Analysis of HIV-1 Viremia and Provirus in Resting CD4+ T Cells Reveals a Novel Source of
Residual Viremia in Patients on Antiretroviral Therapy
Timothy P. Brennan1, John O. Woods2, Ahmad R. Sedaghat3, Janet D. Siliciano3, Robert F.
Siliciano3,4, and Claus O. Wilke2, 5*
1. Department of Molecular Biology and Genetics, The Johns Hopkins University
School of Medicine, Baltimore, MD 21205
2. Institute for Cell and Molecular Biology, The University of Texas at Austin, Austin,
3. Department of Medicine, The Johns Hopkins University School of Medicine,
Baltimore, MD 21205
4. Howard Hughes Medical Institute, Baltimore, MD 21205
5. Center for Computational Biology and Bioinformatics and Section of Integrative
Biology, The University of Texas at Austin, Austin, TX 78712
* Corresponding author
Section of Integrative Biology
University of Texas at Austin
1 University Station C0930
Austin, TX 78712
Phone: 512 471 6028
Fax: 512 471 3878
Copyright © 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.
J. Virol. doi:10.1128/JVI.02568-08
JVI Accepts, published online ahead of print on 17 June 2009
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Abstract: Highly active antiretroviral therapy (HAART) can reduce HIV-1 viremia to clinically
undetectable levels. Despite this dramatic reduction, some virus is present in the blood.
Additionally, a long-lived latent reservoir for HIV-1 exists in resting memory CD4+ T cells.
This reservoir is believed to be a source of the residual viremia and is the focus of eradication
efforts. Here, we employ two measures of population structure, analysis of molecular variance
and the Slatkin-Maddison test, to demonstrate that the residual viremia is genetically distinct
from proviruses in resting CD4+ T cells, but that proviruses in resting and activated CD4+ T cells
belong to a single population. Residual viremia is genetically distinct from proviruses in
activated CD4+ T cells, monocytes, and unfractionated peripheral blood mononuclear cells. The
finding that some of the residual viremia in patients on HAART stems from an unidentified
cellular source other than CD4+ T cells has implications for eradication efforts.
Successful treatment of HIV-1 infection with highly active antiretroviral therapy
(HAART) reduces free virus in the blood to levels undetectable by the most sensitive clinical
assays (18, 36). However, HIV-1 persists as a latent provirus in resting, memory CD4+ T
lymphocytes (6, 9, 12, 16, 48) and perhaps in other cell types (45, 52). The latent reservoir in
resting CD4+ T cells represents a barrier to eradication because of its long half-life (15, 37, 40-
42) and because specifically targeting and purging this reservoir is inherently difficult (8, 25,
In addition to the latent reservoir in resting CD4+ T cells, patients on HAART also have a
low amount of free virus in the plasma, typically at levels below the limit of detection of current
clinical assays (13, 19, 35, 37). Because free virus has a short half-life (20, 47), residual viremia
is indicative of active virus production. The continued presence of free virus in the plasma of
patients on HAART indicates either ongoing replication (10, 13, 17, 19), release of virus
following reactivation of latently-infected CD4+ T cells (22, 24, 31, 50), release from other
cellular reservoirs (7, 45, 52), or some combination of these mechanisms. Finding the cellular
source of residual viremia is important because it will identify the cells that are still capable of
producing virus in patients on HAART, cells that must be targeted in any eradication effort.
Detailed analysis of this residual viremia has been hindered by technical challenges
involved in working with very low concentrations of virus (13, 19, 35). Recently, new insights
into the nature of residual viremia have been obtained through intensive patient sampling and
enhanced ultra-sensitive sequencing methods (1). In a subset of patients, most of the residual
viremia consisted of a small number of viral clones (1, 46) produced by a cell type severely
underrepresented in the peripheral circulation (1). These unique viral clones, termed
predominant plasma clones (PPCs), persist unchanged for extended periods of time (1). The
persistence of PPCs indicates that in some patients there may be another major cellular source of
residual viremia (1). However, PPCs were observed in a small group of patients who started
HAART with very low CD4 counts, and it has been unclear whether the PPC phenomenon
extends beyond this group of patients. More importantly, it has been unclear whether the
residual viremia generally consists of distinct virus populations produced by different cell types.
