinfection by the same serotype, making vaccination a possibility. Since the 1940s vaccines for S. flexneri have been developed with little success, however, the growing understanding of S. flexneri's pathogenesis and the host's immune response is assisting in the generation of more refined vaccine strategies. Current research encompasses a variety of vaccine types, which despite disparity in their efficacy and safety in humans represent promising progress in S. flexneri vaccine development.

During evolutionary adaptation in the immune system, host defense is traded off against autoreactivity. Signals through the costimulatory receptor CD28 enable T cells to respond specifically to pathogens, whereas those through the related costimulatory receptor, ICOS, which arose by gene duplication, are critical for affinity maturation and memory antibody responses. ICOS ligand, unlike the pathogen-inducible CD28 ligands, is widely and constitutively expressed in the immune system. Here, we show that crosstalk between these two pathways provides a mechanism for obviating the normal T cell dependence on CD28. Several CD28-mediated responses-generation of follicular helper T cells, germinal center formation, T helper 1 cell-dependent extrafollicular antibody responses to Salmonella and bacterial clearance, and regulatory T cell homeostasis-became independent of CD28 and dependent on ICOS when the E3 ubiquitin ligase Roquin was mutated. Mechanisms to functionally compartmentalize ICOS and CD28 signals are thus critical for two-signal control of normal immune reactions.

Previous studies have shown that Shigella flexneri bacteriophage X (SfX) encodes a glucosyltransferase (GtrX, formerly Gtr), which is involved in 0 antigen modification (serotype Y to serotype X). However, GtrX alone can only mediate a partial conversion. More recently, a three-gene cluster has been identified next to the attachment site in the genome of two other 5. flexneri bacteriophages (i.e. S N and Sfll). This gene cluster was postulated to be responsible for a full 0 antigen conversion. Here it is reported that besides the gtrX gene, the other two genes in the gtr locus of SfX were also involved in the 0 antigen modification process. The first gene in the cluster (gtrA) encodes a small highly hydrophobic protein which appears to be involved in the translocation of lipid-linked glucose across the cytoplasmic membrane. The second gene in the cluster (gtrl3) encodes an enzyme catalysing the transfer of the glucose residue from UDP-glucose to a lipid carrier. The third gene (gtrx) encodes a bacteriophage-specif ic glucosyltransferase which is largely responsible for the final step, i.e. attaching the glucosyl molecules onto the correct sugar residue of the 0 antigen repeating unit. A three-step model for the glucosylation of bacterial 0 antigen has been proposed.

Salmonella group A, group B, and group D strains have paratose, abequose, and tyvelose, respectively, as the immunodominant sugar in their 0 antigens, which are otherwise identical; only the final steps differ in the biosynthetic pathways of these sugars. The gene rjbJ from a group B strain, encoding abequose synthase, the final and only unique step in the biosynthesis of CDP-abequose, has been cloned and sequenced (P. Wyk and P. Reeves, J. Bacteriol. 171:5687-5693, 1989 The evolutionary origin of these genes is discussed.We are studying the genetics of a major polymorphism in bacteria, and in the accompanying paper (13), we described and gave the sequence of the rfbJ gene of Salmonella strain LT2 (serovar typhimurium), which encodes the enzyme abequose synthase and determines the 04 epitope characteristic of the 0 antigen of serogroup B strains of salmonellae. In this paper we describe the identification and sequencing of the genes which determine the distinguishing immunodominant sugars for the related 0 antigens of serogroups A and D.The 0 antigen, a polysaccharide on the cell surface, is in groups A, B, and D a polymer with a repeat unit of four sugars, of which three form a backbone (mannosyl-rhamnosyl-galactose) common to all three groups, while the fourth sugar is a dideoxyhexose linked (al,3) to the mannose residue. The dideoxyhexose is paratose in group A, abequose in group B, and tyvelose in group D. The biosynthetic pathways for these three sugars diverge only at the last steps (Fig. 1). The genes encoding paratose synthase and CDPtyvelose-2-epimerase have not been named previously, and we propose the names rflS and rJbE, respectively. See the accompanying study (13) for a more detailed introduction.The genes for the biosynthetic pathways of the dideoxyhexoses are in the rfb gene cluster of the respective strains, and we have shown that there is a region of low homology in the general area of the genes for the latter part of the pathway (12, 13). In the accompanying study (13) fied in a group D strain and is shown to be present also in a group A strain but in a mutant form. The inferred amino acid sequences for RfbS and RfbE show that both proteins have homology with well-documented dehydrogenases. MATERIALS AND METHODSStrains used as sources of DNA or for cloning are described in Table 1. All plasmids used contained DNA from the rib regions of Ty2 (Ty2la was used as the source) or IMVS1316, cloned into pUC9 or pUC18. The extent of the cloned DNA has been indicated in the summary diagram (see Fig. 6), as have the vector name and the orientation of the insert.The serovars and sources of group A, B, and D strains used for probing with ribE DNA were as given below (see Acknowledgments for full names of sources). Group A, 5 paratyphi A were from ICPMR and 3 paratyphi A were from MDUM; and one nitra, one kiel, and one paratyphi A var. durazzo were from Le Minor. Group B, 2 typhimurium, one stanley, one derby, and one budapest were from IMVS; one agona, one heidelberg, one bredeney, one hessarek, one derby,...

Studies of cellular biology in recent decades have highlighted the crucial roles of glycans in numerous important biological processes, raising the concept of glycomics that is now considered as important as genomics, transcriptomics and proteomics. For millions of years, viruses have been co-evolving with their hosts. Consequently, during this co-evolution process, viruses have acquired mechanisms to mimic, hijack or sabotage host processes that favour their replication, including mechanisms to modify the glycome. The importance of the glycome in the regulation of host-virus interactions has recently led to a new concept called 'glycovirology'. One fascinating aspect of glycovirology is the study of how viruses affect the glycome. Viruses reach that goal either by regulating expression of host glycosyltransferases or by expressing their own glycosyltransferases. This review describes all virally encoded glycosyltransferases and discusses their established or putative functions. The description of these enzymes illustrates several intriguing aspects of virology and provides further support for the importance of glycomics in biological processes.

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