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. IX. Variance in complement requirement among IgG and IgM from early and late sera under different sensitization conditions.

Early and late sera of rabbits immunized with herpes simplex virus were fractionated into IgG and IgM, and the minimal concentration of complement (C) required for full enhancement of neutralizing activity was determined for each by the plaque reduction method. In tests employing simultaneous mixing of virus, antibody and C, C-requiring neutralizing (CRN) antibody in IgM required 2–8 times more C than that in IgG. When virus-antibody mixtures were incubated at 0 C overnight before addition of C, a marked enhancement of CRN endopoint especially of late IgG and IgM was exhibited, in contrast to materially unchanged titers of the ordinary neutralizing antibody. This result suggested an abundance of slow-reacting CRN-virus complexes. The CRN antibody so detected required about 4 times more C than that detectable by the usual test in the case of late IgG and IgM. When virus sensitized with late IgG at 0 C overnight was further incubated at 37 C for 1 hr, the C requirement changed but slightly without showing any more increase of the endpoint, whereas sensitization at 0 C for 2 to 3 days further increased the CRN antibody endpoint but the C requirement was equal to that after 1 day's sensitization at 0 C. Based on these and earlier findings, a hypothesis is proposed that binding of a single antibody molecule with virus may cause a series of changes of the virus particle or part of those changes depending on the nature of antibody and on the sensitization condition, and C added to such complexes at an appropriate stage of the changes can accelerate the procession of the changes leading eventually to inactivation.     

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.     

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.     

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.     

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 IgG (7 S γ₂). 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-mercaptoethanol. 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.     

Complement-requiring neutralizing antibody in guinea pigs with primary and recurrent genital herpes

Guinea pigs inoculated intravaginally with herpes simplex virus type 2 (HSV-2) strain 1868 produced a serum complement-requiring neutralizing (CRN) antibody during primary acute infection, i.e., 10 days postinoculation. The CRN antibody titers in the guinea pig sera decreased to less than 1:10 after heating at 56 degrees C for 30 min. It was found that 32 units of complement were necessary to obtain a satisfactory HSV-2 neutralizing antibody titer. Nonheated sera significantly reduced virus infectivity titers when mixed with 3.5 log10 PFU of HSV-2 and incubated at 37 degrees C for 20 to 60 min (P less than 0.001), whereas the same sera after heating at 56 degrees C for 30 min showed no inhibitory effect. Only 27.3% of infected guinea pigs had low serum non-CRN antibody titers ranging from 1:20 to 1:40. In addition, no evidence of increase in CRN antibody titers was noted during spontaneous recurrent genital herpes infection.     

Structural and Functional Diversity of Novel Coronin 1C (CRN2) Isoforms in Muscle

Coronin 1C (synonyms: coronin-3, CRN2), a WD40 repeat-containing protein involved in cellular actin dynamics, is ubiquitously expressed in human tissues. Here, we report on the identification and functional characterization of two novel coronin 1C isoforms, referred to as CRN2i2 and CRN2i3, which also associate with F-actin. Analyses of the coronin 1C gene disclosed a single promoter containing binding sites for myogenic regulatory factors and an alternative first exon 1b present in intron 1, which give rise to the novel isoforms. Chromatin immunoprecipitation studies demonstrate MyoD binding to a region of the CRN2 gene, which contains a highly conserved E-box element in exon 1a. Gel-filtration assays suggest that the largest isoform 3 exists as a monomer, in contrast to isoform 1 and isoform 2 appearing as trimers. CRN2i3, which can be induced by MyoD, is exclusively expressed in well-differentiated myoblasts as well as in mature skeletal muscle tissue. In human skeletal muscle, CRN2i3 is a novel component of postsynaptic neuromuscular junctions and thin filaments of myofibrils. Together, our findings postulate a role for CRN2 isoforms in the structural and functional organization of F-actin in highly ordered protein complexes.     

Different roles of IgG and complement receptors in phagocytosis by polymorphonuclear leukocytes.

The functions of IgG and complement receptors in phagocytosis of immune complexes by mouse polymorphonuclear leukocytes were examined by in vitro experiments. The immune complexes were sheep red cells (E) sensitized with IgG antibody (EA) or with antibody and complement (EAC). Inhibition experiments with Fab fragments of rabbit IgG antibody anti-mouse IgG have shown that the complement receptor is primarily involved in the attachment phase, whereas participation of the IgG receptor is necessary for inducing the mechanism of phagocytosis. The possible relevance of these findings for the in vivo mechanism of defense infection, and for the control of antibody synthesis is discussed.     

