Human cytomegalovirus (HCMV) infection is largely asymptomatic in the immunocompetent host, but remains a major cause of morbidity in immunosuppressed individuals. Using the recently described technique of staining antigen-specific CD8 ؉ T cells with peptide-HLA tetrameric complexes, we have demonstrated high levels of antigen-specific cells specific for HCMV peptides and show that this may exceed 4% of CD8 ؉ T cells in immunocompetent donors. Moreover, by staining with tetramers in combination with antibodies to cell surface markers and intracellular cytokines, we demonstrate functional heterogeneity of HCMV-specific populations. A substantial proportion of these are effector cytotoxic T lymphocytes, as demonstrated by their ability to lyse peptide-pulsed targets in "fresh" killing assays. These data suggest that the immune response to HCMV is periodically boosted by a low level of HCMV replication and that sustained immunological surveillance contributes to the maintenance of host-pathogen homeostasis. These observations should improve our understanding of the immunobiology of persistent viral infection.

GPR84 is a member of the metabolic G protein-coupled receptor family, and its expression has been described predominantly in immune cells. GPR84 activation is involved in the inflammatory response, but the mechanisms by which it modulates inflammation have been incompletely described. In this study, we investigated GPR84 expression, activation, and function in macrophages to establish the role of the receptor during the inflammatory response. We observed that GPR84 expression in murine tissues is increased by endotoxemia, hyperglycemia, and hypercholesterolemia. Ex vivo studies revealed that GPR84 mRNA expression is increased by LPS and other pro-inflammatory molecules in different murine and human macrophage populations. Likewise, high glucose concentrations and the presence of oxidized LDL increased GPR84 expression in macrophages. Activation of the GPR84 receptor with a selective agonist, 6-(octylamino) pyrimidine-2,4(1H,3H)-dione (6-n-octylaminouracil, 6-OAU), enhanced the expression of phosphorylated Akt, p-ERK, and p65 nuclear translocation under inflammatory conditions and elevated the expression levels of the inflammatory mediators TNFα, IL-6, IL-12B, CCL2, CCL5, and CXCL1. In addition, GPR84 activation triggered increased bacterial adhesion and phagocytosis in macrophages. The enhanced inflammatory response mediated by 6-OAU was not observed in GPR84−/− cells nor in macrophages treated with a selective GPR84 antagonist. Collectively, our results reveal that GPR84 functions as an enhancer of inflammatory signaling in macrophages once inflammation is established. Therefore, molecules that antagonize the GPR84 receptor may be potential therapeutic tools in inflammatory and metabolic diseases.

Chitin is a skeletal cell wall polysaccharide of the inner cell wall of fungal pathogens. As yet, little about its role during fungus-host immune cell interactions is known. We show here that ultrapurified chitin from Candida albicans cell walls did not stimulate cytokine production directly but blocked the recognition of C. albicans by human peripheral blood mononuclear cells (PBMCs) and murine macrophages, leading to significant reductions in cytokine production. Chitin did not affect the induction of cytokines stimulated by bacterial cells or lipopolysaccharide (LPS), indicating that blocking was not due to steric masking of specific receptors. Toll-like receptor 2 (TLR2), TLR4, and Mincle (the macrophage-inducible C-type lectin) were not required for interactions with chitin. Dectin-1 was required for immune blocking but did not bind chitin directly. Cytokine stimulation was significantly reduced upon stimulation of PBMCs with heat-killed chitin-deficient C. albicans cells but not with live cells. Therefore, chitin is normally not exposed to cells of the innate immune system but is capable of influencing immune recognition by blocking dectin-1-mediated engagement with fungal cell walls.

