Reduced cytotoxic activity of lymphocytes in mice carrying heterozygous mutations in several HLH-causing genes. Control (black bars), Prf1^−/− (lightest gray bars), Rab27a^+/−Stx11^+/− (AS, light gray bars), Rab27a^+/−Prf1^+/− (AP, medium gray bars), and Rab27a^+/−Prf1^+/−Stx11^+/− mice (ASP, dark gray bars) were infected with 200 PFU of LCMV-WE. (A) LCMV titers in the liver of infected mice 14 and 21 days postinfection. Data (mean ± SEM) are representative of 3 to 4 independent experiments with at least 3 mice in each group. Control (black bars), Prf1^−/− (lightest gray bars), Rab27a^+/−Stx11^+/− (AS, light gray bars), Rab27a^+/−Prf1^+/− (AP, medium gray bars), and Rab27a^+/−Prf1^+/−Stx11^+/− mice (ASP, dark gray bars) were injected with a mix of control and β2-m–deficient spleen cells labeled with different fluorescent dyes (CFSE and Violet, respectively). Sixteen hours later, the presence of injected cells was quantified by FACS analysis. (B) FACS analysis of input cells (upper left) and MNCs from spleen of control (upper middle), Prf1^−/− (upper right), Rab27a^+/−Stx11^+/− (AS, lower left), Rab27a^+/−Prf1^+/− (AP, lower middle), and Rab27a^+/−Prf1^+/−Stx11^+/− mice (ASP, lower right panel). Data are representative of 2 independent experiments with at least 3 mice in each group. (C) Relative cytotoxic index. Data (mean ± SEM) are representative of 2 independent experiments. *P < .05; **P < .01; ***P < .001. A.U., arbitrary units; MHC, major histocompatibility complex; mRNA, messenger RNA.