Elevated levels of cell-free NKG2D-ligands modulate NKG2D surface expression and compromise NK cell function in severe COVID-19 disease

Introduction

It is now clear that coronavirus disease 19 (COVID-19) severity is associated with a dysregulated immune response, but the relative contributions of different immune cells is still not fully understood. SARS CoV-2 infection triggers marked changes in NK cell populations, but there are contradictory reports as to whether these effector lymphocytes play a protective or pathogenic role in immunity to SARS-CoV-2.

Methods

To address this question we have analysed differences in the phenotype and function of NK cells in SARS-CoV-2 infected individuals who developed either very mild, or life-threatening COVID-19 disease.

Results

Although NK cells from patients with severe disease appeared more activated and the frequency of adaptive NK cells was increased, they were less potent mediators of ADCC than NK cells from patients with mild disease. Further analysis of peripheral blood NK cells in these patients revealed that a population of NK cells that had lost expression of the activating receptor NKG2D were a feature of patients with severe disease and this correlated with elevated levels of cell free NKG2D ligands, especially ULBP2 and ULBP3 in the plasma of critically ill patients. In vitro, culture in NKG2DL containing patient sera reduced the ADCC function of healthy donor NK cells and this could be blocked by NKG2DL-specific antibodies.

Discussion

These observations of reduced NK function in severe disease are consistent with the hypothesis that defects in immune surveillance by NK cells permit higher levels of viral replication, rather than that aberrant NK cell function contributes to immune system dysregulation and immunopathogenicity.

Preclinical immune efficacy against SARS-CoV-2 beta B.1.351 variant by MVA-based vaccine candidates

The constant appearance of new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VoCs) has jeopardized the protective capacity of approved vaccines against coronavirus disease-19 (COVID-19). For this reason, the generation of new vaccine candidates adapted to the emerging VoCs is of special importance. Here, we developed an optimized COVID-19 vaccine candidate using the modified vaccinia virus Ankara (MVA) vector to express a full-length prefusion-stabilized SARS-CoV-2 spike (S) protein, containing 3 proline (3P) substitutions in the S protein derived from the beta (B.1.351) variant, termed MVA-S(3Pbeta). Preclinical evaluation of MVA-S(3Pbeta) in head-to-head comparison to the previously generated MVA-S(3P) vaccine candidate, expressing a full-length prefusion-stabilized Wuhan S protein (with also 3P substitutions), demonstrated that two intramuscular doses of both vaccine candidates fully protected transgenic K18-hACE2 mice from a lethal challenge with SARS-CoV-2 beta variant, reducing mRNA and infectious viral loads in the lungs and in bronchoalveolar lavages, decreasing lung histopathological lesions and levels of proinflammatory cytokines in the lungs. Vaccination also elicited high titers of anti-S Th1-biased IgGs and neutralizing antibodies against ancestral SARS-CoV-2 Wuhan strain and VoCs alpha, beta, gamma, delta, and omicron. In addition, similar systemic and local SARS-CoV-2 S-specific CD4+ and CD8+ T-cell immune responses were elicited by both vaccine candidates after a single intranasal immunization in C57BL/6 mice. These preclinical data support clinical evaluation of MVA-S(3Pbeta) and MVA-S(3P), to explore whether they can diversify and potentially increase recognition and protection of SARS-CoV-2 VoCs.

N'-Phenylacetohydrazide Derivatives as Potent Ebola Virus Entry Inhibitors with an Improved Pharmacokinetic Profile

Ebola virus (EBOV) is a single-strand RNA virus belonging to the Filoviridae family, which has been associated to most Ebola virus disease outbreaks to date, including the West African and the North Kivu epidemics between 2013 and 2022. This unprecedented health emergency prompted the search for effective medical countermeasures. Following up on the carbazole hit identified in our previous studies, we synthetized a new series of compounds, which demonstrated to prevent EBOV infection in cells by acting as virus entry inhibitors. The in vitro inhibitory activity was evaluated through the screening against surrogate models based on viral pseudotypes and further confirmed using replicative EBOV. Docking and molecular dynamics simulations joined to saturation transfer difference-nuclear magnetic resonance (STD-NMR) and mutagenesis experiments to elucidate the biological target of the most potent compounds. Finally, in vitro metabolic stability and in vivo pharmacokinetic studies were performed to confirm their therapeutic potential.

Selection of phage-displayed antibodies with high affinity and specificity by electrophoresis in microfluidic devices

A method development aimed for high-throughput and automated antibody screening holds great potential for areas ranging from fundamental molecular interactions to the discovery of novel disease markers, therapeutic targets and monoclonal antibody engineering. Surface display techniques enable efficient manipulation of large molecular libraries in small volumes. Specifically, phage display appeared as a powerful technology for selecting peptides and proteins with enhanced, target-specific binding affinities. Here, we present a phage-selection microfluidic device wherein electrophoresis was performed under two orthogonal electric fields through an agarose gel functionalized with the respective antigen. This microdevice was capable of screening and sorting in a single round high affinity phage-displayed antibodies against virus glycoproteins, including Human Immunodeficiency Virus-1 (HIV-1) glycoprotein (gp120) or the Ebola Virus glycoprotein (EBOV-GP). Phages were differentially and laterally swept depending on their antigen affinity; the high affinity phages were recovered at channels proximal to the application site, whereas low affinity phages migrated distal after electrophoresis. These experiments proved that the microfluidic device specifically designed for phage-selection is rapid, sensitive and effective. Therefore, this is an efficient and cost-effective method that allowed highly controlled assay conditions for isolating and sorting high affinity ligands displayed in phages. This article is protected by copyright. All rights reserved.

Intranasal administration of a single dose of MVA-based vaccine candidates against COVID-19 induced local and systemic immune responses and protects mice from a lethal SARS-CoV-2 infection

Current coronavirus disease-19 (COVID-19) vaccines are administered by the intramuscular route, but this vaccine administration failed to prevent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus infection in the upper respiratory tract, mainly due to the absence of virus-specific mucosal immune responses. It is hypothesized that intranasal (IN) vaccination could induce both mucosal and systemic immune responses that blocked SARS-CoV-2 transmission and COVID-19 progression. Here, we evaluated in mice IN administration of three modified vaccinia virus Ankara (MVA)-based vaccine candidates expressing the SARS-CoV-2 spike (S) protein, either the full-length native S or a prefusion-stabilized [S(3P)] protein; SARS-CoV-2-specific immune responses and efficacy were determined after a single IN vaccine application. Results showed that in C57BL/6 mice, MVA-based vaccine candidates elicited S-specific IgG and IgA antibodies in serum and bronchoalveolar lavages, respectively, and neutralizing antibodies against parental and SARS-CoV-2 variants of concern (VoC), with MVA-S(3P) being the most immunogenic vaccine candidate. IN vaccine administration also induced polyfunctional S-specific Th1-skewed CD4+ and cytotoxic CD8+ T-cell immune responses locally (in lungs and bronchoalveolar lymph nodes) or systemically (in spleen). Remarkably, a single IN vaccine dose protected susceptible K18-hACE2 transgenic mice from morbidity and mortality caused by SARS-CoV-2 infection, with MVA-S(3P) being the most effective candidate. Infectious SARS-CoV-2 viruses were undetectable in lungs and nasal washes, correlating with high titers of S-specific IgGs and neutralizing antibodies against parental SARS-CoV-2 and several VoC. Moreover, low histopathological lung lesions and low levels of pro-inflammatory cytokines in lungs and nasal washes were detected in vaccinated animals. These results demonstrated that a single IN inoculation of our MVA-based vaccine candidates induced potent immune responses, either locally or systemically, and protected animal models from COVID-19. These results also identified an effective vaccine administration route to induce mucosal immunity that should prevent SARS-CoV-2 host-to-host transmission.

