We performed a detailed evaluation of T cells and CAVL on a large, well-characterized cohort of individuals with long-term follow-up. The strength of this study is the comprehensive evaluation of T cells at early time points after seroconversion, with a large number of endpoints. Our primary objective was to evaluate whether central memory T cell (TCM) quantification predicts disease progression; secondary endpoints included evaluation of other T cell subsets, immune proliferation (activation), and CAVL.
are a cornerstone of the adaptive immune response; the SIV model of infection points to the critical role played by these cells in the pathogenesis of disease [11
]. However, in our well-characterized cohort of recently-infected subjects, we did not observe an association between the proportion of TCM
and disease progression. Nonetheless, we identified other potential correlates of progression including proportions of naïve or CD127+ memory CD8 T cells, Ki-67 expression in CD4 or CD8 T cells (correlating with immune activation), and the CD4 CAVL.
We considered several possible explanations for the lack of an association between TCM
cells and disease progression. Since the proportions of memory cells vary with time since infection [10
], it is possible that mixing subjects identified during both acute and early HIV infection might mask associations. We therefore analyzed the predictive value of the proportion of TCM
cells among subjects identified within 3 months, 3–6 months, etc., from imputed seroconversion; no significant association was observed for any group. While our study was powered to demonstrate differences overall, this subgrouping significantly limits statistical power. Longitudinal studies with repeated sampling are likely necessary to associate the dynamics of TCM
cells with disease progression. Nonetheless, in humans, remodeling of the memory compartment is sufficiently variable during early infection such that enumeration of memory cells may not provide a predictor for progression.
As part of our secondary objectives we evaluated the predictive value of other T cell subset representation. After HIV infection, the naïve T cell pool progressively declines throughout the course of the disease [19
]. In our cohort, subjects who best maintained naïve CD8 T cells showed significantly slower progression of disease. This suggests that, while destruction of naïve T cells was thought to be a consequence of chronic disease, the loss of these cells starts early and may be related to the mechanisms accounting for progression throughout disease.
Within the CD8 T cells, another marker predictive of progression is expression of the IL7-receptor (CD127). IL7 plays an integral role in the differentiation and survival of TCM
and naïve T cells [20
], and CD127 expression appears necessary for the generation of long-lived memory T cells, a hallmark of protective immunity [25
]. Disturbances in the IL-7, IL-7 receptor axis have been reported during both acute and chronic HIV infection [26
]; levels of CD127−
CD8 T cells correlate with VL and inversely with CD4 counts [27
]. Here we show that decreased levels of CD127+
CD8 T cells during early disease are associated faster disease progression. Thus, the early depletion of long-lived cells in general (such as naïve or memory CD127+
cells, ) may more accurately predict subsequent progression to AIDS than the loss of TCM
as defined by commonly-used phenotypic markers.
Immune activation (IA) plays a key role in the pathogenesis of both SIV and HIV-infection [30
], and correlates with loss of CD4 T cells and progression to AIDS. IA has been studied extensively in the setting of chronic HIV infection [2
]; together with some small studies of early infection [1
], it was suggested that early levels of IA may set the stage for future disease. In our study, the proportion of proliferating (Ki-67+
) CD8 and CD4 T cells, a measure of IA, varied among individuals, but was low overall (mean = 1.7%). Nonetheless, having higher levels of proliferation was strongly predictive of progression. Specifically, the median time to AIDS and death among subjects with the highest levels (top quartile) of proliferation in CD8 T cells was 4 years, in contrast to 10 years for subjects with the lowest levels. Further, though proliferation correlated with plasma VL (data not shown), it still provides independent predictive power. While the mechanisms by which IA exerts deleterious effects remain unclear, they may be closely related to altered CD127 expression, since T cell activation is associated with reduced CD127 expression and altered T cell homeostasis [27
In our study, Ki-67 expression in CD8 cells identified subjects that progressed faster to AIDS and death, even after adjusting for CD4 count and VL. This argues for the inclusion of a marker of proliferation and activation to the markers used to assess vaccine efficacy and prognostic significance following HIV infection. Similarly, measures of immune proliferation and activation could be used to identify subjects most at risk for progression and likely to benefit from early therapeutic intervention.
The basic impetus for our study was the finding that acute SIV infection is accompanied by a massive destruction of memory T cells. As discussed above, cross-sectional enumeration of TCM
subsets in early HIV disease did not provide a correlate of subsequent progression. However, since the early destruction of CD4 T cells is mediated by viral infection, we hypothesized that a signature of the extent of the destruction during acute phase would still be found in the cell-associated viral load at early time points. Indeed, we found that CAVL was predictive of disease progression – in that individuals with higher rates of infection of CD4 T cells progressed more rapidly. Remarkably, we found relatively high rates of infection of naïve CD4 T cells (albeit 10-fold less than memory CD4 T cells). Naïve CD4 T cells do not express CCR5, the obligate HIV co-receptor for nearly all transmitted viruses. In this early-stage cohort, we expect that most circulating virus is CCR5-dependent. Since naïve CD4 T cells can be infected with CCR5-dependent viruses in vitro
, with T cell activation [37
], the infection in vivo
that we observed may be a direct consequence of IA.
In otherwise asymptomatic individuals current practice guidelines recommend initiation of antiretroviral therapy at a CD4 count<350 [40
]. However, there is a growing body of literature that supports the initiation of ART at higher CD4 counts, demonstrating reductions in both AIDS and non AIDS-related morbidity and mortality [4
]. Our studies provide some mechanistic clues to why these beneficial effects with HAART are observed. Specifically, we suggest that immunological disturbances (altered homeostasis and increased proliferation/activation) are established early in HIV infection, and are observed even among subjects with relatively preserved CD4 counts. The magnitude of these disturbances correlate with progression. Since HAART has been shown to reduce immune activation, preserve naïve T cell populations, and maintain CD127 expression [26
], our data indirectly support early initiation of such therapy.
In conclusion, we find that quantification of TCM in early infection does not provide predictive power for progression. However, measures of homeostasis and activation, including CD127 expression and Ki-67 do provide such information, and should be studied further to determine their role in clinical monitoring of HIV-1 progression. In addition, CAVL provides predictive power, but is not as easily implemented on a routine basis. Future efforts at identifying markers of subsequent progression should focus on measures of activation and homeostasis during the earliest stages of infection.