This longitudinal dissection of the cellular immune response in a person who died within 45 months of presenting with the acute HIV-1 infection syndrome provides important insights into virus-specific cellular immune responses during rapidly progressive infection. Despite strong in vivo activated CTL responses at the time of seroconversion, viremia was not contained and progressive CD4 cell decline ensued. Although strong CTL responses were observed at the time of lowest viremia, increasing viremia was not associated with the development of detectable CTL escape mutations in the immunodominant Env epitope, and the dominant CTL responses in the earliest stages of infection remained present in vivo until the time of death. CTL responses declined progressively in peripheral blood; however, these CTL could be readily expanded in vitro to large numbers, arguing against senescence and exhaustion as contributors to the CTL decline. In addition, no new epitopes were targeted as the disease progressed. Together, these data suggest a defect in in vivo activation of CTL as a contributor to disease progression. The lack of detectable HIV-1-specific CD4 proliferative responses and the lack of broadening of the CTL response over time provide further support for this conclusion.
The results of this study demonstrate a strikingly static CTL response over the course of progressive infection. The initial detectable CTL response was directed against a single epitope and broadened to target a second epitope within 3 months of presentation. However, despite ongoing viral replication, the response never broadened further and these two responses could still be detected at the time of death. Clones obtained at the time of initial presentation with the acute HIV-1 infection syndrome were identical in epitope specificity and in their ability to recognize in vivo virus variants, as demonstrated by experiments with limiting peptide concentrations. Furthermore, both of the epitopes were presented in the context of HLA B7. No other class I alleles were identified to be involved in presenting viral proteins for CTL recognition. Importantly, this included the lack of detectable CTL responses to well-defined epitopes presented by HLA-A*0201, which are targeted by more than 70% of A*0201-positive persons (4
). CTL directed against the dominant HLA-A*0201 Gag epitope are potent inhibitors of HIV-1 replication in vitro and have been associated with control of viremia in vivo (33
). Epitope competition is not likely to have hindered the presentation of other epitopes (42
), since we have observed A2-restricted responses in other persons who coexpress HLA B7 and target the same gp41 epitope as our subject (unpublished data). The functional CTL data presented extend studies based on T-cell receptor TCR analysis suggesting that a narrowly directed initial response is associated with more rapidly progressive infection (21
). The finding of a second patient with rapidly progressive infection and a monospecific response to the HLA-B7-restricted epitope within gp41 further suggests that the specificity of the initial response as well as its breadth may play a role in determining the rate of disease progression. The present results also indicate that narrowly directed initial CTL specificities can be maintained throughout the course of progressive infection. The inability of these patients to form CTL with new specificities may be related to the phenomenon of original antigenic sin, as has been described in mouse models (25
These longitudinal studies also provide evidence that viral escape from CTL recognition is not a requirement for disease progression. Consistent with a study of acute infection (2
), we noted sequence variation within CTL epitopes, consistent with selection pressure mediated by CTL. However, within the limitations of the assays performed, these mutations did not confer escape from recognition by established CTL responses. Observed mutations did not alter the binding motif which has been suggested to play a role in effective immune system escape (9
), and CTL clones recognized epitopes representing dominant in vivo virus variants as well as wild-type virus. Moreover, some species that were detected as escape variants never became dominant in vivo. Thus, we conclude that mutations within CTL epitopes did not account for the inability of the CTL response to control viremia.
The continued presence of CTL in the peripheral blood from the time of presentation until the patient’s death argues against clonal exhaustion as a mechanism for immune system evasion. CTL clonal exhaustion, resulting from chronic exposure to high antigen levels, has been shown to result in immune system escape in a murine model of lymphocytic choriomeningitis virus infection (31
), but its role in HIV-1 infection is not clear. Clearly, CTL were not completely lost in this rapid progressor as his disease progressed, since HIV-specific CTL of the identical epitope specificity and HLA restriction were readily obtainable from PBMC harvested as late as 48 h before his death. Such persistence of CTL clones has been observed in other studies, with individual clones being detected for up to 5 years (20
). Sequencing of the T-cell receptors of these clones will be necessary, however, before it can be stated definitively that clonal exhaustion did not occur in this case. Additionally, early data on soluble-factor production by the immunodominant Env-specific CTL clones suggest that they are capable of producing large quantities of soluble inhibitory factors when stimulated, arguing against an intrinsic defect in effector function in these activated clones.
The continued presence of CTL able to recognize autologous virus, along with the lack of broadening of the CTL response, suggests that a central immunologic defect in this subject was lack of in vivo activation of CTL. CD4+
helper T cells are known to contribute to the maturation of virus-specific CTL and have recently been shown to be important in the control of viremia (17
). Lack of a detectable CD4 proliferative response to HIV-1 antigens suggests that lack of T-helper-cell function may have played an important role in preventing in vivo activation of HIV-specific CTL and additionally in preventing the formation of CTL targeted against a larger array of epitopes. Findings here are consistent with murine models of LCMV infection in mice lacking CD4+
T lymphocytes. In these models, CTL are vigorously induced early in infection but do not persist in the absence of help, and viremia is not controlled during the chronic phase of infection (28
). This is precisely what was observed in our study in that a strong CTL response was observed initially but declined rapidly and virus breakthrough occurred.
These data provide a rationale for efforts to augment immunity in HIV-1-infected individuals by broadening the CTL response to include multiple epitopes and augmenting T-helper-cell responses. Earlier studies suggested that CTL directed against Gag regions of HIV-1 are associated with less rapid progression (24
), while CTL directed against Env and Pol epitopes may be less efficient at inhibition of HIV-1 replication in in vitro models (51
). Further comparison of rapidly progressive HIV-1 infection to nonprogressive infection may therefore allow a more specific targeting of vaccine strategies to ensure that augmented immune responses are directed against a variety of epitopes which most efficiently inhibit viral replication.