In all but 19 subjects, CD4 counts were substantially reduced during TI. In total, over a median TI duration of 5.5 months, the median (IQR) CD4 loss was 335 (167-512) cells/μL. Among those with reduced CD4 counts at treatment restart, 66 (40.2%) subjects experienced an increase in CD4 to pre-TI levels. The estimated cumulative probability (95% CI) of experiencing an increase in CD4 to pre-TI levels at 3, 6, 12, 18, and 24 months was 11.6% (7.6%, 17.6%), 23.6% (17.7%, 30.9%), 31.3% (24.6%, 39.3%), 38.3% (30.7%, 47.1%), and 45.5% (36.7%, 55.4%), respectively (Fig. 1).
In total, 125 (68.3%) subjects experienced an immunological response (an increase in CD4 count from baseline of >200 cells/μL) with the median (95% CI) time to immunological response being 8 (5.7, 11.6) months. The cumulative probabilities (95% CI) of gaining at least 200 CD4 cells at 3, 6, 12, 18, and 24 months after treatment resumption were 24.1% (18.5%, 30.9%), 43.1% (36.2%, 50.6%), 58.1% (50.8%, 65.6%), 68.3% (60.7%, 75.7%), and 76.0% (68.1%, 83.1%), respectively (Fig. 1).
Patients who initiated their first HAART regimen when they were ART naive, thus not exposed to mono or dual therapies before their first HAART regimen, were more likely to experience an immunologic response compared with those with pre-HAART ART experience, although this effect was marginally nonsignificant [hazard ratio 1.39 (0.94, 2.06), P = 0.10). Patients with higher pre-TI CD4 counts were more likely to experience an immunologic response (HR per 100 cells/μL higher: 1.25 (1.16, 1.34), P < 0.001]. However, after adjusting for the pre-TI CD4 count, subjects who reinitiated HAART with higher CD4 counts at the time of treatment resumption (ie, those with smaller CD4 losses during TI) were less likely to experience an immunologic response [HR per 100 cells/μL higher: 0.88 (0.79, 0.98), P = 0.02]. Subjects who initiated a PI-based HAART regimen after TI were more likely to experience an immunological response than those who initiated a NNRTI or triple NRTI-based regimen [2.51 (1.54, 4.10), P < 0.001]. At the same time, patients who initiated the same class of therapy as they were receiving immediately before TI were less likely to experience an immunological response [0.60 (0.41, 0.87), P = 0.007] than those who initiated a regimen based on a different drug class.
CD4 Trends After Treatment Resumption
Observed cross-sectional medians (IQR) before and after treatment resumption are shown in Figure 2A, whereas modeled values (with 95% CI) are shown in Figure 2B. The CD4 increase after treatment resumption was biphasic with the slope being steeper during the first 3 months after resumption (Fig. 2B) than subsequently. The median (95% CI) CD4 increase (on the square root scale) per month during the first 3 months after treatment resumption was 1.08 (0.91, 1.25). In contrast, the increase after the first 3 months was relatively slow and only marginally different from 0 [mean (95% CI) rate of increase: 0.32/yr (−0.05, 0.70) on the square root scale]. At 3, 6, 12, 18, and 24 months after treatment resumption, the estimated median (95% CI) CD4 increases were 149 (125, 173), 153 (130, 177), 161 (136, 186), 170 (140, 199), and 178 (141, 214) cells/μL, respectively.
