Optimize fibro dT mRNA purification

To maximize the efficiency of Cytiva’s fibro oligo(dT) media in the purification of in vitro transcribed (IVT) messenger RNA (mRNA), it is essential to optimize running conditions. Two of the critical parameters for this application are loading conditions and residence time. In this study, we present a summary of chromatographic data that demonstrates the impact of salt concentration and residence time on binding capacity. Optimization of these parameters maximizes productivity and performance by using the medium’s high capacity and mass transfer capabilities, characteristics of the cellulose fiber matrix technology. Furthermore, we present a rapid, simple method for determining mRNA concentration in complex samples prior to fibro dT experiments. These optimization strategies are central to oligo(dT) mRNA purification workflows and are particularly relevant when translating to fibro dT mRNA purification using advanced membrane-based oligo(dT) chromatography systems.

Introduction

Oligonucleotide deoxythymidine (Oligo(dT)) based purification is a fundamental affinity chromatography technique that isolates polyadenylated (poly(A)) mRNA from total RNA samples. This technology is based on the hybridization of a short strand of the dT nucleotides to the polyadenylated poly(A) tail of mRNA.  In advanced formats such as fibro dT mRNA purification, this approach is implemented using membrane-based oligo(dT) chromatography to support high throughput and rapid processing. These systems enable effective mRNA binding capacity optimization under controlled salt concentration and residence time chromatography conditions. This technique underpins modern mRNA downstream workflows and is widely used for scalable purification of IVT-derived mRNA.

Historically, this technique has evolved from batch-based cellulose methods to highly efficient and automated methods driven largely by the need for high-purity mRNA for molecular biology applications and therapeutic mRNA drugs. Cytiva fibro dT is an affinity chromatography product based on an oligo(dT) ligand and a membrane technology with electrospun cellulose fibers, highly suitable for isolating polyadenylated mRNA molecules. The unique characteristics of the fibro base matric enable the Cytiva fibro dT to provide higher binding capacities at higher flow rates for efficient and fast mRNA isolation. These attributes make fibro dT highly suitable for modern oligo(dT) chromatography workflows where scalability and residence time chromatography control are critical.

The salt concentration in the sample and the binding buffer is one of the most critical running parameters when using Cytiva’s 0.4 mL fibro dT unit. Salt concentration affects the stability of hybridization between the oligo(dT) ligand and the poly(A) tail of mRNA by screening electrostatic repulsion between the negatively charged phosphate backbones. At moderate salt levels, cations act as counter-ions and promote base pairing (A–T) and hydrogen bond formation, improving capture efficiency.

If the salt concentration is too low, electrostatic repulsion increases and hybridization becomes inefficient. This can cause losses of target mRNA in the flow-through.

Conversely, very high salt concentrations can increase the risk of undesired co-binding of non-target nucleic acids and can reduce overall process robustness. In addition, high salt may increase the risk of precipitation, particularly for long mRNA constructs or self-amplifying RNA (saRNA).

Importantly, Cytiva’s fibro dT units can maintain higher binding capacity at low to moderate salt concentrations, enabling purification of a broad range of poly(A)-containing RNA types, including mRNA and self-amplifying RNA (saRNA). This flexibility supports robust mRNA binding capacity optimization across a range of process conditions.

Discover how fibro dt units can seamlessly integrate into your downstream mRNA platform.

Materials and methods

RNA is commonly transcribed into RNA from a linear DNA template in an enzymatic reaction called in vitro transcription. In this study, the linearized DNA template carries the sequence of a T7 promotor followed by the DNA sequence for a poly(A)-tailed mRNA (~2000 nucleotides) and was purified using a lab-scale Xampler™ ultrafiltration hollow fiber cartridge (NMWC 100,00) before transcribed into RNA.

