The immune response to pathogens must be powerful enough to clear the infection but controlled enough to limit the scope of damage. Because an overactive response can result in autoimmune disease or organ transplant rejection, a fine balance is needed to maintain immune system homeostasis. Regulatory T cells (Tregs) are a subset of T cells that help suppress the immune response and maintain this balance. It has become increasingly clear over the years that Tregs play an important role in control of infections. VIDD Associate Member Dr. Jenny Lund studies adaptive immunity and specifically the role Tregs play in promoting successful responses to virus infection.
“Understanding immune mechanisms of virus control has always been a big interest of mine,” says Lund, “especially for mucosal pathogens.”
The Lund lab focuses on investigating anti-viral immune responses in the context of West Nile virus (WNV), HSV-2, influenza and HIV.
Immune responses to HIV
Initiated in 2008, the Partners PrEP Study evaluated the efficacy of pre-exposure prophylaxis (PrEP) in preventing HIV infection over a two year period in 4,700 HIV serodiscordant couples. The study found a 75% reduction in HIV incidence in persons who received combination PrEP versus placebo. Lund’s group was interested in whether there were differences in HIV-specific immune responses in subjects who received PrEP. They analyzed a panel of several T-cell markers, such as activation markers, as well as cytokine expression. After a tremendous amount of work, they did not find any differences between the subject groups.
“But then we wondered if there a difference in immune response between people who contracted HIV versus people with similar demographics who remained uninfected,” said Lund.
They examined pre-infection blood samples taken from subjects who acquired HIV (cases) and control individuals who remained HIV negative. They assessed the magnitude, frequency and breadth of T-cell responses and found no statistical differences between cases and controls for either CD4+ or CD8+ T cells. However, when they looked specifically at Tregs they found an association. There was a higher frequency of Tregs in controls than cases: 3.6% vs 3.1%, respectively (p = 0.04). Stated another way, a 1% increase in Tregs corresponds to 35% lower odds of HIV acquisition. When comparing Treg frequency with HIV-specific CD4+ T-cell responses, they found an inverse correlation for Env, Gag and Tat responses. This suggests that an immune system in a low activation state, which corresponds to higher numbers of Tregs, could provide a protective environment against HIV acquisition and thus serve as an immune correlate of protection from HIV infection.
Discovering improved animal models for WNV
WNV, a mosquito-born virus, results in a relatively wide range of disease outcomes in humans. Many people who get infected with WNV will never know it; either they don’t have any symptoms or it manifests as a flu-like illness. But at the other end of the spectrum, people can get severe neuroinvasive disease and die or have lifelong central nervous system (CNS) infection. It remains a mystery as to why in some people WNV can be controlled and in others leads to chronic, sometime fatal infection.
Much of what we know about the in vivo immune response to WNV has been learned from the mouse model of infection. In a common inbred lab mouse model, C57BL/6J, the disease course is pretty predictable: about 30% of the mice die and 70% get a little sick and they survive. Unfortunately, this doesn’t model the variation we see in human WNV disease. So why use C57BL/6J mice at all? Using inbred, in other words genetically identical, mice to study infectious diseases removes confounding factors based on allelic differences between mice when analyzing data.
Adapted from Graham et al, mBio, 2015.
“However, that [inbreeding] is also a big weakness,” says Lund, “in that we are only looking at one mouse genome and don’t see the influences of other polymorphisms/alleles that are found in the human population.”
The Collaborative Cross (CC) panel of mice was developed as an effort to overcome this obstacle. These mice were created from 8 founder lines: 5 are classical inbred lab lines and 3 lines are derived from wild mice. After crossing (mating) pairs of mice over and over, the 8 original mouse lines have so far expanded to ~1,000 different lines that are genetically distinct, but have different recombinations of the founder line sequences. And because these mice are inbred, they are essentially a limitless resource. The mice are all sequenced, so scientists can correlate different mouse genomes with immune phenotypes and disease outcomes.
Lund is collaborating with several labs on a large NIH U19 grant to use CC mice as a tool for investigating pathogenesis of several viral diseases: WNV, SARS and flu. The Lund lab is analyzing adaptive immune responses to WNV infection, hoping to discover a better mouse model than C57BL/6J.
Lund explains, “We’re 3 years into this grant and almost done with the screens. We’ve done about 100 mouse lines already.“
So far, they have identified several mouse lines that when infected with WNV present with a novel phenotype distinct from C57BL/6J. The lab recently published a study identifying WNV infected CC mice that fell into 3 different disease categories: Asymptomatic, Symptomatic, and Asymptomatic/CNS involvement (see figure). Interestingly, the Asymptomatic/CNS involvement mice did not show any clinical symptoms of disease, such as weight loss or increased cytokine levels, while histological exam of brain tissue uncovered a dense infiltrate of WNV-specific CD8+ T cells in the CNS; like that found in neuroinvasive WNV infection. This mouse line could provide insight into immune correlates of protection that help prevent severe disease.
Now Lund’s group can use these mouse lines to ask questions such as: what are the host genomic factors and immune phenotypes that lead to chronic stage infection? What causes neuroinvasive disease in some people while some get just a flu-type illness? The lab is currently at the stage where they don’t have enough manpower to look at all of the piles of data, so they are working with systems biology modelers who can scan through data for each mouse efficiently.
“We think these CC mice will be incredibly useful for any type of research, really,” says Lund. “They can be used for anything where you want to map genes to a specific phenotype; whether it is an immune response to disease or even behavioral research.”
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