A new study just published by Nature’s advance online website suggests that it may be possible to reinvigorate exhausted, dysfunctional CD8 T cell responses. The authors argue that the findings may be relevant for chronic viral infections in humans (such as HIV and hepatitis B & C) where failure to control viral replication may be associated with virus-specific CD8 T cells becoming exhausted. The research was conducted by Rafi Ahmed’s group at Emory University using a mouse model of chronic infection with a virus called LCMV (lymphocytic choriomeningitis virus). Previous work has shown that when mice are infected with a particular strain of LCMV (called clone 13) a chronic infection develops and LCMV-specific CD4 & CD8 T cells become exhausted (i.e. they lose their ability to function properly). In contrast, infection of mice with a different LCMV strain (the Armstrong strain) only leads to a time-limited acute infection that is rapidly cleared by LCMV-specific CD4 & CD8 T cells; these T cells subsequently persist for the life of the animal and retain a full spectrum of functional properties (e.g. proliferation, cytokine production and, for CD8 T cells, the ability to kill virus-infected cells).
The researchers began by comparing the gene expression profiles of exhausted versus functional CD8 T cells (taken from mice infected with LCMV clone 13 and LCMV Armstrong strain, respectively). The most notable difference that they uncovered was that exhausted CD8 T cells strongly expressed the gene encoding a receptor called PD-1. This receptor can interact with one of two ligands: PD-L1 (formerly called B7-H1), which is widely expressed on both haematopoetic and parenchymal cells, and PD-L2 (B7-DC), which is predominantly expressed on macrophages and dendritic cells. Interactions between PD-1 and PD-L1 are considered to be inhibitory, meaning that they play a role in switching off or dampening down the CD8 T cell response. The researchers found that PD-L1 was expressed at very high levels in spleen cells from mice chronically infected with LCMV clone 13, particularly on virally infected cells.
Having uncovered this evidence that the PD-1 receptor is involved CD8 T cell exhaustion, the researchers next studied the effects of blocking PD-1/PD-L1 interactions using an antibody that specifically targets PD-L1. The strategy proved successful: in chronically infected mice, the function of exhausted LCMV-specific CD8 T cells improved and LCMV viral load declined. Because CD8 T cell exhaustion is also seen in settings where CD4 T cell responses are absent or impaired, the researchers also assessed the effect of blocking PD-L1 on LCMV-specific CD8 T cells that were generated in the absence of CD4 T cell help. Again, the functionality of these CD8 T cells (as measured by proliferation, cytokine production and ability to kill LCMV-infected cells) was restored when PD-L1 was blocked.
No obvious adverse effects of blocking PD-L1 were seen in these studies. Mice bred to genetically lack PD-L1 were able to clear LCMV Armstrong strain like normal mice, but when infected with LCMV clone 13 they died from severe immunopathology (an immune reaction against their own tissues). The study authors note that: “the PD-1 inhibitory pathway may have evolved to regulate immune-mediated damage during a persistent infection by turning 'off' the virus-specific T cells. The pathogen ends up taking advantage of this inhibitory pathway to establish persistence in the host. However, as we have shown, one can turn the tables on the pathogen by strategically blocking this inhibitory pathway and turning the T cells back on. However, one needs to carefully monitor the potential for autoimmunity and immunopathologic damage under such conditions.”
The researchers conclude that further studies of this approach are warranted, and argue that “our finding that PD-1/PD-L1 blockade was effective in enhancing CD8 T-cell responses even under conditions of CD4 T-cell deficiency is of particular relevance to the situation seen during HIV infection.” Some additional support for this argument can be found in a study published a couple of years ago in the journal Blood which reported that PD-L1 expression (or B7-H1 as it was referred to at the time) is increased in individuals with HIV infection. It has also been reported that individuals with HIV infection accumulate more and more dysfunctional HIV-specific CD8 T cells as disease progresses; the researcher Mark Connors has shown that >50% of all CD8 T cells can be HIV-specific in a person with an AIDS diagnosis. This offers the intriguing (but highly speculative) possibility that a therapy capable of restoring HIV-specific CD8 T cell function might work better in advanced than early disease.
Study abstract:
Nature advance online publication; published online 28 December 2005
Restoring function in exhausted CD8 T cells during chronic viral infection
Daniel L. Barber, E. John Wherry, David Masopust, Baogong Zhu, James P. Allison, Arlene H. Sharpe, Gordon J. Freeman and Rafi Ahmed
Functional impairment of antigen-specific T cells is a defining characteristic of many chronic infections, but the underlying mechanisms of T-cell dysfunction are not well understood. To address this question, we analysed genes expressed in functionally impaired virus-specific CD8 T cells present in mice chronically infected with lymphocytic choriomeningitis virus (LCMV), and compared these with the gene profile of functional memory CD8 T cells. Here we report that PD-1 (programmed death 1; also known as Pdcd1) was selectively upregulated by the exhausted T cells, and that in vivo administration of antibodies that blocked the interaction of this inhibitory receptor with its ligand, PD-L1 (also known as B7-H1), enhanced T-cell responses. Notably, we found that even in persistently infected mice that were lacking CD4 T-cell help, blockade of the PD-1/PD-L1 inhibitory pathway had a beneficial effect on the 'helpless' CD8 T cells, restoring their ability to undergo proliferation, secrete cytokines, kill infected cells and decrease viral load. Blockade of the CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) inhibitory pathway had no effect on either T-cell function or viral control. These studies identify a specific mechanism of T-cell exhaustion and define a potentially effective immunological strategy for the treatment of chronic viral infections.
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