Degranulation of mast cells (MC) is triggered by direct infection <i>in vivo</i>.

<p>(A) MC are first-hit target cells of mCMV infection <i>in vivo</i>. Peritoneal exudate cells (PEC) were recovered from the peritoneal cavity at 4 h or 24 h after intraperitoneal infection with fluorescent reporter virus mCMV-GFP. This timing does not allow completion of the viral infection cycle and thus precludes virus spread to secondary target cells. In the cytofluorometric analysis, a gate was set on FcεRI⁺CD117⁺ MC for identification of infected MC expressing the reporter GFP. (B) Selective degranulation of infected, GFP⁺ MC. PEC were pre-gated for expression of FcεRI and analyzed for the expression of CD117 and the degranulation marker CD107a. GFP-expression in FcεRI⁺CD117⁺CD107a⁺ MC identifies degranulating MC as those which are infected. The black-shaded area represents cells within the gate of CD107a⁺ MC, the gray-shaded area represents the “reverse gate” of MC not or no longer expressing CD107a. Shown are the original data from one mouse as an example. Percentages of degranulating CD107a⁺ MC of n = 4 mice tested individually ranged between 9.0% and 24.1% with a mean value of 17.5% and a median value of 18.5%.