Since the HIV-1 infection in most patients is initially established by a single viral clone
(23, 51), with subsequent diversification (29), the presence of genetically distinct populations of
virus in a single individual can reflect entry of viruses into compartments where replication
occurs with limited subsequent intercompartmental mixing(32). Sophisticated genetic tests can
detect such population structure in a sample of viral sequences (4, 39, 49). Using two
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complementary tests of population structure (14, 43), we analyzed viral sequences from multiple
sources within individual patients in order to determine whether a source other than circulating
resting CD4+ T cells contributes to residual viremia and viral persistence. Our results have
important clinical implications for understanding HIV-1 persistence and treatment failure, and
for improving eradication strategies, which are currently focusing only on the latent CD4+ T cell
Patient populations: In the analysis of population structure between circulating activated and
resting CD4+ T cells, three distinct sources of patient sequences were used. These datasets
included a set of patient sequences obtained from Chun et al. (10) (patients 1, 2, 4, 5, 7, and 8),
Bailey et al. (1) (patient 154), and Chun et al. (11) (patients J2 and J7). We failed to obtain
complete datasets from either of the two Chun et al. (10, 11) studies. Additionally, patient
identifiers used in our study do not match those used in the two Chun et al. (10, 11) studies, due
primarily to randomization of patient data. We had complete access to all patients and sequences
used in the Bailey et al. (1) study, of which patient 154 was used in the analysis of circulating
activated versus resting CD4+ T cells.
In the analysis of population structure between proviruses in circulating resting CD4+ T
cells and free plasma virus, we utilized two distinct sources of patient data. The first set of
patient sequences was obtained from the Bailey et al. study (1). The second source of patient
data was derived in our lab, through the sequence analysis of newly enrolled patients. We
enrolled asymptomatic HIV-1-infected adults who maintained suppression of viremia on anti-
retroviral drugs to below the limit of clinical detection (<50 copies/mL). These patients had all
maintained stable suppression for at least 6 months prior to enrolling in our study, and most had
maintained suppression for much longer. A summary of all patient characteristics can be found
in Supplementary Table 2. Sampling from these newly enrolled patients occurred at study entry,
as well as subsequent time points. Patients who volunteered donated 180 mL of blood/visit.
Some patients returned periodically over the course of 3 years to provide additional blood
samples. Supplementary Table 3 contains the periodic sampling data for all newly enrolled
patients. Obtaining sequences from the plasma of patients with clinically undetectable viral
loads is technically challenging, and it is often not possible to recover many, if any, sequences.
Therefore, we included in this study only those patients from whom we were able to obtain a
sufficient number of sequences. We chose this sufficient number to mean no less than 20 plasma
and 20 proviral sequences from any single patient. Our protocol was approved by a Johns
Hopkins institutional review board, and informed consent was obtained from all study
Amplification, cloning, and sequencing of the env gene from free plasma virus and
proviruses in circulating resting CD4+ T cells: 180 mL of blood was collected at each study
visit using an acid-citrate-dextrose anticoagulant and separated using a Ficoll density gradient.
After gradient separation, the plasma layer was quickly removed, centrifuged to remove any
contaminating cells, and immediately frozen and stored at -80°C until further use. The buffy
coat layer was subsequently removed from the Ficoll tubes, and resting CD4+ T cells were
purified from total peripheral blood mononuclear cells (PBMC) via magnetic bead depletion, as
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previously described (16). Purified resting CD4+ T cells were lysed with a commercial detergent
cell lysis solution (Gentra), and the lysate was frozen at -80 °C until further use.
To analyze free virus in the plasma, 6 mL aliquots of plasma were thawed and subjected
to ultracentrifugation at 170,000 × g for 30 minutes at 4°C. Pelleted virus was subsequently
resuspended in 400 oL of phosphate buffered saline (PBS) (Invitrogen) and lysed, and the RNA
extracted via a silica bead-based RNA isolation protocol, implemented on an EZ1 Biorobot
(Qiagen). The RNA was eluted in 60 oL of elution buffer and subsequently treated with
amplification grade DNase I (Invitrogen), per the manufacturer’s instructions. To amplify region
C2-V4 of the env gene from RNA isolated from free plasma virus, the RNA was subjected to a
one-step reverse transcriptase (RT)-PCR using a Superscript III reverse transcriptase/Platinum
Taq high fidelity DNA polymerase one-step RT-PCR kit (Invitrogen), followed by a nested PCR,
using Platinum Taq high fidelity DNA polymerase, and 2.5 oL of the outer reaction as template.
Control reactions were carried out for all experimental amplifications, including a no RT control
to rule out DNA contamination and a no template control. Primers for the outer and nested
reactions were as follows: (outer forward) 5' - CTGTTAAATGGCAGTCTAGC - 3', (outer
reverse) 5' - CACTTC TCCAATTGTCCCTCA - 3', (nested forward) 5' -
ACAATGCTAAAACCATAATAGT - 3', (nested reverse) 5' - CATACATTGCTTTTCCTACT -
3'. PCR conditions were as follows: (one-step RT-PCR) reverse transcription at 50°C for 30
minutes, denaturation at 94°C for 3 minutes, followed by 40 cycles of 94°C for 30 seconds, 55°C
for 30 seconds, and 68°C for 1 minute, (nested reaction) denaturation at 94°C for 3 minutes,
followed by 40 cycles of 94°C for 30 seconds, 55°C for 30 seconds, and 68°C for 1 minute.
