Domains 1 and 2 of ICAM-1 are sufficient to bind human rhinoviruses

The Coxsackievirus and Adenovirus Receptor: Glycosylation and the Extracellular D2 Domain Are Not Required for Coxsackievirus B3 Infection

The coxsackievirus and adenovirus receptor (CAR) is a member of the immunoglobulin superfamily (IgSF) and functions as a receptor for coxsackie B viruses (CVBs). The extracellular portion of CAR comprises two glycosylated immunoglobulin-like domains, D1 and D2. CAR-D1 binds to the virus and is essential for virus infection; however, it is not known whether D2 is also important for infection, and the role of glycosylation has not been explored. To understand the function of these structural components in CAR-mediated CVB3 infection, we generated a panel of human (h) CAR deletion and substitution mutants and analyzed their functionality as CVB receptors, examining both virus binding and replication. Lack of glycosylation of the CAR-D1 or -D2 domains did not adversely affect CVB3 binding or infection, indicating that the glycosylation of CAR is not required for its receptor functions. Deletion of the D2 domain reduced CVB3 binding, with a proportionate reduction in the efficiency of virus infection. Replacement of D2 with the homologous D2 domain from chicken CAR, or with the heterologous type C2 immunoglobulin-like domain from IgSF11, another IgSF member, fully restored receptor function; however, replacement of CAR-D2 with domains from CD155 or CD80 restored function only in part. These data indicate that glycosylation of the extracellular domain of hCAR plays no role in CVB3 receptor function and that CAR-D2 is not specifically required. The D2 domain may function largely as a spacer permitting virus access to D1; however, the data may also suggest that D2 affects virus binding by influencing the conformation of D1.

IMPORTANCE

An important step in virus infection is the initial interaction of the virus with its cellular receptor. Although the role in infection of the extracellular CAR-D1, cytoplasmic, and transmembrane domains have been analyzed extensively, nothing is known about the function of CAR-D2 and the extracellular glycosylation of CAR. Our data indicate that glycosylation of the extracellular CAR domain has only minor importance for the function of CAR as CVB3 receptor and that the D2 domain is not essential per se but contributes to receptor function by promoting the exposure of the D1 domain on the cell surface. These results contribute to our understanding of the coxsackievirus-receptor interactions.

Absence of the Kilifi mutation in the rhinovirus-binding domain of ICAM-1 in a Caucasian population

Human rhinoviruses (HRV), responsible for approximately 60% of the common colds, are divided into two groups, according to their receptor specificity. The major group of HRVs gains access to human cells by binding to the intercellular adhesion molecule-1 (ICAM-1), whereas HRVs of the minor group use members of the low-density lipoprotein receptor (LDLR) family for cell entry. Previous studies confirmed that the HRV-binding region of ICAM-1 is located in the amino-terminal immunoglobulin-like (Ig) domain 1, which is encoded by exon 2 of the ICAM-1 gene. An A --> T transversion in codon 29 of ICAM-1 exon 2 causes a lysine to methionine substitution (K29M), and was found at a high frequency (33.2%) in Kilifi (Kenya), as well as in other African populations. In this study we examined whether polymorphisms in exon 2 of ICAM-1 could be detected in a Caucasian population, assuming that these could be of importance in HRV binding. DNA from 100 healthy, unrelated, Belgian volunteers was obtained through a noninvasive swish-and-spit method. Using a primer set in the adjacent intron sequences, the full-length ICAM-1 exon 2 was amplified by polymerase chain reaction (PCR), followed by direct sequencing of the PCR product. No polymorphisms could be demonstrated in exon 2 of the ICAM-1 gene among all 100 tested individuals. The rhinovirus-binding Ig domain 1 of ICAM-1 seems to be a highly conserved region in the Caucasian population.

Peripheral nerve protein, P0, as a potential receptor for Theiler's murine encephalomyelitis virus

Theiler's murine encephalomyelitis virus (TMEV) belongs the family Picornaviridae. TMEV not only replicates in the gastrointestinal tract but also spreads to the central nervous system (CNS) either by a hematogenous or a neural pathway during natural infection. The DA strain of TMEV infects neurons during the acute phase, and glial cells and macrophages during the chronic phase, leading to a demyelinating disease similar to multiple sclerosis. Different virus-host receptor interactions in the peripheral and the neuronal cells could explain the pathways of viral spread from the peripheral to the CNS and neurons to glial cells. However, the receptor for TMEV remains unknown. P0 protein, a 28-31 kD glycoprotein, belongs to the immunoglobulin superfamily and constitutes 50% of the total myelin protein in the peripheral nerve. Other picornaviruses use members of the immunoglobulin superfamily as receptors. Thus we hypothesized P0 protein could act as a receptor for TMEV. In a virus overlay assay, radiolabeled TMEV bound to a 28-30 kD protein from the peripheral nerve of wild-type C57BL/6, but no binding was found in the peripheral nerve from P0-knockout mice. TMEV replicated fourfold higher in P0-transfected BW5147.G.1.4 cells than in mock-transfected cells. The increase in virus replication in the P0-transfected cell line was blocked by preincubation of the cells with anti-P0 antibody. A virus binding study showed that TMEV bound to P0-transfected cells but not to mock-transfected cells. The use of the P0 protein in Schwann cells as a receptor may be one mechanism by which TMEV spreads from the gastrointestinal tract to the CNS.

