Antigen-Antibody Interactions in vitro: II. The Non-Neutralizing Antibodies are by far the Most Potent Virus Inactivators

Author(s): Viggo Bitsch

Inactivation of the virus in a conventional neutralization test was in early comprehensive studies found to be a bifactorial process, consisting of a prompt and short-lasting reaction originally termed “over-neutralization” and an enduring but slowly progressing reaction of first order. The reaction of first order follows the lines of the formula of the regular antigen-antibody interactions not including aggregation. These lines could be recognized because antigens and their antibodies are bound irreversibly under physiological conditions. Tests for demonstration of virus and antibodies of very high sensitivity and specificity have been elaborated on basis of these reaction lines and used in diagnosing, controlling and eradicating viral infections in the veterinary field for a long time. The early and rapid “over-neutralization” reaction could later be concluded to be neutralization by aggregation of viruses.

The non-neutralizing antibodies inactivate viruses by aggregation of the agents. In-vitro studies have demonstrated an extremely high virus-inactivating potency of these antibodies. The principal characteristics of virus aggregation are the following: 1) the antibodies to various antigenic determinants aggregate virions synergistically and rapidly, 2) the virus-inactivating aggregation by the polymerized isotypes of antibodies is greatly enlarged by their polyvalency, and 3) the complement component C1q will promptly attach to antibodies sensitized by being bound to their antigenic determinant on a virus and inactivate such virus-antibody complexes by including them in aggregates. In complement-enriched neutralization tests, C1q will promptly aggregate antigen-antibody complexes formed almost immediately and with increasing reaction times aggregate the test virus following the first-order binding of non-neutralizing antibodies to their antigenic determinants, inactivating viruses with the same rate as neutralizing antibodies.

In a herpesvirus complement-enriched neutralization test, the titers of reacting non-neutralizing IgG antibodies in the highest concentration was found to be approx. 8 times higher than that of the neutralizing antibodies. The total concentration in blood of non-neutralizing antibodies in blood by far exceeds that of the neutralizing ones, being largely proportional to the number of antigenic determinants on a virion. The neutralizing capability of the non-neutralizing antibodies by aggregation, most pronounced in cooperation with the C1q complement component, implies that the non-neutralizing antibodies have a much greater neutralizing potency than neutralizing antibodies. One non-neutralizing antibody bound to an antigenic determinant will result in almost immediate inactivation of the virus due to aggregation created by the C1q component.

The formation of the pentameric and decavalent IgM antibodies is the first humoral immune response after infection and the virus-inactivating potential of these antibodies is enormous. A significant neutralizing IgM antibody response could be detected in blood by a complement-enriched neutralization test 4 days after nasal infection and samples taken after 8 to 15 days were positive in dilutions 1:10.000 or higher. The IgM antibodies, in cooperation with the C1q component of complement, must be considered the ultimate virus inactivators and of almost unimaginable importance in the defeat of infections.

Unidentified specific forces attracting 1) the binding sites of antibodies and their specific determinants on viruses, and 2) the binding sites of the C1q component and the Fc fragment of sensitized antibodies bound to their antigenic determinants, are concluded to be one reason for the prompt reaction, which is characteristic of the virus aggregation process.

Further investigations into the effector mechanisms determining 1) the specificity and attractive binding of the antibodies to their antigenic determinants and 2) the reaction between the complement component C1q and sensitized antibodies are urgently needed.

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