Virus sensitization

Epitope location in the Cryptococcus neoformans capsule is a determinant of antibody efficacy

Monoclonal antibodies (mAbs) to the polysaccharide capsule of Cryptococcus neoformans can prolong survival in mice. However, the properties of antibodies that mediate protection are not fully understood. The IgM mAbs 12A1 and 13F1 originated from the same B cell and differ only by somatic mutations in their variable regions; yet mAb 12A1 protects against serotype D infection, while mAb 13F1 does not. Phage peptide display libraries were used to analyze the fine specificity of these two mAbs. The selection of distinct peptide motifs from identical libraries confirmed that mAbs 12A1 and 13F1 bound to two distinct epitopes. Immunofluorescence and immunoelectron microscopy studies revealed differences in antibody localization within the capsule of serotype D strain; mAb 12A1 bound to the outer rim of the capsule resulting in an annular pattern, whereas mAb 13F1 bound throughout the capsule and had a punctate appearance. The difference in the binding pattern of mAb 12A1 and 13F1 was not observed on serotype A organisms, where both mAbs bound to the capsule with an annular fluorescence pattern. The fluorescence pattern of binding correlated with protective efficacy; mAb 13F1 prolonged survival of mice infected with the J11 serotype A strain (annular fluorescence), but not serotype D strains (punctate pattern). Annular binding, but not punctate binding, was associated with increased opsonic efficacy for phagocytosis of C. neoformans by J774.16 macrophage-like cells. The correlation between capsular binding pattern, opsonic activity, and ability to prolong survival suggests that the efficacy of anticryptococcal antibodies is dependent upon where they bind in the polysaccharide capsule.

Postabsorption neutralization of poliovirus

Nineteen neutralizing murine monoclonal antibodies against poliovirus type 1, including representatives reacting with each of the antigenic sites on the virion, were tested for their abilities to neutralize the virus either before or after attachment to susceptible cells. All antibodies neutralized unattached virus; six had reasonable titers of postabsorption neutralization (PAN). Experiments with antibodies lacking PAN activity showed that Fc-specific rabbit anti-mouse antibodies could confer PAN activity. PAN was shown to involve the prevention of the cell-mediated conversion of virus to 135S and 80S particles. Evidence that one of the PAN-positive antibodies probably bound bivalently to preabsorbed virions, whereas a PAN-negative antibody bound monovalently, is presented. Two PAN-positive antibodies were added to an excess of virus in suspension, and only one antibody caused the virus to aggregate.

Neutralization of adenoviruses: kinetics, stoichiometry, and mechanisms

Kinetic curves for neutralization of adenovirus type 2 with anti-hexon serum revealed no lag periods even when the serum was highly diluted or when the temperature was lowered to 4 degrees C, thus indicating a single-hit mechanism. Multiplicity curves determined with anti-hexon serum displayed a linear correlation between the degree of neutralization and dilution of antiserum. Neutralization values experimentally obtained under steady-state conditions fully fitted a single-hit model based on Poisson calculations. Quantitation of the amount of 125I-labeled type-specific anti-hexon antibodies needed for full neutralization of adenovirus showed that 1.4 antibodies were attached per virion under such conditions. Virions already attached to HeLa cells at 4 degrees C were, to a large extent, neutralizable by anti-hexon serum, whereas anti-fiber and anti-penton base antisera were negative. It is suggested that adenovirus may be neutralized by two pathways: aggregation of the virions (extracellular neutralization) as performed by anti-fiber antibodies and blocking of virion entrance from the acidic endosomes into the cytoplasm (intracellular neutralization). The latter effect could be obtained by (i) covering of the penton bases, as performed by anti-penton base antibodies, thereby preventing interaction between the penton bases and the endosomal membrane, which results in trapping of virions within endosomes, and (ii) inhibition of the low-pH-induced conformational change of the viral capsid, which seems to occur in the endosomes and is necessary for proper exposure of the penton bases, as performed by anti-hexon antibodies.

Augmentation of the neutralisation test for type 1 HSV: evidence of high representation of neutralising antibody in the adult community

Optimal neutralisation of type 1 herpes simplex virus was obtained by reacting undiluted human serum with virus for 4 h at 37 degrees C, followed by addition of antihuman globulin for 20 min; under these conditions it was possible to detect neutralising antibody activity in 40 of 45 human sera (88%) previously adjudged to be negative by conventional neutralisation tests.

Macrophage Fc receptors control infectivity and neutralization of canine distemper virus-antibody complexes

Dogs that are persistently infected or that become moribund after exposure to canine distemper virus (CDV) have antibody that neutralized CDV when tested in dog lung macrophage cultures but failed to neutralize CDV when tested in epithelial, fibroblastic, or lymphatic cells. The antibody attached to protein A and was found in the immunoglobulin G fraction. The antibody bound complement and lysed CDV-infected target cells. The neutralizing activity in macrophages could be abolished (i) by pepsin digestion and removal of Fc portions from the antibody, (ii) by blocking the Fc receptors of macrophages with heat-treated normal dog serum, and (iii) by binding of protein A to Fc portions of the antibody. It was concluded that attachment of the CDV-antibody complex to Fc receptors of macrophages was essential for virus neutralization. If this attachment was hindered, the CDV-antibody complex became infectious for macrophages. In contrast, serum from recovering dogs neutralized CDV when tested in epithelial, fibroblastic, or lymphatic cells as well as in macrophages.

