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Journal of Virology, January 2002, p. 411-415, Vol. 76, No. 1
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.76.1.411-415.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Residual Viral Replication during Antiretroviral Therapy Boosts Human Immunodeficiency Virus Type 1-Specific CD8+ T-Cell Responses in Subjects Treated Early after Infection
Gabriel M. Ortiz,1* Jennifer Hu,1 Joshua A. Goldwitz,1 Rohit Chandwani,1 Marie Larsson,2 Nina Bhardwaj,2 Sebastian Bonhoeffer,3 Bharat Ramratnam,1 Linqi Zhang,1 Martin M. Markowitz,1 and Douglas F. Nixon1,
Aaron Diamond AIDS Research Center, The Rockefeller University, New York, New York 10016,1
Laboratory for Cellular Immunology, The Rockefeller University, New York, New York 10021,2
Ecology and Evolution, ETH Zurich, CH-8092 Zurich, Switzerland3
Received 25 May 2001/
Accepted 27 September 2001

ABSTRACT
Human immunodeficiency virus type 1 (HIV-1)-infected subjects
treated early after infection have preserved HIV-1-specific
CD4
+ T-cell function. We studied the effect of highly active
antiretroviral therapy (HAART) on the frequency of HIV-1-specific
CD8
+ T cells in patients treated during early (
n = 31) or chronic
(
n = 23) infection. The degree of viral suppression and time
of initiation of treatment influenced the magnitude of the CD8
+ T-cell response. HIV-1-specific CD8
+ T cells can increase in
number after HAART in subjects treated early after infection
who have episodes of transient viremia.

