The Role of CD8+ Lymphocytes in the Control of SIV Viremia

Jeffrey T. Safrit, Ph.D.
Yerkes Research Center &
Emory University School of Medicine

The debate about the pathogenic or protective role of CD8+ T cells in lentiviral infections may be leaning more and more toward one side of the argument. Understanding the true role of these cells in vivo is critical to both our understanding of HIV pathogenesis and for the design of vaccine candidates. To this end, clinical evidence has been accumulating for the past few years on the potential of the cellular immune response to control viral replication in HIV-1 infected individuals. Now, a recent report by Jörn Schmitz et al. from the laboratories of Drs Keith Reimann and Norman Letvin has provided direct evidence for the positive role of CD8+ T cells in controlling virus in SIV-infected macaques ⁽¹⁾.

These investigators successfully depleted CD8+ lymphocytes in rhesus macaques infected with SIV-mac251 and examined the effect of this depletion on viral load, Gag p27 antigenemia, and SIV-Gag-specific CD8+ T cells during both the acute and chronic phases of infection, and on survival time.

Acute Infection

In an experimental setting that mimics primary HIV-1 infection in humans, rhesus macaques infected with SIVmac251 manifest extremely high viremias that peak between days 7 and 10 post infection. This high viral load then rapidly declines over the next few days presumably due to the onset of the SIV-specific immune response. Others have argued, however, in favor of an exhaustion of appropriate target cells as a reason for this decline ⁽²⁾. In the present study, six acutely infected animals given a humanized anti-CD8 monoclonal antibody were completely depleted of CD8+ lymphocytes for a variable duration. In three of the animals, CD8+ cells were depleted for 17-21 days while the remaining three animals were depleted of CD8+ cells from 4 to as much as 9 weeks. Despite the variability in time of depletion, the results were very consistent. In the CD8 depleted animals, viral RNA and Gag p27 antigenemia remained higher for longer than in animals treated with a control antibody. In fact, in the animals that were depleted of CD8+ lymphocytes for >4 weeks, viral RNA and gag p27 antigen were never controlled and all three developed disease rapidly. These results are more consistent with the hypothesis that reduction of acute viremia is due to the SIV-specific CD8+ T cell response than due to exhaustion of target cells.

Previous attempts at assessing the consequences of in vivo CD8+ cell depletion on the course of SIV infection have been hampered both by the variability of the viral loads and the incomplete depletions that were achieved ^{(3, 4)}. In a study of protection provided by attenuated SIV strains, Stebbings et al. determined that CD8+ cells were not necessary for protection from subsequent wild type virus infection ⁽³⁾. In addition, Matano et al. depleted CD8+ cells prior to or during early SHIV infection ⁽⁴⁾. They found ensuing viral loads to be higher while CD4+ cells were lower than controls, suggesting a role for CD8+ cells in clearance of viremia. However, the CD8+ cells depletions in both sets of experiments were partial and thus the results must be interpreted with caution. In addition, the number of macaques per experiment prevented any statistically significant results. Both of these hurdles were overcome in the study by Schmitz et al. ⁽¹⁾. The use of a "humanized" monoclonal may have been the difference as the half-life and thus effectiveness of the antibody would likely be increased due to the reduction of any anti-monoclonal immune responses in the macaques. The previous studies used unmodified murine monoclonal antibodies.

Chronic Infection

Two recent studies have addressed the role of CD8+ lymphocytes in controlling virus during chronic SIV infection of macaques. Schmitz et al extended their findings in the acute infection model to examine the effects of CD8+ cell depletion in animals infected with SIVmac251 for greater than nine months ⁽¹⁾. Jin et al. depleted CD8+ lymphocytes in seven macaques (six infected with SIVmac251 or 239 for 1 to 4 years and one uninfected control) ⁽⁵⁾. Both studies achieved transient depletions of CD8 cells lasting from 8-14 days. Interestingly, the humanized anti-CD8 monoclonal antibody appeared to be no more efficient than a murine anti-CD8 when used in the chronic infection setting. Nonetheless, both studies were able to show dramatic rises in viremia during the window of depletion. As expected, when the CD8+ cells returned, viremia fell back to the original chronic levels, consistent with the conclusion that CD8+ lymphocytes are responsible for controlling levels of virus.

