From Soil to Synovium: The Microbial Roots of Rheumatoid Arthritis

By Lorraine Gonizzi

The Microbiota-Rheumatoid Arthritis Connection: Spotlight on Prevotella copri

Can a bacterium be the hidden culprit behind a chronic disease that destroys joints? Though it may sound unbelievable, emerging research suggests that an imbalance in our gut microbiota, specifically the overgrowth of Prevotella copri (P. copri), may play a crucial role in the development of rheumatoid arthritis (RA), an autoimmune disease affecting the joints ^1,2,3,4.

Picture your gut as a vibrant garden, teeming with trillions of bacteria. This garden is home to trillions of bacteria, forming a complex ecosystem known as the gut microbiota. These organisms fuel our bodies with energy, produce vital compounds and support immune function ⁵. Just like in a garden, balance is crucial for maintaining a healthy microbiota. When an invasive microbe, like a weed in a garden, disrupts this balance the vital microbiota is disturbed. This phenomenon, known as dysbiosis, leads to a loss of beneficial bacteria, a reduction in vital metabolites and a weakening of the gut barrier, serving as the protective soil (Figure 1) ⁶.

Dysbiosis is observed in RA, which is characterized by chronic joint inflammation. This inflammation is primarily driven by autoantibodies, including rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPA), which target the synovium, a tissue within the joint. Along with self-reactive T cells, RF and ACPA attack joint tissues, ultimately leading to joint destruction ^2,7. RA negatively impacts both the physical and mental well-being of patients affected by it ⁷. Unfortunately, current treatments focus on symptom management rather than addressing the root causes, as the mechanisms of RA development remain unclear ⁸.

However, scientists have discovered that dysbiosis occurs even before RA symptoms appear ^1,2,3,9. One bacterium, P. copri, tends to grow uncontrollably in the gut of people at risk for developing RA ¹⁰. P. copri is usually involved in breaking down complex carbohydrates and producing short chain fatty acids ¹¹. Several studies suggest a positive correlation between P. copri and a pro-inflammatory environment, indicating its potential role in RA development ^1,12. Given that this dysbiosis occurs at an early stage, it may represent a potential target for the treatment, detection and prevention of RA. But how can a single bacterium contribute to the onset of an autoimmune disease affecting the joints? To unravel this mystery, let’s stroll through our microbial garden and see how the delicate balance in our bacterial landscape might be the key to understanding RA.

The Overactive Gardener: the Immune System at Work 

A healthy gut microbiota resembles a thriving botanical garden where diverse species work together to maintain harmony. In RA, this balance is disrupted as P. copri overtakes the ecosystem, resulting in a loss of beneficial bacteria and weakening the gut barrier ⁶. This creates an environment that favors its overgrowth. The immune system, akin to a gardener, is responsible for maintaining balance in the microbiota. In RA patients, the overgrowth of P. copri triggers the immune system to respond, gathering in the intestines — causing the gardener to go rogue and launch an aggressive inflammatory response (Figure 2) ^9,13.

Researchers observed that in RA patients, the immune system launches a pro-inflammatory response within the gut, including the infiltration of T-helper (Th) cells, which are immune cells that exacerbate inflammation and damage gut barrier integrity ^9,13,14,15. This results in a ‘leaky gut’, meaning everything from the stool, such as P. copri, has a higher chance of penetrating the tissue of the intestines. Importantly, increased Th cell activity doesn’t necessarily mean these cells recognize P. copri. To test this, researchers isolated gut Th cells from a mouse model of RA and exposed them to P. copri. Remarkably, these Th cells recognized P. copri and exhibited a strong pro-inflammatory reaction. A similar reaction was observed when Th cells from RA patients were exposed to P. copri but not in those from healthy individuals ^9,13. This suggests that only RA patients’ Th cells recognize P. copri as a threat, triggering chronic inflammation in the gut. In essence, the gardener becomes hyper-vigilant, specifically targeting P. copri, but this targeted response in one area may have collateral damage elsewhere (Figure 2).

Molecular Mimicry: Mistaking a Friend for a Foe

The focus on the gut raises the question of how inflammation and P. copri recognition can damage distant joints. Studies in RA mice have shown that these activated Th cells travel from the gut to the spleen and eventually to the joints where they continue to promote inflammation ^9,15. Once in the joints, these Th cells can promote ongoing inflammation, suggesting that their effects persist beyond the gut ^9,15. In essence, the gardener doesn’t just patrol the garden, but starts searching the house for invasive weeds too.

