Anaemia and iron deficiency in patients with rheumatoid arthritis and other chronic diseases^*

The prevalence of RA in the general population is estimated at between 0.3% and 2%. The disease affects approximately three times as many women as men. The mean onset age of RA is approximately 40; the onset of the disease occurs earlier in women than in men. The prevalence of RA varies depending on the geographical latitude and the research methodology used. In Europe, the prevalence is between 0.5% and 1% and is lower in the southern part of the continent (3.3/10 thousand in the south; 5.9/10 thousand in the north) [2, 68].

The etiopathogenesis of RA is not completely understood. A number of factors play a role in the development of RA, including genetic factors, e.g. the presence of major histo-compatibility complex (MHC) class II antigens, including in particular DRB antigens, environmental factors (e.g. exposure to viral and bacterial infections) and immune disorders [13, 56].

One of the environmental factors linked to a higher risk of developing RA is smoking, which probably increases the citrullination of peptides [22].

The literature also includes studies which indicate that obesity plays a role in the development of seronegative RA and suggest that adipose tissue has an impact on immune response. Most likely, Porphyromonas gingivalis, a bacterium which is implicated in the development of periodontal diseases and which is capable of producing specific antibodies against citrullinated enolase, which react with human citrullinated antibodies through molecular mimicry, is involved in the pathogenesis of RA [40].

Apart from histocompatibility antigens (DRB*0401, DRB*0404, DRB*0101, DRB*1402), the genetic factors which play a role in the development of RA include the polymorphism of the protein tyrosine phosphatase non-receptor type 22 gene, polymorphisms of the TNF-α promoter gene, interleukin-1 gene, interleukin-1 receptor antagonist and chemokine receptor CCR5 as well as the polymorphism of the gene coding for peptidylarginine deiminase (PAD14), which is mainly prevalent in the Asian population [18, 27, 47, 68].

The initiation and persistence of inflammation is most likely linked to the response of memory cells (TCD45RO+) to an antigen in genetically predisposed individuals. Stimulation of T cells occurs through the formation of the antigen-MHC complex. The costimulatory proteins in this reaction include CD4 and CD28 molecules on the surface of T cells [53].

Early changes in the synovial tissue of joints show a Th1 response linked to the production of cytokines such as: IL-2, IFN-ɣ, TNF-α and GM-CSF. Activated (macrophage-like) synoviocytes start to produce IL-1β, TNF-α and IL-6, which contribute to joint destruction through the activation of osteoclasts and matrix metalloproteinases. In addition, IL-6 is responsible for the differentiation of B cells and the production of antibodies. In turn, TNF-α activates surface adhesion molecules, which are involved in the migration of inflammatory cells. TNF-α also stimulates the release of other proinflammatory cytokines, such as IL-1, IL-6 and IL-8, as well as matrix metalloproteinases, which contribute to cartilage damage. TNF-α has an impact, both direct and indirect (through the RANKL/RANK system), on osteoclasts [9, 46, 57, 64].

On the one hand, all the aforementioned factors constitute a mechanism for the formation of inflammatory pannus, which is involved in cartilage damage and bone erosions in joints. On the other hand, some cytokines are the targets of certain drugs used in the treatment of RA (e.g. tocilizumab – an IL-6 pathway inhibitor) [59].

Additional tests show reduced serum ferritin levels (<12 ng/ml), reduced serum iron levels, reduced transferrin saturation, increased TIBC, increased transferrin receptor levels and reduced serum hepcidin levels [4, 17, 51]. Rarely, iron deficiency may also be caused by the autosomal recessive mutation of the matriptase-2 gene (a protein involved in hepcidin regulation), which leads to iron-refractory iron deficiency anaemia (IRIDA) [21].

Iron deficiency (ID) is defined as a reduction in ferritin levels and transferrin saturation (TSAT) of less than 20%. ID may lead to anaemia, defined as a reduction of more than two standard deviations below the mean of the haemoglobin and haematocrit concentrations, and the red blood cell count. As a result of this process, the ability of red blood cells to carry oxygen is reduced.

Iron deficiency is the most common dietary deficiency. It is estimated that it affects over 2 billion people around the world [74]. The prevalence of ID varies depending on such factors as age, sex as well as physiological, pathophysiological, environmental and socioeconomic factors [16, 41]. Iron deficiency is a major problem in both developing and well-developed countries. For instance, in the USA, ID affects 2% of men and 9% of women [31]. Iron deficiency is commonly associated with low haemoglobin levels. In addition to anaemia, ID leads to poor pregnancy outcomes, impaired school performance and lower productivity [74].

The major causes of anaemia include the loss of erythrocytes as a result of bleeding or haemolysis as well as reduced or impaired erythropoiesis in bone marrow.

The following tests are used to assess iron deficiency and determine its type: ferritin, hepcidin, TIBC (total iron binding capacity) and UIBC (unsaturated iron binding capacity).

Ferritin is both an acute-phase protein and the main protein which stores iron in the body. A total of 1 µg/l of ferritin is equivalent to 8 mg of stored Fe. As ferritin is an acute-phase protein, its concentration is increased in patients with infectious disease, inflammation (rheumatic diseases), malignancy, liver disease or chronic kidney disease. Particularly high ferritin levels, exceeding even 10.000 µg/l, are observed in patients with Still’s disease or haemophagocytic syndrome [54, 55].