Products of the nested reaction were separated on 1% agarose gels, bands of appropriate size
were excised, and the corresponding amplicons were eluted using QIAquick gel extraction kits
(Qiagen). Isolated amplicons were subsequently cloned using a PCR2.1 TOPO cloning vector
(Invitrogen), and at least 6 clones were sequenced from each PCR using an ABI Prism 3700
DNA analyzer (Applied Biosystems). All sequences generated for newly enrolled patients have
been deposited in Genbank.
To analyze provirus in circulating resting CD4+ T cells, DNA from purified, lysed cells
was isolated using the Puregene method (Gentra). An outer and nested PCR designed to amplify
full-length env was then carried out on the isolated DNA in a limiting dilution fashion, as
previously described (1). The amplification was carried out using Accuprime Pfx DNA
polymerase (Invitrogen), and 5 oL of template DNA. The following primers were used in these
reactions: (outer forward) 5' – ATGGCAGGAAGAAGCGGAGACAG - 3', (outer reverse) 5' –
GCTCAACTGGTACTAGCTTGAAGCACC - 3', (nested forward) 5' –
GATAGACGCGTAGAAAGAGCAGAAGACAGTGGCAATG - 3', (nested reverse) 5' –
CCTTGTGCGGCCGCCTTAAAGGTACCTGAGGTCTGACTGG - 3'. PCR conditions were
as follows: denaturation at 94°C for 3 minutes, followed by 40 cycles of 94°C for 30 seconds,
60°C for 30 seconds, and 68°C for 3 minutes. Products of the nested reaction were then
separated on 0.8% agarose gels. Bands of the appropriate size were excised, and the amplicons
eluted using QIAquick gel extraction kits (Qiagen). Because the reactions were set up in a
limiting dilution fashion, with > 90% Poisson probability of being clonal, isolated amplicons
could be directly sequenced as outlined above, without the need for a cloning step. Clonality
was ensured after direct sequencing via a manual inspection of the corresponding
electropherograms for doublet peaks. Only those sequences that were verified as being clonal
were included in our analysis.
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Sequence analysis: Several steps were taken to ensure that we were working with quality
sequences, devoid of contamination, PCR error, and PCR resampling (28). We also took
measures to ensure that all sequences analyzed were clonal, and derived from independent
reactions. All these procedures have been previously described (1).
Phylogenetic analysis: Sequences were subjected to multiple sequence alignment with the
HXB2 reference sequence using Gene Cutter
(http://www.hiv.lanl.gov/content/sequence/GENE_CUTTER/cutter.html), preserving codon
positions. The quality of the alignments was manually inspected and adjustments were made
when necessary, using the sequence editor BioEdit version 7.0.9 (Tom Hall, Ibis Biosciences,
Carslbad, CA). Gaps in multiple alignments were removed prior to estimating phylogenies in
cases where there were length polymorphisms. Duplicate identical sequences derived from
independent reactions were removed from the alignment for phylogenetic reconstruction, but
subsequently added back to the trees. Phylogenies were estimated using both a “classical”
approach and a Bayesian approach, both functioning under a maximum likelihood optimality
criterion. The classical approach was implemented using a web-based version of RAxML (44)
available through the CIPRES supercomputing cluster (http://www.phylo.org/). We used the
general time-reversible model of nucleotide substitution with an estimation of the proportion of
invariant sites and with gamma-distributed rate variation, and included an M-group ancestral
sequence as an outgroup. The precision of phylogenetic reconstruction (nodal support) was
assessed via bootstrap analysis, with the number of bootstrap pseudo-replicates determined
empirically by the software. The Bayesian approach (21) was implemented using a web-based
version of MrBayes, also available through the CIPRES web portal. Again, we used the general
time-reversible model of nucleotide substitution with an estimation of the proportion of invariant
sites and with gamma distributed rate variation, and included an M-group ancestral sequence
used as an outgroup. For each patient, we carried out the Bayesian inference by running ten
Markov-chain-Monte-Carlo chains, each starting from a random tree. Each chain ran for 2.0 ×
107 generations, with samples taken every 100th generation. Phylogenetic trees were visualized
using Tree View version 1.6.6 (33, 34).