Murine monoclonal antibody to a single protein neutralizes the infectivity of human cytomegalovirus

Murine monoclonal antibodies to human cytomegalovirus (CMV) strain AD169 were selected that neutralized virus infectivity. One monoclonal antibody-producing hybridoma, 1G6, was used to produce ascites fluid from which immunoglobulin was isolated. This antibody efficiently neutralized CMV AD169, other laboratory strains (Towne, Davis), and clinical isolates of CMV in early tissue culture passage (less than 10) in the absence of complement. The antibody immunoprecipitated a single 86,000-dalton protein from both laboratory and clinical strains. This viral protein was demonstrated by indirect immunofluorescence to be localized in the cytoplasm of CMV-infected cells.

Neutralizing antibody response of rabbits and goats to caprine arthritis-encephalitis virus

Rabbits were immunized with purified caprine arthritis-encephalitis virus and examined for neutralizing activity. Analysis of virus-antiserum interaction at 37 degrees C demonstrated little loss of viral infectivity after incubation with heat-inactivated rabbit antiserum for 60 min. However, sensitization of virus (as assessed by the addition of complement) occurred almost immediately and was 95% complete after 10 min. The complement-dependent neutralizing activity was associated with the immunoglobulin G fraction of rabbit antiserum. Addition of goat anti-rabbit immunoglobulin G to the immune rabbit serum-caprine arthritis-encephalitis virus mixture also resulted in neutralization of infectivity when unbound antibody was removed before addition of the anti-immunoglobulin. Serum from most caprine arthritis-encephalitis virus-infected goats contains antibody activity to the core protein p28, as demonstrated by immunodiffusion and enzyme-linked immunosorbent assay. However, attempts to demonstrate neutralizing activity in the serum of goats up to 1.5 years post-inoculation or in serum of hyperimmunized goats were unsuccessful when the sera were examined alone or in combination with complement or rabbit anti-goat immunoglobulin or both.

Monoclonal antibodies to the glycoprotein of vesicular stomatitis virus: comparative neutralizing activity

Nineteen independently isolated hybridomas producing monoclonal antibodies to the glycoprotein of vesicular stomatitis virus were isolated and studied for their capacity to neutralize viral infectivity. By measuring competitive binding of 125I-labeled monoclonal antibodies in a radioimmunoassay. 11 different, non-cross-reacting antigenic determinants were identified on the vesicular stomatitis virus G protein. All monoclonal antibodies reacting with determinants 1, 2, 3, and 4 resulted in viral neutralization, whereas those binding to the other seven determinants did not neutralize infectivity. The mixture of two monoclonal antibodies binding to different determinants resulted in a more rapid neutralization than either antibody alone, suggesting that different antibodies can exert a synergistic effect on viral neutralization. Kinetic experiments revealed biphasic neutralization curves similar to those expected for heterologous antibody. No evidence could be obtained to relate biphasic kinetics of viral neutralization to heterogeneous populations either of antibody molecules or of virus. The possible significance of the kinetic data with monoclonal antibodies is discussed.

Neutralization and sensitization of cytomegalovirus by IgG antibody, anti-IgG antibody, and complement

Human cytomegalovirus (CMV) strain Ad 169 was reacted with IgG antibody obtained from infected renal transplant patients, and the degree of neutralization was determined. The mixture of antibody and virus was then incubated with anti-human IgG (A-IgG) or complement (C) to measure additional neutralization by these agents and thus to estimate the concentration of infectious virus-antibody complexes which had been formed. Neutralization and formation of infectious complexes susceptible to neutralization by A-IgG or C decreased with lower concentrations of antibody. The rate of neutralization of preformed complexes by A-IgG or C was more rapid than the formation of complexes or the neutralization of native virus. If equally infectious suspensions of native virus and virus plus antibody were challenged with additional CMV antibody both solutions were neutralized to the same degree. The data are most compatible with the suggestion that the formation of infectious virus-antibody complexes is the initial step in the neutralization process. These complexes do not appear to be protected from further neutralization.

Viral epitopes and monoclonal antibodies: isolation of blocking antibodies that inhibit virus neutralization

The inability of pathogenic animal viruses to be completely neutralized by antibodies can lead to chronic viral infections in which infectious virus persists even in the presence of excess neutralizing antibody. A mechanism that results in this nonneutralized fraction of virus was defined by the topographical relationships of viral epitopes identified with monoclonal antibodies wherein monoclonal antibodies bind to virus and sterically block the binding of neutralizing antibodies.