Neutralizing antibodies against hepatitis C virus and the emergence of neutralization escape mutant viruses.

We developed an in vitro assay for antibodies to hepatitis C virus (HCV) that bind to virions and prevent initiation of the replication cycle in susceptible cells in vitro. These antibodies therefore appear to be capable of neutralizing the virus. Using this assay and a standard inoculum of HCV of known infectivity, we have measured the antibody in serial serum samples obtained from the same chronically infected patient over 14 years following onset of his hepatitis. Such antibody was found in sera collected within 5 years of onset of hepatitis but not in later sera. In double immunoprecipitation experiments with anti-human immunoglobulin, the same sera that contained neutralizing antibody were found to contain antibody that bound to HCV to form antigen-antibody complexes immunoprecipitable with anti-human globulin. Similarly, plasma collected from this patient in 1990, 13 years after onset of hepatitis, and which contained HCV that had diverged genetically from the 1977 strain, did not contain antibody capable of neutralizing either the 1977 or the 1990 strain of HCV. However, plasma collected a year later (1991, 14 years after onset of hepatitis) contained neutralizing antibody to the 1990, but not the 1977, strain of HCV. These results suggest that HCV does induce antivirion antibody, as measured by blocking of initiation of the replication cycle of virus in cells and by the formation of immunoprecipitable antigen-antibody complexes but that these antibodies are isolate specific and change over time. Thus, these antivirion antibodies function as neutralizing antibodies and are probably in vitro correlates of the attempt of the host to contain the emergence of neutralization-resistant variants of HCV over time.     

A human immunodeficiency virus type 1 (HIV-1) infection-enhancing factor in seropositive sera

While testing sera for Human Immunodeficiency Virus neutralizing antibody titers, three sera were identified which had the ability to enhance infectivity of the virus. The sera were from three different individuals residing in Nashville, TN. The enhancing factor was not removed by either filtration through 0.05 micron filters or by incubation for one hour with a stoichiometric amount of protein A sepharose. Two of the sera were able to enhance infection by two divergent isolates (HTLV-IIIB and HTLV-IIIRF) while one was only capable of enhancing infection of target cells by HTLV-IIIB. None of the sera induced syncytium formation in chronic HIV-infected cells. The findings suggest that the substance is neither a virus nor an IgG class 1 or 2.     

Differences in antibody production against mouse hepatitis virus (MHV) among mouse strains

Several strains of mice were examined for antibody production after intranasal inoculation with a low virulence strain of mouse hepatitis virus (MHV), MHV-NuU. C57BL/6N mice were shown to be high responders in the production of complement fixing (CF) antibody as compared to C3H/HeN, BALB/c-AnN, DBA/2N mice. F1 hybrids B6C3 and BDF1 from C57BL/6N mice, showed CF antibody responses as high as C57BL/6N, suggesting that high responsiveness is genetically controlled. All these mouse strains were able to produce high titred neutralizing antibody to MHV.     

Determination of a statistically valid neutralization titer in plasma that confers protection against simian-human immunodeficiency virus challenge following passive transfer of high-titered neutralizing antibodies

We previously reported that high-titered neutralizing antibodies directed against the human immunodeficiency virus type 1 (HIV-1) envelope can block the establishment of a simian immunodeficiency virus (SIV)/HIV chimeric virus (SHIV) infection in two monkeys following passive transfer (R. Shibata et al., Nat. Med. 5:204-210, 1999). In the present study, increasing amounts of neutralizing immunoglobulin G (IgG) were administered to 15 pig-tailed macaques in order to obtain a statistically valid protective neutralization endpoint titer in plasma. Using an in vitro assay which measures complete neutralization of the challenge SHIV, we correlated the titers of neutralizing antibodies in plasma at the time of virus inoculation (which ranged from 1:3 to 1:123) with the establishment of infection in virus-challenged animals. Ten of 15 monkeys in the present experiment were virus free as a result of neutralizing IgG administration as monitored by DNA PCR (peripheral blood mononuclear cells and lymph node cells), RNA PCR (plasma), virus isolation, and the transfer of lymph node cell suspensions (10(8) cells) plus 8 ml of whole blood from protected animals to na?ve macaques. The titer of neutralizing antibodies in the plasma calculated to protect 99% of virus-challenged monkeys was 1:38.     