Analysis of Population Structure: We defined population structure as the presence of more
than one distinct genetic population in a group of sequences. To ascertain the presence or
absence of population structure in sequences derived from distinct sources, we employed two
complementary statistical tests. The first test, subsequently referred to as the Slatkin-Maddison
test (SM) (43), is implemented in the software package HyPhy (26). This test is a phylogeny-
based test that enumerates the minimum number of inferred migration events between two or
more populations on the basis of the reconstructed phylogeny. Briefly, each population is
assigned a character state, and each sequence derived from that population is labeled with the
corresponding character state. The enumeration process begins at the terminal leaves of the tree
and moves up, inferring an ancestral character state to each ancestral node. For each character-
state mismatch between an ancestral node and one of its descendants, a migration event is
inferred. We emphasize that the number of inferred migration events in itself is not meaningful,
because the HIV-1 life cycle violates the assumptions of the two-island models in whose context
the SM test was originally formulated. Nevertheless, we can use this quantity reliably to test for
evidence against incomplete mixing, as explained next.
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The SM null hypothesis states that the two (or more) character states are randomly
sampled from one large intermixing population. If the null hypothesis is true, then sequences
with randomly permuted character states should yield a comparable number of inferred migration
events as found in the original data. If the null is false, however, then randomly permuting group
assignments should increase the number of inferred migration events. The SM test assumes both
the lack of recombination and the lack of selection. Both of these assumptions may be violated in
HIV. To address this limitation, we also carried out a genetic-distance-based test of population
structure that does not rely on these assumptions (see below).
We carried out SM tests by adding back all of the duplicate, identical clones to the
reconstructed phylogenetic trees, removing the M-group ancestral outgroup, and inputting the
resulting trees into HyPhy. The software then enumerated the minimum number of migration
events and calculated statistical support against the null hypothesis non-parametrically by
randomly permuting group assignments (character states) 1000 times, recalculating the minimum
number of migration events, thus generating the sampling distribution. From this distribution,
we estimated the p-value by determining the cumulative weight of migration events in the
sampling distribution ø the number of inferred events in the original data. We subsequently
subjected the raw p-values for all patients to a Benjamini-Hochberg false discovery rate (FDR)
correction for multiple significance tests (2).
The second test of population structure involved the analysis of molecular variance
(AMOVA) (4, 14, 39). This test is implemented in the software package Arlequin (2). It is a
genetic-distance-based test that first calculates Euclidean pair-wise distances within and between
predefined groups and then partitions covariance components to the respective groups. The test
is analogous to a nested ANOVA, except that the normality assumption is not required.
Statistical support for the observed population structure is determined non-parametrically by
permuting group assignments 1000 times and re-calculating all statistics to generate their
sampling distributions. We again used the FDR method to correct for multiple testing.
We carried out AMOVAs by first formatting aligned sequences into batch files for input
into the Arlequin program. Briefly, using a text editor, we partitioned sequences into groups
defined by their source, removed the outgroup sequences, and specified the copy number of each
sequence. The batch file also contained information regarding the specific population structure
to be tested. In all cases, we were testing whether or not sequences from two distinct sources
represented a single intermixing population or not. Once the batch files were set up
appropriately, they were inputted into Arlequin. We let Arlequin compute a distance matrix
using Tamura and Nei corrected distances and an empirically-defined g shape parameter for
gamma-distributed rate variation.
Nucleotide sequence accession numbers. All newly obtained sequences for this study are
available at GenBank under accession numbers GQ256402 - GQ256627 and GQ261350 -
Proviruses in circulating activated and resting CD4+ T cells belong to one intermixing
population: We first tested for the presence of population structure in sets of proviral sequences
derived from circulating activated and resting CD4+ T cells. Because activated and resting CD4+
T cells represent one single population of cells at different stages of activation, genetically
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distinct populations of proviruses are not expected unless activated CD4+ T cells are frequently
infected de-novo by virus from a distinct origin. To assess population structure, we utilized two
complementary tests, the Slatkin-Maddison (SM) test (43) and analysis of molecular variance
(14), on published (1, 10, 11) and newly obtained datasets. The Slatkin-Maddison (SM) test (43)
assesses whether sequences belonging to two or more groups are randomly distributed over the
leaves of a phylogenetic tree encompassing all sequences. We used two methods of
phylogenetic reconstruction, classical maximum likelihood and Bayesian inference. In most
cases, both methods yielded identical tree topologies with only minor differences in branch
lengths. Figure 1 shows representative maximum likelihood trees consisting of proviral
sequences derived from activated and resting CD4+ T cells from (a) a patient with a defined PPC
(1) and (b) a patient without a PPC. Both trees show evidence of intermingling between proviral
sequences derived from activated and resting CD4+ T cells, devoid of any obvious population-
level structure. Similar results were obtained for other patients (Supplementary Figures 1a – 1e,
The results from the SM test were consistent with the intermingled nature of the
sequences illustrated in the reconstructed phylogenies. In all but one patient, we could not reject
the null hypothesis that no population structure existed between the two groups of sequences
(Table 1 and Figure 2). The phylogenetic tree for patient J2, the only patient for whom we could
reject the null hypothesis, contained a clade of free plasma virus sequences devoid of any
proviral sequences, as well as a clade of proviral sequences devoid of related free plasma
sequences (Supplementary Figure 1f). However, this pattern was atypical. For all other the
patients, the phylogenetic analysis showed a lack of compartmentalized structure. Similar
results were obtained when the SM test was performed using phylogenies constructed with the
Bayesian inference method (Table 1 and Figure 2).
