Rapid AAV-Neutralizing Antibody Determination with a Cell-Binding Assay

Abstract

Recombinant adeno-associated virus (rAAV) has been developed as a successful vector for both basic research and human gene therapy. However, neutralizing antibodies (NAbs) against AAV capsids can abolish AAV infectivity on target cells, reducing the transduction efficacy. Absence of AAV NAb has become a prerequisite qualification for patients enrolled in gene therapy trials. Nevertheless, accurate assessment of AAV NAb has remained a challenging task. Here we developed a rapid assay based on the observations that AAV NAb inhibits rAAV binding to the host cell surface and NAb titers are negatively related to the amount of AAV genomes binding to the target cells. By quantifying the AAV genome on the target cells in the presence of anti-sera, AAV NAb titers can be accurately determined. The titer determined by this assay correlates well with the classical transduction-based assays. A major advantage of this method is that it can be carried out with a 30-min binding assay without the lengthy wait for a transduction outcome. This assay is independent of transduction performance of AAV serotype in the target cells. Therefore, the AAV cell-binding assay for NAb determination offers an alternative method for in vivo NAb assay.

Keywords: AAV; gene therapy; neutralizing antibody assay; vector.

Figure 1

Correlation of AAV2 Vector-Host Cell Binding to GFP Reporter Gene Expression Level (A) Measurement of AAV2 genomes binding to the host cells at different MOIs. scAAV2-EGFP at the MOIs indicated on the x axis were incubated with GM16095 cells at 4°C for 30 min, and then the vector genome copy numbers were determined by qRT-PCR, which is shown on the y axis. Data are expressed as log per cell means ± SD. (B) The GFP expression at 48 hr post-infection was observed by fluorescence microscopy. A typical view image is shown for each MOI.

Figure 2

Sensitivity Assessment of AAV8 Vector-Host Cell Binding Compared to Reporter Gene Expression (A) AAV8 genomes binding to the host cells at different MOIs. scAAV8-EGFP vectors at the MOIs indicated on the x axis were incubated with GM16095 cells at 4°C for 30 min, and then the vector genome copy numbers were measured by qRT-PCR, which is shown on the y axis. Data are expressed as log per cell means ± SD. (B) The GFP expression at 48 hr post-infection was observed by fluorescence microscopy. A typical view image is shown for each MOI.

Figure 3

AAV-Binding Assay Procedure and NAb Calculation Demonstration (A) Flowchart of AAV-neutralizing antibody determination using an AAV-binding assay. (B and C) An example of NB50 calculation. ssAAV2-EGFP (B) or ssAAV8-EGFP (C) and varying dilutions of neutralizing antibodies were incubated at 37°C for 1 hr. The resulting complex of AAV vector and neutralizing antibodies (NAbs) was then applied to GM16095 cells and incubated at 4°C for 30 min. The genome copy number of AAV bound to the cell was determined by qRT-PCR. The NB50 values were then calculated, as represented by the antibody dilutions needed to neutralize 50% of the input vector, which was 1:453 for AAV2 and 1:114 for AAV8. NC, negative control with no antibody added. Data are expressed as mean percent neutralized vector ± SD.

Figure 4

Determination of Neutralizing Titer in Two Typical Human Serum Samples for AAV2 Serotype (A) The neutralizing activity of human serum against AAV2 vectors. Neutralizing titers (NTs) were calculated as the serum dilution required to neutralize 50% of the input vector quantified by genome copy numbers obtained from qRT-PCR assay. Graphic determination for patients 1 and 7 yielded NTs against AAV2 of 1:38 and 1:602, respectively. NC, negative control with no serum added. Data are expressed as mean percent neutralized vector ± SD. (B) Correlation between NB50 and NT50 values. 21 patient samples were analyzed to determine the r value, which was 0.93. Data are expressed as mean dilution ratio ± SD. (C) Observation of GFP gene expression through fluorescence microscopy following the transduction-based assay of NAb against AAV2. GM16095 cells were incubated with the mixture of ssAAV2-EGFP (MOI: 1,000) and several dilutions of human serum from patients 1 and 7, as described in the Materials and Methods.