Factors associated with paradoxical immune response to antiretroviral therapy in HIV infected patients: a case control study

Highly active antiretroviral therapy (HAART) has been introduced in the therapeutic arsenal for HIV infection since the 90's. The efficacy of HAART turned HIV infection into a chronic condition which made it possible to considerably improve its survival rate over the last 20 years [1–3]. Such an improvement was in consequence of the virologic suppression that occurs in approximately 70% of patients during the first regimen of HAART [4], thus making their immunologic reconstitution possible [1, 5].

The frequency of a paradoxical immunologic response to HAART, defined as viral suppression without CD4 cell-count improvement, has been reported in the literature as 8 to 42%, around 15% in most instances [6–25].

Several outcomes have been related to this poor immunologic response. Most of the studies had demonstrated worse outcomes when compared to those with complete response such as the increased risk of AIDS-defining illness or death [14, 16, 23, 25]. Some authors, however, did not observe statistically significant differences between these outcomes in full responders and non-immunologic responders [26, 27].

The efforts to determine the prevalence of this condition and its associated factors are hampered by the absence of standardization in its definitions. Different studies establish different boundaries in the CD4 cell counts or different time frames to cross such boundaries in the characterization of an adequate immune response, making it difficult to compare most of the results [6, 8, 10, 14, 16, 22, 28].

Different associated factors have also been pointed out by several authors, like older age [7, 10, 12, 18, 20, 22, 28–35], lower [19, 20, 23, 24, 32, 34–37] or higher [10, 18, 21, 28, 38–40] baseline CD4 cell count, lower nadir CD4 count [20, 22]; poor adherence to HAART [10, 41]; regimen of HAART [10, 31, 32, 38, 42]; levels of baseline viral load [8, 10, 12, 15, 18, 21–23, 28, 32, 33, 36] and even comorbidities [20, 43] and HIV transmission category [6, 8, 28, 34].

The present study aims to determine, in a cohort of HIV infected patients in Brazil, what factors were independently associated with such a discordant response to HAART.

Out of 934 patients on HAART during the study period (April-September 2009) 563 had undetectable HIV viral load (HIVVL) (limit of detection: 50 copies/ml). Out of 563 with viral suppression, 39 cases and 160 controls were consecutively selected, with individual matching by gender. Controls were derived from 272 medical records sequentially verified. In the group of cases, there were eight females and 31 males, whereas in the control group 36 females and 124 males.

Sampling of cases and controls according to the eligibility criteria of the study is represented in Figure 1.

Logistic Regression Analysis

Figure 2 presents the final hierarchical theoretical model used for multivariate analysis of the results.

Variables that remained significantly different after logistic regression were: total time in use of HAART (adjusted OR 0.981; CI 95%: 0.96-0.99), nadir CD4-count (adjusted OR 0.985; CI 95%: 0.97-0.99) and time of undetectable HIVVL (adjusted OR 0.969: CI 95%: 0.94-0.99) (Table 4).

Characteristics	Univariate analysis			Multivariate analysisb
	p-value	OR	CI 95%	p-value	Aj OR	CI 95%
Total time in use of HAART	0.000a	-	-	0.013a	0.981	0.96-0.99
Concomitant medications over 30 consecutive days in previous year	0.002a			0.109
Yes			-			-
No		0.29	0.13-0.65		0.423	0.14-1.21
Modifications in HAART before previous year due to abandonment or poor adherence	0.018a			0.159
No			-			-
Yes		3.39	1.33-8.65		2.852	0.66-12.25
Regimen containing zidovudine	0.021a			0.359
Yes			-			-
No		2.28	1.11-4.66		1.601	0.58-4.37
AIDS defining illness prior to HAART	0.040a			0.515
Yes		0.88	0.28-2.77		1.475	0.45-4.74
No			-			-
Irregular use of HAART in previous year	0.071a			0.403
Yes		2.22	0.91-5.39		1.888	0.42-8.36
No			-			-
Nadir CD4-count	0.000a	-	-	0.000a	0.985	0.97-0.99
Time of undetectable HIVVL	0.000a	-	-	0.037a	0.969	0.94-0.99

OR, odds ratio; CI 95%, confidence interval of 95%; Aj OR, adjusted odds ratio; HAART, highly active antiretroviral therapy; nadir CD4-count, the lowest CD4 ever presented by patient; HIVVL, HIV viral load; undetectable HIVVL, over time the most common limit of detection was less than 400 copies/ml.

^a p-value < 0,050.

^b Multivariate analysis was performed by the binary logistic regression model using Enter method for all variables with a p-value < 0.1 in the bivariate analysis model who fitted the requirements for logistic regression. It is important to note that the inclusion of variables in this model was performed according to a hierarchical causal theoretical tree previously developed for this study.

For total time in use of HAART, the odds ratio for each additional month was 0.981, i.e., for each additional month of ART use there is a reduction of 1.9% in the risk for paradoxical immune response (PIR). For each additional month of undetectable HIVVL there is a reduction of 3.1% in the risk of PIR (Table 4).

Regarding nadir CD4-count, every increase of one cell corresponded to a reduction of 1.5% in the risk of PIR (Table 4). Otherwise, each increase of 100 cells/mm³ in nadir CD4-count corresponds to a risk reduction of PIR of 77.7%.