The IVT reaction was performed on an 800 mL scale in a WAVE™ 25 2-liter single-use bag for 2.5 hours at 37°C, using T7 RNA polymerase, Inorganic phosphatase, and Ribonuclease inhibitors manufactured by Aldevron (Cat. no. 9135, 9132 and 9134). DNase I (Aldevron, Cat. no. 9136) digest was performed for 1 hour in the same bag, and enzymatic activity was quenched by the addition of EDTA before harvest. Crude mRNA samples were stored at -80°C until use.

For the determination of the mRNA concentration in the crude IVT samples, a small sample of the IVT reaction mixture was diluted 50x in TE buffer. Two mL of the diluted sample was injected on a HiPrep™ 26/10 Sepharose™ 6 Fast Flow column (product number 28405268) followed by an isocratic elution operated by ÄKTA pure™ 25 system (2 mm UV flow cell). The area of the first peak in the chromatogram was quantified, and the mRNA concentration was calculated using a mass extinction coefficient of 25 (mg/mL)^-1cm^-1 at 260 nm using the peak analysis tool in Cytiva’s UNICORN™ Evaluation software (version 7). Accurate quantification at this stage is essential for downstream oligo(dT) mRNA purification efficiency and reproducibility.

For the purification, two 40 mL frozen crude mRNA stocks were thawed in a water bath at 37°C. For a total of 50 mL thawed crude sample, 12.5 mL 2 M KCl was added, resulting in a final concentration of 400 mM KCl. This 62.5 mL sample was further diluted with binding buffer containing 400 mM KCl to a final volume of 600 mL. The concentration of the thawed and diluted mRNA sample was 0.318 mg/mL (n=3), analyzed using the HiPrep™ 26/10 Sepharose™ 6 Fast Flow column as described above. The diluted crude mRNA samples were thereafter purified on a Cytiva fibro dT 0.4 mL unit (product number 17553101, following the procedure described in Chapter 3 of the Fibro dT 0.4 mL Instructions for Use (document number 29806853), available on the product web page, using the buffers listed in Table 1. This workflow reflects a typical fibro dT mRNA purification setup leveraging membrane-based oligo(dT) chromatography.

To assess the impact of different salt concentrations, pre-purified mRNA was adjusted with 2 M KCl solution to generate samples with salt concentrations ranging from 100 to 600 mM KCl. Matching binding buffers were prepared for each sample. The final mRNA concentrations were analyzed with a NanoDrop™ spectrophotometer. These samples were used to determine the dynamic binding capacity at 10% breakthrough (QB10) for varying salt concentrations, as well as QB10 at varying residence times (performed with 400 mM KCl) with the buffers listed in Table 1. This approach supports systematic mRNA binding capacity optimization and evaluation of residence time chromatography effects.

Table 1. Chromatography buffers prepared from stocks of ultrapure 1 M Tris-HCl (pH 7.5), 0.5 M EDTA (pH 8.0), 2 M KCL, and DNase/RNase-Free Distilled Water.

Results

RNA quantification before fibro dT purification

Before the fibro dT purification runs, the mRNA concentration of the crude IVT sample was determined using size exclusion chromatography with Sepharose™ 6 Fast Flow resin, packed to a 10 cm column bed height, as exemplified in Figure 1. This step ensures controlled loading during oligo(dT) mRNA purification.

Figure 1. Separation performed on a diluted crude IVT sample with a HiPrep™ 26/10 Sepharose™ 6FF column. While the mRNA molecules fully elute in the first peak, smaller impurities like unincorporated NTPs, DNA fragments and proteins elute in a second, well-separated peak from the RNA peak. Integration of the area of the first peak area was used to calculate the concentration total RNA.

Impact of salt concentration on fibro dT performance

The effect of salt concentration in mRNA purification was evaluated to understand its role in binding efficiency. The binding capacity (QB10) for a polyadenylated mRNA (2000nt) was determined at four different salt levels at 15 seconds of residence time using six different fibro dT units. The binding capacity increases with an increase in salt concentration (Figure 2). The QB10 values obtained were 3.9 mg/mL, 6.1 mg/mL, 8.3 mg/mL, and 10.0 mg/mL at 100, 200, 400, and 600mM, respectively (n=6 for each salt condition). The difference in calculated QB10 value at different KCL concentrations were all statistically significant, 100 mM and 200mM (p = <0.0001), 200mM and 400mM (p = <0.0001), 400mM and 600mM KCl (p = 0.002). These findings highlight how salt tuning can directly drive mRNA binding capacity optimization in oligo(dT) chromatography.

