Somatic STAT3 Mutations in CD8+ T Cells of HTLV-2 Positive Blood Donors
Authors: Kim Daehong , Myllymäki Mikko , Kankainen Matti , Jarvinen Timo , Park Giljun , Bruhn Roberta , Murphy Edward L , Mustjoki Satu 
Published: November 2021

Abstract
Introduction: T-cell large granular lymphocyte (T-LGL) leukemia is a rare lymphoproliferative disorder with recurrent somatic STAT3 mutations. It has been suggested that viral antigens act as the initial stimuli resulting in clonal expansion of CD8+ cells in the disease. However, less is known whether chronic exposure to viral antigens is associated with acquisition of somatic mutations in CD8+ T cells among individuals without clinically detectable lymphoproliferations.
Human T-cell leukemia virus type 2 (HTLV-2) antibody positivity has been detected in patients with T-LGL leukemia. Here, we examined whether CD8+ T cells from HTLV-2 positive healthy blood donors harbor somatic mutations in STAT3 or other immune-associated genes, potentially identifying individuals at risk of subsequent lymphoproliferative diseases.
Methods: We analyzed HTLV-2 infected (n=30) and uninfected (n=35) healthy blood donor samples obtained from University of California San Francisco and Vitalant Research Institute, which were enrolled in the United States-based HTLV Outcomes Study (HOST) cohort. All cases had serologic evaluation for HTLV-2 infection at the time of sampling. We examined somatic mutations of STAT3 in CD4+ and CD8+ T-cell populations using ultra-deep targeted amplicon sequencing. In addition, we applied a custom sequencing panel covering the coding regions of 2,533 immune-related genes to characterize a larger spectrum of somatic mutations in CD8+ T cells.
Results: Somatic STAT3 mutations were detected in CD8+ but not in CD4+ T cells of four (13.3%, 4/30) HTLV-2 positive healthy blood donors (p.Y640F, p.N647I, p.D661Y, and p.Y657_K658insY with variant allele frequencies of 11.9%, 0.5%, 4.9%, and 1.2%, respectively) using amplicon sequencing. The detected STAT3 mutations have been previously described and reported in T-LGL leukemia. Total white blood cell and lymphocyte counts were similar between STAT3 mutated and non-mutated cases. No STAT3 mutations were discovered in HTLV-2 negative donors with amplicon sequencing.
Of the 28 HTLV-2 positive cases, 19 had at least one somatic variant in CD8+ T cells based on the immunogene panel sequencing (n=28). 8 cases had variants in genes previously identified in T-LGLL (STAT3, KMT2D, TYRO3, DIDO1, BCL11B, CACNB2, KRAS, LRBA and FANCA), and 5 cases had variants in genes involved in JAK-STAT signaling (NFKBIA, PIK3R5, MAPK14, EP300, MPL, IFNAR1, IL6ST and IL20RA). Three recurrently mutated genes were detected: VWF, SMAD7 and MXRA5. The four HTLV-2 positive blood donors harboring STAT3 mutations had more somatic mutations (median=6) than HTLV-2 positive donors without STAT3 mutations (median=1, p=0.06).
Conclusion: We report the presence of somatic gain-of-function STAT3 mutations in CD8+ T cells of 13% of HTLV-2 positive healthy blood donors. We identified additional somatic mutations in genes associated with JAK-STAT signaling, immune regulation and lymphoproliferation in CD8+ T cells of HTLV-2 positive cases. While STAT3 activation, with or without mutations, is considered as a hallmark of T-LGLL, our results reveal the presence of STAT3 mutations in CD8+ T cells of healthy blood donors harboring HTLV-2 without clinical history of lymphoproliferative disease. Additional research is warranted to elucidate whether HTLV-2 carriers harboring STAT3 and other mutations are at increased risk of subsequent T-LGL leukemia or other lymphoproliferative diseases.

Disclosures
Mustjoki: Pfizer: Research Funding; BMS: Research Funding; Novartis: Research Funding; Janpix: Research Funding.