New Tick-Borne Viruses Evade Human Immune System Defense Mechanisms

Warmer climates are contributing to the increased activity of ticks, notorious vectors for diseases like Lyme disease and Rocky Mountain spotted fever. However, a less well-known but emerging threat lies in a distinct group of tick-borne pathogens known as nairoviruses. These viruses can provoke severe fever and compromised organ function following tick bites. Recent research published in ACS Infectious Diseases sheds light on the sophisticated mechanisms nairoviruses employ to evade the human immune system, with significant implications for surveillance and public health preparedness.

In this study, Scott Pegan, along with his colleagues, explores how nairoviruses circumvent the host’s innate defenses. A notable example of this family is the Crimean-Congo hemorrhagic fever virus (CCHFV), a pathogen with a high fatality rate posing a global threat, particularly to civilians and military personnel in endemic areas across Africa, the Middle East, and Asia. Understanding the immune evasion strategies of such viruses is critical for developing effective countermeasures and improving biosurveillance capabilities.

Nairoviruses produce a specialized enzyme—an ovarian tumor domain-containing protease—that selectively cleaves ubiquitin and ISG15 molecules attached to human proteins. Ubiquitin and ISG15 modifications typically function as alarm signals to the immune system, marking infected cells for destruction. By removing these small protein tags, nairoviruses render themselves invisible to host defenses, enabling uninterrupted replication and pathogenesis.

The investigators analyzed enzymatic activity from four different nairovirus species: three recently identified orthonairoviruses isolated from patients in Asia and an additional virus known as Pacific Coast tick nairovirus (PCTNV) found in ticks but not yet confirmed in human infections. Their assays revealed that the PCTNV enzyme exhibited the highest efficiency in cleaving ubiquitin and ISG15 from human proteins, suggesting a superior ability to suppress immune detection compared to its counterparts.

Given that PCTNV is transmitted by a tick species known to bite humans and capable of carrying other diseases such as Rocky Mountain spotted fever, these findings raise concerns about potential human exposure risks, especially along the U.S. West Coast. This highlights the urgent need for vigilance, not only regarding tick bites themselves but also concerning the species of ticks involved, as they may harbor previously unrecognized pathogens.

Expanding on this work, Pegan’s team collated ubiquitin cleavage data from 13 nairovirus species to train computational models aimed at distinguishing pathogenic viruses from less harmful ones. This represents a promising avenue toward automated biosurveillance systems capable of real-time detection and risk assessment of emerging tick-borne viruses.

Overall, this study emphasizes the complex interplay between nairoviruses and human immune defenses and underscores the importance of integrated molecular and computational approaches to tackle the growing threat of these stealthy viral pathogens.

Keywords

Viruses, Nairoviruses, Tick-borne pathogens, Immune evasion, Ubiquitin, ISG15, Protease, Biosurveillance