As the SM test relies on phylogenies, which are difficult to estimate in situations where
there is low diversity and possible recombination, we also included a phylogeny-independent test
of population structure, a genetic-distance-based analysis of molecular variance (AMOVA) (14).
This test analyzes within and among-group molecular variation in a nested ANOVA-like
framework. The results of the AMOVA matched those of the SM tests. For all patients but
patient J2, the null hypothesis of no population structure could not be rejected (Table 1 and
Figure 2). In other words, proviral sequences derived from circulating resting and activated
CD4+ T cells in a typical patient comprise one intermixing genetic population with no significant
structure. This result is consistent with the known biology of these cells.
Provirus derived from circulating resting CD4+ T cells and free plasma virus represent
distinct genetic populations in most patients: To assess the presence of population structure
between free plasma virus and provirus derived from resting CD4+ T cells, we carried out the
same tests as described above. Figure 3 shows representative maximum likelihood trees of
proviral sequences derived from resting CD4+ T cells and sequences from free plasma virus for
(a) a patient with a defined PPC (1) and (b) a patient without a PPC. A PPC was previously
defined as a clonal, free-plasma-virus–derived sequence which represented >50% of the total
free plasma virus sequences, while also representing <1% of the total proviral sequences
obtained from circulating CD4+ T cells. In both cases, some of the plasma sequences were
identical to sequences found in the CD4+ reservoir. Thus, at least some of the plasma virus may
have been produced by latently infected CD4+ T cells that became activated. But both trees
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reveal a tendency for some plasma and reservoir sequences to segregate, in contrast to the
completely intermingled pattern observed for provirus in resting and activated CD4+ T cells.
The SM analysis revealed significant population structure between the two sources of
virus in all but two patients, patient 134 and patient 202 (Table 2 and Figure 2). For patients 134
and 202, the phylogenetic trees show a pattern of intermingling between proviral sequences and
free plasma virus sequences not unlike the pattern we found for proviral sequences derived from
activated and resting CD4+ T cells (Supplemental Figures 1k and 1l). However, for all of the
remaining patients, the phylogenetic trees show segregating proviral and free-plasma sequences
(Figure 3 and Supplemental Figures 1h – 1j, and 1m – 1s). The results were not affected by the
method of phylogeny reconstruction (Table 2).
The AMOVA results again matched the SM results (Table 2 and Figure 2). Taken
together, these data suggest that the residual viremia is compartmentalized and includes one or
more virus populations that are genetically distinct from the proviruses in circulating resting
CD4+ T cells.
In a subset of patients on HAART, the residual viremia is dominated by a PPC (1). PPCs
were detected in 6 of 13 patients studied (Table 2). The presence of a PPC would be expected to
influence the analysis of population structure. In fact, for the RT gene in patients with PPC,
AMOVA found that between 10% and 40% of all molecular variation was among groups (i.e.,
distinguished proviral from free-plasma sequences), while between 60% and 90% of all
molecular variation was shared between the two sequence groups (Table 2). For the RT gene in
patients without PPC, on the other hand, the among-group variation was only between 2% and
3%. (In patient 134, the percent variation among populations is negative. Negative percentages
can arise in AMOVA (14) but usually coincide with non-significant p-values.) By contrast, for
the Env gene, the absence of a PPC seems to increase rather than decrease the percent variation
among populations (Table 2).
