Effects of age and sex on immunological and virological responses to initial highly active antiretroviral therapy

Introduction

Women account for half of the estimated 46 million HIV-infected adults worldwide [1], and are increasingly affected by the US HIV epidemic [2]. Prior to receiving antiretroviral therapy, younger HIV-infected women have higher CD4 cell counts [3,4] and lower HIV RNA levels [5,6] when compared with HIV-infected men. Levels of endogenous female hormones, or other sex-linked factors, may be an underlying cause of this difference [7,8] and differences in short or longer term treatment responses. Therefore, premenopausal women may respond to a first highly active antiretroviral therapy (HAART) regimen differently from either postmenopausal women or men, regardless of age. In this study, we characterized the 6-month immunological and virological responses to initial HAART by age and sex to clarify sex differences or similarities.

Methods

We relied on the Johns Hopkins University (JHU) and the University of North Carolina (UNC) observational clinical cohorts, both previously described [9,10]. In brief, data were collected through available electronic institutional databases and structured medical record reviews. For this study, patients must have entered HIV care at JHU or UNC between 1 January 1998 and 1 January 2004, initiated antiretroviral therapy with HAART during follow-up, and have available pre-HAART [baseline (BL)] and 6-month post-HAART initiation (6 m) CD4 cell counts and HIV RNA levels. HAART was defined as: (1) two nucleoside reverse transcriptase inhibitors (NRTIs) and at least one nonnucleoside reverse transcriptase inhibitor (NNRTI) or protease inhibitor (PI); or (2) at least three NRTIs. CD4 cell counts and HIV RNA levels obtained closest to 6 months, and within 5–7 months, from therapy initiation were included in the 6 m analysis. All patients ≥50 years old at HAART initiation were included as cases, and all patients <50 years old at HAART initiation were eligible to be controls and were randomly selected within sex strata at a 1: 3 case-to-control ratio. We excluded 31% of cases (29 of 93) and 36% of controls (108 of 298) because of unavailable BL or 6 m CD4 cell counts and/or HIV RNA levels.

Immunological reconstitution was defined as a >25% increase in CD4 cell count from BL to 6 m. As secondary endpoints, we also assessed absolute change in CD4 cell count, a >50 cells/μL CD4 count increase, and a >10% CD4 cell count increase. Virological response was defined as an HIV RNA level <400 HIV-1 RNA copies/mL at 6 m. We used Pearson's χ² test and the Kruskal–Wallis test to compare patient demographic and clinical characteristics across age and sex strata. All analyses were intention-to-treat (ITT). We used multivariable log-linear risk models fitted by binomial maximum likelihood to estimate adjusted relative risks (RRs) and 95% confidence intervals (CIs). Log-linear risk models were relied on as our outcomes were common and odds ratios from logistic regression do not approximate RRs well when the incidence of an outcome is more than about 10% [11].

Results

In the JHU and UNC cohorts, 63 individuals ≥50 years old met the study criteria, and 183 subjects <50 years old were randomly selected as controls. Women comprised 28% of the subjects in each of the case (n=18) and control (n=52) groups. Sixty-two per cent (n=152) and 38% (n=94) of subjects were from JHU and UNC, respectively. The median ages of patients ≥50 years old and <50 years old were 54 years [interquartile range (IQR) 51, 60] and 38 years (IQR 32, 48), respectively. Race varied across age and sex strata, with men <50 years old and women ≥50 years old less likely to be black than men ≥50 years old and women <50 years old (58% vs. 84%, respectively; P<0.01) (Table 1). Diabetes mellitus was more common among older than younger patients (21% vs. 8%, respectively; P<0.01). Although calendar year of HAART initiation was comparable across sex and age strata, women <50 years old and men ≥50 years old were more likely to receive a PI-based regimen than women ≥50 years old and men <50 years old (51% vs. 37%, respectively; P=0.04). Median BL HIV RNA levels were similar across age and sex strata; however, subjects ≥50 years old had higher BL CD4 cell counts than subjects <50 years old (median 152 vs. 73 cells/μL, respectively; P=0.04).

