In polymyositis, “myos-“ refers to the muscles, “poly-“ means many, and “-itis” refers to inflammation, so polymyositis is an inflammatory disorder which involves many muscle groups around the body.
Polymyositis is an immune- mediated disease, meaning that the immune system attacks the muscles in our own body.
Normally, the cells of the immune system are ready to spot and destroy anything foreign that could cause the body harm.
To help with this, most cells in the body have a set of proteins that combine together to form something called a major histocompatibility complex, or MHC, class I molecule that sits on the surface of their cell membrane.
These surface proteins act kind of like a serving platter, presenting molecules from within the cell for the immune system to continually sample.
Normally though the molecule’s just a sample from the cell, and the immune system recognizes it as harmless, and this is known as a self-antigen, and there’s no response.
But when a cell is actually invaded by a pathogen like a virus, viral antigens are presented on the MHC class I molecule, and that sparks a different immune response.
A type of T-lymphocyte, called a CD8+ T-cell, also known as a cytotoxic T-cell, uses its T-cell receptors to bind to the antigen presented by the MHC class I molecule.
If the cytotoxic T-cell binds strongly, than the antigen is recognized as foreign, and the cytotoxic T-cell secretes a whole lot of perforin and granzymes.
Perforin forms big holes in the infected cell and that allows the granzymes to enter the cell.
Once inside, the granzymes induce apoptosis, or programmed cell death.
As if that weren’t enough, the cytotoxic T-cells have a protein called Fas ligand on their surface, and it binds to a molecule called Fas on the surface of the infected cell.
When these two combine, it triggers a cascade of signaling events inside the target cell that also leads to apoptosis.
Meanwhile, B- lymphocytes that react to the pathogen, can also start producing a whole lot of antibodies.
These antibodies bind the pathogen, and typically prevent it from attacking the host’s cells and, at the same time, “tag” the pathogen for further destruction by other immune cells.
In polymyositis, healthy muscle cells present normal muscle proteins on the MHC class I molecule, and the cytotoxic T-cells inappropriately react and get activated.
That’s because it’s thought that the muscle proteins might look similar to a foreign pathogen.
This is called molecular mimicry, because from the perspective of the cytotoxic T-cell, a host protein is mimicking a foreign protein.