Thymus and thymoma in myasthenia gravis patients

Abstract

The primary function of the normal thymus is to generate, select and export T lymphocytes - which it does during fetal life, childhood and even into the fourth decade [1, 2]. In general, T cell receptors (TcRs) bind antigenic peptide fragments presented by self HLA molecules; CD4+ "helper" T cells (Th1) are "restricted" to class II molecules (HLA-DR, -DQ or -DP), and CD8+ cytotoxic T cells to class I molecules (HLA-A, -B or -C). These highly polymorphic heterodimers differ most in amino acids lining their peptide-binding grooves, so the epitopes they present tend to show allele-specific binding motifs. In the thymic cortex, bone marrow-derived haemopoietic progenitors generate immature thymocytes in great numbers. As their TcRs rearrange, they generate a vast polyclonal repertoire with very diverse specificities for peptides/HLA variants. Those that can recognise peptides in the individual's own class I or class II alleles on the epithelial cells in the cortex are "positively selected" there, while the remaining 80-90% (that cannot do so) die by neglect [3]. The random process of TcR gene rearrangement inevitably generates some T cells with high affinities for self HLA molecules. So, as the surviving cells mature and move into the thymic medulla, they are "screened" for potential autoreactivity. When they encounter bone marrow- derived dendritic cells, macrophages or medullary epithelial cells at the cortico-medullary junction, those with autoaggressive potential are "negatively selected" (deleted). Occasional medullary epithelial cells express autoantigens from peripheral tissues, e.g. insulin [21]; they too may delete T cells with high affinities for their epitopes. As a result, the T cells eventually exported into the peripheral pool recognise self HLA molecules very weakly - except for those occupied by peptides (derived from foreign antigens) that have optimal contact sites for their particular TcRs. Finally, 5-10% of the nascent CD4+ T cells in the medulla develop locally into CD25+ regulatory T cells (T reg) that can dampen or suppress responses of other T cells and prevent them from expanding out of control [4]. "Myasthenia" means weakness of voluntary muscles, and results from defects in neuromuscular transmission. These usually cause loss of acetylcholine receptors (AChRs) on the post-synaptic (muscle) membrane; reserves of AChR are so limited in humans that a reduction of >60% is enough to cause weakness. In the rare inherited (congenital) myasthenias, there may be mutations in the AChR subunits or in adjacent proteins [5, 6]. By contrast, in myasthenia gravis (MG) receptor loss is caused by autoantibodies specific either to the AChR itself (>80%), or (apparently) to the nearby muscle-specific kinase (MuSK) [7, 8]. These high affinity IgG antibodies can readily be detected in very sensitive and specific radio-immunoassays (RIAs), which are invaluable for diagnosis [9,10]; false positives are very rare. Currently, however, 5-10% of patients are negative in both assays. Since their typical generalised MG clearly improves on plasma exchange, they apparently have auto-antibodies to distinct neuromuscular target(s) whose identification is a high priority. As reviewed in chapter 5, the weakness in MG is characteristically "fatiguable", meaning it increases with increasing effort. It usually first affects the extra-ocular muscles, causing ptosis and/or diplopia, which remain the only defects in ∼15% of cases. In the other ∼85%, the myasthenia generalises, usually within the next three years, to affect movements of the face, mouth, throat, neck and/or limbs [11]. More seriously, there may be problems with swallowing (even leading to inhalation pneumonia) and/or breathing (demanding assisted ventilation) so MG can still be life-threatening. The AChR comprises two a subunits and plus single βl, γ and δ subunits; the γ is replaced by an ε during the third trimester of fetal life. These subunits are all related in evolution [12]; each has about 210 extracellular amino acids, three transmembrane segments in close order, about 100 - residue cytoplasmic loops, a fourth transmembrane segment and a short extracellular C-terminal tail. Together, they form a channel with a central pore that opens when AChs occupy both of their binding sites at the α/δ and α/ε interfaces. The α subunit also includes major epitopes for MG auto-antibodies [13], though they are very hard to map because of the conformational complexity of the extracellular domain. The anti-AChR antibodies in MG are almost entirely specific for extracellular epitopes in the native molecule [13]. They are mainly IgG1 and IgG3, and cause loss of receptors partly by accelerating their degradation, but mainly by activating complement; the ensuing damage tends to flatten the junctional folds in the muscle membrane and reduce total surface area [14]. Only a minority of antibodies in a minority of patients causes pharmacological blockage of ACh binding. The anti-MuSK antibodies are mainly IgG4, which does not activate complement, and their role in pathogenesis is not fully understood [10]. Nevertheless, they clearly identify a separate MG subgroup (Table 6.1) [15], and have not yet been detected in other patients with anti-AChR antibodies, with pure ocular MG, or with thymomas. Classifying MG patients into subgroups is highly informative. Patients with early-onset MG (EOMG; onset before age 40) form a very distinctive subgroup in Caucasians. They have characteristic thymic changes, and are the likeliest to show benefits from thymectomy [16, 17]. Though these have still not been rigorously proven [18], many clinicians believe that the myasthenia remits in about a quarter of thymectomised patients (usually within 2-3 years), improves in about one half of patients, and is unaffected in the remaining quarter. The well known HLA-DR3-B8 associations are especially strong in the 70-80% of females [19]; since their EOMG almost always starts between puberty and menopause, hormonal influences seem highly likely. Autoimmunisation apparently involves the thymus, and may be a two-step process (see below). The EOMG thymus consistently shows medullary lymph node-like infiltrates that expand from perivascular spaces and compress the epithelial cells into characteristic bands and cords [20]. Some of these epithelial cells express HLA-class II and sometimes also AChR subunits [21]; since they also bear CXCR13 and produce IL-6 [22], they may be well placed to attract and autoimmunise mature T cells (Step I) [20]. Rare nearby muscle-like thymic myoid cells are haphazardly scattered in the normal and EOMG medulla, and express striated muscle proteins; these include intact AChR of the fetal isoform that is often preferred by the patients' auto-antibodies [23]. The myoid cells are clearly implicated in provoking the infiltrates in EOMG, especially the germinal centres (Step II) [20]. Since these are the essential sites of all antibody mutation/ diversification [24], it is no surprise that anti-AChR antibodies cloned from these thymi have proved to be highly mutated [25]; the very heterogeneity of the anti-AChR auto-antibodies may enhance their pathogenicity. There is also local activation of AChR-specific plasma cells, which are readily detectable in most EOMG thymi by their spontaneous production of anti-AChR antibodies, especially in patients with the highest serum anti-AChR titres [26]. If thymectomy does, indeed, prove to be beneficial, one might predict that its role is to interrupt the above steps in auto-immunisation. Pre-treatment with corticosteroids can grossly deplete thymocytes, though the effects vary widely [27]. Since the cortex is especially steroid-susceptible, the medullary epithelium and the infiltrates often appear enriched, even though their total bulk is reduced [27]. Especially after steroids have been combined with azathioprine, the thymus may be so drastically collapsed as scarcely to warrant resection at all. Unfortunately, since it may then be replaced by fat, it is not possible to assess the completeness of this "hormonal thymectomy" radiologically, even with enhancement, so pre-operative measurement of residual thymus tissue is not yet possible. In patients with neither anti-AChR nor anti-MuSK auto-antibodies, the thymus often shows changes very similar to those in EOMG; like their myasthenia, these are somewhat milder, with fewer, smaller, germinal centres, though similar infiltrating T cell areas are still obvious [28, 29]. © Springer-Verlag Berlin Heidelberg 2007.