Inflammation in pulmonary arterial hypertension

Pulmonary arterial hypertension in connective tissue diseases

PAH is a common complication of systemic inflammatory conditions, such as scleroderma and systemic lupus erythematosus. Pulmonary arterial lesions in the lungs of patients suffering from connective tissue diseases (CTD) with isolated PH are often similar to those found in lungs displaying PPH, including plexogenic arteriopathy. Resemblance in pathological anatomy may suggest an identical pathophysiology. Besides medial hypertrophy, intimal “onion bulb” lesions and characteristic glomoid-like plexiform lesions, Cool et al. 8 have reported that in patients with scleroderma-related PH, mononuclear inflammatory cells surround vascular sites of plexiform growth, but not uninvolved vessels or extravascular lung structures. In addition, Tuder et al. 9 were the first to identify inflammatory infiltrates in the range of plexiform lesions in the lungs of patients displaying severe PPH. Thiscommon denominator ofPPH and PAH associated with CTD underlines a possible role for vascular inflammation in PAH. In in vitro experiments, auto-antibodies from patients with CTD (anti-U1-ribonucleoprotein antibodies, anti-double-stranded deoxyribonucleic acid antibodies) have been shown to induce up-regulation of immuno-active molecules, such as intercellular adhesion molecule‐1, endothelial leukocycte adhesion molecule‐1 and major histocompatibility complex class II, onhuman pulmonary endothelial cells, suggesting that such immunitary/inflammatory processes could lead to a proliferative and inflammatory pulmonary vasculopathy 10. Some studies have reported a significant improvement in PAH associated with CTD after immunosuppressive therapy 11. However, this clinical observation still needs to be confirmed by large prospective studies (see below).

Disturbances of the immune system can be complicated by pulmonary arterial hypertension

The course of other immunological disturbances, such asHIV infection 2 or plasma cell dyscrasia with polyneuropathy, organomegaly, endocrinopathy, M protein and skin changes (POEMS syndrome) 12, can be complicated with significant PAH. Inflammatory action in the range of affectedvessels has been observed in HIV patients with PAH, although development of severe PAH seems to be unrelated tothe degree of immune deficiency 8. Nevertheless, in a previous study it was demonstrated that a group of HIV patients displaying PAH had significantly higher auto-antibody levelsthan a matched HIV non-PAH control group 13. This couldindicate a role for a complicating auto-immunity statusin the evolution of some seropositive patients, triggering the development of PAH and thereby worsening prognosis 14. Excessive production of immune mediators inthe rare POEMS syndrome with PH has been reported byFeinberg et al. 15. Increased baseline levels of tumour necrosing factor (TNF)‐α, soluble TNF-receptor type I (sTNF-RI), interleukin (IL)‐6, interferon gamma, IL‐2, soluble IL‐2 receptor (sIL-2R) and abnormally low levels of sIL-6R normalised with steroid application and plasmapheresis, with an improvement of disease status. As the interplay of IL‐6 and its receptor sIL-6R appears to be relevant to the pathogenic manifestations of POEMS syndrome with PH, it is remarkable that excessive IL‐1 and ‐6 serum levels have been described in pure PPH, as compared with PH secondary to chronic obstructive pulmonary disease 16. The authors have recently reported an exaggerated production of CX3C-chemokine fractalkine (FKN) and an increased interaction with its receptor, CX3C-R1, in the lungs of patients suffering from severe PPH 17.

Primary pulmonary hypertension patients show a pattern of autoimmunity and inflammation

A large proportion of “pure” PPH patients without immunodeficiency or other associated systemic diseases haveevidence of autoimmunity and/or active inflammation, including detectable levels of circulating antinuclear antibodies 18, elevated serum levels of the pro-inflammatory cytokines IL‐1 and ‐6 16, and increased pulmonary expression of platelet-derived growth factor 19 or macrophage inflammatory protein‐1α 20. For example, anti-fibrillin-auto-antibodies (anti-Fbn‐1) are commonly found in systemic sclerosis, calcinosis, Raynaud's phenomenon, oesophageal involvement, sclerodactyly, telangiectasia syndrome (CREST) and mixed connective tissue disease. Morse et al. 21 have reported elevated frequency of anti-Fbn‐1 in patient groups of adults with PPH (93%, n=75), children with PPH (84%, n=33) and patients with appetite suppressant-associated PPH (67%, n=18), as compared with healthy individuals. In addition, recent data providing evidence for a close association of PPHand autoimmune thyroid disease, such as Grave's disease or Hashimoto-thyreoiditis, has revealed the possibility of an autoimmunitary pathomechanism in PPH 22. Further evidence supporting the concept of a systemic inflammatory component in PPH was recently given by Balabanian et al. 17, who demonstrated significantly increased plasma levels of various inflammatory markers in patients with severe PPH, as compared with normal controls (table 1⇓).

