This study addresses one of the central conflicts in the engagement between HIV and the immune response, namely, the balance between the opposing forces driving mutational escape and those driving sequence conservation: this may be decisive in influencing outcomes from HIV-1 infection. HLA-B*57 and HLA-B*5801 are associated with effective control of HIV-1 (21
), and a proposed mechanism to contribute to this instance of successful immune control (24
) is that B*57/5801-restricted CTL target a conserved epitope, TW10, from which escape comes only at a cost to viral replicative capacity. This study demonstrates an in vitro fitness cost resulting from the T242N mutation. Alternative escape options for the virus are limited, given that all alternative sites of TW10 escape are associated with linked mutations, suggesting required functional compensation. The mechanisms for these associations between TW10 escape mutations and their linked polymorphisms are presently unknown. However, these structural data indicate the critical role of Thr-242 in nucleating and stabilizing helix 6 in the capsid protein and highlight the interdependence of key residues within TW10, helix 6, and the CypA loop to explain the observed inflexibility of sequence changes in this region. As with other epitopes associated with successful HIV/SIV control (4
), the capsid protein is a promising target for effective CTL, perhaps because of selection pressure against single escape mutations arising in the absence of linked compensatory changes.
Recent studies from the SIV macaque model of HIV infection have demonstrated the similar phenomenon of pyrrhic escape, that is, viral escape from a particular CTL specificity followed by immune control (26
). However, it was noted from follow-up studies that escape mutation within a single capsid protein epitope, albeit occurring at a fitness cost, may not be sufficient to explain the subsequent immune control of immunodeficiency virus enjoyed by those macaques (22
). In other words, HLA-B*57 and -B*5801-restricted CTL responses in addition to that to TW10 may be required to explain the successful control of HIV observed in persons expressing these HLA alleles.
The putative compensatory mutations that are described in this study also warrant further discussion. The associations between particular Gag capsid protein polymorphisms and the TW10-related escape mutations identified suggest a biological interdependence between the linked mutations. However, it is not known how this interdependence operates. Although the data presented have emphasized a potential structural role for these reciprocal changes, the compensatory mutations may operate via a wholly different mechanism. For example, the H219X mutation observed almost exclusively in association with the T242N mutation in B*57/5801-positive subjects (Fig. ) (only 1 of 48 B*57/5801-positive subjects having the H219X mutation did not have the T242N mutation; P
= 2.5 × 10−6
) may be selected as a means by which viral replicative capacity is restored to its original level, but in a way that is independent of the structural impact of the T242N substitution. Our own preliminary data (not shown) indicate that the introduction of the substitution H219Q increases the replicative capacity of the virus carrying the T242N mutation in vitro. This would be consistent with a recent paper demonstrating that H219Q and H219P mutations increase viral replicative capacity independent of any change at T242 (15
). Similarly, under pressure from particular protease inhibitors, a necessary requirement for the replication competence of drug-resistant virus was the development of substitutions in Gag such as H219Q (16
Furthermore, these data emphasize the predictable nature of escape mutations, prompting the hypothesis that prior vaccination with both wild-type virus and the anticipated escape mutants would limit viral escape options even further than in natural infection. Primary responses in individual subjects can be generated both to wild-type TW10 and to the escape forms (10
). The absence of TW10 escape is associated with long-term control of HIV (27
), so simultaneously shutting down all viable escape options may prove a highly successful strategy against HIV-1, if it can be implemented. The use of structural data to aid in identification of the limits of the capacity of the virus to evade potent early CTL responses thus forms a valuable contribution to vaccine design.