Factors found to affect initial and subsequent rates of CD4 change after resumption of HAART are shown in Table 3. Subjects who had experienced a virologic response (viral load <500 copies/mL) on their pre-TI HAART regimen tended to have higher CD4 counts at the time of treatment resumption (data not shown); these individuals then tended to experience more rapid increases in CD4 counts in the first 3 months after resumption of therapy but slower increases subsequently. Individuals who had experienced less rapid CD4 decline during TI (ie, quartiles 2, 3, and 4 compared with quartile 1) tended to experience less rapid increases in CD4 count in the first 3 months after treatment resumption. The CD8 slope during TI was positively correlated with the CD4 count at treatment resumption (data not shown) but was negatively associated with the rate of CD4 increase during the first 3 months. Subjects who resumed HAART with a higher (≥5,000 copies/mL) viral load experienced a more rapid rate of CD4 increase during the first 3 months after treatment resumption. Although initial (ie, during the first 3 months after HAART resumption) rate of CD4 increase was similar among those with sustained, initial, or nonresponse to HAART resumption, subsequent rates of CD4 increase were significantly higher in sustained responders (Table 3). It is worth noting that when we looked at the gain of >200 CD4 cells/μL, the percentage was larger in those with sustained virologic response followed by those with initial virologic response.
Patients who resumed treatment with a PI-based regimen tended to experience a more rapid rate of CD4 increase during the first 3 months after resuming therapy than patients starting other regimens; in contrast, those who reinitiated HAART with the same class of drug as they had been receiving pre-TI tended to experience a less rapid rate of CD4 increase in the first 3 months.
Despite the publication of findings from several important studies reporting the negative short- and long-term impacts of TIs, it is unlikely that all patients will manage to maintain lifelong HAART, particularly if there is a move toward earlier initiation of HAART.16 The best illustration of this was reported by SMART trial investigators, who reported that 1 year after the premature closure of the study, and against the advice of trial investigators, only 84% of the patients who had originally been randomized to the treatment discontinuation arm had restarted treatment.17
The present study was set-up with the aspiration of providing some help to clinicians when advising their patients on the possible immunological consequences of taking a TI. An increase of 200 cells/μL was chosen to define immunological response because even for patients restarting HAART with a very low CD4 count, such an increase could permit them to discontinue prophylactic regimens. Whilst over two-thirds of patients included in our study experienced a CD4 increase of at least 200 cells/μL after a median of 8 months after resuming therapy, only 45.5% of the patients reached their pre-TI CD4 level by 2 years after resuming therapy. These findings take on greater relevance when considering 2 methodological aspects of our study. First, to be eligible, all patients had to have a pre-TI CD4 count of >500 cells/μL; this restriction was included because a TI at lower CD4 counts would be associated with an unacceptable level of clinical risk,4-6,8 and also because we wanted to ensure that included patients had a strong immune system that could be reconstituted on resumption of HAART. Second, despite the high pre-TI CD4 counts of patients included in the study, the duration of TIs were shorter than those in most trials, including our own.4-8 These patients often decided to discontinue treatment by themselves, thus it could be plausible that they were nonadherent and that immunological response was a direct consequence of an uncontrolled viremia after treatment was restarted. On the contrary, more than 90% of the patients reached an undetectable viral load after resuming HAART. A possible explanation for the differences among effect of viral suppression on CD4 gain could be the CD4 values at HAART initiation among these groups of patients.
Our data have shown that the first phase of CD4 increase seems to be independent of whether one has complete virologic suppression or not, whereas in the second phase CD4 increases are higher in those with a complete virologic suppression. This finding could be explained by the fact that, as shown in naive patients, the first increase in CD4+ cells after therapy is due to the redistribution of memory cells18 that are already present in the organism and dissequestered from lymphoid organs. The second phase of increase, due to naive cells, requires the presence of an intact pool of lymphoid precursors that are generated in the thymus. Most of these cells express the CD4 molecule, can be infected by HIV and thus undergo cell death.19 It is therefore likely that even a low amount of the virus can impair the intrathymic process of T-cell maturation, and that a complete suppression of viral production is essential to have functional T-cell precursors able to generate peripheral blood CD4+ lymphocytes.