The monocrotaline model

Pathological changes in lungs of patients displaying PAH do not concern the whole pulmonary arterial tree, but remain restricted to certain levels of the vessel. The classical hypertensive pulmonary arteriopathy concerns overall muscular arteries of ≤500 µm in diameter, corresponding to the subsegmentary arteries and their down-stream colaterals. Different and characteristic lesions, such as isolated medial hypertrophy, concentric intimal fibrosis, in situ thrombosis, pulmonary arteritis and typical plexiform lesions with their glomoid-like exhuberant endothelial cell proliferation, are found 23. Perivascular inflammatory infiltrates with macrophages and lymphocytes in the range of occlusive lesions canbe observed in PAH 9. Endothelial cell dysfunction withderegulated expression of vasoactive, mitogenic and pro-inflammatory mediators may be the cause of these changes 24, 25. Monocrotaline, a plant-derived toxine, causes endothelial cell injury and subsequently a massive mononuclear infiltration into the perivascular regions of arterioles and musculary arteries when injected into rats. These animals develop severe PH after monocrotaline exposure 26. Although typical plexiform lesions are not normally found in monocrotaline-induced PH, it is used as a standard model for PAH and PPH. The important role of inflammation in this model has led to several studies focusing on immunosuppressive and anti-cytokine treatment (see below), and, therefore, raises the question of how involved inflammatory cascades are in the installation and evolution of PAH lesions in humans.

The role of chemokines in pulmonary arterial hypertension

Identification of perivascular inflammatory cell infiltrates, comprised of T‐ and B‐lymphocytes and macrophages, has supported the concept that inflammatory cells may play a role in PAH. The involvement of leukocytes, such as macrophages and lymphocytes, in complex lesions of PPH was initially described by Tuder et al. 9. Recent studies by Dorfmüller et al. 27 have confirmed this observation, stressing the possible role of perivascular lymphocytic infiltrates. This inflammatory pattern has been demonstrated in plexiform lesions, as well as in other vascular lesions of PAH-affected lungs (fig. 1⇓).

The role of T‐lymphocyte recruitment by chemotactic cytokines in PAH has also been previously evaluated. Leukocyte trafficking involves successive events, including rolling, firm adhesion and extravasation, in response to a chemoattractant gradient that may involve chemokines 28. Chemokines are soluble, secreted basic proteins that direct the migration of specific subsets of leukocytes 28, 29. They play a major role in the different steps of leukocyte recruitment, including rolling, activation, adherence and extravasation into the inflammed tissue. The above-mentioned studies by Dorfmüller et al. 27 and Balabanian et al. 17 have attempted to analyse those chemokine-dependent mechanisms leading to inflammatory cell recruitment in the lungs of patients displaying PAH. FKN/CX3CL1 is a unique chemokine, since it exists inboth a soluble form as a chemotactic protein and ina membrane-anchored form as a cell-adhesion molecule on endothelial cells 30, 31. Its actions are mediated by CX3CR1, a seven transmembrane receptor that is expressed by monocytes, microglial cells, neurons, natural killer cells, mast cells and subpopulations of T‐lymphocytes 32–37. FKN promotes CX3CR1-expressing leukocyte recruitment, but, unlike other chemokines, it can mediate the rapid-capture, integrin-independent adhesion and activation of circulating CX3CR1+ leukocytes under high blood flow 38–40. Several recent studies have reported a polymorphism in CX3CR1 associated with a reduced risk of acute coronary artery disease, suggesting that FKN plays a critical role in monocyte/T‐cell recruitment to the vessel wall 41, 42. The authors were able to demonstrate the following: 1) CX3CR1 was upregulated incirculating CD4+ and CD8+ T‐lymphocytes from PAH patients, as compared with controls; 2) this deregulation of CX3CR1 expression accounted for the increased sensitivity ofthese cells to soluble FKN (sFKN) (fig. 2⇓); 3) the abnormal response of T‐lymphocytes to FKN was not the mere consequence of PH, as it was not present in patients with PHsecondary to chronic thrombembolic PH (CTEPH); 4)elevated sFKN plasma concentrations were measured in PAH patients, as compared with CTEPH patients and normal controls; 5) lung samples from PAH patients showed anincreased FKN messenger ribonucleic acid (mRNA) expression, as compared with controls, and pulmonary artery endothelial cells from PAH patients expressed FKN (fig. 3⇓).

Regulated upon activation, normal T‐cell expressed and secreted (RANTES) is an important chemoattractant for monocytes and T‐cells 43, 44. RANTES presumably plays akey role in a number of arterial inflammatory processes, such as glomerulonephritis 45, Kawasaki disease 46 and Takayasu arthritis 47. In addition, successful antagonisation of RANTES has been reported in animal models of inflammatory disease 48–50. RANTES may also play an indirect role in PAH through the induction of endothelin-converting enzyme‐1 and endothelin‐1, a potent endothelium-derived factor with strong vasoconstrictive and mitogenic action 51. In their recent work, Dorfmüller et al. 27 have found new evidence for a possible involvement of this potent mediator in the evolution of PAH. Experiments on patients and healthy controls showed the following: 1) RANTES mRNA was detected by competitive reverse transcriptase-polmerase chain reaction in lung samples from all PAH patients and controls; 2) the number of RANTES mRNA copies was significantly elevated in the lungs of PAH patients as compared with controls (fig. 4⇓); and 3) endothelial cells were the major source of RANTES, identified by in situ hybridisation and immunohistochemistry in PAH lung samples (fig. 5⇓) 27.