Nanobodies Protecting From Lethal SARS-CoV-2 Infection Target Receptor Binding Epitopes Preserved in Virus Variants Other Than Omicron

The emergence of SARS-CoV-2 variants that escape from immune neutralization are challenging vaccines and antibodies developed to stop the COVID-19 pandemic. Thus, it is important to establish therapeutics directed toward multiple or specific SARS-CoV-2 variants. The envelope spike (S) glycoprotein of SARS-CoV-2 is the key target of neutralizing antibodies (Abs). We selected a panel of nine nanobodies (Nbs) from dromedary camels immunized with the receptor-binding domain (RBD) of the S, and engineered Nb fusions as humanized heavy chain Abs (hcAbs). Nbs and derived hcAbs bound with subnanomolar or picomolar affinities to the S and its RBD, and S-binding cross-competition clustered them in two different groups. Most of the hcAbs hindered RBD binding to its human ACE2 (hACE2) receptor, blocked cell entry of viruses pseudotyped with the S protein and neutralized SARS-CoV-2 infection in cell cultures. Four potent neutralizing hcAbs prevented the progression to lethal SARS-CoV-2 infection in hACE2-transgenic mice, demonstrating their therapeutic potential. Cryo-electron microscopy identified Nb binding epitopes in and out the receptor binding motif (RBM), and showed different ways to prevent virus binding to its cell entry receptor. The Nb binding modes were consistent with its recognition of SARS-CoV-2 RBD variants; mono and bispecific hcAbs efficiently bound all variants of concern except omicron, which emphasized the immune escape capacity of this latest variant.

Cross-reactive cellular, but not humoral, immunity is detected between OC43 and SARS-CoV-2 NPs in people not infected with SARS-CoV-2: Possible role of cTFH cells

Multiple questions about SARS-CoV-2 humoral and cellular immunity remain unanswered. One key question is whether preexisting memory T or B cells, specific for related coronaviruses in SARS-CoV-2-unexposed individuals, can recognize and suppress COVID-19, but this issue remains unclear. Here, we demonstrate that antibody responses to SARS-CoV-2 antigens are restricted to serum samples from COVID-19 convalescent individuals. In contrast, cross-reactive T cell proliferation and IFN-γ production responses were detected in PBMCs of around 30% of donor samples collected prepandemic, although we found that these prepandemic T cell responses only elicited weak cT_FH activation upon stimulation with either HCoV-OC43 or SARS-CoV-2 NP protein. Overall, these observations confirm that T cell cross-reactive with SARS-CoV-2 antigens are present in unexposed people, but suggest that the T cell response to HCoV-OC43 could be deficient in some important aspects, like T_FH expansion, that might compromise the generation of cross-reactive T_FH cells and antibodies. Understanding these differences in cellular responses may be of critical importance to advance in our knowledge of immunity against SARS-CoV-2.

A Single Dose of an MVA Vaccine Expressing a Prefusion-Stabilized SARS-CoV-2 Spike Protein Neutralizes Variants of Concern and Protects Mice From a Lethal SARS-CoV-2 Infection

We generated an optimized COVID-19 vaccine candidate based on the modified vaccinia virus Ankara (MVA) vector expressing a full-length prefusion-stabilized SARS-CoV-2 spike (S) protein, termed MVA-CoV2-S(3P). The S(3P) protein was expressed at higher levels (2-fold) than the non-stabilized S in cells infected with the corresponding recombinant MVA viruses. One single dose of MVA-CoV2-S(3P) induced higher IgG and neutralizing antibody titers against parental SARS-CoV-2 and variants of concern than MVA-CoV2-S in wild-type C57BL/6 and in transgenic K18-hACE2 mice. In immunized C57BL/6 mice, two doses of MVA-CoV2-S or MVA-CoV2-S(3P) induced similar levels of SARS-CoV-2-specific B- and T-cell immune responses. Remarkably, a single administration of MVA-CoV2-S(3P) protected all K18-hACE2 mice from morbidity and mortality caused by SARS-CoV-2 infection, reducing SARS-CoV-2 viral loads, histopathological lesions, and levels of pro-inflammatory cytokines in the lungs. These results demonstrated that expression of a novel full-length prefusion-stabilized SARS-CoV-2 S protein by the MVA poxvirus vector enhanced immunogenicity and efficacy against SARS-CoV-2 in animal models, further supporting MVA-CoV2-S(3P) as an optimized vaccine candidate for clinical trials.

Full efficacy and long-term immunogenicity induced by the SARS-CoV-2 vaccine candidate MVA-CoV2-S in mice

Two doses of the MVA-CoV2-S vaccine candidate expressing the SARS-CoV-2 spike (S) protein protected K18-hACE2 transgenic mice from a lethal dose of SARS-CoV-2. This vaccination regimen prevented virus replication in the lungs, reduced lung pathology, and diminished levels of pro-inflammatory cytokines. High titers of IgG antibodies against S and receptor-binding domain (RBD) proteins and of neutralizing antibodies were induced against parental virus and variants of concern, markers that correlated with protection. Similar SARS-CoV-2-specific antibody responses were observed at prechallenge and postchallenge in the two-dose regimen, while the single-dose treatment does not avoid vaccine breakthrough infection. All vaccinated animals survived infection and were also protected to SARS-CoV-2 reinfection. Furthermore, two MVA-CoV2-S doses induced long-term memory S-specific humoral and cellular immune responses in C57BL/6 mice, 6 months after immunization. The efficacy and immunological benefits of the MVA-CoV2-S vaccine candidate against COVID-19 supports its consideration for human clinical trials.

CD4+ T Cell Immune Specificity Changes After Vaccination in Healthy And COVID-19 Convalescent Subjects

The immune response promoted by SARS-CoV-2 vaccination is relevant to develop novel vaccines and optimized prevention strategies. We analyzed the adaptive immunity in healthy donors (HD) and convalescent individuals (CD), before and after administering BNT162b2 vaccine. Our results revealed specific changes in CD4+ T cell reactivity profile in vaccinated HD and CD, with an increase in S1 and S2 positive individuals, proportionally higher for S2. On the contrary, NCAP reactivity observed in HD and CD patients was no longer detectable after vaccination. Despite the substantial antibody response in CD, MPro-derived peptides did not elicit CD4+ lymphocyte activation in our assay in either condition. HD presented an increment in anti-S and anti-RBD IgG after first dose vaccination, which increased after the second vaccination. Conversely, anti-S and anti-RBD IgG and IgA titers increased in already positive CD after first dose administration, remaining stable after second dose inoculation. Interestingly, we found a strong significant correlation between S1-induced CD4+ response and anti-S IgA pre-vaccination, which was lost after vaccine administration.

COVID-19 vaccine candidates based on modified vaccinia virus Ankara expressing the SARS-CoV-2 spike induce robust T- and B-cell immune responses and full efficacy in mice

Vaccines against SARS-CoV-2, the causative agent of the COVID-19 pandemic, are urgently needed. We developed two COVID-19 vaccines based on modified vaccinia virus Ankara (MVA) vectors expressing the entire SARS-CoV-2 spike (S) protein (MVA-CoV2-S); their immunogenicity was evaluated in mice using DNA/MVA or MVA/MVA prime/boost immunizations. Both vaccines induced robust, broad and polyfunctional S-specific CD4+ (mainly Th1) and CD8+ T-cell responses, with a T effector memory phenotype. DNA/MVA immunizations elicited higher T-cell responses. All vaccine regimens triggered high titers of IgG antibodies specific for the S, as well as for the receptor-binding domain; the predominance of the IgG2c isotype was indicative of Th1 immunity. Notably, serum samples from vaccinated mice neutralized SARS-CoV-2 in cell cultures, and those from MVA/MVA immunizations showed a higher neutralizing capacity. Remarkably, one or two doses of MVA-CoV2-S protect humanized K18-hACE2 mice from a lethal dose of SARS-CoV-2. In addition, two doses of MVA-CoV2-S confer full inhibition of virus replication in the lungs. These results demonstrate the robust immunogenicity and full efficacy of MVA-based COVID-19 vaccines in animal models and support its translation to the clinic.IMPORTANCE The continuous dissemination of the novel emerging SARS-CoV-2 virus, with more than 78 million infected cases worldwide and higher than 1,700,000 deaths as of December 23, 2020, highlights the urgent need for the development of novel vaccines against COVID-19. With this aim, we have developed novel vaccine candidates based on the poxvirus modified vaccinia virus Ankara (MVA) strain expressing the full-length SARS-CoV-2 spike (S) protein, and we have evaluated their immunogenicity in mice using DNA/MVA or MVA/MVA prime/boost immunization protocols. The results showed the induction of a potent S-specific T-cell response and high titers of neutralizing antibodies. Remarkably, humanized K18-hACE2 mice immunized with one or two doses of the MVA-based vaccine were 100% protected from SARS-CoV-2 lethality. Moreover, two doses of the vaccine prevented virus replication in lungs. Our findings prove the robust immunogenicity and efficacy of MVA-based COVID-19 vaccines in animal models and support its translation to the clinic.

