# Lentiviral Titer via p24 (viral coat protein) ELISA Assay

## Introduction

We determine the transducing units per milliliter (TU/mL) of lentivirus based upon the p24 assay from ZeptoMetrix. The titer calculation is determined by using the conversion factor from Didier Trono to convert from concentration of p24 to viral titer. There are approximately 1 x 104 physical particles of lentivirus for every pg of p24 antigen. Based on ELISA results, this works out to be 100 transducing units (TU) for every pg of p24 antigen present.

## Materials

• Lentivirus targeting your favorite gene
• p24 ELISA from ZeptoMetrix

## Protocol

1. Dilute samples between 750 and 3000 fold.
2. Utilize the p24 ELISA kit from ZeptoMextrix and follow the manufacturer's instructions.
3. ALWAYS include blanks to “zero” the plate reader.
4. ALWAYS utilize a p24 standard curve for calibration.
5. For the standard curve, utilize two independent dilutions with five concentrations of p24 plus a concentration of 0.
6. Plot the standard curve as concentration of p24 vs. OD450
7. The fit in the standard curve should yield an R2 > 0.95.
8. Determine the slope m and the intercept b. (You may also download and utilize the Calibration Sheet in the attached spreadsheet.)
9. Calculate TU/mL for your diluted virus based upon the following equation (You may also download and utilize the Data Sheet in the attached spreadsheet.)

## Tips

Derivation of conversion factor:

a. (2 x 103 molecules) x (24 x 103 Da of p24 per PP) = (48 x 106)
b. 48 x 106/Avogadro’s constant = (48 x 106) / (6 x 1023) = 8x10-17 g of p24 per PP
c. There is approximately 1 PP per 1 x 10-16 g of p24
d. 1 x 104 PP per pg of p24
e. 100 TU per 1000 PP