Avian Influenza A Tolerates Mammalian Fever
A pivotal study published in Science elucidates a critical mechanism governing the severity of zoonotic influenza infections. Researchers have demonstrated that the core body temperature differential between birds and mammals acts as a primary barrier to viral replication, which is effectively reinforced by the mammalian febrile response. However, avian influenza A viruses (IAVs), having evolved to replicate within the higher physiological temperatures of avian hosts (400C to 420C), possess a thermal resilience that allows them to bypass this innate immune defense. This thermotolerance is driven largely by the viral polymerase subunit PB1, a finding that offers new insights into the pathogenicity of historical pandemics and current zoonotic threats.
Fever is an evolutionarily conserved response intended to inhibit pathogen replication. In this study, investigators established that human-adapted seasonal influenza viruses are highly temperature-sensitive; they thrive in the cooler upper respiratory tract (~330C) but are significantly inhibited by febrile temperatures (~400C). Using a murine model of "simulated fever," researchers maintained mice at an ambient temperature that elevated their core body temperature by ~20C. This physiological hyperthermia effectively protected mice infected with human-adapted virus strains, transforming a typically severe infection into a mild disease presentation. The protective capacity of fever failed completely against viruses carrying avian-origin genetic material. The study identified the PB1 subunit of the viral polymerase as the critical determinant of temperature sensitivity.
By mapping amino acid substitutions, the team identified specific residues substitutions that confer "avian-like" temperature resistance. Mutant viruses engineered with these thermotolerant PB1 subunits caused severe disease and weight loss in mice, even in the presence of a simulated fever. The virus effectively ignored the host's thermal defense mechanism. This suggests that the ability to replicate efficiently at febrile temperatures is a key factor enabling zoonotic spillover viruses to cause severe pathology in humans.
