DENV isolates passed serially from brain to brain led to increased neurovirulence and neurotropism in mice[44] and a clear attenuation in human volunteers.[45] However, viral encephalitis is not a major clinical symptom in human dengue disease, as nervous system involvement in DENV infections is

rare and few cases are reported.[46] The IFN system is critical to the host antiviral response, which led to the use of AG129 mice, which are type I and II IFN-R-deficient 129 mice, immune deficient and highly susceptible.[47] Intraperitoneal infection with the mouse-adapted neurotropic DENV-2 strain, New Guinea C, led to 100% lethality in AG129 mice, all of them presenting paralysis.[48] The neuroinflammatory changes led to alterations in motor behaviour and muscle tone and strength in DENV-3-infected mice. The neuroinflammatory process was marked by up-regulation of the chemokines Epigenetics inhibitor CCL2, CCL5, CXCL1 and CXCL2, and of the cytokines TNF-α and IFN-γ, which occurs in parallel with increased leucocyte rolling and adhesion in meningeal vessels and infiltration of immune cells into the brain.[49] In summary, even if these models were used to study antiviral compounds or behaviour, the major limitation involving immune-compromised mice is that paralysis is not a major clinical observation in DENV infection. Initial tropism studies using the

AG129 (IFN type I and II receptor-deficient) model demonstrated that clinical isolates from all four DENV serotypes replicate BGB324 nmr efficiently in spleen, lymph node, bone marrow and muscle.[50] Negative-strand

viral RNA was detected in dendritic cells and macrophages of the lymph node and spleen.[50] To develop an experimental model where viral encephalitis was not the major clinical observation, Shresta et al.[47] infected AG129 mice intravenously with the DENV-2 strain PL046. Infected AG129 mice succumbed to DENV infection, RNA Synthesis inhibitor presenting increased levels of TNF-α and vascular leakage syndrome. AG129 mice are able develop cross-reactive and long-lasting antibody responses to DENV.[51] Sequential DENV infection in AG129 mice results in decreased viral load of the second serotype and full protection against lethal infection. AG129 and other mouse strains have been used to study ADE by passive transfer of anti-DENV monoclonal antibodies, cross-reactive immune serum, or diluted homotypic serum before infection.[52, 53] Mortality was associated with vascular leakage syndrome, high levels of TNF-α and thrombocytopenia, similar to the clinical findings observed in DHF/DSS in humans. No memory response was observed in mice receiving passive transfer of serum or antibody. Hence, models of sequential DENV infection may be useful to study ADE in the presence of a cellular memory immune response.