GENETIC-MAPPING OF REOVIRUS VIRULENCE AND ORGAN TROPISM IN SEVERE COMBINED IMMUNODEFICIENT MICE - ORGAN-SPECIFIC VIRULENCE GENES

We used reovirus reassortant genetics and severe combined immunodefici ent (SCID) mice to define viral genes important for organ tropism and virulence in the absence of antigen-specific immunity. Adult SCID mice infected with reovirus serotype 1 strain Lang (T1L) died after 20 +/- 6 days, while infection with serotype 3 strain Dearing (T3D) was leth al after 77 +/- 22 days. One hundred forty-five adult SCID mice were i nfected with T1L, T3D, and 25 different T1L x T3D reassortant reovirus es, and gene segments associated with the increased virulence of T1L w ere identified. Gene segments S1, L2, M1, and L1 accounted for >90% of the genetically determined increase in T1L virulence. Gene segment M1 was independently important for virulence, with S1, L2, and L1 alone or in combination also playing a role. T1L grew to higher titers in mu ltiple organs and caused more severe hepatitis than T3D. Seventy adult SCID mice, TIL, T3D, and 15 T1L x T3D reassortant viruses were used t o map genetic determinants of viral titers in the brain, intestines, a nd liver, as well as the severity of hepatitis. Different sets of gene segments were important for determining viral titers in different org ans. Gene segments L1 (encoding a core protein) and L2 (encoding the c ore spike of the virion) were important in all of the organs analyzed. The M1 gene segment (encoding a core protein), but not the S1 gene se gment, was a critical determinant of reovirus titer in the liver and s everity of hepatitis. The S1 gene segment (encoding the viral cell att achment protein and a nonstructural protein), but not the M1 gene segm ent, was a critical determinant of titers in intestines and brains. Th ese studies demonstrate that viral growth in different organs is depen dent on different subsets of the genes important for virulence. The vi rion-associated protein products of the four gene segments (L1, L2, M1 , and S1) important for virulence and organ tropism in SCID mice likel y form a structural unit, the reovirus vertex. Organs (the brain and i ntestines versus the liver) differ in properties that determine which virulence genes, and thus which parts of this structural unit, are imp ortant.