Effects on human complement activation. Human sera was incubated with Ad (5 × 1010 vp ml−1) in the presence or absence of 40 μg ml−1 Xbp for 90 min at 37 °C. (a) Human sera with high pre-existing neutralising IgG titres (high NAbs) and sera samples with low IgG titres which exhibited a dependence on FX for protection (low NAbs Group A), (b) FX-depleted human plasma (Quadratech Diagnostics, UK) and (c) C1q depleted human sera were used. C3a-desArg levels were quantified by ELISA. Graphs show transduction as a percentage of serum+phosphate-buffered saline (PBS). *P<0.05 vs matched PBS or Ad5.