The antiviral impact of CD8 T cells: much ado about the mechanism
Richard Jefferys, TAG
The primary mechanism by which CD8 T cells contribute to controlling pathogens is by killing infected cells. The afflicted cells display pathogen epitopes on their surface via class I HLA molecules that constantly shuttle disused protein fragments to the cell surface in a manner akin to taking out the garbage. CD8 T cells that recognise the pathogen epitope in the context of an HLA molecule that matches the HLA molecule expressed by the CD8 T cell initiate a cascade of events that results in the infected cell being killed. Examples of the process have been captured on video (the Howard Hughes Medical Institute website has an excellent excerpt from a lecture by Bruce Walker showing such a video and explaining it ).
However, although this is the canonical mechanism of CD8 T cell activity (and the reason for their alternate moniker of cytotoxic T cell lymphocyte or CTL), CD8 T cells can also suppress pathogens by other means such as release of chemokines and cytokines and, in the case of HIV, an as-yet-unidentified antiviral factor that goes by the acronym of CAF.
While there is a vast amount of evidence demonstrating the importance of CD8 T cells in suppressing replication of HIV (and other similar viruses such as SIV), the relative contributions of direct killing and indirect suppression have not been clearly delineated. Attempting to do so is a complex task, as the contribution of these activities may vary based on factors such as the extent of immunologic control and disease stage.
Two new studies in PLoS Pathogens make a valiant first attempt to shed light on this issue by evaluating whether artificial depletion of CD8 T cells impacts the speed of SIV suppression by antiretroviral therapy (ART) in macaques. [2, 3]
The rationale of both studies is that the kinetics of ART-mediated viral suppression should be slower in the absence of CD8 T cells because infected cells will have a longer lifespan. In both experiments, no such effect is demonstrated, leading the researchers to argue that CD8 T cells do not impact the lifespan of productively infected cells and that therefore their primary mechanism of action in chronic SIV infection is indirect suppression as opposed to direct killing. In an accompanying commentary, Miles P. Davenport and Janka Petravic invoke the tortured language of former US Secretary of Defense Donald Rumsfeld and suggest that CD8 T cell activity against HIV may qualify as a known unknown. 
But there are a number of issues that make interpreting these data complicated. Both studies involve chronically SIV-infected macaques, and CD8 T cell dysfunction and exhaustion in the setting of chronic infection is well documented. It could be that in a milieu in which viral control is only partial and both functional and dysfunctional SIV-specific CD8 T cells co-exist, indirect suppression plays a larger role than in the setting of robust immunological control (e.g. elite controllers). The selection pressure imposed upon SIV and HIV by HLA-restricted CD8 T cell responses is also extremely well documented, and although the authors of these new papers argue that indirect suppression could also exert selective pressure this scenario is somewhat difficult to envision. It is straightforward to grasp how a virus with an immune escape mutation could survive as a result of CD8 T cells being unable to kill the cell it is occupying. But in the case of indirect suppression, the escape mutation would have to abrogate localized release of suppressive substances by virus-specific CD8 T cells; it seems unlikely that the epitope specificities of CD8 T cells in a localized environment would be so uniform that a single virus carrying an escape mutation would switch off the suppression to a sufficient degree to obtain a survival advantage (although this question may be amenable to study).
Both papers note the possibility that CD8 T cells might kill virus-infected cells prior to the release of new virions, therefore making their activity essentially invisible in this particularly experimental system. The data cited in support of this possibility comes from Jonah Sacha, who has shown that epitopes from incoming virions can be processed and presented to CD8 T cells prior to virus integration, leading to killing of infected cells before the establishment of productive infection. 
Another possibility is that CD8 T cell-mediated killing reduces the average number of virions produced by each infected cell while having only a minor impact on the average lifespan (I think the mathematical model used in these papers assumes the same average virion production or burst size for every infected cell). Due to considerable variation from animal to animal, relatively subtle differences in infected cell lifespan may not be easy to capture.
Despite all the complexity and caveats, the papers are provocative and highlight the need to better understand the mechanism of action of CD8 T cells in SIV, and by extension, HIV infection.
In a related development, a group of researchers headed by members of the Human Immunology Laboratory at the International AIDS Vaccine Initiative (IAVI) have just published details of an assay that measures HIV inhibition by CD8 T cells in vitro. 
Although the numbers of people studied is small, the paper reports that inhibition measured by the assay correlated with viral load control, i.e. CD8 T cell-mediated inhibition was strong among untreated individuals with viral loads <10,000 copies/mL but weak in those above that threshold. The researchers also use the assay to measure the inhibitory capacity of CD8 T cells from seven uninfected individuals immunised with a DNA/Ad5 HIV vaccine regimen, reporting that significant inhibition could be documented but only after receipt of the Ad5 boost. Importantly, there was no correlation between the degree of in vitro virus inhibition and the numbers of vaccine-induced CD8 T cell responses measured by ELISpot, which up until now has been the standard way to measure the immunogenicity of T cell-based vaccine candidates.
The new assay takes three weeks to run and involves less than 2 million cells, making it more practical than those developed previously. The study authors write: We believe the viral inhibition assay will be a useful tool in the study of HIV-1 pathogenesis and vaccine development, complementing existing methods used to prioritise candidates for further trials.
Source: TAG Basics Science Blog. T he antiviral impact of CD8 T cells: much ado about the mechanism. (9 February 2010).
1. Walker B. Lecture excerpt.
2. Klatt NR et al. CD8+ lymphocytes control viral replication in SIVmac239-infected rhesus macaques without decreasing the lifespan of productively infected cells. PLoS Pathog 6(1): e1000747. January 201. doi:10.1371/journal.ppat.1000747.
3. Wong JK et al. In vivo CD8+ T-cell suppression of SIV viremia is not mediated by CTL clearance of productively infected cells. PLoS Pathog 6(1): e1000748. doi:10.1371/journal.ppat.1000748.
4. Davenport MP, Petravic J. CD8+ T cell control of HIVa known unknown. PLoS Pathog 6(1): e1000728. doi:10.1371/journal.ppat.1000728.
5. Sacha JB et al. Gag-specific CD8+ T lymphocytes recognise infected cells before AIDS-virus integration and viral protein expression. Journal of Immunology, 2007, 178: 2746-2754.
6. Aggeliki Spentzou A et al. Viral inhibition assay: a CD8 T cell neutralisation assay for use in clinical trials of HIV-1 vaccine candidates. JID 2010;201:720729. DOI: 10.1086/650492.