Deregulated cellular circuits driving immunoglobulins and complement consumption associate with the severity of COVID-19 patients

SARS‐CoV‐2 infection causes an abrupt response by the host immune system, which is largely responsible for the outcome of COVID‐19. We investigated whether the specific immune responses in the peripheral blood of 276 patients were associated with the severity and progression of COVID‐19. At admission, dramatic lymphopenia of T, B, and NK cells is associated with severity. Conversely, the proportion of B cells, plasmablasts, circulating follicular helper T cells (cTfh) and CD56^–CD16⁺ NK‐cells increased. Regarding humoral immunity, levels of IgM, IgA, and IgG were unaffected, but when degrees of severity were considered, IgG was lower in severe patients. Compared to healthy donors, complement C3 and C4 protein levels were higher in mild and moderate, but not in severe patients, while the activation peptide of C5 (C5a) increased from the admission in every patient, regardless of their severity. Moreover, total IgG, the IgG1 and IgG3 isotypes, and C4 decreased from day 0 to day 10 in patients who were hospitalized for more than two weeks, but not in patients who were discharged earlier. Our study provides important clues to understand the immune response observed in COVID‐19 patients, associating severity with an imbalanced humoral response, and identifying new targets for therapeutic intervention.

Oxidized Low-Density Lipoprotein Receptor in Lymphocytes Prevents Atherosclerosis and Predicts Subclinical Disease

BACKGROUND

Although the role of Th17 and regulatory T cells in the progression of atherosclerosis has been highlighted in recent years, their molecular mediators remain elusive. We aimed to evaluate the association between the CD69 receptor, a regulator of Th17/regulatory T cell immunity, and atherosclerosis development in animal models and in patients with subclinical disease.

METHODS

Low-density lipoprotein receptor-deficient chimeric mice expressing or not expressing CD69 on either myeloid or lymphoid cells were subjected to a high fat diet. In vitro functional assays with human T cells were performed to decipher the mechanism of the observed phenotypes. Expression of CD69 and NR4A nuclear receptors was evaluated by reverse transcription-polymerase chain reaction in 305 male participants of the PESA study (Progression of Early Subclinical Atherosclerosis) with extensive (n=128) or focal (n=55) subclinical atherosclerosis and without disease (n=122).

RESULTS

After a high fat diet, mice lacking CD69 on lymphoid cells developed large atheroma plaque along with an increased Th17/regulatory T cell ratio in blood. Oxidized low-density lipoprotein was shown to bind specifically and functionally to CD69 on human T lymphocytes, inhibiting the development of Th17 cells through the activation of NR4A nuclear receptors. Participants of the PESA study with evidence of subclinical atherosclerosis displayed a significant CD69 and NR4A1 mRNA downregulation in peripheral blood leukocytes compared with participants without disease. The expression of CD69 remained associated with the risk of subclinical atherosclerosis in an adjusted multivariable logistic regression model (odds ratio, 0.62; 95% CI, 0.40-0.94; P=0.006) after adjustment for traditional risk factors, the expression of NR4A1, the level of oxidized low-density lipoprotein, and the counts of different leucocyte subsets.

CONCLUSIONS

CD69 depletion from the lymphoid compartment promotes a Th17/regulatory T cell imbalance and exacerbates the development of atherosclerosis. CD69 binding to oxidized low-density lipoprotein on T cells induces the expression of anti-inflammatory transcription factors. Data from a cohort of the PESA study with subclinical atherosclerosis indicate that CD69 expression in PBLs inversely correlates with the presence of disease. The expression of CD69 remained an independent predictor of subclinical atherosclerosis after adjustment for traditional risk factors.

Measles Virus Hemagglutinin epitopes immunogenic in natural infection and vaccination are targeted by broad or genotype-specific neutralizing monoclonal antibodies

Measles virus (MV) remains a leading cause of vaccine-preventable deaths in children. Protection against MV is associated with neutralizing antibodies that preferentially recognize the viral hemagglutinin (MV-H), and to a lesser extent, the fusion protein (MV-F). Although MV is serologically monotypic, 24 genotypes have been identified. Here we report three neutralization epitopes conserved in the more prevalent circulating MV genotypes, two located in the MV-H receptor binding site (RBS) (antigenic site III) and a third in MV-H/MV-F interphase (antigenic site Ia) which are essential for MV multiplication. In contrast, two MV-H neutralization epitopes, showed a genotype-specific neutralization escape due to a single amino acid change, that we mapped in the "noose" antigenic site, or an enhanced neutralization epitope (antigenic site IIa). The monoclonal antibody (mAb) neutralization potency correlated with its binding affinity and was mainly driven by kinetic dissociation rate (k_off). We developed an immunoassay for mAb binding to MV-H in its native hetero-oligomeric structure with MV-F on the surface of a MV productive steady-state persistently infected (p.i.) human cell lines, and a competitive-binding assay with serum from individuals with past infection by different MV genotypes. Binding assays revealed that a broad neutralization epitope, in RBS antigenic site, a genotype specific neutralization epitopes, in noose and IIa sites, were immunogenic in natural infection and vaccination and may elicit long-lasting humoral immunity that might contribute to explain MV immunogenic stability. These results support the design of improved measles vaccines, broad-spectrum prophylactic or therapeutic antibodies and MV-used in oncolytic therapies.

Allosteric inhibition of aminopeptidase N functions related to tumor growth and virus infection

Cell surface aminopeptidase N (APN) is a membrane-bound ectoenzyme that hydrolyzes proteins and peptides and regulates numerous cell functions. APN participates in tumor cell expansion and motility, and is a target for cancer therapies. Small drugs that bind to the APN active site inhibit catalysis and suppress tumor growth. APN is also a major cell entry receptor for coronavirus, which binds to a region distant from the active site. Three crystal structures that we determined of human and pig APN ectodomains defined the dynamic conformation of the protein. These structures offered snapshots of closed, intermediate and open APN, which represent distinct functional states. Coronavirus envelope proteins specifically recognized the open APN form, prevented ectodomain progression to the closed form and substrate hydrolysis. In addition, drugs that bind the active site inhibited both coronavirus binding to cell surface APN and infection; the drugs probably hindered APN transition to the virus-specific open form. We conclude that allosteric inhibition of APN functions occurs by ligand suppression of ectodomain motions necessary for catalysis and virus cell entry, as validated by locking APN with disulfides. Blocking APN dynamics can thus be a valuable approach to development of drugs that target this ectoenzyme.

A structural view of coronavirus-receptor interactions

In the coronavirus (CoV), the envelope spike (S) glycoprotein is responsible for CoV cell entry and host-to-host transmission. The S is a multifunctional glycoprotein that mediates both attachment of CoV particles to cell surface receptor molecules as well as membrane penetration by fusion. Receptor-binding domains (RBD) have been identified in the S of diverse CoV; they usually contain antigenic determinants targeted by antibodies that neutralize CoV infections. To penetrate host cells, the CoV can use various cell surface molecules, although they preferentially bind to ectoenzymes. Several crystal structures have determined the folding of CoV RBD and the mode by which they recognize cell entry receptors. Here we review the CoV-receptor complex structures reported to date, and highlight the distinct receptor recognition modes, common features, and key determinants of the binding specificity. Structural studies have established the basis for understanding receptor recognition diversity in CoV, its evolution and the adaptation of this virus family to different hosts. CoV responsible for recent outbreaks have extraordinary potential for cross-species transmission; their RBD bear large platforms specialized in recognition of receptors from different species, which facilitates host-to-host circulation and adaptation to man.