Effect of selective complement deficiency on the rate of neutralization of enveloped viruses by human sera.

The capacity of human sera genetically deficient in selective complement (C) components to enhance neutralization of enveloped viruses was examined by kinetic plaque reduction assays. Vaccinia virus, a DNA virus, and vesicular stomatitis virus (VSV), an RNA virus, were studied. Exogenous rabbit: or human antibody to vaccinia virus, and guinea pig or human antibody to VSV were provided in limiting, C-dependent concentrations. IgG antibodies predominated in most of the antisera employed. C5-deficient and C6-deficient human sera consistently supported normal rates of neutralization of either virus; this effect was heat-labile. C4-deficient human serum did hot exceed heat-inactivated serum in any neutralization assay. C1r-deficient serum displayed slight heat-labile neutralizing capacity against vaccinia but none against VSV. C2- and C3-deficient sera consistently exhibited measurable but clearly subnormal rates of neutralization. Two fresh agammaglobulinemic sera failed to inactivate either virus in the absence of added antibody. These results confirm and extend earlier evidence, based on neutralization of herpes simplex and Newcastle disease viruses in the presence of early (IgM) antibody and functionally pure guinea pig C components or C-deficient animal sera, that the late-acting components C5-C9 are not required for C-dependent neutralization. Data on four enveloped viruses now agree that this function is mediated by C1-C3, although C1 plus C4 appear to have some neutralizing capacity. This requirement for C1-C3 is overcome, however, in the presence of higher antibody cohcentrations, suggesting that the contribution of the C system to viral neutralization in vivo may be chiefly in the early phase of infection when antibody is limited.     

Virus-neutralizing antibodies of immunoglobulin G (IgG) but not of IgM or IgA isotypes can cure influenza virus pneumonia in SCID mice.

The ability of monoclonal antibodies (MAbs) to passively cure an influenza virus pneumonia in the absence of endogenous T- and B-cell responses was investigated by treating C.B-17 mice, homozygous for the severe combined immunodeficiency (SCID) mutation, with individual monoclonal antiviral antibodies 1 day after pulmonary infection with influenza virus PR8 [A/PR/8/34 (H1N1)]. Less than 10% of untreated SCID mice survived the infection. By contrast, 100% of infected SCID mice that had been treated with a single intraperitoneal inoculation of at least 175 micrograms of a pool of virus-neutralizing (VN+) antihemagglutinin (anti-HA) MAbs survived, even if antibody treatment was delayed up to 7 days after infection. The use of individual MAbs showed that recovery could be achieved by VN+ anti-HA MAbs of the immunoglobulin G1 (IgG1), IgG2a, IgG2b, and IgG3 isotypes but not by VN+ anti-HA MAbs of the IgA and IgM isotypes, even if the latter were used in a chronic treatment protocol to compensate for their shorter half-lives in vivo. Both IgA and IgM, although ineffective therapeutically, protected against infection when given prophylactically, i.e., before exposure to virus. An Fc gamma-specific effector mechanism was not an absolute requirement for antibody-mediated recovery, as F(ab')2 preparations of IgGs could cure the disease, although with lesser efficacy, than intact IgG. An anti-M2 MAb of the IgG1 isotype, which was VN- but bound well to infected cells and inhibited virus growth in vitro, failed to cure. These observations are consistent with the idea that MAbs of the IgG isotype cure the disease by neutralizing all progeny virus until all productively infected host cells have died. VN+ MAbs of the IgA and IgM isotypes may be ineffective therapeutically because they do not have sufficient access to all tissue sites in which virus is produced during influenza virus pneumonia.     

Antibody-mediated in vitro neutralization of human immunodeficiency virus type 1 abolishes infectivity for chimpanzees.

This study was undertaken to establish whether antibody directed against the human immunodeficiency virus type 1 (HIV-1) principal gp120 type-specific neutralization determinant can abolish the infectivity of HIV-1 in chimpanzees. Challenge inocula of the IIIb virus isolate were mixed in vitro with either immunoglobulin G (IgG) from an uninfected chimpanzee, nonneutralizing IgG from an HIV-seropositive human, a virus-neutralizing murine monoclonal antibody directed against the HIV-1 IIIb isolate, or virus-neutralizing IgG from a chimpanzee infected with the IIIb isolate. Both neutralizing antibodies were directed against the principal neutralization determinant of the challenge isolate. Establishment of infection following inoculation of each virus-antibody mixture into chimpanzees was assessed by virus-specific antibody development and by virus isolation. No protective effect was noted either with the control IgG or with the nonneutralizing anti-HIV IgG. By contrast, the polyclonal chimpanzee virus-neutralizing IgG prevented HIV-1 in vivo infection, while the neutralizing monoclonal antibody notably decreased the infectivity of the challenge virus. Hence, antibody to the gp120 principal neutralization determinant is able both to prevent HIV-1 infection in vitro and to inhibit infection in vivo.     