Summary The outcome of an encounter between a cytotoxic cell and a potential target cell depends on the balance of signals from inhibitory and activating receptors. Natural Killer group 2D (NKG2D) has recently emerged as a major activating receptor on T lymphocytes and natural killer cells. In both humans and mice, multiple different genes encode ligands for NKG2D, and these ligands are non‐classical major histocompatibility complex class I molecules. The NKG2D–ligand interaction triggers an activating signal in the cell expressing NKG2D and this promotes cytotoxic lysis of the cell expressing the ligand. Most normal tissues do not express ligands for NKG2D, but ligand expression has been documented in tumour and virus‐infected cells, leading to lysis of these cells. Tight regulation of ligand expression is important. If there is inappropriate expression in normal tissues, this will favour autoimmune processes, whilst failure to up‐regulate the ligands in pathological conditions would favour cancer development or dissemination of intracellular infection.

The murine molecule dectin-1 (known as the b-glucan receptor in humans) is an immune cell surface receptor implicated in the immunological defense against fungal pathogens. Sequence analysis has indicated that the dectin-1 extracellular domain is a C-type lectin-like domain, and functional studies have established that it binds fungal b-glucans. We report several dectin-1 crystal structures, including a high-resolution structure and a 2.8 Å resolution structure in which a short soaked natural b-glucan is trapped in the crystal lattice. In vitro characterization of dectin-1 in the presence of its natural ligand indicates higher-order complex formation between dectin-1 and b-glucans. These combined structural and biophysical data considerably extend the current knowledge of dectin-1 structure and function, and suggest potential mechanisms of defense against fungal pathogens.Keywords: immune recognition; fungal pathogen; b-glucan; protein crystallography; C-type lectin-like domain Dectin-1 is a cell-surface immune receptor for b-glucans, which are major structural cell wall components that are conserved in fungi (Brown and Gordon 2001). Recognition of b-glucans by dectin-1 can trigger phagocytosis of fungal pathogens and protective inflammatory responses. Originally identified as a dendritic cell receptor in the mouse (Ariizumi et al. 2000), dectin-1 is now known to be widely expressed in both mouse and human cells, particularly on monocytes/macrophages and neutrophils (Brown 2006). Primary sequence analysis indicates that dectin-1 is a 28-kDa type II membrane protein. An extracellular Ctype lectin-like domain (CTLD) is connected by a stalk to a transmembrane region, followed by a cytoplasmic tail containing an immunoreceptor tyrosine-based activation (ITAM)-like motif (Ariizumi et al. 2000). First recognized as a calcium-dependent carbohydrate-binding domain, the CTLD fold is also seen in non-calciumdependent protein recognition interactions (Drickamer 1999). Few of the residues required for calcium coordination in classical CTLDs are conserved in dectin-1. The dectin-1 CTLD has two potential N-linked glycosylation sites, whereas its human homolog, the b-glucan receptor (bGR), has none. In addition to its role in b-glucan binding, it has been suggested that dectin-1 recognizes an endogenous T-cell ligand in a carbohydrate-independent manner (Ariizumi et al. 2000), but there is no evidence as to the nature of this potential ligand. Abbreviations: AUC, analytical ultracentrifugation; bGR, human b-glucan receptor; BGC, b-glucan; CTLD, C-type lectin-like domain; DLS, dynamic light scattering; TTX, Triton X-100.Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi

