Pathogenic breaches: how viruses compromise blood-tissue barriers.

Blood-tissue barriers (BTBs) are highly specialized, selectively permeable surfaces that separate the circulatory system from delicate tissues and organs. Critical examples include the blood-brain barrier (BBB), blood-retinal barrier (BRB), blood-testis barrier (BTB), and other organ-specific barriers, including the alveolar-capillary interface in the lungs and the glomerular filtration barrier in the kidneys. These barriers regulate the bidirectional transport of nutrients, gases, and waste while restricting pathogens, toxins, and immune cells to maintain physiological balance. Nevertheless, viruses have evolved multiple strategies to circumvent or compromise these barriers, facilitating viral entry, evading immune surveillance, and establishing infection within protected compartments. Neurotropic viruses, including the West Nile virus and Japanese encephalitis virus, impair the blood-brain barrier by disrupting tight junction proteins and cytokine storms. In contrast, respiratory viruses such as influenza and SARS-CoV-2 affect the lung barrier, resulting in alveolar injury and systemic inflammation. Other viruses, such as the Zika virus, affect the BTB and placental barriers, presenting significant risks to fetal development and reproductive health. Such breaches facilitate viral spread, exacerbate tissue damage, and complicate therapeutic interventions. This review provides a comprehensive overview of blood-tissue barrier architecture, function, and mechanisms of viral disruption, highlighting their dual role in protection and susceptibility during viral infections. By elucidating interactions between viruses and blood-tissue barriers, this work highlights emerging research directions to mitigate viral pathogenesis and enhance treatment efficacy for barrier-associated diseases.