Virological suppression at 6 m was similar across age and sex strata, in both unadjusted and adjusted analyses (Table 2). Older subjects were slightly more likely to be virologically suppressed at 6 m than younger subjects (75% vs. 67%, respectively), although this difference was not statistically significant in either unadjusted analyses or analyses adjusted for site, type of HAART, and BL CD4 cell count and HIV RNA level (unadjusted RR=1.12; 95% CI 0.94, 1.34; and adjusted RR=1.08; 95% CI 0.94, 1.23). Comparable results were obtained when analyses were restricted to women only (unadjusted RR=1.16; 95% CI 0.85, 1.58; and adjusted RR=1.09; 95% CI 0.80, 1.48), and the effect of age on virological suppression did not differ by gender (test of interaction P=0.81).

Irrespective of the definition of immunological reconstitution used, no differences across age and sex strata were observed in either unadjusted or adjusted analyses, or in analyses restricted to patients with HIV RNA levels <400 copies/mL at 6 m (Table 2). Although older patients had slightly lower increases in CD4 cell counts than younger patients on average, this difference was not statistically significant in either unadjusted or adjusted analyses. For example, when comparing older and younger subjects who achieved virological response, the unadjusted and adjusted RRs for obtaining a >25% CD4 cell count increase were 0.92 (95% CI 0.77, 1.09) and 1.05 (95% CI 0.87, 1.28), respectively. Analogous findings were obtained when we restricted the analyses to women (unadjusted RR=0.83; 95% CI 0.58, 1.19; and adjusted RR=0.97; 95% CI 0.55, 1.74), and the effect of age on immunological response did not differ by gender (test of interaction P=0.53).

Discussion

In this collaboration between two large clinical observational cohorts, we found overall an excellent response to an initial HAART regimen, with over two-thirds of subjects virologically suppressed at 6 months from therapy initiation. The overall immunological response was also relatively robust, especially in those who achieved virological suppression. These results reinforce the finding that both younger and older populations respond well to currently prescribed potent combination antiretroviral therapy.

The hypothesis that sex and age differences exist in immunological and virological responses to HAART is plausible. Other sex differences related to antiretroviral therapy, including pharmacokinetics, pharmacodynamics and toxicities [12–16], have been described. Sex differences in response to HIV infection prior to antiretroviral therapy initiation have also been reported, including differences in CD4 cell counts [3,4], HIV RNA levels [5,6], and possibly HIV-1 disease progression [17,18]. Additionally, sex differences in virological, immunological, and clinical HAART outcomes have also been observed in some [19–21], but not all [22–28], previous studies. Age differences in immunological recovery [29,30] and virological response [31] have also been well described.

In this first report comparing the effect of sex by age strata, we did not find differences at 6 months from the initiation of a first HAART regimen among previously antiretroviral therapy-naïve patients in either immunological reconstitution or virological response. The potency of a first HAART regimen in controlling HIV-1 replication and subsequent immunological reconstitution probably masks any more subtle effect of biological sex differences on treatment responses. In this report, older subjects, especially older women, were initiated on HAART at much higher CD4 cell counts than in other reports [24,32,33]. Additionally, older women were more likely to receive a triple-NRTI regimen. Both of these factors may also blunt any differences. In the 6 months of initial HAART, HIV RNA levels drop rapidly and CD4 cell counts increase, primarily as a result of redistribution from solid lymphoid tissue [34,35]. Therefore, sex-related biological differences may be more likely to be detected with longer antiretroviral therapy use and possibly with subsequent HAART regimens rendered less potent by resistant virus.

We chose the age of 50 years to indicate the menopause in women [36,37], with possible misclassification if menopause occurs earlier in HIV-infected women. We also relied on observational clinical data, and residual bias cannot be ruled out, although we controlled for baseline characteristics in multivariable analyses. Despite the limited sample size, the very low risk ratios with narrow confidence intervals would suggest that the likelihood of missing large or moderate differences in immunological or virological responses in the first 6 months of initial HAART is very low. We are relatively confident that there is unlikely to be much of a difference even if a larger number of subjects were studied.

In conclusion, virological and immunological responses to HAART among previously antiretroviral-naïve patients appear to be similar across age and sex strata in the initial 6 months. However, longer term monitoring of HAART outcomes by age and sex is indicated, when biological and behavioural factors may become more apparent.