Crystal structures of an ICAM-5 ectodomain fragment show electrostatic-based homophilic adhesions

Crystal structures of Intercellular Cell Adhesion Molecule-5 (ICAM-5) show charge-based homotypic interactions and a potential ICAM-5 cell adhesion complex in neurons.

Intercellular cell adhesion molecule-5 (ICAM-5) is a member of the ICAM subfamily that is exclusively expressed in the telencephalon region of the brain. The crystal structure of the four most N-terminal glycosylated domains (D1–D4) of ICAM-5 was determined in three different space groups and the D1–D5 fragment was modelled. The structures showed a curved molecule with two pronounced interdomain bends between D2 and D3 and between D3 and D4, as well as some interdomain flexibility. In contrast to ICAM-1, ICAM-5 has patches of positive and negative electrostatic charge at D1–D2 and at D3–D5, respectively. ICAM-5 can mediate homotypic interactions. In the crystals, several charge-based intermolecular interactions between the N-terminal and C-terminal moieties of the ICAM-5 molecules were observed, which defined an interacting surface in the D1–D4 fragment. One of the crystal lattices has a molecular assembly that could represent the homophilic ICAM-5 cell adhesion complex in neurons.

Fructose 1-phosphate is the preferred effector of the metabolic regulator Cra of Pseudomonas putida

The catabolite repressor/activator (Cra) protein is a global sensor and regulator of carbon fluxes through the central metabolic pathways of gram-negative bacteria. To examine the nature of the effector (or effectors) that signal such fluxes to the protein of Pseudomonas putida, the Cra factor of this soil microorganism has been purified and characterized and its three-dimensional structure determined. Analytical ultracentrifugation, gel filtration, and mobility shift assays showed that the effector-free Cra is a dimer that binds an operator DNA sequence in the promoter region of the fruBKA cluster. Furthermore, fructose 1-phosphate (F1P) was found to most efficiently dissociate the Cra-DNA complex. Thermodynamic parameters of the F1P-Cra-DNA interaction calculated by isothermal titration calorimetry revealed that the factor associates tightly to the DNA sequence 5'-TTAAACGTTTCA-3' (K(D) = 26.3 ± 3.1 nM) and that F1P binds the protein with an apparent stoichiometry of 1.06 ± 0.06 molecules per Cra monomer and a K(D) of 209 ± 20 nM. Other possible effectors, like fructose 1,6-bisphosphate, did not display a significant affinity for the regulator under the assay conditions. Moreover, the structure of Cra and its co-crystal with F1P at a 2-Å resolution revealed that F1P fits optimally the geometry of the effector pocket. Our results thus single out F1P as the preferred metabolic effector of the Cra protein of P. putida.

Antigenic modules in the N-terminal S1 region of the transmissible gastroenteritis virus spike protein

The N-terminal S1 region of the transmissible gastroenteritis virus (TGEV) spike (S) glycoprotein contains four antigenic sites (C, B, D and A, from the N- to the C-terminal end) and is engaged in host-cell receptor recognition. The most N-terminal portion of the S1 region, which comprises antigenic sites C and B, is needed for the enteric tropism of TGEV, whereas the major antigenic site A at the C-terminal moiety is required for both respiratory and enteric cell tropism, and is engaged in recognition of the aminopeptidase N (APN) receptor. This study determined the kinetics for binding of a soluble S1 protein to the APN protein. Moreover, the S1 region of the TGEV S protein was dissected, with the aim of identifying discrete modules displaying unique antigenic sites and receptor-binding functions. Following protease treatments and mammalian cell expression methods, four modules or domains (D1-D4) were defined at the S1 region. Papain treatment identified an N-terminal domain (D1) resistant to proteolysis, whereas receptor binding defined a soluble and functional APN receptor-binding domain (D3). This domain was recognized by neutralizing antibodies belonging to the antigenic site A and therefore could be used as an immunogen for the prevention of viral infection. The organization of the four modules in the S1 region of the TGEV S glycoprotein is discussed.

Structure of the extracellular portion of CD46 provides insights into its interactions with complement proteins and pathogens

The human membrane cofactor protein (MCP, CD46) is a central component of the innate immune system. CD46 protects autologous cells from complement attack by binding to complement proteins C3b and C4b and serving as a cofactor for their cleavage. Recent data show that CD46 also plays a role in mediating acquired immune responses, and in triggering autophagy. In addition to these physiologic functions, a significant number of pathogens, including select adenoviruses, measles virus, human herpes virus 6 (HHV-6), Streptococci, and Neisseria, use CD46 as a cell attachment receptor. We have determined the crystal structure of the extracellular region of CD46 in complex with the human adenovirus type 11 fiber knob. Extracellular CD46 comprises four short consensus repeats (SCR1-SCR4) that form an elongated structure resembling a hockey stick, with a long shaft and a short blade. Domains SCR1, SCR2 and SCR3 are arranged in a nearly linear fashion. Unexpectedly, however, the structure reveals a profound bend between domains SCR3 and SCR4, which has implications for the interactions with ligands as well as the orientation of the protein at the cell surface. This bend can be attributed to an insertion of five hydrophobic residues in a SCR3 surface loop. Residues in this loop have been implicated in interactions with complement, indicating that the bend participates in binding to C3b and C4b. The structure provides an accurate framework for mapping all known ligand binding sites onto the surface of CD46, thereby advancing an understanding of how CD46 acts as a receptor for pathogens and physiologic ligands of the immune system.

The human membrane cofactor protein (MCP, CD46) is expressed on all nucleated cells and serves as a marker that prevents host cells from destruction by the immune system. It functions as a cofactor that helps to inactivate the C3b and C4b molecules, which are central components of the complement system. In addition to its role in regulation complement activation, CD46 is also used by a large number of pathogens, including measles virus and adenovirus, as a receptor to allow these pathogens to attach to the cell surface and initiate an infection. We have determined the three-dimensional structure of the bulk of the extracellular region of CD46 using X-ray crystallography. This structure provides detailed information about the location of previously identified residues that play a role in the interactions with C3b, C4b, and several pathogens, advancing an understanding of the function of the CD46 protein as a host and pathogen receptor. Moreover, the structure also reveals an unexpected, bent conformation of the protein that has implications for how the binding sites are presented at the cell surface.

T cell/transmembrane, Ig, and mucin-3 allelic variants differentially recognize phosphatidylserine and mediate phagocytosis of apoptotic cells

T cell/transmembrane, Ig, and mucin (TIM) proteins, identified using a congenic mouse model of asthma, critically regulate innate and adaptive immunity. TIM-1 and TIM-4 are receptors for phosphatidylserine (PtdSer), exposed on the surfaces of apoptotic cells. Herein, we show with structural and biological studies that TIM-3 is also a receptor for PtdSer that binds in a pocket on the N-terminal IgV domain in coordination with a calcium ion. The TIM-3/PtdSer structure is similar to that of TIM-4/PtdSer, reflecting a conserved PtdSer binding mode by TIM family members. Fibroblastic cells expressing mouse or human TIM-3 bound and phagocytosed apoptotic cells, with the BALB/c allelic variant of mouse TIM-3 showing a higher capacity than the congenic C.D2 Es-Hba-allelic variant. These functional differences were due to structural differences in the BC loop of the IgV domain of the TIM-3 polymorphic variants. In contrast to fibroblastic cells, T or B cells expressing TIM-3 formed conjugates with but failed to engulf apoptotic cells. Together these findings indicate that TIM-3-expressing cells can respond to apoptotic cells, but the consequence of TIM-3 engagement of PtdSer depends on the polymorphic variants of and type of cell expressing TIM-3. These findings establish a new paradigm for TIM proteins as PtdSer receptors and unify the function of the TIM gene family, which has been associated with asthma and autoimmunity and shown to modulate peripheral tolerance.

