Angela R. Manser

Ruxolitinib is a small-molecule inhibitor of the JAK kinases, which has been approved for the treatment of myelofibrosis, a rare myeloproliferative neoplasm (MPN), but clinical trials are also being conducted in inflammatory-driven solid tumors. Increased infection rates have been reported in ruxolitinib-treated patients, and natural killer (NK) cells are immune effector cells known to eliminate both virus-infected and malignant cells. On this basis, we sought to compare the effects of JAK inhibition on human NK cells in a cohort of 28 MPN patients with or without ruxolitinib treatment and 24 healthy individuals. NK cell analyses included cell frequency, receptor expression, proliferation, immune synapse formation, and cytokine signaling. We found a reduction in NK cell numbers in ruxolitinib-treated patients that was linked to the appearance of clinically relevant infections. This reduction was likely due to impaired maturation of NK cells, as reflected by an increased ratio in immature to mature NK cells. Notably, the endogenous functional defect of NK cells in MPN was further aggravated by ruxolitinib treatment. In vitro data paralleled these in vivo results, showing a reduction in cytokine-induced NK cell activation. Further, reduced killing activity was associated with an impaired capacity to form lytic synapses with NK target cells. Taken together, our findings offer compelling evidence that ruxolitinib impairs NK cell function in MPN patients, offering an explanation for increased infection rates and possible long-term side effects associated with ruxolitinib treatment.

Killer cell immunoglobulin-like receptors (KIRs) on natural killer (NK) cells are crucially involved in the control of cancer development and virus infection by probing cells for proper expression of HLA class I. The clonally distributed expression of KIRs leads to great combinatorial diversity that develops in the presence of the evolutionary older CD94/NKG2A receptor to create highly stochastic but tolerant repertoires of NK cells. These repertoires are present at birth and are subsequently shaped by an individuals' immunological history toward recognition of self. The single most important factor that shapes functional NK cell repertoires is the genetic diversity of KIR, which is characterized by the presence of group A and B haplotypes with complementary gene content that are present in all human populations. Group A haplotypes constitute the minimal genetic entity that provides high affinity recognition of all major human leukocyte antigen class I-encoded ligands, whereas group B haplotypes contribute to the diversification of NK cell repertoires by providing sets of stimulatory KIR genes that modify NK cell responses. We suggest a cooperative model for the balancing selection of A and B haplotypes, which is driven by the need to provide a suitable corridor of repertoire complexity in which A/A individuals with only 16 different KIR combinations coexist with A/B and B/B donors expressing up to 2048 different clone types.

Natural killer cells are well known to mediate anti-leukemic responses in myeloid leukemia but their role in myelodysplastic syndromes is not well understood. Here, in a cohort of newly diagnosed patients (n=75), widespread structural and functional natural killer cell defects were identified. One subgroup of patients (13%) had a selective deficiency of peripheral natural killer cells (count <10/mm(3) blood) with normal frequencies of T and natural killer-like T cells. Natural killer cell-deficient patients were predominantly found in high-risk subgroups and deficiency of these cells was significantly associated with poor prognosis. In the second subgroup, comprising the majority of patients (76%), natural killer cells were present but exhibited poor cytotoxicity. The defect was strongly associated with reduced levels of perforin and granzyme B. Notably, natural killer cell function and arming of cytotoxic granules could be fully reconstituted by in vitro stimulation. Further phenotypic analysis of these patients revealed an immature natural killer cell compartment that was biased towards CD56(bright) cells. The residual CD56(dim) cells exhibited a significant increase of the unlicensed NKG2A(-)KIR(-) subset and a striking reduction in complexity of the repertoire of killer cell immunoglobulin-like receptors. Taken together, these results suggest that the widespread defects in natural killer cell function occurring in patients with myelodysplastic syndromes are mostly due to either unsuccessful or inefficient generation of mature, functionally competent natural killer cells, which might contribute to disease progression through impaired immune surveillance.

A role for HLA class I polymorphism in childhood acute lymphoblastic leukemia (ALL) has been suggested for many years, but unambiguous associations have not been found. Here, we show that the HLA-C-encoded supertypic epitope C2, which constitutes a high-affinity ligand for the inhibitory natural killer (NK) cell receptor KIR2DL1, is significantly increased in ALL patients (n = 320; P = .005). Stratification for ethnicity and disease subtype revealed a strong association of C2 with B-ALL in German cases (P = .0004). The effect was independent of KIR2DS1 and KIR2DL1 allelic polymorphism and copy number. Analysis of clinical outcome revealed a higher incidence of late relapse (> 2.5 years) with increasing number of C2 alleles (P = .014). Our data establish C2 as novel risk factor and homozygosity for C1 as protective for childhood B-ALL supporting a model in which NK cells are involved in immunosurveillance of pediatric B-ALL via interaction of KIR with HLA-C ligands.