Mechanisms of neutralization of animal viruses: monoclonal antibodies provide a new perspective

Immunological factors that affect the in vivo fate of T7 phage in the mouse

Phage display is a powerful method to study organ and tissue specific addresses. As part of our studies on the in vivo panning of tissue-homing peptide libraries, we examined the survival of T7 phage in the blood of C57BL/6J mice to estimate the half-life of T7 phage and the factors responsible for its inactivation. Amplified and purified T7 phage particles with or without random peptide library inserts were injected intravenously into the tail vein of wild-type (C57BL/6J) and immunocompromized (C57BL/6J) female mice. In wild-type mice, both the parent phage as well as phage carrying a peptide library were eliminated quickly from the blood, with only approximately 1% survival of detectable infectious phage after 60 min of injection. In SCID (C57BL/6J-Prkdc(scid)) mice, phage titers were stable over the same period of time with or without peptide library, suggesting a role for either B- or T cells or both in phage inactivation. The presumed role of B cell was indicated by demonstration of stable phage in the B-cell deficient mouse (C57BL/10-Igh-6(tm1Cgn)). In other immunocompromized mice, the phage titers were unstable, similar to that found in wild-type mice. In no case, was there a difference between phage with or without random peptide library. These data indicate that the presence of random C-X7-C peptides on the T7 phage coat protein does not affect the clearance of the phage in murine blood. Most likely, host immune factors play a role in the neutralization of T7 phage in blood by reacting with B-cell dependent immunoglobin.

Neutralizing antiviral antibody responses

Neutralizing antibodies are evolutionarily important effectors of immunity against viruses. Their evaluation has revealed a number of basic insights into specificity, rules of reactivity (tolerance), and memory—namely, (1) Specificity of neutralizing antibodies is defined by their capacity to distinguish between virus serotypes; (2) B cell reactivity is determined by antigen structure, concentration, and time of availability in secondary lymphoid organs; and (3) B cell memory is provided by elevated protective antibody titers in serum that are depending on antigen stimulation. These perhaps slightly overstated rules are simple, correlate with in vivo evidence as well as clinical observations, and appear to largely demystify many speculations about antibodies and B cell physiology. The chapter also considers successful vaccines and compares them with those infectious diseases where efficient protective vaccines are lacking, it is striking to note that all successful vaccines induce high levels of neutralizing antibodies (nAbs) that are both necessary and sufficient to protect the host from disease. Successful vaccination against infectious diseases such as tuberculosis, leprosy, or HIV would require induction of additional long-lasting T cell responses to control infection.

The synergistic neutralization of Rift Valley fever virus by monoclonal antibodies to the envelope glycoproteins

A panel of monoclonal antibodies (MAbs) mapping to different antigenic sites on the RVFV G1 and G2 proteins were used to examine the mechanisms involved in neutralization of the virus. Three types of synergistic neutralization of RVFV were observed on mixing various pairs of MAbs. Firstly, enhanced neutralization occurred for two MAb pairs that showed augmented binding for G2. These comprised a combination of a neutralizing MAb with a non-neutralizing antibody, as well as two antibodies which were non-neutralizing individually. In the second category, synergistic neutralization was observed between combinations of MAbs for which increased binding had not been detected. Lastly, mixtures of G1 and G2-specific MAbs were also capable of enhancing neutralization. Post-adsorption neutralization assays revealed that some MAbs neutralized cell-attached virus efficiently, indicating that they can neutralize by inhibiting the infection process after virus attachment. MAbs mapping to G1 IIe, G2I b and G2I c were unable to neutralize adsorbed virus and thus probably neutralize by preventing virus attachment to cells. Several G1-reactive MAbs displayed low level post-adsorption activity, suggesting they may be capable of inhibiting RVFV infectivity at different stages of the replication cycle.

Mode of neutralization of lactate dehydrogenase-elevating virus by polyclonal and monoclonal antibodies

Neutralization of the infectivity of [3H]uridine-labeled lactate dehydrogenase-elevating virus (LDV) by polyclonal mouse or rabbit antibodies to the envelope glycoprotein of LDV, VP-3, or by neutralizing monoclonal antibodies (mAb) that recognize a different epitope on VP-3 than the polyclonal antibodies correlated with an increase in the sedimentation rate of LDV from 230 S to greater than or equal to 270 S. Incubation of LDV with normal mouse plasma or non-neutralizing mAbs to LDV VP-3 had no effect on its sedimentation rate. Similarly, incubation of a neutralization escape variant of LDV with the mAb used in its selection had no effect on its sedimentation rate, whereas neutralization of this variant by polyclonal mouse or rabbit anti-VP3 antibodies increased the sedimentation rate. Neutralization of LDV infectivity was only observed at high antibody/virion ratios and often was followed by loss of the viral RNA. The results suggest that neutralization of LDV infectivity results from binding of multiple antibody molecules that recognize specific epitopes on the viral envelope glycoprotein and ultimately leads to disintegration of the virions.

