How does the body lose its immune protection during HIV infection? Is HIV the big viral killer or could the immune system itself be the culprit in cell suicide? If you ask Warner Greene, a leading HIV immunologist and cure researcher, the answer is number two. “When you look at what is going on in lymphoid tissues, the host immune response to the virus is playing an important role in the demise of CD4 T-cells,” says Greene. When it comes to tackling HIV latency and our goal of remission, he adds, “The action is also taking place in lymphoid tissues, not in the blood stream.”
Greene is Director of the Gladstone Center for HIV Cure Research and co-chair of UCSF’s Center for AIDS Research (with Paul Volberding), among the many professional hats he wears. He’s an eminence grise in the HIV field and sits on the Board of Directors for the coming SCOPE remission trial.
In November 2016, Greene and lead author Gilad Doitsh published a paper in the journal Cell Host Microbe arguing that the immune system’s response to HIV, not the virus, is responsible for most of the programmed cell death that leads to immune decline in HIV-positive individuals.[i] In their studies, using human tonsil and spleen tissue (key HIV lymphoid tissue reservoir sites), the duo found that resting, or dormant, immune memory T-cells harboring incomplete, or defective, HIV gene fragments (the product of abortive HIV infection) triggered a highly inflammatory form of cell death called pyroptosis. Both memory T cells and other types of CD4 T cells die by this pathway.
Their work has provided a potential direct link between two signature pathways of HIV disease, or pathogenesis: CD4 T cell depletion and chronic inflammation. They think it’s possible that this cell death process plays a critical role in early seeding of the latent HIV reservoir by luring central memory CD4 T cells to sites of cell death and inflammation in the tissue, where they then become latently infected.
In their lab studies, the duo found that 95% of the dying T cells were actually bystander cells. They further identified a culprit: caspase-1, which becomes active when assembled into a multi-protein complex also called an inflammasome, that can ultimately trigger pyroptosis. (Another key component of this inflammasome is IFI16; it’s actually responsible for detecting the small HIV DNA fragments generated during abortive infection of the bystander memory T cells). Cell death occurs when pores form in the outer lipid plasma membrane of the cell and it loses its cytoplasm—its inner living material contents.
The duo then tried capsase-1 inhibitors and were able to block T cell death. In Greene’s lab, Thomas Packard is a talented young postdoc who identified a chemokine, or chemical messenger, called CCL2; it attracts T cells that carry a receptor called CCR2.
“Among the cells that come flying in are central memory T cells because they express CCR2,” adds Greene about CCR2-positive T cells. Such central memory T cells form an important part of the latent reservoir, he explains, adding, “We are currently studying whether CCL2 production depends on IFI16 action. It might even be possible to block this seeding of the reservoir with small molecules that block CCL2 binding to its CCR2 receptor.”
The renewed focus on the host immune side of HIV disease has also revealed an additional hurdle: B cell follicles. These are clusters of B cells that also contain a subset of immune cells called T-follicular helper cells. An immunology team led by Louis Picker at the Oregon Health & Science University, working with colleagues Jake Estes, Jeff Lifson, and others, has suggested that T-follicular helper cells form an important part of the latent reservoir. But the body’s immune soldiers—HIV-specific T cells (also known as cytotoxic T-lymphocytes or CTLs) and natural killer cells—can’t penetrate the B cell follicle compartment to get to the latently infected T cells.[ii]
“The B cell follicle appears to be an immune privileged site,” says Greene, stressing, “We really need to figure out a way to get killer cells in there.” Over at the National Institutes of Health, HIV vaccine researchers Barbara Felber and George Pavlakis think the cytokine interleukin 15 (IL-15) may be the key to helping CTL cells breach the B cell follicle. [iii] “It’s really important that we are able to deliver the kill mechanism to where the latently infected cells are,” Greene adds, pointing to a critical target of new remission and cure studies.
Greene is eager to study whether capsase-1 inhibitors might be able to curb inflammation and limit the initial seeding of the reservoir in acutely-infected individuals. His lab is also collaborating with researchers looking at the effects of capsase-1 inhibitors in other viral diseases including influenza, avian influenza, and Venezuelan equine encephalitis (VEE). “I think host dependent pathways of cell death and inflammation are likely playing out in many types of viral infections. HIV may simply be the first of many,” adds Greene. He began his immunology career working on congenital immunodeficiency and seeks insights from other diseases to understand HIV latency.
Coronaviruses may offer a model: “Most of the inflammatory response in the lungs is not the virus itself but the host response to the virus,” Greene explains. In HIV, he says, “What the inflammation does is ramp up the disease, causing changes in the lungs.”
Given these insights, can we try to reprogram the immune system to block the cellular messages that lead to cell death and inflammation? And in the process, can we block development of various viral diseases? Greene hopes so.
Here, nature offers an evolutionary model—more than one. Many studies show that non-human primates infected with simian viruses in the same family as HIV don’t get sick and have only minimal levels of inflammation, even though they have viral loads higher than seen in untreated human AIDS patients. “It’s not an attenuation of the virus; it’s attenuation of some host process that produces the disease,” explains Greene, who says animal studies offer important comparative models for new human remission studies. “I guess if we gave humans another 100,000 to a million years of evolution, we’d get with HIV where many monkeys are with their SIVs,” he sums up. “But we’d like to get there faster.”