To determine to what extent PPCs were influencing the analysis, and to determine
whether the plasma sequences other than the PPC were genetically distinct from sequences in
resting CD4+ T cells, we applied our analyses to datasets stripped of all PPCs (Table 4). We
found that even though PPCs do influence our analyses, in most patients there remains
significant population structure even after removal of all PPC sequences. For the SM tests, in all
but one patient, patient 113, we can reject the null hypothesis of no population structure after
removing the PPCs (Table 4). The dataset for patient 113 had the least number of sequences,
and the change of result for this patient may simply reflect loss of statistical power. In the
AMOVA analyses, all but two patients, patients 113 and 209, still exhibit a significant difference
after removing the PPC. Removal of the PPCs resulted in a reduction of among-population
variation of approximately 10-30% for RT and 5-6% for env. Surprisingly, for RT, the among-
population variation for patients 139, 148, 154 after removal of the PPC still exceeded the
among-population variation of any of the patients without PPC. Taking all results together, we
find a consistent pattern of free plasma virus and provirus from resting CD4+ T cells showing
significant population structure in the majority of patients, regardless of the presence or absence
of a PPC and regardless of the gene sequenced.
Previous studies have demonstrated that the total pool of resting CD4+ T cells harboring
integrated HIV-1 DNA consists of a mixture of both replication-competent proviruses and
defective proviruses, with the former representing only a small fraction of the total (6, 9, 15, 16).
Therefore, it may be possible that the population of free plasma viruses appears genetically
different from the total population of integrated proviruses, while simultaneously appearing
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similar to the small fraction of replication competent viruses. We were able to address this
possibility using replication competent sequences obtained from the extensively characterized
patient 154 (1). We found that the two potentially distinct populations (total integrated provirus
and replication-competent integrated provirus) appeared to belong to one intermixing population
of proviruses (Supplementary Table 1). Moreover, our results when comparing free plasma virus
to the total population of integrated provirus (Table 2) were near identical to our results when
comparing free plasma virus to only a subset represented by replication competent virus
(Supplementary Table 1).
In our analysis of population structure between free plasma virus and provirus derived
from circulating resting CD4+ T cells, we combined samples obtained from multiple time points.
Thus, our analysis of this relationship reflects a time-averaged sampling from these
compartments. To determine whether we could consider our data sets as temporally
homogeneous, we tested for the presence or absence of temporal population structure in several
representative patients. We found no evidence of population structure when comparing proviral
populations from different time points (data not shown); however, we found evidence for
population structure when comparing populations of free plasma virus from different time points
(data not shown).
In addition to comparing free plasma virus to proviruses derived from resting CD4+ T
cells, for one patient (pt. 154), we were also able to compare free plasma virus to proviruses
derived from activated CD4+ T cells, monocytes, and unfractionated peripheral blood
mononuclear cells (PBMC). Our analysis revealed that the residual plasma virus forms a
significantly distinct genetic population from all these cellular sources in this patient (Table 3).
Again, to determine the extent to which the PPCs were affecting the results in these analyses, we
repeated all analyses with all PPCs removed (Table 4). Table 4 shows that for the SM test, in all
but one scenario, free plasma virus compared to provirus from activated CD4+ T cells, the results
remained significant. The AMOVA analyses found significant population structure for all
scenarios (Table 4). Taken together, these results are also largely consistent regardless of the
presence or absence of a PPC.
There is great current interest in the nature of the HIV-1 viruses that persist in patients on
HAART and that cause the rebound in viremia that follows cessation of treatment. Previous
studies have compared the rebound viremia in patients following interruption of HAART
interruption to proviruses in the resting CD4+ T cell reservoir (6, 7, 22, 50). However, studies of
the rebound viremia suffer from the fact that the virus that initially rebounds when treatment is
stopped at a particular time point may be different than the virus that would have rebounded if
treatment was stopped at a later time point. In other words, the rebound viremia may reflect the
stochastic activation of some stable reservoir. In addition, the rebound viremia cannot be
attributed to a particular cellular source without extensive sampling of both compartments and
rigorous genetic comparisons, features that are missing from previous studies.
A more comprehensive approach to understanding viral persistence is to examine the
nature of the free viruses that continue to be produced in patients on HAART and to compare
features of this residual viremia with known cellular reservoirs (1, 24, 31). We used population
genetics in a statistical framework to systematically analyze the relationship between free plasma
virus and the provirus in resting CD4+ T in patients receiving HAART. We found that, in all but
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two patients, the residual free virus in the plasma was, in general, genetically distinct from
proviral sequences in resting CD4+ T cells. In contrast, proviral sequences derived from
activated and resting CD4+ T cells comprised one intermixing genetic population. A recently-
published study (38) found similar results, using the SM test to show significant
compartmentalization between plasma and CD4+ T cell-derived sequences. Here, we addressed a
number of issues missing in that study. First, whenever amplifying virus from a patient with
low-level viremia, one must be aware of PCR re-sampling (28) and steps must be made to ensure
that this phenomenon does not dominate the results. Our study utilized both patients derived
from Bailey et al. (1) and newly enrolled patients, all of which had their samples processed in a
manner to avoid PCR re-sampling. Second, different tests of population structure can yield
contradictory results, and a conservative analysis should therefore employ at least two
complementary tests of compartmentalization (49). For this reason, we employed an additional
statistical test of population structure, the AMOVA. Third, we addressed the issue of
predominant plasma clones (PPCs), first described by Bailey et al. (1), and investigated their
relationship to the more general phenomenon of compartmentalization.