Paradoxical Immune Response (PIR) has been surrounded by uncertainties, both regarding its determinant factors and its consequences for the outcome of HIV infection. Despite its retrospective design and use of preexisting data, this study was able to uncover a negative association between PIR and total time in use of HAART as well as time for undetectable HIV viral load. Such an association was not adequately revealed by other studies because of their rigid eligibility criteria in terms of time frames, thus preventing any analysis of the influence of time frames on the outcomes [6, 12, 16, 19–21, 23].

Time of HIVVL suppression can be a surrogate marker of adherence to HAART. Thus, patients who have good adherence and, therefore, a longer time of undetectable HIVVL are less likely to come forward with paradoxical immune response. Such an association has also been reported by others [10, 41]. Long time in use of HAART increases chances of optimal immune response. Several studies regarding this subject used predetermined goals to be met in terms of CD4-count. As an example, in the cohort study by Florence et al (2003) [20], it was considered adequate the immune response gain of ≥ 50 CD4-cells/mm3 or ≥ 75 cells/mm3 versus use of HAART for six (6) months or twelve (12) months, respectively. Dronda et al (2002) [6] considered adequate the immune response elevation of 100 cells/mm3 or higher at 24 months of HAART whereas for Kaufmann et al (2005) [23] the ideal immune response to be achieved after five years of ART was an elevation of 500 CD4 cells/mm3 or higher.

Another variable independently associated with PIR in this study was the nadir CD4-count. In other studies, both baseline CD4-count (measured before starting HAART) [10, 18–21, 23, 24, 28, 32, 34–40, 46] and nadir CD4-count [20, 22, 37, 47] have been associated with paradoxical immune response. As nadir CD4-count and baseline CD4-count are correlated, the present study used nadir CD4-count as an independent variable because it represents the lowest CD4 count value ever observed in a patient, along with his/her medical history, thus representing in fact the worst immune status he/she had ever had. The finding of an association between the increase in cell count at nadir CD4 determination and a decreasing risk of PIR conforms to Florence et al (2003) [20], Kaufmann et al (2002) [22] and Zoufaly et al (2010) [47] studies data.

Studies have been undertaken to elucidate the mechanisms involved in the occurrence of this poor immune response. Some of these studies have found direct association between the frequency of paradoxical immune response and the increase in T-cell activation (or persistence) and, consequently, excessive apoptosis of these cells [4, 48, 49]. Brenchley et al (2006) [50] observed that the bacterial translocation in these patients on HAART was the causation for the activation of their immune system.

Other studies have reported a relationship between PIR and cytokines profile. As an example, Sachdeva et al (2008) [36] demonstrated a low capacity to produce interferon alpha in patients with paradoxical immune response. Other authors have found associations with a reduced performance of interleukin 7 (IL-7) in the body, which was probably due to a down regulation of its receptor (CD127) during HIV infection [51]. Based on these conclusions, Levy et al (2009) [52] conducted a phase I/II clinical trial using IL-7 in patients with PIR, and demonstrated that this cytokine induced for a higher increase of CD4 cells when compared to that observed in the placebo group. A positive effect was also observed in the functional activity of CD4 cells in the group receiving IL-7 [52].

A relationship between age and PIR has been emphasized by studies highlighting a correlation with thymus size and function [7, 10, 12, 18, 21, 22, 28, 29, 31–34]. In the present study, age had not the importance evidenced by others.

Recent guidelines for the treatment of HIV infection do not establish a rigid CD4 cell count threshold at which therapy should be initiated. The International AIDS Society-USA Panel, for example, states that deciding when to initiate HAART is mainly a matter of the readiness of the patient [1]. Such recommendations are based on evidence that the benefits of treatment outweigh any additional risk posed by anti-retroviral drugs, as a long-standing untreated viremia implies a chronic inflammatory state resulting in end-organ damage and comorbid conditions. According to the literature, life expectancy in individuals with HIV infection is reduced even at higher CD4 cell counts [53]. Consequently, there is a concern to tailor the initiation of therapy according to other events related to HIV replication apart from CD4 cell-count limits, such as indications of rapid progression, older age and comorbid conditions [1]. Although the basic criteria in the present study are related to CD4 count as a determining factor for treatment, the results support the conclusion that early therapy could improve the immune response, as demonstrated by the inverse relationship between nadir CD4 count and the occurrence of PIR.

The limitations of the present study are those inherent to the case-control design, particularly regarding the vulnerability to selection bias and misclassification. Such possibilities are even more evident when one considers the preexistence of the data and the selection of prevalent cases. However, the consistence of the associations observed between the candidate variables of exposure and the outcome reinforces the validity of the conclusions, even if some potential risk factors could have persisted undetected.

The main contribution of this study lies in the demonstration of an independent association of PIR with markers of intense immunossupression such as low nadir CD4 counts and short periods of time for undetectable viral loads. It seems that immune reconstitution is a process that depends on a residual immune function to be effective. In other words, if the immunossupression had gone too far, it could be too late to obtain a consistent reconstitution.

Therefore, there is a need for more studies, mainly with a prospective design to fully establish the consequences of PIR. If there is an important impact over morbidity and quality of life, as anticipated, the conclusions from studies like this one should strengthen the need for early intervention with HIV infected patients regarding HAART. The more we learn about it, the more we understand the complex nature of HIV infection, and better strategies are made available to allow for safe and effective intervention for the immune reconstitution. In the future, inadequate responses like PIR should be eliminated at the best scenario.