STUDIES ON THE NEUTRALIZATION OF HERPES SIMPLEX VIRUS. I. APPEARANCE OF NEUTRALIZING ANTIBODIES HAVING DIFFERENT GRADES OF COMPLEMENT REQUIREMENT

Functions of Antibodies

Antibodies can impact pathogens in the presence or in the absence of effector cells or effector molecules such as complement, and experiments can often sort out with precision the mechanisms by which an antibody inhibits a pathogen in vitro . In addition, in vivo models, particularly those engineered to knock in or knock out effector cells or effector molecules, are excellent tools for understanding antibody functions. However, it is highly likely that multiple antibody functions occur simultaneously or sequentially in the presence of an infecting organism in vivo . The most critical incentive for measuring antibody functions is to provide a basis for vaccine development and for the development of therapeutic antibodies. In this respect, some functions, such as virus neutralization, serve to inhibit the acquisition of a pathogen or limit its pathogenesis. However, antibodies can also enhance replication or contribute to pathogenesis. This review emphasizes those antibody functions that are potentially beneficial to the host. In addition, this review will focus on the effects of antibodies on organisms themselves, rather than on the toxins the organisms may produce.

IgG fc receptors provide an alternative infection route for murine gamma-herpesvirus-68

Background

Herpesviruses can be neutralized in vitro but remain infectious in immune hosts. One difference between these settings is the availability of immunoglobulin Fc receptors. The question therefore arises whether a herpesvirus exposed to apparently neutralizing antibody can still infect Fc receptor⁺ cells.

Principal Findings

Immune sera blocked murine gamma-herpesvirus-68 (MHV-68) infection of fibroblasts, but failed to block and even enhanced its infection of macrophages and dendritic cells. Viral glycoprotein-specific monoclonal antibodies also enhanced infection. MHV-68 appeared to be predominantly latent in macrophages regardless of whether Fc receptors were engaged, but the infection was not abortive and new virus production soon overwhelmed infected cultures. Lytically infected macrophages down-regulated MHC class I-restricted antigen presentation, endocytosis and their response to LPS.

Conclusions

IgG Fc receptors limit the neutralization of gamma-herpesviruses such as MHV-68.

Lansing poliovirus infection in mice: antibody demonstrable by enzyme-linked immunosorbent assay (ELISA) and immunoprecipitation but not by neutralization

Adult mice infected intracerebrally (i.c) with the Lansing strain of type 2 human poliovirus (HPV2) failed to develop a systemic neutralizing antibody response until 2 months post-infection (p.i). In contrast, an enzyme-linked immunosorbent assay (ELISA) demonstrated an antibody response of IgM and IgG classes beginning at day 1 p.i. with peak levels reached by 5 weeks p.i. This response was slightly greater in paralyzed than in nonparalyzed animals. Immunoprecipitation of poliovirus proteins from cytoplasmic extracts and disrupted purified virion preparations revealed antibodies to three capsid proteins, two capsid precursor proteins, and one nonstructural protein. Finally, neither neutralizing antibody nor definite virus replication was detected after oral, intraperitoneal, or intravenous routes of inoculation. We conclude that the lack of a systemic neutralizing antibody response in mice is probably due to an insufficient amount of infectious virus and consequently viral neutralizing epitopes reaching extraneural lymphoid tissues.

Selection of unique antigenic variants of Newcastle disease virus with neutralizing monoclonal antibodies and anti-immunoglobulin

Monoclonal antibodies were used to isolate nonneutralizable antigenic variants in the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus. It had been found that a large percentage of virus retains infectivity despite binding neutralizing antibody. This high persistent fraction of nonneutralized virus precluded the isolation of variants by the standard treatment with antibody alone. Rabbit anti-mouse immunoglobulin was used to reduce the percentage of virus that remains infectious despite the presence of bound antibody. This procedure made possible the isolation of variants of two distinct types: classical variants, not neutralized because they do not bind the antibody used to select them; and unique variants that, although still capable of binding the selecting antibody, are only slightly neutralized. The general applicability of this method for the isolation of antigenic variants in nonneutralizing epitopes is also discussed.