Immunosupressants in the therapy of pulmonary arterial hypertension

The effectiveness of corticosteroids and immunosuppressants in the treatement of PAH associated with connective tissue disorders has been discussed previously. In the absence of a larger placebo-controlled study, evaluations about efficacy of such treatment rely on case reports and observations in smaller groups. Nevertheless, many of these publications have reported an improvement after treatment, usually with a combination of immunosuppressants and corticoids 11, 52. Interestingly, convincing results with decreasing mean pulmonary arterial pressures of <28 mmHg have been frequently reported in patients suffering from systemic lupus erythematosus 53, 54. Conversely, treatment in patients with systemic sclerosis seems to be less effective on PH, suggesting that different mechanisms may be involved in the pathogenesis ofPH secondary to scleroderma. Usually, corticosteroids associated with immunosuppressants, such as cyclophosphamide in bolus infusion, seem to be the most effective treatment. However, immunosuppressive protocols vary from one study to another and comparison is difficult. Lastly, there can be difficulties in evaluating the effects of immunosuppressive therapy alone because of the frequent use of vasodilator therapy 11. In conclusion, immunosuppressants should be considered for patients with CTD and PAH, with the important exception of scleroderma. Here, strict clinical and haemodynamical criteria are necessary to evaluate the efficacy of such a treatment. If there is no clinical and haemodynamical improvement after 3–6 months of therapy then treatment should be stopped because of possible complications, including infections and neoplasms.

Recent studies on animal models with induced PH have highlightened two other substances with immunosuppressive effects in the treatment of PH. Rapamycin, a macrolide immunosuppressant currently being used in therapy for chronic allograft rejection, and triptolide, a herb used in traditional chinese medicine to treat rheumatoid arthritis andother autoimmune diseases, have been tested on rats being pneumectomised and subsequently treated with monocrotaline 26, 55. Both rapamycin and tryptolide showed significant effects with lower mean pulmonary arterial pressures, as compared with vehicle-treated controls. As a morphological correlate to these findings, a significantly less right ventricular hypertrophy and pulmonary arterial neointimal formation were demonstrated. It is noteworthy that tryptolide shows anti-inflammatory properties by inhibiting T‐cell activation at the level of cytokine gene transcription 56.

Anti-cytokine treatment in pulmonary arterial hypertension

Cytokine antagonists have been mainly tested on the monocrotaline rat model. Voelkel et al. 6 have studied the role of IL‐1, a strong pro-infammatory cytokine, in monocrotaline-induced PH as compared with chronic hypoxia PH, showing that IL‐1 is excessively produced in the lungs of rats treated with monocrotaline. Repeated injections of IL‐1 receptor antagonist reduced PH and right heart hypertrophy in the monocrotaline model but not in the chronic hypoxia model. If studies showing elevated circulating levels of IL‐1 and ‐6 in patients displaying PPH, but not in patients with PHsecondary to COPD are considered, a possible role for mediators of inflammation in some forms of PH can be assumed 16. However, the authors are not aware of any studies attempting to evaluate anti-cytokine therapies in PH patients.

Numerous studies on inflammatory diseases and the effects of anti-cytokine treatment have stressed the importance of such therapeutic alternatives. Animal model experiments with RANTES-receptor antagonists (Met-RANTES), show relevant anti-inflammatory properties in the treatment of chronic allograft nephropathy. In their latest study, Song et al. 57 showed that Met-RANTES diminishes the early infiltration and activation of mononuclear cells in grafts of transplanted rat kidneys accompanied by a local decrease of IL‐1, ‐2 and TNF‐α, as well as RANTES, and thereby reduces the pace ofchronic allograft nephropathy. Moreover, recent studies demonstrated the prevention of crescentic glomerulonephritis in animal models by immunoneutralisation of the FKN-receptor CX3CR1 40. Considering these studies on FKN and RANTES, and the possible role for these two chemokines in PAH, further studies on animal models should evaluate a possible role for anti-cytokine treatment in this condition.

When assessing these different observations, inflammation and autoimmunity seem to have at least some influence on theevolution of the disease. The frequent association of pulmonary arterial hypertension and well-defined inflammatory conditions, as well as the presence of complicating pulmonary arterial hypertension in autoimmunitary disturbances, indicate a possible role for inflammatory cascades, leading to inflammatory infiltrates and remodeling of the vessel. Latest results from the authors suggest that increased activation of circulating inflammatory cells in affected individuals is a primary event, rather than a pure response to the altered physiological condition of the patient.