Figure 2. Binding capacity (QB10) is a function of salt concentration at 15-second residence time of a representative mRNA (2000 nt). The results are presented as average values ± standard deviation (n = 6). Experiments were performed with a sample loading flow rate of 1.6 mL/min, i.e., 4 membrane volumes per minute, which corresponds to a residence time of 15 seconds.

Fibro dT performance at varying residence times

To study how residence time affects Cytiva fibro dT performance, the capacity was determined at the residence times of 15, 30, 60, and 120 seconds, corresponding to flow rates of 1.6, 0.8, 0.4, and 0.2 mL/min at sample loading. These principles are broadly applicable across oligo(dT) mRNA purification platforms, including fibro dT mRNA purification.

The QB10 of the mRNA (2000 nt) increased achieving 8.3, 9.1, 9.9, and 11.0 mg/mL at 15, 30, 60 and 120 seconds of residence time, respectively (Figure 3). The statistical multiple comparison test indicates that QB10 values between adjacent residence times were not significant. Only the difference between QB10 values at RT 15 seconds and 120 seconds was statistically significant (p = 0.033). The statistical test for trend showed a clear statistical significance (p = < 0.01).

Figure 3. mRNA QB10 at different residence times (15, 30, 60, and 120 seconds) obtained for three separate batches of HiTrap units. KCl concentration used was 400mM. The results are presented as the values for individual HiTrap units and the Average ± Standard Deviation (n=3).

Conclusions

These experiments demonstrate how salt concentration and residence time influence mRNA binding capacity on Cytiva fibro dT 0.4 mL units and provides guidance for selecting starting conditions and performing rapid, data-driven optimization.

Across the evaluated range, increasing KCl concentration during loading increased binding capacity (QB10) for a representative ~2000 nt mRNA (100–600 mM KCl). At a fixed salt concentration of 400 mM KCl, all residence times tested (15–120 seconds) met the acceptance criteria, and longer residence time increased QB10, with the largest difference observed between 15 and 120 seconds.

For practical method development, moderate salt concentration provides a robust starting point. In this study, 400 mM KCl in both sample and binding buffer delivered high capacity while avoiding unnecessarily high salt. A residence time of 15 seconds can be used as a productivity-focused starting condition and can be increased when higher capacity is required.

When optimizing a fibro dT capture step, salt concentration is a primary lever to adjust binding capacity. After selecting a salt concentration that meets the required capacity and supports stable operation, residence time can be tuned to balance throughput and capacity. During optimization, it is recommended to match the ionic strength of the sample and binding buffer to minimize shifts in binding behavior during loading. This iterative approach enables efficient mRNA binding capacity optimization.

Reliable RNA quantification prior to loading supports consistent method development and comparison between runs. In this work, size exclusion chromatography on Sepharose™ 6 Fast Flow provided a practical approach to quantify mRNA in crude IVT samples (Figure 1) and to verify that the mRNA eluted in a well-resolved first peak relative to smaller impurities.

Size exclusion chromatography (SEC) on Sepharose™ 6 Fast Flow can be used to quantify mRNA concentration in crude IVT samples prior to fibro dT purification (Fig. 1). In addition to quantification, SEC provides a rapid check that the mRNA elutes in a well-resolved first peak relative to smaller impurities, supporting informed decisions on loading amount and process conditions.

As salt concentration is increased, the risk of precipitation and reduced operability may increase, particularly for long mRNA constructs or self-amplifying RNA (saRNA). If precipitation or turbidity is observed, reduce the salt and/or condition the sample to limit precipitation (e.g., dilution, addition of more EDTA). If capacity is insufficient at low salt, increasing salt and/or residence time can reduce flow-through losses while maintaining short cycle times.