The cell surface molecules CD4 and CD8 greatly enhance the sensitivity of T-cell antigen recognition, acting as "co-receptors" by binding to the same major histocompatibility complex (MHC) molecules as the T-cell receptor (TCR). Here we use surface plasmon resonance to study the binding of CD8␣␣ to class I MHC molecules. CD8␣␣ bound the classical MHC molecules HLA-A*0201, -A*1101, -B*3501, and -C*0702 with dissociation constants (K d ) of 90 -220 M, a range of affinities distinctly lower than that of TCR/peptide-MHC interaction. We suggest such affinities apply to most CD8␣␣/classical class I MHC interactions and may be optimal for T-cell recognition. In contrast, CD8␣␣ bound both HLA-A*6801 and B*4801 with a significantly lower affinity (>1 mM), consistent with the finding that interactions with these alleles are unable to mediate cell-cell adhesion. Interestingly, CD8␣␣ bound normally to the nonclassical MHC molecule HLA-G (K d ϳ150 M), but only weakly to the natural killer cell receptor ligand HLA-E (K d > 1 mM). Site-directed mutagenesis experiments revealed that variation in CD8␣␣ binding affinity can be explained by amino acid differences within the ␣3 domain. Taken together with crystallographic studies, these results indicate that subtle conformational changes in the solvent exposed ␣3 domain loop (residues 223-229) can account for the differential ability of both classical and nonclassical class I MHC molecules to bind CD8. Cytotoxic T lymphocytes (CTL)1 expressing the cell surface glycoprotein CD8 play an important role in immune protection against intracellular pathogens such as viruses. CD8 greatly enhances antigen recognition by CTL (1) and is referred to as a co-receptor, since it binds to the same peptide-major histocompatibility complex class I molecules as the T-cell receptor (TCR) (1). Generation of mature CTL also depends upon the presence of CD8 at their surface (2) and upon the interaction of CD8 with MHC (3, 4). CD8 exists at the cell surface as a homodimer of two ␣ chains (CD8␣␣) and as a heterodimer or an ␣ and a ␤ chain (CD8␣␤). Expression of the heterodimeric form of CD8 is limited to the ␣␤ T-cell population, whereas homodimeric CD8␣␣ is present not only on ␣␤ T-cells but also on subsets of ␥␦ T cells and natural killer (NK) cells (5).Direct binding of CD8␣␣ to classical MHC alleles was initially demonstrated using cell-cell adhesion assays (6). Mutagenesis data suggested that CD8 and the TCR bind to separate sites on the MHC molecule (7). Recently, crystal structures of human and murine CD8␣␣-MHC complexes (8, 9) have shown that CD8␣␣ binds to an extended site on classical MHC molecules, which includes nonpolymorphic residues from the ␣2 and ␣3 domain as well as ␤ 2 -microglobulin. The two CD8␣ subunits contact the ␣3 domain in a manner resembling antibody-antigen interaction, with their complementarity determining region-like loops forming a pocket which accommodates an exposed loop (residues 223-229) linking the C and D strands. This mode of interaction prevents the simultaneous bin...

We have recently shown that the C-type lectin-like receptor, CLEC-2, is expressed on platelets and that it mediates powerful platelet aggregation by the snake venom toxin rhodocytin. In addition, we have provided indirect evidence for an endogenous ligand for CLEC-2 in renal cells expressing HIV-1. This putative ligand facilitates transmission of HIV through its incorporation into the viral envelope and binding to CLEC-2 on platelets. The aim of the present study was to identify the ligand on these cells which binds to CLEC-2 on platelets. Recombinant CLEC-2 exhibits specific binding to HEK-293T (human embryonic kidney) cells in which the HIV can be grown. Furthermore, HEK-293T cells activate both platelets and CLEC-2-transfected DT-40 B-cells. The transmembrane protein podoplanin was identified on HEK-293T cells and was demonstrated to mediate both binding of HEK-293T cells to CLEC-2 and HEK-293T cell activation of CLEC-2-transfected DT-40 B-cells. Podoplanin is expressed on renal cells (podocytes). Furthermore, a direct interaction between CLEC-2 and podoplanin was confirmed using surface plasmon resonance and was shown to be independent of glycosylation of CLEC-2. The interaction has an affinity of 24.5+/-3.7 microM. The present study identifies podoplanin as a ligand for CLEC-2 on renal cells.

Gag and p24 Gag sequences tested contain two-thirds of the dominant Gag-specific epitopes, irrespective of the clade, ethnicity, or age group studied. However, there were distinctive differences between the dominant responses made by Caucasoids and Africans. Dominant responses in Caucasoids were more often within p17Gag peptide residues 16 to 30 (38 versus 12%; P < 0.01), while p24 Gag peptide residues 41 to 60 contained the dominant Gag epitope more often in the African subjects tested (39 versus 4%; P < 0.005). Within this 20-mer p24Gag , an epitope presented by both B42 and B81 is defined which represents the dominant Gag response in >30% of the total infected population in Durban. This epitope is closely homologous with dominant HIV-2 and simian immunodeficiency virus Gag-specific CTL epitopes. The fine focusing of dominant CTL responses to these few regions of high immunogenicity is of significance to vaccine design.