A micro-neutralization system for detection of s-CRN antibody to herpes simplex virus

It was learned that the ordinary micro-neutralization system with herpes simplex virus (HSV) gave a composite result of the initial neutralization and the effect of antibody on subsequent growth of unneutralized virus. In the case of slow-reacting complement-requiring neutralizing (s-CRN) antibody, which was detected by incubating virus-serum mixtures at 4 C for 3 days before addition of complement, the titer obtained was lower than expected from the result of the plaque reduction test. This was thought ascribable to its low ability to prevent viral breakthrough caused by growth of unneutralized virus. This was overcome by adding an appropriate amount of hyperimmune antibody at 3 hr after addition of cells. The endpoint of s-CRN antibody so determined was but slightly lower than that obtained by the plaque reduction test. Early (1-week) rabbit sera, which were negative in the ordinary micro-neutralization test, titered 1:2,560 to 1:5,120 when tested by this method. When the 3-day sensitization in the cold was substituted by 5-hr incubation at 37 C, the titer obtained was 2 to 4-fold lower; in this case, however, the whole process could be finished within 3 days. Also, s-CRN antibody reactive with type 2 HSV in homologous and heterologous sera could be detected by the same method using type 1 hyperimmune serum as the additional antibody.

Studies on the role of humoral and cell-mediated immunity in pigs after vaccination with Aujeszky's disease virus

Humoral and cellular immunity in pigs vaccinated twice with Aujeszky's disease virus (ADV) was studied by seroneutralizing test and direct leucocyte migration inhibition technique. Significant migration inhibition of leucocytes (LMI) was found on the fifth day, whereas specific antibodies began to appear at that time only in very low titers. Anamnestic reaction due to the second injection of ADV did not bring about a significant increase of migration inhibition of leucocytes, instead the level of antibodies elevated markedly.

Studies on the neutralization of herpes simplex virus. XI. Differences between slow-reacting complement-requiring neutralizing (s-CRN) antibodies in IgG and IgM

Late IgG and IgM from a rabbit immunized with herpes virus were tested for ordinary neutralizing (N) antibody, complement-requiring neutralizing (CRN) antibody and in addition CRN antibody detectable by overnight sensitization at 0 C (s-CRN antibody). Heat stability tests showed that IgG s-CRN antibody was slightly less resistant to heating at 70 C than were N and CRN antibodies, whereas all three activities of IgM were quickly degraded at this temperature. Dose-response curves with varying amounts of complement (C) or anti-antibody revealed a marked difference between IgG s-CRN and IgM s-CRN antibodies. While 1-hr sensitization at 37 C was insufficient to detect IgG s-CRN antibody, it had the same effect as overnight sensitization at 0 C for IgM s-CRN antibody. When sensitization at 0 C was prolonged to 3 days, unexpectedly high endpoints exceeding 1:10,000 were obtained even with IgM. consequently, enhancement by C was several hundred-fold with IgM in contrast to 5- to 10-fold enhancement of IgG s-CRN antibody, which was similar to that after overnight sensitization. Also IgM obviously required more C than did IgG. These results suggest that IgM of late serum is slower reacting and more C-dependent than IgG s-CRN antibody. Tests with early rabbit sera indicated that the s-CRN antibody detectable by 3-day sensitization reaches a high level before the appearance of N antibody.

Studies on the neutralization of herpes simplex virus. X. Demonstration of complement-requiring neutralizing (CRN) and slow-reacting CRN (s-CRN) antibodies in late IgG

A sample of late IgG from a rabbit hyperimmunized with herpes simplex virus was analyzed for neutralizing (N) and complement-requiring neutralizing (CRN) antibodies. In a usual endpoint test, N and CRN titers were 1: 40 and 1: 160, respectively, but when virus-IgG mixtures were incubated at 0 C overnight before addition of complement (C), an endpoint of 1:1280 was obtained. Virus sensitized at 0 C overnight required more C for inactivation than did sensitized virus formed earlier. Sensitization kinetic curve experiments employing a proper initial virus concentration, which permitted differentiation of sensitized viruses requiring different amounts of C, indicated that formation of sensitized virus detectable only with a relatively large amount of C proceeded slowly at IgG dilutions where the ordinary CRN antibody requiring a smaller amount of C was negligible. The results strongly suggested that the IgG sample contained slow-reacting CRN (s-CRN) antibody in excess of the hitherto known CRN antibody. As to the mechanism of formation of s-CRN complexes, experiments failed to prove the occurrence of complexes initially insensitive to C, and it appears more likely that s-CRN antibody has a comparatively low avidity for virus.