Respiratory infections and asthma

Clinical and experimental evidence suggests an important role for respiratory infections in the development of asthma attacks. Viral upper respiratory infections have been associated with 80% of asthma exacerbations in children and 50% of all asthma episodes in adults. Human rhinovirus has been implicated as the principal virus associated with asthma episodes. Separate studies indicate that atypical bacteria such as Chlamydia pneumoniae and Mycoplasma pneumoniae may precipitate asthma symptoms. Although not completely clarified, the intricate pathogenetic mechanisms by which viral infections promote asthma attacks have been extensively investigated in recent years. By contrast, it has not yet been established whether atypical bacterial infections are an epiphenomenon or a pathogenic event in asthma.

Th1/Th2 lymphocyte polarization in asthma

Asthma is a complex inflammatory disease of the lung characterized by variable airflow obstruction, bronchial hyperresponsiveness, and airway inflammation. Inflammation in asthma consists of airway infiltration by mast cells, lymphocytes, and eosinophils. There is accumulating evidence that CD4+ lymphocytes with a Th2-cytokine pattern play a pivotal role in the pathogenesis of asthma. These cells orchestrate the recruitment and activation of the primary effector cells of the allergic response (mast cells and eosinophils), through the release of cytokines such as IL-4, IL-5, and IL-13. Allergic inflammation is also implicated in airway epithelium changes, although the mechanisms by which inflammatory cells and, in particular, T cells interact with the epithelium are not completely clarified. This paper explores the role of T cells in the allergic inflammation of asthma.

Multiple sclerosis: the increased frequency of the ICAM-1 exon 6 gene point mutation genetic type K469

Intracellular adhesion molecule-1 (ICAM-1) plays an important role in the cascade of adhesion events in the homing of inflammatory cells to the central nervous system (CNS) in experimental autoimmune encephalomyelitis (EAE) and in multiple sclerosis (MS). Two single-base ICAM-1 polymorphisms have been described, in exons 4 and 6, changing codons 241 and 469 in the ICAM-1 gene, respectively. Both polymorphisms result in amino acid changes and can potentially lead to different interactions of ICAM-1 with its ligands. To detect ICAM-1 gene polymorphisms in MS, we have developed a highly sensitive and site-specific, two-stage, nested polymerase chain reaction. Genomic DNA was extracted from blood cells of 79 MS patients and 68 control subjects. The results were confirmed by direct dideoxy chain termination sequencing. The frequency of exon 6 allele T was found to be significantly higher in MS patients than in controls (68% vs 49%). Most interesting, the frequency of exon 6 homozygote K469 was significantly higher in MS patients than in controls (53% vs 34%). Higher frequency of the K469 genotype was found to be independent of possible linkage with the previously described MS susceptibility factor, the HLA class II DR2 allele. In the present study, we have shown for the first time the ICAM-1 gene polymorphisms in MS. The results indicate increased frequency of ICAM-1 exon 6 allele T in MS patients, which may contribute to the MS genetics background.

Chinese hamster ovary cells are non-permissive towards infection with coxsackievirus B3 despite functional virus-receptor interactions

Viral infection is a complex process which includes binding and interaction of the virus with specific cell surface receptors, uptake and uncoating of the virus, and finally replication. Chinese hamster ovary (CHO) cells are non-permissive towards infection with coxsackieviruses of group B (CVB), although they do express a putative CVB-specific receptor protein. In order to localize the block of infection in CHO cells, these cells were tested for binding of radiolabelled CVB3, receptor-mediated transformation of virions into A-particles, and replication of the viral genome. Binding of CVB3 to CHO cells was found to be comparable to the binding of this virus to permissive cell lines. Detergent-solubilized membrane proteins of CHO cells were tested in virus overlay protein-binding assays (VOPBAs) and shown to express a 100 kDa CVB-binding membrane protein similar to the CVB receptor protein which we recently described for permissive HeLa cells. Incubation of CVB3 with intact CHO cells resulted in transformation of cell-bound virions into non-infectious A-particles (deprived of capsid protein VP4), demonstrating the functional activity of the CVB receptor protein on CHO hamster cells. Transfection of recombinant CVB3 cDNA or viral RNA into CHO cells resulted in the production of infectious CVB3 virions, implying that the failure of CVB to infect CHO cells is not caused by a defect in virus replication but results from a block in the uptake of virus particles into the cell after the initial steps of virus-receptor interactions.

Characterization of a 100-kilodalton binding protein for the six serotypes of coxsackie B viruses

Viral infection of host cells primarily depends on binding of the virus to a specific cell surface protein. In order to characterize the binding protein for group B coxsackieviruses (CVB), detergent-solubilized membrane proteins of different cell lines were tested in virus overlay protein-binding assays. A prominent virus-binding protein with a molecular mass of 100 kDa was detected in various CVB-permissive human and monkey cell lines but was not detected in nonpermissive cell lines. The specificity of CVB binding to the 100-kDa protein on permissive human cells was substantiated by binding of all six serotypes of CVB and by competition experiments. In contrast, poliovirus and Sendai virus did not bind to the 100-kDa CVB-specific protein. A fraction of HeLa membrane proteins enriched in the range of 100 kDa showed functional activity by transforming infectious CVB (160S) into A-particles (135S). In order to purify this CVB-binding protein, solubilized membrane proteins from HeLa cells were separated by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by elution of the 100-kDa protein. Amino acid sequence analysis of tryptic fragments of the CVB-binding protein indicated that this 100-kDa CVB-specific protein is a cell surface protein related to nucleolin. These results were confirmed by immunoprecipitations of the CVB-binding protein with nucleolin-specific antibodies, suggesting that a nucleolin-related membrane protein acts as a specific binding protein for the six serotypes of CVB.