Programmed death I (PD-1), an inhibitory receptor on immune system cells, has long been known to play an important dual role in immune regulation: Preventing the immune system from attacking the self and keeping an activated immune system in check.
In 2006, however, Rafi Ahmed, of the Emory University School of Medicine, and colleagues discovered that the PD-1 pathway could also be exploited by pathogens to repress normal T cell function during chronic viral infection. In mice infected with lymphocytic choriomeningitis virus (LCMV), Ahmed found PD-1 upregulated in functionally-impaired CD8 T cells. The team then showed that preventing PD-1 activation by blocking its ligands with antibodies restored T cell function and decreased viral load in the mice.
The research was...
In 30 to 40 subsequent papers, says Ahmed, upregulated expression of the PD-1 pathway has been firmly documented as a mechanism of T cell dysfunction in the three main human chronic viral infections - HIV, hepatitis B, and hepatitis C - as well as SIV infection in non-human primates.
Further investigations of PD-1 expression on exhausted T cells led to the discovery of a plethora of inhibitory pathways active during chronic infection. Last year, Ahmed and Wherry examined the molecular signature of T cell exhaustion by comparing gene expression profiles of infected and functional CD8 T cells in mice.
Around the same time, researchers identified a second reversible pathway involved in HIV T cell inhibition.
How PD-1 blocks T cell activation is still an open question, says Ahmed. Researchers know that PD-1 has two tyrosine molecules on its cytoplasmic tail that get phosphorylated when PD-1 binds a ligand. The tail then binds phosphatases in the T cell, triggering a downregulation of antigen receptor signaling. But few additional details are known, says Freeman.
Recent research seems only to be further muddying the waters. Last year Freeman and colleagues implicated an additional co-stimulatory molecule in the inhibition pathway, B7-1.
Also, this past July researchers revealed the crystal structures of the PD-1/PD-L1 and PD-1/PD-L2 complexes.
But before investigating molecular mechanisms, researchers are focused on turning the research into therapy. The next step is to determine the side effects of blocking the PD-1 pathway in vivo. "All indications are if you block it, the immune system will be more active. The big question is if you do that, is it safe?" says Freeman. Ahmed and his team are beginning experiments in non-human primates with SIV. He expects results on in vivo blockades to be published within the next 6-12 months. "Those papers will be extremely critical in taking [the research] to the next step," says Ahmed.