Immunogenicity of subviral herpes simplex virus preparations. I. Formation of neutralizing antibodies in different animal species after administration of herpes simplex virus solubilized antigens

Production of neutralizing antibodies was followed in guinea pigs, rabbits, hamsters and mice immunized with crude antigen extracts (AM) from human diploid cells infected with herpes simplex virus type 1. The AM induced relatively high levels of neutralizing antibodies in all four species. The antibodies were predominantly complement-requiring and remained so even after administration of repeated AM doses. With the strains used, the antibody response was predominantly type specific and, surprisingly, the type specificity of sera usually increased after administration of repeated doses of AM. Guinea pigs seemed to be the best responsive animal species. They developed the highest levels of antibodies and complement-nonrequiring antibodies were seen in them earlier than in the other animal species. The dose-response experiments carried out in guinea pigs indicated that after a single dose administration the ratio between complement-requiring and complement-nonrequiring antibodies was dependent on the amount of antigen administered. When AM was given without adjuvant less efficient antibody production wws observed than after the administration of the same amount of antigen with adjuvant.

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.

Immunological studies of heat-labile virus inhibitors. I. Specificity of absorption onto sensitive viruses and specific response after virus infections

Heat-labile virus inhibitor (HLI) in normal sera of various mammalian species capable of neutralizing variola (VRV) and Newcastle disease viruses (NDV) was studied immunologically. After sucrose density gradient centrifugation of guinea pig serum, the HLI activity against VRV and that against NDV were both demonstrated in the same region sedimenting fastor than IgM. Absorption with partially purified VRV or NDV removed the HLI activity on the homologous virus but not that on the other. Prior saturation of virions with specific antibody blocked the absorption of HLI, suggesting a specific competition for binding site (s) between specific antibody and HLI. The HLI level against variola virus was checked in connection with immunization with vaccinia virus. In human primary vaccination, the HLI level rose sharply within 4 weeks after vaccination, turning to decline gradually to settle at a level higher than that of the conventional neutralizing antibody (NA). In cases of human revaccination, a sharp rise of HLI started 4 days after vaccination and reached the highest level within 7 days, preceding the rise of conventional NA level which occurred about 3 days later. Three rabbits with negative HLI activity prior to vaccinia immunization obtained an HLI activity within 2 weeks, which showed a sharp rise up to 6-8 weeks. One rabbit with a positive prior HLI activity also showed a sharp rise of the HLI activity after immunization. In all rabbits the final HLI level was identical with that of conventional NA. Groups of guinea pigs were immunized with either VRV or NDV. Rises of the HLI level after immunization were observed in all animals, the activity being restricted to the homologous virus used in the immunization. Complement requiring NA was detected during the course of immunization but its behavior was different from that of HLI. The above observations were interpreted to suggest a ubiquitous presence of HLI as a specific reactive agent and its role at an earliest stage of immune response.

Studies on the neutralization of herpes simplex virus. VIII. Significance of viral sensitization for inactivation by complement

Early and late IgG of rabbits immunized with herpes virus showed, respectively, 8-fold and 2-fold enhancement of neutralization endpoint in the presence of complement (C). Kinetic curve experiments employing an appropriate amount of virus revealed that both neutralization and sensitization followed first-order reaction, and each IgG possessed a certain range of concentration where neutralization was negligible while sensitization was marked. Dose responses of neutralization and sensitization velocities demonstrated that the C enhancement of late IgG was about 7-fold and that of early IgG more than 20-fold. These facts suggested that the IgGs contained two different entities of complement-requiring (CRN) and non-requiring neutralizing (N) antibodies at different proportions, only the former being responsible for sensitization. The different CRN: N ratios obtained by the endpoint and kinetic methods may mean either that the two antibodies differ in avidity for the virus or that the number of critical sites per virion for CRN antibody is greater than that for N antibody. In this interpretation, sensitization by CRN antibody as well as neutralization by N antibody is thought to result from attachment of a single antibody molecule to the viral critical site. Alternative explanations, ascribing the mechanism of neutralization to steric hindrance of critical sites or to multiple hit of those sites by antibody, were denied by analyses of the present data.

Cell-mediated immunity in Aujeszky disease virus infected pigs. I. Lymphocyte stimulation

The appearance of cell-mediated immunity was studied in Aujeszky diseased pigs with the aid of the in vitro stimulation of sensitized lymphocytes. The first cell-mediated immunity reaction of lymphocytes occurred 4 days after infection. From day 7 to 35, the latest day tested, the reactions were most marked with lymphocytes from lymph nodes and spleen, whereas blood and thymus lymphocytes reacted less frequently; bone marrow lymphocytes showed no response. Reinfection did not considerably enhance lymphocyte reactivity. Humoral immunity was demonstrated a few days later than cell-mediated immunity. Neutralizing antibodies were first detected at day 7, reaching optimal titers at day 14. Complement fixing antibodies were detected from day 14 onward. Reinfection caused a very weak booster effect only on neutralizing antibody production. The sensitivity of the neutralization test could be enhanced up to sixfold by the addition of fresh guinea pig complement. It is concluded that cell-mediated immunity influences the early stage of infection with Aujeszky disease virus when humoral immunity is not yet demonstrable or yet rather low. Lymph nodes and spleen are apparently of special importance for the appearance of ADV-reactive lymphocytes.

