STAT3-Mutations Indicate the Presence of Subclinical Self-Reactive Cytotoxic T Cell Clones in Aplastic Anemia and Myelodysplastic Syndromes

Abstract 646

Large granular lymphocyte leukemia (LGL) is often associated with immune cytopenias, but can also occur in the context of myelodysplastic syndrome (MDS). LGL shares certain pathogenetic similarities with aplastic anemia (AA) and some forms of MDS, in which cytopenias are related to immune suppression of normal hematopoiesis. In these conditions, the inhibition of hematopoietic progenitor and stem cells has been described to be mediated by mostly polyclonal cytotoxic T lymphocytes (CTL). Previously, using molecular analysis of TCR VB repertoire in these diseases we have demonstrated oligoclonal skewing clonal of CTL spectrum that was reminiscent (albeit less pronounced) to that seen in LGL. These observations support the theory that these CTL expansions correspond to a cellular reaction against autologous hematopoietic targets. Detection of STAT3 mutations would substantiate the hypothesis that autoimmune reactions can be due to intrisic genetic lesions in autoimmune cells.

The recent discovery of recurrent somatic STAT3 mutations appears to be the key molecular lesion promoting clonal outgrowth of autonomous CTL clones in LGL. This finding raised the hypothesis of whether those mutations could be found in other bone marrow failure (BMF) states and whether they could be diagnostically useful and associated with distinct clinical features. Initially, we have directly sequenced STAT3 exons in 120 T-LGL cases and identified 33 mutations in 32 cases (27%). All mutations were located in the domain of STAT3 (residues 585–688) that shares homology with Src homology 2 (SH2) domains. The STAT3 SH2 domain mediates STAT3 dimerization via binding of phosphotyrosine residue Y705. Two mutations, Y640F and D661Y, accounted for 80% of the somatic variations found, enabling the design of a more sensitive ARMS-PCR method for each of these alterations, suitable for the massive screening we have envisioned for AA and MDS.

In BMF, we have first identified 21 MDS patients with known LGL and screened them for the presence of STAT3 mutations: in the CTLs of 6/21 of these patients STAT3 mutations were found and thus less clinically apparent LGL expansion could also be present in more classical MDS. Thus, we extended our screening to CTLs from an additional 368 patients with MDS and no suspected concomitant LGL: we identified 9 additional patients with STAT3 mutated clones. MDS patients carrying STAT3 mutant CTL clones had both advanced and low risk disease (low, n=1; int-1, n=4; int-2, n=8; High, n=2). These patients were characterized by a higher frequency of hypocellular bone marrow (55 vs. 10.5%; p<.001), and neutropenia at diagnosis (p<.04). No significant differences were found in overall survival. By analogy we also searched for STAT3 CTL clones in AA (N=148) and PNH (N=30). In total, we have identified 17 (10%) AA patients with STAT3 mutant CTL clones: all of these patients did not display manifest LGL. Clinically, these patients had a higher proportion of non severe AA (40% vs. 23%) and were more likely to respond to first line immunosuppression (76 vs., 65%) though no statistical significance was reached. In addition, cases with BMF and a subclinical mutant CTL clones were retrospectively tested for the presence of a TCR rearrangement: an oligoclonal (40% of cases) or monoclonal pattern (20% of cases) was seen in most of patients. In sum, the surprising discovery of STAT3 mutated clones in BMF states seems to be predominantly in AA patients and can be also found MDS cases with mainly, hypoplastic features, suggesting that STAT3 mutated self reactive CTL clones may play a role in immune pathogenesis of these conditions.