ACTG 709 is one of the largest studies to date to evaluate prospectively the impact of ART followed by TI on HIV-specific responses in acutely and recently infected individuals. This study was designed to investigate the findings of Rosenberg et al.4
, which were subsequently confirmed by Kaufmann et al.14
, that the majority of treated seroconverters with robust HIV-specific lymphoproliferative responses during ART were able to maintain viral loads below 5,000 copies/mL for at least 6 months after one or more TIs. The present study is distinct from these two previous studies in that subjects were not limited to those with robust HIV-specific lymphoproliferative responses, but instead all seroconverters who received early ART and underwent TI were included. Thus, the present study provides information on the impact of this strategy on immune responses in a wider population of seroconverters than those two previous studies. Results of the present study revealed: 1) a minority of subjects treated during primary HIV infection had detectable HIV-specific lymphoproliferative responses; 2) HIV-specific CD8+ T-cell interferon-gamma responses increased in magnitude and breadth during one year of suppressive ART, but were not related to HIV-specific lymphoproliferative responses; and 3) HIV-specific CD8+ T-cell interferon-gamma responses were not associated with virologic success during TI. Collectively, these data do not support the hypotheses that early treatment of HIV infection frequently results in robust HIV-specific lymphoproliferative responses, that induction of robust HIV-specific lymphoproliferativeresponses in individuals treated during primary infection augments HIV-specific CD8+ T cell interferon-gamma responses, or that virus-specific CD8+ T-cell interferon-gamma responses mediate viral control during TI.
HIV-specific lymphoproliferative responses were observed in 24% of subjects in the present study and in most instances were transient. The frequency of lymphoproliferative responses in individuals treated during primary infection has varied widely in other studies ranging from 25% to 100% of those treated for one year 4, 15, 16
. Differences in types and concentrations of HIV antigens used in assays may partially account for these discrepancies. A strength of our study is that seronegative subjects who were studied concurrently demonstrated no evidence of HIV-specific responses, thereby confirming the specificity of the assay. Lymphoproliferative responses in the present study may have been blunted because specimens for immune function assays were shipped overnight or held for 24 hours prior to performance of the assays, procedures that we have previously shown reduce lymphoproliferative responses8
. The abundance of lymphoproliferative responses to non-HIV antigens, however, indicates that loss of lymphoproliferative responses was relative and not absolute. Importantly, robust lymphoproliferative responses are reduced, but not ablated by shipment of specimens8
, suggesting that most individuals in our study did not develop robust HIV-specific lymphoproliferative responses despite early initiation of ART.
All three subjects who harbored HIV-specific lymphoproliferative responses prior to TI achieved virologic success, whereas only 39% of the 18 subjects without HIV-specific lymphoproliferative responses maintained low level viremia for at least 6 months during TI. These findings are consistent with previous studies in acute 4, 15
and chronic HIV infection17, 18
that have demonstrated an association between the presence of HIV-specific lymphoproliferativeresponses, which are largely mediated by CD4+ T cells11, 12
, and virologic success upon TI. Initially, many interpreted this as evidence of “immune control” of HIV replication4
. Nevertheless, multiple lines of evidence including data from this study suggest that HIV-specific CD4+ T-cells do not control viremia. HIV-specific CD4+ T-cells are highly susceptible to HIV infection in vivo19
. Although short bursts of viremia boost virus-specific CD4+ T-cells, the majority of these cells are depleted in association with loss of HIV-specific lymphoproliferative responses during prolonged TI20
, consistent with the transient nature of HIV-specific lymphoproliferative responses we observed. Furthermore, induction of HIV-specific lymphoproliferative responses through therapeutic vaccination in a previous study of individuals treated during primary infection failed to reduce viremia during TI21
. Subjects with HIV-specific lymphoproliferative responses in the present study tended to have lower baseline viral loads than those without HIV-specific responses, and lower baseline viral load was also related to virologic success in the parent study7
. Similarly, Kaufmann et al.14
reported that the major predictor of low level viremia during the first treatment interruption was a low viral load prior to initiation of ART. Collectively, these data suggest that preserved HIV-specific lymphoproliferative responses are the consequence of a relatively less virulent HIV infection and not the cause of it. An entry criterion for both Rosenberg’s and Kaufmann’s studies was a robust HIV-specific lymphoproliferative response (SI≥10) prior to TI 4
. Selection of subjects with less virulent HIV infection may explain why more subjects achieved virologic success in their studies, 88% and 57%, respectively, compared to ACTG 371 (40%)7
HIV-specific CD8+ T-cell interferon-gamma responses increased in both magnitude and breadth during one year of ART, but were not predictive of virologic success, similar to what has been reported in other studies of TI in primary infection5, 14, 15
. Importantly, the present study illustrates that CD8+ interferon-gamma responses can be augmented during TI even in subjects without vigorous HIV-specific lymphoproliferative responses prior to TI. Marked epitope- and allele-specific differences in the ability of HIV-specific CD8+ T cells to neutralize HIV-1 in vitro have been observed despite similar interferon-gamma responses22
, suggesting that interferon-gamma responses are not reflective of CD8+ T cell antiretroviral activity. More recently, the interaction between peptide and MHC class I alleles has been linked to durable virologic control in chronic HIV infection23
. Associations between low-level viremia and HIV-specific CD8+ T-cell maturation levels24, 25
, cytokine profiles26, 27
, and release of a soluble, non-cytotoxic factor28
have been described as well. Whether these characteristics of CD8+ T cells account for the virologic control seen in subjects in the present study, and whether they are modifiable by early initiation of ART are unanswered questions.
