Altered virulence of vaccine strains of measles virus after prolonged replication in human tissue

To understand the molecular determinants of measles virus (MV) virulence, w e have used the SCID-hu thymus/liver xenograft model (SCID-hu thy/liv) in w hich in vivo MV virulence phenotypes are faithfully duplicated. Stromal epi thelial and monocytic cells are infected by MV in thymus implants, and viru lent strains induce massive thymocyte apoptosis, although thymocytes are no t infected. To determine whether passage of an avirulent vaccine strain in human tissue increases virulence, we studied a virus isolated from thymic t issue 90 days after infection with the vaccine strain Moraten (pMor-1) and a virus isolated from an immunodeficient child with progressive vaccine-ind uced disease (Hu2). These viruses were compared to a minimally passaged wil d-type Edmonston strain (Ed-wt) and the vaccine strain:Moraten. pMor-1, Hu2 , and Ed-wt displayed virulent phenotypes in thymic implants, with high lev els of virus being detected by 3 days after infection (10(5.2), 10(2.8), an d 10(3.4), respectively) and maximal levels being detected between 7 and 14 days after infection. In contrast, Moraten required over 14 days to grow t o detectable levels. pMor-1 produced the highest levels of virus throughout infection, suggesting thymic adaptation of this strain. Similar to other v irulent strains, Ed-wt, Hu2, and pMor-1 caused a decrease in the number of viable thymocytes as assessed by trypan blue exclusion and fluorescence-act ivated cell sorter analysis. Thymic architecture was also disrupted by thes e strains. Sequence analysis of the hemagglutinin (H) and matrix (M) genes showed no common changes in Hu2 and pMor-1. M sequences were identical in p Mor-1 and Mor and varied in H at amino acid 469 (threonine to alanine), a p osition near the base of propeller 4 in the propeller blade/stem model of H structure. Further study will provide insights into the determinants of vi rulence.