Insights into neutralization of animal viruses gained from study of influenza virus

It has long been known that the binding of antibodies to viruses can result in a loss of infectivity, or neutralization, but little is understood of the mechanism or mechanisms of this process. This is probably because neutralization is a multifactorial phenomenon depending upon the nature of the virus itself, the particular antigenic site involved, the isotype of immunoglobulin and the ratio of virus to immunoglobulin (see below). Thus not only is it likely that neutralization of one virus will differ from another but that changing the circumstances of neutralization can change the mechanism itself. To give coherence to the topic we are concentrating this review on one virus, influenza type A which is itself well studied and reasonably well understood [1–3]. Reviews of the older literature can be found in references 4 to 7.

Effects of specific monoclonal antibodies on La Crosse virus neutralization: aggregation, inactivation by Fab fragments, and inhibition of attachment to baby hamster kidney cells

At high concentrations, several monoclonal antibodies to the G1 glycoprotein of La Crosse (LAC) virus aggregated the virus. To determine whether this accounted for the neutralization, the monoclonal antibodies were digested to make Fab fragments. With one exception, each monovalent antibody neutralized LAC virus to the same extent that bivalent antibody did, although higher concentrations were needed. Fab fragments of synergistic pairs of antibodies also exhibited enhanced binding in a competition binding assay but did not increase neutralization. To determine specific mechanisms for neutralization, the effects of polyclonal or monoclonal antibodies on virus attachment were examined. Polyclonal antibody to LAC virus reduced virus attachment by only 68% although it neutralized 99.99% of the virus. When virus was preincubated with a neutralizing monoclonal antibody to each of seven antigenic regions on G1, only antibody to one region reduced attachment of virus by as much as 92%. Antibodies to two regions that neutralize virus by 90-98% only inhibited attachment by 9 and 13%, respectively. The other antibodies showed intermediate degrees of neutralization and inhibition of attachment. Pairs of antibodies previously shown to be synergistic in neutralizing activity did not inhibit attachment any more than the single antibodies did.

Persistent infection of mice by lactate dehydrogenase-elevating virus: effects of immunosuppression on virus replication and antiviral immune responses

Maximum plasma titers (10(9)-10(10) ID50/ml) of lactate dehydrogenase-elevating virus (LDV) in mice are observed one day after infection, but then decrease 4-5 log during the next 5 weeks to attain a persistent steady-state level for the remainder of the life of the animal. The decrease in plasma LDV level during the first 5 weeks after infection and long-term viremia were not affected by lethal X-irradiation of the mice, daily injections of cyclosporin A or depletion of the mice of T cells by treatment with anti-CD4, anti-CD8, or anti-Thy1.2 monoclonal antibodies, although these treatments inhibited the formation of anti-LDV antibodies. LDV viremia was also the same in nu/nu and nu/+ Swiss mice, though the former did not mount an anti-LDV immune response, while the latter did. The appearance of anti-LDV neutralizing antibodies in infected mice 1-2 months after infection or the injection of infected mice with high doses of anti-LDV neutralizing monoclonal antibodies also did not affect the level of LDV viremia. Repeated treatments of infected mice with either cyclophosphamide or dexamethasone caused 1-2 log increases in plasma LDV titers. Although cyclophosphamide treatment prevented the formation of anti-LDV antibodies, dexamethasone caused an increase in plasma LDV levels without affecting anti-LDV antibody formation. We conclude that an anti-LDV immune response does not play a significant role in controlling LDV replication in mice. The data support the view that within 1 day after infection of a mouse, all LDV-permissive macrophages, which appear to be the only cells supporting LDV replication in the mouse, are destroyed as a result of a cytocidal infection by LDV. Subsequently, LDV replication is limited by the rate of generation of new permissive macrophages. The steady-state viremia attained about 5 weeks after infection reflects a balance between LDV replication in permissive macrophages as they arise and LDV inactivation and clearance.

Functional and neutralization profile of seven overlapping antigenic sites on the HN glycoprotein of Newcastle disease virus: monoclonal antibodies to some sites prevent viral attachment

We have previously identified five antigenic sites on the hemagglutinin-neuraminidase (HN) glycoprotein of the Australia-Victoria isolate of Newcastle disease virus (Iorio and Bratt, J. Virol. 48, 440-450; Iorio et al., J. Gen. Virol. 67, 1393-1403). Two additional sites (designated 12 and 23) are now described, bringing to a total of seven the number of antigenic sites defined by our panel of neutralizing anti-HN antibodies. Competition antibody binding and additive neutralization assays reveal that each of these newly-identified sites overlaps two previously-defined ones. The seven HN antigenic sites thus form a continuum in the three-dimensional conformation of the molecule. Studies on the inhibition of hemagglutination (HA), neuraminidase (NA) and the attachment of virus to chick cell monolayers have been used to construct a functional profile of each antigenic site. Monoclonal antibodies (mAbs) to three overlapping sites (12, 2 and 23) inhibit HA and NA and prevent viral attachment to chick cell monolayers. These findings are consistent with the domains recognized by these mAbs being close to the NA and receptor-binding sites. MAbs to two other overlapping sites, 14 and 1 (which in turn, overlap site 12), inhibit HA quite effectively, and attachment to a lesser extent. Sites 14 and 1 probably identify a second domain involved in receptor recognition. MAbs to the two remaining sites (3 and 4), though neutralizing, are negative in all three assays, thus recognizing domains not involved in HA or NA or attachment to chick cells.