A noted difference among the studies above was the effect of the depletion and increased viremia on the macaque CD4+ lymphocytes. While Schmitz et al made no reference to changes in the CD4+ subset in either the acute or chronic infection experiments, the studies of both Jin et al. and Matano et al. noted declines in CD4+ lymphocytes coincident with the rise in viremia. The CD4+ cell levels returned to pre-CD8 depletion levels as virus was again controlled by the CD8+ cells. The later results seem to be more in line with what would be expected during even a transient dramatic increase in viremia. These results are also consistent with the typical pattern seen in acute HIV infection when the high viral loads result in declines in CD4 cells that usually return to baseline as virus is controlled ⁽⁶⁾.

Role of Antigen Specific CD8+ Lymphocytes

As CD8+ lymphocytes may be able to control virus by a number of specific and nonspecific mechanisms, it was reasonable to ask to what extent SIV-specific CD8+ cytotoxic T lymphocytes (CTL) might be involved in the control of virus in vivo. Schmitz et al followed SIV gag-specific CTL in Mamu A*01 animals during and after the depletions by flow cytometric staining with SIV peptide tetramers (p11c, C-M) ⁽¹⁾. During acute infection, the appearance of tetramer positive CTL was delayed by CD8+ cell depletion but arose coincident with the control of viremia in the Mamu A*01 animal where depletion lasted < 21 days. In contrast, in a Mamu A*01 animal that was depleted of CD8+ cells > 28 days, tetramer positive CTL were never observed and virus was never controlled. Interestingly, when gag-specific CTL were followed during the chronic infection experiment, a significant boost of the pre-depletion levels of tetramer positive cells occurred that was coincident with the control of viremia following the transient depletion.

Implications

These studies emphasize the potential relevance of virus-specific CD8+ lymphocytes in the context of prophylactic HIV vaccines. By their nature, induced CD8+ cells would serve as a second line of defense in situations where virus broke through antibody containment and was able to infect cells in the new host. The above results also suggest that therapeutic vaccines capable of boosting antigen-specific cellular responses may productively modulate viral replication in already infected individuals.

References

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2. Phillips AN. 1996. Reduction of HIV concentration during acute infection: independence from a specific immune response. Science 271:497-499.
3. Stebbings R; Stott J; Almond N; Hull R; Lines J; Silvera P; Sangster R; Corcoran T; Rose J; Cobbold S; Gotch F; McMichael A; Walker B. 1998. Mechanisms of protection induced by attenuated simian immunodeficiency virus. II. Lymphocyte depletion does not abrogate protection. AIDS Res Hum Retroviruses 14:1187-1198.
4. Matano T; Shibata R; Siemon C; Connors M; Lane HC; Martin MA. 1998. Administration of an anti-CD8 monoclonal antibody interferes with the clearance of chimeric simian/human immunodeficiency virus during primary infections of rhesus macaques. J Virol 72:164-169.
5. Jin X; Bauer DE; Tuttleton SE; Lewin S; Gettie A; Blanchard J; Irwin CE; Safrit JT; Mittler J; Weinberger L; Kostrikis LG; Zhang L; Perelson AS; Ho DD. 1999. Dramatic Rise in Plasma Viremia after CD8(+) T Cell Depletion in Simian Immunodeficiency Virus-infected Macaques. J Exp Med 189:991-998.
6. Safrit JT; Koup RA. 1995. The immunology of primary HIV infection: which immune responses control HIV replication? Curr Opin Immunol 7:456-461.

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