Retrovirus Infectivity Assays

Microtest offers a number of cell-based assays for detecting different classes of murine retroviruses. Our procedures can detect viruses from the four classes of these retroviruses — originally isolated from mice — that are of concern for various biological products. This Microtest screening is fully validated according to guidelines of the International Conference on Harmonisation (ICH).

Direct and extended MiCl1 S+L- assays

For the detection of xenotropic retroviruses (X-MLV), Microtest uses two versions of the reliable S+L- focus-forming assay utilizing MiCl1 cells.

In the direct assay, the number of foci that form in a monolayer after inoculation with a sample is proportional to the number of infectious units in the inoculum. This assay can be used to quantitate the amount of virus in a sample

The extended assay can detect low levels of virus not detected in a direct assay.

After passaging, Microtest virologists observe the monolayers. The presence of foci indicates the presence of virus in the sample.

Extended Mus dunni assay

Mus dunni cells are broadly susceptible to different classes of murine leukemia virus (MLV): amphotropic, ecotropic, polytropic (also called mink cell focus forming or MCF), and xenotropic viruses. These cells are therefore used in assays for the detection of MLV in biological samples. As MLV does not typically cause cytopathic effect in the cells, Microtest follows an antibody staining technique that utilizes monoclonal antibodies against MLV. M. dunni cells are inoculated with samples, then passaged to amplify low-level virus that may be present in the inoculum.

After incubation, the cells are stained with the antibodies, using a fluorescent antibody technique. If Microtest virologists observe any fluorescent cells present after staining, this indicates that virus was present in the sample.

Extended XC plaque assay

The extended XC plaque assay is utilized to detect infectious ecotropic murine retroviruses (E-MLV). E-MLV infects only cells of mouse and rat origin. Cells from the mouse embryo cell line, SC-1, are inoculated with the sample, and passaged to amplify any low level of virus present. Then the cells are overlaid with XC cells, which will form giant multinucleated cells (syncytia) where virus was present in the SC-1 cells.