Crystallization and preliminary crystallographic analysis of the measles virus hemagglutinin in complex with the CD46 receptor

The measles virus (MV) hemagglutinin (MV-H) mediates the attachment of MV particles to cell-surface receptors for entry into host cells. MV uses two receptors for attachment to host cells: the complement-control protein CD46 and the signalling lymphocyte activation molecule (SLAM). The MV-H glycoprotein from an Edmonston MV variant and the MV-binding fragment of the CD46 receptor were overproduced in mammalian cells and used to crystallize an MV-H-CD46 complex. Well diffracting crystals containing two complexes in the asymmetric unit were obtained and the structure of the complex was solved by the molecular-replacement method.

Specificity switching in virus-receptor complexes

Several structures of complexes between viral attachment proteins and their cellular receptors have been determined recently, enhancing our understanding of the molecular recognition processes that guide formation of virus-receptor complexes. Moreover, these structures also highlight strategies by which highly similar viral proteins within a single virus family can adapt to engage different receptors. Consequences of such differences are altered tropism and pathogenicity. An improved understanding of the molecular details of this specificity switching in receptor binding will help to establish links between receptor tropism, spread, and disease. Moreover, it also has relevance for the design and use of viruses as gene delivery vehicles with altered properties as well as for the identification of target viral epitopes of new vaccines.

Structures of T cell immunoglobulin mucin protein 4 show a metal-Ion-dependent ligand binding site where phosphatidylserine binds

The T cell immunoglobulin and mucin domain (TIM) proteins are important regulators of T cell responses. Crystal structures of the murine TIM-4 identified a metal-ion-dependent ligand binding site (MILIBS) in the immunoglobulin (Ig) domain of the TIM family. The characteristic CC' loop of the TIM domain and the hydrophobic FG loop shaped a narrow cavity where acidic compounds penetrate and coordinate to a metal ion bound to conserved residues in the TIM proteins. The structure of phosphatidylserine bound to the Ig domain showed that the hydrophilic head penetrates into the MILIBS and coordinates with the metal ion, whereas the aromatic residues on the tip of the FG loop interacted with the fatty acid chains and could insert into the lipid bilayer. Our results also revealed an important role of the MILIBS in the trafficking of TIM-1 to the cell surface.

Structures of T Cell immunoglobulin mucin receptors 1 and 2 reveal mechanisms for regulation of immune responses by the TIM receptor family

The T cell immunoglobulin mucin (TIM) receptors are involved in the regulation of immune responses, autoimmunity, and allergy. Structures of the N-terminal ligand binding domain of the murine mTIM-1 and mTIM-2 receptors revealed an immunoglobulin (Ig) fold, with four Cys residues bridging a distinctive CC′ loop to the GFC β-sheet. The structures showed two ligand-recognition modes in the TIM family. The mTIM-1 structure identified a homophilic TIM-TIM adhesion interaction, whereas the mTIM-2 domain formed a dimer that prevented homophilic binding. Biochemical, mutational, and cell adhesion analyses confirmed the divergent ligand-binding modes revealed by the structures. Structural features characteristic of mTIM-1 appear conserved in human TIM-1, which also mediated homophilic interactions. The extracellular mucin domain enhanced binding through the Ig domain, modulating TIM receptor functions. These results explain the divergent immune functions described for the murine receptors and the role of TIM-1 as a cell adhesion receptor in renal regeneration and cancer.

Adenovirus type 11 binding alters the conformation of its receptor CD46

Adenoviruses (Ads) are important human pathogens and valuable gene delivery vehicles. We report here the crystal structure of the species B Ad11 knob complexed with the Ad11-binding region of its receptor CD46. The conformation of bound CD46 differs profoundly from its unbound state, with the bent surface structure straightened into an elongated rod. This mechanism of interaction is likely to be conserved among many pathogens that target CD46 or related molecules.

Methods for preparation of low abundance glycoproteins from mammalian cell supernatants

Proteins secreted to mammalian cell supernatants are usually in a low concentration and purity, due to the limitation of the expression systems or the presence of a large amount of contaminant proteins from the cell medium. So, initial protein recovery from cell supernatants requires of a highly specific chromatography step. We compared several purification methods based on affinity chromatography for purification of proteins from cell culture supernatants: metal chelate affinity, strep-tag and immunopurification with a monoclonal antibody. Soluble receptor glycoproteins were engineered with the corresponding peptide tag at their C-terminal end. The proteins were expressed in 293T cells and secreted to the cell supernatant, as monitored by sandwich ELISA. Supernatants were run through the different chromatography columns and several purification-related parameters determined. While all column-retained proteins were easily eluted, the chelating and immunopurification chromatography gave the highest yield and the latest method provided a sample with the highest purity. So, in spite of its cost, immunopurification chromatography gave optimal results for purification of a low abundance protein from a cell supernatant. Finally, we applied a protein expression system together with immunopurification chromatography for preparation of a glycoprotein for crystallization.

Rhinovirus-stabilizing activity of artificial VLDL-receptor variants defines a new mechanism for virus neutralization by soluble receptors

Members of the low-density lipoprotein receptor family possess various numbers of ligand binding repeats that non-equally contribute to binding of minor group human rhinoviruses. Using an artificial concatemer of five copies of repeat 3 of the human very-low density lipoprotein receptor, we demonstrate protection of HRV2 against low-pH mediated uncoating and inhibition of penetration of an RNA-specific fluorescent dye into the intact virion. This indicates that the recombinant receptor inhibits viral breathing and irreversible conformational modifications of the capsid that precede RNA release, providing a new mechanism for rhinovirus neutralization by soluble receptor molecules.

Contribution of N-Linked Glycans to the Conformation and Function of Intercellular Adhesion Molecules (ICAMs)

The crystal structures of the glycosylated N-terminal two domains of ICAM-1 and ICAM-2 provided a framework for understanding the role of glycosylation in the structure and function of intercellular adhesion molecules (ICAMs). The most conserved glycans were less flexible in the structures, interacting with protein residues and contributing to receptor folding and expression. The first N-linked glycan in ICAM-2 contacts an exposed tryptophan residue, defining a conserved glycan-W motif critical for the conformation of the integrin binding domain. The absence of this motif in human ICAM-1 exposes regions used in receptor dimerization and rhinovirus recognition. Experiments with soluble molecules having the N-terminal two domains of human ICAMs identified glycans of the high mannose type N-linked to the second domain of the dendritic cell-specific ICAM-grabbing nonintegrin lectin-ligands ICAM-2 and ICAM-3. About 40% of those receptor molecules bear endoglycosidase H sensitive glycans responsible of the lectin binding activity. High mannose glycans were absent in ICAM-1, which did not bind to the lectin, but they appeared in ICAM-1 mutants with additional N-linked glycosylation and lectin binding activity. N-Linked glycosylation regulate both conformation and immune related functions of ICAM receptors.

Receptor priming of major group human rhinoviruses for uncoating and entry at mild low-pH environments

Receptor priming of low-pH-triggered virus entry has been described for an enveloped virus (15). Here we show with major group human rhinoviruses (HRV) and its intercellular adhesion molecule-1 receptor that nonenveloped viruses follow this novel cell entry principle. In vitro the receptor primed HRV for efficient uncoating at mild low pH (5.5 to 6.0). Agents preventing endosomal acidification reduced or blocked rhinovirus cell infection, while nocodazole had no effect on infection of any serotype tested. The entry inhibitory effect of lysosomotropic agents was overcome by exposing cell-internalized HRV to mild low pH (5.5 to 6.0). We therefore conclude that receptor priming of major group HRV must occur in vivo as well. Cooperation of a cellular receptor and low pH in virus uncoating will polarize the exit of the genome to the receptor-bound, membrane-proximal region of the virus particle during acidification of endosomes. This process must be required for efficient penetration of the cellular membrane by viruses.

