Impact of STAT3 Mutations on Clinical Features and Treatment Outcomes in Large Granular Lymphocyte Leukemia

Large granular lymphocyte leukemia (LGL) is a generally indolent clonal/oligoclonal lymphoproliferation of cytotoxic T cells (CTLs) associated with autoimmune disorders and B cell dyscrasias. From an immunologic standpoint, clonal CTL expansions in LGL represent a natural model for cytotoxic immune responses in which the clinical manifestation may be dependent upon the target recognition spectrum of clonal cells. The presence of STAT3 mutations in the context of autoimmune disease explains how the breach of tolerance may solidify a pathologic immune response. Conversely, the clonal proliferative drive due to chronic antigen stimulation may create a selective milieu for evolution of STAT3 mutant clones. Recent description of germ line STAT3 mutations in families with an early-onset autoimmune syndromes further solidifies the role of STAT3 in the regulation of immune responses.

A proportion of patients with LGL display severe cytopenias, the most common indication for therapy initiation. Therapy choices in LGL are mostly empiric and thus the ability to rationally select the most effective primary therapy would help to optimize outcomes and avoid futile cycles in refractory patients. We hypothesized that the mutational status and other clinical and molecular parameters may allow for better selection of therapy in LGL.

By uniform stringent criteria we diagnosed 181 patients with LGL (90% T, 10% NK LGLs). The median follow up was 35 months, and 85% of cases received therapy for their cytopenias manifested as anemia in 47%, neutropenia in 50%, and bi-and multi lineage cytopenia in 32%. Somatic STAT3 mutations were detected by targeted next generation sequencing in 36% of patients, with >2 mutations (biclonal) found in 5%. No germ line STAT3 mutations were found in this cohort. 60% of STAT3 positive and 42% of STAT3 negative cases were associated with neutropenia (p=0.02); the severity of neutropenia strongly correlated with the Y640F (p=0.0001) and D661Y (p=0.012) clonal burden as assessed by variant allelic frequency.

Cyclosporine A (CsA), oral cyclophosphamide (CTX) or methotrexate (MTX) were used in 61, 42, and 53 patients respectively. As initial therapy, CsA, CTX and MTX were associated with response rates of 41%, 59%, and 39%, while overall response rates were 42%, 60% (p=.06), and 36%, respectively. Salvage therapies included alemtuzumab (N=21, 29% responses), tacrolimus (N=6/8 and 62% responses), abatacept (6/8 responders) and tofacitinib (6/9 responses) and ATG (2/4). When comparing STAT3 mutant cases to those without a STAT3 mutation (WT), if treated with CsA, 50% mutant cases responded vs. 35% WT (p=.25), with CTX 68% mutant cases responded vs. 44% WT (p=.11), and with MTX 46% mutant cases responded vs. 32% WT (p=.3). Of note is that 50% of STAT3 mutant cases responded to JAK inhibitor tofacitinib. STAT3 mutational status did not affect survival (p=0.69), but as expected female LGL patients had better overall survival than males (p=0.02).

In conclusion, the clonal burden for STAT3 Y640F and D661Y mutations correlate with the severity of neutropenia but do not affect the probability of response to common therapies of this disease. Female LGL patients had better overall survival.