Grasping Immune-Signaling Mechanisms of Yellow Fever Infection

Researchers have uncovered a critical role for a new immune signaling pathway in controlling infection by the yellow fever virus, which is spread by mosquitoes. [CDC]

Vaccines play an essential role in controlling the spread of infectious diseases. Yet, the underlying immune activation mechanisms for many of these prophylactic therapies remains elusive. Yellow fever—a deadly viral infection spread by mosquitoes—devastated much of the world until a vaccine was developed in the late 1930s. Now researchers at Princeton University have just uncovered a critical role for a new immune signaling pathway in controlling infection by the yellow fever virus (YFV). Findings from the new study—published online today in mBio in an article entitled “Type III Interferon-Mediated Signaling Is Critical for Controlling Live Attenuated Yellow Fever Virus Infection In Vivo”—are not only important for understanding immune control mechanisms of YFV but may help scientists elucidate the signaling pathways that control other flaviviruses, such as Dengue and Zika.  

Infection with YFV causes a devastating illness, with mortality rates soaring to 50% in some areas. Fortunately, there is an effective vaccine for YFV: a live-attenuated strain of the virus called YFV-17D, which differs by only a few amino acids from the virulent viral strain YFV-Asibi but provokes a potent and durable protective immune response in humans.

“An improved understanding of the complex mechanisms regulating YFV-17D attenuation will provide insights into key viral–host interactions that regulate host immune responses and infection outcomes, [and] open novel avenues for the development of innovative vaccine strategies,” explained senior study investigator Alexander Ploss, Ph.D., assistant professor in Princeton's department of molecular biology. Still, research efforts have been hampered because mice, which are used in the study of viral infections, are resistant to YFV infection. Nonetheless, recent mouse experiments have pointed to a key role for cytokines called interferons (IFNs) in controlling the virus.

Interestingly, much like humans, mice possess three types of IFNs, molecules produced by the immune system during infection: type I IFNs, which signal through the widely distributed IFN-α/β receptor; type II IFNs that act on IFN-γ receptors present in most tissues; and type III IFNs, which activate signaling by IFN-λ receptors found on epithelial cells.

Mice lacking type I receptors die after infection by YFV-Asibi, but survive YFV-17D infection despite extensive viral replication at an early stage of infection. Type II IFN signaling has also been shown to be important for clearing up late-stage YFV-Asibi and YFV-17D infection when type I IFN signaling is defective. Conversely, the contribution of type III IFN signaling to control of YFV infection was unknown. The researchers sought to address this question by studying YFV-17D infection in mice lacking the type III receptor.

Historical video produced by the predecessor agency to the CDC about how to control the spread of yellow fever (1945). [NIH].

(Left panel) Loss of type I IFN signaling leads to active replication of the attenuated YFV strain (YFV-17D), which is accompanied by viral invasion of the brain and damage to brain tissues (spongiosis). (Right panel) The additional loss of type III IFN signaling in type I IFN-deficient mice impairs the integrity of the blood–brain barrier and alters immune cell function, which aggravates spongiosis and is ultimately lethal. [Florian Douam and Alexander Ploss/Princeton University]

“…we report that while wild-type (WT) and IFN-λ receptor knockout (λR^−/−) mice were largely resistant to YFV-17D, deficiency in type I IFN signaling resulted in robust infection,” the authors wrote. “Although IFN-α/β receptor knockout (α/βR^−/−) mice survived the infection, mice with combined deficiencies in both type I signaling and type III IFN signaling were hypersusceptible to YFV-17D and succumbed to the infection. Mortality was associated with viral neuroinvasion and increased the permeability of the blood–brain barrier (BBB). α/βR^−/− λR^−/− mice also exhibited distinct changes in the frequencies of multiple immune cell lineages, impaired T-cell activation, and severe perturbation of the proinflammatory cytokine balance.”

The initial experiments showed that these mice could control viral replication and rapidly cleared YFV-17D, indicating that type III signaling alone wasn't necessary for resistance to YFV-17D. However, mice lacking both type I and type III receptors succumbed after YFV-17D infection, suggesting type III signaling does contribute to the antiviral immune response.

As they dug deeper, the research team examined YFV-17D levels in various tissues. Early in the infection, the virus was present in every tissue of each mouse model examined. However, although viral loads were low in wild-type mice and type III receptor-deficient mice, they were much higher in type I and type I/III receptor-deficient mice.

Unexpectedly, investigators found that the viral loads in brains of type I/III receptor-deficient mice increased over time in comparison to type I receptor-deficient mice, showing that loss of type III IFN signaling enhances the susceptibility of type I receptor-deficient animals to a brain infection. This was a significant find as the presence of viruses in the brain can cause brain damage such as spongiosis or encephalitis. The low level of YFV-17FD brain invasion in wild-type mice caused mild spongiosis, whereas type I/III receptor-deficient mice had severe spongiosis—potentially explaining YFV-17D lethality in those animals. However, this raised the question of why YFV-17 was present at such high levels in the animals' brains.

Previous work showed that type III IFN signaling affects the epithelial cells that make up the BBB and modulates BBB integrity during infection by another flavivirus, West Nile virus. This was consistent with the Princeton team’s findings that the BBB of type I/III receptor-deficient mice was especially leaky to a blue dye. However, this wasn't the only way that loss of type III IFN signaling impaired the body's response to YFV. The researchers also found evidence that type III receptor deficiency provokes strong imbalances in several distinct kinds of immune cells during YFV-17D infection. In particular, type I/III receptor-deficient mice were defective in the activation of T cells, critical immune cells that control YFV-17D infection.

“We uncovered a critical role of type III IFN-mediated signaling in preserving the integrity of the BBB and preventing viral brain invasion,” Dr. Ploss stated, noting that work is needed to explore how type III IFN signaling affects YFV infection in primates.