Neutralizing activities of early and late IgG fragments from rabbits immunized with herpes simplex virus

Rabbits immunized with herpes virus were bled periodically and bivalent and univalent fragments of IgG from each serum sample were prepared by enzymatic digestion. The 2-week F(ab')2 showed a low neutralizing activity only after addition of anti-IgG. F(ab')2 of the 4-week serum retained almost all of the neutralizing activity of IgG, while its univalent fragments demonstrated none even when tested with anti-IgG. In contrast to these early IgG fragments, univalent fragments of the 9-week and 20-week IgG neutralized the virus to considerable extents in the absence of anti-IgG; after addition of anti-IgG the activity equaled that of intact IgG in the cases of Fab' and Fab-II, though the activity of Fab-I was relatively low. Three three univalent fragments were all sensitive to heating at 70 C and to ultraviolet irradiation, whereas intact IgG resisted these treatments. F(ab')2 was resistant to the heating and less sensitive to ultraviolet irradiation than univalent fragments. Neutralization kinetic curve experiments to test blocking effects of IgG fragments against the neutralization by intact IgG suggested that the early Fab' did combine with the virus and that the late Fab' exerted a higher blocking effect than the early Fab'.

Complement requirement of neutralizing antibodies in different classes of immunoglobulin appearing in rabbits and guinea pigs after primary and booster immunizations with herpes simplex virus

Rabbits and guinea pigs were immunized with herpes simplex virus and bled periodically. The sera were fractionated into slow IgG, fast IgG and IgM by DEAE-cellulose column chromatography, and complement-requiring (CRN) and nonrequiring neutralizing (N) antibody activities were estimated. In early sera of rabbits, the two IgG and IgM fractions possessed about equal CRN activities, although some animals showed a slightly lower activity in fast IgG. In guinea pigs, the early CRN activity resided mainly in slow IgS (7 S gamma2). The early IgG antibody of guinea pigs differed from that of rabbits in that it resembled IgM in resistances to heating at 70 C and to 2-merceptoethanol. The level of CRN IgM antibody in rabbits declined following a peak reached in 2 to 3 weeks, whereas such a decline was never observed in guinea pigs. N IgG antibody was developed a few weeks after the first immunization in rabbits and much retarded in guinea pigs. In both species, booster immunization quickly evoked N antibody in the two IgG fractions and also CRN IgM antibody, but in the case of rabbits the IgM antibody disappeared soon. It is concluded that IgG plays an important role in humoral immunity from the initial stage of the immunization course.

Effect of heat-labile factors on the neutralization of vaccinia virus by human

The effect of unheated guinea pig and human prevaccination serum on the neutralization of vaccinia virus was studied. Enhancement of neutralization was found in all sera containing antibody and was more marked (20- to 150-fold) in sera obtained in the first weeks after primary vaccination than in sera from immune adults or cord sera (4- to 25-fold). The enhancing factor was thought to be complement because it was destroyed by heating and ethylenediaminetetraacetic acid treatment. The fresh serum-enhanced neutralization test represents a highly sensitive and specific test which can be applied to measure low levels of vaccinia antibody not detectable by other means.

In vitro studies with Modoc virus in Vero cells: plaque assay and kinetics of growth, neutralization, and thermal inactivation

A sensitive and quantitative assay system is described for plaquing Modoc virus in Vero cells. Neutralizing antibodies to Modoc virus could be detected by using this in vitro system by their interference with viral plaque formation. Virus was readily neutralized within 30 min at 37 C by a 1:10 dilution of hyperimmune hamster serum. The rate of neutralization and the total amount of virus neutralized was not altered significantly by the addition of 20 U of guinea pig complement to the hyperimmune hamster serum. A study of the growth of Modoc virus in Vero cells is also presented. After an initial latent period of 20 h, viral titer increased exponentially for 20 h. By 83 h after infection, 8,000 plaque-forming units of virus were detected per cell. The stability of viral infectivity in phosphate-buffered saline at pH 7.4 was evaluated. No reduction in viral titer was detected after 3 days at 7 or 22 C. A continuous decrease in infectivity at 37 C was observed, however, throughout the observation period.

