Our immune system helps protect us against harmful and foreign invaders, such as bacteria and viruses, to keep us healthy. However, our immune systems can sometimes attack our bodies and damage our healthy cells, which gives rise to autoimmune diseases. Autoimmune diseases are conditions that occur as a result of the immune system attacking the body’s cells and tissues. Mixed connective tissue disease (MCTD) is one of these autoimmune diseases. MCTD is an autoimmune condition that shares similar overlapping symptoms with other autoimmune diseases, such as lupus and scleroderma. It is a rare condition and can affect multiple organs in the body. Autoantibodies play a crucial role in the development of mixed connective tissue disease and its associated symptoms. Understanding how autoantibodies work and how they cause mixed connective tissue disease and its symptoms is important when considering diagnosis, treatment and management. 

Continue reading to learn more about MCTD, autoantibodies, the role of autoantibodies in causing MCTD and its implications for disease management. 

Mixed connective tissue disease (MCTD) 

MCTD is a rare systemic autoimmune disease that tends to overlap with other autoimmune conditions, including lupus, scleroderma, and polymyositis. Its overlap with other autoimmune diseases, particularly regarding symptoms and clinical manifestations, can make it more difficult to diagnose. Mixed connective tissue disease primarily affects women in their 20s or 30s.^1,2

Symptoms of MCTD can include:^2,3

• Fatigue or tiredness 
• Mild fever 
• Unexplained muscle pain or muscle weakness
• Raynaud’s phenomenon: A condition that occurs when blood vessels narrow and limit blood flow to the skin.⁴ This will usually cause numbing, coldness, and a colour change in fingers and toes.  
• Swollen fingers
• Presence of a rash

Symptoms of mixed connective tissue disorder tend to progress over several years, and disease severity can range, with some cases causing severe complications that could be damaging long-term or possibly even fatal. The exact cause and risk factors that lead to the development of MCTD are still unknown. Diagnosis for MCTD typically includes a clinical examination of symptoms, the patient’s medical & family history, as well as a blood test to examine whether certain antibodies are involved. Diagnosing mixed connective tissue disease at an early stage is crucial, particularly because the condition can cause several complications such as pulmonary hypertension (high blood pressure in vessels between the heart and lungs), inflammatory conditions in the lungs, cardiovascular complications, and infections.¹

Autoantibodies: what happens when our immune system fails? 

A key aspect of mixed connective tissue disease, and autoimmune diseases in general, is a dysregulation in one of the body’s most important systems, the immune system. Normally, the immune system can distinguish between self (body’s cells and proteins) vs nonself (foreign pathogens), and attack harmful invaders (e.g bacteria, viruses) to protect itself. One of how this self-protection is achieved is through the immune system’s production of antibodies. Antibodies are a key component of the body’s immune response, and they are produced primarily to fight against harmful pathogens.⁵

However, in autoimmune diseases, such as mixed connective tissue disorder, this self-protective mechanism of the immune system breaks down. Rather than targeting foreign invaders, the immune system mistakenly identifies the body’s cells and tissues as threats. This leads to the production of autoantibodies. Autoantibodies are antibodies that bind to self-antigens(molecules on your cells that signal the immune system that they’re part of your own body and should not be attacked) and attack the body’s immune system.⁶ They are implicated in various health conditions, with one of the most prominent being autoimmune conditions. Mixed connective tissue disease is primarily associated with the presence of the autoantibody ‘U1 ribonucleoprotein (U1 RNP). The presence of autoantibodies is a key diagnostic factor, and it plays a role in the disease’s pathogenesis. 

Autoantibodies involved in MCTD

The most important autoantibody involved in mixed connective tissue disease is anti-U1 RNP. It is highly associated with MCTD and is a key diagnostic factor. However, their presence is not exclusive to MCTD and can be associated with other health conditions, especially those involving the connective tissues, such as lupus.⁷ 

Ribonucleotides (RNPs), in simple terms, are complexes or structures that help regulate the processing of RNA(Ribonucleic Acid, is a message copy of DNA that instructs cells to make proteins to keep the body running)and gene expression.⁸ In individuals affected by MCTD, the immune system recognises U1-RNP as a ‘foreign invader’ or threat, which results in the production of the anti-U1 RNP autoantibody. These autoantibodies that are produced will attack U1-RNP in the body. Moreover, this production of autoantibodies will lead to the symptoms observed in mixed connective tissue disease. 

Another autoantibody involved in mixed connective tissue disorder, although less common, is anti-U1 small nuclear ribonucleoprotein particle (snRNP). The presence of this autoantibody can be observed in some cases of mixed connective tissue disease along with other systemic autoimmune diseases.⁹ 

Autoantibodies & MCTD symptoms

Once autoantibodies such as anti-U1 RNP are produced, the immune system is involved in a dangerous cycle of attacking itself and creating inflammation. When autoantibodies begin binding to normal proteins in the body, they form immune complexes(clusters formed when antibodies in your body stick to bacteria or viruses, helping the immune system find and remove them) and accumulate in tissues, joints, muscles, etc This creates an inflammatory environment that could involve multiple organs and organ systems. As a result, several complications can occur, including pulmonary hypertension, interstitial lung disease, cardiovascular complications and other inflammatory conditions. 

Specific symptoms will vary from individual to individual, as different organs and systems may be impacted. However, the core of MCTD is a dysregulated immune system that produces autoantibodies and attacks itself. Understanding the root of mixed connective tissue disorders is crucial for shaping diagnostic processes, managing patients’ symptoms, limiting complications, and improving care overall. 

Summary

• Mixed connective tissue disease is a rare autoimmune disorder that shares overlapping symptoms and clinical features with other systemic autoimmune disorders, such as lupus
• Common symptoms include a mild fever, fatigue, rash, Raynaud’s phenomenon, and muscle pain or weakness
• MCTD, similarly to other autoimmune conditions, occurs when the body’s immune system begins to recognise the body’s cells as threats and attacks itself by producing autoantibodies
• The key autoantibody involved in mixed connective tissue disorder is anti-U1 RNP
• The presence of anti-U1 RNP is one of the primary diagnostic factors of mixed connective tissue disease, and this autoantibody helps drive the condition
• Autoantibodies in MCTD can help drive inflammation and create an inflammatory environment, affecting multiple organs. 
• As a result, MCTD can cause several complications such as pulmonary hypertension, cardiovascular conditions, interstitial lung disease, and inflammation
• Understanding how these autoantibodies play a role in the disease development of mixed connective tissue disorder is crucial for developing better diagnostic tools and treatment strategies, ultimately enhancing a patient's quality of life and improving patient outcomes