CD4/CD8 ratio normalization rates and low ratio as prognostic marker for non-AIDS defining events among long-term virologically suppressed people living with HIV

Survival of HIV-infected individuals has significantly improved with the introduction of combination antiretroviral therapy (cART). Nowadays, since antiretrovirals (ARVs) are initiated in earlier stages of infection, HIV-associated illnesses or complications of acquired immunodeficiency syndrome (AIDS) are less frequently observed in most HIV-infected individuals [1]. Despite low risks of AIDS in many patients who are treated with cART, immune recovery or restoration is rarely achieved [2, 3]. The incidence of several serious non-AIDS defining events (NAEs) including cardiovascular disease, cerebrovascular disease, renal disease and non-ADIS-related cancers have risen as longevity has increased among PLHIV [4]. Lifestyle factors and ageing play a role, but the impact of HIV infection and associated chronic inflammatory states on the incidence of these NAEs is still not fully understood.

Chronic inflammation includes persistent ongoing immune activation, bacterial translocation caused by injured mucosa-associated lymphoid tissue, asymptomatic replication of HIV itself and co-pathogens such as cytomegalovirus infection. These conditions lead to immune activation, and hence potentially increase the risk for all-cause mortality in HIV patients [5‐8]. The ratio of CD4⁺ T cells to CD8⁺ T cells (CD4/CD8 ratio) has been used as a surrogate marker of immunosenescence in both the general population [9, 10] and PLHIV [11]. Additionally, the CD4/CD8 ratio has shown an independent association with NAEs and mortality in people living with HIV (PLHIV) whereas CD4 counts alone do not predict the risk of NAEs [12, 13]. Studies have reported the ratio remains low, with a slow rate of recovery in a substantial proportion of patients, including those with adequate CD4⁺ T cell count recovery, over years of cART [12, 14]. The clinical significance of this ongoing immune dysfunction needs further exploration.

The majority of previous published studies exploring associations with CD4/CD8 ratios and their prognostic value for predicting NAE have been from Western countries, and evidence to inform an association in HIV-infected Asians is still limited. A study comparing Asian and Caucasian cohorts has shown that the baseline CD4/CD8 ratio before cART initiation was significantly lower in Asians but there were no differences in the odds of achieving the ratio value of > 1 between two cohorts [15].

In this study, we aimed to firstly evaluate the incidence and probability of CD4/CD8 normalization and to explore factors associated with normalization among patients from a Thai HIV cohort with sustained virologic suppression and long-term follow-up. Secondly, we explored the value of CD4/CD8 ratio in predicting NAEs including cardiovascular or cerebrovascular diseases, kidney diseases, non-AIDS malignancies and deaths.

In our cohort with median duration cART of 8.9 years and median duration of virological suppression of 6.1 years, the probability of achieving CD4/CD8 normalization at 5 and 10 years was 19% and 39% respectively. These Kaplan–Meier probabilities are consistent with our incidence rate of approximately 4% per year. Our data confirmed the previous few available data demonstrating the prolonged duration of recovery to normal CD4/CD8 ratio among PLHIV with suppressive cART, with a median of 10 years to reach normalization target [12]. This study also demonstrated that low ratio was a higher risk for clinical progression of NAEs. This can be translated into that persistent disparity between CD4 and CD8 cell counts, even with CD4 recovery after cART, could be a critical marker for evaluating the prognosis of long-term virologically suppressed PLHIV. Our patients had a relatively lower incidence of CD4/CD8 ratio normalization compared to an Italian cohort. The Italian cohort that included only patients with undetectable HIV viral load (HIV-RNA level < 80 copies/mL) after cART initiation, showed 29% achieved ratio normalization by 5 years whereas it was 19% by 5 years from our study [12]. However, the baseline median CD4/CD8 ratio and median nadir CD4 count in the Italian cohort were higher than those of our Thai cohort (0.39 vs. 0.22, and 378 vs. 206 cells/mm3, respectively). Our study showed that higher chances of normalization were associated with higher baseline CD4 counts (> 350 cells/mm³). This is consistent with previous studies suggesting baseline CD4 counts < 200 cells/mm³ was an unfavorable predictor towards CD4/CD8 ratio normalization [3, 12], and that low CD4 T-cell counts at baseline before cART are associated with incomplete immune response even after years of viral suppression [20]. Moreover, improvement of CD4/CD8 ratio was seen in both normalization and non-normalization groups after having suppressed viremia, compared to baseline values before cART initiation. This suggests that current ART guidelines recommending initiation of cART treatment with higher CD4 cell counts will have benefits in increasing the number of participants achieved higher CD4/CD8 ratios earlier.

