Altered T-cell subset repertoire affects treatment outcome of patients with myelofibrosis.

Phenotypic characterization of T cells in myelofibrosis (MF) is intriguing owing to increased inflammation, markedly elevated pro-inflammatory cytokines, and altered distribution of T-cell subsets. Constitutive activation of Janus kinase-2 (JAK2) in the majority of MF patients contributes to the expression of the programmed cell death protein-1 (PD1) and T-cell exhaustion. We wondered whether T-cell activation affects treatment outcome of patients with MF and sought to determine whether the JAK1/2 inhibitor ruxolitinib affects the activation of T-cell subsets. T cells from 47 MF patients were analyzed and the percent of either helper (CD4+) or cytotoxic (CD8+) naive, central memory, effector memory, or effector T cells; and fractions of PD1-expressing cells in each subset were assessed. An increased number of T cells coexpressing CD4/PD1 and CD8/PD1 in MF compared to healthy controls (n=28) was found, and the T cells were significantly skewed toward an effector phenotype in both CD4+ and CD8+ subsets, consistent with a shift from a quiescent to an activated state. Over the course of ruxolitinib treatment, the distribution of aberrant T-cell subsets significantly reversed towards resting cell phenotypes. CD4+ and CD8+ subsets at baseline correlated with monocyte and platelet counts, and their PD1-positive fractions correlated with leukocyte counts and spleen size. Low numbers of PD1+/CD4+ and PD1+/CD8+ cells were associated with complete resolution of palpable splenomegaly and improved survival rate, suggesting that low levels of exhausted T cells confer a favorable response to ruxolitinib treatment.