But the gardener runs into a problem. He begins to mistake innocent houseplants for the invasive garden weeds. This confusion mirrors a process known as molecular mimicry, where microbial molecules closely resemble host molecules, leading the immune system to attack the body’s own tissues (Figure 3) ¹⁶. Scientists have identified several host molecules originating from joints that share over 70% similarity to Pc-p27, a P. copri molecule. In addition, exposing RA patients’ Th cells to these joint molecules, they recognize it to the same extent as they recognize the Pc-p27 of P. copri. In contrast, healthy individuals do not exhibit such recognition ¹⁷. This suggests that RA patients’ Th cells recognize P. copri in the gut but mistakenly attack joints due to molecular mimicry.

The Right Tools and Environment

But if the immune system’s confusion stems from a microbial mimic, why doesn’t everyone with P. copri develop RA? In non-westernized populations who consume high-fiber diets, P. copri is abundant yet these individuals do not develop RA ¹¹. This suggests that the gardener’s response depends not only on the presence of weeds but also on the broader environmental conditions to promote disease.

One important factor influencing this environment is HLA-DRB1, a variant of the antigen-presenting molecule (MHC) that helps activate the immune response; just like a gardener needing the right hoe to remove invasive weeds effectively. Studies have linked the HLA-DRB1 variant to RA ^7,18. Both Pc-p27 and similar self-molecules are presented on the HLA-DRB1 molecule, reinforcing its role in disease progression ^7,17. Research in mice expressing HLA-DRB1 has shown that they develop more severe RA than those with HLA-DRB2, further supporting the crucial role of HLA-DRB1 in RA ¹⁹.

Beyond microbial and immune factors, sex also influences RA susceptibility. RA is more common in women, with female mice also showing more severe disease. This suggests that sex hormones play a key role in RA risk ^8,19. Estrogen strengthens immune responses by increasing HLA-DRB1 expression, acting like fuel for the gardener by boosting its ability to tackle weeds ²⁰. Interestingly, when male mice are given estrogen, their HLA-DRB1 levels rise to those of females. Male hormones appear to be protective against RA, explaining why men are less affected ^21,22. Estrogen also influences the microbiota, with higher levels of estradiol linked to lower Prevotella levels ²³. After menopause, women’s microbiota shifts, increasing Prevotella and aligning with the typical onset of RA, around age 55 ^24,25. The loss of estrogen’s influence on both the immune system and the microbiota may create an environment that accelerates RA development, particularly through the rise in Prevotella, which interacts with immune cells to fuel inflammation. Together, these findings suggest that these hormonal shifts, along with microbiota changes like increased Prevotella, may together heighten RA risk. 

Even before RA symptoms appear, the immune system is already on high alert. Autoantibodies like ACPA and RF can be present up to a decade before diagnosis, suggesting silent inflammation is brewing beneath the surface ²⁶. Some bacteria, including P. copri, thrive in inflammatory environments, suggesting that inflammation may act as fertilizer for the invasive weeds ^27,28,29,30. Studies have shown that inflammation increases gut permeability, allowing immune signals to interact with gut microbes ^15,31,32. RA patients with P. copri show higher inflammatory levels, and when they receive anti-inflammatory treatments, their gut microbiota often rebalances, reducing P. copri-dominated dysbiosis ^{9,14,33,34,35,36,37,38}. This suggests that inflammation is not just a byproduct of RA but actively contributes to its development.

Conclusion and Future Directions 

If our gut is a garden, then restoring balance may be the key to controlling autoimmune diseases like RA. Emerging evidence highlights the gut-joint axis as a key player in RA, with P. copri-driven dysbiosis potentially initiating molecular mimicry and sustaining autoimmune inflammation. This offers promising new avenues for early detection, prevention and treatment of RA.

With growing insight into the gut-joint axis and its potential role in the origins of RA, future strategies are increasingly focused on addressing root causes rather than solely managing symptoms. Early detection methods, such as analyzing the gut microbiota for P. copri overgrowth, have shown potential in assessing RA risk ³⁹. Preventive approaches, including precision probiotics and high-fiber diets, appear promising in restoring microbial balance and lowering RA risk, although further studies are needed to confirm their effectiveness ^{40,41,42,43,44}. When integrated with conventional therapies, dietary interventions have also been linked to improved RA disease management and symptom relief ^45,46,47. Additionally, advancing our understanding of how genetics and hormones influence immune responses and shape the microbiota may pave the way for more personalized treatment strategies. By nurturing a balanced gut ecosystem early, our internal microbial garden, we may not only improve RA outcomes but also help prevent the disease from taking root altogether.

About the author

Further reading

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