Polymyositis is caused by the immune system mistakenly attacking healthy muscle tissue. Specifically, a type of white blood cell called CD8+ T cells infiltrates skeletal muscle fibers and destroys them, leading to progressive muscle weakness. While this immune attack is well documented, what sets it off in the first place involves a mix of genetic susceptibility, environmental triggers, viral infections, and sometimes medications.
Polymyositis is rare, affecting roughly 0.5 to 8.4 people per 100,000 in the United States. It almost never appears in childhood and typically develops after age 20. Understanding its causes matters because identifying the trigger can shape treatment and help predict how the disease will behave.
How the Immune System Attacks Muscle
In a healthy body, immune cells ignore your own tissues. In polymyositis, that tolerance breaks down. Muscle fibers begin displaying a surface molecule called MHC class I, which is essentially a molecular flag that tells the immune system “inspect me.” Healthy muscle cells don’t normally display this flag in significant amounts, but when they do, CD8+ T cells recognize the muscle tissue as foreign and release toxic molecules that damage and kill the fibers.
This isn’t limited to already-damaged cells. Non-necrotic (otherwise healthy) muscle fibers expressing MHC class I can also be invaded and destroyed. The result is chronic inflammation inside the muscle itself, which progressively weakens it over weeks to months. This process distinguishes polymyositis from conditions like dermatomyositis, where the immune attack targets blood vessels supplying the muscle rather than the fibers directly.
Genetic Risk Factors
Not everyone’s immune system makes this mistake, and genetics play a significant role in who is vulnerable. The strongest genetic link comes from a cluster of immune-related genes known as the HLA 8.1 ancestral haplotype. A large genome-wide association study across geographically diverse Caucasian populations found that alleles within this haplotype account for essentially all the identifiable genetic risk for major myositis subtypes, including polymyositis.
Within this gene cluster, the allele HLA B*08:01 showed a particularly strong association with polymyositis specifically, while other alleles in the same haplotype were more closely tied to dermatomyositis. However, no single gene variant is sufficient on its own. Multiple alleles within the HLA 8.1 haplotype need to be present together for the full risk effect. This means polymyositis isn’t inherited in a simple way. You can carry these genetic variants and never develop the disease, but they make your immune system more prone to the kind of misfire that triggers it.
Viral and Infectious Triggers
Genetics load the gun, but something in the environment often pulls the trigger. Several viruses have been linked to the onset of inflammatory myopathies like polymyositis. The most consistently cited include Coxsackie B virus, enteroviruses, parvovirus, HTLV-1 (a virus that infects immune cells), and HIV.
The leading theory is molecular mimicry: viral proteins resemble proteins found on muscle cells closely enough that the immune system, while fighting the infection, accidentally learns to attack muscle tissue too. Once that autoimmune response is established, it can persist long after the original infection clears. Some infections may also directly damage muscle cells, causing them to display MHC class I molecules and making them visible targets for immune attack.
COVID-19 has also been reported as a potential trigger for inflammatory myopathy, though documented cases remain rare. Hepatitis C is another virus of interest, particularly in related conditions like inclusion body myositis.
Autoantibodies and Their Role
Many people with polymyositis produce specific antibodies that target their own cellular machinery. These are called myositis-specific autoantibodies, and the most common one in polymyositis is the anti-Jo-1 antibody, which attacks an enzyme involved in protein building inside cells (histidyl-tRNA synthetase). Anti-Jo-1 antibodies are found in roughly 15% to 30% of all myositis patients and appear more frequently in polymyositis than in other forms.
Other autoantibodies found in myositis patients target the signal recognition particle (involved in moving proteins within cells) and a nuclear enzyme called Mi-2. These antibodies aren’t just diagnostic markers. They likely contribute to ongoing tissue damage and help define distinct clinical patterns. For example, people with anti-Jo-1 antibodies often develop a specific combination of muscle inflammation, lung disease, joint pain, and cracked skin on the hands, sometimes called antisynthetase syndrome.
Medications That Can Trigger Polymyositis
Certain drugs can induce or worsen inflammatory myopathies that closely resemble polymyositis. Statins, the widely prescribed cholesterol-lowering medications, are the best-known culprit. Since their introduction in the 1980s, statins have been associated with a range of muscle problems, from mild soreness to full-blown immune-mediated inflammatory myopathy, including cases that meet the criteria for polymyositis or dermatomyositis.
Immune checkpoint inhibitors, a newer class of cancer drugs that work by removing the brakes on the immune system, are an increasingly recognized cause. By design, these drugs amplify immune activity, which can spill over into autoimmune attacks on muscle tissue. As checkpoint inhibitors become more widely used in oncology, drug-induced myopathy cases are expected to rise.
Other medications linked to immune-mediated myopathies include interferon-alpha (used for hepatitis C and certain cancers), TNF-alpha blockers used for rheumatoid arthritis and inflammatory bowel disease, and less commonly, D-penicillamine and procainamide. In a review of myopathy cases caused by TNF-alpha blockers, 12 out of 24 patients developed polymyositis specifically, with the remainder developing dermatomyositis. The drugs most often involved were etanercept, infliximab, and adalimumab.
The Link to Cancer
A small but notable percentage of polymyositis cases occur alongside cancer, raising the question of whether the cancer itself can provoke the autoimmune response. In a large Taiwanese study tracking over 1,100 polymyositis patients across nearly two decades, 3.3% were diagnosed with cancer after their myositis diagnosis. The most common types were colorectal, lung, and liver cancers.
This cancer association is stronger in dermatomyositis (where 5.6% developed malignancies in the same study), but it’s relevant enough in polymyositis that doctors typically screen for underlying cancer, particularly in older adults with new-onset disease. The relationship may work in both directions: tumors can express proteins that resemble muscle tissue, triggering a cross-reactive immune attack, or chronic immune dysregulation from myositis may itself contribute to cancer development.
Why No Single Cause Explains Most Cases
For the majority of people with polymyositis, no single identifiable trigger is found. Instead, the disease likely results from a combination of genetic predisposition, an environmental event (often a viral infection that may have gone unnoticed), and an immune system that, for reasons not fully understood, fails to shut down the inflammatory response once it starts. The presence of autoantibodies in many patients suggests the immune system has been fundamentally reprogrammed to see muscle tissue as a threat, which is why polymyositis tends to be a chronic condition rather than a one-time event.
Drug-induced cases are the exception, where removing the offending medication can sometimes halt or reverse the process. In all other cases, the autoimmune mechanism, once activated, typically requires ongoing management to control.