IFNG and infection: Four days post infection with the Sterne ΔhtrAlefMUTΔcya live attenuated vaccine strain [12,13] approximately 25 IFNγ spots per 106 splenocytes were observed upon stimulation of splenocytes with a mixture of B. anthracis antigens (urea-extracted B. anthracis antigens, to be referred as “core” antigen, [12,18]) and 10-15 spots when stimulated with the PA protein (Figure 2A).