Hepcidin, a peptide hormone, or more precisely its 25-amino acid form (hepcidin-25), is involved in iron metabolism [19]. As a ferroportin antagonist, it reduces serum iron levels. Its concentration is lower in iron deficiency anaemias [54].

Total iron-binding capacity (TIBC) is a measure of the total amount of iron that can bind to transferrin. One gram of transferrin binds 1.41 mg of Fe. TIBC is increased in both latent and overt anaemia [7].

Unsaturated iron-binding capacity (UIBC) represents the portion of iron binding sites on transferrin that are not occupied by iron. UIBC values are increased in patients with iron deficiency and are reduced in patients with anaemia of chronic disease [7].

ID may be absolute or functional, depending on its pathophysiology [73]. Absolute ID occurs when body iron stores are depleted. Therefore, in such a case, iron measurement is based on the measurement of peripheral blood ferritin levels, which correspond to stored iron, mainly in hepatocytes and reticuloendothelial cells. In functional ID, there are iron stores in the body, but the iron is stored in compartments where it is not available for metabolic processes; it is mainly trapped inside the reticuloendothelial system [29, 48].

It is believed that this is due to excessive inflammatory activation and the resulting increased production of proinflammatory cytokines (mainly interleukin-6) and hepcidin [8, 25]. Hepcidin is the main protein which regulates systemic iron metabolism. As a result of its excessive expression, iron becomes “trapped” in reticuloendothelial cells and the absorption of iron by enterocytes in the gastrointestinal tract is inhibited. As a consequence, tissues do not receive the amount of iron they need for metabolic processes [28, 29, 48].

In heart failure, iron prevents anaemia and is essential for the functioning of cells that have a high mitotic rate and metabolic turnover (including haematopoietic cells) and high-energy demands due to the intensive mechanical work performed (cardiomyocytes, skeletal muscle cells). It was shown that in patients with heart failure, iron deficiency correlates with impaired exercise tolerance, aggravation of depression symptoms, lower quality of life and increased risk of hospitalisation and death (including cardiovascular death) [32, 33, 48].

According to Okonko et al., the risk of death in patients with iron deficiency is twice as high as that observed in patients with anaemia without iron deficiency [49].

The results of the multicentre FAIR-HF trial confirm the effectiveness of treatment with iron supplementation in patients with heart failure with or without anaemia [5].

Iron deficiency – both absolute and functional – is one of the two leading causes of anaemia in patients with chronic kidney disease. A reduction in iron stores is secondary to blood loss relating to frequent blood draws or occult gastrointestinal bleeding as well as blood loss during haemodialysis, which may be between ten and twenty times higher compared to that of healthy individuals [24, 50]. Iron deficiency affects 48% to 98% of pre-dialysis patients with chronic kidney disease [62].

Other causes of iron deficiency include impaired iron absorption from the gastrointestinal tract caused by uremic gastropathy, the use of drugs that suppress the secretion of hydrochloric acid or bind phosphates in the gastrointestinal tract, chronic inflammation and vitamin B and C deficiency [15].

Functional iron deficiency is linked to the unavailability of iron for erythropoiesis despite normal iron stores in the body. This occurs during inflammation, which results in iron being bound in ferritin and hemosiderin complexes in the reticuloendothelial system [38].

In the early stages of chronic kidney disease, oral supplementation is used, whereas intravenous supplementation is recommended where oral supplementation proves ineffective. In patients with stage 4 or stage 5 chronic kidney disease, intravenous iron supplementation is used [57].

Both oral and intravenous iron supplementation improves red blood cell parameters and complements treatment with erythropoietin in patients with chronic kidney disease [39].

However, some researchers claim that iron supplementation in patients with chronic kidney disease has adverse effects, i.e. a higher risk of serious cardiovascular incidents and infectious complications [1].

Inflammatory bowel disease is often accompanied by iron deficiency, which is caused by insufficient supply or supplementation of iron or its impaired absorption or excessive loss. Iron deficiency anaemia affects 49% to 74% of patients with inflammatory bowel disease, and over half of patients with inflammatory bowel disease suffer from chronic inflammation [63].

Iron deficiency and the resulting anaemia are associated with a lower quality of life, fatigue, impaired exercise tolerance, higher risk of cardiovascular incidents, lower work capacity (76%, 63% and 60% of patients) as well as an increased risk of hospitalisation and blood transfusions [66]. It was found that oral iron supplementation is an effective treatment method. However, 21% of patients discontinue the treatment due to its side effects [36].

In the active stage of inflammatory bowel disease, oral supplementation is ineffective due to increased hepcidin and TNF-alpha levels [67].

In patients with inflammatory bowel disease, indications for intravenous iron supplementation include: intolerance or ineffectiveness of oral supplementation, severe anaemia and high bowel inflammatory activity. It has been found that this form of treatment is successful [3].

