C HA-specific activation of FcγR by AAV-HA, AAV-cHA or WIV pooled pre-challenge sera (n = 18 mice per group). MDCKII cells were transfected with wildtype HA (pAAV-HA) or a stalk-only HA (pAAV-mHL1) before mouse serum and FcγR-I-expressing reporter cells were added (n = 2, technical duplicates). Mean ± SE. 