October - December 2007: 
Volume 20, Issue 4

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Impaired T cell regulation in interstitial lung disease: a new culprit for an old case?
Keywords: Tregs, Sarcoidosis, NSIP, IPF
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The immune system is in a state of perpetual balance between control of invading pathogens and maintenance of immunological tolerance. Immunological tolerance is the process of discrimination between self and non-self and is responsible for the supression of inordinate immune response and reactivity against self antigens1. The mechanisms of immunological tolerance can be divided into three main types. The first is immunological ignorance, in which T and B cells are unaware of the presence of their autoantigen, because of either its low concentration or the presence of blood-tissue barriers that separate the cells from antigens2. The second type is central tolerance, which results from clonal deletion or clonal inactivation, while anergy and active suppression by a group of lymphocytes designated as T regulatory cells constitute the third type, namely, peripheral tolerance3. Among the various subpopulations of immunosuppressive T regulatory cells, the CD4+CD25highFoxp3+ (Treg) cells are those that have been the most extensively studied in both humans and animals. Forkhead/winged helix transcription factor (Foxp3) represents their most definite marker and is thought to be crucial for Treg development and function3. In contrast to CD4+CD25- cells Treg cells are anergic and produce only low amounts of IL-10, TGF-β and IFN-γ4, and no IL-2 and IL-4. Tregs control both innate and adaptive immune responses and are essential for the prevention of autoimmunity5.

Recent findings support a role for Tregs in the pathophysiology and pathogenesis of interstitial lung disease (ILD). Miyara et al observed a substantial increase of Tregs in the peripheral blood (PB) and bronchoalveolar lavage (BAL) and at disease sites of patients with active sarcoidosis6. Despite their high numbers and their potent suppression of cell proliferation, which Miyara’s group believe may be responsible for the anergic state seen in sarcoidosis, Tregs are unable to completely abrogate the secretion of proinflammatory cytokines such as TNF-α and IFN-γ, thus permitting the development of granulomas. A major limitation of the study was the inadequate characterization of Tregs; levels of Foxp3 in the patients’ PB and BAL were not determined either by flow cytometry or real-time quantitative PCR, therefore the observed increase of CD4+CD25high T cells could possibly be attributed to an increase in activated T cells without immunoregulatory properties, which merely express the early activation marker CD25. In support of this concept, other investigators found reduced Foxp3 expression in BAL CD4+ cells of sarcoidosis patients compared to controls, indicative of low numbers, or, alternatively, a qualitative Treg defect7. The authors have observed that sarcoidosis patients exhibit a wide range, both in BAL and in PB, of levels of CD4+CD25highFoxp3+ Treg, which exerted normal suppressive activity, as determined by multicolour flow cytometry and suppression assays, respectively (unpublished observation). It is speculated that the inconsistency between the three studies is a reflection of the heterogeneity in disease presentation in sarcoidosis, although the small size of the patient cohort in the authors’ unpublished study currently precludes conclusions regarding an association between disease severity and Treg levels.

Idiopathic pulmonary fibrosis (IPF) is another condition in the same disease group, commonly encountered in everyday clinical practice, for which there is growing evidence of immune system involvement. Although the pathogenesis of IPF is not yet understood, local and systemic immune activation, remitting after steroid therapy, has been demonstrated in IPF/UIP patients8 and also recent data support the involvement of an autoreactive immune process9. Given the cardinal role of Tregs in the prevention of autoimmunity the authors are currently exploring their possible alterations in patients with IPF and non-specific interstitial pneumonia (NSIP). A significant impairment of Treg suppressor function is evident in both the PB and BAL in IPF patients, most of whom also display low numbers of PB and BAL Treg compared both to control subjects and NSIP patients. In contrast, Tregs are increased in the BAL of NSIP patients, while their suppressor function remains intact. Identical findings were derived from lung tissue biopsies assessed for the presence of Tregs by combined staining for Foxp3 and CD4.

A shift towards a Th2 response seems to predominate in IPF, promoting fibrosis primarily via secretion of profibrotic cytokines into the injured epithelium10. Tregs can suppress not only Th1, but also Th2 responses, as was recently demonstrated in various in-vivo models of airway hyperreactivity (AHR) and inflammation, in which the absence of Tregs promoted AHR11, while adoptive transfer of ovalbumin (OVA)-specific CD4+CD25+ cells to OVA-sensitized mice significantly decreased the expression of Th2 cytokines in the lung after airway antigen challenge12.

It can therefore be surmised that the low numbers and the Treg dysfunction, both systemic and local, found in IPF patients in the authors’ series may either result in inefficient control of an earlier excessive Th2 response or contribute to a Th2 skew. On the other hand, Tregs were observed to be fully functional in NSIP patients, implying the presence of distinct pathogenetic pathways between the two disorders. In addition, the intriguing finding of Treg increase only in the BAL of NSIP deserves further consideration as this may indicate excessive suppression of the local immune response. It is postulated that this local accumulation of Tregs in NSIP may result either from vigorous recruitment from the PB, or from a local increase due to recognition of lung tissue antigens. Alternatively, the high levels of inflammation and the production of proinflammatory cytokines such as IL-2 may release the anergic state of Tregs, inducing them to proliferate. Whatever the mechanism, it remains to be elucidated whether the observed increase of Tregs in the lung epithelium of NSIP occurs as a primary event inducing aberrant inhibition of local immunity or merely represents a reactive process.

Collectively, the data from previous studies and those of the authors clearly suggest a form of systemic immune deregulation in IPF, further fostering the notion of an immune-mediated pathogenesis. In contrast, NSIP appears to follow a different pathogenetic pattern, possibly determined by local phenomena. The findings, if confirmed by further studies, could give rise to new pathways for therapeutic manipulation of Tregs in these incurable types of ILD, either by adoptive transfer of exvivo expanded Tregs13 or by their in-vivo induction by monoclonal antibodies14,15.


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