Long-term CD4+ lymphocyte response following HAART initiation in a U.S. Military prospective cohort

Among those with human immunodeficiency virus (HIV) infection, the CD4+ T-lymphocyte count is the major indicator of immunodeficiency, a main factor in deciding whether to initiate highly active antiretroviral therapy (HAART), and an important parameter in monitoring treatment response [1, 2]. Failure to restore a normal CD4+ count following HAART is associated with increased morbidity due to both AIDS and non-AIDS events, as well as increased mortality [3–5].

Studies of the kinetics of CD4+ count response post-HAART indicate that the CD4+ count increases rapidly during the first 3-6 months, in part due to release of memory T-cells from lymphoid tissue, and then increases slowly during the next 3-4 years, reflecting reconstitution of the immune system [6–10]. The magnitude of CD4+ recovery may depend on a variety of factors, including maintenance of virologic suppression, age, and CD4+ count at HAART initiation [1, 7, 9, 11–20].

The question of whether those initiating HAART will continue to increase their CD4+ count after 4-5 years or will plateau has been debated in the literature, and remains unclear. Some studies have suggested that normalization of CD4+ counts in HIV-infected persons can be achieved if viral suppression with HAART can be maintained for a sufficiently long period of time [19]. In one study, after > 5 years on HAART, patients with viral suppression who started at ≤200 cells/mm³ had an adjusted annual increase of 32 cells/mm³, attaining an average CD4+ count of 497 cells/mm³[19]. Another study statistically estimating the CD4+ trajectory concluded that those starting HAART at ≤200 CD4+ cells who remained on therapy would continue to increase through 7 years, although 25% still had ≤350 cells at 7 years [20]. One small study of 16 patients followed for up to 10 years with strict viral control based on HIV RNA detection using ultrasensitive techniques showed continued positive increases in CD4+ counts, although this study represented a small group of highly selected patients [21]

On the other hand, other studies report that the average CD4+ count may level off after 4-6 years following HAART initiation, even among patients with viral suppression [12, 13]. Given this leveling off, many patients who start at lower CD4+ counts, even after years on HAART with early CD4+ increases, may fail to reach a normal CD4+ threshold. In one study of those with sustained viral suppression who started HAART at ≤200 CD4+ cells/mm³, after 6 years only 42% had ≥ 500 CD4+ cells/mm³, and only 12% had >750 cells/mm³[12]. In another study, 44% of those starting therapy with a CD4+ count <100 cells/mm³ and 25% of those starting HAART with a CD4+ count of 100-200 cells were unable to achieve a CD4+ cell count >500 cells/mm³ over a mean follow-up of seven years, and many did not reach this threshold by year 10 [18].

The important question of the long-term CD4+ count response therefore remains unresolved. This question is especially relevant for those who start HAART at lower CD4+ counts. Despite current recommendations to start HAART at CD4+ counts of 350 cells/mm³ or greater [1, 2], the reality is that many patients, even in developed countries, are still being diagnosed and initiate treatment late in the course of their HIV infection [22, 23].

An additional methodological challenge in using observational data to evaluate the long-term effect of CD4+ count at HAART initiation on subsequent response is that those starting HAART at lower CD4+ levels may have been infected for longer periods of time. If the post-HAART response is affected by duration of HIV infection, comparing different strata without accounting for the fact that those initiating HAART at lower CD4+ levels may have a longer lead-time can result in biased group comparisons [24].

We were able to address both of these issues by analyzing data from the U.S. Military HIV Natural History Study (NHS) [25]. This prospective cohort of HIV-infected U.S. military personnel has followed some participants for up to twelve years after availability of HAART. Because all active duty personnel are confirmed to be HIV-negative prior to enlistment and undergo routine HIV screening, HIV seroconversion (SC) date can be reliably determined for the majority of members. All cohort members have free access to care and availability of therapy. Data from this cohort were analyzed to determine the long-term CD4+ count trajectory after HAART initiation, as well as the influence of baseline CD4+ count, duration of HIV infection, and other covariates on post-HAART CD4+ response.

Among HIV-positive persons starting HAART, we identified a rapid average increase of 93-151 cells during the first six months in all baseline CD4+ strata, followed by a continued average increase of 22-36 cells per year through the first four years. Among VL suppressors, these increases were even greater, with an average of 119-177 cells during the first phase, followed by an average of 51-66 cells per year during the second phase, through 4 years. For example, a patient who starts HAART with a CD4+ count of 125 cells/mm³ and who maintains viral suppression will on average have an increase to about 500 cells/mm³ at the end of four years.

A major purpose of this analysis was to identify whether after four years the CD4+ response continues to increase or plateaus. Among all participants, the average third-phase response was slightly positive (8.6 cells/year) in the lowest CD4+ baseline strata (≤200 cells), slightly negative (-8.1 cells/year) in the highest strata (≥500 cells), and essentially flat (with 95% CI overlapping zero) in the two middle strata. However, among VL suppressors, we identified positive average increases of approximately 12-16 cells/year, with no significant differences in third-phase slopes between any of the baseline strata. This supports the general conclusion that if viral suppression can be maintained through effective and uninterrupted HAART, a continued pattern of CD4+ count improvement may occur in most patients, irrespective of CD4+ count at HAART initiation.

There are several caveats to this overall conclusion. Although increases four years after starting HAART in viral suppressors continued to be positive, they were small. This provides support for current guidelines to start HAART at higher CD4+ levels, before severe immune suppression has occurred [1, 2, 30]. In addition, our analysis indicates that a variety of other factors may affect and modulate the CD4+ response curve, including nadir CD4+ cell count, AIDS prior to HAART start and pre-HAART duration of HIV infection.

