TACI-induced expression of AID mRNA. To examine whether single or compound mutations in TACI affect TACI activation that lead to isotype switch, the induction of AID mRNA by an agonist anti-TACI antibody, with or without added IL-4 or IL-10, was examined for B cells of subjects from Families C, E, and F compared with similarly treated cells from a healthy control. The index patients with CVID (C1, E1, and F1) have 2 mutations in TACI, whereas other members are either normal or have 1 mutation. For this B cells were cultured with biotin-labeled anti-TACI on antibiotin beads for 24 hours, and AID mRNA was quantitated by real-time PCR. Results for AID mRNA for Family C is given in A, Family E in B, and Family F in C. The significant differences (Mann-Whitney test) for induction of AID mRNA for controls compared with subjects for each condition are indicated on the figure.