Structural analysis of human rhinovirus complexed with ICAM-1 reveals the dynamics of receptor-mediated virus uncoating

Intercellular adhesion molecule 1 (ICAM-1) functions as the cellular receptor for the major group of human rhinoviruses, being not only the target of viral attachment but also the mediator of viral uncoating. The configurations of HRV3-ICAM-1 complexes prepared both at 4 degrees C and physiological temperature (37 degrees C) were analyzed by cryoelectron microscopy and image reconstruction. The particle diameters of two complexes (with and without RNA) representing uncoating intermediates generated at 37 degrees C were each 4% larger than that of those prepared at 4 degrees C. The larger virus particle arose by an expansive movement of the capsid pentamers along the fivefold axis, which loosens interprotomer contacts, particularly at the canyon region where the ICAM-1 receptor bound. Particle expansion required receptor binding and preceded the egress of the viral RNA. These observations suggest that receptor-mediated uncoating could be a consequence of restrained capsid motion, where the bound receptors maintain the viral capsid in an expanded open state for subsequent genome release.

Distinct kinetics for binding of the CD46 and SLAM receptors to overlapping sites in the measles virus hemagglutinin protein

Measles virus (MV) is a human pathogen using two distinct cell surface receptors for entry into host cells. We present here a comparative analysis for binding of the MV receptors CD46 and SLAM to the measles virus hemagglutinin protein (MVH, Edmonston strain). Soluble monomeric and dimeric MVH variants were prepared in mammalian cells and their conformation assessed using a panel of monoclonal antibodies. The two receptor molecules specifically bound to the MVH protein with distinct binding modes. The association rate (k(a)) for SLAM binding to MVH was very low ( approximately 3000 m(-1)s(-1)), about 20 times lower that the k(a) determined for CD46 binding. However, SLAM bound tighter to the virus protein than CD46, as revealed by a 5-fold lower dissociation rate (k(d), approximately 1.5 x 10(-3) s(-1)). These data suggest that the SLAM receptor binds to a less accessible and more hydrophobic surface on MVH than the CD46 receptor, as illustrated in a binding model. Despite the differences in kinetics, receptor competition binding experiments revealed that they recognize overlapping sites in MVH. Indeed, a panel of anti-MVH monoclonal antibodies equally inhibited binding of both receptor molecules. The similar immune reactivity of the two receptor binding sites suggests that the shift in receptor usage by MV may not be driven by immune responses.

Distinct cellular receptor interactions in poliovirus and rhinoviruses

Receptor binding to human poliovirus type 1 (PV1/M) and the major group of human rhinoviruses (HRV) was studied comparatively to uncover the evolution of receptor recognition in picornaviruses. Surface plas- mon resonance showed receptor binding to PV1/M with faster association and dissociation rates than to HRV3 and HRV16, two serotypes that have similar binding kinetics. The faster rate for receptor association to PV1/M suggested a relatively more accessible binding site. Thermodynamics for receptor binding to the viruses and assays for receptor-mediated virus uncoating showed a more disruptive receptor interaction with PV1/M than with HRV3 or HRV16. Cryo-electron microscopy and image reconstruction of receptor-PV1/M complexes revealed receptor binding to the 'wall' of surface protrusions surrounding the 'canyon', a depressive surface in the capsid where the rhinovirus receptor binds. These data reveal more exposed receptor-binding sites in poliovirus than rhinoviruses, which are less protected from immune surveillance but more suited for receptor-mediated virus uncoating and entry at the cell surface.

Crystal structure of two CD46 domains reveals an extended measles virus-binding surface

Measles virus is a paramyxovirus which, like other members of the family such as respiratory syncytial virus, is a major cause of morbidity and mortality worldwide. The cell surface receptor for measles virus in humans is CD46, a complement cofactor. We report here the crystal structure at 3.1 A resolution of the measles virus-binding fragment of CD46. The structure reveals the architecture and spatial arrangement of two glycosylated short consensus repeats with a pronounced interdomain bend and some flexibility at the domain interface. Amino acids involved in measles virus binding define a large, glycan-free surface that extends from the top of the first to the bottom of the second repeat. The extended virus-binding surface of CD46 differs strikingly from those reported for the human virus receptor proteins CD4 and intercellular cell adhesion molecule-1 (ICAM-1), suggesting that the CD46 structure utilizes a novel mode of virus recognition. A highly hydrophobic and protruding loop at the base of the first repeat bears a critical virus-binding residue, thereby defining an important recognition epitope. Molecules that mimic the conformation of this loop potentially could be effective anti-viral agents by preventing binding of measles virus to CD46.

Lymphocyte function-associated antigen-1 binding residues in intercellular adhesion molecule-2 (ICAM-2) and the integrin binding surface in the ICAM subfamily

The crystal structure of intercellular adhesion molecule-2 (ICAM-2) revealed significant differences in the presentation of the critical acidic residue important for integrin binding between I and non-I-domain integrin ligands. Based on this crystal structure, we mutagenized ICAM-2 to localize the binding site for the integrin lymphocyte function-associated antigen-1 (LFA-1). The integrin binding site runs diagonally across the GFC beta-sheet and includes residues on the CD edge of the beta-sandwich. The site is oblong and runs along a flat ridge on the upper half of domain 1, which is proposed to dock to a groove in the I domain of LFA-1, with the critical Glu-37 residue ligating the Mg2+ in the I domain. Previous mutagenesis of ICAM-1 and ICAM-3, interpreted in light of the recently determined ICAM-1 and ICAM-2 structures, suggests similar binding sites. By contrast, major differences are seen with vascular cell adhesion molecule-1 (VCAM-1), which binds alpha4 integrins that lack an I domain. The binding site on VCAM-1 includes the lower portion of domain 1 and the upper part of domain 2, whereas the LFA-1 binding site on ICAM is confined to the upper part of domain 1.

The domain structure of ICAM-1 and the kinetics of binding to rhinovirus

Fragments of intercellular adhesion molecule 1 (ICAM- 1) containing only the two most N terminal of its five immunoglobulin SF domains bind to rhinovirus 3 with the same affinity and kinetics as a fragment with the entire extracellular domain. The fully active two-domain fragments contain 5 or 14 more residues than a previously described fragment that is only partially active. Comparison of X-ray crystal structures show differences at the bottom of domain 2. Four different glycoforms of ICAM- 1 bind with identical kinetics.

The structure of immunoglobulin superfamily domains 1 and 2 of MAdCAM-1 reveals novel features important for integrin recognition

BACKGROUND

Mucosal addressin cell adhesion molecule 1 (MAdCAM-1) is a cell adhesion molecule that is expressed on the endothelium in mucosa, and guides the specific homing of lymphocytes into mucosal tissues. MAdCAM-1 belongs to a subclass of the immunoglobulin superfamily (IgSF), the members of which are ligands for integrins. Human MAdCAM-1 has a unique dual function compared to other members in the same subclass in that it binds both the integrin alpha4beta7, through its two IgSF domains, and a selectin expressed on leukocytes, via carbohydrate sidechains. The structure determination of the two IgSF domains and comparison to the N-terminal two-domain structures of vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecules (ICAM-1 and ICAM-2) allow us to assess the molecular basis of the interactions between integrins and their preferred ligands.

RESULTS

The crystal structure of a fragment containing the two IgSF domains of human MAdCAM-1 has been determined to 2.2 A resolution. The structure of MAdCAM-1 reveals two separate integrin-recognition motifs. The key integrin-binding residue, Asp42, resides in the CD loop of domain 1; a buried arginine residue (Arg70) plays a critical role in maintaining the conformation of this loop. The second binding site is associated with an unusual long D strand in domain 2. The D and E strands extend beyond the main body of the domain, forming a negatively charged beta ribbon unique to MAdCAM-1. This ribbon is located on the same face as the key aspartate residue in domain 1, consistent with evidence that it is involved in integrin binding.