Our study also addressed the relationship between proviral sequences derived from
activated and resting CD4+ T cells. A previous study (10, 11) derived asymmetric migration
rates of HIV-1 sequences between activated and resting CD4+ T cells, implicitly assuming
compartmentalization between these two cell types. Since in the majority of patients we could
not reject the null hypothesis of no compartmentalization, we conclude that the migration rates
calculated in (10, 11) have no meaningful interpretation.
Although HAART can halt ongoing replication, memory CD4+ T cells harboring
replication competent HIV-1 provirus can still produce progeny virus after reactivation. Thus,
the latent reservoir likely contributes to the residual viremia in patients on HAART. Yet we
show here that at the population level, proviruses derived from resting CD4+ T cells and free
viruses in the plasma exist as two distinct genetic populations. There are several possible
explanations. Because we only sampled the circulating, resting CD4+ T cell reservoir, one
explanation is that resting CD4+ T cells sequestered in various lymphoid tissues could be the
source of the free viruses observed in the plasma of these patients. The assumption that
sampling from the periphery can yield a good representation of the archived quasispecies present
in the CD4+ T cell reservoir, while a potential limitation of this study, is not uncommon in this
area of research. Another potential explanation is that other cell types or anatomical
compartments could also function as long-term stable reservoirs (1, 3, 31, 32, 45). Recent
studies identifying PPCs provide additional evidence suggesting that an alternate reservoir for
HIV-1 may be responsible for some of the observed residual viremia (1). We suggest that the
presence of PPCs may be a manifestation of a more general phenomenon in which a major
source of the free virus in the plasma is some cellular compartment severely underrepresented in
the peripheral blood.
Whether we found population structure between free plasma virus and provirus from
resting CD4+ T cells did not depend on the presence of a PPC. However, the percent of
molecular variation among (i.e., not shared by) these two groups of sequences was strongly
influenced by the PPCs. We studied the impact of PPCs by re-analyzing all datasets after
removing all PPCs. We found that removal of the PPC reduced the percent variation among
groups, but in most cases significant population structure remained even after removal of the
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Our analysis encompassed two genes, RT and Env. Our finding of population structure
between free plasma virus and provirus derived from resting CD4+ T cells did not seem to
depend on the gene under study. However, the percent of molecular variation among groups, as
calculated by AMOVA, was different for the two genes, as was the effect of the presence of PPC
sequences or removal thereof. The observed difference in percent variation among groups for
RT and Env may be explained by the difference in the overall diversity found in these two genes.
AMOVA is based on molecular diversity, which is much lower in RT than in Env. Since the
initial HIV-1 infection of a patient was likely established by a single clone (23, 51), two distinct
subpopulations can display little among-population variation if the time to accumulate diversity
has been short, and this effect would be stronger in RT than in Env. Under this scenario, we can
conclude that there is a major source of residual viremia other than reactivated CD4+ T cells, but
that this source becomes clearly visible in the RT gene only when it produces a PPC.
Alternatively, if we reject the notion that the Env gene is significantly more diverse than RT, we
can explain the correlation between the presence or absence of a PPC and the percent variation
among populations by a model where the majority of plasma virus originates from resting CD4+
T cells but a subset (exemplified in the PPC) originates from an as-of-yet unidentified source.
Based on our analyses of patients involving Env sequences, we feel that the former model
explaining the source of residual viremia best fits the data. We did not have the data to analyze
both the RT and Env genes for each patient, but we did have sufficient data for one patient
(patient 154). For this patient, we found that our results were comparable for the two genes.
In our analysis of population structure between free plasma virus and provirus derived
from resting CD4+ T cells, we used samples derived from multiple time points. Thus, our
analysis reflects a time-averaged sampling from the compartments under study. Temporal
sampling is a logistical necessity when carrying out analyses of free plasma virus in patients on
suppressive HAART, because of the very small numbers of sequences obtainable at any given
blood draw. We found no evidence for temporal variation in proviral populations from different
time points. We did, however, observe that plasma viruses isolated at different time points
seemed to be sampled from distinct subpopulations. This observation lends further support to our
overall conclusion that free plasma virus does not exclusively derive from circulating CD4+ T
cells. Our analysis of longitudinally-sampled plasma viral sequences is in agreement with
previously published work by Joos et al. (22), who found temporal variation but no evidence of
continued evolution in free plasma virus sampled at different time points.