Anaemia is one of the most common complications in patients with rheumatoid arthritis. Its prevalence is 33% to 60%, depending on the research methodology used [26, 72]. The causes of anaemia in RA patients include the following: iron deficiency caused by blood loss, e.g. as a result of long-term treatment with non-steroidal anti-inflammatory drugs, bone marrow hypoplasia resulting from the use of drugs, e.g. cyclophosphamide, vitamin B12 deficiency or folic acid deficiency, e.g. during treatment with methotrexate. Approximately 60% of anaemia in patients with RA is anaemia of chronic disease (ACD). There is no clear data in the literature with regard to iron deficiency in RA patients.

The pathomechanism of anaemia in patients with RA is complex and multifactorial. On the one hand, in RA, immune-competent cells become activated and the production of cytokines increases. The latter, i.e. TNF alpha, interferon gamma (most potent inhibitor), and interleukin-1, have direct suppressive effects on the red blood cell system in bone marrow, which results in decreased erythropoiesis. In addition, the production of erythropoietin and iron management become impaired. TNF alpha and interferon gamma inhibit the production of erythropoietin in the kidney. Moreover, the half-life of erythrocytes is decreased as a result of increased erythrophagocytosis, which is of lesser importance [43]. A lower sensitivity of bone marrow to erythropoietin has been observed in patients with ACD, which most likely results from the reduced number of erythropoietin receptors and increased apoptosis of precursor cells in bone marrow [26, 70]. The cytokines such as IL-1, IL-6, IL-10, interferon gamma and TNF-alpha are secreted in RA. This, in turn, leads to the activation of macrophages and the deposition of iron in macrophages [26, 69]. Interferon gamma increases the expression of divalent metal transporter 1 (DMT1) on macrophages, which stimulates the uptake of ferrous iron. The interleukin-10 increases transferrin receptor expression and uptake of transferrin bound iron. TNF alpha induces macrophages to phagocytose erythrocytes for the recycling of iron. Interferon gamma decreases the expression of the iron transporter ferroportin 1, which inhibits iron export from macrophages. This process is also induced by hepcidin. TNF alpha, interleukin-1, interleukin-6, and interleukin-10 induce ferritin expression, which stimulate the retention of iron in macrophages [70]. The aforementioned processes result in reduced serum iron levels and anaemia.

On the other hand, during inflammation, the release of hepcidin, which binds to ferroportin and initiates its destruction, is increased. As a result, hepcidin inhibits the absorption of iron in the gastrointestinal tract and the release of iron from the reticuloendothelial system. IL-6 through STAT3 induces the transcription of the hepcidin-coding gene [26, 44]. As a result of both the processes which take place in RA, iron is not sufficiently available for erythroblasts, despite normal iron storage in the body. Therefore, in patients with RA, increased anaemia severity correlates with excessive production of IL-6 [20, 52].

It was observed that both anaemia and iron deficiency (without anaemia) in inflammatory diseases other than RA, e.g. heart failure, may be associated with reduced exercise tolerance, a worse prognosis and thus reduced work capacity [23, 33].

There is evidence that low haemoglobin levels in RA patients are significantly correlated with disability, activity and duration of the disease as well as damage to joints and joint pain [60]. Treatment of anaemia in RA patients includes iron supplementation, blood transfusions and the use of erythropoiesis-stimulating agents. Importantly, the treatment of the underlying condition itself may lead to an increase in haemoglobin levels [14].

In chronic inflammatory diseases, iron deficiency, even without anaemia, may cause fatigue as well as the aggravation of a given chronic condition, which leads to increased morbidity and mortality. Iron deficiency is very common. However, it is often disregarded, especially in patients with chronic conditions, even though it has an impact of the wellbeing of patients. In the literature, there are reports concerning iron deficiency in patients with heart failure, renal failure and chronic bowel diseases. There are no such reports or treatment recommendations for patients with RA [32].

Biological treatments used in patients with RA, e.g. infliximab (anti-TNF-α monoclonal antibody), tocilizumab (anti-IL-6 receptor antibody) and anakinra (anti-IL-1 antibody), not only effectively inhibit the progression of joint involvement, but may also prevent anaemia [61].

Tocilizumab is a humanised monoclonal antibody targeted at both membranous and soluble (Mil-6R, Sil-6R) IL-6 receptors. The drug was initially used in the treatment of Castelman disease. In 2009, it was registered in the European Union for the treatment of moderate and severe RA in combination with methotrexate (MTX) or in monotherapy. It is administered by intravenous infusion every 4 weeks at a dose of 8 mg/kg of bodyweight [35]. Blockade of IL-6, a key inflammatory cytokine produced mainly by monocytes and macrophages, which is mediated mainly by IL-1, makes it possible, among other things, to block the activation of antigen-recognising T cells and stimulate differentiation of B cells into plasma cells that synthesize antibodies and to block the synthesis of acute-phase proteins in the liver [37]. However, despite the pathophysiological relationship between ID with anaemia and inflammation, data on iron management in RA patients is quite enigmatic. There are only a few studies which analyse anaemia in RA patients with iron deficiency [12]. It is also not clear whether anti-inflammatory treatment (e.g. blocking the IL-6 pathway) may have an impact on iron management in RA patients.