This increased demand for insect cell products leads, in turn, to greater need for effective harvesting methods for large-scale cultures. Techniques have been established for harvesting insect cells using batch centrifugation,2 continuous centrifugation,3 and tangential flow filtration using either hollow fibers4 or flat sheets.5 Batch centrifugation is the simplest method for small-scale cultures, but it is very difficult to scale up to larger sizes because each batch usually is limited to only a few liters. Continuous centrifugation permits larger scale operation, but requires considerable initial investment. Also, the concentrate contains many particles that are 1 µm or larger, which means an additional filtration step is necessary before further processing. Finally, the product that remains in the sediment-containing phase is not recovered.6
An alternative to centrifugation is tangential flow filtration (TFF). TFF has the advantage of being linearly scalable from the bench top to large scale production.7 A major drawback to TFF is the tendency of microfiltration membranes to foul with particles in the media, which lowers membrane performance and makes cleaning difficult.8,9 Another complication of using TFF is the shear sensitivity of infected insect cells.5 If TFF is operated at too high a flow rate, the cells will break apart as a result of the shear. These cell parts further foul the membrane.This study tests the performance of a modified TFF system, "SmartFlow TFF" (SFTFF, NCSRT, Apex, NC), in obtaining a high yield of a target protein from insect cell culture. In SFTFF, ribs create uniform retentate channels with uniform flow patterns over the entire functional surface of the membrane module. These uniform flow patterns maximize membrane efficiency and minimize fouling of the membrane module. The uniform flow also makes it possible to scale the system linearly from the laboratory bench to commercial production. In installed systems for other applications, the system has been scaled up directly from 5-m2 laboratory studies to commercial operations that process process 1.4 million L/d.
In this work, a single tank, single-module process was developed using SFTFF to harvest the SARS-CoV spike protein, which was expressed intracellularly from an insect cell culture. One microfiltration module was used for the following steps: (i) clarification; (ii) diafiltration to replace the medium with buffer; (iii) lysis of cells and release of the desired protein by the addition of extraction buffer; and (iv) passage of the released protein through the membrane. Then, an ultrafiltration membrane was used to concentrate the protein and perform a buffer exchange before loading the protein onto an HPLC column. No lysis was observed, which means that SFTFF did minimal harm to the cells during clarification. The one tank–one module method simplifies the harvest in comparison to multiple centrifugation steps. The implications of these results for both small-scale and large-scale harvesting of insect cells are discussed below.