TEXT
Highly active antiretroviral therapy (HAART) can reduce levels
of human immunodeficiency virus type 1 (HIV-1) in plasma to
below the limit of detection. However, viral replication persists,
as suggested by ongoing viral sequence evolution (
14,
43), the
presence of two-long-terminal-repeat episomal circles (
36),
the expression of viral mRNA in lymphoid cells and peripheral
blood mononuclear cells (PBMCs) (
16), the persistence of proviral
DNA (
10,
17), and the presence of low but detectable levels
of viral mRNA and infectious virions in the plasma of infected
subjects (
9,
10,
42). Depending on the magnitude of residual
viral replication during HAART, viral sequence evolution and
drug resistance may result, thereby contributing to disease
progression (
24). Although the source of the residual virus
is unknown, possibilities include latently infected CD4
+ T cells
and de novo production from cellular or anatomic reservoirs
protected from optimal levels of antiretroviral agents.
The relationship between antigen load and viral replication is complex (18), and the effect of residual viral replication on HIV-1-specific CD8+ T-cell responses is not well understood. Antiviral CD8+ T-cell responses correlate inversely with levels of plasma HIV-1 RNA in some subjects and usually decrease in response to HAART (7, 8, 13, 19, 2527, 30, 32, 34, 39, 41). However, antiviral CD8+ T-cell responses can remain constant or even increase after initiation of HAART (2, 8, 25, 30, 32, 38). Whether responses are functional and capable of responding to antigen remains controversial (1, 3, 5, 1113, 21, 35, 40).
To determine the effects of low-level virus replication on HIV-1-specific CD8+ T-cell responses, we undertook a longitudinal study of subjects treated during early HIV-1 infection (n = 31) and subjects treated during the chronic phase of infection (n = 23). All were enrolled in HAART trials. The clinical and virologic characteristics of some of these subjects have been described elsewhere (33). The demographic, virologic, and immunologic characteristics of our subjects are shown in Table 1. The mean age of the subjects and the time required to suppress plasma virus load to <50 copies/ml on two consecutive clinical visits did not differ significantly between the groups. However, different patterns of viral load suppression within the groups were observed after initiation of HAART.
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TABLE 1. Demographic, virologic, and immunologic characteristics of subjects with early or chronic HIV-1 infectiona
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Plasma samples were assayed for HIV-1 RNA with a reverse transcriptase
PCR assay (Roche Diagnostics, Alameda, Calif.) (lower limit
of detection, 50 copies/ml) or a branched-DNA assay (Chiron,
Emeryville, Calif.) (lower limit of detection, 50 copies/ml).
Most subjects exhibited stable suppression of HIV-1 plasma viral
load after initiation of HAART. Some subjects on HAART had intermittent
viremic episodes, defined as time points at which the plasma
HIV-1 load was detectable after having been stably suppressed
below detection limits. In others, such suppression was not
achieved and ongoing viremia was observed. In the latter subjects,
viremic episodes were counted after 150 days on HAART, the time
by which viremia had been stably suppressed in the majority
of subjects. The subjects were divided into those with less
than two and those with two or more viremic episodes per year
while on HAART. A recent study has shown that the occurrence
of at least two intermittent viremic episodes per year is associated
with an increase in the half-life of the latent reservoir of
HIV-1 (
33). Twenty-five patients treated during early infection
and 15 treated during chronic infection had fewer than two viremic
episodes per year, whereas six treated during early infection
and eight treated during chronic infection had two or more viremic
episodes per year. Plasma viral profiles of patients treated
during early infection who had at least two viremic episodes
per year are shown in Fig.
1.
HIV-1-specific CD8
+ T-cell responses were measured with a recombinant
vaccinia virus-based gamma interferon enzyme-linked immunospot
(ELISPOT) assay as described elsewhere (
22), with minor variations.
PBMCs were infected with recombinant vaccinia viruses expressing
the HIV
IIIB proteins Env-gp160, Gag-p55, Pol, and Nef (Therion
Biologics, Cambridge, Mass.). The negative control was a vaccinia
virus with a deletion in the thymidine kinase gene, where recombinant
genes were inserted (Therion Biologics), and the positive control
was phytohemagglutinin (Sigma, St. Louis, Mo.) at 10 µg/ml.
CD8
+ T cells are the predominant cell type producing gamma interferon
in this assay (
15,
22,
37).
Immune responses were measured longitudinally during HAART. Interestingly, some members of each group had increases in HIV-1-specific CD8+ T-cell responses while on HAART (Fig. 1). PBMC samples obtained closest to 1 year after initiation of HAART were used for direct cross-sectional comparison of HIV-1-specific CD8+ T-cell responses.
Among patients treated during early HIV-1 infection, the total HIV-1-specific CD8+ T-cell response was greater in the six subjects who averaged two or more viremic episodes per year while on HAART than in those in whom HIV-1 replication was relatively well suppressed (Fig. 2). A similar trend was noted among patients treated during chronic infection, but the difference was not statistically significant. The total HIV-1-specific CD8+ T-cell response increased in only three of eight subjects treated during chronic infection who had two or more viremic episodes per year while on HAART (data not shown). In subjects treated during early HIV-1 infection with at least 300 days of immunologic follow-up, a positive correlation was found between total HIV-1-specific as well as Pol-specific CD8+ T-cell response and number of viremic episodes (P = 0.05 and 0.03, respectively).
We propose an antigenic-threshold model for the relationship
between HIV-1-specific CD8
+ T-cell responses and residual viral
replication. Augmentation of memory CD8
+ T-cell responses requires
sufficient levels of antigen, functional T-helper-cell responses,
and intact antigen presentation networks (
6,
20,
29,
31). HIV-1
infection appears to disturb both the antigen presentation networks
and the T-helper-cell responses (
2,
4,
23,
28), and the disturbances
may be greater in subjects treated during chronic infection
than in those treated during early infection (
2,
6). These disturbances
would affect the ability of CD8
+ T cells to respond to antigen.
In our study, only subjects treated during early infection had
significant increases in the magnitude of HIV-1-specific CD8
+ T-cell responses associated with viremic episodes. Thus, subjects
treated during early infection may have a lower antigenic threshold
because their immune responses are better preserved, as reflected
in their higher T-helper-cell responses and intact antigen presentation
networks. These findings may partly explain why some subjects
have increases in HIV-1-specific CD8
+ T-cell responses after
initiating HAART.

ACKNOWLEDGMENTS
We gratefully acknowledge the assistance of Arlene Hurley and
the clinical team at the Aaron Diamond AIDS Research Center
and Rockefeller University Hospital. We thank Chris Chung for
viral load measurements, Annika Karlsson for helpful comments,
and Stephen Ordway and Gary Howard for editorial assistance.
This work was supported by NIH grants U01AI41534, MO1-RR00102, R01 AI44595, and R37 AI36082. Gabriel M. Ortiz, a student in the Tri-Institutional M.D.-Ph.D. Program, is supported by NIH Medical Scientists Training Program grant GM 0773 and Minority Predoctoral Fellowship F31 GM20068-01. Douglas F. Nixon is an Elizabeth Glaser Scientist of the Elizabeth Glaser Pediatric AIDS Foundation.

FOOTNOTES
* Corresponding author. Present address: Gladstone Institute of Virology and Immunology, University of California, San Francisco, CA 94110-9100. Phone: (415) 695-3826. Fax: (415) 826-8449. E-mail:
gortiz{at}gladstone.ucsf.edu.

Present address: Gladstone Institute of Virology and Immunology, University of California, San Francisco, CA 94110-9100. 

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Journal of Virology, January 2002, p. 411-415, Vol. 76, No. 1
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.76.1.411-415.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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