Comparison of fluorescent-antibody, neutralizing-antibody, and complement-enhanced neutralizing-antibody assays for detection of serum antibody to respiratory syncytial virus

A comparison of three assays for the detection of serum antibody to respiratory syncytial virus (RSV) was carried out on 47 serum samples obtained sequentially from infants and young children with RSV infection. Neutralizing-antibody (NA) activity was determined by a semimicromethod of plaque reduction. Complement-enhanced NA activity was determined by the addition of guinea pig complement to NA assays. RSV antibody responses in immunoglobulin G, immunoglobulin M, and immunoglobulin A classes were determined by using indirect immunofluorescence techniques for fluorescent-antibody (FAb) assay. Antibody to RSV was detectable by all three techniques as early as 4 days after the onset of illness. At all phases of illness, titers obtained by complement-enhanced NA assays were significantly greater than those obtained by NA or FAb assays (P less than 0.01). RSV-FAb titers determined in the immunoglobulin G class correlated well with those determined by complement-enhanced NA or NA assays. The data suggest that the FAb assay for detection of RSV antibody in serum is somewhat less sensitive but also less laborious and more rapid than NA assays.

Enhanced antiglobulin-mediated neutralization of herpes simplex virus-IgG complexes by complement and heterologous anti-immunoglobulin

The ability of IgG anti-Fc and anti-Fab to neutralize infectious herpes simplex virus-IgG (HSV-IgG) complexes was determined. When limiting amounts of antiglobulin were used, antibody directed against the Fab portion of human IgG was significantly more effective than anti-Fc antibodies in neutralizing the HSV-IgG complexes. The detection of viral bound antibody was enhanced by the incorporation of heterologous antiglobulin or complement in the antiglobulin neutralization test. Specifically, HSV-IgG which had been incubated with rabbit antihuman globulin was further neutralized by goat antirabbit IgG or guinea pig serum complement. This augmented neutralization test could prove useful in detecting small amounts of antibody bound to virus in infectious isolates from patients or experimental animals with viral diseases.

Neutralization of animal viruses

Various aspects of the interaction of bacterial viruses and antibody were studied by Andrewes and Elford in England. Similar studies, as well as studies on animal viruses, were carried out in Australia by Burnet and his colleagues. One result of their extensive studies, which were summarized in great detail, was the conclusion that, with respect to their interaction with antibody, bacterial and animal viruses were basically different. Specifically, the difference resided in the stability of the union of virus and antibody, whereas bacterial viruses formed stable complexes, animal viruses formed complexes that tended to dissociate readily. The introduction of animal cell cultures as host systems greatly aided in the study of animal viruses, with respect to fewer and more readily controlled variables, and by the use of the plaque assay in enhanced quantitative reliability. In 1956, Dulbecco et al. described the interaction of two animal viruses with their respective antibodies. The results of these studies led these investigators to conclude, among other things, that animal viruses, at least the two they studied, reacted with antibodies to form complexes that did not dissociate spontaneously. This interpretation was challenged by Fazekas de St. Groth and Reid. As more animal virus-antibody systems were studied by many investigators, there seemed to be a greater accord for irreversible, rather than reversible, interaction. For this reason, in this chapter it is assumed that there are no differences between bacterial viruses, as one category, and animal viruses, as a separate category, concerning their interaction with antibodies. Rather, differences, when they exist, are considered to be related to the viruses per se. Although this chapter is intended to survey the neutralization of animal viruses, occasional reference is made to the studies on bacterial viruses when these studies are pertinent and illuminating to the topic at hand.

Infectious virus-antibody complexes of sindbis virus

Infectious virus-antibody complexes were formed when Sindbis virus was reacted with antibodies raised against purified viral envelope glycoproteins E1 and E2 as well as against preparations of intact virus. Results from rate zonal centrifugation in sucrose gradients of the complex formed with anti-E1 sera showed this complex to be about the same size as virions. A test of virus neutralization, based on direct plaque assay, by antibodies raised in rabbits and mice given virus in complete Freund adjuvant indicated the presence of antibodies able to complex but not neutralize virus. Conditions were found in which most of the virus was complexed and protected fron neutralization, suggesting that these sera may contain a mixed population of antiviral antibodies with different specificities and different avidities.

Shedding of infectious virus/antibody complexes from vesicular lesions of patients with recurrent herpes labialis

The concentration of herpes-simplex virus (H.S.V.) in the lesions of adults with recurrent herpes labialis was determined. On the 1st day the vesicle appeared, the fluid within the lesion contained 10(5-3) plaque-forming units (P.F.U.) of H.S.V./mul. By swabbing the surface of the lesions with a sterile pledget, 10(6-2) P.F.U. of virus was isolated from the inflamed labial mucosa. The amount of virus obtained from the labial surface decreased on the 2nd and 3rd day to 10(5-0) and 10(3-0) P.F.U., respectively. In two patients on immunosuppressive drugs, high concentrations of virus (greater than 10(4-0) P.F.U.) were obtained per swab for more than 3 weeks. The presence of infectious virus-antibody (V.A.) complexes in herpetic lesions was demonstrated by examining fifty-two isolates from twenty-eight patients at various times during the course of their disease. 71% of the patients had V.A. complexes in their lesions on the 1st day of the vesicular eruption, and by the 3rd day all of the lesions examined contained complexes. It is concluded that patients with active lesions shed high concentrations of virus and that natural infection may be transmitted by an infectious V.A. complex.