The nuts and bolts of retrovirus safety testing

by Dr. Ray Nims


Retroviruses may integrate into the genome of host animals. For this reason they are often referred to as endogenous viruses. Viral particles may or may not be expressed in the host cell. Expressed viruses may be infectious or non-infectious, and infectious virus may have tropism for (ability to infect) the same or different animal species relative to the host cell of origin. Infection results from a process of reverse transcription of the viral RNA leading to proviral DNA. To accomplish this, retroviruses have a specialized enzyme known as reverse transcriptase. Through this process (see figure below), the infected cell may be enlisted to produce viral progeny. Certain of the retroviruses are known to be oncogenic (e.g., human T-lymphotropic virus 1, feline leukemia virus, Raus sarcoma virus, etc.). Other retroviruses are of concern as a result of disease syndromes caused in humans (e.g., human immunodeficiency virus 1 in acquired immunodeficiency syndrome, and the possible role of xenotropic murine leukemia virus-related virus in chronic fatigue syndrome). From a biosafety standpoint, there is a worry that under some conditions, integrated viruses in cell substrates employed to produce biopharmaceuticals which do not normally express their presence may be induced to produce infectious particles.

Source: AccessExcellence

Retrovirology safety testing for biologics manufacture can be confusing to those not familiar with the subject. Here is a brief overview.

Demonstrating retroviral safety typically involves a combination of the following three components:
• detecting infectious retrovirus through cell culture assays (XC plaque, cocultivation with mink lung or Mus dunni cells, etc.).
• measuring reverse transcriptase enzyme activities either through tritiated thymidine incorporation into templates, or through product amplification (PCR) techniques (PERT, etc.). This is not required if infectious retrovirus is detected.
• Visualizing and enumerating retroviral particles in supernatants or in fixed cells using transmission electron microscopy.

The various assays are applied during cell bank characterization (including end of production cell testing), during evaluation and validation of purification processes, and in some instances, as bulk harvest lot-release assays (results from 3 lots at pilot or commercial scale are submitted with the marketing application). For processes using well-characterized rodent cells known to contain endogenous retrovirus (CHO, C127, BHK, murine hybridoma), retroviral infectivity testing of the processed bulk is not required provided that adequate downstream clearance of the particles has been demonstrated.

Infectivity testing can be particularly confusing, due to the variety of cell-based assays employed. These include both direct and indirect assays. An example of a direct assay is the XC-plaque assay for ecotropic (a term meaning the virus is infectious for mouse cells) murine retroviruses. By definition, therefore, this would only be used to assay production cells of mouse origin.

Indirect assays are those in which a second endpoint is required to assess positive or negative outcome. Indirect assays include the various co-cultivation assays in which the test cells are co-cultivated with host cells such as mink lung, Mus dunni, and any of a number of human cells (see Table 4 within USP <1237> Virology Tests for a list of commonly used host cells). The indirect assays are performed to detect xenotropic retroviruses (retroviruses which are capable of infecting only animals other than the species of origin). The secondary endpoints used to assess outcome include reverse transcriptase activity, sarcoma virus rescue (S+L- focus formation assays), or enzyme immunoassay. The indirect assays are used in the retrovirus testing of mouse, hamster, monkey, and human production cell substrates. The selection of the host cell for the cocultivation assay is dependent upon the species of origin of the production cell, recognizing that cocultivation host cells from a species other than that of the production cells must be used. For production processes using rodent or other non-human cells, one or more human host cells are typically used for the cocultivation assay, as xenotropic retroviruses infectious for human cells are of obvious concern.

Still confused? Don’t worry. An individual with virology testing expertise can assist in designing the appropriate retrovirus testing battery for your biologic.