It is known that the total pool of resting T cell-derived HIV-1 provirus contains a mixture
of both replication competent and incompetent species, with the former representing only a small
fraction of the total (6, 9, 15, 16). Thus, it is possible that the free plasma virus sampled could be
genetically similar to a subset of the total proviral pool, represented by the replication competent
population, while appearing genetically distinct when compared to the total population. Methods
developed to isolate replication competent provirus in patients are technically challenging, and
often yield too few sequences to be of any use for a detailed phylogenetic analysis (30). Thus,
we only had sufficient data from replication competent proviruses to carry out an analysis in one
extensively characterized patient, patient 154. Our results showed that replication competent
provirus and total integrated provirus comprise one intermixing genetic compartment.
Furthermore, when comparing free plasma virus to replication competent provirus, we found that
these two populations form distinct genetic compartments. While limited to one patient, our data
support the hypothesis that infected T cells isolated from the peripheral circulation represent one
single compartment. Therefore, mutations or other events resulting in replication incompetence
by guest on June 22, 2009
are occurring within the context of one intermingling genetic population, and thus, on a
population level, there should be little overall genetic difference.
We have also shown that proviruses derived from activated and resting CD4+ T cells form
one intermixing genetic population. We can explain this observation in the context of basic T
cell biology. Activated and resting CD4+ T cells represent the same population of cells but fixed
at different stages of activation. Therefore, proviruses derived from activated or resting CD4+ T
cells come from the same cellular compartment and should form one intermixing genetic
population. This will only be true, however, if activated CD4+ T cells are not newly infected by
free plasma virus. If there were ongoing replication, one might expect discordance between
proviruses derived from activated and resting CD4+ T cells, and might expect little evidence for
population structure between free plasma virus and provirus derived from activated CD4+ T
cells. We had the data to test the latter hypothesis for only one patient, patient 154. In this
patient, free virus was significantly different from provirus derived from both activated and
resting CD4+ T cells. By contrast, provirus derived from activated and resting CD4+ T cells
showed no evidence of population structure, consistent with the overall pattern we found in this
Our study did not address the phylogenetic relationship between plasma virus and virus
in resting CD4+ T cells of patients who have active viral replication. In patients with active viral
replication, most of the plasma virus is derived from recently infected cells that turnover very
rapidly (20, 47). The dominant viral variants in the plasma are typically those that are the most
fit under the existing condition. In contrast, the latent reservoir in resting CD4+ T cells harbors a
stable archive of preexisting viral variants. For example, we have previously shown that in
patients failing therapy, the plasma contains drug resistant variants while the latent reservoir
harbors the original wild type form and earlier drug resistant variants (30). Thus, in viremic
patients, it is expected that there will be differences in the viral quasispecies detected in the
plasma and the latent reservoir. However, these reflect differences between active replication vs.
production from stable reservoirs rather than differences between stable reservoirs.
Unfortunately, technical difficulties currently preclude a detailed analysis of this problem.
Stevenson and colleagues showed that most of the HIV-1 DNA in resting CD4+ T cells of
viremic patients is a labile unintegrated form in recently infected cells (5). This unintegrated
HIV-1 DNA greatly complicates the detection of the much rarer integrated form. We have
developed an experimental approach to isolate integrated HIV-1 DNA (30), but the number of
sequences that can be obtained by this approach is generally too limited for a detailed
phylogenetic analysis. It is however sufficient to confirm the general impression that in viremic
patients, there are substantial differences between the actively replicating pool of viruses
observed in the plasma and the stable archival pool of integrated proviruses in resting CD4+ T
That a major part of free plasma virus may be derived from some as of yet unidentified
cellular source has several important clinical implications with respect to HAART regimen
management, virologic failure, rebound viremia associated with treatment interruption, and
strategies aimed at eradication. Numerous laboratories are actively pursuing various eradication
strategies, most of which involve some aspect of targeting and purging the latent reservoir in
resting memory CD4+ T cells. If much of the residual viremia of patients undergoing HAART
comes from another reservoir or compartment as suggested here, then eradication strategies will
have to include ways to target and purge this additional reservoir to be successful.
by guest on June 22, 2009
We thank D.C. Nickle and T.W. Chun for providing us with the sequences for patients 1-5, 7, 8,
J2, and J7. This research was supported by NIH grant AI43222, the Doris Duke Charitable
Foundation, and the Howard Hughes Medical Institute (R.F.S.) and by NIH grant AI065960
by guest on June 22, 2009
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