Human immunodeficiency virus type 1 mutants that escape neutralization by human monoclonal antibody IgG1b12. off.

IgG1b12, a human monoclonal antibody (MAb) to an epitope overlapping the CD4-binding site on gp120, has broad and potent neutralizing activity against most primary human immunodeficiency virus type 1 (HIV-1) isolates. To assess whether and how escape mutants resistant to IgG1b12 can be generated, we cultured primary HIV-1 strain JRCSF in its presence. An escape mutant emerged which was approximately 100-fold more resistant to neutralization by IgG1b12. Both virion-associated and solubilized gp120 from this variant had a reduced affinity for IgG1b12, and sequencing of its env gene showed that amino acid substitutions had occurred at three positions within gp120. Two (D164N and D182N) were located in V2, and one (P365L) was in C3. By site-directed mutagenesis, we demonstrated that the D182N and P365L mutations, but not D164N, contribute to the IgG1b12-resistant phenotype. However, the former two substitutions, individually or in combination, hinder the replication of the neutralization-resistant virus. Introduction of the D164N substitution into the P365L variant results in a nonviable virus (D164N/P365L). In contrast, addition of D164N to the D182N or D182N/P365L mutant partially restored replicative function to near wild-type levels. Furthermore, we found that all of the IgG1b12-resistant mutant viruses remained sensitive to other human MAbs, such as 2G12 and 2F5, and to the CD4-IgG molecule, except that the P365L-containing mutant was slightly resistant to CD4-IgG. These results suggest that escape from IgG1b12 neutralization is due to a local rather than a global modification of the gp120 structure. Our findings have implications for the therapeutic and prophylactic applications of antibodies for HIV-1 infection.     

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'.     

Studies on the Neutralization of Herpes Simplex Virus

Herpes virus was reacted with an early rabbit antiserum containing predominantly complement-requiring neutralizing (CRN) antibody to produce CRN-antibody-sensitized virus (SV), and the action of complement (C') upon SV was studied. Reduction of infectivity due to C' was about equal with undiluted and 1000-fold diluted SV. Even higher dilutions which contained about 10 to 100 infectious units per 0.05 ml were also completely inactivated by C'. Kinetic experiments revealed that the velocity of titer reduction in the presence of C' of 100-fold diluted SV was not slower than that of undiluted SV. When SV was first treated with C' and then diluted 100-fold, the surviving virus showed but a slightly reduced efficiency of filtration through the 0.45 μ Millipore membrane as compared with SV first diluted 1: 100 and then treated with C'. The titer reduction of SV–C'1 complexes in the presence of C'4 followed a one-hit curve. These results indicated that the reduction of infectivity of SV due to C' was not a result of immunoaggregation of infectious SV. Alternative possible mechanisms of the action of C' are discussed.     

A new microplate neutralization test for typing of herpes simplex virus.

A microplate serum neutralization test for estimation of complement-requiring neutralizing (CRN) antibody was established as the first step for simplification of typing of herpes simplex virus (HSV). When guinea pigs were immunized with type 2 HSV, the late sera could mostly differentiate the types of HSV better than hyperimmune rabbit sera, the CRN titer against the heterologous type 1 HSV being much lower than the homologous titer. Sera of guinea pigs immunized with type 1 HSV showed about the same level of cross reaction against type 2 HSV as did rabbit antisera. Guinea pig sera having minimal levels of cross reaction were selected, and their high dilution (1:160) and complement were added to serial 10-fold dilutions of virus in the microplate titration of virus infectivity. Selective reduction of virus titer by either antiserum could determine the type of HSV. No equivocal intermediate case was found among a number of stock strains including many fresh isolates. The typing result coincided with that determined by a modification of Yang et al's method based on virus titers obtained with Vero and primary chick embryo cells. The typing based on plaquing in chick embryo cells sometimes failed to identify type 1 HSV.