CONCLUSIONS

The structural comparison of MAdCAM-1 to other members of the same IgSF subclass reveals some interesting features. Firstly, MAdCAM-1, like VCAM-1, has the key integrin-binding residue located on the protruding CD loop of domain 1 and binds to an integrin that lacks an I domain. This is in contrast to ICAM-1 and ICAM-2 where the key residue is located at the end of the C strand on a flat surface and which bind to integrins that contain I domains. Secondly, architectural differences in the CD loops of MAdCAM-1 and VCAM-1 cause an 8 A shift in position of the critical aspartate residue, and may partly determine their binding preference for different integrins. Finally, the unusual charge distribution of the two-domain fragment of MAdCAM-1 is predicted to orient the molecule optimally for integrin binding on the top of its long mucin-like stalk.

A dimeric crystal structure for the N-terminal two domains of intercellular adhesion molecule-1

The 3.0-A structure of a 190-residue fragment of intercellular adhesion molecule-1 (ICAM-1, CD54) reveals two tandem Ig-superfamily (IgSF) domains. Each of two independent molecules dimerizes identically with a symmetry-related molecule over a hydrophobic interface on the BED sheet of domain 1, in agreement with dimerization of ICAM-1 on the cell surface. The residues that bind to the integrin LFA-1 are well oriented for bivalent binding in the dimer, with the critical Glu-34 residues pointing away from each other on the periphery. Residues that bind to rhinovirus are in the flexible BC and FG loops at the tip of domain 1, and these and the upper half of domain 1 are well exposed in the dimer for docking to virus. By contrast, a residue important for binding to Plasmodium falciparum-infected erythrocytes is in the dimer interface. The presence of A' strands in both domains 1 and 2, conserved hydrogen bonds at domain junctions, and elaborate hydrogen bond networks around the key integrin binding residues in domain 1 make these domains suited to resist tensile forces during adhesive interactions. A subdivision of the intermediate (I) set of IgSF domains is proposed in which domain 1 of ICAM-1 and previously described I set domains belong to the I1 set and domain 2 of ICAM-1, ICAM-2, and vascular cell adhesion molecule-1 belong to the I2 set.

Role of alpha L beta 2 integrin avidity in transendothelial chemotaxis of mononuclear cells

The leukocyte integrin alpha L beta 2 (LFA-1) is important in transendothelial migration. Since it is not fully understood how LFA-1 mediates transmigration, we studied the effects of alpha L and beta 2 cytoplasmic domain mutants that alter LFA-1 adhesiveness for intercellular adhesion molecule-1. Monocyte chemotactic protein-1 (MCP-1) induced LFA-1-dependent transendothelial migration of Jurkat and J-beta 2.7 transfectants coexpressing the MCP-1 receptor CCR2B and wild-type alpha L. No transendothelial chemotaxis was observed with truncation mutants of the alpha L cytoplasmic tail, which rendered LFA-1 constitutively active or locked LFA-1 in a low avidity state unresponsive to cellular activation. Moreover, transendothelial chemotaxis of lymphoblastoid SLA transfectants was abolished by truncation of the beta 2 cytoplasmic domain, but not by mutation of its TTT motif, which is important in phorbol ester-induced adhesion. These data indicate that transmigration may require both alpha L and beta 2 cytoplasmic domains. We further show that MCP-1-induced transendothelial chemotaxis of PBMC was inhibited by sustained activation of LFA-1 with Mn2+ or a stimulatory mAb to beta 2. Dimeric soluble intercellular adhesion molecule-1 also reduced transendothelial chemotaxis of PBMC. Taken together, our data suggest that transendothelial chemotaxis of mononuclear cells may involve dynamic changes in LFA-1 avidity.

Role of alpha L beta 2 integrin avidity in transendothelial chemotaxis of mononuclear cells

The leukocyte integrin alpha L beta 2 (LFA-1) is important in transendothelial migration. Since it is not fully understood how LFA-1 mediates transmigration, we studied the effects of alpha L and beta 2 cytoplasmic domain mutants that alter LFA-1 adhesiveness for intercellular adhesion molecule-1. Monocyte chemotactic protein-1 (MCP-1) induced LFA-1-dependent transendothelial migration of Jurkat and J-beta 2.7 transfectants coexpressing the MCP-1 receptor CCR2B and wild-type alpha L. No transendothelial chemotaxis was observed with truncation mutants of the alpha L cytoplasmic tail, which rendered LFA-1 constitutively active or locked LFA-1 in a low avidity state unresponsive to cellular activation. Moreover, transendothelial chemotaxis of lymphoblastoid SLA transfectants was abolished by truncation of the beta 2 cytoplasmic domain, but not by mutation of its TTT motif, which is important in phorbol ester-induced adhesion. These data indicate that transmigration may require both alpha L and beta 2 cytoplasmic domains. We further show that MCP-1-induced transendothelial chemotaxis of PBMC was inhibited by sustained activation of LFA-1 with Mn2+ or a stimulatory mAb to beta 2. Dimeric soluble intercellular adhesion molecule-1 also reduced transendothelial chemotaxis of PBMC. Taken together, our data suggest that transendothelial chemotaxis of mononuclear cells may involve dynamic changes in LFA-1 avidity.

Crystal structure of ICAM-2 reveals a distinctive integrin recognition surface

Recognition by integrin proteins on the cell surface regulates the adhesive interactions between cells and their surroundings. The structure of the 'I' domain that is found in some but not all integrins, has been determined. However, the only integrin ligands for which structures are known, namely fibronectin and VCAM-1, are recognized by integrins that lack I domains. The intercellular adhesion molecules ICAM-1, 2 and 3 are, like VCAM-1, members of the immunoglobulin superfamily (IgSF), but they are recognized by an I domain-containing integrin, lymphocyte-function-associated antigen 1 (LFA-1, or CD11a/CD18). Here we present the crystal structure of the extracellular region of ICAM-2. The glutamic acid residue at position 37 is critical for LFA-1 binding and is proposed to coordinate the Mg2+ ion in the I domain; this Glu 37 is surrounded by a relatively flat recognition surface and lies in a beta-strand, whereas the critical aspartic acid residue in VCAM-1 and fibronectin lie in protruding loops. This finding suggests that there are differences in the architecture of recognition sites between integrins that contain or lack I domains. A bend between domains 1 and 2 of ICAM-2 and a tripod-like arrangement of N-linked glycans in the membrane-proximal region of domain 2 may be important for presenting the recognition surface to LFA-1. A model of ICAM-1 based on the ICAM-2 structure provides a framework for understanding its recognition by pathogens.

A highly efficacious lymphocyte chemoattractant, stromal cell-derived factor 1 (SDF-1)

Chemotactic factors are postulated to direct emigration of lymphocytes from the blood stream into sites of inflammation. Members of a family of chemotactic cytokines, termed chemokines, have been shown to attract lymphocytes but efficacy, i.e., the maximal percentage of attracted cells, has been low. We have identified a highly efficacious lymphocyte chemotactic activity in the supernatants of the murine bone marrow stroma cell line MS-5 which attracts 10-fold more lymphocytes in vitro than currently described lymphocyte chemoattractants. Purification of this chemotactic activity revealed identity to stromal cell-derived factor 1 (SDF-1). SDF-1 acts on lymphocytes and monocytes but not neutrophils in vitro and is both a highly efficacious and highly potent mononuclear cell attractant in vivo. In addition, SDF-1 induces intracellular actin polymerization in lymphocytes, a process that is thought to be a prerequisite for cell motility. Since SDF-1 is expressed constitutively in a broad range of tissues it may have a role in immune surveillance and in basal extravasation of lymphocytes and monocytes rather than in inflammation.

