Neutralization of HIV-1: a paradox of humoral proportions.

The production of immunoglobulin capable of neutralizing the infectivity of a virus represents one of the most remarkable molecular accomplishments of the host's available immune defenses. It should be no surprise that a virus that has existed in the parenchyma of the immune system has evolved as an equally dynamic molecule (i.e., viral envelope) for survival. Neutralizing immunoglobulin (Ig) can best serve the host under conditions where the invading pathogen requires a well-defined cell-free state for establishing an infection or transmission. Evidence for a controlling and therefore protective role of neutralizing Ig against lentiviruses has been defined in natural and experimental infections with equine infectious anemia virus of ungulate members in the family equidae. Rapid replication of the virus immediately after infection and its release in a cell-free state leads to the production of neutralizing Ig and subsequent control of the primary viremia. A similar cause-effect relationship exists in humans between the high-titered viremia, observed shortly after HIV-1 infection, and the subsequent production of neutralizing Ig. Partially controlling this acute stage of viral replication by neutralizing Ig and thus preventing an otherwise acute form of immunosuppression or immune complex disease may be viewed paradoxically as a survival property of the virus. Immunologically mediated control, in a Darwinian sense, selects for viruses that have optimized the parameters of longevity and transmission from host to host. This paradox of neutralization in HIV-1 infection appears to be mediated by the convergence of structural and functional roles of the third variable domain (V3) of the external envelope glycoprotein. During infection or envelope-based vaccination, antibody to this cross-reactive, immuno-dominant epitope dominates the antigenic repertoire. Once this occurs, the host is less able to respond to emerging viruses containing closely related V3 structures. Thus a relatively restricted clonal-dominance of the neutralization response results. The V3 domain, apparently in concert with the rest of the molecule, provides an epitope that can tolerate and utilize its conformational flexibility to allow immune escape while maintaining its functional role in infectivity. Sixteen other putative epitopes have been described as being involved in the induction of neutralizing Ig. Currently the biologically functional role of neutralizing Ig to these other epitopes are complicated by a prior lack of knowledge and appreciation of the in vitro variables affecting their measurements.(ABSTRACT TRUNCATED AT 400 WORDS)