Because most of these patients restarted treatment while their CD4 count was above 350 cells/μL, it could be argued that a return to pre-TI CD4 count levels by 2 years may have minimal clinical significance as most patients would already be protected from complications. However, it is important to evaluate the capability of the immune system to reconstitute after a break in therapy. In that regard, the finding that the time taken for the CD4 count to return to pre-TI levels after resuming therapy was substantially longer than the duration of the TI itself (and therefore that the rate of CD4 loss during TI is more rapid than the rate of CD4 gain after resumption of therapy) is concerning. The factors that determine CD4 response in patients resuming therapy are only partially known, but clearly depend on both the host and the virus.20,21 Apparently, the posttreatment resumption immune response was more rapid in patients who had started HAART while ART naive. This may indicate that the immune system may be less damaged, but further studies on the quality of CD4+ T cells are urgently needed to ascertain, for example, the degree to which the CD4 counts that are restored are of memory or naive phenotype. In ARV-naive patients starting HAART for the first time, the CD4 increase is biphasic, with a far more rapid increase in CD4 count in the first 3 months. This initial increase follows a reduction in T-cell activation, and primarily consists of a release of memory CD4 cells that have been trapped in lymphoid tissue (ie, cell redistribution).22,23 It is currently unknown whether the same phenomenon occurs during treatment resumption after TI, but the fact that patients with steeper CD4 declines during TI have steeper CD4 increases in the first 3 months following resumption of treatment supports, at least in part, a similar CD4 redistribution.
In our patients, immune reconstitution was less marked in those who had experienced slower CD4 losses during TI, or who restarted treatment with a higher CD4 count. This phenomenon has also been described in ARV-naive patients starting treatment for the first time,24 although these findings are not universal.25,26
A higher viral load at treatment resumption was associated with a better immunologic response as is also seen in patients initiating HAART for first time.24,27
Interestingly, we found that patients with steeper CD8 drops during TI had faster CD4 increases after restarting HAART, even after adjusting for the CD4 slope during TI. It is interesting that this CD8 effect persists even after adjusting for the CD4 slope during TI. An important predictor of immune response is the level of immune activation which contributes to CD4+ T cell death via apoptosis. Indeed, it has been shown in both animal models28 and in ARV-naive patients that a greater increase in CD4 cells is associated with lower CD8 cell counts at baseline.29 There may be 2 different, although not mutually exclusive, explanations for this phenomenon. First, during HIV infection, CD8+ T cells have the role of controlling viral production by killing infected CD4+ T cells. However, it has been shown that they not only kill such cells, but also eliminate cells that are not infected but bound to viral peptides, or those that are undergoing excessive activation.30
We found that patients who reinitated HAART with the same class of drug that they had been receiving immediately before TI tended to experience poorer immunologic responses after resumption of therapy. It is possible that this finding may reflect the presence of resistance mutations at the time of TI which then compromise the individuals' response to the same regimen in the future, or to another regimen of the same class. Alternatively, as this strategy may not now be recommended (particularly if patients have experienced virologic failure on treatment) then rather than representing genuine structured TIs, treatment breaks in these patients may reflect periods of particularly poor adherence to treatment; responses to treatment may be poorer in these patients once they restart treatment if adherence problems remain. Unfortunately, information about the specific drugs received before TI and after treatment resumption was limited and therefore we cannot investigate these trends in more detail.
Our study has several other limitations. First of all, it was an observational study, thus the criteria for interruption was not established neither was the duration of follow-up after restarting HAART. Second, it was not possible to analyze the T-cell subsets in detail, and in particular the degree of differentiation of either CD4+ or CD8+ T cells in terms of newly produced, virgin, or memory lymphocytes. Thus, we can only give a partial explanation, quantitative and not qualitative, of immune reconstitution. Finally, we studied only one cycle of TI, thus we do not know what could happen after several TIs.
In conclusion, the immune system responds to HAART in the same manner in naive patients as in patients after 1 cycle of TI. It is, therefore, possible to predict the response to antiretroviral treatment reintroduced after TI on the basis of the previous response to HAART. Finally, data on qualitative immune reconstitution are not available, but, at least from a quantitative point of view, we can say that even 1 cycle of TI could have a detrimental effect on immune system.
No author had any conflict of interest relevant to the article.
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Keywords:© 2009 Lippincott Williams & Wilkins, Inc.
HAART resumption; virologic response; immunological response; prognostic factors; treatment interruption; HIV infection