Kinetics and thermodynamics of virus binding to receptor. Studies with rhinovirus, intercellular adhesion molecule-1 (ICAM-1), and surface plasmon resonance

We have studied the kinetics and thermodynamics of a virus interacting with its receptor using human rhinovirus serotype 3 (HRV3), soluble intercellular adhesion molecule-1 (ICAM-1, CD54) containing Ig superfamily domains 1-5 (sICAM-1), and surface plasmon resonance. There were two classes of binding sites for sICAM-1 on HRV3, each comprising about 50% of the total sites, with association rate constants of 2450 +/- 300 and 134 +/- 11 M-1 s-1. These rates are low, consistent with binding to a relatively inaccessible site in the rhinovirus canyon. By contrast, three monoclonal antibodies bound to sICAM-1 with a single rate constant of 17,000-48,000 M-1 s-1. The dissociation rate constant for HRV3 was 1.7 +/- 0.1 x 10(-3) s-1, giving calculated dissociation constants of 0.7 +/- 0.1 and 12.5 +/- 1.2 microM. Agreement was good with saturation binding in solution, which showed two sites of similar abundance with KD of 0.55 +/- 0.2 and 5.7 +/- 2.0 microM. A bivalent chimera of ICAM-1 with the IgA1 Fc region bound with KD = 50 and 410 nM, showing 17-fold enhanced affinity. Lowering pH from 8.0 to 6.0 reduced affinity by approximately 50-fold, primarily by reducing the on rate. Thermodynamic measurements showed that binding of ICAM-1 to HRV3 is endothermic, by contrast to binding to monoclonal antibody. The heat that is absorbed of 3.5 and 6.3 kcal/mol for the two classes of ICAM-1 binding sites may contribute to receptor-mediated disruption of virions, which has an activation energy of about 42 kcal/mol.

Introduction of a loss-of-function point mutation from the SH3 region of the Caenorhabditis elegans sem-5 gene activates the transforming ability of c-abl in vivo and abolishes binding of proline-rich ligands in vitro

We have introduced two loss-of-function point mutations from highly conserved regions of the src homology 3 (SH3) domains of the Caenorhabditis elegans sem-5 gene into the SH3 domain of the murine type IV c-abl tyrosine kinase proto-oncogene. One of the mutations, P131L, activated abl to transform fibroblasts while the other, G128R, did not. When combined with independent activating mutations in the c-abl kinase domain or NH2-terminus, the G128R mutation blocked transformation by the double mutant, suggesting that the G128R mutant was unable to transform cells for trivial reasons. The c-Abl G128R mutant, like wild type c-Abl protein, was localized to the nucleus and actin cytoskeleton and had normal tyrosine kinase activity in vitro, while the transforming c-Abl P131L protein was localized exclusively to the cytoplasm and exhibited decreased in vitro kinase activity. By real-time biospecific interaction analysis, the wild type Abl SH3 domain bound to two proteins containing proline-rich motifs with dissociation constants of 0.2 and 17 microM; the G128R mutant bound with 50-fold lower affinity, and no binding was detected by the P131L mutant. Both mutations completely abolished binding of the Abl SH3 domain to proline-rich target proteins in a filter-binding assay. These results suggest that the transforming activity of Abl is regulated in vivo by an inhibitor protein which associates with the SH3 domain via a proline-rich sequence.

Pathway of rhinovirus disruption by soluble intercellular adhesion molecule 1 (ICAM-1): an intermediate in which ICAM-1 is bound and RNA is released

We have examined the pathway of rhinovirus interaction with soluble intercellular adhesion molecule 1 (sICAM-1). Binding of sICAM-1 to rhinovirus serotypes 3 and 14 gives particles with sedimentation coefficients from 145 to 120S, depending on the amount of sICAM-1 bound. The formation of 120S particles is faster and more extensive at a neutral pH than at an acidic pH. A large number of receptors (> 30) can bind to human rhinovirus 3 without disruption. Disruption by sICAM-1 of rhinovirus that yields 80S particles is strongly temperature dependent and is antagonized by a low pH. Interestingly, sICAM-1 remains bound to the viral capsid after RNA is released, although in smaller amounts than those observed for the native virus. We have found heterogeneity both between and within 80S particle preparations in the VP4 content and number of bound receptors. The ability of the virus to remain bound to its receptor during the uncoating process may facilitate the transport of the viral genome into the cytoplasm in vivo.

Structural polymorphism of d(GA.TC)n DNA sequences. Intramolecular and intermolecular associations of the individual strands

Alternating d(GA.TC)n sequences are highly structurally polymorphic. Most of their conformational flexibility is likely to reside in the structural properties of the individual strands themselves. In this paper the conformational behaviour of the d(GA)20 and d(TC)20 oligonucleotides was analysed. Formation of d(GA)20 intramolecular duplexes is observed at any pH value, from 8.3 to 4.6. On the other hand, intramolecular d(TC)20 duplexes are formed only under acidic conditions. The acid d(TC)20 intramolecular duplex is likely to be stabilized through the formation of C+C pairs, the thymine residues remaining unpaired. The d(GA)20 oligonucleotide also forms intermolecular duplexes which coexist with the intramolecular forms at any pH, from 8.3 to 4.6. The structural conformation adopted by the d(TC)20 oligonucleotide at neutral pH is uncertain. Under these conditions, this oligonucleotide shows an electrophoretic apparent molecular weight consistent with the formation of a bimolecular complex. However, no hydrogen bonding was observed to occur under these conditions. Implications of these results for an understanding of the molecular principles behind the conformational flexibility of alternating d(GA.TC)n sequences are discussed. The possible biological significance of these results is also discussed.

Alternating d(GA)n DNA sequences form antiparallel stranded homoduplexes stabilized by the formation of G.A base pairs

Alternating d(GA)n DNA sequences form antiparallel stranded homoduplexes which are stabilized by the formation of G.A pairs. Three base pairings are known to occur between adenine and guanine: AH+ (anti).G(syn), A(anti).G(anti) and A(syn).G(anti). Protonation of the adenine residues is not involved in the stabilization of this structure, since it is observed at any pH value from 8.3 to 4.5; at pH < or = 4.0 antiparallel stranded d(GA.GA) DNA is destabilized. The results reported in this paper strongly suggest that antiparallel stranded d(GA.GA) homoduplexes are stabilized by the formation of alternating A(anti).G(anti) and G(anti).A(syn) pairs. In this structure, all guanine residues are in the anti conformation with their N7 position freely accessible to DMS methylation. On the other hand, adenines in one strand adopt the anti conformation, with their N7 position also free for reaction, while those of the opposite strand are in the syn conformation, with their N7 position hydrogen bonded to the guanine N1 group of the opposite strand. A regular right-handed helix can be generated using alternating G(anti).A(syn) and A(anti).G(anti) pairs.

Efficient neutralization and disruption of rhinovirus by chimeric ICAM-1/immunoglobulin molecules

The intercellular adhesion molecule 1 (ICAM-1) is used as a cellular receptor by 90% of human rhinoviruses (HRVs). Chimeric immunoadhesin molecules containing extracellular domains of ICAM-1 and constant regions of immunoglobulins (Igs) were designed in order to determine the effect of increased valency, Ig isotype, and number of ICAM-1 domains on neutralization and disruption of rhinovirus structure. These immunoadhesins include ICAM-1 amino-terminal domains 1 and 2 fused to the hinge and constant domains of the heavy chains of IgA1, IgM, and IgG1 (IC1-2D/IgA, -/IgM, and -/IgG). In addition, all five extracellular domains were fused to IgA1 (IC1-5D/IgA). Immunoadhesins were compared with soluble forms of ICAM-1 containing five and two domains (sICAM-1 and ICI-2D, respectively) in assays of HRV binding, infectivity, and conformation. In prevention of HRV plaque formation, IC1-5D/IgA was 200 times and IC1-2D/IgM and IC1-2D/IgA were 25 and 10 times more effective, respectively, than ICAM-1. The same chimeras were highly effective in inhibiting binding of rhinovirus to cells and disrupting the conformation of the virus capsid, as demonstrated by generation of approximately 65S particles. The results show that the number of ICAM-1 domains and a flexible Ig hinge are important factors contributing to the efficacy of neutralization. The higher efficiency of chimeras that bound bivalently in disrupting HRV was attributed to higher binding avidity. The IC1-5D/IgA immunoadhesin was effective at nanomolar concentrations, making it feasible therapy for rhinovirus infection.