The literature is conflicting regarding the use of CD4/CD8 ratio as a surrogate marker to describe immune dysfunction and to predict the progression towards NAEs and mortality [12, 13, 21]. A report from Mussini et al. [12] suggested the predictive value of CD4/CD8 ratio for clinical progression of severe NAEs and deaths among patients with prolonged cART treatment in a large Italian cohort, with the same cutoff values of the ratio used in our study. In contrast, in a large collaborative study of European and North American cohorts failed to show a link between low CD4/CD8 ratio and non-AIDS related mortality among virologically suppressed patients on cART [21]. Of note, the study showed low ratio and high CD8 counts were associated with AIDS-related mortalities. We sought to carry out an additional analysis to explore whether CD4/CD8 ratio has an association with development of NAEs. Our case definition of NAEs was similar to the ones used in previous studies, which included cardiovascular and cerebrovascular diseases, non-AIDS defining cancers, kidney diseases and deaths [12, 13]. The incidence rates of composite non-AIDS endpoints were significantly different among categories with different CD4/CD8 ratio cutoff values in our study. Patients with low CD4/CD8 ratio had higher incidence rates of NAEs than those with higher ratios. In multivariate analysis, we also found the independent associations of low CD4/CD8 ratio with composite end point or deaths after adjusting for potential confounders that were significant in the univariate analysis.

Previous reports have also suggested the association of CD4/CD8 ratio in T-cell activation or senescence [22, 23]. A plausible explanation associations with NAEs is that low CD4/CD8 ratio could be a marker of ongoing immune dysfunction among prolonged virologically suppressed HIV patients on stable cART since immune activation is been associated with clinical events and mortality in PLHIV [24].

One of the strengths of our study is the prolonged follow-up time with a median of 8.9 (IQR, 5.2–13.7) years of cART treatment. Our participants were taking cART with prolonged duration of virological suppression of 6.1 (IQR, 3–10.8) years. Another strength of our study is that we evaluated the probabilities of normalization among virologically suppressed participants up to 10 years. This is relatively longer than other studies which showed the normalization probabilities just up to 2 years [3] and 5 years [12], respectively.

Several important limitations in this study should be acknowledged. First, we could not provide the association of low CD4/CD8 ratio with each particular non-AIDS incident due to low numbers of events in each category. Second, we could not include some potential factors that have previously suggested may have influences on immune activation or persistent inflammation such as cytomegalovirus (CMV) co-infection [25, 26] in our study analysis. Third, our study population could be considered low risk, since they started cART, and maintained good adherence and virological suppression. Nevertheless, this also allowed us to assess CD4/CD8 ratio recovery without the confounding effect of viremia. Fourth, the study was a retrospective analysis from a prospective cohort. The conclusion for causal inference between low CD4/CD8 ratio and NAEs is therefore low due to the observational nature of the cohort. Lastly, although CD4/CD8 ratios may be a useful marker, there is no known strategy apart from taking cART to improve ratios that are low. Further analyses should explore whether newer therapies for HIV treatment such integrase strand inhibitors or monoclonal antibodies have additional impact on CD4/CD8 ratio recovery.

Probabilities of immune recovery among patients on cART is relatively low in our cohort, suggesting the ongoing immune dysfunctions even after years of treatment. Patients with low baseline CD4 counts were less likely to achieve ratio normalization. Earlier ART initiation may result in increased normalization rates. We also sought to use multiple NAEs as a composite endpoint to determine its association with the ratio. The finding of low ratio with the linkage to NAEs highlights the potential use of CD4/CD8 ratios as a marker for the clinical progression of non-communicable diseases and mortality among PLHIV with prolonged suppression of viremia.