OX6-positive cells became progressively rounder during the first week (Fig.  1E) , as reflected by an increase in shape index from 0.42 in pathogen-free rats to 0.66 at 2 days after infection and to 0.77 at 1 week (Fig. 2) . At 2 and 4 days, the shape of many of the cells was intermediate between stellate and round.