Focal Infectivity Assay

Lately in lab I've been doing a lot of focal infectivity assays or FIAs. Focal Infectivity Assays can be used to quantitatively determine the production of virus by infected cells.

To the right is a very clear example of this. You just count the blue dots. When a lot of them are touching and aggregated together they form what is called a syncitia. The official definition of a syncitia is goes something like "HIV-infected patients, macrophages fuse into multinucleated giant cells that produce copious amounts of virus. T-tropic isolates, or syncitia-inducing (SI) strains replicate in primary CD4+ T-cells as well as in macrophages and use the alpha-chemokine receptor, CXCR4, for entry." Thank you wikipedia.

Syncytia formation refers to the observation that HIV infected monocytes and macrophages can fuse together in a rather large conglomeration of cells. This syncitium may occur in lymphoid tissues, bone marrow, and brain. The bad news about syncytia formation is that it enables an HIV-infected macrophage to bring dozens of previously uninfected macrophages into the fold (so to speak) and render them useless. This syncitium of immune cells does not participate in further general immune function and probably represents an attempt to neutralize the HIV-infected macrophage that started the process. The capacity for syncitia formation is not universal in HIV infection. Syncitia forming virus tends to emerge later in the course of infection and is associated with a more rapid decline in CD4 cells.

HIV can be divided into two general categories CCR5 tropic virus and CXCR4 tropic virus. This distinction is based on the predominant type of co-receptors. HIV attaches to cells via a CD4 receptor and requires one of these two co-receptors to get in. Studies have clearly demonstrated that the CXCR4 tropic virus is capable of syncytia formation, but CCR5 tropic virus is not. Patients with low CD4 counts are more likely to have both CXCR4 and CCR5 (dual tropic) virus. CCR5 inhibitors are now in large scale clinical trials and we are now routinely testing patients in these trials to see which type of virus they harbor. It is likely that this testing will become available for clinical care if these agents live up to their promise.


Counting these by eye under a microscope is so tedious I feel that my eyes will burn out and bleed. It's really quite awful. You start seeing dots everywhere. Yes, we have a plate reader. But it's not calibrated correctly and the camera lens is dirty. So clean the camera lens right? Wrong. You need a metric screwdriver set and the company has told me it's in the mail. I sent them a strong email. Let me see how good that'll do me.

In the meantime I suppose I'll just continue counting and trying to do comparisons with the plate reader which is actually called an AID virureader.

I'm coming across so many more new words daily. Hopefully to keep up with them I'll post them here and that will help me memorize and further understand.