If timing of initiation of ART in primary infection is critically related to preservation of lymphoproliferative responses or magnitude of interferon-gamma responses, one would anticipate that subjects treated earliest would have the most vigorous immune responses. In the present study there was no evidence for more robust immune responses to HIV or other antigens in those treated in acute versus recent infection. In the parent study ACTG 371 7
and in the study by Kaufmann et. al.14
, the timing of initiation of ART was not related to durability of virologic control during TI. Because subjects were not randomized to treatment according to disease stage, it is not possible to derive definitive conclusions from these observations. Nevertheless, these data suggest that there is no dramatic enhancement of antigen-specific lymphocyte proliferative responses or HIV-specific CD8+ T cell responses from early initiation of ART in primary HIV infection.
Impairments in lymphoproliferative responses to all non-HIV microbial antigens were demonstrated at baseline in HIV-infected subjects compared to seronegative subjects. ART initiated during primary infection was associated with normalization of lymphoproliferative responses to the ubiquitous antigens Candida
and CMV, whereas neither tetanus nor MAC-specific lymphoproliferative responses normalized on ART, consistent with findings in chronic infection 29, 30
. Importantly, lymphoproliferative responses to all non-HIV microbial antigens and alloantigens worsened during TI and were impaired 4 weeks after TI compared to those in seronegative controls. This is the first study to report the impact of TI on lymphoproliferative responses to multiple non-HIV microbial antigens and alloantigens in individuals treated during acute HIV infection. These findings contrast with those of Rosenberg et. al.4
who reported that tetanus-specific lymphoproliferative responses were not altered in the context of TI, and concluded from these limited data that the intervention was safe. Progressive impairments in lymphoproliferation to HIV antigens, recall antigens, alloantigens, and mitogens were the first functional defects described in association with HIV infection 31
. These impairments correlate with CD4+ T-cells, but are also predictive of disease progression independent of viral load and CD4+ T-cell count 32
. Taken together, these data suggest that TI in individuals treated during early HIV infection results in significant immune impairments.
Enthusiasm for TI as a strategy to reduce antiretroviral drug use has waned substantially over the past five years. TI during chronic HIV infection resulted in worse clinical outcomes in subjects enrolled in the SMART study33
. In addition, observational studies suggest that initiation of ART at CD4+ T cell counts above 500 cells/mm3
is associated with better clinical outcomes than when ART is initiated at conventional CD4+ T cell threshold levels34
. Data from the present study demonstrating impairments in antigen-specific lymphoproliferative responses during TI in subjects with primary infection further support the notion that continued ART is beneficial even in early disease. New initiatives to enhance HIV prevention by early treatment of all individuals with ART35
further weigh against strategies that use TI. Recent data from a randomized controlled study of nine months of ART followed by TI versus standard of care treatment for primary HIV-infection suggest that early treatment confers a 4 month delay in initiation of ART36
, consistent with observations from Kaufmann et. al’s uncontrolled study that virologic control conferred by early ART is not durable14
. If additional data from larger randomized controlled studies confirm this modest benefit, it seems unlikely that this strategy will continue to be pursued. Nevertheless, the improvement in virologic control conferred by ART in primary infection remains unexplained. A better understanding of the mechanisms underlying the transient virologic control conferred by early treatment and TI could nonetheless provide important insight into HIV pathogenesis and potentially lead to development of novel and more durable therapies to treat and prevent HIV infection.