Immunocompetent mice are the well-characterized and widely accepted experimental animal model for JEV infection research [
70], however, this kind of mouse model has above-mentioned limitations which limit their widespread application in some flaviviruses such as DENV, ZIKV, WNV, YFV, and TBEV [
71,
72,
73,
74,
75]. Therefore, the scientists constructed immunodeficient mice models that support flavivirus infection and demonstrate more efficiency than immunocompetent mice in evaluating vaccine and antiviral agents [
76]. Immunodeficient mice refer to mice with deficiencies in one or more components of the immune system either due to congenital genetic mutations or artificial methods.
For examples, Johnson AJ et al. evaluated the infectivity of DENV in AG129 mice (lacking IFN-α/β and IFN-γ receptors genes) of different ages (4-, 6-, 8-, or 12-week-old). They administered the DENV via i.p. injection into the mouse's abdominal cavities, and then found that mice of different age groups all exhibited neurological abnormalities (hind-limbs paralysis and blindness) on the 7
th day post-infection and all mice died on the 12
th day after infection. In comparison, no WT mice had clinical symptoms [
77]. Similarly, Aliota MT et al. reported that ZIKV infection could cause 100% mortality in 3- to 8-week-old AG129 mice. Although having no paralysis during the whole experimental period, all infected mice displayed signs of illness like weight loss, lethargy, and hunching by the 5
th day post-infection [
78]. In addition, Zhou D et al. compared the pathogenicity of two strains (pSA14 and rG66A) of JEV using 5-week-old C57BL/6 mice and INFAR
-/- mice. All C57BL/6 mice infected with the pSA14 strain via i.p. injection succumbed within nine days post-infection whereas mice infected with rG66A strain just exhibited a 30% mortality. In contrast, all INFAR
-/- mice, irrespective of virus strains, died within six days post-infection, suggesting that INFAR
-/- mice are more susceptible to JEV infection than WT mice [
79]. In fact, AG129 mice are also applicable animal models for YFV infection[
80]. To understand the role of type III IFN in YFV infection, Douam F et al. constructed a range of mouse models including αβR
-/-(IFN-α/β receptor knockout), λR
-/-( IFN-λ receptor knockout), and αβR
-/-λR
-/- mice (both IFN-α/β and IFN-λ receptors knockout) mice. Two different doses (1 × 10
6 and 1 × 10
7 PFU, representing low and high doses, respectively) of YFV (YFV-17D strain) were injected to the tail vein of mice. Collected data revealed the obvious difference in mortality rate among three mouse models, with αβR
-/-λR
-/-mice exhibiting increased susceptibility to YFV. Specifically, αβR
-/-λR
-/-mice presented a mortality rate of 30% at the low dose of YFV and a mortality rate of 60% at the high dose of YFV. In contrast, the mortality rates in the other two mouse models were limited to 10% even at the high dose of YFV [
81].
STING (stimulator of interferon genes), primarily recognized for initiating the production of type I interferon and the innate immune response to cytosolic DNA, is essential for defending against neurotropic RNA viruses. Studies have shown that mice lacking STING (STING
-/- mice) exhibit more significant neurological symptoms compared to WT mice following WNV infection. These neurological symptoms in affected STING
-/- mice include loss of balance, diminished muscle tone and reflexes, especially in the pelvic limbs, along with increased paresis and paralysis. This suggests more severe damage to the hindbrain and spinal cord [
82].