Cytotoxicity of adenovirus-antibody aggregates: sensitivity to different cell strains, and inhibition by hexon antiserum and by complement

Adenovirus-antibody aggregates under defined conditions are cytotoxic in vitro. All members of adenovirus groups I, II, and III caused toxicity upon aggregation. The toxicity of the clusters is exerted by the virions. Toxicity is temperature dependent and may be caused by a mechanism similar to that used in viral penetration. Cells permitting direct viral penetration were all sensitive to the toxic aggregates. The toxicity seems to be related to hexon antigens on the surface of the virions since antihexon sera neutralized the toxicity. No evidence was obtained showing that pentons are required for this kind of cytotoxicity. Adenovirus types 3, 5, and 9 were used in the experiment. Cytotoxicity was estimated by the (51)Cr release assay. Complement factors could be excluded as mediators of the cytolytic reactions. Instead, complement was shown to prevent the formation of toxic aggregates or to neutralize the toxicity of preformed ones.

Enhancement of hemagglutination inhibition by complement

Experiments were conducted to determine the effect of specific antibody and complement on hemagglutination inhibition of influenza virus. It was found that early immunoglobulin M antibody to influenza virus inhibited the hemagglutinating capacity of the virus. When fresh guinea pig serum was added, the inhibiting capacity of the serum was elevated from four- to eightfold When guinea pig serum was treated with known complement inhibitors, it lost its capacity to enhance hemagglutination. The use of functionally purified complement components indicated that the first, second, and fourth components were necessary and sufficient for enhancement of hemagglutination inhibition by specific antibody to influenza virus.

Significance of bivalence of antibody in viral neutralization

The role of bivalence of antibody in its capacity to neutralize virus was studied with rabbit antibodies to the bacteriophage, phiX174. Univalent Fab or Fab' fragments of IgG isolated from antiviral antisera obtained early in the immunization schedule had virtually no activity compared to that of the intact IgG. When the antibodies were isolated from antisera of the same rabbits several months later, the univalent fragments and IgG were essentially equal in activity. The results are interpreted on the basis that an IgG molecule, because of its bivalence, has a higher effective combining affinity (avidity) than a univalent fragment. After prolonged immunization, however, the affinity of univalent antibody becomes sufficiently high that it exceeds a threshold value, above which further increase in affinity, through bivalence, is no longer significant. The results could explain the variability in relative effectiveness of univalent antibodies observed in previous studies. These data, and the fact that F(ab')(2) fragments from either "early" or "late" antisera were as effective as IgG, indicate that fragment Fc is not a significant factor in neutralization. No differences in dissociation from the virus of univalent antibody from early and late antisera could be demonstrated by dilution at temperatures up to 47 degrees C. The attachment at sites of neutralization on the virus appears to be functionally almost irreversible in this system.

Complement components required for virus neutralization by early immunoglobulin antibody

Virus neutralization by early antisera usually requires or is enhanced by the presence of complement. The particular components of the complement system which are needed for neutralization of Newcastle disease virus by early IgM antibodies were studied. It was found in this system that participation of only the first four of the nine complement components (C1, C2, C3, and C4) was necessary and sufficient for neutralization to occur. It was concluded that the role of complement in neutralization was most likely that of contributing bulk in the form of large protein molecules to the virus-antibody complex.

Effect of complement and viral filtration on the neutralization of respiratory syncytial virus

The addition of 10 hemolytic units of guinea pig complement has been shown to enhance the neutralizing capacity of respiratory syncytial (RS) immune sera produced in guinea pigs and ferrets. This same immune sera, when tested without complement, had little or no neutralizing capacity. The addition of complement to RS immune horse serum did not significantly increase its neutralizing capacity. Immune horse serum effectively neutralized RS virus without complement. Other studies indicated that a 50% tissue culture infective dose of between 30 and 100 should be used in RS serum neutralization tests and that incubation should be for 90 to 105 min at room temperature. The neutralizing capacity of guinea pig immune serum was not increased by the use of filtered virus. The rate of virus neutralization, however, was increased with the addition of 10 hemolytic units of complement. The neutralizing capacity of RS immune horse serum was much greater for filtered than for unfiltered RS virus. The addition of complement increased the rate of virus neutralization but did not increase the neutralizing capacity of the horse immune serum.