Our analysis is consistent with other studies identifying nadir CD4+ count as a predictor of CD4+ cell response [31, 32]. A lower nadir CD4+ count may reflect a more profound disturbance of T-cell homeostasis, with more severe immunological deficits that cannot be reversed even with HAART-induced viral suppression [33]. For example, in one analysis of response to immunization in those with normal CD4+ counts and viral suppression after more than a year, a lower CD4+ nadir before HAART predicted poorer vaccination response [34].

Our analysis also identified a clinical AIDS diagnosis preceding HAART as a predictor of a poorer CD4+ response. This may be another reflection of functional or other immune deficiencies in response to HIV infection that lead to a less robust immunologic recovery. This finding supports current recommendations to initiate HAART in all patients with a history of an AIDS-defining illness, irrespective of their current CD4+ count [1, 2].

A major finding of this analysis was the strong negative effect of pre-HAART duration of HIV infection on CD4+ cell response to HAART, even after controlling for viral suppression, CD4+ count and other factors. A previous study [14] also identified duration of infection as a predictor of CD4+ response, but duration was based upon time from the first recorded HIV test rather than the entire estimated period of HIV infection, as this analysis was able to do. Potential immunopathogenic explanations for why a longer time from HIV SC to HAART start results in a more impaired capacity for immunologic recovery include decreased CD4+ cell production or excessive CD4+ cell destruction. For example, it has been proposed that CD4+ T-cell hyperactivation may persist even after HAART virologic suppression, and that this results in greater apoptotic cell death [32, 33, 35–37]. Our finding that both higher baseline VLs and longer duration of pre-HAART infection were predictive of poorer immunologic response suggests that long-standing high levels of viral replication may lead to persistent T-cell activation or other T-cell dysfunction which cannot be fully reversed even after HAART introduction.

This analysis has several potential limitations. First, by definition those who were followed for >5 years represent "healthy survivors"; those who died or who dropped out of the study because of illness soon after starting HAART would not be captured in the third-phase analysis. Although for the great majority of enrollees death was uncommon, it was more common among those in the lowest CD4+ cell strata. For example, a preliminary analysis of mortality among those initiating HAART at ≤200 CD4+ cells/mm³ identified a 6-year death rate of 18%, compared to rates of 3%-5% for those initiating HAART at the higher CD4+ cell strata (IDCRP, unpublished data). Although many patients are still followed in the military health care system even if they are no longer on active duty after an AIDS diagnosis, some patients with advanced disease may have separated from the military system and had their health care transferred to the Veteran's Affairs or other health systems. However, even if such a healthy survivor effect did occur, we do not believe that it significantly affected our overall conclusions. Such an effect would most likely occur in the lowest (≤200 cell) CD4+ stratum, and among viral suppressors, the third-phase CD4+ increases we saw in this stratum were not significantly different from those seen in the other baseline strata. Nonetheless, the increased mortality seen in the lowest group provides additional support for current guidelines to start HAART before severe immune suppression has occurred.

Second, in this observational study, the four baseline strata were not randomized, and group differences may be due to unmeasured confounding. We tried to limit the extent of confounding by adjusting for many HIV-related factors, as well as time-dependent covariates including VL and HAART use. Analyzing our data in different ways, including through several different exploratory analyses, did not change our overall conclusions.

Third, although we adjusted for different classes of drug therapy at HAART initiation in our exploratory sensitivity analyses, we did not present data on specific ART drugs. However, this was not the intent of this analysis. Even within a given antiretroviral class, there is considerable variation depending on potency, drug-drug interactions, use of ritonavir boosting for PIs, and multiple other factors. Clinicians may select individual drugs for a HAART regimen based on a variety of factors, and information about the efficacy of specific ART drugs and regimens is best obtained through randomized trials.

Finally, this cohort is characterized by a number of specific demographic and clinical factors, and results may vary for other populations with different characteristics. For example, 96% of our study sample was male, and the median age was 36 years. Also, given the specific structured testing schedule in the military, it is likely that many patients in this cohort were diagnosed with HIV earlier than typically seen in clinical practice.

This analysis also has several strengths. First, in contrast to many other HIV cohort studies, we were able to estimate SC date and time from HIV SC to HAART start. The fact that this variable consistently emerged as a significant predictor of CD4+ response supports the importance of including this covariate in our analysis.

Second, follow-up in this analysis extended for some patients out past 8 years, considerably longer than most other observational studies. This analysis therefore provides an important contribution to the literature concerning the long-term third-phase CD4+ response to HAART, especially in those patients who maintain virologic suppression.

Third, because HIV treatment in the military is free, availability of care and access to therapy were not barriers confounding our results. In fact, viral suppression rates in this cohort have previously been reported as approaching those in clinical trials [25].

Among HIV-infected persons who initiated HAART at different CD4+ levels and who were followed in some cases for over ten years, we identified a rapid followed by a more gradual increase in CD4+ cells for the first four years. After this time, among those who maintain viral suppression, our results suggest that in all strata, there will on average be a positive but small average increase of about 12-16 cells per year. However, multiple factors may influence this immunologic response, including CD4+ nadir, a preceding AIDS diagnosis, and, importantly, time from HIV infection to HAART start. Our findings strongly support the conclusion that immunologic response to HAART is maximized if treatment is started with virally suppressive therapy as early as possible.